Fuel Cells: Hydrogen and Ethanol Technologies

Fuel Cells: Hydrogen and Ethanol Technologies Mauro C dos Santos, Luanna S Parreira, Felipe De Moura Souza, José Camargo Junior, and Tuani Gentil, Federal University of ABC, Santo André, Brazil r 2017 Elsevier Inc. All rights reserved.

1 2 3 4 5 6 7 8 8.1 8.1.1 8.1.2 9 10 11 12 References Further Reading

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Worldwide Energy Demand Types of (Environmental) Pollutants Direct Ethanol Acid Fuel Cell Direct Ethanol AFC Crossover Effect Catalytic Poisoning by Reaction Intermediates Characteristics of Nafion Membranes Electrocatalysts Support Carbon-based supports Supports based on other materials Metal Alloys Nanoparticles Methods of Preparation News and Perspectives

1 1 6 6 7 9 9 9 11 11 13 14 14 15 16 16 21

Worldwide Energy Demand

Energy production is one of the crucial aspects of modern society. Currently, the energy matrix mainly comes from nonrenewable primary sources, including fossil fuels and their derivatives (oil, coal, and natural gas). Since 1930, energy consumption has been growing exponentially, mainly due to the growth of the world population. Data obtained in the 2016 Energy Outlook report indicates that global population will increase by approximately 34% between 2014 and 2035, reaching 8.8 billion people by 2035. Consequently, the gross domestic product (GDP) will rise to more than double in (by) the same period. A fifth of this increase will be the result of population growth, while the other four-fifth will be due to improvements in productivity (i.e., GDP per person) [1]. The autonomous International Energy Agency (IEA), an organization created in November 1974 with the aim of promoting the energy security of countries, reports that fossil fuels remain the main source of energy, with oil contributing 43%, natural gas 17%, and coal 13% in 2014 [2]. Large amounts of chemical compounds are emitted into the atmosphere from human activities, especially those related to energy. Such compounds may be divided into two groups: primary pollutants, which are emitted by a source of pollution, such as cars; and secondary pollutants, which undergo chemical reactions with the primary pollutants in the environment (atmosphere) [3,4]. Currently, more than half the world’s population lives in cities and urban areas and is exposed to air pollutants from burning fuels for stationary sources, such as industries, and mobile sources, such as autos and other vehicles. The other portion of the population also suffers exposure, but from burning solid fuels derived from biomass (wood, charcoal, and agricultural waste) and liquid fuels (kerosene and vegetable oil) used for heating, lighting, and cooking. As a result of this exposure, there has been an increase in cardiovascular and respiratory diseases. In addition to problems related to public health, the environment also suffers consequences, for example, the increase of greenhouse gases, which has been identified as a serious environmental problem exacerbating global warming [3].

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Types of (Environmental) Pollutants

The main pollutants emitted into the atmosphere are sulfur dioxide (SO2), ozone (O3), nitrogen dioxide (NO2), hydrocarbons, carbon monoxide (CO), and particulate matter [1,2]. The IEA reports that the carbon dioxide (CO2) concentration in the atmosphere has increased significantly over the last century compared to the Pre-Industrial age (about 280 ppm), reaching 337 ppm in 2014, with an average growth of 2 ppm/year in the last decade. Significant increases have also occurred in the levels of methane (CH4), nitrous oxide (N2O) and inhalable particulate matter. Since two-thirds of all greenhouse gas emissions and 80% of CO2 emissions originate from the energy production sector, efforts to reduce emissions and mitigate environmental problems must be considered [1,2,4]. The increasing concentration of these pollutants in the atmosphere and the high probability of oil depletion has boosted research in this area, with studies aiming to reduce emissions and partially replace the use of fossil fuels [1]. The Kyoto Protocol,

Reference Module in Materials Science and Materials Engineering

doi:10.1016/B978-0-12-803581-8.09263-8

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Fuel Cells: Hydrogen and Ethanol Technologies

Fig. 1 Schematic illustration of proton exchange membrane fuel cell. Adapted from Hartmut, W., Michael, G., Marcelo, L., 2000. Tecnologia de células a combustível. Quimica Nova, 23 (4), 538–546.

ratified in 2015, proposes to reduce greenhouse gas emissions in developed countries, relative to emission levels of the 1990s. Thus, alternative energy sources, such as biofuels, have been developed from renewable sources (natural or biological) as well as fuel cells [5,6]. Fuel cells are electrochemical devices that directly convert chemical energy into electrical energy and heat. The operating principle of a fuel cell is similar to that of batteries. The only difference is related to the storage of reagents because of the external shape (form) of the fuel cell. Fuel cells are batteries in continuous operation, in which the anode (negative electrode) is supplied with an oxidizing gas, as shown in Fig. 1. When using hydrogen as the fuel and oxygen as the oxidant, energy conversion occurs through the oxidation of H2 to produce protons on a gas diffusion electrode, releasing electrons [7] according to Reaction (I): Anode : 2 H2 þ 4 H2 O-4 H3 Oþ þ 4 e

ðIÞ

On the opposite electrode, which is also a gas diffusion electrode, O2 is reduced by the protons transported through the electrolyte. Thus, water is obtained as the product, in addition to free electrons that generate current. This current is conducted through an external circuit. Reaction (II) occurring at this electrode is: Cathode : O2 þ 4 H3 Oþ -6 H2 O

ðIIÞ

The overall reaction is accompanied by the release of heat, as described below: Overall reaction : 2 H2 þ O2 -2 H2 O

ðIIIÞ

Gas diffusion electrodes (GDEs) have the ability to conduct electrical current. They are permeable to reactantcharacteristics of several gases and are separated from each other by an ionic conductor that prevents mixing of the gases. For good performance in fuel cells, GDEs must exhibit high catalytic activity, thus promoting high current densities. Additionally, their pores should not be sturdy during operation in order to prevent absorption of the electrolyte. Historically, the discovery of fuel cell operation, which currently occurs by the combination of hydrogen and oxygen, is attributed to the German scientist Christian Friedrich Schönbein, who discovered this process in January 1839. While Schönbein was conducting his theoretical research, a Welsh scientist named William Robert Grove was also investigating the effects of this combination. The latter developed the first fuel cell, which he called “voltaic battery gas.” This battery consisted of pairs of platinum electrodes immersed in sulfuric acid in a closed system containing hydrogen and oxygen, as shown in Fig. 2 [8]. Grove made two relevant observations about this experiment: first, he observed the flow of current when the electrodes were connected in sequence, indicating the generation of electric current; and second, he observed that the containers containing gases were filled with water [8,9]. In 1889, the term “fuel cell” was adopted by chemists Ludwig Mond and Charles Langer in an attempt to construct the first functional gas battery device composed of air and industrial coal gas. In the 1930s, Francis Bacon continued studies on fuel cells at the University of Cambridge, changing the electrolyte from acid to alkaline (potassium hydroxide) [8]. The theoretical efficiency (n) of electrochemical processes of energy production, which do not follow Carnot’s principle, was demonstrated by Wilhelm Ostwald and Walther Nernst in the latter part of the last century. It is given by the ratio between the

Fuel Cells: Hydrogen and Ethanol Technologies

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Fig. 2 Schematic illustration of Grove cell. Adapted from Ref. Grove, W.R., 1842. LXXII. On a gaseous voltaic battery. Philosophical Magazine Series 3 21 (140), 417–420.

Table 1

Characteristics of well-known fuel cells [7] Polymer electrolyte fuel cells (PEFC)

Alkaline fuel cell (AFC)

Phosphoric acid fuel cells (PAFC)

Molten carbonate fuel cells (MCFC)

Solid oxide fuel cells (SOFC)

Electrolyte

Hydrated polymeric ion exchange membranes

Immobilized liquid phosphoric acid in SiC

Immobilized liquid molten carbonate in LiAlO

Perovskites (ceramics)

Electrodes

Carbon

Mobilized or immobilized potassium hydroxide in asbestos matrix Transition metals

Carbon

Nickel and nickel oxide

Catalyst Operating temperature Charge carrier Prime cell components Product water management Product heat management

Platinum 40–801C

Platinum 65–2201C

Platin 2051C

Electrode material 6501C

Perovskite and perovskite/metal cermet Electrode material 600–10001C

Hþ Carbon-based

OH Carbon-based

Hþ Graphite-based

CO3 Stainless-based

O Ceramic

Evaporative

Evaporative

Evaporative

Gaseous product

Gaseous product

Process gas H þ liquid cooling medium

Process gas þ electrolyte circulation

Process gas þ liquid cooling medium or steam generation

Internal reforming þ process Gas

Internal reforming þ process gas

Gibbs free energy and the enthalpy of the reaction (Eq. (1)). n¼

DG DH

ð1Þ

Driven by the space race, which occurred after the launch of the first artificial satellite Sputnik by the Soviet Union in 1957 and the manned space flights by NASA, interest in fuel cells was revived. With further technological advances, the electrodes, which until then were made of permeable metal plates, were replaced by GDEs. GDEs allowed the reduction of the amount of platinum by increasing the surface area. Triggered by the discovery of Teflon in the 1970s, Nafion membranes were also applied in fuel cells. This resulted in the development of several types of fuel cells, which can be classified according to the electrolyte present in each operating system [8]. In general, the electrolyte defines the operating temperature range of the cell and dictates the physicochemical and thermodynamic properties of the materials used in its components, such as electrodes, interconnections, and current collectors. Table 1 presents a summary of the characteristics of several fuel cells [7]. The type of electrolyte and the operating temperature define the two main groups of fuel cells, namely: alkaline fuel cells (AFC), proton exchange membrane fuel cells (PEMFC), and phosphoric acid fuel cells (PAFC), which operate at low temperatures; and molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC), which operate at high temperatures [9].

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Fuel Cells: Hydrogen and Ethanol Technologies

Fig. 3 Planar structure of Nafion molecule.

PEMFCs have been a subject of research in this area because they exhibit the interesting features of high power densities and efficiency, relatively low work temperatures (80–901C) and great robustness. In these cells, the electrolyte consists of a polymeric membrane (a polymer from sulfonic acid fluoride or another similar polymer) through which protons are exchanged, promoting proton conductivity [10]. They consist of two segments, depending on the applied fuel. There are also devices that use hydrogen as the fuel and allow direct liquid oxidation. These are called direct liquid fuel cells (DLFCs), and they operate by oxidizing small organic molecules (SOMs) [11]. In DLFCs, the electrolyte used is a fluoro-copolymer sulfonate based on tetrafluoroethylene [12]. Being a hydrated Nafion membrane, it has a structure similar to that of Teflon, which promotes ionic conduction [13], as shown in Fig. 3. In fuel cells that apply hydrogen as the fuel, the sources are gases from reforming or synthesis. These gases also contain water vapor, CO2 and CO. They are obtained by the transformation of primary fuels, such as ethanol. In this way, fuel cells are classified as indirect, as mentioned above, or direct, which allow the use of fuel without the reforming process [15]. The technology for the use of hydrogen as a fuel faces some difficulties, related mainly to the pressure required for its storage. Car makers developing vehicles capable of oxidizing this fuel need a system capable of compressing hydrogen at a pressure of approximately 700 bar [16]. Research at the U.S. Department of Energy’s (DOE), Office of Energy Efficiency and Renewable Energy (EERE), and Fuel Cell Technologies Office (FCTO) focuses on the development and improvement of such devices, enabling future suppliers to master this technology in order to enable its implementation and commercialization [16]. Fuel cell electric vehicles (FCEV) use containers capable of withstanding high pressures that are enveloped by a carbon fiber composite (CFC) material. However, the current cost for the development of these containers is $17/kWh, representing approximately 90% of the total cost [16,17]. Because of this problem, the Hydrogen Storage Program, together with the FCTO, has invested in overcoming the storage challenges of H2 at 700 bar. Advanced technologies, including cold/cryo-compressed H2 and materials-based (metal hydrides, chemical hydrogen storage, and sorbents) hydrogen storage, improve the characteristics of value, volume, weight, time of supply, and life cycle. This is done in several ways, from the development of lower-cost carbon fibers to improvements in the design and manufacture of the systems [16]. Groups researching advanced materials have invested in computer modeling systems, engineering projects, and performance validation. For example, the Oak Ridge National Laboratory (ORNL) is developing precursor polyacrylonitrile (PAN) fibers, which have the potential to reduce the cost of carbon fibers by 25% [16–18]. According to research conducted by the “Technology Roadmap Hydrogen and Fuel Cells” in Japan and the United States, it is estimated that 400,000 FCEVs will be available for sale in 2025, increasing annual sales to about 2.3 million by 2030 [19]. Due to difficulties, such as the transport and storage [20] of hydrogen fuel, fuel cells capable of oxidizing SOMs, including methanol and ethanol, were developed. These include direct methanol fuel cells (DMFCs) and direct ethanol fuel cells (DEFCs). The use of liquid fuels is viable in terms of transportation and storage, in addition to providing relatively high energy levels. The use of methanol triggered the development of DLFCs in the alternative energy research field. However, since methanol is toxic to humans, the use of ethanol as a fuel is favorable. In addition to the low toxicity of ethanol, data show that complete oxidation generates 8.0 kWh kg1, compared to 6.1 kWh kg1 from the oxidation of methanol. Therefore, it is possible to obtain more energy from the ethanol compared to the same amount of methanol, thus reducing fuel consumption [13,21]. From an environmental point of view, ethanol is a renewable biomass source of energy and provides an alternative to petroleum-derived fuels and natural gas, which are applied for different types of energy generation. In Brazil, the main source of ethanol is sugar cane, and the country is a world pioneer in the use of biofuels. Approximately 45% of the energy and 18% of the fuels consumed in Brazil are renewable. In contrast, in the rest of the world, 86% of energy comes from nonrenewable sources. In the United States, ethanol is mainly derived from corn, while in Europe, ethanol is obtained from beet sugar [20]. Another point to be highlighted is the natural sequestration of large amounts of CO2 achieved through the combustion of ethanol. The growth of trees demands a large amount of carbon by absorption. Since ethanol is obtained from natural sources, the generated CO2 is consumed and the cycle is closed. This prevents emission to the atmosphere and consequently controls the

Fuel Cells: Hydrogen and Ethanol Technologies

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Fig. 4 Schematic illustration of neutral carbon cycle. Adapted from Olah, G.A., Goeppert, A., Prakash, G.K.S., 2009. Chemical recycling of carbon dioxide to methanol and dimethyl ether: From greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons. Journal of Organic Chemistry 74 (2), 487–498.

Fig. 5 Proton exchange membrane fuel cell [21].

increase of the greenhouse effect. Data show that approximately 150–200 t of carbon can be absorbed per hectare of forest in development [18–20]. Fig. 4 shows a representative scheme of the neutral carbon cycle. The operation of this type of fuel cell is based on the use of a noble metal in the anode. The noble metal can split molecular hydrogen into positive ions (protons) and negative ions (electrons). The membrane allows the passage of ions to the cathode to be used for water production by combination with oxygen. The passage of electrons occurs through an external circuit, as described in Fig. 5. This type of fuel cell has the capacity to produce approximately 1.1 V. It is equipped with a channel that distributes hydrogen and a collector for the generated current. The operation of a fuel cell is based on the reverse electrolysis principle, i.e., the opposite process to that seen in the electrolysis of water. Taking the example of a PEMFC, a hydrogen molecule dissociates and is input at the anode covered by a catalyst. The catalyst separates the atoms into protons and electrons. The electrons produced in the anode

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Fuel Cells: Hydrogen and Ethanol Technologies

Fig. 6 Proton exchange membrane fuel cells (PEMFC) and alkaline anion exchange membrane fuel cells (aaEMFCs) [24].

make the anode electronegative. In turn, the cathode is electropositive due to the presence of oxygen. Therefore, there is a potential difference between the electrodes, resulting in the circulation of current (electrons) through an external circuit. The protons (H þ that cross the electrolyte in the direction of the cathode) combine with the hydroxyl ions at the cathode, producing water [21]. As mentioned earlier, DEFCs use ethanol as the fuel. Here, the goal is to convert the alcohol molecule into CO2 by a complete reaction, producing 12 electrons. When methanol is applied in DMFCs, only six electrons are produced. However, DEFCs have a lower rate of total oxidation compared to DMFCs due to the structural characteristics of ethanol. These characteristics and their effects are discussed in detail in the following paragraphs. However, it is important to note that ethanol in PEMFCs can be oxidized using an acid or alkaline medium, each producing electrons by different mechanisms [17,22]. There are two main types of fuel cells for ethanol oxidation, namely PEMFC and alkaline anion exchange membrane fuel cells (AAEMFC) [22,23]. The operation schemes of these fuel cells are shown in Fig. 6.

3

Direct Ethanol Acid Fuel Cell

This type of fuel cell operates at pH levels below 5, using a membrane that allows cation mobility in the electrolyte. As mentioned previously, the membrane is made of Nafion, most commonly Nafion 115 and 117. The structure is composed of weakly linked cations that complete the circuit throughout the stage. With the use of Pt-based electrocatalysts, which are more efficient for ethanol oxidation (see Section 8), acidic conditions provide a high concentration of H þ ions in the device. Thus, proton exchange is better than in AFC [21]. For DEFCs in an acid medium, the reactions [25] are as follows: Reaction at the anode : CH3 CH2 OH þ 3H2 O-2CO2 þ 12Hþ þ 12e Reaction at the cathode : 3O2 þ 12Hþ þ 12e -6H2 O Global reaction : CH3 CH2 OH þ 3O2 -2CO2 þ 3H2 O The mechanism of possible routes for ethanol oxidation in an acidic medium and the intermediaries generated are presented in Fig. 7.

4

Direct Ethanol AFC

AFCs, which use electrolytes with high pH values (between 8 and 12), were developed and widely studied from 1960 to 1980. In this type of fuel cell, due to the low concentration of H þ ions in the middle, the use of proton exchange membrane (PEM) is rendered ineffective due to the slow kinetics of diffusion. Therefore, the use of a membrane that allows anion exchange from the anode to the cathode is crucial. Such anion exchange membranes (AEM) were introduced, thus boosting research on these fuel cells. Unlike PEM-DEFCs, in AEM-DEFCs, reforming water occurs at the anode and its consumption occurs at the cathode. The movement of ions in the membrane is also reversed, as shown in Fig. 8. However, problems, such as the passage of fuel, may also occur if the concentration in both components of the electrodes is neglected [27].

Fuel Cells: Hydrogen and Ethanol Technologies

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Fig. 7 Possible routes for ethanol oxidation in an acidic medium [26].

Fig. 8 Schematic diagram of an anion exchange membranes (AEM) for a hydrogen fuel cell [9].

Alkaline DEFCs may be preferable to acidic DEFCs for a few reasons, such as increased efficiency and kinetics of the oxidation reaction. This is because the higher concentration of OH ions favors the adsorption of OH ions on the surface of the electrocatalysts, thus facilitating the oxidation reaction. In these cells, it is possible to change the electrocatalyst so that the fuel is preferentially oxidized instead of the dissociation of water molecules to OH ions at the anode. Thus, the expensive noble metal platinum may be replaced by another lower-cost metal, such as palladium, gold, and silver. In addition, the less corrosive nature of the alkaline medium allows greater durability of the electrodes [21]. However, an undesirable effect occurs with a large concentration of OH ions in the system: these ions can react with CO2, producing carbonate and bicarbonate ions. These ions and the alkali species from the medium (NaOH or KOH) react to form carbonate salts (Na2CO3 or K2CO3), which block and decrease the spread of ions in the device environment, resulting in a low cell efficiency. In addition, these carbonate salts can also affect the electroactive surface of the electrocatalysts. However, to achieve an AFC with a better efficiency, electrocatalysts and electrodes resistant to salts and their ions may be used [27].

5

Crossover Effect

“Crossover” is one of the common effects occurring in alcohol fuel cells. This effect involves permeation of the fuel from the anode, across the electrolyte membrane and to the cathode. With this passage, the cathode potential decreases, thus reducing the overall cell efficiency and wasting fuel during the operation [24,25].

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Fuel Cells: Hydrogen and Ethanol Technologies

Fig. 9 Effect of current density on the crossover rate at different temperatures and ethanol concentrations [28].

Fig. 10 Ethanol crossover rate vs. ethanol concentration at different temperatures and helium flow rates [28].

Both methanol and ethanol fuel cells undergo “crossover” effects, but ethanol performs better due to its lower diffusion rate through the membrane. However, once filled, the catalytic layer is polluted due to intermediate products from the oxidation. The increase of temperature, current density and ethanol supply intensifies the “crossover” effect [28], as seen in Figs. 9 and 10.

Fuel Cells: Hydrogen and Ethanol Technologies

9

The crossover effect occurs when the intermediates generated by ethanol oxidation (acetic acid and acetaldehyde) have higher concentrations than O2 at the cathode [24,25].

6

Catalytic Poisoning by Reaction Intermediates

Some characteristics of the ethanol molecule inhibit the oxidation process compared to the use of methanol. For example, the difficulty of breaking the C–C bond results in the formation of intermediates, such as acetaldehyde and acetic acid. Another source of interference is the formation of CO during the process, resulting in catalytic poisoning [26,27]. Platinum is one of the metals most widely used as an electrocatalyst for ethanol oxidation due to its ability to dehydrogenate organic molecules when present at the anode of the fuel cell. However, its interaction with CO is too strong, implying adsorption on active sites and consequently catalytic poisoning. Therefore, studies have aimed to improve the efficiency of fuel cells by the development of new nanostructures for electrocatalysts and support materials [28,29].

7

Characteristics of Nafion Membranes

As mentioned previously, the Nafion polymeric membrane is more applicable in fuel cells due to its mechanical and electrochemical properties, which allow the conductivity of protons. On the other hand, it also has many disadvantages. Nafion is considerably permeable to fuel and can cause a loss, resulting in swelling of the membrane. Additionally, it is relatively expensive. Therefore, one of the challenges is the development of new technologies for application to polymeric membranes in order to mitigate the aforementioned disadvantages [25]. Membranes of sulfonated polymers (ether-ether-ketone) – (SPEEK) have emerged as a viable alternative for replacing the Nafion membrane in DE-PEMFC, offering proton conductivity as well as chemical and thermal stability. Depending on the approach, it is possible to make specific modifications to the SPEEK electrolytic membrane, with the aim of further improving its performance. These modifications include the application of polyamides (PI) and coatings with carbon molecular sieves (CMS) along with the use of a reinforcement phase with N-(3-triethoxysilylpro pyl)-4,5-dihydroimidazole (DHIM), which consists of inorganic as well as organic functional groups [25]. Polyvinyl alcohol membranes (PVAs) are another type of membrane that have been studied. PVA can be used with the electrolyte in both PEMs and AEMs. It possesses good resistance to corrosion, reduced permeability to ethanol and high hydrophilicity and is inexpensive. Modifications to the PVAs were made by doping with phosphotungstic acid (HPW) and crosslinking with diethylenetriamine penta-acetic acid (DTPA) [25].

8

Electrocatalysts

We now discuss the kinetics of electrochemical processes and the effects of electrode materials on the reaction rates. The electrodes in a fuel cell, in the first instance, are not transformed into products. Instead, they participate as reagents in chemical reactions, acting only as receivers or electron donors. The electrodes can also participate in the adsorption of reagents and in some cases intermediates. Then, the role of the electrode in fuel cells can be classified specifically as that of a catalyst or an electrocatalyst [30]. The study of electrocatalysts or electrocatalysis intends to uncover how the electrode material affects the reaction kinetics, with the aim of accelerating the reaction process by decreasing the activation energy of the process. This means that for the same overpotential, the electrochemical reaction will occur at different speeds [31]. Then, electrocatalysis will depend on the electrode material; the solution in which it is immersed; temperature; area and morphology of the electrocatalyst; and many other variables. Some of the main variables will be discussed in this section. An appropriate way to select a good electrocatalyst is by the Tafel curve. Julius Tafel (1862–1918) published a paper in 1905 presenting a plot of overpotential versus current density. The Tafel curve for a system that is far from equilibrium in the anodic direction, which is the case for electrocatalysts for fuel oxidation in fuel cells, is based on the linear function following Eq. (2) below: Z¼ where: R is the universal gas constant. T is the absolute temperature. Z is the overpotential. b is the coefficient symmetry. i is the current density. i0 is the so-called “exchange current density.”

RT RT ln i0 þ ln i ð1  bÞF ð1  bÞ

ð2Þ

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Fuel Cells: Hydrogen and Ethanol Technologies

Fig. 11 Assumed experimental diagram for different electrocatalysts with their Tafel curves.

Fig. 12 Volcano curve for hydrogen adsorption on different metals.

The value of exchange current (i0) indicates the electrocatalytic activity of the electrocatalyst and is a specific parameter. The coefficient of symmetry (b) is also an intrinsic parameter of the electrocatalyst but is related to the geometrical shape of the fuel molecule and consequently its approach and absorption on the electrocatalyst surface to be oxidized. A generic Tafel curve is shown in Fig. 11. It is seen that electrocatalysts have different linear coefficients (i0), following Eq. (2) [32]. Polarization curves obtained from experimental electrodes can be compared to identify the electrode with a higher electrochemical performance. This is the electrode with the highest current density (i) under the same overpotential (Z). More information about the Tafel curve and its construction can be found in the following Refs. [30,33,34]. Chronoamperommetric curves at different potentials are also used to consider electrocatalytic behavior. For a given potential, the higher the current density per area or per mass of the electrocatalyst, the higher its electrocatalytic activity in experiments. In these experiements, the oxidation of hydrogen, methanol, or ethanol at half cells are studied as examples of reactions occurring at the anode. Conversely, for the oxygen reduction reaction (ORR), rotating ring-disk experiments or rotating-disk experiments were also used in order to discuss the electrocatalytic behavior. In these cases, the following parameters are compared: (1) the highest current density per area or per mass of the catalyst at the highest potential and (2) the lowest ring current, indicating a mechanism in which water is produced (with the transfer of four electrons) rather than hydrogen peroxide. These cases show the electrocatalysis effect in a half-cell experiment for the ORR. Polarization curves, in which the fuel cell overpotential is plotted versus current density and/or power density is plotted versus current density, are the most appropriate means of discussing catalysis in a fuel cell device. The detailed conditions for a good anode or cathode electrocatalyst are discussed here. In this case, a higher power density or cell overpotential at small values of current densities indicates the best performance of the device. The lower the reduction of the cell potential with current, the higher the capacity of the cell to furnish electrical energy at a given time. Other important information about catalytic performance is the fuel adsorption capacity for subsequent oxidation. A volcano curve is a graph that correlates the exchange current versus the hydrogen absorption enthalpy of different metal surfaces. It is established knowledge that the best catalysts have the highest values of exchange current and adsorption enthalpy. A common graph is the volcano curve for the hydrogen reagent in fuel cells, as shown in Fig. 9. Thus, metal catalysts based on platinum and palladium are most widely used in research on hydrogen fuel cells (Fig. 12).

Fuel Cells: Hydrogen and Ethanol Technologies

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Fig. 13 Ideal histogram illustrating the average size of metal nanoparticles for improved electrochemical performance.

It is common to search alloys based on these effective metals, as presented in the volcano curve, with other inexpensive metals that can improve the electrocatalytic activity of the material. After choosing a metal with improved electrochemical activity for a specific fuel, we seek to study other factors that influence the performance of the electrocatalyst. These include the shape, size, and distribution of the nanoparticles and the material for the support, among many other factors. The commercial viability of the developed technology, the cost of the raw material of the electrocatalyst, the preparation method, and other costs are also considered. Regarding the size of metal nanoparticles on the support surface, studies indicate that these particles should have an average diameter of 2–5 nm, as shown in the illustrative histogram of an ideal or semi-ideal result presented in Fig. 10. Nanoparticles with diameters larger than 5 nm represent a loss of active area, and this low surface area affects the performance of the catalyst. Nanoparticles smaller than 2 nm are unstable and tend to agglomerate so as to form particles larger than 5 nm with decreased surface area (Fig. 13).

8.1

Support

Due to the high cost of effective metals, such as palladium and platinum, it is not feasible to use a homogeneous layer of these noble and rare metals as electrocatalysts [35]. Therefore, the current idea is to use the least amount of catalytic metal to obtain the best electrocatalyst performance. For this reason, research on material supports has advanced in order to obtain good electrocatalytic performances, even with smaller amounts of catalytic metals. The most desirable property of support materials is a high surface area to provide the maximum possible active sites for electrooxidation [31,32]. It has been shown that the role of the nanostructured support material in the operation of fuel cells is beyond the efficient distribution of the metal catalyst on its surface. The support should also have good conductivity because this influences the flow of electrons. The durability of the support is also a criterion for assessing its performance, in addition to high stability for fuel cell operation and easy recovery of noble metals from the spent electrocatalyst [31,32]. Furthermore, a pore size of 20–40 nm is desirable in order to increase the surface area of the support, since this will generate more locations for forming catalytic metal active sites [36].

8.1.1

Carbon-based supports

The most common supports for fuel cells are based on carbon. Supports with Vulcan XC-72 carbon black, graphene, single- and multi-walled nanotubes, carbon nanofibres (CNFs) and carbon mesoporous materials have been most researched as electrocatalystic supports for fuel cells [35,37–39]. Vulcan XC-72 carbon black has been substantially overtaken by carbon nanotubes (CNTs) as an electrocatalytic support [40,41]. The structure and electrical properties of CNTs provide high electrical conductivity and a specific interaction between catalytic metals due to the promotion of delocalized electrons between the “d” orbital of the metal and the CNTs, which increases the catalytic activity. CNTs can have a few impurities, depending on the preparative process. On the other hand, Vulcan XC-72 contains significant amounts of organosulfur impurities, which can poison the metal electrocatalyst. CNTs also have a homogeneous structure and other properties that are better compared to Vulcan XC-72 [42,43]. Recent results involve a covalent functionalization treatment using an acid for carboxyl group formation on the CNT structure (COOH-CNTs) and a noncovalent treatment with the addition of polymers, such as poly(allylamine hydrochloride) (PAH). These modifications have shown results better than those for non-functionalized CNTs. This is due to the formation of effective active sites for anchoring metal nanoparticles, resulting in high dispersion and better control of electrocatalyst metal loading [44,45].

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Fuel Cells: Hydrogen and Ethanol Technologies

Fig. 14 Scheme with carbon nanoparticle structures [46].

There are many subgroups of CNTs, including single-walled carbon nanotubes (SWCNTs) [47], double-walled carbon nanotubes (DWCNTs) [48], multi-walled carbon nanotubes (MWCNT) [49], hollow-structured MWCNT/CNFs, and MWCNT structured bamboo/CNFs, among many other variations [50]. Some of these carbon structures are shown in (Fig. 14). In 1994, CNFs were produced from carbon structures by Rodriguez et al. [51]. Even today, CNFs are studied as support materials for fuel cell applications. In principle, these structures are produced from the decomposition of hydrocarbons. The CNF structure is differentiated because it is desirable to obtain either a carbon material with a minimized hollow cavity or without it. CNFs have larger diameters and lengths than CNTs and are classified into three types: (1) ribbon-like CNFs, (2) platelet CNFs, and (3) herringbones (stacked-cup), depending upon the orientation of the nanofibres with respect to the growth axis [48–50]. The main contribution of these CNFs is their low susceptibility to CO poisoning compared to usual carbon materials. These results have been explained by the specific position and crystallographic orientation of the metal nanoparticles when dispersed in graphite nanofibres [46,52]. Vulcan XC-72 carbon has an average micropore size generally smaller than 2 nm. This makes it efficient as a support for Vulcan nanocatalysts. However, it has been demonstrated that it is difficult to access reactants in micropores of this size. Thus, a compromise is required in terms of pore size. In contrast, macroporous carbon materials, with an average pore size larger than 50 nm, have excellent mass transfer characteristics but a low surface area and low conductivity [53,54]. Mesoporous carbon nanoparticles are carbon materials with an average size between 2 and 50 nm, with properties intermediate to those of microporous and macroporous structures. Many studies have focused on the equilibrium of these mesoporous carbon supports. Some results indicate that mesoporous carbon promotes nanoparticles with a significantly higher electrochemically active surface area (ECSA) compared to amorphous and microporous carbon. Mesoporous carbon materials exhibit higher conductivity and mass transport than macroporous carbon materials [55–59]. Graphene is a monolayer graphite carbon with a hexagonal closed packed lattice. It has found extensive applicability in research on carbon materials [59,60]. The physicochemical properties of graphene are very similar to those of CNTs, and it can be

Fuel Cells: Hydrogen and Ethanol Technologies

13

regarded as a SWCNT unrolled sheet structure. Advances in graphene research have shown that it is a support with high surface area and, most importantly, a promising anti-poisoning catalyst [61,62]. There have been many other studies on carbon support materials, with some materials showing better performances than others in different aspects. Additionally, there have been attempts to apply multiple carbon structures on a single support [63]. The incorporation of heteroatoms, for example, sulfur, nitrogen, boron, etc., on carbon nanoparticle structures has been performed, with the objective of maintaining the physical and chemical features of carbon nanoparticles, while improving their stability, reactivity, electronic structure, conductivity, etc. [64–66]. There are numerous carbon nanostructures, and it would be impossible to discuss the particular characteristics each of these. Therefore, some are merely mentioned in this work. Others, whose results have been shared more in the scientific community, are discussed in greater detail. Metal oxides added to carbon support structures have produced results relevant to catalysis because they avoid catalytic surface poisoning by acting as adsorbents of oxygenated species, such as an oxygen source for CO oxidation. Moreover, carbon support structures can increase the catalytic activity of metal nanopartícles by a bifunctional mechanism [66,67]. The most advanced research has been on CeO, MnO2, NiO, TiO2, Co3O4, Mn3O4, RuO2, WO2, NbO2, Nb2O5, ZnO, and other metal oxides [63,68]. Carbon functionalization using polymers has been highlighted in recent years, and such polymers can be divided into two groups: conductive polymers and polyelectrolytes. Their contributions to carbon nanoparticles have not yet been identified precisely. However, it is known that they are very useful for controlling the growth of nanoparticles [69]. Conductive polymers have shown higher peak current densities than graphene or Vulcan support due to an increased charge transfer rate and the dispersion of catalytic metal particles [70]. The most representative conductive polymers used to fabricate hybrid catalysts are polyaniline (PANI), polypyrrole (PPy), poly(2-amino-5-mercapto-1,3,4-thiadiazole) (PAMT), and poly(pyrogallol) (PPG), among others. All of the aforementioned conductive polymers show greater potential oxidation peaks than conventional catalysts in the cyclic voltammetry technique. The electrocatalyst Pt/PPG/graphene has shown reduced CO poisoning [69–73]. Examples of polyelectrolytes are poly(amidoamine) (PAMAM), poly(diallyldimethylammonium chloride) (DADMAC) and heteropolyacids (HPAs) of H3PMo12O40. They provide a way to produce highly dense and homogeneously distributed metal nanoparticles without damaging the electronic structure of carbon [53–55,74].

8.1.2

Supports based on other materials

Research into supports for use in fuel cells has also been conducted on materials other than those mentioned above, showing that supports extend beyond those with the molecular structure of carbon materials. There are studies on metal nitrides, carbides, electronically conducting polymers (ECPs), sulfoxides, and conductive metal oxides, among others [75]. Nitrides of transition metals have not been studied as supports for application in fuel cells, but some preliminary studies already show that these materials may be promising. Titanium nitride (TiN), boron nitride (BN), and molybdenum nitride (Mo2N) have been shown to have advantages. TiN has high electrical conductivity and corrosion resistance in an acidic medium; BN possesses favorable thermal properties, such as stability, thermal inertia, and thermal conductivity; and Mo2N exhibits excellent properties for ethanol reforming [57–61]. Carbides are compounds composed of carbon and a less electronegative element, although in these carbon compounds, all elements possess some degree of covalent character [76]. Boron carbide (B4C), silicon carbide (SiC), titanium carbide (TiC), and tungsten carbide (WC) are some carbides that have been explored as supports. Among these, WC has been studied more extensively and shows greater promise. Its main characteristics are better conductivity and a high electrochemical oxidation activity compared to commercial carbon supports [77–80]. ECPs are materials with electronic and physicochemical properties suited for use as supports. These materials are also easy to prepare. Due to their ability to disperse metal particles, ECPs, including PANI, PPY, poly(3,4-ethylene-dioxythiophene) (PEDOT), polythiophene, etc., have been used for applications in fuel cells [76,81]. Several nanostructured conductive polymers have been studied, and the results demonstrate improvements and advances, mainly related to the excellent dispersion of noble metals on the polymer [82–89]. Conductive metal oxides are the most widely studied supports, after carbon nanostructures, for application in fuel cells. Usually these materials have good conductivity, homogeneous catalytic metal dispersion, thermal and chemical stability, mechanical strength, and a high surface area, among other aspects [78]. There is a wide variety of metal oxides for supports, for example, TiO2, SnO2, CeO2, NiO, WO3, ZrO2, RuO2, SiO2, In2O3, and many others. In addition, there is the possibility of doping these structures with another metal [90–92]. In general, each compound is characterized by a change of crystallographic phases, nanostructure morphology, and oxidation number. The morphology of the nanostructures is an important property that can greatly enhance the dispersion of catalytic metal nanoparticles and thus, the electrocatalytic activity of the material. All the supports discussed here have been researched using different morphologies with different reported catalytic effects. Until now, the importance of the support for anchoring metal nanoparticles has been reported. However, the metal nanoparticles may have different morphologies and chemical compositions. Therefore, the next section discusses the importance of the formation of metal alloys for better nanoeletrocatalysis, good chemical stability, and a longer duration of useful life.

14

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Fuel Cells: Hydrogen and Ethanol Technologies

Metal Alloys

In order to enhance electrocatalytic activity, the minimization of CO poisoning on the electrocatalyst and reduction of the amount of noble metal in the catalyst have been studied. These changes are applied during the synthesis of alloys based on palladium and/ or platinum, most commonly with inexpensive auxiliary metals that possess adequate catalytic properties for specific applications. The alloy can be binary, i.e., composed of two metallic elements; ternary, i.e., comprising three metallic elements; and so on. The mass ratio of each metallic element in the alloy may also vary. The alloys most frequently reported to show improved catalytic results have a noble metal and auxiliary metal in a ratio of approximately 50:50 or 25:75 [93–97]. Some examples of such alloys are PtSn, PtNi, PtIr, PtCu, PtCo, PtRu, PtRh, PtAg, PtAu, PtPd, PdFe, PdCu, PdNi, PdSn, and a wide variety [98–107], including the ternary alloy formed from the combination of two good auxiliary metals with a noble metal (Pt and/or Pd). In all of these, it is important to evaluate whether the inclusion of auxiliary metals helps the electrocatalytic activity of these nanoparticles. Two mechanisms are most widely accepted to explain the increase in catalytic activity: the bifunctional mechanism and the electronic effect. In the bifunctional mechanism, the effective metal, Pt or Pd, operates as a site of adsorption and dehydrogenation when the fuel is ethanol, and the auxiliary metal is activated by water, forming oxygenated species at potentials lower than the effective metal in nearby sites with adsorbed CO. These oxygenated species from auxiliary metal promote the oxidation of CO to CO2, thus reducing catalytic metal poisoning. Then, the addition of the auxiliary metal, in this case, promotes a release of the active catalytic sites of the noble metal for subsequent reactions by supplying oxygen species that oxidize CO to CO2. In this mechanism, the role of the auxiliary metal is to maintain the maximum catalytic activity of the noble metal [108–111]. One of the most promising alloys and a main example of the bifunctional mechanism in an electrocatalyst is the alloy of platinum and ruthenium on a carbon support. The bifunctional effect is promoted by ruthenium atoms, which produce oxygen species that react with water molecules at a less positive potential than platinum, as shown in the following chemical reactions [112,113]: Ru þ H2 O-RuOH þ Hþ þ 1e

E0 E0:2V NHE

ðIVÞ

The formation of ruthenium hydroxide sites (RuOH) can oxidize CO to CO2 because the RuOH species provide oxygen to CO. This effect occurs because the RuOH species are close to and randomly dispersed around the platinum sites. Thus, the platinum sites are freed for the oxidation reaction of a new fuel molecule, which was sterically hindered previously. The chemical reaction representing this is shown below [114–116]. PtðCOÞads þ RuOH-Pt þ Ru þ CO2 þ Hþ þ e

ðVÞ

If the CO is also adsorbed on the ruthenium site and blocks it chemically, then, as soon the RuOH oxygen species oxidizes, the CO is adsorbed, similar to the case of platinum poisoning. This releases the Ru active sites for a new activation, with water becoming an adsorbate species that acts as an oxygen supplier [115,117–120]. RuðCOÞads þ RuOH-2Ru þ CO2 þ Hþ þ e

ðVIÞ

On the other hand, the electronic effect is caused by changes of the electronic density by the auxiliary metal in the alloy due to the “d”-band emptying/filling of the catalytic noble metal. This change implies a low CO adsorption energy on the catalytic noble metal sites, releasing these active sites for the oxidation of intermediates during the fuel oxidation process [111,121–123]. In the 1990s, Iwasita–Vielstich’s team used in situ Fourier transform infrared spectroscopy (FTIR) studies to consider the change of energy of the electronic states of the d orbital of platinum caused by cocatalysts, for example, Ru, resulting in a weakening of the Pt–COads bond. This mechanism is widely recognized [124–129]. Numerous alloys have been studied, and a simple change of any parameter of the alloy structure can generate different effects that may be unknown in the literature. Moreover, different chemical elements can be used to form new alloys that are yet to be studied for fuel cell applications. Thus, this topic is yet to be well-established and is still in the formative stage.

10

Nanoparticles

The shape of a nanoparticle, the distribution of alloy atoms and the crystal structure of the unit cell are properties that directly influence the performance of the nanoelectrocatalyst. Therefore, the study of these morphological variations of nanoparticles of a catalyst alloy is very important and determines the state of the art of the synthesis of new nanoelectrocatalysts. The formation of catalytic metal nanoparticles of platinum or palladium is affected by various factors, which modify the size and shape of these nanoparticles. In general, these nanoparticles are produced by nucleation from a solution with a subsequent growth process [130]. In the first stage, the zero-valence metal must be obtained by the reduction of metal ions or bond cleavage of compounds, resulting in metal atoms with a zero valence. Then, these metal atoms collide to produce small clusters that are thermodynamically unstable and can dissolve before they reach a critical radius. If the clusters do not dissolve, they exceed a free critical energy barrier and eventually become thermodynamically stable nuclei. These nuclei should grow into nanoparticles through the consumption of dissolved free atoms or unstable agglomerates [130–134].

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15

Fig. 15 Schematic illustration of the three common types of alloys (a) random, (b) grouped, and (c) ordered forms.

Nanoparticles and their alloys may be controlled by thermodynamic and kinetic factors, which are determined by both the intrinsic structural properties of platinum or palladium and the reaction systems. The reaction conditions can be influenced by a simple modification of the solvent, reducing agent, surfactant, or precursor reagents of the catalytic metals. The metal nanoparticles grow different shapes of crystallographic faces to minimize the surface energy and excess total free energy. Platinum and palladium, which have a face-centered cubic symmetry (FCC), are generally connected by three low-index planes, namely, the {100}, {110}, and {111} surfaces. Among these three, the {111} face has the lowest energy, while the surface of {110} planes has the highest energy in the case of platinum [106–140]. These metals can have different shapes according to the method of preparation. The following structures have already been reported for platinum: spheres, cubes, tetrahedra, octahedra, tetrahexahedra, cuboctahedra, truncated octahedra, triangles, squares, tetragons, hexagons, nanorods, nanowires, nanotubes, necklace structures, core/shell structures, snowflake-like particles, dendrites, porous particles, nanohorns, and hollow structures [111–125]. For palladium, it has been reported that structures with single-seed crystals can develop into octahedra, cubes, or cuboctahedrons, depending on the relative rates of growth along the [111] and [100] Miller indices. In the case of anisotropic growth, cuboctahedral cubic structures can develop into seeds and octagonal stems or rectangular bars. Additionally, other structures are possible because of the existence of multiple twins [126–135]. Alloys are solid solutions of two or more metals. They can be synthesized with a wide range of compositions, yielding different properties for each prepared metal atomic composition. This is an advantage over pure catalytic metals. Metal alloys are commonly classified into three types: random, clustered, ordered, and core–shell [140–145]. A schematic illustration of this alloy classification is shown in Fig. 15. Alloys with long-range atomic order are classified as intermetallic compounds. There have been many advances in the control of composition and structure of metal-based catalysts, and this field is at the forefront of innovations in nanoparticle research. Finally, nanoparticle features, such as shape, crystal structure, composition, distribution, and average particle size can significantly affect the efficiency of the electrocatalyst. Thus, research of the synthesis of nanoparticles is much needed in order to control and improve the production of nanoelectrocatalysts.

11

Methods of Preparation

This section presents a general view of the most common methods of synthesis of noble metal-based electrocatalysts using platinum or palladium. Each method has its advantages and disadvantages, and no method can be established as superior to another. The determination of the preferability of a method depends on the context, including the reagents and available instrumentation, specific applications, and other such factors. In general, there are two ways to obtain nanoparticles. The first begins with a macrostructure, which is broken down to micropieces and subsequently to the nanometric scale. Methods using this approach are called top-down methods. The other category includes bottom-up methods, which begin from atoms (angstrom scale) and grow into nanostructures. Fig. 16 illustrates the two types of processes [146–148]. Bottom-up methods are more commonly used by chemists for chemical synthesis, while top-down methods are more commonly used by physicists and engineers. Bottom-up methods are most commonly used for the preparation of nanoparticles to be applied in fuel cells.Some examples of bottom-up methods include chemical reduction [149,150], chemical precipitation [151,152], sol–gel [153,154], polymer precursors [155,156], hydrothermal [157–159], colloidal [105,160], impregnation [154,161], microemulsions [162,163], green methods [164,165], and polyols [166,167], among many others. Each method has its particular features and differs from the others at some stage of the preparative process. Some provide fine control of temperature and pressure for the formation of nanostructures, as in the case of hydrothermal methods; others use ultrasonic baths for long periods, for example, the polymeric precursor method; yet others employ overnight magnetic stirring, for example, some cases of polyol preparations or long calcination processes. Some methods use acidic media, while others require

16

Fuel Cells: Hydrogen and Ethanol Technologies

Fig. 16 Diagram showing the ways for obtaining nanostructures: bottom-up and top-down methods.

alkaline media. The sequence of addition of metal precursors can also be varied. Many other parameters can be changed, and each affects the final nanostructural properties [120,155,168–170]. Therefore, the choice of preparative method depends on the atomic distribution in the alloy, the morphology desired for a specific support, availability of reagents and instruments, cost of preparation, the electrocatalytic reaction of interest, etc. Thus, it is necessary to examine the most appropriate method of preparation according to the resources available to the researcher and the properties desired in the final synthesized product.

12

News and Perspectives

There have been many recent advances in hydrogen- and ethanol-based technologies for fuel cells. Buses and hybrid cars powered by hydrogen fuel cells are increasingly common [171–174]. This indicates how this has shown greater advances for stationary applications in which more electric charge is required. While ethanol fuel cell technology is a growing field, it focuses on mobile devices that require less electric charge, such as batteries, cellphone chargers and electronic devices [175]. The choice of electrocatalyst is crucial for commercial applications of fuel cells. In this context, complex materials are being studied for electrochemical oxidation reactions; for example, there has been growth in the research of trimetallic alloy nanoparticles in recent years [176]. In addition, the amount of catalytic metal in these nanoparticles has been rapidly decreased in order to lower the cost of the device. The current proposal is to prepare catalysts with less than 0.35 mg/cm2 [177–179]. It has also been indicated that the presence of defects increases the catalytic activity of nanoparticles, even though this behavior has not yet been fully understood [175,176]. The synthesis of nanostructures with three dimensions or multiple dimensions has shown excellent results for electrocatalysis [180,181]. Doping is now performed in Nafion, with the objective of combining properties of other materials with the known properties of Náfion. Nafion is aimed at improving desired properties, usually to decrease ohmic resistance and increase durability. In some cases, materials are synthesized to have specific properties desired for certain applications, for example, Nafion-TiO2 and CNTs doped in Nafion [180–185]. In short, there are still challenges to overcome for the construction of fuel cells using both hydrogen and ethanol. However, studies have shown significant advances, which may enable this technology to dominate applications in electric motors and batteries of electronic devices in the near future [186].

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Further Reading Antolini, E., Perez, J., 2011. The renaissance of unsupported nanostructured catalysts for low-temperature fuel cells: from the size to the shape of metal nanostructures. Journal of Materials Science 46 (13), 4435–4457. Baldocchi, D., Ryu, Y., Keenan, T., 2016. Open peer review terrestrial carbon cycle variability [version 1; referees: 2 approved] F1000 Faculty Reviews 2371-2375. Bang, J.H., Han, K., Skrabalak, S.E., Kim, H., Suslick, K.S., 2007. Porous carbon supports prepared by ultrasonic spray pyrolysis for direct methanol fuel cell electrodes. The Journal of Physical Chemistry C 111 (29), 10959–10964. Cao, L., Scheiba, F., Roth, C., et al., 2006. Novel nanocomposite Pt/RuO2⋅xH2O/carbon nanotube catalysts for direct methanol fuel cells. Angewandte Chemie International Edition 45 (32), 5315–5319. Chen, J., Lim, B., Lee, E.P., Xia, Y., 2009. Shape-controlled synthesis of platinum nanocrystals for catalytic and electrocatalytic applications. Nano Today 4 (1), 81–95. Chen, S., Wei, Z., Guo, L., et al., 2011. Enhanced dispersion and durability of Pt nanoparticles on a thiolated CNT support. Chemical Communications 47 (39), 10984–10986. Chen, X., Li, N., Eckhard, K., et al., 2007. Pulsed electrodeposition of Pt nanoclusters on carbon nanotubes modified carbon materials using diffusion restricting viscous electrolytes. Electrochemistry Communications 9 (6), 1348–1354. Chen, Z., Qiu, X., Lu, B., et al., 2005. Synthesis of hydrous ruthenium oxide supported platinum catalysts for direct methanol fuel cells. Electrochemistry Communications 7 (6), 593–596. Cheong, S., Watt, J.D., Tilley, R.D., 2010. Shape control of platinum and palladium nanoparticles for catalysis. Nanoscale 2 (10), 2045–2053. Chhina, H., Campbell, S., Kesler, O., 2006. An oxidation-resistant indium tin oxide catalyst support for proton exchange membrane fuel cells. Journal of Power Sources 161 (2), 893–900. Cochell, T., Manthiram, A., 2012. Pt@ PdxCuy/C core–shell electrocatalysts for oxygen reduction reaction in fuel cells. Langmuir 28 (2), 1579–1587. Cui, Z., Guo, C.X., Li, C.M., 2013. Self-assembled phosphomolybdic acid–polyaniline–graphene composite-supported efficient catalyst towards methanol oxidation. Journal of Materials Chemistry A 1 (22), 6687–6692. Drillet, J.F., Dittmeyer, R., Jüttner, K., 2007. Activity and long-term stability of PEDOT as Pt catalyst support for the DMFC anode. Journal of Applied Electrochemistry 37 (11), 1219–1226. Feng, C., Chan, P.C., Hsing, I.M., 2007. Catalyzed microelectrode mediated by polypyrrole/Nafion® composite film for microfabricated fuel cell applications. Electrochemistry Communications 9 (1), 89–93. Feng, C., Takeuchi, T., Abdelkareem, M.A., Tsujiguchi, T., Nakagawa, N., 2013. Carbon–CeO2 composite nanofibers as a promising support for a PtRu anode catalyst in a direct methanol fuel cell. Journal of Power Sources 242, 57–64. Formo, E., Peng, Z., Lee, E., et al., 2008. Direct oxidation of methanol on Pt nanostructures supported on electrospun nanofibers of anatase. The Journal of Physical Chemistry C 112 (27), 9970–9975. Gao, H., He, J.B., Wang, Y., Deng, N., 2012. Advantageous combination of solid carbon paste and a conducting polymer film as a support of platinum electrocatalyst for methanol fuel cell. Journal of Power Sources 205, 164–172. Gomes, R.S., De Bortoli, A.L., 2016. A three-dimensional mathematical model for the anode of a direct ethanol fuel cell. Applied Energy 183, 1292–1301. Grigoriev, S.A., Kuleshov, N.V., Grigoriev, A.S., Millet, P., 2015. Electrochemical characterization of a high-temperature proton exchange membrane fuel cell using doped-poly benzimidazole as solid polymer electrolyte. Journal of Fuel Cell Science and Technology 12 (3), 031004. Haque, M.A., Sulong, A.B., Loh, K.S., et al., 2016. Acid doped polybenzimidazoles based membrane electrode assembly for high temperature proton exchange membrane fuel cell: A review. International Journal of Hydrogen Energy. doi:10.1016/j.ijhydene.2016.03.086. He, H., Xiao, P., Zhou, M., et al., 2013. PtNi alloy nanoparticles supported on carbon-doped TiO2 nanotube arrays for photo-assisted methanol oxidation. Electrochimica Acta 88, 782–789. Higgins, D.C., Choi, J.Y., Wu, J., Lopez, A., Chen, Z., 2012. Titanium nitride–carbon nanotube core–shell composites as effective electrocatalyst supports for low temperature fuel cells. Journal of Materials Chemistry 22 (9), 3727–3732. Kim, H.S., Jeon, S.W., Cha, D., Kim, Y., 2016. Numerical analysis of a high-temperature proton exchange membrane fuel cell under humidified operation with stepwise reactant supply. International Journal of Hydrogen Energy 41 (31), 13657–13665. Lee, E., Manthiram, A., 2010. One-step reverse microemulsion synthesis of Pt−CeO2/C catalysts with improved nanomorphology and their effect on methanol electrooxidation reaction. The Journal of Physical Chemistry C 114 (49), 21833–21839. Lee, E.P., Peng, Z., Cate, D.M., et al., 2007. Growing Pt nanowires as a densely packed array on metal gauze. Journal of the American Chemical Society 129 (35), 10634–10635. Li, W., Liang, C., Zhou, W., et al., 2003. Preparation and characterization of multiwalled carbon nanotube-supported platinum for cathode catalysts of direct methanol fuel cells. The Journal of Physical Chemistry B 107 (26), 6292–6299. Liu, R., Zhou, H., Liu, J., et al., 2013. Preparation of Pd/MnO2-reduced graphene oxide nanocomposite for methanol electro-oxidation in alkaline media. Electrochemistry Communications 26, 63–66. Mallikarjuna, N.N., Varma, R.S., 2007. Microwave-assisted shape-controlled bulk synthesis of noble nanocrystals and their catalytic properties. Crystal Growth & Design 7 (4), 686–690. ́ ́ Martınez-Rodrı guez, R.A., Vidal-Iglesias, F.J., Solla-Gullón, J., Cabrera, C.R., Feliu, J.M., 2014. Synthesis of Pt nanoparticles in water-in-oil microemulsion: Effect of HCl on their surface structure. 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