Supercritical fluids in fuel cell research and development

J. of Supercritical Fluids 62 (2012) 1–31

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Review

Supercritical fluids in fuel cell research and development S.E. Bozbag, C. Erkey ∗ Department of Chemical and Biological Engineering, Koc¸ University, 34450, Sariyer, Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 2 February 2011 Received in revised form 31 July 2011 Accepted 16 September 2011 Keywords: Fuel cell Supercritical Electrocatalyst Membrane Fuel processing

a b s t r a c t Fuel cells (FCs) are emerging as devices for electricity generation in a new economic era where energy is increasingly obtained from renewable sources. FCs operate with relatively higher efficiencies as compared to internal combustion engines due to the direct conversion of chemical energy to electricity by electrochemical reactions. However, there exist a number of obstacles for their widespread acceptance and integration in our daily lives. These obstacles can be summarized as the high cost of FCs due to the high costs of materials, the need to process fuels to very high-purity levels, the unacceptable declines in performance with time as well as the absence of a H2 infrastructure. Applications of supercritical fluids (SCFs) in synthesis of novel materials and development of new processing techniques offer a wide range of opportunities that can help commercialization of FCs. These include the preparation of micro or nanoarchitectured materials in a highly controllable manner for electrolyte-electrode assemblies of a wide variety of FCs including proton exchange membrane FCs (PEMFCs) and solid oxide FCs (SOFCs). In this extent, materials synthesized using SCFs are (at least) comparable or superior in performance as compared to their conventional counterparts. The synthesis and processing of novel materials necessary for efficient hydrogen storage/processing and design of novel processes for H2 production may also benefit from the use of SCFs. © 2011 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4. 5.

6.

7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Types of FCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Thermodynamics of FCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Performance analysis of FCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Challenges in achieving widespread usage of FCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supercritical fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Properties of SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PEMFCs and the obstacles for widespread commercialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of SCFs in development of membrane electrode assemblies for PEMFCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Preparation of composite Nafion® membranes and its alternatives using SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Preparation of carbon supported electrocatalysts using SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Preparation of carbon supports using SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Future research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of SCFs in development of electrolyte–electrode assemblies for SOFCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Preparation of ceramic membranes using SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Future research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fuel processing and storage using SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Preparation of membranes for hydrogen purification using SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Hydrogen production by reforming and gasification in supercritical and near-critical water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Preparation of catalysts for fuel processing using SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +90 2123381866; fax: +90 2123381548. E-mail address: [email protected] (C. Erkey). 0896-8446/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2011.09.006

2 2 3 4 5 6 6 6 8 10 10 13 19 20 21 22 22 22 23 24 26

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8.

7.4. Gas clean-up using mesoporous adsorbents prepared using SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5. Use of SCFs in development of materials for hydrogen storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6. Future research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Studies by many groups at universities, research institutes and companies around the world indicate that the crude oil and natural gas production rates in our planet have peaked or will peak in the coming years. The reserves of the most abundant fossil fuel, coal, are also limited. Furthermore, it is now widely accepted that the use of fossil fuels causes an increase of CO2 concentration in the atmosphere. This increase is believed to cause a net warming of the earth’s surface, which can have dramatic consequences around the globe. An increase in global temperature is expected to cause sea levels to rise and to change the amount and pattern of precipitation. Other likely effects include changes in the frequency and intensity of extreme weather events, species extinctions and changes in agricultural yields. Even though the predictions of the magnitude of these changes are uncertain, immediate action is required to combat global warming [1]. As a result of these two factors that are depletion of fossil fuel reserves and global warming, energy in the world is going to be produced increasingly from renewable sources in the coming decades. These include biomass, geothermal, hydropower, solar, ocean-based and wind energy. However, most of these renewable energy sources are variable on a daily basis and are not available in all regions on earth, which necessitates the utilization of energy storage systems. The use of the stored energy requires energy carriers such as electricity or hydrogen [1]. The electricity generated from renewable sources can be used to produce hydrogen from splitting of water by electrolysis as shown in Fig. 1. The generated hydrogen can then be stored on-site for later use or pumped into a pipeline grid. A fuel cell (FC) can then be used to generate electricity from hydrogen and oxygen from air. The emissions from such a FC consist of only water.

26 26 27 27 27 27

The scheme presented in Fig. 1 can be termed sustainable in which energy from renewables provides the carbon free energy to make hydrogen for use as energy carrier. FCs are very important components of this cycle and therefore, their development is necessary for successful transition into hydrogen economy. Currently, such a scheme is not practical because of the high cost of FCs which are often associated with expensive materials that are used in their manufacture and the low durability of FCs. As a result, research and development efforts in the FC field focuses mainly in the areas shown in Fig. 2. Supercritical fluids (SCFs) have been used as synthesis and processing media for many materials and compounds [2,3]. In this article, the studies associated with FCs applications are reviewed. We first describe the basics of the FCs and SCFs in Sections 2 and 3, respectively. Section 4 is dedicated to an analysis of the obstacles for the widespread commercialization of PEMFCs. The use of SCFs in the production and processing of FC electrode–electrolyte assemblies for PEMFCs and SOFCs have shown very promising results and are described in Sections 5 and 6, respectively. Materials synthesized using SCFs such as catalysts, membranes and adsorbents can also be very useful in H2 storage and fuel processing for FCs which are reviewed in Section 7. Futuristic perspectives are also included at the end of every chapter. 2. Fuel cells A FC is an electrochemical device that continuously converts the chemical energy in a fuel into electricity and heat. The basic component a FC consists of an electrolyte layer in contact with a porous anode and cathode on either side. A schematic representation of a typical proton exchange membrane FC (PEMFC) with the reactant/product gases and the ion conduction flow directions through the cell is shown in Fig. 3.

Fig. 1. Electricity production from renewable sources using hydrogen.

S.E. Bozbag, C. Erkey / J. of Supercritical Fluids 62 (2012) 1–31

3

Fig. 2. Main focuses of fuel cell research and development.

In a typical FC, fuel is fed continuously to the anode (negative electrode) compartment and an oxidant (i.e., oxygen from air) is fed continuously to the cathode (positive electrode) compartment. The electrochemical oxidation of the fuel at the anode generates electrons, which flow through an external load to the cathode. The electrons cause the electrochemical reduction of the oxidant at the cathode. The ions that are generated can be either positive or negative ions, meaning that an ion carries either a positive or negative charge (surplus or deficit of electrons). In theory, any fluid can be used as a fuel and can be subjected to oxidation at the FC’s anode as long as the fluid is supplied continuously and its its chemical oxidation is possible. Similarly, the oxidant can be any fluid that can be reduced at a sufficient rate. Fuel can be pure H2 and also any hydrocarbon such as natural gas, methanol, gasoline or even biomass if the FC system is coupled with a fuel processor. The electrolyte conducts ionic charge between the electrodes and thereby completes the cell electric circuit, as illustrated in Fig. 3. It also provides a physical barrier to prevent the fuel and oxidant gas streams from directly mixing. The functions of porous electrodes in FCs are: (1) to provide a surface where gas/liquid ionization or de-ionization reactions can take place, (2) to conduct ions away from or into the solid–fluid interface(s) once they are formed (so an electrode must be made of materials that have good electrical conductivity), and (3) to provide a physical barrier that separates the bulk gas phase and the electrolyte. The conversion in a FC virtually continues for as long as fuel and oxidant are supplied. Therefore, a FC is like a battery and is a thermodynamically open system compared to conventional cell

Fig. 3. Schematic of a proton exchange membrane fuel cell.

batteries. FCs have high energy efficiency, no emissions, no noise, and they are modular [4,5]. Because of these remarkable properties, the use of FCs as power sources is expected to increase in the coming decades. In the subsequent three subsections of this section, different types of FCs (Section 2.1), as well as the thermodynamics (Section 2.2) and the performance analysis (Section 2.3) of FCs are described. The current obstacles against the widespread utilization of FCs (Section 2.4) are also discussed. 2.1. Types of FCs FCs are generally classified based on the electrolyte used. Currently available FCs for commercial or research use and their properties are presented in Table 1. Proton exchange (or polymer electrolyte) membrane FCs (PEMFC) and direct methanol FCs (DMFC) are operated at low temperatures (50–100 ◦ C). DMFCs are elegant since the use of a liquid fuel (methanol) provides advantages in transportation and storage of fuel. In these FCs, the most commonly used electrolyte is a perfluorinated ionomer consisting of a fluorinated-carbon backbone with fluoro-ether side chains ending in sulfonic acid groups, commercially known as Nafion® . Pure H2 fuel is used in PEMFC and an electrical efficiency range of 35–45% is generally achieved for applications from automotive to the combined heat and power systems (5–250 kW). According to the U.S. Department of Energy (DOE), PEMFCs are the primary candidates for powering light-duty vehicles and buildings, and have potential for much smaller applications such as replacements for rechargeable batteries. Typically, a PEMFC operating at atmospheric pressure generates more than 0.6 A/cm2 at 0.6 V with a typical operating temperature range as low as 60–80 ◦ C. Commercially, a limited number of PEMFCs have been deployed into buses in cities in Europe, USA, Canada and Japan. These FCs were manufactured by companies including UTC Power, Ballard and Toyota with power ranges varying from 90 to 300 kW. As far as the fuel specifications are concerned, phosphoric acid FCs (PAFCs) are considerably less sensitive to CO poisoning (up to 0.5% CO as diluent) as compared to PEMFCs (10 ppm of CO is enough the poison the catalyst). However, both of these FCs suffer from slow oxygen reduction reaction kinetics at the cathode which necessitates the use of platinum based catalysts. PAFCs are quite similar in electrode components and working principle to PEMFCs except that a liquid electrolyte is used in PAFCs. The PAFC stationary systems produced by UTC Power is another commercially available example of FCs. Solid oxide FCs (SOFCs) are often used in stationary power generation thanks to their superior power range but they do operate at high temperatures (>700 ◦ C) and are not convenient for mobile applications due to their long initiation period as compared to PEMFCs. A solid oxide electrolyte such as yttria stabilized zirconia (YSZ) is used as the electrolyte in SOFCs which can achieve MW ranges and can

Adapted from the references [4,6]. PEMFC: Proton exchange membrane fuel cell; DMFC: Direct methanol fuel cell; AFC: Alkaline fuel cell; PAFC: Phosphoric acid fuel cell; MCFC: Molten carbonate fuel cell; SOFC: Solid oxide fuel cell.

Electric utility 2 kW–MW >50% O2− SOFC

Lithium and potassium carbonate Solid oxide electrolyte (yttria, zirconia) MCFC

Co-ZrO2 Ni-ZrO2 anode, Sr-LaMnO3 cathode

700–1000

H2 , CO, CH4 , other hydrocarbons

Gaseous products

200 kW–MW Gaseous products >50%

Phosphoric acid PAFC

CO3 2−

KOH AFC

Ni anode, NiO cathode

Solid polymer (Nafion® ) DMFC

H+

650–850

H2 , CO, CH4 , other hydrocarbons

5–250 kW Evaporate 40% 100–220

Pure H2

<5 kW – OH−

Ni/Ag metal oxides, noble metals Pt

60–120

Pure H2

35–55%

∼5 kW Evaporate H+ Transition metals

30–130

Methanol

20–30%

5–250 kW Evaporate Solid polymer (Nafion® ) PEMFC

Transition metals

H+

30–130

Pure H2

35–45%

Common power ranges Water management Electric efficiency (system) Fuel Operating temperature range (◦ C) Charge carrier Electrodes Electrolyte FC type

Table 1 Currently present fuel cells and some typical properties.

Electrical utility, portable power, transportation Portable devices such as notebooks and cell-phones Military, space and residential plants Electric utility and transportation Electric utility

S.E. Bozbag, C. Erkey / J. of Supercritical Fluids 62 (2012) 1–31

Potential applications

4

operate with a good number of fuels such as H2 , CO, CH4 or other hydrocarbons. We note that energy efficiencies reach >50% in SOFCs [5,6]. The relatively high operating temperature allows for highly efficient conversion to power, internal reforming, and high quality by-product heat for cogeneration or for use in a bottoming cycle. There are two types of SOFCs depending on stack design: tubular and planar. Planar SOFC systems with high power densities operate at relatively lower temperatures (700–850 ◦ C). Combined with the ability of SOFC to use conventional fossil fuels, working at lower temperatures could help reduce the cost of the FC because less-expensive materials of construction could be use [7]. The commercially available tubular design of Siemens-Westinghouse’s combined heat and atmospheric pressure fuel cell system can generate power in the range of 200–250 kW. Alkaline FCs (AFCs) outshine with excellent rates of electrocatalytic reactions due to the fast O2 electrode kinetics and the flexibility to use a wide range of electrocatalysts. However, they are very delicate due to CO and CO2 poisoning and therefore require highly purified H2 and oxidant streams. AFCs manufactured by UTC Power were used in the NASA’s Space Shuttle Orbiter and Apollo missions. Molten carbonate FCs (MCFCs) with high operating temperatures do present some benefits such as the lack of expensive transition metal electrocatalysts as the nickel containing electrodes are sufficient to provide good activities. However, utilization of high temperatures presents some problems for materials, mechanical stability and stack life associated with corrosion. MCFCs are also commercially available and FCs manufactured by FuelCell Energy provide a power range from 300 kW to 2.8 MW. 2.2. Thermodynamics of FCs In a FC operating at a constant pressure and temperature, the maximum electrical work (We ) is given by the change in Gibbs free energy, dG, of the electrochemical reaction: We = dG = −nFE

(1)

where n is the number of electrons associated with the reaction, F is the Faraday constant and E is the ideal potential of the FC. For single phase gas systems, the Gibbs free energy change of a reversible reaction is given by:



fi,reactants

i



0

G = G + RT ln

(2)

fi,products

i

where G◦ is the standard Gibbs free energy of the reaction, R is the gas constant and fi is the fugacity of the gaseous species. Substituting equation (1) in equation (2) leads to the Nernst equation:



fi,reactants

RT ln E=E + nF 0

i



(3)

fi,products

i

For FCs which operate at low pressures, one can assume that the mixture is an ideal gas and the fugacities can be approximated by the partial pressures. In equation (3), E0 , the Nernst potential, is the ideal open circuit cell potential at reference conditions and E is the actual open circuit potential at given conditions, which determines the maximum achievable performance of the FC. Therefore, depending on the electrochemical reaction occurring in the FC and the reaction temperature, the ideal cell potential can be determined. For example, for a FC that operates through the following reaction: H2 + 21 O2 → H2 O

(R1)

S.E. Bozbag, C. Erkey / J. of Supercritical Fluids 62 (2012) 1–31

5

Fig. 4. Basic working principles and anode/cathode reactions of different types of fuel cells. Adapted from the reference [6].

the ideal cell potential at 25 ◦ C is 1.229 V for liquid H2 O and 1.18 V for H2 O vapor as the products, the difference between voltage values corresponding to the heat of vaporization of water. At 80 ◦ C, the ideal cell potential drops to 1.17 V for H2 O vapor as product. The thermal efficiency of an internal combustion engine is defined as the ratio of network output to the amount of heat absorbed. In a conventional combustion engine, the chemical energy in the fuel is first converted to heat and then converted to mechanical energy which is convertible to electrical energy. However, in FCs, the chemical energy is directly converted into electrical energy by the electrochemical reactions that take place in the anode and the cahode. Therefore, for a FC operating reversibly, the ideal efficiency is given by: id =

G H

(4)

where H is the enthalpy change of the electrochemical reaction. Different reactions occur in various FCs depending on the fuel supplied. These reactions, reactants and products are depicted in Fig. 4. Depending on the total reaction occurring in a FC, the ideal efficiency can be calculated by Eq. (4). At standard conditions the ideal efficiency of a FC which operates based on electrochemical oxidation of hydrogen with air is 0.83 [7]. We note that for internal combustion engines, the Carnot efficiency is 0.77 when an isothermal hot reservoir of 1000 ◦ C and heat rejection at average ambient conditions of 25 ◦ C and 1 bar is assumed. 2.3. Performance analysis of FCs FC performance is often demonstrated using the plots of FC voltage and power density as a function of current density as shown in Fig. 5. Power density is the product of the current density and the voltage. FCs are designed to operate at or slightly below the power density maximum. At current densities lower than the power density maximum, voltage efficiency (the ratio of actual

cell voltage to the theoretical cell voltage) is appreciable but power density is low. At current densities above the power density maximum, both of them decrease. We should also mention that the current obtained from FC is proportional to the amount of the fuel consumed. As a consequence, the electric power produced per unit of fuel decreases when FC voltage decreases. Thus, maintaining high voltage values under high current densities is a challenge and is very difficult to achieve in reality. Moreover, the voltage output of a FC is always lower than the thermodynamic predictions because of the irreversible losses [8]. These losses cause the efficiency of the FC to decrease. As can be seen in Fig. 5, the three main reasons associated with the irreversible losses in a FC are: i. Activation losses (kinetic losses) ii. Ohmic losses (charge transport losses) iii. Concentration losses (mass transport losses) One may also notice the gap between the thermodynamic ideal voltage and the open circuit voltage (OCV) at zero current density in Fig. 5. This gap usually originates due to the H2 cross-over through the electrolyte to the cathode or because of some corrosion that might take place at the electrodes [4]. As shown in Fig. 5, kinetic losses are associated with low current densities and they arise from the electrochemical reactions taking place in the surface of the electrodes. An activation barrier is associated with these electrochemical reactions and a portion of the FC voltage is lost to overcome this barrier and this voltage loss is called activation overpotential. Minimizing the activation overpotential can be achieved by utilizing a high activity catalyst, high surface area electrode materials together with the increase in temperature. For PEMFCs and PAFCs, the activation overpotential is due to the slow kinetics of the oxygen reduction reaction (ORR) occurring at the cathode. In AFCs, the overpotential is generally relatively low. Significant amount of research effort is directed towards minimizing this loss in PEMFCs. For the ORR taking place in these FCs, the reaction pathway affects the activation overpotential remarkably and depending on the pathway the reaction product can be H2 O, H2 O2 or even OH− in AFCs. Therefore, the reaction potentials associated with various pathways are also different. There are 2 common pathways of ORR; four electrons (4e− ) and two electrons (2e− ) pathways. According to the 4e− pathway, the ORR proceeds through [9]; O2 + 2H2 O + 4e− → 4OH− (in alkaline media) ; E 0 = 0.401 V (R2) O2 + 4H+ + 4e− → 2H2 O

(in acidic media) ; E 0 = 1.229 V

The reaction may also follow the 2e−

(R3)

pathway in alkaline media

according to; Fig. 5. Typical performance curves for FCs.

O2 + H2 O + 2e− → HO2 − + OH−

E 0 = −0.065 V

(R4)

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Ohmic losses are driven by a voltage gradient and occur due to the ion flow resistances in electrolyte and electron flow resistances through the electrode as well as due to contact resistances. Any of these resistance types can dominate the overall resistance depending on stack geometry, materials used and operation temperature [7]. At high current densities, the voltage loss with increasing current density is caused by mass transport losses, which occur when the reactants are consumed by the electrochemical reaction faster than they can diffuse into the porous electrode. Other reasons for mass transport losses are variations in bulk flow composition and the reduction in mass fluxes due to formation of water pools. In FCs with purely gas phase reactants and products such as SOFCs, the mass transport is controlled by gas diffusion and the losses are relatively small whereas in FCs with multi-phase flow within the porous electrodes such as PEMFCs, they may be severe [7]. Fig. 6. Variation of oxygen reduction activity with the oxygen binding energy. Reprinted with permission from [11]. Copyright 2011 American Chemical Society.

which can be followed by either the further reduction reaction. HO2 − + H2 O + 2e− → 3OH− ; E 0 = 0.867 V

(R5)

or the decomposition reaction. 2HO2 − → 2OH− + O2

(R6)

In acidic media, O2 + 2H+ + 2e− → H2 O2 ; E 0 = 0.670 V

(R7)

can be followed by either H2 O2 + 2H+ + 2e− → 2H2 O; E 0 = 1.77 V

(R8)

or 2H2 O2 → 2H2 O + O2 4e−

(R9)

2e−

or pathways depend on the electrode surfaces The and leads to the complete water or hydrogen peroxide formation, respectively. In PEMFCs, 2e− pathway is not preferred since the H2 O2 is not gaseous in FC operation conditions and poisons the catalyst. In Reactions (2–9), the potentials correspond to the standard state values versus the normal hydrogen electrode (NHE) at 25 ◦ C and these values affect the FC efficiency directly according to the Eq. (4). The reaction proceeds through the 4e− pathway when Pt, Pt alloys, Ag or Pd are used as catalysts whereas peroxide pathway is dominant on graphite and most carbons along with Au, and transition metal oxides (NiO). The details of these reaction schemes are given elsewhere [9,10]. The ORR activity as a function of the oxygen binding energy is plotted for transition metals in Fig. 6. Here, it is easy to notice the superiority of Pt over other metals. Although ORR activity of Pt over other transition metals is superior, the voltage loss due the activation overpotential is always present. Nørskov et al. [11] showed that the origin of the overpotential for Pt (1 1 1) is due to the tendency of the adsorbed oxygen to be very stable at high potentials so that the proton and the electron transfer become impossible. If the stability of the adsorbed oxygen may be decreased, the overpotential can thus be reduced. Another factor that deteriorates the catalytic activity of Pt is the fact that it is very sensitive towards trace amounts of impurities (i.e. CO) in the H2 stream. The FC kinetics at low current densities are often approximated by the Tafel equation below: act = a + b log j

(5)

where act is the activation overpotential, b is the Tafel slope and j is the net current which is related to the exchange current density.

2.4. Challenges in achieving widespread usage of FCs Even though the technical feasibility of FCs have been demonstrated in numerous applications, there exist some challenges in achieving widespread commercial utilization [4,5]: • Nonexistence of H2 fuel infrastructure, and difficulties related to H2 processing (purification and clean-up), storage and transportation. • Relatively high cost of FCs and consequently the high cost of the electricity generated by FCs. The cost issue is partly related to the materials (electrolyte, electrodes as the major contributors to the cost) as well as the immature manufacturing techniques. • Fuel processing necessities for some FC types. • Challenges associated with maintaining the performance for an extended period of time. • Difficulties in thermal and water management of the system. • Irreversible potential losses associated with materials such as kinetic or activation overpotential effects which limit the electrode reactions and mass transport effects in PEMFCs, poor electrolyte materials that cause reactant and potential losses in DMFCs and ohmic losses often faced in i.e. SOFCs. • Insufficient lifetime for some applications (particularly for stationary power generation). • Issues related to the social and political acceptance of the new technology. SCFs can provide solutions for some of the above-mentioned challenges, particularly for the improvement of essential materials associated with FCs as well as for the fuel production process. Thus, this present review aims to present the studies associated with the SCFs in the R&D of FCs and the very next section is dedicated to the analysis of the areas where the use of SCFs could be beneficial. 3. Supercritical fluids 3.1. Properties of SCFs A SCF is a fluid simultaneously heated and compressed above its critical temperature and pressure (Tc , Pc ). In practice, the term is used to denote fluids in the approximate reduced temperature and pressure range Tr = 0.95–1.10; Pr = 1.01–1.5 (Tr = T/Tc , Pr = P/Pc ). The critical points of some of the SCFs are depicted in Fig. 7. Among the SCFs, supercritical carbon dioxide (scCO2 ) is particularly attractive due to its low Tc (31.1 ◦ C) and Pc (7.38 MPa) and also because it is abundant, inexpensive, nonflammable, nontoxic and environmental benign.

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Fig. 7. Critical points of some fluids.

The thermophysical properties of a SCF are intermediate between those of a gas and a liquid and can be adjusted by slight changes in temperature and/or pressure. SCFs have liquid-like densities, gas-like viscosities and kinematic viscosities, much higher diffusivities than liquids and very low surface tension [12]. The liquid-like density of SCFs makes them appropriate as solvents for a wide range of solutes such as organic and inorganic materials, polymers, pharmaceuticals, biomolecules and organometallic compounds. Since the solute solubility varies generally as a power law with respect to density [13], a modest change in either pressure or temperature, in the vicinity of the critical point, can alter this solvating power as well as its selectivity as a solvent over a wide range of compounds. In Fig. 8a, we give a demonstration of the adjustability of a solute’s solubility in scCO2 with changes in pressure. At 60 ◦ C, the solubility of organometallics increases with increasing CO2 pressure [14]. Compared with conventional liquid solvents, high diffusivities in SCFs combined with their low viscosities result in enhanced mass transfer characteristics. The low surface tension of SCF not only permits better penetration and wetting of pores than liquid solvents do, but also avoids the pore collapse which can occur when certain nanostructured materials such as organic and silica aerogels are contacted with liquid solvents. scCO2 also displays high permeation rate in virtually all polymers and the exposure to scCO2 results in various extents of swelling and enhanced chain mobility of the polymers, which makes it possible to impregnate a wide variety of chemicals into various polymers. Moreover, the degree of polymer swelling, diffusion rates within the substrate, and the partitioning of penetrants between the SCF and the swollen polymer can be controlled by density mediated adjustments of solvent strength via changes in temperature and pressure [15–19]. Solubility of CO2 inside numerous polymers can be found in the excellent review of Tomasko et al. [17]. In Fig. 8b we give a representative plot of the effects of pressure and temperature on the solubility of CO2 in polyethylene terephthalate (PET). According to this, CO2 uptake of polymers increases appreciably with increasing pressure. One can observe that at a particular pressure, the CO2 sorption increases with decreasing temperature up to a particular pressure then increases with increasing temperature. The former is due to the decline of the density with increasing temperature and the latter is observed due the plasticization of the polymer by the CO2 [20]. As a result of the CO2 sorption, a decline is observed in the polymer’s glass transition temperature (Tg ) whose magnitude depends on CO2 concentration inside the polymer. The extent of the Tg depression

Fig. 8. (a) Variation of solubility of various organometallic compounds with pressure in CO2 . Ni(cp)2 : bis(cyclopentadienyl)nickel; Ru(thd)2 cod: bis(2,2,6,6-tetramethyl3,5-heptanedionato)(1,5-cyclooctadiene)ruthenium(II); Pt(cod)me2 : dimethyl(1,5cyclooctadiene)platinum(II). Adapted from the reference [14]. (b) The solubility of CO2 in polyethylene terephthalate (PET) [20]. Reprinted from the cited references with permission from Elsevier.

strongly depends on the polymer’s nature (molecular weight, functional groups, crystallinity etc.). scCO2 is also miscible with gases including hydrogen, oxygen and carbon monoxide at temperatures above its Tc . Therefore, significantly higher concentrations of such gases can be obtained in scCO2 compared to organic solvents in which gases are sparingly soluble. Therefore, higher reaction rates can be obtained and mass transfer limitations can be prevented. Subsequent to CO2 treatments, no residue is left in the processed product due to the gaseous character of CO2 near ambient conditions, which eliminates the purification step such as filtration or drying etc. otherwise necessary when organic solvents are used. Currently, large-scale chemical manufacturing is facing a serious solvent problem in connection with environmental concerns. Regulations concerning the use of hazardous organic solvents such as chlorinated hydrocarbons are becoming increasingly stringent and are thus spurring the development of environmentally friendly and economical processing media. scCO2 has excellent potential as an environmentally friendly and cheap medium. Increasing numbers of applications involving scCO2 have been demonstrated in a variety of fields including polymerizations [21], pharmaceutical applications [22], textile processing and dyeing [23], coatings [24], specialized materials fabrications

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[25,26], cleaning [27], chromatography [28], microelectronics processing [29], heterogeneous catalysis [30,31] and regeneration of adsorbents and other solid matrices [32–35]. Although specialized pressure vessels, high pressure pumps, and other equipment are necessary for processing, successful commercial-scale processes such as natural products/food extractions [36,37], wood impregnation [3] and aerogel production have been deployed. For almost two decades, SCFs are being investigated in development of new materials or new processes to make materials. By using SCFs, micro/nanoparticles, fibers, films, nanotubes, nanowires, nanorods, nanofoams/aerogels, inorganic and organic composites can be prepared. Using different approaches described in the literature [2,38,39], materials with sizes from 1 nm up to 10 ␮m can be controllably obtained. The geometry of these materials can also be architectured. In SCF processing of materials, the parameters affecting the size, the morphology, structure and the composition of materials include the properties of the solvent reactant nature and concentrations, the operating pressures, temperatures and residence times and the reactor technology employed [40–42]. Literature shows that nanoarchitectured materials prepared using SCFs can be incorporated into FCs and they are comparable or superior in performance as compared to the materials prepared using traditional techniques. These materials include new proton exchange membranes as electrolytes, nanostructured electrocatalysts and porous carbon aerogels/nanotubes as electrodes for PEMFCs (Section 5), new electrode/electrolyte materials for SOFCs (Section 6), catalysts, membranes and adsorbents for fuel processing (Section 7). The production of H2 is also possible through the utilization of supercritical water (scH2 O) and will be elaborated as well in Section 7.

4. PEMFCs and the obstacles for widespread commercialization An essential component of a PEMFC is a membrane electrode assembly (MEA), which is typically prepared by sandwiching an ionically conducting polymeric membrane (usually Nafion® ) between two electrically conductive electrodes (anode and cathode). A simplified representation of a single PEMFC stack is given in Fig. 9. The membrane must be highly proton conductive, must present an adequate barrier to mixing of fuel and reactant gases, and must be chemically and mechanically stable. Additionally, the conductivity of Nafion® depends on its water content and a higher

GDL 7% Bipolar plate 9% Seal 7% BOS 3% Electrode 54%

Final Assembly 12% Membrane 8%

Membrane Electrode GDL Bipolar plate Seal BOS Final Assembly

Fig. 10. Proton exchange membrane fuel cell stack cost breakdown. Adapted from the reference [46].

water concentration in the Nafion® results in higher conductivity. In the electrodes, there exist catalytic particles (usually carbon supported platinum), which are essential for catalyzing the electrochemical reactions (oxidation of H2 at the anode and the reduction of O2 at the cathode). Pt is the conventional metal of choice in a PEMFC electrode due to its remarkable electrocatalytic properties as mentioned previously [9,43]. The activity of the electrocatalysts are governed by a wide variety of properties such as the Pt content and dispersion, Pt nanoparticle size, orientation of the surface planes in the nanoparticles, pore volume, pore size, interface between electrolyte, Pt and the support [44,45]. MEA’s structure and composition should be such that all forms of overpotential should be minimized and the power density should be maximized (cf. Fig. 5). The catalytic metal loading should also be minimal (thus, the cost per kW of PEMFC). Effective water and thermal management of the MEA system should be attained in order to achieve compatible lifetimes [9]. In Fig. 10, the stack cost breakdown is presented for a PEMFC system capable of delivering 80 kW net power for automotive applications with a price of 29$/kW when manufactured in high-volume (500 000 units/year). Even though the aforementioned FC price is very optimistic considering where the technology is today, it can be seen that the electrodes represent the major percentage of the costs. On the other hand, when produced in small quantities (1000 units/year), the percentage cost of the electrode can drop to 10–15% and the percentage cost of membrane increases to ∼47% [46]. The major cost in the electrode is the cost of Pt. The DOE’s target for Pt utilization is 0.2 g Pt/kW by 2015 [47]. Pt is an expensive metal and the reduction in the amount of Pt would bring tremendous declines in total FC cost. The price of Pt had an increasing trend

Fig. 9. Components of a proton exchange membrane fuel cell (a simplified schematic of the stack).

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over the past 20 years. The yearly average Pt price per gram was 11.7$ and 17.4$ in 1992 and 2002, respectively. As of May 2011 the price is about 59$/g. Therefore, the amount of Pt used in MEAs needs to be reduced using i.e. smaller nanoparticles or by alloying it with cheaper metals. Along the first approach, significant research efforts have been directed towards reducing particle diameters in carbon supported platinum and also to increase the electrochemically active surface area (ESA) (which is the net area where the electrochemical reactions take place) [43,44]. For instance, an otherwise inert metal Au becomes very active towards CO oxidation when particles are in 1–3 nm size range [54]. We should mention that only the catalyst particles, which are located at the electrolyte/catalyst interfacial region, are effective in electrochemical reactions. Therefore, further improvements in platinum utilization can be achieved by incorporating a certain amount of Nafion into the electrode with the hope that this will enable the transfer of the protons in the catalyst layer [48]. Nafion can be incorporated into electrodes in several distinct ways: (i) brushing a Nafion solution onto the catalyst loaded electrode surface to form a thin Nafion layer and (ii) mixing Nafion into the catalyst solution ink [48]. Unfortunately however, even with the incorporation of the Nafion, the Pt utilization may not be enough. Along this line, promising results were obtained using layer by layer assembly technique [49]. Along the second approach for reducing the Pt amount (alloying it with cheaper metals), many different combinations of metals such as Pd and Ni with Pt have been investigated [50,51]. Essentially, using bimetallic catalysts may provide advantages in catalytic activity, stability and economy. The intrinsic catalytic activity change originates from two cumulative phenomena called the ligand and the strain effect. The addition of another metal causes alterations in the electron density of the system and the alteration of the metal–metal bond length that may lead to improve catalytic activity [52]. The geometry has also important effects. For instance, one can improve the catalytic activity per amount of metal used by using a core–shell structure to provide the same catalytically active area in the shell with active and expensive metal (i.e. Pt) and with less active and cheaper metal in the core (i.e. Cu, Ni). For the case of supported PtRu catalysts, the presence of Ru in the mixture prevents the CO poisoning in PEMFCs and DMFCs [53]. Carbon supports with high surface area are commonly used in low temperature FCs. Good crystallinity and the high surface area of carbon supports provide high dispersion of electrocatalyst particles (Pt or Pt alloys) and bring facile electron transfer. The high surface area and the electrical conductivity arise because of high porosity and the conductive nature of the carbon due to the mobile ␲ electrons, respectively [55,56]. On the other hand, carbon supports should also possess mesoporosity in the 20–40 nm region in order to provide accessible surface area for the catalyst particles and for Nafion (which may not access to pores smaller than 20 nm) as well as to facilitate the diffusion of species [55]. Carbon blacks (CB) are widely used as catalyst support in low temperature FCs due to their low cost and high availability as well as their high surface areas (58–1500 m2 /g depending on the process type) and nanometric particle sizes (15–50 nm). Different CBs from various suppliers result in different FC performance. The microstructure of carbon support in the catalyst layer and the type of carbon support are among the key characteristics for efficient PEMFC performance. The polarization curves obtained with catalysts prepared using 12 different carbon supports at 50 ◦ C and 0.1 MPa at a Pt loading of 0.5 mg/cm2 is given in Fig. 11 and the differences in the polarization curves are remarkable [57]. The results indicate that the type of carbon support affects the cell performance dramatically. Currently, most commercial electrocatalysts are based on carbon black supports that are produced from a wide variety of hydrocarbon sources by high temperature processes. The properties of such supports are not tailored for fuel

9

Fig. 11. Polarization curves for electrocatalysts prepared from various CBs [57]. Reproduced by permission of ECS – The Electrochemical Society.

cell applications. Therefore, they have serious disadvantages when used as a catalyst support in FCs such as the carbon corrosion in acidic environment [58]. Carbon nanotubes (CNT) along with other non-conventional carbonaceous materials such as micro and mesoporous carbon aerogels (CA) exhibit high potential in near future as catalyst support in FCs. The performance of DMFCs is hindered due to the methanol cross-over which is the transport of methanol from the anode to the cathode through the electrolyte membrane, Nafion® without being oxidized at the anode. The phenomenon is caused by the protonic drag of methanol, which is similar to the electro-osmotic drag of water. In DMFCs, consequently, loss of fuel is encountered. But more importantly, the further oxidation of methanol at the cathode on catalytic nanoparticles limits the oxygen adsorption and reduction due the decreasing the number of available sites. Therefore, a reduced cell voltage is encountered because of the mixed potential at the cathode. Substantial efforts are spent in the research community to eliminate this phenomenon. Furthermore, unlike PEMFCs which consume hydrogen, kinetic limitations appear in both the anode and the cathode in DMFCs. With the utilization of current catalysts comes the slow anode reaction near the thermodynamic potential. Therefore, a large overpotential is encountered. Moreover, the currently used catalysts in anode are sensitive to poisoning by the impurities and even to the by products of the reaction. Lifetime, thermal stability and high cost of Nafion® are other important issues. Therefore, composites of or alternatives to Nafion® are needed. Various membranes such as organic–inorganic and acidic–basic polyaryl composites of fluorinated or non-fluorinated membranes have been prepared by different manufacturers or researchers [59]. Generally, organic–inorganic composites are associated with reducing methanol cross-over whereas acid–base composites can both reduce the methanol cross-over and increase the conductivity. An excellent review on investigations on polymer electrolyte membranes for DMFCs is given by Neburchilov et al. [59]. Modern FC electrodes are gas diffusion electrodes (GDEs) that consist of a porous layer of high surface area catalyst and again porous, electrically conducting gas diffusion layer (GDL), or electrode substrate (or electrode backing material). The electrodes, anode and cathode, are in closey contact with the polymer electrolyte membrane. The GDL allows the flow of both reactant gases and product water. Thus, it must be sufficiently porous and further

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Fig. 12. Areas where supercritical fluid processes can be used for proton exchange membrane fuel cell applications.

electrically and thermally conductive, and also must be sufficiently rigid to support MEA film [5]. However, most of the commercial materials have high costs and display stability problems at high temperatures. Bipolar collector/separator plates have several functions in a FC stack (cf. Fig. 9) such as connecting cells electrically in series, separating the gases in adjacent cells, providing structural support of the cell, conducting heat from active cells to the cooling cells and housing the flow fields [5]. A water jacket for cooling is often placed at the back of each reactant flow field followed by a metallic current collector plate. The cell can also contain a humidification section for the reactant gases, which are kept close to their saturation level in order to prevent dehydration of the membrane electrolyte which can otherwise provoke severe mechanical and conductivity problems due to the decrease of water activity within the membrane [60,61].

5. Use of SCFs in development of membrane electrode assemblies for PEMFCs The areas where SCF based processes can be applied for PEMFCs are presented in Fig. 12. In order to reduce methanol crossover, composite Nafion® membranes or alternatives of Nafion® have been synthesized by using scCO2 based processes including supercritical fluid extraction (SFE), supercritical deposition (SCD) and grafting (Section 5.1). Furthermore, carbon supported electrocatalysts can be prepared using SCD method (which will be presented in Section 5.2 in detail). Preparation of carbon (CNT, Black pearl, VXR, etc.) supported electrocatalysts is the most deeply investigated subject among SCFs applications in FC R&D. By using such electrocatalysts, it is possible to improve the electrochemical activity and the performance of PEMFCs. Synthesis of new carbon nanostructured materials as electrocatalyst supports with controllable architecture using scCO2 is also a promising research area and will be elaborated in Section 5.3.

5.1. Preparation of composite Nafion® membranes and its alternatives using SCFs The first study on preparing Nafion® composites using SCFs was carried out by Kim et al. [62]. The authors cast membranes from blends of polyethylene glycol (PEG) with polycarbonate (PC) with PC:PEG ratios 7:3 and 9:1. This process was followed by the removal of PEG from the blend by extraction with scCO2 as a result of the considerable solubility of PEG in scCO2 and due to the ability of scCO2 to swell the polymer. The resulting structure was a porous PC where PEG had acted as porogen. Then, Nafion was impregnated into the PC membrane which resulted in PC/Nafion composites. They showed that the initial polymer ratio affected the membrane structure as well as the average pore diameter and the 7:3 composite was more porous which led to an improved Nafion® sorption capacity for these membranes compared to the 9:1 membranes. Membranes that absorbed more Nafion® (7:3) had higher ionic conductivity than that of the 9:1 membranes. However, the 9:1 membranes had less water uptake (15% instead of 47.1% in 7:3). Furthermore, the prepared composite membranes were thinner (42.3 ␮m) than that of the commercial Nafion® membranes. Even though the composite membranes had lower ionic conductivity than the commercial Nafion® , the authors showed that the smaller thickness of the composite membranes could provide better conductance than that of the commercial Nafion® i.e. 15.2 S/cm3 for the 7:3 membrane, whereas 6.7 S/cm3 for the commercial Nafion® . Porous polypropylene (PP) membrane is also promising because of its low cost and easy handling. Kim et al. [63] prepared porous PP for PEMFCs using camphene as a porogen which was extracted using scCO2 . The optimum conditions for camphene extraction from the membranes prepared by solution casting of PP/camphene mixtures were found to be at 45 ◦ C and 15 MPa. At these conditions, 94 and 99% of the camphene were extracted within 5 and 10 min, respectively. The resulting material had high porosity and mechanical strength. The average pore size in the membrane was about 2–3 ␮m and the pore sizes increased with decreasing PP/camphene ratio. The porosities were 80, 76, and 71% for the samples prepared

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with initial PP wt.% of 10, 20, and 30 in the PP/camphene starting solution, respectively. The results showed a decline in porosity with increasing PP concentration. The breaking points of the PP membranes with 10, 20, and 30 wt.% polypropylene were 0.17, 0.24, and 0.46 kgf /mm, respectively. The thickness of polypropylene membrane was 70 ± 3 ␮m and that of the composite membrane impregnated with Nafion® solution was 105 ± 3 ␮m. The water uptake of the polypropylene composite membrane was 25 ± 3%. The ion conductivity of the polypropylene composite membrane was 0.0030 ± 0.0005 S/cm. Removal of a polymerization solvent by extraction with scCO2 to create a porous structure can also be an alternative route to prepare proton conductive membranes [64]. Copolymers of hydrophobic diglycidyl ether of bisphenol A (DGEBA) vinyl ester (VE) and hydrophilic 2-acrylamido 2-methyl 1-propane sulfonic acid (AMPS) were synthesized using free radical copolymerization in dimetyl formamide (DMF). The extraction of the solvent by the scCO2 was found to lead to pores smaller than 60 nm. Increasing porosity promoted proton mobility, probably due to a combined effect of decreased tortuosity and increased sulfonic acid group concentration. We note that since the ion conductivity of the membrane depends on its water content, the physical or chemical state of water and the effect of the porosity to the state of water are important parameters. There exist two states of water in the membrane; freezable and nonfreezable. The freezable water is the unbound water present in the matrix which interacts weekly with the membrane and possesses freezing/melting transitions in differential scanning calorimetry. The nonfreezable water is strongly bound to the ionic groups and aids the proton transport through the membrane. In this study, the porosity was also found to alter the water uptake behavior of the membranes by causing an agglomeration of freezable water within the pore volume while not affecting the nonfreezable water fraction. Investigation of the association of water with sulfonic acid groups showed that the freezable water ratio per sulfonic acid groups was strongly affected by the membrane pore volume, while the increased concentration of sulfonic acid groups lowered the nonfreezable water ratio per sulfonic acid groups. Palladium-impregnated Nafion® membranes were synthesized via supercritical deposition (SCD) technology for the first time by Erkey and colleagues [65], which involved impregnation of Nafion® membranes with palladium(II) hexafluoroacetylacetonate Pd2 (hfac)2 from scCO2 solution followed by subjecting the impregnated membrane to hydrogen (SCD technology will be elaborated in the next section). The impregnation was carried out at 80 ◦ C and 13.6 MPa for 12 h. The conversion of the precursor to the metal form was carried out by hydrogen, which was injected into the vessel at high pressure, and the samples were subjected to hydrogen for another 8 h. The resulting membranes had Pd loadings of 1.19 and 2.65 mg/cm2 . According to SEM analysis, the membrane had a uniform surface morphology and no cracks could be observed. TEM image of the cross-section of the composite membrane indicated a thin (0.3 ␮m) film of Pd around the membrane surface and a significant number of isolated Pd particles deeper in the membrane. The particle size of the Pd particles ranged from 5 to 10 nm. Membrane–electrode assemblies with these palladinized Nafion® membranes were prepared and evaluated in DMFCs to determine methanol cross-over, proton conductivity as well as DMFC performance. The Pd-impregnated Nafion® membranes showed reduced methanol cross-over and gave higher cell performance than that of pure Nafion® membrane, although the proton conductivity was decreased with the impregnation of palladium. The palladinized membrane showed more significant benefits in performance with higher concentration (5 M) of methanol. Using the same technique, Kim et al. [66] investigated the incorporation of palladium particles onto Nafion® 117 membranes.

11

The membranes were impregnated with palladium(II) acetylacetonate at 20 MPa and 80 ◦ C for 4 h in scCO2 followed by chemical decomposition. The images from the electron probe micro analysis (EPMA) of the cross-section of Pd/Nafion® membranes are presented in Fig. 13. The results indicate that the Pd/Nafion® composite membranes showed different morphologies depending on the concentration of the reducing agent (NaBH4 ) in the injected solution. They obtained Pd particles of 4–5 nm using a 0.5 mM solution of NaBH4 , whereas for 10 and 100 mM solutions, the average diameter of the particles was 20–70 nm. They observed that the ion conductivity of Pd/Nafion® reduced using a 0.5 mM (0.0833 S/cm) NaBH4 solution was higher than that of the Nafion® 117 (0.0740 S/cm). However, the increase of the reducing agent concentration led to a decrease in the ion conductivity to a level even less than that of the Nafion® 117 which was attributed to the increase of the Pd particle size. It was suggested that the ability of the particles with diameters of 20–70 nm to conduct protons were less than that of 4–5 nm particles. Additionally, the permeability of the composite membranes decreased with the increasing NaBH4 concentration. They also tested the composite membranes in a DMFC with 2 M methanol at 80 ◦ C with air in the cathode. As a result of the competition phenomenon between the ionic conductivity (0.0765 S/cm) and permeation rate (4.38 × 10−7 cm2 s−1 ), the composite membranes prepared using 2 mM reducing agent had the best cell performance and 44% higher current density than that of Nafion® 117. Grafting is another technique, which can be used to prepare improved membranes with reduced methanol cross-over. Several grafting studies are present in the literature where polymers such as poly(styrene-block-butadiene-block-styrene) (SPS) or polystyrene sulfonic acid (PSSA) were grafted on a variety of hydrophobic polymers, such as poly(ethylenetetrafluoroethylene) (ETFE), poly(vinylidene fluoride) (PVDF), and low-density poly(ethylene) (LDPE) [67]. Grafting copolymerization usually consists of exposing the hydrophobic polymer membrane to a radiation source, which promotes the formation of radicals and functional groups on the membrane. In most of the studies in the literature, methanol cross-over was reduced as compared to Nafion with grafted polymers. However, problems such as instability and delamination were encountered which resulted in poor DMFC performance. Using SCFs as grafting media can bring a number of advantages. Since a wide range of monomers have high solubility in scCO2 [68], the monomer and the polymerization initiators can be dissolved in scCO2 . Then, they can be incorporated into another polymer matrix controllably aided by the sorption of scCO2 in that polymer. Subsequently, the polymerization can be carried out within this matrix leading to a polymer–polymer composite. To improve the conductivity of Nafion® and decrease the cross-over of methanol, composite membranes were formed by grafting styrene using scCO2 impregnation and polymerization procedures. Sauk et al. [69] prepared proton exchange membranes by impregnating Nafion® membranes with styrene in scCO2 and subsequent polymerization at 10 MPa for 4 h. They were able to control the mass uptake (polystyrene grafted Nafion® ) by the initial amount of styrene in the fluid phase (∼30% mass uptake for 0.5 g of monomer at 25 MPa and 38 ◦ C). With a methanol permeability of 2.15 × 10−6 cm2 s−1 , the polystyrene grafted Nafion® membrane had also lower methanol cross-over than Nafion® 115 (3.29 × 10−6 cm2 s−1 ). Cell performance tests showed that at 0.35 V, the performance of polystyrene grafted Nafion® membrane was better than Nafion® 115 because of the lower methanol permeability and higher ionic conductivity which make these proton exchange membranes promising candidates for use in DMFCs. Additionally, it is worthwhile to mention that there is a competitive phenomena between the methanol permeability and the mass gain. As mentioned by Sauk et al., an increase in mass gain would increase membrane thickness, which is good for preventing

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Fig. 13. Electron probe micro analysis of the palladium element at the Pd/Nafion® membrane cross-section prepared using supercritical deposition and reduced with (a) 0.5 mM, (b) 2 mM, (c) 10 mM and (d) 100 mM of NaBH4 . Reprinted from [66] with permission from Elsevier.

methanol cross-over. However, this would also cause a decrease in the ionic conductivity because of increased membrane resistance. In a subsequent study by the same group, styrene was grafted to a polypropylene membrane in scCO2 . The membrane was impregnated at 38 ◦ C and 12 MPa for 16 h using a solution of styrene, divinylbenzene (DVB), and 2,2 -azoisobutyronitrile (AIBN) in scCO2 [70]. After impregnation, the polymerization process was carried out at 78 ◦ C in scCO2 at 11 MPa for 8 h. Then, the grafted membranes were sulfonated. A Nafion® laminated polypropylene grafted polystyrene sulfonic acid (PP-g-PSSA) membrane was made by hot pressing the PP-g-PSSA membrane and Nafion® 112. The permeability measurements indicated that as the DVB concentration increased, the permeability decreased. The permeability of Nafion® 112 was 3.45 × 10−6 cm2 /s, while that of the laminated membrane with a DVB content of 10 wt.% was 1.7 × 10−6 cm2 /s, corresponding to a 49% reduction. The DMFC tests were carried out at 90 ◦ C with air in the cathode. The best cell performance was observed with the PP-g-pssa/Nafion® laminated membrane with a DVB content of 5 wt.%. At 0.35 V, with the styrene grafted membrane, a current of 220 mA was observed whereas this value was 180 mA for Nafion® . These results were attributed to the degree of grafting of the polystyrene. However, the performance of the PP-g-PSSA/Nafion® laminated membrane with a DVB content of 10 wt.% was lower than that of the membrane with a DVB content of 2.5 wt.%. This discrepancy can be explained by the fact that

the latter membrane had a higher membrane resistance, due to the greater amount of crosslinking that occurred at the surface. Byun et al. [71] have recently synthesized poly(vinylidene fluoride) (PVDF)/PSSA membranes by impregnating and polymerizing styrene with divinylbenzyne into PVDF membranes in scCO2 . They have found that SO3 − groups were uniformly distributed throughout the membrane structure based on EDS measurements. Su et al. conducted some experiments on the preparation of polysiloxane modified PFSA membranes where scCO2 was used as a swelling and impregnation medium to incorporate the functional SiO2 precursor – (3-mercaptopropyl) methyldimethoxysilane (MPMDMS) in an aqueous mixture into PFSA membrane. The condensation polymerization was catalyzed by carbonic acid which was produced via the reaction of water and scCO2 [72]. Subsequent to the grafting, the membrane was treated with H2 O2 in order to oxidize the thiol groups to sulfonic acid groups. The methanol permeability of the 13.9% polysiloxane impregnated PFSA membranes showed a remarkable decrease without any reduction in proton conductivity in the temperature range 25–65 ◦ C. Zeolites are promising candidates for use in development of ion conductive membrane composites. Their incorporation into the membrane leads to a selective transport of hydrogen ions over methanol molecules. Tricoli and Nannetti [73] prepared zeolite–Nafion composites and obtained promising results in conductivity, selectivity and methanol permeability as compared to

S.E. Bozbag, C. Erkey / J. of Supercritical Fluids 62 (2012) 1–31

sole Nafion. The scCO2 activation of a Nafion® membrane prior to zeolite deposition was used to modify the structure of the membrane [74]. Gribov et al. [74] treated Nafion® membranes in scCO2 at 13.1–13.8 MPa and 45 ◦ C for 2 h. Their experiments with pure Nafion® in scCO2 showed that CO2 treated Nafion® had lower methanol permeability and proton conductivity. Furthermore, according to Gribov et al. [74], the incorporation of zeolite affects the selectivity (proton conductivity over methanol permeability ratio). They were able to produce membranes with higher selectivity compared to the commercial Nafion® 115 membrane using a colloidal and in situ route to synthesize Fe-silicalite nanoparticles and incorporating them into the membrane. Recently, Su et al. [75] prepared perfluorosulfonic acid (PFSA) membranes and treated them with scCO2 at 14 MPa and 80–160 ◦ C. The applied depressurization rate was 0.5 MPa/h. Their results indicated an increase in the crystallinity of the membrane, development of long-range order and a decrease in the size of the ion-cluster in the membrane as a consequence of the scCO2 treatment. Additionally, the volume increase of the membrane due to water uptake decreased. The study showed that the prepared membrane was partially soluble in ethanol/water mixture unlike commercial thermo-extruded Nafion membrane and the solubility was found to decrease with increasing scCO2 treatment temperature. They suggested that more physical cross-links were formed in the membrane after the scCO2 treatment, which may be the cause of the increase in the mechanical strength. Finally, the methanol permeability of the membrane was decreased by 42% without any reduction in proton conductivity and mechanical strength.

5.2. Preparation of carbon supported electrocatalysts using SCFs Traditional routes for the preparation of supported metal nanoparticles include wet impregnation, co-precipitation, deposition–precipitation, sol–gel, microemulsions and chemical vapor impregnation. All of these methods have some limitations associated with the control of the nanoparticle size, distribution and metal loading. Furthermore, none of these methods can be easily applied to polymers or to materials such as aerogels [76,77]. SCFs have been used to deposit and incorporate metallic particles into different inorganic and organic substrates for catalytic applications. They have also been investigated in synthesizing carbon supported electrocatalysts for FCs. SCD involves the dissolution of a metallic precursor in a SCF and the exposure of a substrate to the solution. After incorporation of the precursor with the substrate, the metallic precursor is converted to its metal form by a wide variety of methods resulting in particles or films. Several precursor conversion techniques have been reported in the literature, which include chemical decomposition in the SCF with a reducing agent, such as hydrogen and alcohols, thermal conversion in the SCF and thermal decomposition in an inert atmosphere or chemical conversion with hydrogen or air [77]. The process offers controllable nanoparticle size (within the range of ORR requirements), uniform and narrow size distribution and adjustable metal loading. Table 2 summarizes some properties of the potential FC electrocatalysts prepared via SCFs. One of the first studies on synthesis of catalytic metal nanoparticles on carbon nanotubes (CNTs) using SCD was carried out by Lin et al. [78]. In these experiments, a certain amount of CNT, methanol and Pt(acac)2 (Platinum(II) acetylacetonate) or Pd(hfac)2 (palladium(II) hexafluoroacetylacetonate) was placed in a vessel which was charged with CO2 and kept at 8 MPa and 200 ◦ C. The methanol was used as a co-solvent in order to increase the solubility of precursor in the fluid phase. After 1 h of dissolution and adsorption

13

time, a mixture of H2 + CO2 was injected into the vessel (consequently, the pressure inside the vessel increased to 16 MPa) which enabled the conversion of the adsorbed metal precursor molecules to Pt0 nanoparticles within ∼15 min. The system was depressurized and the resulting CNT supported nanoparticles were taken out of the vessel. The authors prepared electrodes by casting an ink containing Nafion® and the prepared Pt/CNT on glassy carbon electrodes and characterized them by cyclic voltammetry (CV). A typical cyclic voltammogram for the ORR at Pt-CNT electrodes in a solution of 0.1 M H2 SO4 saturated by oxygen is given in Fig. 14a indicating that the CNT supported Pt nanoparticles prepared by SCD are efficient catalysts for ORR. They also investigated the efficiency of the Pt based electrocatalysts for the methanol oxidation reaction, which occurs in DMFCs. The reaction mechanism for methanol oxidation involves many intermediate steps and CO is believed to be the most abundant surface intermediate species [79]. It is known that the ratio of the forward anodic peak current (If ) to the reverse anodic peak current (Ib ) can be used to describe the catalyst tolerance towards the carbon accumulation and a high If /Ib value represents a tendency of oxidation of methanol to CO2 . The authors have found high values like 1.6 and 1.3 that indicate the oxidation of most of the carbonaceous species to CO2 in the forward scan. The high catalytic activity towards both ORR and MOR was attributed to the larger surface area of carbon nanotubes and the decrease in the overpotential for methanol oxidation and oxygen reduction reaction. For CV scans, Lin et al. [78,80,91] noted that the cathodic current flowing during the reduction of oxygen should also contain the contribution from the reduction of platinum oxide that might be formed because of the adsorption of oxygen on the electrode surface. It was impossible to separate these two contributions accurately. However, with experiments carried out under a nitrogen atmosphere, they concluded that the major contribution to the cathodic current in oxygen saturated solution was from the oxygen reduction instead of the probable reduction of metal oxide nanoparticles. As a result of their experiments with different scan rates (Fig. 14b and c), they suggested that the ORR process on Pt-CNT and Pd-CNT is controlled by the diffusion of oxygen to the electrode surface since the peak current increases with the square root of the scan rate as shown in the inset of Fig. 14b. The authors proposed that the possible course of the ORR on a nanoparticle of Pd deposited CNT electrode proceeded according to the reactions (7) and (8) [80]. Erkey’s group also reported on synthesis of supported platinum nanoparticles using SCD with particle size as small as 1 nm and metal content of 10–40 wt.% on carbon aerogels which had average pore diameters of 4 and 22 nm (CA-4 and CA-22) [81]. Smirnova et al. [82] used CA supported Pt catalysts in the preparation of cathode catalyst layers in PEMFCs using screen-printing technique and tested them at ambient pressure and fully saturated conditions. The prepared catalysts with Pt particle size ranging from 1 to 2 nm showed high catalytic activity, relatively high OCV, close to theoretical Tafel slope, and high electrochemical surface area, which was twice as much as the ESA of the commercial catalysts. The PEMFC performance was studied for the aerogel with 16 nm of average pore diameter at different cathode catalyst loadings in the range 0.06–0.6 mg/cm2 . Increase in pore size of aerogel support from 16 to 20 nm resulted in a significant increase of cell performance and maximum power density of the cell. The 20 nm catalyst with low Pt loading (0.1 mg/cm2 ) showed good power density (close to 0.8 mW/cm2 ) with air at ambient pressure. This was attributed to the better penetration of Nafion® into the pores of the aerogel carbon support where Pt nanoparticles resided. The excellent properties of the electrocatalysts were attributed to the structure of the supported Pt catalyst having large pores, even distribution of Pt particles inside the porous carbon structure, and less tendency for agglomeration and sintering during the cell operation.

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Table 2 Summary of the properties of the fuel cell electrocatalysts prepared using supercritical fluids. Metal deposited

Support

Wt.% metal

Average particle size (nm)

ESA (m2 /g)

Remarks

Application

References

Pt

MWCNT

25

5–10

–

DMFC

[78]

Pt

10 20 20 40 16.3

1.0 2.5 3.0 3.0 2.6

–

–

[81]

Pt

CA CA CA CA Nafion® –CB

PEMFC

[48]

Pt

CA

37

1–2

PEMFC

[82]









20



One of the first studies which demonstrated the feasibility of using SCD for FC electrocatalyst preparation Controllable Pt dispersion and nano-particle size on CA were demonstrated Pt–Nafion® –CB nanocomposites displayed significant activity in PEMFCs Low Pt loaded (0.1 mg Pt/cm2 ) CA with 20 nm pores showed a maximum power density of 800 mW/cm2 in PEMFC Superior performance of the electrocatalysts prepared via SCD was demonstrated over their commercial counterparts Electrocatalyst prepared using W/C NRs had 30% more ESA than that of the commercial electrocatalyst No detectable ESA loss and high stability against the ORR High Pt loaded SWCNTs showed very promising PEMFC activity with a peak power density of 449 mW/cm2 with a Pt utilization of 1.09 g Pt/kW Superior performance of Pt-Ru over the other metal combinations towards MOR. Furthermore, high If /Ib values were obtained – EDX analysis showed Pd dissolution after the potential cycling degradation tests High activity as anode catalyst for DMFC

PEMFC

[83,86]

PEMFC

[87]

PEMFC

[88]

PEMFC

[89]

DMFC

[90,91]

DMFC PEMFC

[95] [92]

DMFC

[96,97]

20

72 69 77

Pt

VXR MWCNT BP2000

9.0 9.9 47.5

1.2 2.0 1.0

173 130 102

Pt

MWCNT

–

8.7

31.1

Pt

CB

24

3.7

–

Pt

XC-72 XC-72 + SDS CF CF + SDS SWCNT SWCNT + SDS

85.4 77.0 68.1–82.6 81.7 70.6–87.2 78.8

4.5 3.0 3.4–4.5 3.1 4.3–6.9 4.5

Pt-Ru (48:52) Pt-Ni (62:38) Pt-Au (37:63) Pt-Pd (50:50) Pt-Cu (66:34)

MWCNT MWCNT MWCNT MWCNT MWCNT

21.30 23.13 4.28 31.77 21.75

2.8 ± 0.8 6.6 ± 3.5 9.3 ± 3.7 9.2 ± 3.3 5.7 ± 2.2

–

Pt-Ru (43:57) Pt-Pd (2:1) Pt-Pd (1:2) Pt-Pd (1:4)

MWCNT CB CB CB

– 18 (Pt)/6 (Pd) 15 (Pt)/10 (Pd) 8 (Pt)/7 (Pd)

5 3.2 3.9 5.1

– 55–38* 30–17* 36–17*

Pt-Ru

–

10–20

–

ESA: Electrochemically active surface area; MWCNT: Multi walled carbon nanotube; SCD: Supercritical deposition; FC: Fuel cell; PEMFC: Proton exchange membrane fuel cell; DMFC: Direct methanol fuel cell; CB: Carbon blacks; CA: Carbon aerogel; W/C NRs: H2 O-in-CO2 microemulsion nanoreactors; CF: Carbon fiber; ORR: Oxygen reduction reaction; MOR: Methanol oxidation reaction; SDS: sodium dodecyl sulfate; If /Ib : Ratio of forward anodic peak current to the reverse anodic peak current. * After 50 and 300 potential cycles, respectively.

Bayrakceken et al. [83] prepared MWCNT-supported Pt particles using SCD. (Pt(cod)me2 ) was dissolved in scCO2 and impregnated into MWCNTs at 24.2 MPa and 343 K for 6 h. The precursor was converted thermally under flowing N2 (100 cm3 /min) for 4 h at 473 K. Loadings of up to 15.2 wt.% Pt could be obtained with uniformly sized particles with diameters around 2 nm. XRD was used to confirm the presence of metallic Pt. The authors suggested that the conversion in scCO2 using hydrogen led to particle formation not only at the surface but also in the fluid phase, resulting in precipitation of these particles onto the surface. On the other hand, thermal conversion of Pt(cod)me2 occured at the MWCNT surface alone, leading to greater uniformity in the particle size. They also

demonstrated the thermodynamic control of the loading based on a Langmuir type adsorption isotherm. Hiramatsu et al. [84] synthesized CNT supported Pt nanoparticles using (methylcyclopentadienyl)trimethyl platinum in scCO2 at 9 MPa and substrate temperature ranging from 70 to 170 ◦ C. At such experimental conditions, the utilized Pt precursor thermally decomposed enabling the one-pot synthesis of supported Pt nanoparticles. The process is similar to the SCD with simultaneous thermal conversion described by Bozbag et al. [85] where the Pt(cod)me2 molecules decomposed to elemental Pt on organic aerogels in scCO2 vacating surface sites for more Pt(cod)me2 molecules. The newly adsorbed molecules decomposed as well

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15

Fig. 15. (a) Transmission electron microscope (TEM) image of a carbon black supported Pt electrocatalyst prepared via supercritical deposition. (b) TEM image of a commercially available carbon black supported electrocatalyst (ETEK). Reprinted from [86] with permission from Elsevier.

Fig. 14. Electrochemical characterizations of carbon supported catalysts prepared using supercritical deposition. (a) Pt-carbon nanotube electrode for oxygen reduction reaction in 0.1 M H2 SO4 saturated with oxygen at 20 mV/s. Cyclic voltammograms of oxygen reduction with different scan rates in oxygen saturated 1 M H2 SO4 solution (b) at the glassy carbon/Pt–CNT and (c) at glassy carbon/Pd–CNT electrodes. Reprinted with permission from [78]. Copyright 2011 American Chemical Society. Also reprinted from [80] with permission from Elsevier.

and the process continued until the surface was saturated with Pt nanoparticles. Hiramatsu et al. [84] pointed out the influence of substrate temperature on Pt nanoparticles formed on CNTs. The nanoparticle size increased to 1.5–3 nm when the deposition temperature increased from 120 to 170 ◦ C. The electrochemical characterization of the Pt/CNT composites was not done. However, given the small particle size and narrow size distributions, they hold promise for FC applications. Pt based electrocatalysts with different supports such as MWCNTs, Vulcan XC 72R (VXR) and black pearl 2000 (BP2000) were also prepared by SCD by Bayrakceken et al. [86]. Pt nanoparticles, about 1–2 nm in diameter, were found to be dispersed uniformly on the carbon supports. The TEM images in Fig. 15 give an exemplary comparison between the electrocatalysts prepared by SCD versus a commercially available electrocatalyst with the same support of VXR. It is quite clear that the catalyst prepared by SCD has a smaller average nanoparticle size and a narrower size distribution. ESA and the activity for the oxygen reduction reaction of these catalysts

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0.003 SCD (9 wt. % Pt on CB) 0.002

I/A cm-2

0.001 0.000 -0.001 ETEK (10 wt. % Pt on CB) -0.002 -0.003 -0.004

0.0

0.2

0.4

0.6

0.8

1.0

E/V (vs NHE) Fig. 16. Comparison of cyclic voltammograms of electrocatalysts: commercial and prepared by supercritical deposition (carried out in 0.1 M HClO4 in H2 atmosphere at a scan rate of 50m V/s). Adapted from the reference. Reprinted from [86] with permission from Elsevier.

were compared to that of the commercial ETEK Pt/C (10 wt.% Pt) and Tanaka Pt/C (46.5 wt.% Pt) catalysts. In Fig. 16, the substantially higher areas for hydrogen adsorption and desorption demonstrate the superiority of the electrocatalyst prepared by SCD. Fig. 17 shows CV positive scans for O2 reduction with rotation speeds varying between 100 and 2500 rpm at a 10 mV/s sweep rate for the Pt/VXR. It can be observed that the current increases gradually with the speed of rotation and the limiting current plateaus indicate that the oxygen reduction is fast enough at high overpotentials. The authors further remark that if the electro-catalytically active sites were not distributed uniformly and the electrocatalytic reaction was slow, the current plateau would be more inclined. The ESA and the Pt utilization varied with the type of carbon support at a fixed Pt loading, likely due to the different microporous and meso/macroporous structures of the supports that affect the accessibility of the electrolyte to the metal. Using CV measurements, they calculated the number of electrons transferred per oxygen molecule as 3.5, 3.6 and 3.7 for Pt/BP2000, Pt/VXR and Pt/MWCNT, respectively. These values indicate that the oxygen reduction reaction is close to the 4e− pathway (reaction 3), which indicates almost complete water formation and negligible hydrogen peroxide formation. This behavior

Fig. 17. Hydrodynamic voltammograms of positive scans of Pt/VXR eletrocatalyst prepared using supercritical deposition (for O2 reduction in O2 saturated 0.1 M HClO4 ) (a) Koutecky–Levich plot at 0.2 V. Reprinted from [86] with permission from Elsevier.

is expected for the Pt electrocatalysts as previously mentioned in Section 2.3. The activity towards the ORR of three different electrocatalysts and a commercial Pt/CB electrocatalyst was compared by Shimizu et al. [87]. The electrocatalysts were prepared by using three different methods; (1) nanoreactors of water-in-supercritical CO2 microemulsions (Pt/CNT SCME), (2) SCD (Pt/CNT SC), and (3) water-in-hexane microemulsion (Pt/CNT ME). The preparation sequence for Pt/CNT SCME involved the formation of water in scCO2 microemulsions using AOT surfactant (solubilized by cosolvent hexane) at 19 MPa and 40 ◦ C. Na2 PtCl4 was dissolved in the nanopools of water and was reduced to Pt0 by the injection of a H2 + CO2 mixture to the vessel which brought the final pressure to 25 MPa. Subsequently, the system was depressurized and the composites were removed from the vessel. They were washed with ethanol and then dried in an oven at 80 ◦ C. The activity of the PtCNT SCME system for ORR was found to be the highest at 11.5 A/g of Pt, which was about 1.7 times higher than the commercial reference system based on carbon black. Moreover, the largest ESA was obtained for Pt/CNT SCME at 31.1 m2 /g Pt. This observation suggests that the surfactant used during the synthesis was more effectively removed by using scCO2 as compared to the electrocatalyst prepared with hexane. This surface area was over 3 times larger than the other catalysts tested, including the commercial Pt/CB. Furthermore, the hydrodynamic polarization curves showed a significant potential shift of +350 mV for Pt/CNT SCME at 10 A/g of Pt when compared to the commercial Pt/CB, indicating the possibility for a significant increase in power output in a FC. The same analysis indicated a more complete consumption of oxygen (3.7e− per O2 ) for the newly synthesized Pt/CNT SCME over Pt/CNT SC (3.4e− ), commercial Pt-CB (3.1e− ) and Pt-CNT ME (2.9e− ). The electrocatalytic activity of CB supported Pt catalysts prepared by Ang and Walsh [88] seems promising in terms of stability towards ORR. They synthesized electrocatalysts by dissolving and adsorbing Pt(cod)me2 on CB from scCO2 at 8.3 MPa and 40 ◦ C. The Pt(cod)me2 adsorbed on CB was then decomposed to elemental Pt using post-deposition thermal decomposition under N2 atmosphere at 300 ◦ C. Resulting supported Pt nanoparticles had an average particles size of about 3.7 nm. CV results between 0.6 and 1.1 V at 0.5 V/s and in 0.5 M H2 SO4 showed that the electrocatalysts prepared using SCD had a small reduction in ESA whereas the electrocatalysts prepared via wet impregnation had severe ESA reductions (23%) after 5000 cycles. As mentioned before, nanoenginering the architecture of the Nafion–carbon–platinum system may lead to enhancement of performance since the electrochemical reaction only takes place in the interfacial electro-active region. In Erkey’s group, novel platinum–Nafion® –carbon black nanocomposites were synthesized via a supercritical fluid route as electrocatalysts for PEMFC [48]. First, Nafion® and CB composites were prepared. They were then impregnated with Pt(cod)me2 in scCO2 followed by thermal conversion of the Pt precursor to obtain platinum–Nafion® –carbon black composite. The average size of the platinum metal particles in the Nafion® –carbon black composite was found to be around 2.6 nm. CV tests demonstrated that the Pt–Nafion® –carbon black nanocomposite displayed promising electroactivity as an electrocatalyst for FCs. They suggested that the electrochemical area (20 m2 /g) can possibly be further increased by optimization of the preparation conditions and the ratio of Nafion® to carbon black. The developed strategy takes advantage of the fact that Nafion® does not decompose at the low temperatures employed for SCD. A novel approach of decorating carbon supports with platinum using Pt(acac)2 , which exhibit high performance for PEMFC applications have been demonstrated by Taylor et al. [89]. Carbon black (CB), carbon fiber (CF), and SWCNT supported Pt nanoparticles were prepared in liquid or supercritical methanol as the medium

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17

Fig. 18. Performance curves for a PEMFC at 80 ◦ C in which electrocatalysts were prepared using SCD (anode and cathode humidifiers at 90 ◦ C, with H2 and O2 flow rates of 100 sccm). (a) SWCNTs with sodium dodecyl sulfate (SDS) (solid points) and without SDS (outlined points). (b) XC-72 with SDS (outlined points) and without SDS (solid points). Reprinted from [89] with permission from Elsevier.

at pressure conditions ranging from 0.4 to 8.5 MPa and temperature range of 200–300 ◦ C. The technique may be advantageous over conventional SCD since it does not require additional steps for precursor conversion, which may lead to a reduction in the time and cost of synthesis. These materials were characterized and showed promising catalytic properties in PEMFC. Under supercritical conditions, they found that higher fluid densities suppressed the growth of the particles. By lowering the temperature to 230 ◦ C (just below the critical temperature of methanol), they also demonstrated that high-temperature methanol can be a viable medium for synthesizing FC catalysts. Pt/CF FC catalysts synthesized under these conditions had an average particle size of 2.8 ± 2.8 nm, and 90% of the particles was less than 5 nm. The catalysts showed comparable Pt nanoparticle structures based on high-resolution TEM imaging and demonstrated good PEMFC activity, with a platinum utilization of 27% for Pt/CFs, 36% for Pt/SWCNTs, and 63% for XC-72 (CB). They also demonstrated the benefit of using a surfactant such as sodium dodecyl sulfate (SDS) in the synthesis of Pt nanoparticles in supercritical methanol. As shown in Fig. 18, Pt/SWCNT catalysts synthesized in supercritical methanol with SDS had the best performance, with a peak power density of 4.49 kW/m2 and a Pt utilization of 1.088 g Pt/kW whereas the peak power density was 4.26 kW/m2 with a Pt utilization of 1.246 g Pt/kW when XC-72 was used as the support material. The Pt loading was nearly the same for CFs without SDS (78.6%) and with SDS (81.7%). When SDS was used, the Pt loading decreased by 6.6% for the SWCNTs and by 8.4% for XC-72. Additionally, the kinetic studies indicated that formation of Pt nanoparticles was a very fast reaction. Taylor et al. [49] also demonstrated promising methods of SCD on SWCNTs and CFs as well as the use of layer-by-layer (LBL) methods to construct films and MEAs containing SWCNT supported Pt prepared by using supercritical methanol. PANI was used for polycation and Pt/SWCNTs were used as polyanion for the fabrication of LBL films. They were able to use these LBL films as a catalyst layer and a catalyzed gas diffusion layer and achieved remarkable Pt utilization values which were some of the highest reported in the literature such as 2540 mW/mgPt (for PANIPt/SWCNT + Nafion® ) system of 400 bilayers) and 3198 mW/mgPt (for PEI–(Pt/CNF + Nafion® ) system of 100 bilayers). As mentioned before, Pt is very expensive and it constitutes the major percentage in a breakdown of MEA costs. The two alternative approaches that are under consideration to decrease the Pt amount are to decrease the particle size and provide good dispersion on carbon or to use Pt together with a less expensive and

more abundant metal without sacrificing the activity. The second approach can also benefit from using SCFs. However, very few studies were reported in the literature on using SCD to prepare supported binary metal nanoparticles and they include preparation of multiwalled carbon nanotube supported PtRu, PtCu, PtAu, PtPd, and PtNi particles [90,91] as well as CB supported PtPd nanoparticles [92]. Two approaches naturally arise in order to synthesize bimetallic nanoparticles using SCD which are simultaneous [93,94] and sequential [92] deposition. In simultaneous SCD, one starts with two metal precursors by dissolving them simultaneously in scCO2 and then carry out the same steps as in single component SCD. Whereas in sequential SCD, the single component SCD procedure is applied twice as depicted in Fig. 19. Lin et al. synthesized bimetallic PtRu with particle sizes around 5–10 nm on the surface of CNTs using simultaneous SCD and tested them as catalysts for methanol oxidation [91]. The metal precursors (Pt(acac)2 and Ru(acac)2 ) were dissolved in scCO2 at 200 ◦ C and 8 MPa in the presence of a small amount of methanol. Then a H2 + CO2 mixture was injected into the vessel which increased the pressure up to 20 MPa. According to the authors, the reductions from Ru2+ to Ru0 and Pt2+ to Pt0 were fast and took about 15 min. PtRu/CNT composites were subjected to post-deposition sequences involving washing with methanol and ultrasonification. Electrocatalysts showed promising activity for methanol oxidation, which was demonstrated by electrochemical studies including CV, linear, sweep voltammetry and chronoamperometry. The higher If /Ib value and lower onset potential for methanol oxidation showed that the PtRu/CNT exhibited higher activity than that of the Pt/CNT (also synthesized using SCD). The authors suggested that Ru promoted the oxidation of the strongly bound adsorbed CO on Pt by supplying an oxygen source (adsorbed Ru-OH) as proposed in the literature [79]. The high catalytic activity was attributed to the large surface area of CNT and the decrease of the overpotential for methanol oxidation. The fact that Pt-Ru electrocatalyst is more stable than the Pt electrocatalyst needs to be noted. In the work of Yen al. [90], electrochemical activities of the five different binary metal supported electrocatalysts were investigated. The simultaneous SCD sequence was similar to the one used by Lin et al. [91] except for the initial scCO2 pressure (10 MPa) and the pressure after the injection of H2 + CO2 mixture (20 MPa). In this case, the conversion reaction was carried out for 30 min. As shown in Fig. 20, it was found that Pt-Ru catalyst had a higher activity for methanol oxidation than other bimetallic catalysts and furthermore, it had a forward overpeak potential of 913 mV versus NHE and a forward

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Fig. 19. Schematic representation of the sequential supercritical deposition.

peak current of 0.68 A/mg Pt. The MWCNT-supported bimetallic nanoparticle catalysts all exhibited at least 60% higher If /Ib ratios relative to that of Pt monometallic nanoparticles (If /Ib = 1.4). An et al. [95] also prepared PtRu/CNT electrocatalysts by SCD for use in DMFCs but they used different metal precursors (H2 PtCl6 ·6H2 O and RuCl3 ·3H2 O) than that of the Wai’s group (Pt(acac)2 and Ru(acac)2 ). The synthesis medium was a CO2 –methanol–water solution at high pressure. The vessel containing the methanol–water–metal precursor–MWCNT dispersion was charged with CO2 up to a certain pressure (3.5, 6.5 and 9 MPa) and then the high-pressure vessel was placed in an oven at 120 ◦ C and maintained there for 2 h. After the depressurization, the dark precipitate was separated by centrifugation and washed with ethanol and water and then vacuum-dried at 60 ◦ C for 6 h. Characterization of PtRu/MWCNT composites synthesized with an initial weight ratio of RuCl3 ·3H2 O: H2 PtCl6 ·6H2 O: MWCNT of 1:2:2 under an initial CO2 pressure of 6.5 MPa is shown in Fig. 21. Fig. 21a and c reveal that nanoparticles with sizes of around 5 nm were uniformly dispersed on MWCNTs. Further, selected area electron diffraction (SAED) analysis confirmed the crystallinity (Fig. 21b) and energy dispersive X-ray spectroscopy (EDX) analysis confirmed the reduction of the metal precursors (Fig. 21d). An et al. also showed that

Fig. 20. Cyclic voltammetry scans with different bimetallic nanoparticle catalysts produced by supercritical deposition in 2 M CH3 OH and 1 M H2 SO4 under deoxygenated conditions. (Scan rate: 60 mV/s, and the scan direction is shown with an arrow). Reprinted with permission from [90]. Copyright 2011 American Chemical Society.

the Pt-Ru/MWCNT composites had high electrocatalytic activities for methanol oxidation. However, they found that the Pt-Ru electrocatalyst was not stable during methanol oxidation reaction for a long time which was attributed to the leaching of ruthenium during the reaction. Erkey’s group recently prepared bimetallic (PtPd/CB) electrocatalysts for PEMFCs by sequential scCO2 deposition [92]. Their sequence consisted of the SCD of the first metal (Pt) which was followed by the SCD of the second metal (Pd) onto the same substrate. They synthesized supported nanoparticles with Pd:Pt atomic ratios varying between 2:1 and 1:4. The prepared electrocatalyst were found to be rich in Pt for the small nanoparticles (1–4 nm) and Pd rich for larger particles (>5 nm). The comparison of the electrocatalytic activity of various catalysts prepared in the study are presented in Fig. 22 where the hydrodynamic voltammograms of positive scans of prepared catalysts at 2500 rpm before (a) and after (b) potential cycling are plotted. As shown in Fig. 22a, the ORR activity of the prepared catalysts before potential cycling decreased in the order of: PtPd/BP2000 (1:4) < Pt/BP2000 < PtPd/BP2000 (2:1) < PtPd/BP2000 (1:2) up to approximately 0.5 V. At 0.7 V, the worst performance was obtained for Pt/BP2000 catalyst and the activities of the PtPd/BP2000 (1:4) and PtPd/BP2000 (1:2) catalysts became similar. After potential cycling, the order of ORR activity was the same up to 0.35 V. The activities of the PtPd/BP2000 (1:4) and PtPd/BP2000 (1:2) catalysts became nearly identical over 0.6 V. The general trends in activity were similar before and after potential cycling, but similar activities were observed at lower potentials which may be attributed to the activity loss of the catalysts with respect to the change in the composition that affects the system negatively. Based on the analysis of TEM images and EDX data, no Pd existed for the PtPd/BP2000 (2:1) catalyst and a large reduction in the Pd content of the PtPd/BP2000 (1:4) catalyst was observed when subjected to potential cycling between 0.6 and 1.2 V. However, after potential cycling, the activity of PtPd/BP2000 (2:1) catalyst was found to be significantly higher due to the dissolution of Pd leaving only Pt nanoparticles on BP2000. Electrochemical measurements also showed the decline of the ESA for a particular catalysts but the lowest decline was found for the Pt rich ones (29% of the ESA after 300 potential cycles of the CVs for the H2 oxidation reaction). They also found from the post-potential cycling tests that the potential losses increased with increasing Pd content in the electrocatalysts.

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Fig. 21. (a) and (b) Transmission electron microscope (TEM), (c) High resolutition-TEM (HRTEM) and (c) Energy dispersive X-ray (EDX) analysis of electrocatalysts synthesized using supercritical deposition. The inset in (b) is the selected area electron diffraction pattern of the composite denoted in the rectangular area. Reprinted from [95] with permission from Elsevier.

In an interesting study, Wakayama et al. synthesized Pt-Ru nanofibers by the nano-scale casting process using supercritical deposition [96,97]. Pt(acac)2 and Ru(acac)2 precursors were impregnated onto activated carbon fiber cloth in scCO2 at 32 MPa and 150 ◦ C for 24 h. The deposition process was then followed by an oxygen plasma treatment for 20–91 h to remove the activated carbon fiber template. The remaining Pt-Ru nanoporous material was found to have the fibrous shape of the activated carbon template. The fibers consisted of fused particles of 10–20 nm in diameter. They also showed that the chemical composition of Pt-Ru fibers could be controlled by changing the time of the oxygen plasma treatment. However, XRD patterns indicated that Ru was oxidized to RuO2 after the long oxygen plasma treatment. Their results further showed that the Pt-Ru fibers exhibited high activity as an anode catalyst for DMFCs.

5.3. Preparation of carbon supports using SCFs As new generation alternatives to conventionally used CBs as PEMFC electrode materials and electrocatalyst supports, nanostructured and electrically conductive CAs are usually prepared by the carbonization of organic aerogels which are prepared from sol–gel polycondensation of organic monomers (such as resorcinol and formaldehyde) [98,99]. The surface area, pore volume and pore size distribution can be tuned by changing synthesis conditions. The concentration of the micro and mesopores can be controlled independently by the concentration of starting materials, drying

and curing conditions. The preparation of carbon aerogels can be summarized by three steps [99]: • Preparation of gel • Drying of the wet gel • Carbonization Drying of the gels is generally carried out via supercritical extraction since evaporation of the solvent by heating often induces pore collapse due to high capillary forces imposed by the vapor–liquid interface. Supercritical drying results in materials with high surface areas (400–1500 m2 /g). Therefore, scCO2 extraction is widely used in drying of gels that can be used either as catalyst support or adsorbents. Guilminot et al. [100] developed nanostructured CAs by pyrolysis of organic aerogels which were obtained by supercritical drying of cellulose acetate gels. The CAs were found to have high surface areas (above 400 m2 /g) and their surface areas could be increased even more by thermal activation treatment in CO2 atmosphere. This also led to a chemically stable carbon surface, by eliminating most of the OH, CH2 , CH3 and unstable CO groups. Thus, in impregnation of the CA from a solution of H2 PtCl6 in water-isopropanol (1:1), the uptake of the anionic Pt can be facilitated by the CO2 activation since the carbon substrates free of oxygenated surface groups are known to be favored for adsorption. They also showed that CA surface area increased upon carbonization due to the removal of oxygenated groups which could also lead to the improved loading of Pt precursors. The results of CV measurements showed that

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process issues associated with gas phase reactions can be avoided. Lou et al. published a series of papers in which they investigated the synthesis of carbonaceous compounds such as CNTs, fullerenes and carbon spheres [59–61]. They were able to synthesize large amounts of CNTs (55 nm × 1.5 ␮m) from scCO2 at 70 MPa and 550 ◦ C in the presence of metallic lithium. They were also able to manipulate the morphology of the CNTs by changing reaction conditions and synthesized double helical, bamboo-shaped and porous CNTs [102]. Smith et al. [103] synthesized MWCNTs with outer diameters of 10–50 nm and wall thicknesses of 5–20 nm by using a similar approach. However, supercritical toluene was used in this study at temperatures ranging between 600 and 645 ◦ C and at 8.3 MPa. Nickelocene, ferrocene, cobaltocene, cobalt and iron crystals were used as catalysts. According to the authors, cobaltocene seemed to be the optimum catalyst in terms of both the purity of the product and the conversion of toluene to nanotubes. Supercritical CO (Tc = −140.4 ◦ C, Pc = 3.49 MPa) has also been used in CNT synthesis [104] and MWCNTs with diameters of 10–20 nm and lengths of several tens of micrometers were synthesized with a high yield. Recently, a low temperature synthesis method was reported in which near-critical hexane was used at 250 ◦ C and 3.4 MPa in the presence of a cobaltocene catalyst to grow CNTs [105]. The synthesized MWCNTs had an average length of 1.161 ± 0.190 ␮m, average width of 118 ± 17 nm, and wall thickness of 33 ± 9 nm. They were able to incorporate ORR active phytalocyaninces into the CNTs to end up with promising CNT supported non-Pt electrocatalysts. Another interesting material is the composite of CB with scCO2 dried silica aerogel that showed high electrocatalytic activity for methanol oxidation. The electrocatalytic performance of colloidalPt supported carbon–silica composite aerogels increased by four orders of magnitude per gram of Pt for methanol oxidation over that of the native Pt supported carbon powder [106]. 5.4. Future research needs Fig. 22. Comparison of hydrodynamic voltammograms of positive scans of catalysts prepared via supercritical deposition for O2 reduction in O2 saturated 0.1 M HClO4 at 2500 rpm before (a) and after (b) potential cycling between 0.6 and 1.2 V for 1000 cycles. Reprinted from [92] with permission from Elsevier.

the ORR specific activities of the Pt/CA prepared within this study are comparable with that of the Pt/Vulcan XC-72, while their mass activities were slightly higher. Platinum loaded carbon aerogels for PEMFCs were synthesized by Kim et al. [101] by carrying out a sol–gel reaction of phloroglucinol with furfural using propylene oxide as a reducing agent of platinum salt in an alcoholic solution, followed by supercritical drying with CO2 . Subsequent carbonization of the platinum salt–organic aerogels under H2 /Ar gas flow produced platinum–carbon aerogels with a 40 wt.% platinum loading. The aerogel catalyst prepared in this study had a high surface area, high pore volume, a well-developed mesoporous structure, and highly dispersed platinum particles of about 2–3 nm size. Compared with commercial catalysts, the Pt/CA composite had a higher Pt/C ratio. It also had a comparable ESA to those of commercial catalysts. However, some of the platinum surface could not serve as active sites under reaction conditions due to the possible trapping of Pt nanoparticles in micropores. Another explanation of the mediocre performance of the electrocatalyst may be the collapse of the nanosized pores of CAs during solvent impregnation which can cause a decrease in the ESA, a phenomenon that could have been avoided if SCD were used. SCFs can also be alternative media for CNT or fullerene synthesis since the low crystallinity, poor conversion and high temperature

Composites of Nafion with palladium prepared via SCF based routes are promising membranes for DMFCs. Even though metal loading reduces the methanol cross-over, it can also induce a reduction in the ionic conductivity. Further experimental and modeling work is necessary in order to achieve optimal loading amounts which results in maximum FC performance. Acid–base polymeric composites of non-Nafion membranes have potential to reduce the methanol cross-over and increase the conductivity. Preparation techniques of these materials (hydrocarbon membranes, polyaniline, etc.) including polymerization, grafting or processing via SCFs merit future research. We note that the effects of water concentration and temperature on mechanical properties of Nafion® are known. For instance, the elastic modulus and the stiffness decrease with increasing temperature and water concentration [107]. Furthermore, it has also been found that at low temperature, water plasticizes Nafion® and at high temperature, water stiffens Nafion [108]. However, there is not much work in the literature on the mechanical properties of Nafion® composites prepared using SCFs. The electrocatalysts prepared via SCF based routes are promising especially when compared to conventionally prepared commercial catalysts. However, the Pt amounts (mW/mgPt ) must be decreased significantly from current levels for wide-spread commercialization. Further experimental and theoretical studies are needed on the kinetics and thermodynamics of adsorption of organometallic precursors on surfaces of carbonaceous materials to be able to understand the importance of the precursor adsorption on the final properties of the supported electrocatalysts. Computer simulations could be helpful within this topic. Since the final metal particle properties is affected by the conversion process, further investigations in this area would be beneficial. Computer simulations and thermogravimetric analysis (TGA) coupled with mass

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spectroscopy could be helpful for determination of reaction pathways. The use of supercritical solvents other than CO2 has been shown to be promising. Therefore, solubilities of metal precursors in other supercritical solvents need to be measured. Development of models for kinetics of adsorption of the precursors in other supercritical solvents would be beneficial. The understanding of the reduction mechanisms in these new solvents could be very helpful. The development of supported binary metal electrocatalysts by SCD is another area which deserves further attention since very little work has been done. Further investigations on the binary solubilities of metallic precursors in SCFs as well as the understanding of phase behavior of these mixtures are crucial. Furthermore, the modeling of binary adsorption of metal precursors on carbon surfaces from SCF solutions could be very useful for controlling the metal loading and composition. The kinetics and thermodynamics of the conversion reactions of binary metallic precursors and nanoalloy formation should be investigated. Recently, the effects of structure and the composition of the nanoparticles have been shown to be very important [44,109]. Stamenkovic et al. [109] demonstrated that the Pt3 Ni(1 1 1) surface is 10-fold more active for the ORR than the corresponding Pt(1 1 1) surface and 90-fold more active than the current state-of-the-art Pt/C catalysts for PEMFC. They also pointed out the variation in catalytic activity of (1 1 1), (1 0 0) and (1 1 0) planes of Pt3 Ni surfaces. Among these planes (1 1 1) has the highest activity and (1 1 0) has the lowest with a difference of one order of magnitude. The engineering of nanoparticle structures via SCFs should also be considered while preparing carbon supported electrocatalysts. As we previously mentioned, Au nanoparticles of 1–3 nm range protect the electrocatalyst against CO poisoning. PtAu bimetallic electrocatalysts prepared using SCFs may be very promising in that sense. Stability of the aforementioned bimetallic/nanoalloy catalysts needs also further investigation. Combinations with other metals presented in Fig. 6 can be experimented in order to reach conclusive results on this area. Additionally, performance tests in FCs should be carried out using the aforementioned electrocatalyst prepared via SCFs. Finally, we note that the studies on synthesis of supported cheap, non-Pt alloys in scCO2 are rare [110–112] and carbon supported non-Pt alloys have never been produced using SCFs. In this context, SCD can provide new and promising opportunities for development of of non-Pt alloy electrocatalysts for FCs. Novel MEA preparation techniques have been under investigation since the conventional techniques show inefficiencies in incorporating the electrode and the proton exchange membrane. The incorporation of SCFs into new techniques such as LBL [49] can bring advantages and merits future research. Carbon aerogels can be reinforced by planar fibers for an optimal structure suitable for use as asymmetric gas diffusion electrodes in FCs. The use of fiber additives derived from carbon fibers, ceramics or polymers increase the stability of the carbon aerogel and allow preparation of thin planar sheets with thickness between 50 and 300 ␮m. Among different materials carbon fibers are particularly interesting due to their advantageous properties such as high electrical in-plane conductivity, very low shrinkage during the pyrolysis and high mechanical strength which lead to very strong but flexible carbon aerogel sheets when incorporated into aerogels [113,114]. Carbon aerogels are also promising materials for use in bipolar plate manufacturing. These approaches can be beneficial for the aerogel-based electrocatalysts prepared using SCFs. New and functionalized CNTs are synthesizable using surface functionalization from supercritical solutions. Polymer wrapped CNTs or nano hybrid shish–kebabs can be prepared [115]. Dispersion of CNTs in matrices can be carried out in this fashion. The potential use of SCFs in making composites of metal-organic frameworks (MOFs), CNTs and polymers are still

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unknown. Therefore, the need for development of new materials coupled with unique properties of SCFs is expected to lead to novel materials such as electrocatalyst supports and GDL materials for FCs.

6. Use of SCFs in development of electrolyte–electrode assemblies for SOFCs SOFC electrolyte material needs to be stable in both oxidizing and reducing environments and must prevent the mixing of the fuel and air gases. In SOFCs, solid ceramic electrolytes such as yttria-stabilized zirconia (YSZ) are used. YSZ is an oxygen ion conductor and meets the requirements of a SOFC such as high ionic conductivity at temperatures above 700 ◦ C with negligible electronic conductivity. In anode, a nickel–YSZ ceramic metal mixture (cermet) and in cathode, strontium-doped lanthanum manganite (LSM) or lanthanum–strontium ferrite (LSF) are commonly used materials. These materials present good oxidation resistance at high temperature and high catalytic activity. The high operating temperature of SOFCs brings some advantages as well as some disadvantages. Stack hardware, sealing and cell interconnect issues can be stated as the challenges associated with the high material cost due to high temperatures. Sulfur intolerance of the present Ni–YSZ anodes is also another problem. On the other hand, fuel flexibility, high efficiency, and the feasibility of employing cogeneration systems are the advantages [5,8]. Conventional particle formation techniques such as milling, grinding, chemical precipitation and spray drying have several limitations. Mechanical treatment generally damages the product or causes performance degradation that can be imposed by particle traumatization, frictional heat or wide particle size distribution. It is also difficult to obtain particles with the desired particle size and distribution in techniques such as chemical precipitation or spray drying since reaction rates are controlled by mass transfer limitations or due to the droplet sizes formed by spraying [116]. Supercritical fluids have been under investigation for use in synthesizing inorganic materials. Submicron or nanosized inorganic materials can be produced through the hydrothermal reactions in supercritical water (Tc = 373.94 ◦ C and Pc = 22.06 MPa) or the thermal decomposition reactions in various supercritical solvents. The supercritical state of water allows one to vary the reaction rate and reaction equilibrium by shifting the dielectric constant and solvent density with pressure and temperature. Thus, supercritical water can be expected to provide many benefits such as high reaction rates and small particle sizes. Supercritical hydrothermal synthesis (SHS) method was primarily developed by Arai’s group who has studied the hydrolysis of metal salts in supercritical water (scH2 O) to crystallize hydroxide or oxide particles [117]. In this technique, the formation of metal oxide via the hydrothermal crystallization of aqueous metal salt solutions in scH2 O is usually carried out in a continuous reactor. Hydrolysis of metal acetates is also possible in subcritical water [40]. Materials prepared using SCFs that can be used in SOFCs are generally ceramic membranes (which will be elaborated in Section 6.1) even though there are some studies on the synthesis of cathode materials such as La3 Ni2 O7 or La4 Ni3 O10 by SHS [118]. La3 Ni2 O7 − ı and La4 Ni3 O10 − ı appear to possess relatively high oxide ion conductivity, coupled with high mechanical and thermal stabilities at elevated operating temperatures and a good balance between electronic and ionic conductivities as compared to LSM and LSF. Weng et al. [118] were able to prepare La3 Ni2 O7 and La4 Ni3 O10 via single heat treatment in air for 12 h at temperatures 1150 and 1075 ◦ C, respectively from the corresponding metal hydroxide co-crystallites synthesized using SHS at 400 ◦ C and 24.1 MPa. The results look promising, as the higher order

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Ruddlesden–Popper phases (n = 2 and 3) are usually synthesized through several days of multiple regrinding and heating. 6.1. Preparation of ceramic membranes using SCFs SHS has been used to synthesize spherical fine particles of zirconia with particle size of ∼10 nm by the decomposition of ZrOCl2 [119]. The authors also investigated the formation of ceria fine particles from the metal salt, Ce(NO3 )3 ·6H2 O at 573 K and 30 MPa. The size of the particles was found to range from 20 to 300 nm [119–121]. Ultra fine particles could be formed by this method in reactors with residence times less than 1 s. The formation of fine particles was explained by the faster nucleation rate in scH2 O due to the very low solubility of metal oxides in the medium and extremely fast reaction rates. Cabanas et al. prepared solid solutions of powdered CeO2 –ZrO2 in a continuous near-critical water reactor at ca. 300 ◦ C and 25 MPa [185,186]. They were able to control the composition of Ce1 − x Zrx O2 nano-particulates by varying the initial concentrations of Ce4+ and Zr4+ ions in the starting solution ([NH4 ]2 [Ce(NO3 )6 ] (CAN) and [Zr(ac)4 ]). TEM images revealed that the particle size was between 3 and 5 nm and the particles were well dispersed. With the method used, no modifier was necessary and homogenous particles could be produced with surface areas up to ca. 190 m2 /g. Kim et al. [122] also investigated the formation of ceria–zirconia mixed oxide by SHS. They used ammonia/water as pH control agent. The N2 adsorption, temperature programmed reduction and O2 uptake tests revealed that the new composites had superior thermal stability and oxygen storage capacity than composites prepared via conventional co-precipitation method. The synthesis of YSZ using scCO2 and its encapsulation with polydimethylsiloxane (PDMS)–g–polyacrylate (PA) or poly(methyl methacrylate) (PMMA) in scCO2 was demonstrated by Hertz et al. [123]. The YSZ powder was synthesized using a scCO2 aided sol–gel method where the aged/non-aged solution of yttrium and zirconium acetates in pentane and nitric acid was treated with scCO2 at 250 ◦ C and 30 MPa. Subsequently, the encapsulation was also carried out in scCO2 environment where CO2 soluble PDMS-g-PA surfactant was impregnated on YSZ particles at 150 ◦ C and 30 MPa for 30 min. The CO2 soluble surfactant allowed the dispersion of YSZ particles in scCO2 . The two parts of the surfactant had two functions. PDMS part facilitated the dispersion whereas the PA part which was adsorbed on the YSZ particle surface, favored the polymerization by fixing the monomers in the core–shell formed by the surfactant. When the pressure and temperature were reduced, PDMS-g-PA precipitation and encapsulation of particles took place. This method allowed the synthesis of YSZ powders with particle diameters ranging from 30 to 300 nm and crystallite sizes ranging from 10 to 25 nm. Weng et al. [124] also prepared YSZ powders with high crystallinity and NiO/YSZ powder as SOFC electrolyte and anode materials, respectively using SHS at 450 ◦ C and 24.1 MPa. The NiO/YSZ powders were then spark plasma sintered to form dense disks of Ni/YSZ cermets as a result of in situ reduction of the NiO. Ion beam milling tomography on a cermet sample of 24 vol.% Ni loading suggested that the nickel nanoparticles had formed conducting 3D networks. Ni/YSZ samples had significantly higher conductivity (∼102 S/cm) at 24 vol.% Ni compared to cermets with similar Ni contet prepared using conventional methods (∼10−1 S/cm). Authors suggested that the fabrication technique has the potential to make nanostructured anode cermets that can be used in the active and current collecting layers of SOFCs. 6.2. Future research needs The purpose of some of the studies mentioned above was not related to a FC related application. However, we believe that since

YSZ and such oxides of ceria and zirconia are widely used as electrolyte material in SOFC, these methods seem promising. Since new and improved cathode materials are needed to overcome potential losses being faced below 700 ◦ C, SHS may be beneficial in that aspect. Furthermore, there is a lack of knowledge on the surface coverage of catalysts under operating conditions. Influence of the morphology of the anode materials on reduction potential is also largely unknown [54]. Such information may be very beneficial in improving SOFC performance. Design of new sulfur tolerant anode materials would be very beneficial for improving the efficiency of SOFC. Therefore, further studies are needed for testing these materials in SOFCs. SHS may also be used in developing more effective anode and electrolyte materials with higher conductivity and thermal stability. 7. Fuel processing and storage using SCFs Another emerging area where SCFs can be beneficial is in the area of the production and the purification of hydrogen and fuel processing associated with FCs. As previously mentioned, H2 can be fed to a FC directly from a pressurized cylinder or can be produced from hydrocarbons using a fuel processor that is attached to the FC plant. In the latter case, a generic fuel processor in the form that it is generally applied to low temperature FCs is shown in Fig. 23. This diagram shows the main process stages and flows only, dotted devices indicating components that may or may not be present. The area to the left of the solid vertical dividing line is the fuel processor. Here, the main step for H2 production is fuel reforming which can be carried out using methods including steam reforming, partial oxidation and the autothermal reforming. Water–gas shift reaction occurs during reforming as an associated reaction. Unlike the steam reforming reaction, the water–gas shift reaction is exothermic and the equilibrium shifts away from the products at elevated temperatures. As a result, CO is produced which acts as a poison for PEMFC, PAFC and AFC. CO must also be removed from the fuel stream before it reaches to the electrodes where it can prevent the H2 adsorption on Pt or Pt alloys, therefore deactivate the electrocatalysts [4]. Preferential oxidation (PROX), methanation and membrane separation methods are techniques used to reduce the CO level. The only ubiquitous major components of the fuel processor are a reformer and a shift reactor. Depending on FC type, fuel type and choice of reformer technology and operating conditions, other components that may be present upstream or downstream of the reformer are vaporizer, desulphurizer, pre-reformer, further shift stages and a CO polishing unit. Table 3 summarizes some of the fuel composition specifications associated with FCs. Hydrocarbons often contain sulfur containing compounds like triophenes, benzothiophenes and dibenzothiophenes. Sulfur also poisons the FCs and must be reduced from 300 to 500 ppm by weight (sulfur levels of diesel and gasoline, respectively) to values around 0.1 ppm. Approaches like hydrodesulfurization (HDS) of hydrocarbons or selective adsorption of sulfur containing compounds can be applied in order to achieve appropriate H2 purities. The fuel processor in state-of-the-art FC systems is complex, bulky and costly. For example, the fuel processing section of the 250 kW PEMFC system demonstrated by Ballard corporation represents around 40% of the 2.4 m × 2.4 m × 6.1 m system [125]. This represents an overall power density of only 0.018 kWe/l. Cheaper, more compact fuel processors will be an important enabling technology for commercially successful future FC systems. The materials needed for such a fuel processor can be obtained via SCF based processes. For instance, it is possible to purify H2 by using palladium or palladium composite membranes prepared via SCD (Section 7.1). H2 can be produced by gasification in supercritical water (Section 7.2). Furthermore, fuel processing catalysts can be prepared using SCFs (Section 7.3) or fuel desulfurization

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Fig. 23. Generic fuel processor block schematic [125]. Table 3 Required fuel composition specifications for different fuel cell types [5]. FC type

CO

CH4

CO2 and H2 O

Sulfur (as H2 S and COS)

PEMFC AFC PAFC MCFC SOFC

Poison (>10 ppm) Poison Poison (>0.5%) Fuela Fuela

Diluent Diluent Diluent Diluenta,b Diluenta,b

Diluent Poisonc Diluent Diluent Diluent

Poison (>0.1 ppm) Unknown Poison (>50 ppm) Poison (>0.5 ppm) Poison (>1 ppm)

PEMFC: Proton exchange membrane fuel cell; DMFC: Direct methanol fuel cell; AFC: Alkaline fuel cell; PAFC: Phosphoric acid fuel cell; MCFC: Molten carbonate fuel cell; SOFC: Solid oxide fuel cell. a CO is fuel due to water–gas shift reaction and CH4 reacts with H2 O to form H2 and CO faster than reacting as a fuel at the electrode. b A fuel in the internal reforming MCFC and SOFC. c CO2 is a poison for AFC that more or less rules out its use with reformed fuels.

can be carried out by mesoporous adsorbents which are prepared using SCFs (Section 7.4). SCFs can also play an important role in development of solutions to the H2 storage problem (Section 7.5). 7.1. Preparation of membranes for hydrogen purification using SCFs Development of new membranes are important for fuel processing for FCs. Catalytic membrane reactors could be employed for steam reforming to shift the H2 production reaction to the right, thus increasing hydrogen conversion. Membrane separation can also be used anywhere between the reformer and the FC in Fig. 23. High-purity hydrogen could be obtained by dense metallic membranes and especially made of Pd and its alloys [126]. However, due to the poor mechanical strength and low H2 flux, there is a need to develop supported Pd membranes. By using SCD, it is also possible to deposit metallic films such as Pt, Pd, Ru, Ni, Cu, Ag and Au [42]. In SCD, metallic films are obtained by the conversion of metal precursors in SCF in the presence of H2 to ensure steady state transfer of precursor molecules to the surface for film growth and for the removal of reaction products. When H2 is injected to the metal precursor-scCO2 solution, the conversion of precursor molecules occur at the surface of the substrate by the reaction of precursor molecules with H2 [127]. The decomposition of precursor

molecules leads to nucleation and growth to create nanoparticles and these particles further grow to form films until the H2 in the fluid phase or the precursor molecules are consumed in batch operation [42] or as long as reactants are provided for continuous operation [128]. A few studies present in the literature indicate that by manipulating the concentration of the precursor in the SCF, the adsorption phenomenon, the conversion agent (such as H2 ) and the conditions (pressure, temperature, time), one can control the film properties [75,76]. Watkins’ group deposited dense and continuous Pd films within porous alumina substrates using SCD [128,129]. Two metal precursors were used, CpPd(␲–C4 H7 ) (␲-2methylallyl(cyclopentadienyl)palladium(II)) and Pd(hfac)2 in a continuous reactor where H2 /CO2 mixture and precursor/CO2 mixture were fed to the reactor from the opposite sides of the porous alumina substrate that was placed in the middle of the reactor. The reactor pressure was controlled by a back-pressure regulator at 14 MPa and the reaction temperature was 60 ◦ C. The membrane depths, as measured by optical microscopy of the cross-section, ranged from about 80 to 600 ␮m beneath the precursor-side surface. Control of the film depth was achieved by manipulating either the thickness of the barrier or the relative concentration of reactants (H2 and the palladium precursor) or reaction time (∼120–220 min).

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Fig. 24. TEM micrographs of Pd-doped CMS membrane prepared by carbonation of Pd-doped SY film synthesized using supercritical deposition. Reprinted with permission from [130]. Copyright 2011 American Chemical Society.

Yoda et al. [130] synthesized Pt- and Pd-nanoparticle dispersed polyimide films as precursors for metal-doped carbon molecular sieve (CMS) membranes for H2 separation. Impregnation of the films with Pt(acac)2 and Pd(acac)2 dissolved in scCO2 followed by high temperature thermal treatment yielded Pt and Pd nanoparticles (diameters 12 and 5 nm, respectively) that were highly dispersed inside polyimide films. The CMS membrane was fabricated by pyrolysis of metal impregnated polyimide films between graphite blocks at mainly 873–1273 K for 2 h with a heating rate of 10 K/min under a vacuum of 10−5 Torr. Fig. 24 shows the TEM images of Pd-doped CMS membrane prepared by calcination of Pd-doped SY film at 1273 K. The SY films also synthesized by the authors were derived from the casting of poly(amide acid) dissolved in N,N-dimethylacetamide followed by thermal imidization. Pd(acac)2 was impregnated into the SY precursor at 473 K, 19.6 MPa for 9 h. It can be observed that the Pd particles are dispersed all over the membrane and the majority of Pd particles are smaller than 5 nm in diameter. By gas permeability measurements, they showed that the permeabilities of both nitrogen and hydrogen decreased on Pd-doped CMS samples, but the decrease in nitrogen was more pronounced than hydrogen. Also, the H2 selectivity of the synthesized membranes was 17 times higher than that of neat CMS membrane. Recently, a novel hybrid method was proposed to prepare a Pd/␥-alumina composite membrane by electroless plating using an emulsion of scCO2 [131]. The technique consisted of exposing the electroless solution of PdCl2 , sodium formate (HCOONa) and ethylenediamine (NH2 CH2 CH2 NH2 ) to CO2 at 15 MPa and 80 ◦ C with a fluorinated surfactant (F(CF(CH3 )CF2 O)3 CF(CF3 )COO(CH2 CH2 O)CH3 ) for periods ranging from 1 to 7 h. During this period, an emulsion of CO2 , the plating solution and surfactant was formed. Prepared Pd membrane contained continuous Pd microparticles and showed average H2 /N2 selectivity (∼200). 7.2. Hydrogen production by reforming and gasification in supercritical and near-critical water Hydrogen is the most widely used fuel for FCs. Unfortunately, a number of disadvantages are encountered when H2 is produced through the conventional production methods in fuel processors for mobile applications as well as in large-scale production. When H2 is produced via steam reforming of hydrocarbons, tar or char formation is inevitable due to the fact that the raw material does not directly react with steam at low pressures. As an emerging alternative to the conventional processes, supercritical water reforming

or gasification (SCWR or SCWG) has been under investigation for a decade [132]. The popularity of scH2 O in this area cannot be unheeded since the use of scH2 O in H2 production via reforming or gasification brings an important number of advantages which include: • The higher density of scH2 O as compared to that of the steam, which results in a high space–time yield. • Particularly advantageous thermodynamic and physicochemical properties [133]. • Since conventionally, H2 production is also carried out at relatively high pressures. • At reaction conditions, the complete solubility of the hydrocarbons in scH2 O helps reduce the formation of char or slag and thus, the catalyst deactivation is prevented (particularly important in the area of H2 production from diesel) [134]. Catalytic gasification of biomass in scH2 O results also in negligible tar/char formation. • No equilibrium limitations due to the insolubility of hydrogen, which maintains hydrogen concentration in the reaction mixture below the chemical equilibrium of the reaction [135]. • As a result of the low dielectric constant of scH2 O and also the existence of weaker hydrogen bonds, scH2 O behaves like an organic solvent and exhibits very good solubility towards organic compounds containing large nonpolar groups and most permanent gases. • Drying of the wet biomass is not necessary. Costs associated with the water evaporation and energy consumption are eliminated. • A low CO content in the gas phase is obtained in the case of alkali salt-containing feedstock. • In SCWG, water acts as solvent, catalyst or catalyst precursor and reactant all in at once [136,137]. For H2 production, renewable and non-renewable feedstocks can be used. Renewable sources consist of biomass whereas nonrenewable H2 sources can be named as natural gas, gasoline, diesel, methanol, and ammonia produced from fossil reserves [5,134]. Among non-renewable feedstocks, methanol can be considered as a good option for producing H2 since the methanol has a high hydrogen-to-carbon ratio, which favors steam reforming energetically and does not possess C–C bonds. Furthermore, the absence of C–C bonds decreases the soot formation, and since the methanol is in liquid state, it is easy to store, pump and use as a fuel [133]. Ethanol is also an attractive option for hydrogen production, because it is less toxic than methanol and can be produced renewably from biomass with little net addition of carbon dioxide

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to the atmosphere. A particular advantage of using ethanol is that the efficiency triples to 60% from 20% when used to produce H2 for a FC application instead of burning in the combustion engines of automobiles [138,139]. The need to find new and renewable H2 sources comes from environmental and economic exigencies. Biomass being renewable and environmentally abundant is considered as such a source. Studies on biomass gasification of carbohydrates (glucose [140], cellulose [141]) as well as the lignins and glycerol are available in the literature [142,143]. There exist several papers that have reviewed biomass gasification via supercritical route [85,86,94]. Via scH2 O gasification, H2 can be produced from biomass with moisture content above 35%. In addition, the reaction is operated at relatively low temperatures compared to conventional processes. The reaction is usually carried out at temperatures near 600 ◦ C. Resulting gas mixtures are rich in hydrogen (usually >70%). CO2 is also a coproduct, which is produced by the water–gas shift reaction. It is worthwhile to mention again that the CO content of the product stream is very low. Understanding the mechanisms of the reforming or gasification reactions is crucial for further development of the technology and scale-up. These reactions occur in several steps and are discussed elsewhere [144]. The studies on thermodynamics of H2 production by SCWG are equally important and give considerable information on the influence of process variables (pressure, temperature and feed to reactants ratio) [145–148]. Although there are studies conducted in the presence of catalysts, there are also a number of studies in which catalysts were not used. However, in these studies, it is not easy to distinguish whether the reaction proceeds catalytically or not because of the probable catalytic effect of nickel or chrome based ferritic reactor walls (even though the surface area is minimal) [136]. Taylor et al. [149] investigated the hydrogen production from methanol in a compact supercritical water reformer at 550–700 ◦ C and 27.6 MPa in a tubular Inconel 625 reactor. The product stream was rich in H2 , low in CH4 , and near the equilibrium ratio of CO and CO2 . While the product gas composition was not better than that of competitive reforming technologies, they suggested that there are several advantages to use SCWR which include the simplicity resulting from the use of reactor walls as catalyst and also the compactness of the SCWR process. The study by Boukis et al. [144] showed the importance of the pre-oxidation process which leads to a higher conversion of methanol to hydrogen also in an Inconel 625 reactor, where the wall was found to act as a catalyst. They also achieved very high conversion rates, while the hydrogen concentration in the product gas reached the theoretical limit of 75 vol.%. They showed that the residence time required for complete conversion can be as low as 4 s at a reactor temperature of 600 ◦ C. They also carried out experiments in a pilot plant at a flow rate of 100 kg/h [150]. By using an Inconel 600 reactor, Gadhe and Gupta produced hydrogen via SCWGR at 276 bar and 700 ◦ C and investigated the suppression of methane [133]. Both their experimental results and equilibrium calculations showed that as the pressure increased, methanation of CO and CO2 takes place, causing a loss of H2 . They used the following strategies in suppressing methane formation: (i) operation at a low residence time by using a short reactor or a high feed flow rate; (ii) addition of a small amount of K2 CO3 or KOH to the feed; (iii) utilization of the surface catalytic activity of the reactor made of Ni–Cu alloy. These strategies resulted in improved hydrogen production while the formation of methane was suppressed. They also produced hydrogen using in situ-generated copper nanoparticles from cupric acetate as catalyst. Cupric acetate was fed to the reactor along with methanol [151]. DiLeo and Savage investigated the effect of nickel metal on H2 production in scH2 O by methanol gasification at relatively low temperatures compared to the studies mentioned above [152].

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They demonstrated that methanol can be gasified in supercritical water without a heterogeneous catalyst. However, the reaction was slow at 550 ◦ C (k = 4.0 × 10−5 s−1 ) compared to the reaction with Ni catalyst (k = 0.0040 s−1 ). They obtained lower conversion rates for methanol than those in the literature when they used catalysts [133,149]. These results were attributed to the lower operating temperatures than those used by the other studies. Byrd et al. [138] reformed ethanol in order to produce H2 via SCWR using a Ru/Al2 O3 catalyst. They controlled the methane formation by operating at an optimal residence time, high reactor temperature, a low feed concentration of ethanol and a low residence time (4 s). They pointed out that the hydrogen yield increased with increasing temperature and was unaffected by pressure changes in the supercritical region. Furthermore, carbon formation was negligible at ethanol concentrations below 10 wt.%. In subsequent studies by the same group [142,143], the production of H2 from glucose and glycerol was investigated. They obtained significant enhancement in conversion of glucose to H2 by using a 5 wt.% of Ru/Al2 O3 catalyst. Furthermore, coke and other heavier liquid products (tar) were reduced. However, the hydrogen yield decreased as the concentration of glucose in the feed increased beyond 5 wt.% which can be attributed to the formation of heavier molecular weight hydrocarbons and coke, which caused deactivation of the catalyst. Biomass consists of carbohydrates, lignin, salts and other minerals, and sometimes also proteins. In order to simulate the gasification of real biomass in scH2 O, a number of authors used model biomass mixtures [136,137]. These studies showed that the main gas product is CO2 in subcritical and H2 in supercritical conditions. Additionally, CO formation was very low in all conditions primarily due to the high concentration of H2 O. Additionally, there are reports on SCWG in the presence of a carbon catalyst [153] which resulted in the formation of H2 -rich gas. It has also been shown that by catalyzing the water–gas shift reaction, alkali salts in the biomass increase the hydrogen yield and decrease the CO yield. Unfortunately, these salts brings some problems such as plugging [137]. Xu et al. [154] showed that plugging can be prevented by using Na2 CO3 as a catalyst in gasification of glycine and glycerol as model compounds for proteins and fats. Metal catalysts were also shown to induce good hydrogen yields at relatively low temperatures. An excellent review on the catalytic SCWG was published [155]. According to the literature, nickel is the catalyst which provides the best H2 yields along with ruthenium and rhodium [156]. Recently, Resende and Savage [156] investigated the activities of different metal catalysts for SCWG. They showed that nickel and copper catalyzed the SCWG fairly well. Additionally, with all the metals tested (Ni, Fe, Cu, Zn, Zr, and Ru) under SCWG conditions, in the absence of biomass, H2 production was observed from scH2 O except for the experiments with Cu. Yoshida et al. [157] investigated the gasification of cellulose, lignin and mixtures of cellulose and lignin in scH2 O in the presence of a nickel catalyst. They indicated that if sufficient amount of catalyst is used, high gasification rates can be obtained despite the deactivation of the catalyst. They also investigated H2 production from real biomass such as sawdust and rice straw. They indicated that H2 production was lower using real biomass as compared to the calculations carried out using model biomass. Studies on the SCWG of agricultural and leather wastes by Yanik et al. [132] indicated that organic materials other than cellulose, hemicellulose and lignin of which the biomass is constituted may also affect the yield and composition of gas products. So far, the only study on Ni-based catalysts that were synthesized and tested under supercritical conditions was carried out by Taylor et al. [158]. Although the 1-D carbon supported Ni nanoparticles synthesized via supercritical methanol deposition did not have a narrow particle size distribution, the catalysts showed comparable activity to the catalysts used in the literature for SCWG of cellulose.

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The disadvantages of supercritical gasification converge mainly in the technical and engineering challenges. Particularly, for the biomass with a solid content over 30%, pumping can be a considerable challenge and a number of pretreatments such as liquefaction biochemical treatment or size reduction need to be applied before feeding. Additionally, corrosion could be a problem in H2 production using scH2 O at the temperatures and pressures employed. However, the corrosion encountered in SCWG is less severe than that of the supercritical water oxidation (SCWO) and Kruse [136] suggested that since it was possible to solve the corrosion problem in SCWO, it would also be probable to prevent it in SCWG. We refer the reader to the recent reviews written by Kruse on supercritical gasification [136,137] and to Kalinci et al. [159] for a comprehensive review and comparison of different biomass based H2 production systems. 7.3. Preparation of catalysts for fuel processing using SCFs We have already mentioned that H2 production reactions can be carried out in the presence of catalysts. Supercritical drying was used in preparation of cobalt–silica (Co–SiO2 ) aerogel coatings which were successfully grown on the walls of monolith ceramic channels, thus resulting in structured catalytic wall materials which were tested for H2 production by steam reforming of ethanol [160]. The performance of Co–SiO2 aerogel-coated monoliths was outstanding in terms of catalytic activity at low temperature (580–590 K), resistance to oxidation (563–613 K) and stability when compared to conventional catalytic monoliths coated with Co–SiO2 xerogel obtained without supercritical drying. In order to increase the efficiency of the previously mentioned PROX reaction, Kwak et al. [161] prepared alumina aerogel supported Pt-Co catalysts. The gels were dried by different methods which were oven drying, supercritical drying, and vacuum drying. Pt and Co were deposited by two different approaches; single step sol–gel and impregnation using an aqueous solution of hexa chloroplatinic acid and cobalt nitrate. They found that the catalyst obtained by supercritical drying was superior due to the preservation of the active species of the Pt-Co phase in supercritical drying. Haji et al. [162] evaluated Pt/Al2 O3 catalysts prepared by SCD method for the HDS of dibenzothiophene (DBT) at atmospheric pressure. They showed that unlike Mo, Ni or Ru-based catalysts, Pt/Al2 O3 catalysts are active without sulfiding the metal phase. They also recently demonstrated the feasibility of HDS of commercial diesel using Pt/Al2 O3 prepared by SCD [163]. Recently, various HDS catalysts such as mesoporous hexagonal structured alumina supported NiMo and CoMo were prepared via SCD [164,165]. These catalysts showed higher catalytic activity towards HDS compared to commercial ones and considerable selectivity towards cyclohexylbenzene over biphenyl in the HDS of DBT. 7.4. Gas clean-up using mesoporous adsorbents prepared using SCFs The amount of sulfur containing compounds such as thiophenes, benzothiophenes, and dibenzothiophenes in the hydrocarbon fuels can cause important product yield reductions. For example, the presence of 100 ppm of thiophene in a simulated gasoline mixture caused the hydrogen content of the effluent from the fuel processing unit to drop from 60 to 40 vol.% over a 25 h reaction time. Thus, hydrocarbon fuels need to be desulfurized to concentrations less than 0.1 ppm sulfur which necessitates incorporation of desulfurization technologies into the fuel processor [4]. Some specifications associated with different FC types were given in Table 3. It is possible by using mesoporous adsorbents to reduce the sulfur concentration of a hydrocarbon fuel [166,167]. These adsorbents can be prepared by SCF based routes.

Haji and Erkey [168] reported on removal of DBT from model diesel (n-hexadecane) by adsorption on carbon aerogels for FC applications. CA was prepared by the procedure described in Section 5.1. They showed that DBT could be removed using CAs with average pore sizes 4 and 22 nm prepared using two different formulations. The 22 nm CA adsorbed 90% of the total sulfur in 1.6 h. Furthermore, when both DBT and naphthalene were present in the solution, CAs showed selectivity in adsorption for DBT. Jayne et al. investigated the thermodynamics and dynamics of removal of organosulfur compounds from model diesel [169]. They developed a theoretical model to describe the dynamics of adsorption of both DBT and 4,6-dimethyldibenzothiophene (4,6-DMDBT) on CAs from model diesel and it was found to represent the experimental data fairly well. The model takes into account pore diffusion and assumes local equilibrium within the pores. The tortuosities, being the adjustable parameter in the model, were found to be 3.5 and 4.0 for 22 and 4 nm pore diameter CA, respectively. 7.5. Use of SCFs in development of materials for hydrogen storage As the amount of hydrogen consumption increases in the future, its storage will become a critical issue. In order to utilize H2 as a fuel for a typical car, the needed volume of hydrogen at ambient pressure and temperature is approximately 60 m3 which implies that this volume must be decreased to reasonable sizes [170]. This can be accomplished either by compressing hydrogen to very high pressures in specialized containers or using metal hydrides and various adsorbents as hydrogen storage media. The DOE’s target for H2 storage is that the volume per kg of H2 should not exceed 16 L and H2 content should be 6.5 wt.% or more [54]. Various materials such as metal organic frameworks (MOFs) [171], microporous hypercrosslinked polymers [172], superactivated carbons [170] and carbon nanotubes [173,174] are under investigation as promising materials with high hydrogen storage capacity. Among these materials, highly porous MOFs have become very popular in the H2 community due to their excellent H2 storage capacity. Recently, scCO2 drying of solvo-thermally synthesized MOFs [175] was carried out in a manner close to the supercritical drying of aerogels (as previously described in Section 5.3) in order to be able to preserve the pore structure and prevent the collapsing of the pores due to capillary forces. The supercritically dried MOFs had drastically higher surface areas (up to 1200%) leading to paramount H2 storage capacities as compared to MOFs dried conventionally by thermal evacuation of the solvent [176]. Here we note that hydrogen is in supercritical state at the condition of storage at room temperature and the adsorption mechanism is quite different than the bulk liquid. According to the literature, it seems that the monolayer surface coverage is the only possible adsorption mechanism in microporous adsorbents [177] considering that in the supercritical phase it is not possible to liquefy a compound by increasing the pressure [170]. Magnesium is another promising candidate as it can store up to 7.6 wt.% H2 [54]. However, the slow kinetics of hydrogenation reaction at high temperatures is a problem. Additions of various compounds including 3d metals, intermetallics and oxides along with mechanical grinding options have been tried to overcome this issue [178]. Bobet et al. published a series of articles on Mg based mixtures (MgCr2 O3 , MgNi, MgPd) in order to accelerate the hydrogenation of the magnesium by incorporating Cr2 O3 , Pd and Ni by SCD using Cr(acac)3 , Pd(hfac)2 and Ni(hfac)2 [178,179]. In the studies for MgCr2 O3 synthesis, NH3 was used as the supercritical deposition medium at 20 MPa and 200 ◦ C whereas for preparation of MgPd and MgNi mixtures, CO2 /methanol mixture (80/20 M) was used as solvent at 20 MPa with temperatures ranging from 100 to 250 ◦ C. Promising results were obtained for MgCr2 O3 mixture in sorption and desorption of hydrogen. Pd particles with sizes smaller

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than 1 ␮m and particles of Ni with an average size of 900 nm were obtained [180]. The results for composites prepared by SCD were as good as the samples prepared via ball milling in terms of hydrogen sorption properties and even better in the cyclability of hydrogen. In a recent study by the same authors, the Mg particles were decorated with nanosized Cu, Ni and Pd particles using SCD which was carried out in a CO2 /isopropanol (95/5 M) medium [181]. Firstly, the metal precursors (hexafluoroacetylacetonate complexes of the aforementioned metals) were dissolved in a suspension of Mg crystals in isopropanol. After loading the solution to a high-pressure vessel, H2 was introduced at 2 MPa which was followed by the injection of CO2 that raised the pressure to 9 MPa. The temperature was then raised to 130 ◦ C which resulted in an increase of pressure to 25 MPa. The samples were obtained after the depressurization of the vessel. For the experiments with Ni, Pd(hfac)2 precursor was also loaded in order to catalyze the reduction of Ni precursor. By this method, they were able to improve the H2 capacity of magnesium in the range of 200–300 ◦ C for sorption and 260–330 ◦ C for desorption. Furthermore, the same research group demonstrated for the first time [112] the presence of the metal–metal substrate interphases for the Mg supported Cu and Ni nanoparticles with the identification of MgCu2 and Mg2 Ni alloys. These kind of fundamental studies are important for design and development of better H2 storage materials. Another promising way to generate H2 is the reaction of water with alkaline or alkaline earth metal hydrides. NaBH4 stands out among other hydrides with 0.213 g of H2 per 1 g of the compound. A number of catalysts including metal halides, Pt, Ru, Ni and active carbon are known to accelerate the reaction of NaBH4 with water [182]. SCD in the presence of a co-solvent (acetone) was used in the preparation of metal–metal oxide (TiO2 suppported Fe, Ni, Pd, Ru, Rh, Pt) catalysts. The acetylacetonate complex of the desired metal was dissolved in acetone and then the solution was placed in a high-pressure vessel along with 30 g of dry ice and TiO2 powder. The temperature of the vessel was then increased to 150 ◦ C which brought the pressure to 30 MPa. The deposition time was 2 h. This process was followed by the heating of metal precursor supported TiO2 powders to 105 ◦ C for 1 h to end up with TiO2 supported metal nanoparticles. The catalysts accelerated the hydrogen generation from the aforementioned reaction of NaBH4 . Pt-TiO2 catalyst had the highest hydrogen generation rate among the catalysts prepared by SCD [182]. 7.6. Future research needs The SCWG reaction has mainly been studied using model mixtures for biomass. However, the composition of biomass feeds can be very complex and further studies are needed in real systems. Eventhough H2 production using scH2 O seems promising in laboratory and pilot scale, engineering challenges remain for operation on an industrial scale. Further technical studies are needed in pumping of the feedstock and the influence of the water content to the pumping. Sometimes even low concentrations of char or utilized salts can induce plugging of reactor systems. Reforming of biogas using scH2 O can also be a fruitful route for FC grade H2 production. The recent work of Sato et al. [183] showed that such process can be carried out in a Pd membrane reactor. These prospected needs must be realized and aforementioned challenges must be overcome in order to incorporate the technology for stationary or mobile FCs. Even though the hydrogen economy is years away from commercialization, on-demand reformation of hydrogen from military logistics jet fuel (JP-8), coupled with a FC, would enable armed forces personnel to produce electricity in the field with very little noise or heat signature compared to internal combustion electric generators [184]. Previously gained knowledge on preparation of supported catalysts using SCD would be beneficial in development of new catalysts for

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SCWG. To our knowledge, there is only one report of such catalyst prepared in SCF environment. CAs or MOFs dried using scCO2 may be applicable and advantageous as adsorbents for separation and purification problems currently encountered in SCWG. The challenges associated with the purification of H2 can be overcome by improving H2 permeability, selectivity and stability of the Pd based membranes prepared by SCF based routes. H2 storage capacities must be improved by development of new adsorbent materials. Preparation of multicomponent catalysts can help accelerate the H2 sorption and desorption properties of magnesium. 8. Conclusions SCFs show promise for development of novel materials and processes which will aid in widespread acceptance of FCs. Using SCFs, metallic nanoparticles can be incorporated to carbonacaeous supports for use as electrocatalysts or to polymers for use as proton exchange membranes. In most cases, the electrocatalysts prepared by using SCFs had higher electrochemical activities than that of the commercial ones. The work by Bayrakceken et al. [86] demonstrated the superiority of Pt/CB prepared by SCD over commercial Pt/CB. However, the Pt cost needs to be reduced further and bimetallic Pt electrocatalysts will play a very important role in this matter. The results from a few studies on preparation of bimetallic catalysts using SCD are promising and indicate the need for more detailed investigations on other metal pairs. Development of cheap electrocatalysts, which do not contain Pt, is another area where SCFs can contribute. Methanol cross-over, an important challenge encountered in DMFCs, can be reduced by SCD of metals into Nafion® or by preparing alternatives or novel composites of Nafion® by using SCFs. The unique properties of SCFs should be beneficial in further development of new polymeric membrane architectures. The importance of the carbon support on electrocatalysts performance was underlined. However, the interaction of the Nafion® -electrocatalyst-support system is crucial and needs further investigation. SCFs may also play a role in development of conductive nanostructured electrode materials that are not based on carbon. Ionic liquids are very promising candidates as electrolytes in many applications including fuel cells. In the future, CO2 -expanded ionic liquids as FC electrolytes may also be a relevant research area. Preparation of inorganic materials through the utilization of SCFs can also lead to development of superior electrode materials, especially for SOFCs. However, to the best of our knowledge, there is only one study on application of SCFs to preparation of SOFC electrolyte materials. SHS can be fruitful for the development of new inorganic materials that can be used below 700 ◦ C and these materials can enable the reduction of the costs associated with high temperatures. Gasification of biomass or fossil feedstocks by supercritical water to produce hydrogen seems very promising. The use of SCFs in fuel processing for fuel cells is another promising area where very little work has been carried out so far. Materials prepared using SCFs such as catalysts, mesoporous adsorbents and membranes can solve some of the problems encountered in fuel processing and H2 storage. In the next decade, we believe that SCFs will thus play a critical role in FC research and development. Acknowledgement This work was supported by TUBITAK Grant 108M387. References [1] J.W. Tester, E.M. Drake, M.J. Driscoll, M.W. Golay, W.A. Peters, Sustainable Energy Choosing Among Options, The MIT Press, England, 2005.

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[2] E. Reverchon, R. Adami, Nanomaterials and supercritical fluids, Journal of Supercritical Fluids 37 (2006) 1. [3] G. Brunner, Applications of supercritical fluids, Annu. Rev. Chem. Biomol. Eng. 1 (2010) 321. [4] G. Hoogers, Fuel Cell Handbook, CRC Press, USA, 2003. [5] N. Sammes, Fuel Cell Technology Reaching Towards Commercialization, Springer, Germany, 2006. [6] B. Viswanathan, M.A. Scibioh, Fuel Cells Principles and Applications, CRC Press, India, 2007. [7] I. EG&G Technical Services, Fuel Cell Handbook, 7th ed., U.S. Department of Energy, Morgantown, West Virginia, 2004. [8] R. O’Hayre, S.-W. Cha, W. Colella, F.B. Prinz, Fuel Cell Fundamentals, John Wiley & Sons, USA, 2006. [9] E. Yeager, Electrocatalysts for O2 reduction, Electrochimica Acta 29 (1984) 1527. [10] P.S.D. Brito, C.A.C. Sequeira, Cathodic oxygen reduction on noble metal and carbon electrodes, J. Power Sources 52 (1994) 1. [11] J.K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard, H. Jónsson, Origin of the overpotential for oxygen reduction at a fuel-cell cathode, The Journal of Physical Chemistry B 108 (2004) 17886. [12] T.W. Randolph, Supercritical fluid extractions in biotechnology, Trends in Biotechnology 8 (1990) 78. [13] J. Chrastil, Solubility of solids and liquids in supercritical gases, Journal of Physical Chemistry 86 (1982) 3016. [14] O. Aschenbrenner, S. Kemper, N. Dahmen, K. Schaber, E. Dinjus, Solubility of [beta]-diketonates, cyclopentadienyls, and cyclooctadiene complexes with various metals in supercritical carbon dioxide, The Journal of Supercritical Fluids 41 (2007) 179. [15] J.J. Watkins, T.J. McCarthy, Polymer/metal nanocomposite synthesis in supercritical CO2 , Chemistry of Materials 7 (1995) 1991. [16] A.R. Berens, G.S. Huvard, Interaction of polymers with near-critical carbon dioxide, J. American Chemical Society 14 (1989) 207. [17] D.L. Tomasko, H. Li, D. Liu, X. Han, M.J. Wingert, L.J. Lee, K.W. Koelling, A review of CO2 applications in the processing of polymers, Industrial & Engineering Chemistry Research 42 (2003) 6431. [18] E.J. Beckman, Supercritical near-critical CO2 in green chemical synthesis and processing, Journal of Supercritical Fluids 28 (2004) 121. [19] S.G. Kazarian, N.H. Brantley, B.L. West, M.F. Vincent, C.A. Eckert, In situ spectroscopy of polymers subjected to supercritical CO2 : plasticization and dye impregnation, Applied Spectroscopy 51 (1999) 491. [20] J.v. Schnitzler, R. Eggers, Mass transfer in polymers in a supercritical CO2 atmosphere, Journal of Supercritical Fluids 16 (1999) 81. [21] J.L. Kendall, D.A. Canelas, J.L. Young, J.M. DeSimone, Polymerizations in supercritical carbon dioxide, Chemical Reviews 99 (1999) 543. [22] U.B. Kompella, K. Koushik, Preparation of drug delivery systems using supercritical fluid technology, Critical Reviews in Therapeutic Drug Carrier Systems 18 (2001) 173. [23] G.A. Montero, C.B. Smith, W.A. Hendrix, D.L. Butcher, Supercritical fluid technology in textile processing: an overview, Industrial & Engineering Chemistry Research 39 (2000) 4806. [24] K.G. Johns, G. Stead, Supercritical Fluids for Coatings – From Analysis to Xenon. A Brief Overview, Kluwer Academic/Plenum Publishers, New York, 1999. [25] A.I. Cooper, Recent developments in materials synthesis and processing using supercritical CO2 , Advanced Materials 13 (2001) 1111. [26] S. Camy, S. Montanari, A. Rattaz, M. Vignon, J.S. Condoret, Oxidation of cellulose in pressurized carbon dioxide, Journal of Supercritical Fluids 51 (2009) 188. [27] J. McHardy, S.P. Sawan, Supercritical Fluid Cleaning Fundamentals, Technology and Applications, Noyes, Westwood, NJ, 1998. [28] M.D. Palmieri, An introduction to supercritical fluid chromatography. Part 2. Applications and future trends, J. Chemical Education 66 (1989) A141. [29] G.L. Weibel, C.K. Ober, An overview of supercritical CO2 applications in microelectronics processing, Microelectronic Engineering 65 (2003) 145. [30] G. Snyder, A. Tadd, M.A. Abraham, Evaluation of catalyst support effects during rhodium-catalyzed hydroformylation in supercritical CO2 , Industrial & Engineering Chemistry Research 40 (2001) 5317. [31] B. Subramaniam, C.J. Lyon, V. Arunajatesan, Environmentally benign multiphase catalysis with dense phase carbon dioxide, Applied Catalysis B 37 (2002) 279. [32] C.S. Tan, D.C. Liou, Desorption of ethyl acetate from activated carbon by supercritical carbon dioxide, Industrial & Engineering Chemistry Research 27 (1988) 988. [33] C.S. Tan, D.C. Liou, Supercritical regeneration of activated carbon loaded with toluene and benzene, Industrial & Engineering Chemistry Research 28 (1989) 1222. [34] C.S. Tan, D.C. Liou, Regeneration of activated carbon loaded with toluene by supercritical carbon dioxide, Separation Science and Technology 24 (1989) 111. [35] C.S. Tan, D.C. Liou, Adsorption equilibrium of toluene from supercritical carbon dioxide on activated carbon, Industrial & Engineering Chemistry Research 29 (1990) 1412. [36] M. Sihvonen, E. Jarvenpaa, V. Hietaniemi, R. Huopalahti, Advances in supercritical carbon dioxide technologies, Trends in Food Science & Technology 10 (1999) 217.

[37] H.K. Kiriamiti, S. Camy, C. Gourdon, J.S. Condoret, Pyrethrin exraction from pyrethrum flowers using carbon dioxide, Journal of Supercritical Fluids 26 (2003) 193. [38] F. Cansell, B. Chevalier, A. Demourgues, J. Etourneau, C. Even, V. Pessey, S. Petit, A. Tressaud, F. Weill, Supercritical fluid processing: a new route for materials synthesis, Journal of Materials Chemistry 9 (1999) 67. [39] F. Cansell, C. Aymonier, A. Loppinet-Serani, Review on materials science and supercritical fluids, Current Opinion in Solid State & Materials Science 7 (2003) 331. [40] C. Aymonier, A. Loppinet-Serani, H. Reverón, Y. Garrabos, F. Cansell, Review of supercritical fluids in inorganic materials science, Journal of Supercritical Fluids 38 (2006) 242. [41] F. Cansell, C. Aymonier, Design of functional nanostructured materials using supercritical fluids, Journal of Supercritical Fluids 47 (2009) 508. [42] C. Erkey, Preparation of metallic supported nanoparticles and films using supercritical fluid deposition, Journal of Supercritical Fluids 47 (2009) 517. [43] N.M. Markovic, H.A. Gasteiger, P.N. Ross Jr., Oxygen reduction on platinum low-index single-crystal surfaces in sulfuric acid solution: rotating ringPt(hkZ) disk studies, The Journal of Physical Chemistry 99 (1995) 3411. [44] Z. Peng, H. Yang, Designer platinum nanoparticles: control of shape composition in alloy, nanostructure and electrocatalytic property, Nano Today 4 (2009) 143. [45] R. Ferrando, J. Jellinek, R.L. Johnston, Nanoalloys: from theory to applications of alloy clusters and nanoparticles, Chemical Reviews 108 (2008) 845. [46] www1.eere.energy.gov/hydrogenandfuelcells/pdfs/fctt pemfc cost review 0908.pdf. [47] D. Myers, Polymer Electrolyte Fuel Cell Electrocatalysis, Hydrogen and Fuel Cell Technical Advisory Committee Meeting Argonne National Laboratory, 2009. [48] Y. Zhang, C. Erkey, Preparation of platinum-Nafion®-carbon black nanocomposites via a supercritical fluid route as electrocatalysts for proton exchange membrane fuel cells, Industrial & Engineering Chemistry Research 44 (2005) 5312. [49] A.D. Taylor, M. Michel, R.C. Sekol, J.M. Kizuka, N.A. Kotov, L.T. Thompson, Fuel cell membrane electrode assemblies fabricated by layer-by-layer electrostatic self-assembly techniques, Advanced Functional Materials 18 (2008) 3003. [50] Y. Wang, N. Toshima, Preparation of Pd-Pt bimetallic colloids with controllable core/shell structures, Journal of Physical Chemistry B 101 (1997) 5301. [51] W. Vogel, P. Britz, H. Bo1nnemann, J. Rothe, J. Hormes, Structure and chemical composition of surfactant-stabilized PtRu alloy colloids, Journal of Physical Chemistry B 101 (1997) 11029. [52] J.R. Kitchin, J.K. Nørskov, M.A. Barteau, J.G. Chen, Role of strain and ligand effects in the modification of the electronic and chemical properties of bimetallic surfaces, Physical Review Letters 93 (2004) 156801. [53] O.A. Petrii, Pt–Ru electrocatalysts for fuel cells: a representative review, J Solid State Electrochemistry 12 (2008) 609. [54] I. Chorkendorff, J.W. Niemantsverdriet, Concepts of Modern Catalysis and Kinetics, 2nd ed., Wiley-VCH, Weinheim, 2010. [55] E. Antolini, Carbon supports for low-temperature fuel cell catalysts, Applied Catalysis B: Environmental 88 (2009) 1. [56] A.L. Dicks, The role of carbon in fuel cells, Journal of Power Sources 156 (2006) 128. [57] M. Uchida, Y. Fukuoka, Y. Sugawara, N. Eda, A. Ohta, Effects of microstructure of carbon support in the catalyst layer on the performance of polymer electrolyte fuel cells, Journal of the Electrochemical Society 143 (1996) 2245. [58] X. Wang, W.Z. Li, Z.W. Chen, M. Waje, Y.S. Yan, Durability investigation of carbon nanotube as catalyst support for proton exchange membrane fuel cell, Journal of Power Sources 158 (2006) 154. [59] V. Neburchilov, J. Martin, H. Wang, J. Zhang, A review of polymer electrolyte membranes for direct methanol fuel cells, Journal of Power Sources 169 (2007) 221. [60] F. Bauer, S. Denneler, M. Willert-Porada, Influence of temperature and humidity on the mechanical properties of Nafion117 polymer electrolyte membrane, Journal of Polymer Science: Part B: Polymer Physics 43 (2005) 78. [61] M.B. Satterfield, J.B. Benziger, Viscoelastic properties of nafion at elevated temperature and humidity, Journal of Polymer Science: Part B: Polymer Physics 47 (2009) 11. [62] K.-H. Kim, S.-Y. Ahn, I.-H. Oh, H.Y. Ha, S.-A. Hong, M.-S. Kim, Y. Lee, Y.C. Lee, Characteristics of the nafion® impregnated polycarbonate composite membranes for PEMFCs, Electrochimica Acta 50 (2004) 577. [63] M.-S. Kim, S.-J. Lee, J.-U. Kang, K. Jin, Preparations of polypropylene membrane with high porosity in supercritical CO2 and its application for PEMFC, Journal of Industrial & Engineering Chemistry 11 (2005) 187. [64] M.A.M. Rahmathullah, J.D. Snyder, Y.A. Elabd, G.R. Palmese, Nanoporous and proton conductive hydrophobic–hydrophilic copolymer thermoset membranes, Journal of Polymer Science: Part B: Polymer Physics 48 (2010) 1245. [65] R. Jiang, Y. Zhang, S. Swier, X. Wei, C. Erkey, H.R. Kunz, J.M. Fenton, Preparation of Pd-impregnated Nafion® membrane via a supercritical fluid route for direct methanol fuel cells, Electrochemical and Solid State Letters 8 (2005) A611. [66] D. Kim, J. Sauk, J. Byun, K.S. Lee, H. Kim, Palladium composite membranes using supercritical CO2 impregnation method for direct methanol fuel cells, Solid State Ionics 178 (2007) 865. [67] N.W. Deluca, Y.A. Elabd, Polymer electrolyte membranes for the direct methanol fuel cell: a review, Journal of Polymer Science: Part B: Polymer Physics 44 (2006) 2201.

S.E. Bozbag, C. Erkey / J. of Supercritical Fluids 62 (2012) 1–31 [68] R.B. Gupta, J.-J. Shim, Solubility in Supercritical Carbon Dioxide, CRC Press, Boca Raton, FL, 2007. [69] J. Sauk, J. Byun, H. Kim, Grafting of styrene on to Nafion® membranes using supercritical CO2 impregnation for direct methanol fuel cells, Journal of Power Sources 132 (2004) 59. [70] J. Sauk, J. Byun, Y. Kang, H. Kim, Preparation of laminated composite membranes by impregnation of polypropylene with styrene in supercritical CO2 for direct methanol fuel cells, Korean Journal of Chemical Engineering 22 (2005) 605. [71] J. Byun, J. Sauk, H. Kim, Preparation of PVdF/PSSA composite membranes using supercritical carbon dioxide for direct methanol fuel cells, International Journal of Hydrogen Energy 34 (2009) 6437. [72] L. Su, L. Li, H. Li, J. Tang, Y. Zhang, W. Yu, C. Zhou, Preparation of polysiloxane modified perfluorosulfonic acid composite membranes assisted by supercritical carbon dioxide for direct methanol fuel cell, Journal of Power Sources 194 (2009) 220. [73] V. Tricoli, F. Nannetti, Zeolite-Nafion composites as ion conducting membrane materials, Electrochimica Acta 48 (2003) 2625. [74] E.N. Gribov, E.V. Parkhomchuk, I.M. Krivobokov, J.A. Darr, A.G. Okunev, Supercritical CO2 assisted synthesis of highly selective Nafion® zeolite nanocomposite membranes for direct methanol fuel cells, Journal of Membrane Science 297 (2007) 1. [75] L. Su, L. Li, H. Li, Y. Zhang, W. Yu, C. Zhou, Perfluorosulfonic acid membranes treated by supercritical carbon dioxide method for direct methanol fuel cell application, Journal of Membrane Science 335 (2009) 118. [76] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and properties of nanocrystals of different shapes, Chemical Review 105 (2005) 1025–1102. [77] Y. Zhang, C. Erkey, Preparation of supported metallic nanoparticles using supercritical fluids: a review, Journal of Supercritical Fluids 38 (2006) 252. [78] Y. Lin, X.C. Yen, C.M. Wai, Platinum/carbon nanotube nanocomposite synthesized in supercritical fluid as electrocatalysts for low-temperature fuel cells, Journal of Physical Chemistry B 109 (2005) 14410. [79] M. Watanabe, Y.M. Zhu, H. Igarashi, H. Uchida, Mechanism of CO tolerance at Pt-alloy anode catalysts for polymer electrolyte fuel cells, Electrochemistry Communications 68 (2000) 244. [80] Y. Lin, X. Cui, X. Ye, Electrocatalytic reactivity for oxygen reduction of palladium modified carbon nanotubes synthesized in supercritical fluid, Electrochemistry Communications 7 (2005) 267. [81] C.D. Saquing, T.-T. Cheng, M. Aindow, C. Erkey, Preparation of platinum/carbon aerogel nanocomposites using a supercritical deposition method, Journal of Physical Chemistry B 108 (2004) 7716. [82] A. Smirnova, X. Dong, H. Hara, A. Vasiliev, N. Sammes, Novel carbon aerogel supported catalysts for PEM fuel cell application, International Journal of Hydrogen Energy 30 (2005) 149. [83] A. Bayrakceken, U. Kitkamthorn, M. Aindow, C. Erkey, Decoration of multiwall carbon nanotubes with platinum nanoparticles using supercritical deposition with thermodynamic control of metal loading, Scripta Materialia 56 (2007) 101. [84] M. Hiramatsu, T. Machino, K. Mase, M. Hori, H. Kano, Preparation of platinum nanoparticles on carbon nanostructures using metal-organic chemical fluid deposition employing supercritical carbon dioxide, Journal of Nanoscience and Nanotechnology 10 (2010) 4023–4029. [85] S.E. Bozbag, N.S. Yasar, L.C. Zhang, M. Aindow, C. Erkey, Adsorption of Pt(cod)me2 onto organic aerogels from supercritical solutions for the synthesis of supported platinum nanoparticles, Journal of Supercritical Fluids 56 (2011) 105. [86] A. Bayrakceken, A. Smirnova, U. Kitkamthorn, M. Aindow, L. Turker, I. Eroglu, C. Erkey, Pt-based electrocatalysts for polymer electrolyte membrane fuel cells prepared by supercritical deposition technique, Journal of Power Sources 179 (2008) 532. [87] K. Shimizu, I.F. Cheng, J.S. Wang, C.H. Yen, B. Yoon, C.M. Wai, Waterin-supercritical CO2 microemulsion for synthesis of carbon- nanotubesupported Pt electrocatalyst for the oxygen reduction reaction, Energy & Fuels 22 (2008) 2543. [88] S.-Y. Ang, D.A. Walsh, Highly stable platinum electrocatalysts for oxygen reduction formed using supercritical fluid impregnation, Journal of Power Sources 22 (2010) 557. [89] A.D. Taylor, R.C. Sekol, J.M. Kizuka, S. D’Cunha, C.M. Comisar, Fuel cell performance and characterization of 1D carbon supported platinum nanocomposites synthesized in supercritical fluids, Journal of Catalysis 259 (2008) 5. [90] C.H. Yen, K. Shimizu, Y.-Y. Lin, F. Bailey, I.F. Cheng, C.M. Wai, Chemical fluid deposition of Pt-based bimetallic nanoparticles on multiwalled carbon nanotubes for direct methanol fuel cell application, Energy & Fuels 21 (2007) 2268. [91] Y. Lin, X.C.H. Yen, C.M. Wai, PtRu/Carbon nanotube nanocomposite synthesized in supercritical fluid: a novel electrocatalyst for direct methanol fuel cells, Langmuir 21 (2005) 11474. [92] A. Bayrakceken, B. Cangül, L.C. Zhang, M. Aindow, C. Erkey, PtPd/BP2000 electrocatalysts prepared by sequential supercritical carbon dioxide deposition, International Journal of Hydrogen Energy 35 (2010) 11669. [93] C.H. Yen, K. Shimizu, Y.Y. Lin, F. Bailey, I.F. Cheng, C.M. Wai, Chemical fluid deposition of pt-based bimetallic nanoparticles on multiwalled carbon nanotubes for direct methanol fuel cell application, Energy & Fuels 21 (2007) 2268. [94] B. Cangul, L.C. Zhang, M. Aindow, C. Erkey, Preparation of carbon black supported Pd, Pt and Pd-Pt nanoparticles using supercritical CO2 deposition, Journal of Supercritical Fluids 50 (2009) 82.

29

[95] G. An, P. Yu, L. Mao, Z. Sun, Z. Liu, S. Miao, Z. Miao, K. Ding, Synthesis of PtRu/carbon nanotube composites in supercritical fluid and their application as an electrocatalyst for direct methanol fuel cells, Carbon 45 (2007) 536. [96] H. Wakayama, T. Hatanaka, Y. Fukushima, Synthesis of Pt-Ru nanoporous fibers by the nanoscale casting process using supercritical CO2 for electrocatalytic applications, Chemistry Letters 33 (2004) 658. [97] H. Wakayama, Y. Fukushima, Supercritical CO2 for making nanoscale materials, Industrial & Engineering Chemistry Research 45 (2006) 3328. [98] R.W. Pekala, Organic aerogels from polycondensation of resorcinol with formaldehyde, Journal of Materials Science 24 (1989) 3221. [99] C. Moreno-Castilla, F.J. Maldonado-Hodar, Carbon aerogels for catalysis applications: an overview, Carbon 43 (2005) 455. [100] E. Guilminot, F. Fischer, M. Chatenet, A. Rigacci, S. Berthon-Fabry, P. Achard, E. Chainet, Use of cellulose-based carbon aerogels as catalyst support for PEM fuel cell electrodes: electrochemical characterization, Journal of Power Sources 166 (2007) 104. [101] H.-J. Kim, W.-I. Kim, T.-J. Park, H.-S. Park, D.J. Suh, Highly dispersed platinum carbon aerogel catalyst for polymer electrolyte membrane fuel cells, Carbon 46 (2008) 1393. [102] Z.S. Lou, C.L. Chen, Q.W. Chen, J. Gao, Formation of variously shaped carbon nanotubes in carbon dioxide–alkali metal (Li, Na) system, Carbon 43 (2005) 1103. [103] D.K. Smith, D.C. Lee, B.A. Korgel, High yield multiwall carbon nanotube synthesis in supercritical fluids, Chemistry of Materials 18 (2006) 3356. [104] Z.C. Li, J. Andzane, D. Erts, J.M. Tobin, K. Wang, M.A. Morris, G. Attard, J.D. Holmes, A supercritical-fluid method for growing carbon nanotubes, Advanced Materials 19 (2007) 3043. [105] R.L. Arechederra, K. Artyushkova, P. Atanassov, S.D. Minteer, Growth of phthalocyanine doped and undoped nanotubes using mild synthesis conditions for development of novel oxygen reduction catalysts, Acs Applied Materials & Interfaces 2 (2010) 3295–3302. [106] M.L. Anderson, R.M. Stroud, D.R. Rolison, Enhancing the activity of fuelcell reactions by designing three-dimensional nanostructured architectures: catalyst-modified carbon-silica composite aerogels, Nano Letters 2 (2002) 235. [107] Y. Tang, A.M. Karlsson, M.H. Santare, M. Gilbert, S. Cleghorn, W.B. Johnson, An experimental investigation of humidity and temperature effects on the mechanical properties of perfluorosulfonic acid membrane, Materials Science and Engineering: A 425 (2006) 297. [108] P.W. Majsztrik, A.B. Bocarsly, J.B. Benziger, An instrument for environmental control of vapor pressure and temperature for tensile creep and other mechanical property measurements, Review of Scientific Instruments 78 (2007) 103904. [109] V.R. Stamenkovic, B. Fowler, B.S. Mun, G. Wang, P.N. Ross, C.A. Lucas, N.M. Markovic, Improved oxygen reduction activity on Pt3 Ni(111) via increased surface site availability, Science 315 (2007) 493. [110] S.R. Puniredd, S. Weiyi, M.P. Srinivasan, Pd-Pt and Fe-Ni nanoparticles formed by covalent molecular assembly in supercritical carbon dioxide, Journal of Colloid and Interface Science 320 (2008) 333. [111] M. Alibouri, S.M. Ghoreishi, H.R. Aghabozorg, Effect of supercritical deposition synthesis on dibenzothiophene hydrodesulfurization over NiMo/Al2 O3 nanocatalyst, Aiche Journal 55 (2009) 2665. [112] C. Aymonier, A. Denis, Y. Roig, M. Iturbe, E. Sellier, S. Marre, F. Cansell, J.L. Bobet, Supported metal NPs on magnesium using SCFs for hydrogen storage: interface and interphase characterization, Journal of Supercritical Fluids 53 (2010) 102. [113] R. Petricevic, M. Glora, J. Fricke, Planar fibre reinforced carbon aerogels for application in PEM fuel cells, Carbon 39 (2001) 857. [114] C. Schmitt, H. Pröbstle, J. Fricke, Carbon cloth-reinforced and activated aerogel films for supercapacitors, Journal of Non-Crystalline Solids 285 (2001) 277. [115] L.H. He, X.L. Zheng, Q. Xu, Modification of carbon nanotubes using poly(vinylidene fluoride) with assistance of supercritical carbon dioxide: the impact of solvent, Journal of Physical Chemistry B 114 (2010) 5257. [116] Y. Hakuta, H. Hayashi, K. Arai, Fine particle formation using supercritical fluids, Current Opinion in Solid State and Materials Science 7 (2003) 341. [117] T. Adschiri, K. Kanazawa, K. Arai, Rapid and continuous hydrothermal crystallization of metal oxide particles in supercritical water, Journal of the American Ceramic Society 75 (1992) 1019. [118] X. Weng, P. Boldrin, I. Abrahams, S.J. Skinner, S. Kellici, J.A. Darr, Direct syntheses of Lan + 1Nin O3n + 1 phases (n = 1, 2, 3 and [infinity]) from nanosized co-crystallites, Journal of Solid State Chemistry 181 (2008) 1123. [119] T. Adschiri, Y. Hakuta, K. Arai, Hydrothermal synthesis of metal oxide fine particles at supercritical conditions, Industrial & Engineering Chemistry Research 39 (2000) 4901. [120] Y. Hakuta, S. Onai, H. Terayama, T. Adschiri, K. Arai, Production of ultrafine ceria particles by hydrothermal synthesis under supercritical conditions, Journal of Materials Science Letters 17 (1998) 1211. [121] T. Adschiri, Y. Hakuta, K. Sue, K. Arai, Hydrothermal synthesis of metal oxide nanoparticles at supercritical conditions, Journal of Nanoparticle Research 3 (2001) 227–235. [122] J.-R. Kim, W.-J. Myeong, S.-K. Ihm, Characteristics in oxygen storage capacity of ceria–zirconia mixed oxides prepared by continuous hydrothermal synthesis in supercritical water, Applied Catalysis B: Environmental 71 (2007) 57.

30

S.E. Bozbag, C. Erkey / J. of Supercritical Fluids 62 (2012) 1–31

[123] A. Hertz, S. Sarrade, C. Guizard, A. Julbe, Synthesis and encapsulation of yttria stabilized zirconia particles in supercritical carbon dioxide, Journal of the European Ceramic Society 26 (2006) 1195. [124] X. Weng, D. Brett, V. Yufit, P. Shearing, N. Brandon, M. Reece, H. Yan, C. Tighe, J.A. Darr, Highly conductive low nickel content nano-composite dense cermets from nano-powders made via a continuous hydrothermal synthesis route, Solid State Ionics 181 (2010) 827. [125] A. Siddle, K.D. Pointon, R.W. Judd, S.L. Jones, Fuel Processing For Fuel Cells – A Status Review and Assessment of Prospects, Advantica Ltd., 2003. [126] S. Adhikari, S. Fernando, Hydrogen membrane separation techniques, Industrial & Engineering Chemistry Research 45 (2006) 875. [127] C.D. Saquing, D. Kang, M. Aindow, C. Erkey, Investigation of the supercritical deposition of platinum nanoparticles into carbon aerogels, Microporous and Mesoporous Materials 80 (2005) 11. [128] N.E. Fernandes, S.M. Fisher, J.C. Poshusta, D.G. Vlachos, M. Tsapatsis, J.J. Watkins, Reactive deposition of metal thin films within porous supports from supercritical fluids, Chemistry of Materials 13 (2001) 2023. [129] D.P. Long, J.M. Blackburn, J.J. Watkins, Chemical fluid deposition: A hybrid technique for low-temperature metallization, Advanced Materials 12 (2000) 913. [130] S. Yoda, A. Hasegawa, H. Suda, Y. Uchimaru, K. Haraya, T. Tsuji, K. Otake, Preparation of a platinum and palladium/polyimide nanocomposite film as a precursor of metal-doped carbon molecular sieve membrane via supercritical impregnation, Chemistry of Materials 16 (2004) 2363. [131] M. Seshimo, T. Hirai, M.M. Rahman, M. Ozawa, M. Sone, M. Sakurai, Y. Higo, H. Kameyama, Functionally graded Pd/␥-alumina composite membrane fabricated by electroless plating with emulsion of supercritical CO2 , Journal of Membrane Science 342 (2009) 321. [132] J. Yanik, S. Ebale, A. Kruse, M. Saglam, M. Yuksel, Biomass gasification in supercritical water: Part 1. Effect of the nature of biomass, Fuel 86 (2007) 2410. [133] J.B. Gadhe, R.B. Gupta, Hydrogen production by methanol reforming in supercritical water: suppression of methane formation, Industrial & Engineering Chemistry Research 44 (2005) 4577. [134] K. Pinkwart, T. Bayha, W. Lutter, M. Krausa, Gasification of diesel oil in supercritical water for fuel cells, Journal of Power Sources 136 (2004) 211. [135] R.M. Navarro, M.A. Pea, J.L.G. Fierro, Hydrogen production reactions from carbon feedstocks: fossil fuels and biomass, Chemical Reviews 107 (2007) 3952. [136] A. Kruse, Supercritical water gasification, Biofuels Bioproducts Biorefining 2 (2008) 415. [137] A. Kruse, Hydrothermal biomass gasification, Journal of Supercritical Fluids 47 (2009) 391. [138] A.J. Byrd, K.K. Pant, R.B. Gupta, Hydrogen production from ethanol by reforming in supercritical water using Ru/Al2 O3 catalyst, Energy & Fuels 21 (2007) 3541. [139] Y. Matsumura, T. Minowa, B. Potic, S.R.A. Kersten, W. Prins, W.P.M. van Swaaij, B.V. de Beld, D.C. Elliott, G.G. Neuenschwander, A. Kruse, M.J. Antal Jr., Biomass gasification in near- and super-critical water: status and prospects, Biomass and Bioenergy 29 (2005) 269. [140] D. Yu, M. Aihara, M.J. Antal Jr., Hydrogen production by steam reforming glucose in supercritical water, Energy & Fuels 7 (1993) 574. [141] M. Osada, T. Sato, M. Watanabe, T. Adschiri, K. Arai, Low-temperature catalytic gasification of lignin and cellulose with a ruthenium catalyst in supercritical water, Energy & Fuels 18 (2004) 327. [142] A.J. Byrd, K.K. Pant, R.B. Gupta, Hydrogen production from glucose using Ru/Al2 O3 catalyst in supercritical water, Industrial & Engineering Chemistry Research 46 (2007) 3574. [143] A.J. Byrd, K.K. Pant, R.B. Gupta, Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2 O3 catalyst, Fuel 87 (2008) 2956. [144] N. Boukis, V. Diem, W. Habicht, E. Dinjus, Methanol reforming in supercritical water, Industrial & Engineering Chemistry Research 42 (2003) 728. [145] W. Feng, H.J. van der Kooi, J.D.S. Arons, Biomass conversions in subcritical and supercritical water: driving force phase equilibria, and thermodynamic analysis, Chemical Engineering and Processing 43 (2004) 1459. [146] F.A.P. Voll, C.C.R.S. Rossi, C. Silva, R. Guirardello, R.O.M.A. Souza, V.F. Cabral, L. Cardozo-Filho, Thermodynamic analysis of supercritical water gasification of methanol, ethanol, glycerol, glucose and cellulose, International Journal of Hydrogen Energy 34 (2009) 9737. [147] C.C.R.S. Rossi, C.G. Alonso, O.A.C. Antunes, R. Guirardello, L. Cardozo-Filho, Thermodynamic analysis of steam reforming of ethanol and glycerine for hydrogen production, International Journal of Hydrogen Energy 34 (2009) 323. [148] S. Letellier, F. Marias, P. Cezac, J.P. Serin, Gasification of aqueous biomass in supercritical water: a thermodynamic equilibrium analysis, Journal of Supercritical Fluids 51 (2010) 353. [149] J.D. Taylor, C.M. Herdman, B.C. Wu, K. Wally, S.F. Rice, Hydrogen production in a compact supercritical water reformer, International Journal of Hydrogen Energy 28 (2003) 1171. [150] N. Boukis, V. Diem, U. Galla, E. Dinjus, Methanol reforming in supercritical water for hydrogen production, Combustion Science and Technology 178 (2006) 467. [151] J.B. Gadhe, R.B. Gupta, Hydrogen production by methanol reforming in supercritical water: catalysis by in-situ-generated copper nanoparticles, International Journal of Hydrogen Energy 32 (2007) 2374.

[152] G.J. DiLeo, P.E. Savage, Catalysis during methanol gasification in supercritical water, Journal of Supercritical Fluids 39 (2006) 228. [153] A. Tanksale, J.N. Beltramini, G.M. Lu, A review of catalytic hydrogen production processes from biomass, Renewable and Sustainable Energy Reviews 14 (2010) 166. [154] D. Xu, S. Wang, X. Hu, C. Chen, Q. Zhang, Y. Gong, Catalytic gasification of glycine and glycerol in supercritical water, International Journal of Hydrogen Energy 34 (2009) 5357. [155] D.C. Elliott, Catalytic hydrothermal gasification of biomass, Biofuels Bioproducts Biorefining 2 (2008) 254. [156] F.L.P. Resende, P.E. Savage, Effect of metals on supercritical water gasification of cellulose and lignin, Industrial & Engineering Chemistry Research 49 (2010) 2694. [157] T. Yoshida, Y. Oshima, Y. Matsumura, Gasification of biomass model compounds and real biomass in supercritical water, Biomass and Bioenergy 26 (2004) 71. [158] A.D. Taylor, G.J. DiLeo, K. Sun, Hydrogen production and performance of nickel based catalysts synthesized using supercritical fluids for the gasification of biomass, Applied Catalysis B: Environmental 93 (2009) 126. [159] Y. Kalinci, A. Hepbasli, I. Dincer, Biomass-based hydrogen production: a review and analysis, International Journal of Hydrogen Energy 34 (2009) 8799. [160] M. Domínguez, E. Taboada, E. Molins, J. Llorca, Co–SiO2 aerogel-coated catalytic walls for the generation of hydrogen, Catalysis Today 138 (2008) 193. [161] C. Kwak, T.-J. Park, D.J. Suh, Preferential oxidation of carbon monoxide in hydrogen-rich gas over platinum–cobalt–alumina aerogel catalysts, Chemical Engineering Science 60 (2005) 1211. [162] S. Haji, Y. Zhang, D. Kang, M. Aindow, C. Erkey, Hydrodesulfurization of model diesel using Pt/Al2 O3 catalysts prepared by supercritical deposition, Catalysis Today 99 (2005) 365. [163] S. Haji, Y. Zhang, C. Erkey, Atmospheric hydrodesulfurization of diesel fuel using Pt/Al2 O3 catalysts prepared by supercritical deposition for fuel cell applications, Applied Catalysis A: General 374 (2010) 1. [164] M. Alibouri, S.M. Ghoreishi, H.R. Aghabozorg, Hydrodesulfurization activity of NiMo/Al-HMS nanocatalyst synthesized by supercritical impregnation, Industrial & Engineering Chemistry Research 49 (2009) 4283–4292. [165] M. Alibouri, S.M. Ghoreishi, H.R. Aghabozorg, Hydrodesulfurization of dibenzothiophene using CoMo/Al-HMS nanocatalyst synthesized by supercritical deposition, Journal of Supercritical Fluids 49 (2009) 239. [166] R.T. Yang, A. Takahashi, F.H. Yang, New sorbents for desulfurization of liquid fuels by ␲-complexation, Industrial & Engineering Chemistry Research 40 (2001) 6236. [167] X. Ma, L. Sun, C. Song, A new approach to deep desulfurization of gasoline, diesel fuel and jet fuel by selective adsorption for ultra-clean fuels and for fuel cell applications, Catalysis Today 77 (2002) 107. [168] S. Haji, C. Erkey, Removal of dibenzothiophene from model diesel by adsorption on carbon aerogels for fuel cell applications, Industrial & Engineering Chemistry Research 42 (2003) 6933. [169] D. Jayne, Y. Zhang, S. Haji, C. Erkey, Dynamics of removal of organosulfur compounds from diesel by adsorption on carbon aerogels for fuel cell applications, International Journal of Hydrogen Energy 30 (2005) 1287. [170] S. Yan, Z. Li, S. Wei, Z. YaPing, Impact of supercritical adsorption mechanism on research of hydrogen carrier, Chinese Science Bulletin 52 (2007) 1146. [171] E. Poirier, A. Dailly, Thermodynamics of hydrogen adsorption in MOF-177 at low temperatures: measurements and modelling, Nanotechnology 20 (2009) 204006. [172] C.D. Wood, B. Tan, A. Trewin, H. Niu, D. Bradshaw, M.J. Rosseinsky, Y.Z. Khimyak, N.L. Campbell, R. Kirk, E. Stöckel, A.I. Cooper, Hydrogen storage in microporous hypercrosslinked organic polymer networks, Chemistry of Materials 19 (2007) 2034. [173] Q.R. Zheng, A.Z. Gu, X.S. Lu, W.S. Lin, Adsorption equilibrium of supercritical hydrogen on multi-walled carbon nanotubes, Journal of Supercritical Fluids 34 (2005) 71. [174] C.-Y. Chen, K.-Y. Lin, W.-T. Tsai, J.-K. Chang, C.-M. Tseng, Electroless deposition of Ni nanoparticles on carbon nanotubes with the aid of supercritical CO2 fluid and a synergistic hydrogen storage property of the composite, International Journal of Hydrogen Energy 35 (2010) 5490. [175] A.P. Nelson, O.K. Farha, K.L. Mulfort, J.T. Hupp, Supercritical processing as a route to high internal surface areas and permanent microporosity in metalorganic framework materials, Journal of the American Chemical Society 131 (2009) 458. [176] Z. Xiang, D. Cao, X. Shao, W. Wang, J. Zhang, W. Wu, Facile preparation of high-capacity hydrogen storage metal-organic frameworks: a combination of microwave-assisted solvothermal synthesis and supercritical activation, Chemical Engineering Science 65 (2010) 3140. [177] T. Hocker, A. Rajendran, M. Mazzotti, Measuring modeling supercritical adsorption in porous solids. Carbon dioxide on 13X zeolite and on silica gel, Langmuir 19 (2003) 1254. [178] J.-L. Bobet, B. Chevalier, M.Y. Song, B. Darriet, Improvements of hydrogen storage properties of Mg-based mixtures elaborated by reactive mechanical milling, Journal of Alloys and Compounds 356 (2003) 570. [179] J.-L. Bobet, S. Desmoulins-Krawiec, E. Grigorova, F. Cansell, B. Chevalier, Addition of nanosized Cr2 O3 to magnesium for improvement of the hydrogen sorption properties, Journal of Alloys and Compounds 351 (2003) 217.

S.E. Bozbag, C. Erkey / J. of Supercritical Fluids 62 (2012) 1–31 [180] J.L. Bobet, C. Aymonier, D. Mesguich, F. Cansell, K. Asano, E. Akiba, Particle decoration in super critical fluid to improve the hydrogen sorption cyclability of magnesium, Journal of Alloys and Compounds 429 (2007) 250. [181] A. Denis, E. Sellier, C. Aymonier, J.-L. Bobet, Hydrogen sorption properties of magnesium particles decorated with metallic nanoparticles as catalyst, Journal of Alloys and Compounds 476 (2009) 152. [182] Y. Kojima, K. Suzuki, K. Fukumoto, M. Sasaki, T. Yamamoto, Y. Kawai, H. Hayashi, Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide, International Journal of Hydrogen Energy 27 (2002) 1029. [183] T. Sato, T. Suzuki, M. Aketa, Y. Ishiyama, K. Mimura, N. Itoh, Steam reforming of biogas mixtures with a palladium membrane reactor system, Chemical Engineering Science 65 (2010) 451.

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[184] J.W. Picou, J.E. Wenzel, H.B. Lanterman, S. Lee, Hydrogen production by noncatalytic autothermal reformation of aviation fuel using supercritical water, Energy & Fuels 23 (2009) 6089. [185] A. Cabanas, J.A. Darr, E. Lester, M. Poliakoff, A continuous and clean onestep synthesis of nano-particulate Ce1 − x Zrx O2 solid solutions in near-critical water, Chemical Communications 11 (2000) 901. [186] A. Cabanas, J.A. Darr, E. Lester, M. Poliakoff, Continuous hydrothermal synthesis of inorganic materials in a nearcritical water flow reactor; the one-step synthesis of nano-particulate Ce1 − x Zrx O2 (x = 0-1) solid solutions, Journal of Materials Chemistry 11 (2001) 561.