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Direct and Indirect Borohydride Fuel Cells
Direct and Indirect Borohydride Fuel Cells C Ponce de Leon and FC Walsh, University of Southampton, Southampton, UK ã 2015 Elsevier Inc. All rights reserved.
Introduction Chemical Hydrides Direct and Indirect Borohydride Fuel Cells Catalytic Materials Electrode Materials for DBFCs High-surface-area electrodes Hydrogen storage alloys Catalytic Materials for IBFC Mechanism of Borohydride Oxidation Direct Oxidation Indirect Oxidation Rate of hydrogen generation Fuel Cell Configurations Direct Oxidation Borohydride Crossover Electrolyte Parameters Direct Oxidation of Borohydride Temperature Indirect Borohydride Oxidation Fuel Cell Performance Direct Borohydride Oxidation Hydrogen peroxide as oxidant in the DBFC Indirect Oxidation Commercial Ventures and Future Developments Challenges Summary References
1 2 2 3 3 3 3 4 6 6 7 7 8 8 9 10 10 10 10 10 10 14 14 15 15 15 15
Introduction Transportation, storage, and distribution of hydrogen gas remain a matter of concern when compared with traditional energy carriers such as natural gas and gasoline, which can be handled relatively safely with the present distribution infrastructure. Considerable effort has been made to study hydrogen storage technologies to achieve similar or higher energy density as gasoline or diesel for transportation purposes. The storage technologies include high-pressure vessels made of steel or reinforced composites, metal hydrides, metal alloys, activated carbon, graphite nanofibers, and nanotubular materials such as carbon and titanium. The specific energy density of these systems is still insufficient for portable applications and has led to the research for alternative fuels. Ideally, the fuel should be safe and easily transported at high concentrations, should oxidize fast, and should provide large gravimetric and volumetric energy densities. Extensive reviews of hydrogen–oxygen fuel cells have been published over the last decade, and some commercial teaching systems have been on the market over the last decade. Despite the borohydride fuel cell being studied since 1960, scientific papers have tended to be limited to a few electrode materials and the use of borohydride as a hydrogen source for proton exchange membrane fuel cells (PEMFCs). More recently, direct borohydride fuel cells (DBFCs) have been attracting more interest, and published work includes a comparison of different anodes, cathodes, and membranes; cost evaluation; borohydride crossover; and the effect of the borohydride hydrolysis. Although early studies in 1965 pointed out the potential of borohydride as a fuel, there were few publications before 2000. Since then, rapid progress has been achieved in both DBFCs and indirect borohydride fuel cells (IDBFCs), and between 2007 and 2012, nearly 60 papers per year have been published in aspects related to the DBFC. Contemporary scientific papers and reviews also include improvements in cell design and the development of mathematical models. An air-breathing cathode can be used to couple with the direct oxidation of borohydride; this type of electrode has been well characterized in the literature and will not be considered here. Another common oxidant used to complete the borohydride fuel cell is hydrogen peroxide, which can be used in specialized applications such as underwater vehicles and under anaerobic conditions.
Reference Module in Chemistry, Molecular Sciences and Chemical Engineering
http://dx.doi.org/10.1016/B978-0-12-409547-2.11190-4
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Direct and Indirect Borohydride Fuel Cells
Table 1
Theoretical hydrogen yield, specific energy capacity, and maximum percentage of reactant deliverable as hydrogen for various hydrides
Reaction
H2 yield (wt%)
Specific energy capacity of hydride (kW h kg1)
Maximum % of reactant deliverable as H2
LiBH4 + 4H2O ! LiOH + H3BO3 + 4H2 LiH + H2O ! LiOH + H2 NaBH4 + 4H2O ! NaOH + H3BO3 + 4H2 LiAlH4 + 4H2O ! LiOH + Al(OH)3 + 4H2 NaAlH4 + 4H2O ! NaOH + Al(OH)3 + 4H2 CaH2 + 2H2O ! Ca(OH)2 + 2H2 NaH + H2O ! NaOH + H2
8.6 7.7 7.3 7.3 6.4
1.63 1.46–4.73 (Eickhoff) 1.38 1.38–3.95 1.21–2.78 (Eickhoff)
37.0 25.4 21.3 21.2 14.8
5.2 4.8
0.99–1.78 (Eickhoff) 0.91
9.6 8.4
Chemical Hydrides The substitution of hydrogen storage systems mentioned earlier by the in situ generation of hydrogen using a reformer reactor or by hydrolysis of chemical hydrides such as LiNH4, LiH, NaH, NaAlH4, and NaBH4 has also been considered. Chemical hydrides could release hydrogen gas at room temperature and can have storage density up to two to five times more than steel cylinders. Table 1 shows some typical hydride reactions and the percentage of hydrogen deliverable and energy capacity; lithium salts show the highest energy capacity and hydrogen yield, while sodium borohydride has the advantage of been less toxic than lithium with a reasonable energy capacity and hydrogen yield. Other hydride salts contain toxic lithium or have lower hydrogen capacity in comparison with sodium borohydride. On-demand generation of hydrogen using chemical hydrides requires an additional reaction tank containing a catalyst for the hydrolysis reaction and might be unsuitable for portable applications of fuel cells. For this reason, over the last few years, the direct oxidation of hydrides in particular sodium borohydride has been the focus of intense academic research. Sodium borohydride can be shipped as a white solid or as 30% aqueous solution and can be handled in air. Its applications include the manufacture of low tonnage organic compounds, a source of high-purity hydrogen, chemical reduction in the pulp and paper industry, waste treatment, and electroless nickel plating baths.
Direct and Indirect Borohydride Fuel Cells The direct oxidation of borohydride ions coupled with the reduction of oxygen has an equilibrium potential of approximately 1.64 V according to the following reactions: Anode : BH + 8OH 8e ! BO2 + 6H2 O, E ¼ 1:24 V vs: SHE
[1]
Cathode : O2 + 2H2 O + 4e ! 4OH , E ¼ 0:4V vs: SHE
[2]
Cell : BH4 + 2O2 ! BO2 + 2H2 O, Ecell ¼ 1:64V vs: SHE
[3]
The metaborate, BO2 product, can be recycled to borohydride ion and is environmentally friendly. The cell potential of the DBFC reaction [3] leads to a theoretical energy density of 9.3 kW h kg1, which is larger than methanol (6.2 kW h kg1) and hydrogen at 4500 psi (0.45 kW h kg1). The energy density in solution will be lower despite the ability to prepare up to 30 wt% borohydride (>6 mol dm3) solution in concentrated alkaline aqueous electrolytes. The half-life of sodium borohydride is 426 days when stored at pH 14 and 298 K under a nitrogen atmosphere. The high concentration of alkaline is necessary to avoid borohydride hydrolysis, which in neutral solution and in the presence of platinum or cobalt(II) chloride, catalyst can be written as follows: BH4 + 2H2 O ! BO2 + 4H2
[4]
Reaction [4] provides the basis for the IBFCs; borohydride supplies the hydrogen gas that then reacts with oxygen in a PEMFC to produce water, heat, and electrical energy. This is the typical hydrogen–oxygen fuel cell reaction, widely studied over the past 50 years by many academic and industrial research groups: 2H2 + O2 ! 2H2 O, Ecell ¼ 1:18V vs: SHE
[5]
The hydrogen reactant for this fuel cell can be obtained from hydrogen generators consisting of a reactor containing borohydride in presence of a catalyst to carry out reaction [4]. Another technology for hydrogen production, the catalytic steam natural gas reformation, produces a mixture of hydrogen, carbon monoxide, carbon dioxide, water, and sulfur compounds of low molecular mass. This mixture is unacceptable for PEMFC operation, where catalytic metals can become poisoned by small amounts of carbon monoxide (<0.1%) and sulfur. Hydrogen needs to be purified, which increases the number of reactors used. Electrolysis is another common technology to produce high-purity hydrogen; however, technical improvements are required to reduce inefficiencies and cost. Other proposals include using the electricity generated from renewable energy sources such as solar and wind for the electrolysis of water and generation of hydrogen. The reliability of the process depends on geographic location and climate.
Direct and Indirect Borohydride Fuel Cells
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In the DBFC, the borohydride hydrolysis [4] must be suppressed in order to take advantage of the high number of electrons interchanged during the oxidation, whereas in the IBFC, the generation of hydrogen must be enhanced. In the following sections, the characteristics of these two systems will be briefly considered.
Catalytic Materials Electrode Materials for DBFCs The electrochemical oxidation of borohydride was considered for fuel cells applications in the 1960s using porous nickel and palladium anodes. Nickel boride was prepared by precipitating nickel acetate with KBH4 on the pores of a nickel plaque; nickel boride was found to be significantly better than other metals. Other early studies of the oxidation of borohydride were carried out on mercury electrodes. Due to the environmental hazards associated with mercury, it is unlikely that this metal will see any continued use as an anode in borohydride fuel cells. Other catalyst materials studied during the 1960s for borohydride oxidation in strong alkali solutions were nickel, platinum, palladium, and gold. High open-circuit cell potentials and current densities of 0.1 A cm2 indicated the possibility of achieving reasonable energy and power density outputs, but the number of electrons transferred was low (4 at Ni and <4 at Pt). Other works studied high-surface-area electrodes such as Pt and Pd on Ni and C substrates. The oxidation of borohydride yielded six electrons at platinum deposited on a sintered porous Ni plate at 50–200 mA cm2. Another study provided clear evidence of changing boron speciation with time since a new oxidation wave appeared at more negative potentials when the borohydride concentration was low (0.2 mol dm3) in NaOH, at gold electrodes. Direct oxidation of borohydride requires selective anodes with low catalytic activity for the decomposition of borohydride to hydrogen and borates via reaction [4]. More recently, several electrode materials for the direct oxidation of borohydride have been evaluated such as Ni2B and Pd–Ni, Au, colloidal Au and Au alloys with Pt and Pd, MnO2, misch metals, AB5-type hydrogen storage alloys, Ni, Raney Ni, Cu, and colloidal Os and Os alloys. So far, only gold electrodes seem to realize the eight-electron transfer expected from reaction [1]. Less than eight electrons indicate partial oxidation or high rates of the hydrolysis of borohydride through reaction [4]. Figure 1(a) shows the cell potential of different electrode materials in a borohydride fuel cell versus the current density, while Figure 1(b) shows the change of the power density with the current density. Nickel electrodes show higher performance than Au/Pt, Pt inks, and MnO2 electrodes. A detailed investigation of the oxidation of borohydride has been carried out on gold ultra-microelectrodes. Such studies concluded that eight electrons were transferred during the oxidation of borohydride and that the reaction became mass transport-controlled at high potentials. Other studies at gold and platinum electrodes show that the number of electrons transferred is low on platinum but the use of additives such as thiourea could inhibit the evolution of hydrogen and have a positive effect on performance. A rather negative open-circuit voltage can be obtained on nickel electrodes, and acceptable overpotentials at current densities of several hundred mA cm2 are observed, especially at elevated temperatures. The number of electrons remains low at four, indicating that only half the theoretical energy value could be obtained with nickel anodes.
High-surface-area electrodes The preparation of high-surface-area electrodes should include consideration of the influences of catalyst loading and Nafion® content. High-surface-area electrodes containing gold or gold alloy (Au 97% + Pt 3%) catalyst have been used for the oxidation of borohydride. The number of electrons transferred at 0.2 mA cm2 and 343 K approaches 7. Platinum high-surface-area electrodes could achieve current densities in the range of 10–60 mA cm2 with only 5–5.5 electrons transferred. Comparison of six different high-surface-area electrodes has shown that Pd/C and Pt/C are significantly better than nickel in terms of fuel efficiency. Palladium, platinum, and nickel on carbon show current densities up to 800 mA cm2, but the overpotential at the nickel electrode is lower than at palladium or platinum. Table 2 shows a comparison of different anode materials for borohydride oxidation. Gold- or nickel-based materials are the best choices for the oxidation of borohydride; nickel gives the most negative potential for borohydride oxidation but involves a low number of transferred electrons, while gold requires a higher overpotential but gives a better fuel efficiency. Further research work on improved electrocatalytic materials for BH4 oxidation is clearly needed.
Hydrogen storage alloys Anode materials able to store hydrogen have been used in borohydride fuel cells. The role of the borohydride ion is to provide the hydrogen within the lattice of the anode alloy to be stored. The stored hydrogen can then react with oxygen in the fuel cell. Some of the alloys that have been reported include ZrCr0.8Ni1.2; LmNi4.78Mn0.22, where Lm is a lanthanum-rich misch metal; and Zr0.9Ti0.1Mn0.6V0.2Co0.1Ni1.1. These electrodes produce current densities up to 300 mA cm2 at 0.7 V versus SHE, but the efficiency to release the energy from borohydride does not exceed 50% (i.e., z ¼ 4). Other works report that the performance of the borohydride fuel cells is a complex function of the operating conditions and that the efficiency of fuel utilization increases at higher current densities. A two-component alloy catalyst electrode can provide dual functionality: One of the metals in the alloy serves to catalyze hydrolysis of borohydride, while the other metal oxidizes the generated hydrogen. In theory, this combination avoids hydrogen evolution in the cell.
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Direct and Indirect Borohydride Fuel Cells
Figure 1 (a) Cell potential versus current density and (b) power density versus current density for various electrodes in a borohydride fuel cell: (•) commercial air cathode (Johnson Matthey) and anode made of highly dispersed gold/platinum particles supported on high-surface-area carbon silk separated by 2259–60 Pall anion membrane with 5% NaBH4 in 25% NaOH1; ○ Ni + PTFE powders pressed on a Ni foam as anode and air cathode made of Pt or Ag supported on carbon black separated by an NRE211 Nafion membrane, 5% NaBH4 in 6 mol dm3 NaOH2; (■) cathode and anode inks prepared using unsupported Pt black and Nafion, 0.5 mol dm3 NaBH4 in 6 mol dm3 NaOH3; (□) manganese dioxide cathode and standard Pt/Ni anode (Electro-Chem-Technic, the United Kingdom) with 1 mol dm3 NaBH4 in 3 mol dm3 KOH with no membrane4; (r) Anode, MmNi3.35Co0.75Mn0.4Al0.3; cathode, 2 mg cm2 MnO2 in an electrolyte consisted of 0.4 g of KBH4 in 200 cm3 of 6 M KOH.5 All experiments reported at 23 C temperature.
Catalytic Materials for IBFC The generation of hydrogen from the hydrolysis of sodium borohydride has received increasing attention, and many catalysts to improve the hydrolysis reaction rate have been identified. These include dispersed Ru metal on ion exchange beads, dispersed Pt on an oxide or carbon supports, and high-area nickel materials. In the IDBFCs, the concentrated borohydride solution in aqueous alkali is converted to hydrogen gas in a catalytic generator, and the hydrogen gas is then fed to a PEMFC or other hydrogen-fueled
Direct and Indirect Borohydride Fuel Cells
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cell. Such systems have been described for applications as varied as automobile traction and microfabricated fuel cells to power electronic circuits. The hydrolysis of borohydride in aqueous solutions follows zero-order kinetics, when an aqueous solution of sodium borohydride is pumped through a catalyst bed: Catalyst
NaBH4 + 2H2 O ! 4H2 + NaBO2 + heat
[6]
A schematic diagram of the hydrogen generator system is shown in Figure 2. In alkaline solutions, the reaction without the catalytic metal is very slow. The advantages of the in situ hydrogen generation for the PEM cells are as follows: 50% of the hydrogen generated is provided by the water in the electrolyte. Platinum anodes can be used in the PEMFC since only high-purity hydrogen is supplied. Control supply of humidified H2 gas. Continuous generation of H2 by addition of NaBH4 into the hydrolysis reactor. Water recirculation from the fuel cell back to the hydrolysis reactor.
Table 2
Summary of electrode materials performance for the oxidation of borohydride Comment
Anode material
z value
Open-circuit potential at the working electrode versus SHE (V)
Ni Raney Ni Cu Au Pt Dispersed Pd on Ni Dispersed Pt on Ni Dispersed Au on Ni Ni2B H2 storage alloys MmNi3.35Co0.75Mn0.4Al0.3 hydrogen storage alloy
4, 4, 4 NA NA 8, 7 2–4 6 5, 6 NA NA 4 7.5
1.03 1.03 1.02 0.99 1.0 1.00, 0.91 0.91 0.99 1.07 1.15 1.12
Current density (A cm2) 0.6 NA >0.1 possible 0.7 0.1 <0.1 NA 0.1 0.3 0.1
z is the number of electrons transferred during the oxidation reaction. NA ¼ not given in the reference.
Figure 2 Schematic diagram of a system for hydrogen generation from sodium borohydride solution.
Cell potential (V)
References
0.7 0.6 NA NA 0.6 0.92
6–8 8 8 1,8,9 Elder (1962),3 8,10 3,8 8 10 11, Wang (2005) 5
NA 0.99 0.7 0.58
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Direct and Indirect Borohydride Fuel Cells
Some challenges for the development of this system are (a) a cheap catalyst with high conversion and optimum reaction rate, (b) a catalyst tolerant to the borate product, (c) water reuse, and (d) optimal connection to the PEMFC and optimal reactor design. One of the most common catalysts for hydrogen generation from borohydride is Co and Co–P electrodeposited on copper. The catalytic activity of amorphous Co–P deposit can be up to 950 ml min1 g1 of catalyst, 18 times larger than the polycrystalline Co in 1 wt% NaOH + 10 wt% NaBH4 solution at 203 K. Unlike carbon reformers, hydrogen generators from borohydride solutions do not need to be preheated; the reactor is started with a flow rate of borohydride solution of 20 g min1 at 483 kPa at room temperature. The solubility of metaborate is a problem in the reactor since it causes blocking of the active sites of the catalysts and clogging; additional work in heat and water management is required. Table 3 shows a comparison of the hydrogen evolution obtained using different catalysts and conditions; platinum appears to yield the highest hydrogen evolution rate, while Co on Cu gives the lowest one.
Mechanism of Borohydride Oxidation Direct Oxidation Two mechanisms are generally recognized: Mechanism (A): the first step is an electron transfer to form the BH4 radical: BH4 e ! BH4
[7]
The second step involves the decomposition of the radical into BH3 and water, followed by further electron transfer to form diborane, B2H6, which undergoes further electron transfer. Mechanism (B): the first step is the electron transfer, reaction [7], followed by predissociation at surface sites of the catalyst: 2M + BH4 ! M H + M BH3
[8]
This is followed by a series of steps that involve electron transfer, surface reaction, and hydrogen atom combination to produce hydrogen gas as a secondary reaction. Metallic surfaces able to support predissociation are commonly good catalysts, and the adsorption of hydrogen atoms leads normally to the formation of hydrogen gas. Since gold does not support coverage of adsorbed hydrogen atoms, the first type of mechanism would be expected. At nickel, platinum, and palladium, hydrogen adsorbs and the second type of mechanism would predominate. It is likely that the reaction M + H2 O + e ! M H + OH
Table 3
[9]
Comparison of different catalysts for the hydrolysis of NaBH4 Conditions
Hydrogen generation rate (l min1)
Catalyst
NaBH4
NaOH
T
References
44.9 2449.6 1780 394.7 837.5 123.6 38.1 67.2 22.9 8.5 0.4 424.7 9.7 63.0 4.9 17.3 285.0 9.1 234.0 114.2 133.7 324.1
5% Ru on IRA 400 membrane 100 mg of 20% Pt/VULCAN XC-72R 100 mg of 10% Pt/VULCAN XC-72R 100 mg of 5% Pt/VULCAN XC-72R Pt–LiCoO2 50 mg Pt–LiCoO2 256 mg Ru on IRA 400 resin, 256 mg 0.3 NixB cat heat-treated at 150 C in vacuum 0.3 NixB cat heat-treated at 80 C in air Co–P electroplated on Cu at 0.01 A cm2 Co electroplated on Cu at 0.01 A cm2 1.4 mM water-dispersed Ru(0) nanoclusters Ni powder (0.5–1 mm) Co powder (1–2 mm) NiCl2 6H2O (granules) NiF2 4H2O (granules) CoCl2 6H2O (granules) Ni2B Co2B Raney Ni Raney Co Raney Ni50Co50
1% 10 wt% 10 wt% 10 wt% 20% 20%
1% 5 wt% 5 wt% 5 wt% 10% 10%
1.5 wt%? 1:5 wt% 10 wt% 10 wt% 0.15 mol dm3 0.2 g 0.2 g 0.2 g 0.2 g 0.2 g 0.2 g 0.2 g 1g 1g 1g
1 mol dm3 1 mol dm3 1 wt% 1 wt%
298 K – – – – – 298 K – – 303 K 303 K 298 K 293 K 293 K 293 K 323 K 283 K 293 K 293 K 293 K 293 K 293 K
12 13 13 13 14 14 14 15 15 16 16 17 18 18 18 18 18 18 18 18 18 18
20 ml at 10 wt% 20 ml at 10 wt% 20 ml 10 wt% 20 ml 10 wt% 20 ml 10 wt% 20 ml 10 wt% 20 ml 10 wt% 100 ml 10 wt% 100 ml 10 wt% 100 ml 10 wt%
Direct and Indirect Borohydride Fuel Cells
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will occur at the open-circuit potential for borohydride oxidation since the formal potential of borohydride oxidation, reaction [1], is negative to the formal potential of the H2O/H2 couple. As a result, a mixed potential would be observed at good catalytic materials for hydrogen evolution. Taking this into consideration, the estimation of the hydrogen evolution rate as a way to measure the rate of borohydride oxidation becomes more difficult to estimate due to the additional hydrogen evolution reaction [9]. The stability of borohydride restricts the electrolyte to strong alkaline solutions and limits the anode materials to those with low activity for the hydrolysis. A specific material for the direct oxidation of borohydride, reaction [1], should be used, but since most catalytic materials also catalyze its hydrolysis [4], the two reactions are likely to occur at the same time. The electrode potential will acquire a value that corresponds to a mixed potential between the two reactions and will depend on the anode material and temperature. In addition, the reduction of water to hydrogen, which consumes two electrons and is thermodynamically favorable at the potentials of the oxidation of borohydride, reaction [1], could restrict the eight-electron transfer involved in the oxidation of borohydride: 2H2 O + 2e ! H2 + 2OH
[10]
Furthermore, the theoretical number of electrons transferred by the oxidation of borohydride probably decreases due to the existence of partially oxidized species of borohydride and the hydrogen evolution in most electroactive materials. One main oxidation peak is observed in the cyclic voltammogram of BH4 ions at pH 9 followed by a small wave caused by the oxidation BH3OH, which was formed during the partial hydrolysis reaction of BH4 ions: BH4 + H2 O ! BH3 OH + 0:5H2
[11]
The BH3OH ion will have the effect of increasing the open-circuit potential since it is more readily oxidized than BH4 .
Indirect Oxidation The heterogeneous oxidation of borohydride for the generation of hydrogen from alkaline solutions is preferred over the homogenous acid-catalyzed hydrolysis because it is not pH-dependent and the catalyst can be reused. Catalyst metals such as Cu, Co, Ru, Pt, and Ni are generally used, and there is a general agreement that the production of hydrogen is catalyzed by the metal due to the fast proton production during their reduction by BH4 ions. Works on the investigation mechanism of hydrogen production from alkaline sodium borohydride show that the reaction kinetics is of first order when the molar ratio ½BH4 =½Pd is 0.03–0.11 in well-stirred solutions. The experiments used 2.7–10 wt% Pd/C powder catalyst in deuterated sodium borohydride, and the reaction kinetics was followed by nuclear magnetic resonance. The rate constant at a Pd/C loading of 5.5 wt% is 2.4 104 s1, while in the absence of catalyst at pH 13, it is 2.3 107 s1. Other works report that the rate of hydrogen generation during the hydrolysis reaction on carbon-supported ruthenium catalyst is directly proportional to the concentration of NaBH4 (first order). If the amount of catalyst remains constant, the rate of hydrogen generation depended on temperature, NaOH concentration, and the concentration of the metaborate product, NaBO2, which builds up in the reaction vessel and depresses the generation of hydrogen significantly. In other studies, the generation of hydrogen was carried out on 2 wt% Ru/alumina pellets, 3 wt% Ru/carbon extruded, and 0.5 wt% Ru/carbon granules. Most catalysts disintegrated during the experiments due to the evolution of hydrogen except for the Ru/C extruded catalyst. The hydrolysis reaction is assumed to occur in two steps; first, the BH4 ion adsorbs on the surface of the catalyst: M + BH4 ! M BH4
[12]
and second, the adsorbed species react to form adsorbed hydrogen: M BH4 + 2H2 O ! MBO2 + 4H2 M
[13]
The experimental data show that the kinetics of the hydrolysis reaction is of zero order at low temperatures (298 K), whereas first-order kinetics fit the experimental data at higher temperatures (383 K). It should be highlighted the importance of water management for the effective operation of a hydrogen generator system. Other works show that the catalytic generation of hydrogen decreases when the concentration of sodium borohydride increases. A proposed expression for the catalytic oxidation of sodium borohydride is (Zhang, 2007)
d½NaBH4 ¼ k ½NaBH4 0:41 ½NaOH0:13 ½H2 O 0:68 wdt
[14]
where w is the mass of sodium borohydride and k is the rate constant. The equation describes the dependence of hydrogen generation on the concentrations of sodium borohydride, sodium hydroxide, and water.
Rate of hydrogen generation The theoretical hydrogen content of borohydride is 10.9 wt%, and, under ideal conditions, 0.213 g of hydrogen can be obtained per gram of NaBH4. At room temperature in water, however, only a fraction of this amount is released. The amount decreases with time due to the stabilization of the borohydride by the increasing pH of the solution. The rate of hydrolysis of sodium borohydride increases when catalytic active materials, acids, are used or by raising the temperature. The use of heterogeneous catalysts at
8
Direct and Indirect Borohydride Fuel Cells
different temperatures is the most common method to increase the generation of hydrogen gas; however, the available surface area of the catalyst is the limitation. Recently, a new method using ruthenium metal nanoparticles in suspension has been reported. The ruthenium nanoparticles are highly active and show the lowest activation energy for borohydride hydrolysis of 28.51 kJ mol1 in comparison with 75 kJ mol1 for Co, 71 kJ mol1 for Ni, and 63 kJ mol1 for Raney Ni. Kojima et al. (2005) investigated the hydrolysis of NaBH4 in various metal oxides covered with Pt. They found that Pt–LiCoO2 catalyst worked as an excellent H2 generator from NaBH4 solutions. The catalyst was prepared from a honeycomb cordierite monolith, which was immersed in slurry solution containing 1000 g of Pt–LiCoO2, 620 g of 20% Al2O3, and 125 g of H2O and was dried for 24 h at 393 K followed by 3 h calcination at 720 K. The final concentration of Pt in the monolith was 1.5 wt%.
Fuel Cell Configurations Direct Oxidation Most borohydride fuel cells consist of two electrodes separated by a membrane. Both cationic and anionic membranes have been used, and each produces different chemical characteristics within the cell. Cation membranes lead to a chemical imbalance; the oxidation of 1 mol BH4 transfers 8 mol Na+ ions across the membrane, increasing the concentration of NaOH in the catholyte and losing NaOH from the anolyte. Long operations test could cause problems because BH4 ions are only stable in strong alkali concentration. A procedure to return sodium hydroxide from catholyte to anolyte should be implemented. In contrast, the operation with an anion membrane transfers 8 mol of OH from catholyte to anolyte across the membrane for each mole of borohydride oxidized. The chemistry using this membrane is in balance and only borohydride should be replenished. Figure 3 shows sketches of the cell with each type of membrane from a cell with sodium electrolytes. It is assumed that the electrode reaction [1] is occurring without the competing hydrolysis of borohydride, and chemical changes during energy generation are emphasized, even with idealized chemistry. Cation-permeable membranes are commercially available, and the perfluorinated types are very stable in contact with strong reducing agents and concentrated alkali solutions. Nafion® 1100 EW membranes fabricated by DuPont are the most common type. In contrast, the majority of the anion-permeable membranes are unstable in alkali. The perfluorinated anion membrane Tosoh® is
Figure 3 Direct borohydride fuel cells with (a) a cation-permeable membrane and (b) an anion-permeable membrane, drawn to emphasize the chemical balance.
Direct and Indirect Borohydride Fuel Cells
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Figure 4 Comparison of cell potential and power density versus current density for a borohydride fuel cell using three different membranes: cationic (○) Nafion 115 and anionic (□) Asahikasei A-501SB membrane at 25 C. Ni + PTFE powders pressed on Ni foam as anode and air cathode made of Pt or Ag supported on carbon black.2 (▲) CU1 anionic membrane, area 4 cm2; anode, 2 mg cm2 Au/C; cathode, 2 mg cm2 Pt/C. Cell: parallel flow field with 1.32 M NaBH4 in 2.5 M NaOH at 10 cm3 min1. O2 at 200 cm3 min1 at 25 C.19
no longer manufactured because of the very high cost of production. At the present time, the commercially available anion-permeable membranes have not been tested long enough in strong alkali electrolytes, and many are in the development stage. There has been substantial R&D targeted toward improving the stability of membrane polymers to alkali, but, currently, there are no anion membranes on the market stable to hydroxide concentrations above 5%, and higher concentration is required to ensure the stability of sodium borohydride solution. A comparison of a cationic membrane, Nafion N115, and two anionic membranes, Asahikasei A-501SB and CU1, on a borohydride fuel cell is shown in Figure 4 in terms of cell potential and power density versus current density. The thickness of the membranes is approximately 130 mm. The open-circuit potential of the systems is similar, but the fuel cell operated with the cationic membrane presents higher cell potential than the systems operating with the anionic membrane. Power densities are also higher for the system operating with the cationic membrane. Increased ohmic and interfacial resistance between the cathode and the membrane caused by water deficiency in the catholyte leads to the reduction of power density in the system operated with the anionic membrane. Ionic membranes are expensive, and the ion selectivity is not 100%; the transport of other ions causes serious complications in the operation of the cell. A system with no membrane would be ideal and will make the cell design easier and simple. Unfortunately, membranes are necessary and help to avoid interactions between reactant and products at both electrodes. Nevertheless, an undivided borohydride–oxygen fuel cell might be possible since both reactants do not react in homogeneous solution. This will require an inactive cathode catalyst toward borohydride ions and its hydrolysis. The fuel cell chemistry would be as if the system has an anionic membrane. Undivided borohydride fuel cells with air-breathing MnO2 cathode catalyst, which shows no reaction with borohydride, and a dispersed gold catalyst anode, have been described in the literature. The cell potential approaches 0.6 V at 1–5 mA cm2 current densities when operated with 1 mol dm3 KBH4 in 6 mol dm3 KOH electrolyte. Despite the low current density, discharge curves showed that the number of electrons interchanged z is 7.4, and it appeared that the cell would be able to deliver higher current densities. Other works report the operation of a borohydride fuel cell with no membrane with MnO2/C/Ni cathode and Pt black on a Ni mesh anode. Current densities of 35 mA cm2 at a cell potential of 0.8 V in 1 mol dm3 NaBH4 and 5 mol dm3 NaOH have been found. The compact design of these membraneless cells reduces cost and size and improves the power density.
Borohydride Crossover The large concentration of BH4 ions in the anolyte causes strong concentration gradient and the crossover of borohydride ions across the membrane, particularly in anion membranes at open circuit. Chatenet et al.20 studied the reduction of oxygen on carbon-supported platinum, gold, silver, and manganese oxide using oxygen-saturated solutions containing traces of borohydride ions. Their objective was to simulate the conditions at which oxygen would reduce in the fuel cell when crossover of borohydride
10
Direct and Indirect Borohydride Fuel Cells
ions across the membrane occurs. The results indicate that the open-circuit potential of the Au/C and Ag/C cathodes in the presence of 0.01 mol dm3 borohydride falls below 0.3 V versus NHE, while the Pt/C electrode disaggregates due to the hydrogen evolution from the hydrolysis of borohydride and its open-circuit potential falls to 0.8 V versus NHE ( 0.2 V vs. NHE without BH4 ). This potential reflects the mixed potential value of the cathodes that are prone to catalyze the oxidation of borohydride ions rather than reduce oxygen. In the case of manganese oxide electrode, the open-circuit potential changed from 0.075 to 0.05 V versus NHE in presence of borohydride ions, reflecting the selectivity of this electrode to preferentially reduce oxygen. The crossover of borohydride is also due to the potential difference created between the two electrodes, and the current density plays an important role. The potential drop created by the spontaneous electron transfer on the two electrodes determines the direction of the electrical field that drives ion movement by migration. If an anion membrane is used, the borohydride ions are dissuaded from migrating to the catholyte compartment, and the electrical field during operation avoids their migration from anolyte to catholyte; the transport of borohydride ions to the catholyte is low and decreases as the current density increases. At open-circuit conditions, borohydride ions will tend to migrate to the catholyte through the anion-permeable membrane. Hydroxide ions should move in the opposite direction to keep both electrolytes charge-balanced, and if hydroxide is more concentrated in the catholyte, this will be favorable. Lakeman et al.21 analyzed the crossover of borohydride ions across different membranes using a four-electrode cell with gold gauzes as cathode and anode. The study was carried out using 30 wt% NaBH4 in 6 mol dm3 NaOH in one side of the membrane and 30 wt% NaOH in the other. The crossover measurements were taken during 1 h when the electrodes were polarized between 1200 and 200 mV versus SHE at intermittent scans. An anionic membrane identified as 3541P (manufactured at Cranfield University, the United Kingdom) showed the highest crossover value of 4.6 106 mol cm2 s1, while the lowest borohydride transport was observed in a Nafion 117 membrane as 0.4 106 mol cm2 s1. The conclusion was that thicker membranes are more effective in slowing down the migration of borohydride.
Electrolyte Parameters Direct Oxidation of Borohydride The borohydride fuel cell can be operated with sodium or potassium hydroxide electrolytes within the range of 10–40 wt% concentration containing a borohydride salt between 10 and 30 wt%. Although the sodium compounds are cheaper and lighter, ionic membranes work better with potassium salts, which are more conductive. Cationic membranes in their potassium form are more selective to unwanted ions and are significantly less hydrated than membranes in contact with sodium salts. The physical properties of borohydride solutions such as specific gravity, viscosity, and melting point as a function of borohydride and hydroxide ion concentrations are reported in the literature. These investigations report the open-circuit potential, polarization curves, and migration rates of borohydride ions across the membrane as well as the number of electrons transferred in the borohydride oxidation reaction. The investigations indicated that the open-circuit potential at a Zn–Ni alloy anode electrode is not suitable when the concentration of borohydride ion exceeds 5 wt%.
Temperature Unlike methanol fuel cells that operate at between 343 and 373 K, the borohydride fuel cells provide similar energy levels at room temperature 298 K. The energy and cell potentials of the DBFC are higher at elevated temperatures ( 373 K), but the stability of the solutions decreases and less fuel would be available. A clear indication of the better performance of the borohydride fuel cell is presented in Figure 5 in the form of cell potential (5a) and power density (5b) at different temperatures. The highest cell potential and power density occur at 258 K.
Indirect Borohydride Oxidation The hydrolysis of borohydride to produce hydrogen gas is normally carried out in aqueous sodium hydroxide, potassium hydroxide, or neutral solutions. The efficiency of hydrogen generation increases from 92% up to 99% as the concentration of alkali is increased and is also affected by the concentration of sodium borohydride, which is the most common source of hydrogen employed. Other aspects of the generation of hydrogen from NaBH4 include reuse of the catalyst and use of ethylene glycol instead of aqueous solution to dissolve NaBH4, This reduces the temperature of the hydrolysis and allows the regeneration of the reaction products back to borohydride in one step.
Fuel Cell Performance Direct Borohydride Oxidation A comparison between two borohydride fuel cells operating at 60 and 85 C with two methanol–air fuel cells operating at 90 and 110 C is shown in Figure 6(a). The open-circuit value for methanol and borohydride fuel cells is 0.83 and 1.24 V, respectively. The IR drop in the borohydride fuel cells is the dominant effect obscuring the activation polarization overpotential on both cells, while in the methanol fuel cells, the activation polarization overpotential is 0.22 V and is followed by a gradual voltage decrease due to
Direct and Indirect Borohydride Fuel Cells
11
Figure 5 (a) Cell potential versus current density and (b) power density versus current density of a borohydride fuel cell at different temperatures. (○) 25 C, anode: Ni + PTFE powders pressed on a Ni foam and air cathode made of Pt or Ag supported on carbon black, separated by an NRE211 Nafion membrane.2 (□) 50 C and (▼) 85 C anode: AB2 (Zr0.9Ti0.1Mn0.6 V0.2Co0.1Ni1.1) feed with 10 wt% NaBH4 in 20 wt% NaOH at a flow rate of 0.2 l min1. Cathode: Pt/C feed with humidified O2 at 0.2 l min1 at 1 atm and Nafion membrane as electrolyte.11
IR drop but at lower rate than the borohydride fuel cell operated at 85 C. At current density of 400 mA cm2, the cells show approximately 0.4 V cell potentials except for the borohydride fuel cell operating at 60 C, which shows a cell potential of 0.65 V. The maximum power density of the methanol fuel cells is 160 and 180 mW cm2 occurring at 400 mA cm2 when they operate at 90 and 110 C, respectively. The borohydride fuel cells show maximum power densities of 190 mW cm2 at 300 mA cm2 and 290 mW cm2 at 600 mA cm2 for the cells operated at 60 and 85 C, respectively. It should be noted that borohydride fuel cells, operating lower temperatures, show comparable results with direct methanol fuel cells.
12
Direct and Indirect Borohydride Fuel Cells
Figure 6 (a) Cell potential versus current density for two borohydride fuel cells and two methanol fuel cells. Borohydride: (□) 10 wt% NaBH4 20 wt% NaOH, 0.15 ml min1 60 C.22 (■) 10 wt% NaBH4 in 20 wt% NaOH at a flow rate of 0.2 l min1, 85 C.11 Anode: AB2 (Zr0.9Ti0.1Mn0.6V0.2Co0.1Ni1.1) alloy feed with (cathode) Pt/C feed with humidified O2 at 0.2 l min1 at 1 atm and Nafion NE-424 membrane as electrolyte. Methanol: (○) 1 mol dm3 CH3OH (Buttin, 2001). Anode: 85% Pt/Ru in VULCAN XC carbon. Cathode: air feed 85% Pt on Vulcan, temperature 110 C. (•) Anode PtRu/c and cathode 1.37 mg cm2 Pt–Fe/C 90 C.23 (b) Power density versus current density for two borohydride fuel cells and two methanol fuel cells. Borohydride: (□) Li et al.,22 (■) Li et al.11 Methanol: (○) Buttin (2001), (•) Shukla.23 Electrode and operating conditions as in (a).
Typical performance data for the DBFC are presented in Table 4. Different catalysts and separators have been used and many cells use the membrane electrode assembly (MEA) configuration employed in the typical H2/O2 fuel cells. The data show many different electrolytes and conditions and it is difficult to compare; not always the complete picture of the experiments is presented, and the timescale of the experiment is generally omitted in literature reports. Despite the difficulty in comparing the data, it is clear that the performance of the borohydride fuel cells is below theoretical expectations. However, the DBFC compares well with methanol/air and H2/O2 fuel cells; at room temperature; discharge current
Table 4
Some typical performance data for borohydride fuel cells at a current density of approximately 50 mA cm2 Cell components
Conditions
Fuel cell performance
[BH4] (mol dm3)
T (K)
Open-circuit cell potential (V)
Typical current density (A cm2)
Cell potential at 0.1 A cm2
Specific energy density (kW h kg1)
Maximum power density output (W cm2)
References
Cathode
Membrane
Ni2B alloy
Ag on Ni Pt/C
Asbestos
6.2
0.4
298
0.92
0.01–0.06
0.73
NA
NA
10
Anion Pall RAI No. 2259-60 NA. Cation Nafion NE-424 Cation, 5% Nafion binder solution (Undivided) (Undivided)
6
5
343
0.95
0.01–0.3
0.6
0.184
0.06
1
6 5
0.05 2.5
NA 358
NA 1.26
0.12 0.02–0.3
0.7 0.95
0.42 NA
0.09 0.18
24 11
6
0.5
298
1.05
0.01–0.1
0.7
2.8
0.04
3
6 1–5
1 2
298 298
0.8 0.8
0.001–0.005 0.35
NA NA
NA NA
NA 0.019
25 26
6
0.04
298
0.94
0–0.25
0.57
NA
0.07
5
97% Au + 3% Pt particles on carbon cloth ZrCr0.8Ni1.2 Zr0.9Ti0.1Mn0.6V0.2Co0.1Ni1.1
Pt/C Pt/C
Pt/C
Pt/C
Au Pt/Ni
MnO2 MnO2/ C/C 2 mg MnO2
MmNi3.35Co0.75Mn0.4Al0.3 hydrogen storage alloy NA: not given.
(Undivided)
Direct and Indirect Borohydride Fuel Cells
Anode catalyst
[OH] (mol dm3)
13
14
Direct and Indirect Borohydride Fuel Cells
densities of 0.1 A cm2 can be observed at 0.7 V cell potential as shown in Figure 5. If the temperature increases from 323 to 363 K, better performance can be achieved. In addition, borohydride fuel cells appear to perform well when the concentration of borohydride is low, which might reduce the migration of borohydride ions to the catholyte and will allow periodical refueling of the anolyte with NaBH4.
Hydrogen peroxide as oxidant in the DBFC The reduction of hydrogen peroxide in acidic media is 4H2 O2 + 8H + + 8e ! 8H2 O, E0 ¼ 1:77V vs: SHE
[15]
Using this reaction in the borohydride fuel cell will increase the pH gradient across the membrane as well as contribute to a higher cell potential. The coupling of the oxidation of borohydride and the acid reduction of hydrogen peroxide has a theoretical cell potential of 3 V. If alkaline hydrogen peroxide is used, the cell potential decreases to 2 V. Hydrogen peroxide coupled with borohydride has been proposed for underwater-sea applications and was demonstrated by Raman et al.27; dispersed Pt/C and AB5 metal hydrogen storage alloys were used as cathode and anode electrodes, respectively, divided by a pretreated Nafion® 117 membrane sandwiched between the two electrodes. The catholyte was 15% hydrogen peroxide at pHs 0, 0.5, and 1.0, and the anolyte was aqueous borohydride solution with 10 wt% NaBH4 in 20 wt aqueous NaOH. Cell potentials of 1.6 V at 0.1 A cm2 were observed at 343 K and 1.2 V at room temperature. Power density was observed in the order of 0.12 and 0.35 W cm2 at 308 and 343 K, respectively. The data show that most of the overpotential losses in this fuel cell occur at the oxygen cathode. The performance of the two borohydride fuel cells operating at 70 C is shown in Figure 7; one cell operates with air, whereas the other cell with acidic hydrogen peroxide in the catholyte. The open-circuit potential of the cell operating with hydrogen peroxide is 1.21 V, while that of the borohydride–air fuel cell is below 1 V. In both cases, the activation polarization overpotential is obscured by the IR drop. The maximum power density for the hydrogen peroxide and oxygen fuel cells is 166 and 63 mW cm2 at current densities of 300 and 160 mA cm2, respectively. Overall, the performance of the hydrogen peroxide fuel cell is higher than the cell operated with an air-breathing cathode.
Indirect Oxidation On-site hydrogen generator of gas using NaBH4 for a PEMFC is designed for different applications than the DBFC. The competitiveness of the DBFCs and IBFCs can be analyzed in terms of cost29; the conclusion is that borohydride crossover makes DBFC uncompetitive compared with IBFC. If the crossover was resolved, the cost of a six-electron transfer in a DBFC would be equivalent to the IDBFCs, although the volume consumption of NaBH4 would be 1.7 times larger. If an eight-electron transfer borohydride
Figure 7 Comparison of two borohydride fuel cells operating at 70 C with different oxidants: (•) 8.9 mol dm3 H2O2 in contact with a cathode made of 60 wt% Pt/C (1 mg cm2 of Pt) and an AB5 anode made of MmNi3.55Al0.3Mn0.4Co0.75 (5 mg cm2) in contact with 10% aqueous NaBH4 in 20 wt% NaOH.28 (□) Anode: highly dispersed 87% Au + 3% Pt Air on carbon silk.1
Direct and Indirect Borohydride Fuel Cells
15
oxidation can be achieved, the DBFC would be more competitive. Another study showed that a hydrogen generator can produce 120 nl min1 of hydrogen, which is fed into a PEMFC. The cell operates at a cell potential of 0.7 V and could generate 12 kW power.
Commercial Ventures and Future Developments The promising results obtained in the laboratory have not been sufficient to stimulate a large number of commercial activities of this technology. More work has been done in the study of hydrogen generation for IDBFCs in the United States where hydrogen generators and PEMFC have been developed at different scales. In Japan, DBFCs for microelectronics and automobile traction applications have been tested; 20 and 400 W cells for portable applications were designed. In the United Kingdom, the development of a direct borohydride microfuel cell containing a gold anode for special applications has been tested. In Israel, the DBFC has begun commercialization, targeting the portable electronic market and some military applications. Despite the borohydride fuel cell being studied by relatively few research groups, it shows a promising performance. In comparison with the number of person-years that have been invested in the study of methanol and hydrogen fuel cells, DBFCs have progressively emerged as an alternative source of energy.
Challenges Borohydride fuel cells face a number of needs and challenges that need to be addressed, including the following:
• • • • • • •
Oxygen cathodes tolerant to borohydride ions in alkali solutions performing similar to cathodes in acidic media. Low-overpotential anodes able to withdraw eight electrons from borohydride ions. Anion-permeable membranes that are chemically stable in alkali. Long-term studies to assess the effect of metaborate on the catalyst activity and membranes. Issues of mass balance in the electrolyte during long-term operations. Metaborate removal and water management. If cation membranes are used, a strategy to return alkali solutions from catholyte to anolyte should be studied.
Summary This article has focused on fuel cells involving direct and indirect oxidation of borohydride ions used in fuel cells. The DBFCs being inherently more efficient than indirect oxidation produce energy densities comparable to methanol and oxygen/hydrogen fuel cells. The DBFC still has a number of aspects that need to be studied such as borohydride crossover, selective anode materials, and improved anionic membranes. The IDBFCs use the well-studied MEA, which is fed with high-purity hydrogen produced from the efficient hydrolysis of sodium borohydride. The main problems in this cell are the removal of borohydride oxidation products, reuse of catalyst, and heat and water management. Overall, the use of borohydride as an alternative source of energy is promising, but more fundamental data and long-term operational experience with larger cells are necessary.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Amendola, S. C.; Onnerud, P.; Kelly, P. T.; Petillo, P. J.; Sharp-Goldman, S. L.; Binder, M. J. Power Sources 1999, 84(1), 130–133. Liu, B. H.; Li, Z. P.; Arai, K.; Suda, S. Electrochim. Acta 2005, 50(18), 3719–3725. Kim, J.-H.; Kim, H.-S.; Kang, Y.-M.; Song, M.-S.; Rajendran, S.; Han, S.-C.; Jung, D.-H.; Lee, J.-Y. J. Electrochem. Soc. 2004, 151(7), A1039–A1043. Verma, A.; Jha, A. K.; Basu, S. J. Power Sources 2005, 141(1), 30–34. Wang, Y.-G.; Xia, Y.-Y. Electrochem. Commun. 2006, 8, 1775–1778. Indig, M. E.; Snyder, R. N. J. Electrochem. Soc. 1962, 109(11), 1104–1106. Liu, B. H.; Li, Z. P.; Suda, S. J. Electrochem. Soc. 2003, 150(3), A398. Liu, B. H.; Li, Z. P.; Suda, S. Electrochim. Acta 2004, 49(19), 3097–3105. Mirkin, M. V.; Bard, A. J. Anal. Chem. 1991, 63(5), 532–533. Jasinski, R. Electrochem. Technol. 1965, 3(1–2), 40–43. Li, Z. P.; Liu, B. H.; Arai, K.; Suda, S. J. Electrochem. Soc. 2003, 150(7), A868–A872. Amendola, S. C.; Sharp-Goldman, S. L.; Janjua, M. S.; Spencer, N. C.; Kelly, M. T.; Petillo, P. J.; Binder, M. Int. J. Hydrog. Energy 2000, 25(10), 969–975. Wu, C.; Zhang, H.; Yi, B. Catal. Today 2004, 93–95, 477–483. Kojima, Y.; Suzuki, K.-I.; Fukumoto, K.; Sasaki, M.; Yamamoto, T.; Kawai, Y.; Hayashi, H. Int. J. Hydrog. Energy 2002, 27(10), 1029–1034. Hua, D.; Hanxi, Y.; Xinping, A.; Chuansin, C. Int. J. Hydrog. Energy 2003, 28(10), 1095–1100. Cho, K. W.; Kwon, H. S. Catal. Today 2007, 120(3–4), 336–340. O¨zkar, S.; Zahmakiran, M. J. Alloys Compd. 2005, 404–406, 728–731. Liu, B. H.; Li, Z. P.; Suda, S. J. Alloys Compd. 2006, 415(1–2), 288–293. Cheng, H.; Scott, K.; Lovell, K. V.; Horsfall, J. A.; Waring, S. C. J. Membr. Sci. 2007, 288(1–2), 168–174. Chatenet, M.; Micoud, F.; Roche, I.; Chainet, E.; Vondrak, J. Electrochim. Acta 2006, 51, 5452–5458.
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Lakeman, J. B.; Rose, A.; Pointon, K. D.; Browning, D. J.; Lovell, K. V.; Waring, S. C.; Horsfall, J. A. J. Power Sources 2006, 162(2), 765–772. Li, Z. P.; Liu, B. H.; Arai, K.; Suda, S. J. Alloys Compd. 2005, 404–406, 648–652. Shukla, A. K.; Raman, R. K. Annu. Rev. Mater. Res. 2003, 33, 155–168. Lee, S.-M.; Kim, J.-H.; Lee, H.-H.; Lee, P. S.; Lee, J.-Y. J. Electrochem. Soc. 2002, 149(5), A603–A606. Feng, R. X.; Dong, H.; Wang, Y. D.; Ai, X. P.; Cao, Y. L.; Yang, H. X. Electrochem. Commun. 2005, 7(4), 449–452. Verma, A.; Basu, S. J. Power Sources 2005, 145(2), 282–285. Raman, R. K.; Choudhury, N. A.; Shukla, A. K. Electrochem. Solid-State Lett. 2004, 7(12), A488–A491. Choudhury, N. A.; Raman, R. K.; Sampath, S.; Shukla, A. K. J. Power Sources 2005, 143, 1–8. Wee, J.-H. J. Power Sources 2006, 155, 329–339. Chatenet, M.; Micoud, F.; Roche, I.; Chainet, E.; Vondrak, J. Electrochim. Acta 2006, 51, 5449–5467. Cheng, H.; Scott, K. J. Electroanal. Chem. 2006, 596, 117–123. Eickhoff, S.; Zhang, C.; Cui, T. J. Power Sources 2013, 243, 562–568. Hoogers, G., Ed.; Fuel Cell Technology Handbook; CRC Press: Boca Raton, FL, 2003. Kojima, Y.; Susuki, K.; Fukumoto, K.; Kawai, Y.; Kimbara, M.; Nakanishi, H.; Matsumoto, S. J. Power Sources 2005, 125(1), 22–26. Kubokawa, M.; Yamashita, M.; Abe, K. Denki Kagaku 1968, 36, 788. Ponce-de-Leo´n, C.; Walsh, F. C.; Pletcher, D.; Browning, D. J.; Lakeman, J. B. J. Power Sources 2006, 155(2), 172–181. Ponce de Leo´n, C.; Walsh, F. C.; Rose, A.; Lakeman, J. B.; Browning, D. J.; Reeve, R. W. J. Power Sources 2007, 164(2), 441–448. Zhang, Q.; Wu, Y.; Sun, X.; Ortega, J. Ind. Eng. Chem. Res. 2007, 46(4), 1120–1124. Vielstich, W.; Lamm, A.; Gasteiger, H. A., Eds.; In: Handbook of Fuel Cells – Fundamentals, Technology and Applications; Vols. 1–4; Wiley: Chichester, England, 2003. Wang, L. B.; Ma, C. N.; Sun, Y. M.; Suda, S. J. Alloys Compd. 2005, 391(1–2), 318–392. Buttin, D.; Dupont, M.; Straumann, M.; Gille, R.; Dubois, J.-C.; Ornelas, R.; Fleba, G. P.; Ramunni, E.; Antonucci, V.; Arico`, A. S.; Cretı`, P.; Modica, E.; Pham-Thi, M.; Ganne, J.-P. J. App. Electrochem. 2001, 31(3), 275–279. 42. Elder, J. P. Electrochim. Acta 1962, 7(4), 417–426.
Introduction Chemical Hydrides Direct and Indirect Borohydride Fuel Cells Catalytic Materials Electrode Materials for DBFCs High-surface-area electrodes Hydrogen storage alloys Catalytic Materials for IBFC Mechanism of Borohydride Oxidation Direct Oxidation Indirect Oxidation Rate of hydrogen generation Fuel Cell Configurations Direct Oxidation Borohydride Crossover Electrolyte Parameters Direct Oxidation of Borohydride Temperature Indirect Borohydride Oxidation Fuel Cell Performance Direct Borohydride Oxidation Hydrogen peroxide as oxidant in the DBFC Indirect Oxidation Commercial Ventures and Future Developments Challenges Summary References
1 2 2 3 3 3 3 4 6 6 7 7 8 8 9 10 10 10 10 10 10 14 14 15 15 15 15
Introduction Transportation, storage, and distribution of hydrogen gas remain a matter of concern when compared with traditional energy carriers such as natural gas and gasoline, which can be handled relatively safely with the present distribution infrastructure. Considerable effort has been made to study hydrogen storage technologies to achieve similar or higher energy density as gasoline or diesel for transportation purposes. The storage technologies include high-pressure vessels made of steel or reinforced composites, metal hydrides, metal alloys, activated carbon, graphite nanofibers, and nanotubular materials such as carbon and titanium. The specific energy density of these systems is still insufficient for portable applications and has led to the research for alternative fuels. Ideally, the fuel should be safe and easily transported at high concentrations, should oxidize fast, and should provide large gravimetric and volumetric energy densities. Extensive reviews of hydrogen–oxygen fuel cells have been published over the last decade, and some commercial teaching systems have been on the market over the last decade. Despite the borohydride fuel cell being studied since 1960, scientific papers have tended to be limited to a few electrode materials and the use of borohydride as a hydrogen source for proton exchange membrane fuel cells (PEMFCs). More recently, direct borohydride fuel cells (DBFCs) have been attracting more interest, and published work includes a comparison of different anodes, cathodes, and membranes; cost evaluation; borohydride crossover; and the effect of the borohydride hydrolysis. Although early studies in 1965 pointed out the potential of borohydride as a fuel, there were few publications before 2000. Since then, rapid progress has been achieved in both DBFCs and indirect borohydride fuel cells (IDBFCs), and between 2007 and 2012, nearly 60 papers per year have been published in aspects related to the DBFC. Contemporary scientific papers and reviews also include improvements in cell design and the development of mathematical models. An air-breathing cathode can be used to couple with the direct oxidation of borohydride; this type of electrode has been well characterized in the literature and will not be considered here. Another common oxidant used to complete the borohydride fuel cell is hydrogen peroxide, which can be used in specialized applications such as underwater vehicles and under anaerobic conditions.
Reference Module in Chemistry, Molecular Sciences and Chemical Engineering
http://dx.doi.org/10.1016/B978-0-12-409547-2.11190-4
1
2
Direct and Indirect Borohydride Fuel Cells
Table 1
Theoretical hydrogen yield, specific energy capacity, and maximum percentage of reactant deliverable as hydrogen for various hydrides
Reaction
H2 yield (wt%)
Specific energy capacity of hydride (kW h kg1)
Maximum % of reactant deliverable as H2
LiBH4 + 4H2O ! LiOH + H3BO3 + 4H2 LiH + H2O ! LiOH + H2 NaBH4 + 4H2O ! NaOH + H3BO3 + 4H2 LiAlH4 + 4H2O ! LiOH + Al(OH)3 + 4H2 NaAlH4 + 4H2O ! NaOH + Al(OH)3 + 4H2 CaH2 + 2H2O ! Ca(OH)2 + 2H2 NaH + H2O ! NaOH + H2
8.6 7.7 7.3 7.3 6.4
1.63 1.46–4.73 (Eickhoff) 1.38 1.38–3.95 1.21–2.78 (Eickhoff)
37.0 25.4 21.3 21.2 14.8
5.2 4.8
0.99–1.78 (Eickhoff) 0.91
9.6 8.4
Chemical Hydrides The substitution of hydrogen storage systems mentioned earlier by the in situ generation of hydrogen using a reformer reactor or by hydrolysis of chemical hydrides such as LiNH4, LiH, NaH, NaAlH4, and NaBH4 has also been considered. Chemical hydrides could release hydrogen gas at room temperature and can have storage density up to two to five times more than steel cylinders. Table 1 shows some typical hydride reactions and the percentage of hydrogen deliverable and energy capacity; lithium salts show the highest energy capacity and hydrogen yield, while sodium borohydride has the advantage of been less toxic than lithium with a reasonable energy capacity and hydrogen yield. Other hydride salts contain toxic lithium or have lower hydrogen capacity in comparison with sodium borohydride. On-demand generation of hydrogen using chemical hydrides requires an additional reaction tank containing a catalyst for the hydrolysis reaction and might be unsuitable for portable applications of fuel cells. For this reason, over the last few years, the direct oxidation of hydrides in particular sodium borohydride has been the focus of intense academic research. Sodium borohydride can be shipped as a white solid or as 30% aqueous solution and can be handled in air. Its applications include the manufacture of low tonnage organic compounds, a source of high-purity hydrogen, chemical reduction in the pulp and paper industry, waste treatment, and electroless nickel plating baths.
Direct and Indirect Borohydride Fuel Cells The direct oxidation of borohydride ions coupled with the reduction of oxygen has an equilibrium potential of approximately 1.64 V according to the following reactions: Anode : BH + 8OH 8e ! BO2 + 6H2 O, E ¼ 1:24 V vs: SHE
[1]
Cathode : O2 + 2H2 O + 4e ! 4OH , E ¼ 0:4V vs: SHE
[2]
Cell : BH4 + 2O2 ! BO2 + 2H2 O, Ecell ¼ 1:64V vs: SHE
[3]
The metaborate, BO2 product, can be recycled to borohydride ion and is environmentally friendly. The cell potential of the DBFC reaction [3] leads to a theoretical energy density of 9.3 kW h kg1, which is larger than methanol (6.2 kW h kg1) and hydrogen at 4500 psi (0.45 kW h kg1). The energy density in solution will be lower despite the ability to prepare up to 30 wt% borohydride (>6 mol dm3) solution in concentrated alkaline aqueous electrolytes. The half-life of sodium borohydride is 426 days when stored at pH 14 and 298 K under a nitrogen atmosphere. The high concentration of alkaline is necessary to avoid borohydride hydrolysis, which in neutral solution and in the presence of platinum or cobalt(II) chloride, catalyst can be written as follows: BH4 + 2H2 O ! BO2 + 4H2
[4]
Reaction [4] provides the basis for the IBFCs; borohydride supplies the hydrogen gas that then reacts with oxygen in a PEMFC to produce water, heat, and electrical energy. This is the typical hydrogen–oxygen fuel cell reaction, widely studied over the past 50 years by many academic and industrial research groups: 2H2 + O2 ! 2H2 O, Ecell ¼ 1:18V vs: SHE
[5]
The hydrogen reactant for this fuel cell can be obtained from hydrogen generators consisting of a reactor containing borohydride in presence of a catalyst to carry out reaction [4]. Another technology for hydrogen production, the catalytic steam natural gas reformation, produces a mixture of hydrogen, carbon monoxide, carbon dioxide, water, and sulfur compounds of low molecular mass. This mixture is unacceptable for PEMFC operation, where catalytic metals can become poisoned by small amounts of carbon monoxide (<0.1%) and sulfur. Hydrogen needs to be purified, which increases the number of reactors used. Electrolysis is another common technology to produce high-purity hydrogen; however, technical improvements are required to reduce inefficiencies and cost. Other proposals include using the electricity generated from renewable energy sources such as solar and wind for the electrolysis of water and generation of hydrogen. The reliability of the process depends on geographic location and climate.
Direct and Indirect Borohydride Fuel Cells
3
In the DBFC, the borohydride hydrolysis [4] must be suppressed in order to take advantage of the high number of electrons interchanged during the oxidation, whereas in the IBFC, the generation of hydrogen must be enhanced. In the following sections, the characteristics of these two systems will be briefly considered.
Catalytic Materials Electrode Materials for DBFCs The electrochemical oxidation of borohydride was considered for fuel cells applications in the 1960s using porous nickel and palladium anodes. Nickel boride was prepared by precipitating nickel acetate with KBH4 on the pores of a nickel plaque; nickel boride was found to be significantly better than other metals. Other early studies of the oxidation of borohydride were carried out on mercury electrodes. Due to the environmental hazards associated with mercury, it is unlikely that this metal will see any continued use as an anode in borohydride fuel cells. Other catalyst materials studied during the 1960s for borohydride oxidation in strong alkali solutions were nickel, platinum, palladium, and gold. High open-circuit cell potentials and current densities of 0.1 A cm2 indicated the possibility of achieving reasonable energy and power density outputs, but the number of electrons transferred was low (4 at Ni and <4 at Pt). Other works studied high-surface-area electrodes such as Pt and Pd on Ni and C substrates. The oxidation of borohydride yielded six electrons at platinum deposited on a sintered porous Ni plate at 50–200 mA cm2. Another study provided clear evidence of changing boron speciation with time since a new oxidation wave appeared at more negative potentials when the borohydride concentration was low (0.2 mol dm3) in NaOH, at gold electrodes. Direct oxidation of borohydride requires selective anodes with low catalytic activity for the decomposition of borohydride to hydrogen and borates via reaction [4]. More recently, several electrode materials for the direct oxidation of borohydride have been evaluated such as Ni2B and Pd–Ni, Au, colloidal Au and Au alloys with Pt and Pd, MnO2, misch metals, AB5-type hydrogen storage alloys, Ni, Raney Ni, Cu, and colloidal Os and Os alloys. So far, only gold electrodes seem to realize the eight-electron transfer expected from reaction [1]. Less than eight electrons indicate partial oxidation or high rates of the hydrolysis of borohydride through reaction [4]. Figure 1(a) shows the cell potential of different electrode materials in a borohydride fuel cell versus the current density, while Figure 1(b) shows the change of the power density with the current density. Nickel electrodes show higher performance than Au/Pt, Pt inks, and MnO2 electrodes. A detailed investigation of the oxidation of borohydride has been carried out on gold ultra-microelectrodes. Such studies concluded that eight electrons were transferred during the oxidation of borohydride and that the reaction became mass transport-controlled at high potentials. Other studies at gold and platinum electrodes show that the number of electrons transferred is low on platinum but the use of additives such as thiourea could inhibit the evolution of hydrogen and have a positive effect on performance. A rather negative open-circuit voltage can be obtained on nickel electrodes, and acceptable overpotentials at current densities of several hundred mA cm2 are observed, especially at elevated temperatures. The number of electrons remains low at four, indicating that only half the theoretical energy value could be obtained with nickel anodes.
High-surface-area electrodes The preparation of high-surface-area electrodes should include consideration of the influences of catalyst loading and Nafion® content. High-surface-area electrodes containing gold or gold alloy (Au 97% + Pt 3%) catalyst have been used for the oxidation of borohydride. The number of electrons transferred at 0.2 mA cm2 and 343 K approaches 7. Platinum high-surface-area electrodes could achieve current densities in the range of 10–60 mA cm2 with only 5–5.5 electrons transferred. Comparison of six different high-surface-area electrodes has shown that Pd/C and Pt/C are significantly better than nickel in terms of fuel efficiency. Palladium, platinum, and nickel on carbon show current densities up to 800 mA cm2, but the overpotential at the nickel electrode is lower than at palladium or platinum. Table 2 shows a comparison of different anode materials for borohydride oxidation. Gold- or nickel-based materials are the best choices for the oxidation of borohydride; nickel gives the most negative potential for borohydride oxidation but involves a low number of transferred electrons, while gold requires a higher overpotential but gives a better fuel efficiency. Further research work on improved electrocatalytic materials for BH4 oxidation is clearly needed.
Hydrogen storage alloys Anode materials able to store hydrogen have been used in borohydride fuel cells. The role of the borohydride ion is to provide the hydrogen within the lattice of the anode alloy to be stored. The stored hydrogen can then react with oxygen in the fuel cell. Some of the alloys that have been reported include ZrCr0.8Ni1.2; LmNi4.78Mn0.22, where Lm is a lanthanum-rich misch metal; and Zr0.9Ti0.1Mn0.6V0.2Co0.1Ni1.1. These electrodes produce current densities up to 300 mA cm2 at 0.7 V versus SHE, but the efficiency to release the energy from borohydride does not exceed 50% (i.e., z ¼ 4). Other works report that the performance of the borohydride fuel cells is a complex function of the operating conditions and that the efficiency of fuel utilization increases at higher current densities. A two-component alloy catalyst electrode can provide dual functionality: One of the metals in the alloy serves to catalyze hydrolysis of borohydride, while the other metal oxidizes the generated hydrogen. In theory, this combination avoids hydrogen evolution in the cell.
4
Direct and Indirect Borohydride Fuel Cells
Figure 1 (a) Cell potential versus current density and (b) power density versus current density for various electrodes in a borohydride fuel cell: (•) commercial air cathode (Johnson Matthey) and anode made of highly dispersed gold/platinum particles supported on high-surface-area carbon silk separated by 2259–60 Pall anion membrane with 5% NaBH4 in 25% NaOH1; ○ Ni + PTFE powders pressed on a Ni foam as anode and air cathode made of Pt or Ag supported on carbon black separated by an NRE211 Nafion membrane, 5% NaBH4 in 6 mol dm3 NaOH2; (■) cathode and anode inks prepared using unsupported Pt black and Nafion, 0.5 mol dm3 NaBH4 in 6 mol dm3 NaOH3; (□) manganese dioxide cathode and standard Pt/Ni anode (Electro-Chem-Technic, the United Kingdom) with 1 mol dm3 NaBH4 in 3 mol dm3 KOH with no membrane4; (r) Anode, MmNi3.35Co0.75Mn0.4Al0.3; cathode, 2 mg cm2 MnO2 in an electrolyte consisted of 0.4 g of KBH4 in 200 cm3 of 6 M KOH.5 All experiments reported at 23 C temperature.
Catalytic Materials for IBFC The generation of hydrogen from the hydrolysis of sodium borohydride has received increasing attention, and many catalysts to improve the hydrolysis reaction rate have been identified. These include dispersed Ru metal on ion exchange beads, dispersed Pt on an oxide or carbon supports, and high-area nickel materials. In the IDBFCs, the concentrated borohydride solution in aqueous alkali is converted to hydrogen gas in a catalytic generator, and the hydrogen gas is then fed to a PEMFC or other hydrogen-fueled
Direct and Indirect Borohydride Fuel Cells
5
cell. Such systems have been described for applications as varied as automobile traction and microfabricated fuel cells to power electronic circuits. The hydrolysis of borohydride in aqueous solutions follows zero-order kinetics, when an aqueous solution of sodium borohydride is pumped through a catalyst bed: Catalyst
NaBH4 + 2H2 O ! 4H2 + NaBO2 + heat
[6]
A schematic diagram of the hydrogen generator system is shown in Figure 2. In alkaline solutions, the reaction without the catalytic metal is very slow. The advantages of the in situ hydrogen generation for the PEM cells are as follows: 50% of the hydrogen generated is provided by the water in the electrolyte. Platinum anodes can be used in the PEMFC since only high-purity hydrogen is supplied. Control supply of humidified H2 gas. Continuous generation of H2 by addition of NaBH4 into the hydrolysis reactor. Water recirculation from the fuel cell back to the hydrolysis reactor.
Table 2
Summary of electrode materials performance for the oxidation of borohydride Comment
Anode material
z value
Open-circuit potential at the working electrode versus SHE (V)
Ni Raney Ni Cu Au Pt Dispersed Pd on Ni Dispersed Pt on Ni Dispersed Au on Ni Ni2B H2 storage alloys MmNi3.35Co0.75Mn0.4Al0.3 hydrogen storage alloy
4, 4, 4 NA NA 8, 7 2–4 6 5, 6 NA NA 4 7.5
1.03 1.03 1.02 0.99 1.0 1.00, 0.91 0.91 0.99 1.07 1.15 1.12
Current density (A cm2) 0.6 NA >0.1 possible 0.7 0.1 <0.1 NA 0.1 0.3 0.1
z is the number of electrons transferred during the oxidation reaction. NA ¼ not given in the reference.
Figure 2 Schematic diagram of a system for hydrogen generation from sodium borohydride solution.
Cell potential (V)
References
0.7 0.6 NA NA 0.6 0.92
6–8 8 8 1,8,9 Elder (1962),3 8,10 3,8 8 10 11, Wang (2005) 5
NA 0.99 0.7 0.58
6
Direct and Indirect Borohydride Fuel Cells
Some challenges for the development of this system are (a) a cheap catalyst with high conversion and optimum reaction rate, (b) a catalyst tolerant to the borate product, (c) water reuse, and (d) optimal connection to the PEMFC and optimal reactor design. One of the most common catalysts for hydrogen generation from borohydride is Co and Co–P electrodeposited on copper. The catalytic activity of amorphous Co–P deposit can be up to 950 ml min1 g1 of catalyst, 18 times larger than the polycrystalline Co in 1 wt% NaOH + 10 wt% NaBH4 solution at 203 K. Unlike carbon reformers, hydrogen generators from borohydride solutions do not need to be preheated; the reactor is started with a flow rate of borohydride solution of 20 g min1 at 483 kPa at room temperature. The solubility of metaborate is a problem in the reactor since it causes blocking of the active sites of the catalysts and clogging; additional work in heat and water management is required. Table 3 shows a comparison of the hydrogen evolution obtained using different catalysts and conditions; platinum appears to yield the highest hydrogen evolution rate, while Co on Cu gives the lowest one.
Mechanism of Borohydride Oxidation Direct Oxidation Two mechanisms are generally recognized: Mechanism (A): the first step is an electron transfer to form the BH4 radical: BH4 e ! BH4
[7]
The second step involves the decomposition of the radical into BH3 and water, followed by further electron transfer to form diborane, B2H6, which undergoes further electron transfer. Mechanism (B): the first step is the electron transfer, reaction [7], followed by predissociation at surface sites of the catalyst: 2M + BH4 ! M H + M BH3
[8]
This is followed by a series of steps that involve electron transfer, surface reaction, and hydrogen atom combination to produce hydrogen gas as a secondary reaction. Metallic surfaces able to support predissociation are commonly good catalysts, and the adsorption of hydrogen atoms leads normally to the formation of hydrogen gas. Since gold does not support coverage of adsorbed hydrogen atoms, the first type of mechanism would be expected. At nickel, platinum, and palladium, hydrogen adsorbs and the second type of mechanism would predominate. It is likely that the reaction M + H2 O + e ! M H + OH
Table 3
[9]
Comparison of different catalysts for the hydrolysis of NaBH4 Conditions
Hydrogen generation rate (l min1)
Catalyst
NaBH4
NaOH
T
References
44.9 2449.6 1780 394.7 837.5 123.6 38.1 67.2 22.9 8.5 0.4 424.7 9.7 63.0 4.9 17.3 285.0 9.1 234.0 114.2 133.7 324.1
5% Ru on IRA 400 membrane 100 mg of 20% Pt/VULCAN XC-72R 100 mg of 10% Pt/VULCAN XC-72R 100 mg of 5% Pt/VULCAN XC-72R Pt–LiCoO2 50 mg Pt–LiCoO2 256 mg Ru on IRA 400 resin, 256 mg 0.3 NixB cat heat-treated at 150 C in vacuum 0.3 NixB cat heat-treated at 80 C in air Co–P electroplated on Cu at 0.01 A cm2 Co electroplated on Cu at 0.01 A cm2 1.4 mM water-dispersed Ru(0) nanoclusters Ni powder (0.5–1 mm) Co powder (1–2 mm) NiCl2 6H2O (granules) NiF2 4H2O (granules) CoCl2 6H2O (granules) Ni2B Co2B Raney Ni Raney Co Raney Ni50Co50
1% 10 wt% 10 wt% 10 wt% 20% 20%
1% 5 wt% 5 wt% 5 wt% 10% 10%
1.5 wt%? 1:5 wt% 10 wt% 10 wt% 0.15 mol dm3 0.2 g 0.2 g 0.2 g 0.2 g 0.2 g 0.2 g 0.2 g 1g 1g 1g
1 mol dm3 1 mol dm3 1 wt% 1 wt%
298 K – – – – – 298 K – – 303 K 303 K 298 K 293 K 293 K 293 K 323 K 283 K 293 K 293 K 293 K 293 K 293 K
12 13 13 13 14 14 14 15 15 16 16 17 18 18 18 18 18 18 18 18 18 18
20 ml at 10 wt% 20 ml at 10 wt% 20 ml 10 wt% 20 ml 10 wt% 20 ml 10 wt% 20 ml 10 wt% 20 ml 10 wt% 100 ml 10 wt% 100 ml 10 wt% 100 ml 10 wt%
Direct and Indirect Borohydride Fuel Cells
7
will occur at the open-circuit potential for borohydride oxidation since the formal potential of borohydride oxidation, reaction [1], is negative to the formal potential of the H2O/H2 couple. As a result, a mixed potential would be observed at good catalytic materials for hydrogen evolution. Taking this into consideration, the estimation of the hydrogen evolution rate as a way to measure the rate of borohydride oxidation becomes more difficult to estimate due to the additional hydrogen evolution reaction [9]. The stability of borohydride restricts the electrolyte to strong alkaline solutions and limits the anode materials to those with low activity for the hydrolysis. A specific material for the direct oxidation of borohydride, reaction [1], should be used, but since most catalytic materials also catalyze its hydrolysis [4], the two reactions are likely to occur at the same time. The electrode potential will acquire a value that corresponds to a mixed potential between the two reactions and will depend on the anode material and temperature. In addition, the reduction of water to hydrogen, which consumes two electrons and is thermodynamically favorable at the potentials of the oxidation of borohydride, reaction [1], could restrict the eight-electron transfer involved in the oxidation of borohydride: 2H2 O + 2e ! H2 + 2OH
[10]
Furthermore, the theoretical number of electrons transferred by the oxidation of borohydride probably decreases due to the existence of partially oxidized species of borohydride and the hydrogen evolution in most electroactive materials. One main oxidation peak is observed in the cyclic voltammogram of BH4 ions at pH 9 followed by a small wave caused by the oxidation BH3OH, which was formed during the partial hydrolysis reaction of BH4 ions: BH4 + H2 O ! BH3 OH + 0:5H2
[11]
The BH3OH ion will have the effect of increasing the open-circuit potential since it is more readily oxidized than BH4 .
Indirect Oxidation The heterogeneous oxidation of borohydride for the generation of hydrogen from alkaline solutions is preferred over the homogenous acid-catalyzed hydrolysis because it is not pH-dependent and the catalyst can be reused. Catalyst metals such as Cu, Co, Ru, Pt, and Ni are generally used, and there is a general agreement that the production of hydrogen is catalyzed by the metal due to the fast proton production during their reduction by BH4 ions. Works on the investigation mechanism of hydrogen production from alkaline sodium borohydride show that the reaction kinetics is of first order when the molar ratio ½BH4 =½Pd is 0.03–0.11 in well-stirred solutions. The experiments used 2.7–10 wt% Pd/C powder catalyst in deuterated sodium borohydride, and the reaction kinetics was followed by nuclear magnetic resonance. The rate constant at a Pd/C loading of 5.5 wt% is 2.4 104 s1, while in the absence of catalyst at pH 13, it is 2.3 107 s1. Other works report that the rate of hydrogen generation during the hydrolysis reaction on carbon-supported ruthenium catalyst is directly proportional to the concentration of NaBH4 (first order). If the amount of catalyst remains constant, the rate of hydrogen generation depended on temperature, NaOH concentration, and the concentration of the metaborate product, NaBO2, which builds up in the reaction vessel and depresses the generation of hydrogen significantly. In other studies, the generation of hydrogen was carried out on 2 wt% Ru/alumina pellets, 3 wt% Ru/carbon extruded, and 0.5 wt% Ru/carbon granules. Most catalysts disintegrated during the experiments due to the evolution of hydrogen except for the Ru/C extruded catalyst. The hydrolysis reaction is assumed to occur in two steps; first, the BH4 ion adsorbs on the surface of the catalyst: M + BH4 ! M BH4
[12]
and second, the adsorbed species react to form adsorbed hydrogen: M BH4 + 2H2 O ! MBO2 + 4H2 M
[13]
The experimental data show that the kinetics of the hydrolysis reaction is of zero order at low temperatures (298 K), whereas first-order kinetics fit the experimental data at higher temperatures (383 K). It should be highlighted the importance of water management for the effective operation of a hydrogen generator system. Other works show that the catalytic generation of hydrogen decreases when the concentration of sodium borohydride increases. A proposed expression for the catalytic oxidation of sodium borohydride is (Zhang, 2007)
d½NaBH4 ¼ k ½NaBH4 0:41 ½NaOH0:13 ½H2 O 0:68 wdt
[14]
where w is the mass of sodium borohydride and k is the rate constant. The equation describes the dependence of hydrogen generation on the concentrations of sodium borohydride, sodium hydroxide, and water.
Rate of hydrogen generation The theoretical hydrogen content of borohydride is 10.9 wt%, and, under ideal conditions, 0.213 g of hydrogen can be obtained per gram of NaBH4. At room temperature in water, however, only a fraction of this amount is released. The amount decreases with time due to the stabilization of the borohydride by the increasing pH of the solution. The rate of hydrolysis of sodium borohydride increases when catalytic active materials, acids, are used or by raising the temperature. The use of heterogeneous catalysts at
8
Direct and Indirect Borohydride Fuel Cells
different temperatures is the most common method to increase the generation of hydrogen gas; however, the available surface area of the catalyst is the limitation. Recently, a new method using ruthenium metal nanoparticles in suspension has been reported. The ruthenium nanoparticles are highly active and show the lowest activation energy for borohydride hydrolysis of 28.51 kJ mol1 in comparison with 75 kJ mol1 for Co, 71 kJ mol1 for Ni, and 63 kJ mol1 for Raney Ni. Kojima et al. (2005) investigated the hydrolysis of NaBH4 in various metal oxides covered with Pt. They found that Pt–LiCoO2 catalyst worked as an excellent H2 generator from NaBH4 solutions. The catalyst was prepared from a honeycomb cordierite monolith, which was immersed in slurry solution containing 1000 g of Pt–LiCoO2, 620 g of 20% Al2O3, and 125 g of H2O and was dried for 24 h at 393 K followed by 3 h calcination at 720 K. The final concentration of Pt in the monolith was 1.5 wt%.
Fuel Cell Configurations Direct Oxidation Most borohydride fuel cells consist of two electrodes separated by a membrane. Both cationic and anionic membranes have been used, and each produces different chemical characteristics within the cell. Cation membranes lead to a chemical imbalance; the oxidation of 1 mol BH4 transfers 8 mol Na+ ions across the membrane, increasing the concentration of NaOH in the catholyte and losing NaOH from the anolyte. Long operations test could cause problems because BH4 ions are only stable in strong alkali concentration. A procedure to return sodium hydroxide from catholyte to anolyte should be implemented. In contrast, the operation with an anion membrane transfers 8 mol of OH from catholyte to anolyte across the membrane for each mole of borohydride oxidized. The chemistry using this membrane is in balance and only borohydride should be replenished. Figure 3 shows sketches of the cell with each type of membrane from a cell with sodium electrolytes. It is assumed that the electrode reaction [1] is occurring without the competing hydrolysis of borohydride, and chemical changes during energy generation are emphasized, even with idealized chemistry. Cation-permeable membranes are commercially available, and the perfluorinated types are very stable in contact with strong reducing agents and concentrated alkali solutions. Nafion® 1100 EW membranes fabricated by DuPont are the most common type. In contrast, the majority of the anion-permeable membranes are unstable in alkali. The perfluorinated anion membrane Tosoh® is
Figure 3 Direct borohydride fuel cells with (a) a cation-permeable membrane and (b) an anion-permeable membrane, drawn to emphasize the chemical balance.
Direct and Indirect Borohydride Fuel Cells
9
Figure 4 Comparison of cell potential and power density versus current density for a borohydride fuel cell using three different membranes: cationic (○) Nafion 115 and anionic (□) Asahikasei A-501SB membrane at 25 C. Ni + PTFE powders pressed on Ni foam as anode and air cathode made of Pt or Ag supported on carbon black.2 (▲) CU1 anionic membrane, area 4 cm2; anode, 2 mg cm2 Au/C; cathode, 2 mg cm2 Pt/C. Cell: parallel flow field with 1.32 M NaBH4 in 2.5 M NaOH at 10 cm3 min1. O2 at 200 cm3 min1 at 25 C.19
no longer manufactured because of the very high cost of production. At the present time, the commercially available anion-permeable membranes have not been tested long enough in strong alkali electrolytes, and many are in the development stage. There has been substantial R&D targeted toward improving the stability of membrane polymers to alkali, but, currently, there are no anion membranes on the market stable to hydroxide concentrations above 5%, and higher concentration is required to ensure the stability of sodium borohydride solution. A comparison of a cationic membrane, Nafion N115, and two anionic membranes, Asahikasei A-501SB and CU1, on a borohydride fuel cell is shown in Figure 4 in terms of cell potential and power density versus current density. The thickness of the membranes is approximately 130 mm. The open-circuit potential of the systems is similar, but the fuel cell operated with the cationic membrane presents higher cell potential than the systems operating with the anionic membrane. Power densities are also higher for the system operating with the cationic membrane. Increased ohmic and interfacial resistance between the cathode and the membrane caused by water deficiency in the catholyte leads to the reduction of power density in the system operated with the anionic membrane. Ionic membranes are expensive, and the ion selectivity is not 100%; the transport of other ions causes serious complications in the operation of the cell. A system with no membrane would be ideal and will make the cell design easier and simple. Unfortunately, membranes are necessary and help to avoid interactions between reactant and products at both electrodes. Nevertheless, an undivided borohydride–oxygen fuel cell might be possible since both reactants do not react in homogeneous solution. This will require an inactive cathode catalyst toward borohydride ions and its hydrolysis. The fuel cell chemistry would be as if the system has an anionic membrane. Undivided borohydride fuel cells with air-breathing MnO2 cathode catalyst, which shows no reaction with borohydride, and a dispersed gold catalyst anode, have been described in the literature. The cell potential approaches 0.6 V at 1–5 mA cm2 current densities when operated with 1 mol dm3 KBH4 in 6 mol dm3 KOH electrolyte. Despite the low current density, discharge curves showed that the number of electrons interchanged z is 7.4, and it appeared that the cell would be able to deliver higher current densities. Other works report the operation of a borohydride fuel cell with no membrane with MnO2/C/Ni cathode and Pt black on a Ni mesh anode. Current densities of 35 mA cm2 at a cell potential of 0.8 V in 1 mol dm3 NaBH4 and 5 mol dm3 NaOH have been found. The compact design of these membraneless cells reduces cost and size and improves the power density.
Borohydride Crossover The large concentration of BH4 ions in the anolyte causes strong concentration gradient and the crossover of borohydride ions across the membrane, particularly in anion membranes at open circuit. Chatenet et al.20 studied the reduction of oxygen on carbon-supported platinum, gold, silver, and manganese oxide using oxygen-saturated solutions containing traces of borohydride ions. Their objective was to simulate the conditions at which oxygen would reduce in the fuel cell when crossover of borohydride
10
Direct and Indirect Borohydride Fuel Cells
ions across the membrane occurs. The results indicate that the open-circuit potential of the Au/C and Ag/C cathodes in the presence of 0.01 mol dm3 borohydride falls below 0.3 V versus NHE, while the Pt/C electrode disaggregates due to the hydrogen evolution from the hydrolysis of borohydride and its open-circuit potential falls to 0.8 V versus NHE ( 0.2 V vs. NHE without BH4 ). This potential reflects the mixed potential value of the cathodes that are prone to catalyze the oxidation of borohydride ions rather than reduce oxygen. In the case of manganese oxide electrode, the open-circuit potential changed from 0.075 to 0.05 V versus NHE in presence of borohydride ions, reflecting the selectivity of this electrode to preferentially reduce oxygen. The crossover of borohydride is also due to the potential difference created between the two electrodes, and the current density plays an important role. The potential drop created by the spontaneous electron transfer on the two electrodes determines the direction of the electrical field that drives ion movement by migration. If an anion membrane is used, the borohydride ions are dissuaded from migrating to the catholyte compartment, and the electrical field during operation avoids their migration from anolyte to catholyte; the transport of borohydride ions to the catholyte is low and decreases as the current density increases. At open-circuit conditions, borohydride ions will tend to migrate to the catholyte through the anion-permeable membrane. Hydroxide ions should move in the opposite direction to keep both electrolytes charge-balanced, and if hydroxide is more concentrated in the catholyte, this will be favorable. Lakeman et al.21 analyzed the crossover of borohydride ions across different membranes using a four-electrode cell with gold gauzes as cathode and anode. The study was carried out using 30 wt% NaBH4 in 6 mol dm3 NaOH in one side of the membrane and 30 wt% NaOH in the other. The crossover measurements were taken during 1 h when the electrodes were polarized between 1200 and 200 mV versus SHE at intermittent scans. An anionic membrane identified as 3541P (manufactured at Cranfield University, the United Kingdom) showed the highest crossover value of 4.6 106 mol cm2 s1, while the lowest borohydride transport was observed in a Nafion 117 membrane as 0.4 106 mol cm2 s1. The conclusion was that thicker membranes are more effective in slowing down the migration of borohydride.
Electrolyte Parameters Direct Oxidation of Borohydride The borohydride fuel cell can be operated with sodium or potassium hydroxide electrolytes within the range of 10–40 wt% concentration containing a borohydride salt between 10 and 30 wt%. Although the sodium compounds are cheaper and lighter, ionic membranes work better with potassium salts, which are more conductive. Cationic membranes in their potassium form are more selective to unwanted ions and are significantly less hydrated than membranes in contact with sodium salts. The physical properties of borohydride solutions such as specific gravity, viscosity, and melting point as a function of borohydride and hydroxide ion concentrations are reported in the literature. These investigations report the open-circuit potential, polarization curves, and migration rates of borohydride ions across the membrane as well as the number of electrons transferred in the borohydride oxidation reaction. The investigations indicated that the open-circuit potential at a Zn–Ni alloy anode electrode is not suitable when the concentration of borohydride ion exceeds 5 wt%.
Temperature Unlike methanol fuel cells that operate at between 343 and 373 K, the borohydride fuel cells provide similar energy levels at room temperature 298 K. The energy and cell potentials of the DBFC are higher at elevated temperatures ( 373 K), but the stability of the solutions decreases and less fuel would be available. A clear indication of the better performance of the borohydride fuel cell is presented in Figure 5 in the form of cell potential (5a) and power density (5b) at different temperatures. The highest cell potential and power density occur at 258 K.
Indirect Borohydride Oxidation The hydrolysis of borohydride to produce hydrogen gas is normally carried out in aqueous sodium hydroxide, potassium hydroxide, or neutral solutions. The efficiency of hydrogen generation increases from 92% up to 99% as the concentration of alkali is increased and is also affected by the concentration of sodium borohydride, which is the most common source of hydrogen employed. Other aspects of the generation of hydrogen from NaBH4 include reuse of the catalyst and use of ethylene glycol instead of aqueous solution to dissolve NaBH4, This reduces the temperature of the hydrolysis and allows the regeneration of the reaction products back to borohydride in one step.
Fuel Cell Performance Direct Borohydride Oxidation A comparison between two borohydride fuel cells operating at 60 and 85 C with two methanol–air fuel cells operating at 90 and 110 C is shown in Figure 6(a). The open-circuit value for methanol and borohydride fuel cells is 0.83 and 1.24 V, respectively. The IR drop in the borohydride fuel cells is the dominant effect obscuring the activation polarization overpotential on both cells, while in the methanol fuel cells, the activation polarization overpotential is 0.22 V and is followed by a gradual voltage decrease due to
Direct and Indirect Borohydride Fuel Cells
11
Figure 5 (a) Cell potential versus current density and (b) power density versus current density of a borohydride fuel cell at different temperatures. (○) 25 C, anode: Ni + PTFE powders pressed on a Ni foam and air cathode made of Pt or Ag supported on carbon black, separated by an NRE211 Nafion membrane.2 (□) 50 C and (▼) 85 C anode: AB2 (Zr0.9Ti0.1Mn0.6 V0.2Co0.1Ni1.1) feed with 10 wt% NaBH4 in 20 wt% NaOH at a flow rate of 0.2 l min1. Cathode: Pt/C feed with humidified O2 at 0.2 l min1 at 1 atm and Nafion membrane as electrolyte.11
IR drop but at lower rate than the borohydride fuel cell operated at 85 C. At current density of 400 mA cm2, the cells show approximately 0.4 V cell potentials except for the borohydride fuel cell operating at 60 C, which shows a cell potential of 0.65 V. The maximum power density of the methanol fuel cells is 160 and 180 mW cm2 occurring at 400 mA cm2 when they operate at 90 and 110 C, respectively. The borohydride fuel cells show maximum power densities of 190 mW cm2 at 300 mA cm2 and 290 mW cm2 at 600 mA cm2 for the cells operated at 60 and 85 C, respectively. It should be noted that borohydride fuel cells, operating lower temperatures, show comparable results with direct methanol fuel cells.
12
Direct and Indirect Borohydride Fuel Cells
Figure 6 (a) Cell potential versus current density for two borohydride fuel cells and two methanol fuel cells. Borohydride: (□) 10 wt% NaBH4 20 wt% NaOH, 0.15 ml min1 60 C.22 (■) 10 wt% NaBH4 in 20 wt% NaOH at a flow rate of 0.2 l min1, 85 C.11 Anode: AB2 (Zr0.9Ti0.1Mn0.6V0.2Co0.1Ni1.1) alloy feed with (cathode) Pt/C feed with humidified O2 at 0.2 l min1 at 1 atm and Nafion NE-424 membrane as electrolyte. Methanol: (○) 1 mol dm3 CH3OH (Buttin, 2001). Anode: 85% Pt/Ru in VULCAN XC carbon. Cathode: air feed 85% Pt on Vulcan, temperature 110 C. (•) Anode PtRu/c and cathode 1.37 mg cm2 Pt–Fe/C 90 C.23 (b) Power density versus current density for two borohydride fuel cells and two methanol fuel cells. Borohydride: (□) Li et al.,22 (■) Li et al.11 Methanol: (○) Buttin (2001), (•) Shukla.23 Electrode and operating conditions as in (a).
Typical performance data for the DBFC are presented in Table 4. Different catalysts and separators have been used and many cells use the membrane electrode assembly (MEA) configuration employed in the typical H2/O2 fuel cells. The data show many different electrolytes and conditions and it is difficult to compare; not always the complete picture of the experiments is presented, and the timescale of the experiment is generally omitted in literature reports. Despite the difficulty in comparing the data, it is clear that the performance of the borohydride fuel cells is below theoretical expectations. However, the DBFC compares well with methanol/air and H2/O2 fuel cells; at room temperature; discharge current
Table 4
Some typical performance data for borohydride fuel cells at a current density of approximately 50 mA cm2 Cell components
Conditions
Fuel cell performance
[BH4] (mol dm3)
T (K)
Open-circuit cell potential (V)
Typical current density (A cm2)
Cell potential at 0.1 A cm2
Specific energy density (kW h kg1)
Maximum power density output (W cm2)
References
Cathode
Membrane
Ni2B alloy
Ag on Ni Pt/C
Asbestos
6.2
0.4
298
0.92
0.01–0.06
0.73
NA
NA
10
Anion Pall RAI No. 2259-60 NA. Cation Nafion NE-424 Cation, 5% Nafion binder solution (Undivided) (Undivided)
6
5
343
0.95
0.01–0.3
0.6
0.184
0.06
1
6 5
0.05 2.5
NA 358
NA 1.26
0.12 0.02–0.3
0.7 0.95
0.42 NA
0.09 0.18
24 11
6
0.5
298
1.05
0.01–0.1
0.7
2.8
0.04
3
6 1–5
1 2
298 298
0.8 0.8
0.001–0.005 0.35
NA NA
NA NA
NA 0.019
25 26
6
0.04
298
0.94
0–0.25
0.57
NA
0.07
5
97% Au + 3% Pt particles on carbon cloth ZrCr0.8Ni1.2 Zr0.9Ti0.1Mn0.6V0.2Co0.1Ni1.1
Pt/C Pt/C
Pt/C
Pt/C
Au Pt/Ni
MnO2 MnO2/ C/C 2 mg MnO2
MmNi3.35Co0.75Mn0.4Al0.3 hydrogen storage alloy NA: not given.
(Undivided)
Direct and Indirect Borohydride Fuel Cells
Anode catalyst
[OH] (mol dm3)
13
14
Direct and Indirect Borohydride Fuel Cells
densities of 0.1 A cm2 can be observed at 0.7 V cell potential as shown in Figure 5. If the temperature increases from 323 to 363 K, better performance can be achieved. In addition, borohydride fuel cells appear to perform well when the concentration of borohydride is low, which might reduce the migration of borohydride ions to the catholyte and will allow periodical refueling of the anolyte with NaBH4.
Hydrogen peroxide as oxidant in the DBFC The reduction of hydrogen peroxide in acidic media is 4H2 O2 + 8H + + 8e ! 8H2 O, E0 ¼ 1:77V vs: SHE
[15]
Using this reaction in the borohydride fuel cell will increase the pH gradient across the membrane as well as contribute to a higher cell potential. The coupling of the oxidation of borohydride and the acid reduction of hydrogen peroxide has a theoretical cell potential of 3 V. If alkaline hydrogen peroxide is used, the cell potential decreases to 2 V. Hydrogen peroxide coupled with borohydride has been proposed for underwater-sea applications and was demonstrated by Raman et al.27; dispersed Pt/C and AB5 metal hydrogen storage alloys were used as cathode and anode electrodes, respectively, divided by a pretreated Nafion® 117 membrane sandwiched between the two electrodes. The catholyte was 15% hydrogen peroxide at pHs 0, 0.5, and 1.0, and the anolyte was aqueous borohydride solution with 10 wt% NaBH4 in 20 wt aqueous NaOH. Cell potentials of 1.6 V at 0.1 A cm2 were observed at 343 K and 1.2 V at room temperature. Power density was observed in the order of 0.12 and 0.35 W cm2 at 308 and 343 K, respectively. The data show that most of the overpotential losses in this fuel cell occur at the oxygen cathode. The performance of the two borohydride fuel cells operating at 70 C is shown in Figure 7; one cell operates with air, whereas the other cell with acidic hydrogen peroxide in the catholyte. The open-circuit potential of the cell operating with hydrogen peroxide is 1.21 V, while that of the borohydride–air fuel cell is below 1 V. In both cases, the activation polarization overpotential is obscured by the IR drop. The maximum power density for the hydrogen peroxide and oxygen fuel cells is 166 and 63 mW cm2 at current densities of 300 and 160 mA cm2, respectively. Overall, the performance of the hydrogen peroxide fuel cell is higher than the cell operated with an air-breathing cathode.
Indirect Oxidation On-site hydrogen generator of gas using NaBH4 for a PEMFC is designed for different applications than the DBFC. The competitiveness of the DBFCs and IBFCs can be analyzed in terms of cost29; the conclusion is that borohydride crossover makes DBFC uncompetitive compared with IBFC. If the crossover was resolved, the cost of a six-electron transfer in a DBFC would be equivalent to the IDBFCs, although the volume consumption of NaBH4 would be 1.7 times larger. If an eight-electron transfer borohydride
Figure 7 Comparison of two borohydride fuel cells operating at 70 C with different oxidants: (•) 8.9 mol dm3 H2O2 in contact with a cathode made of 60 wt% Pt/C (1 mg cm2 of Pt) and an AB5 anode made of MmNi3.55Al0.3Mn0.4Co0.75 (5 mg cm2) in contact with 10% aqueous NaBH4 in 20 wt% NaOH.28 (□) Anode: highly dispersed 87% Au + 3% Pt Air on carbon silk.1
Direct and Indirect Borohydride Fuel Cells
15
oxidation can be achieved, the DBFC would be more competitive. Another study showed that a hydrogen generator can produce 120 nl min1 of hydrogen, which is fed into a PEMFC. The cell operates at a cell potential of 0.7 V and could generate 12 kW power.
Commercial Ventures and Future Developments The promising results obtained in the laboratory have not been sufficient to stimulate a large number of commercial activities of this technology. More work has been done in the study of hydrogen generation for IDBFCs in the United States where hydrogen generators and PEMFC have been developed at different scales. In Japan, DBFCs for microelectronics and automobile traction applications have been tested; 20 and 400 W cells for portable applications were designed. In the United Kingdom, the development of a direct borohydride microfuel cell containing a gold anode for special applications has been tested. In Israel, the DBFC has begun commercialization, targeting the portable electronic market and some military applications. Despite the borohydride fuel cell being studied by relatively few research groups, it shows a promising performance. In comparison with the number of person-years that have been invested in the study of methanol and hydrogen fuel cells, DBFCs have progressively emerged as an alternative source of energy.
Challenges Borohydride fuel cells face a number of needs and challenges that need to be addressed, including the following:
• • • • • • •
Oxygen cathodes tolerant to borohydride ions in alkali solutions performing similar to cathodes in acidic media. Low-overpotential anodes able to withdraw eight electrons from borohydride ions. Anion-permeable membranes that are chemically stable in alkali. Long-term studies to assess the effect of metaborate on the catalyst activity and membranes. Issues of mass balance in the electrolyte during long-term operations. Metaborate removal and water management. If cation membranes are used, a strategy to return alkali solutions from catholyte to anolyte should be studied.
Summary This article has focused on fuel cells involving direct and indirect oxidation of borohydride ions used in fuel cells. The DBFCs being inherently more efficient than indirect oxidation produce energy densities comparable to methanol and oxygen/hydrogen fuel cells. The DBFC still has a number of aspects that need to be studied such as borohydride crossover, selective anode materials, and improved anionic membranes. The IDBFCs use the well-studied MEA, which is fed with high-purity hydrogen produced from the efficient hydrolysis of sodium borohydride. The main problems in this cell are the removal of borohydride oxidation products, reuse of catalyst, and heat and water management. Overall, the use of borohydride as an alternative source of energy is promising, but more fundamental data and long-term operational experience with larger cells are necessary.
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