FUEL CELLS – EXPLORATORY FUEL CELLS | Sodium Borohydride Fuel Cells

FUEL CELLS – EXPLORATORY FUEL CELLS | Sodium Borohydride Fuel Cells

Sodium Borohydride Fuel Cells C Ponce de Leo´n and FC Walsh, University of Southampton, Southampton, UK & 2009 Elsevier B.V. All rights reserved. Int...

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Sodium Borohydride Fuel Cells C Ponce de Leo´n and FC Walsh, University of Southampton, Southampton, UK & 2009 Elsevier B.V. All rights reserved.

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 that can achieve energy densities comparable to those of 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 nanotube materials such as carbon and titanium. The specific energy density of these systems is still insufficient for portable applications, and this has led to the research for alternative fuels. Ideally, the fuel should be safe and easily transportable at high concentrations, should oxidize fast, and provide large gravimetric and volumetric energy densities. Extensive reviews of fuel cells appeared in 2003, but the discussion of borohydride fuel cells had been limited to some electrode materials and the use of borohydride as a hydrogen source for proton-exchange membrane fuel cells (PEMFCs). More recent reviews include the comparison of different anodes, cathodes, and membranes used in direct borohydride fuel cells (DBFCs), evaluation of the cost of DBFCs and indirect borohydride fuel cells (IBFCs) considering borohydride crossover, and comparison of methanol and borohydride fuel cells. Although early studies in 1965 noted the potential use of borohydride as a fuel, few papers appeared in the literature before 2000. Since then, rapid progress has been achieved in both DBFCs and IBFCs. An air-breathing cathode can be used to couple with the direct oxidation of borohydride; this type of electrode has been well characterized

Table 1

in the literature and will not be analyzed here. Another common oxidant used in the borohydride fuel cell is hydrogen peroxide, which can be used in underwater vehicles and anaerobic applications. Chemical Hydrides The substitution of the hydrogen storage systems mentioned above by the in situ generation of hydrogen using a reformer reactor, or by hydrolysis of chemical hydrides such as LiNH4, LiH, NaH, NaAlH4, or 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 summarizes some typical hydride reactions and the respective percentages of hydrogen deliverable and energy capacities; lithium salts have the highest energy capacity and hydrogen yield, whereas sodium borohydride has the advantage of being 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. The 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, 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.

Some typical hydride reactions and their theoretical performance characteristics

Reaction

H2 yield (wt%)

Specific energy capacity (kWh kg1)

Maximum percent of deliverable H2

LiBH4 þ 4H2 O-LiOH þ H3 BO3 þ 4H2 LiH þ H2 O-LiOH þ H2 NaBH4 þ 4H2 O-NaOH þ H3 BO3 þ 4H2 LiAlH4 þ 4H2 O-LiOH þ AlðOHÞ3 þ 4H2 NaAlH4 þ 4H2 O-NaOH þ AlðOHÞ3 þ 4H2 CaH2 þ 2H2 O-CaðOHÞ2 þ 2H2 NaH þ H2 O-NaOH þ H2

8.6 7.7 7.3 7.3 6.4 5.2 4.8

1.63 1.46 1.38 1.38 1.21 0.99 0.91

37.0 25.4 21.3 21.2 14.8 9.6 8.4

192

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Direct and Indirect Borohydride Fuel Cells The direct oxidation of borohydride ions coupled with the reduction of oxygen has an equilibrium cell potential, Ecell of approximately 1.64 V according to the following reactions: Anode: BH4  þ 8OH  8e -BO2  þ 6H2 O E ¼ 1:24 V vs standard hydrogen electrode ðSHEÞ

½I

Cathode: O2 þ 2H2 O þ 4e -4OH E ¼ 0:4 V vs SHE

½II

Cell: BH4  þ 2O2 -BO2  þ 2H2 O Ecell ¼ 1:64 V vs SHE

½III

The metaborate ion product, BO2  , can be recycled to borohydride ion and is environmentally friendly. The cell potential of the DBFC, described by eqn [III], leads to a theoretical energy density of 9.3 kWh kg1, which is larger than those of methanol (6.2 kWh kg1) and hydrogen at 4500 psi (0.45 kWh kg1). The energy density in solution will be lower despite the fact that borohydride solutions can be prepared up to 30 wt% in concentrated alkaline aqueous solutions (>6 mol dm3). The half-life of sodium borohydride is over 400 days when stored at pH 14 and 298 K under a nitrogen atmosphere. The high concentration of alkaline is necessary to avoid the hydrolysis reaction that in neutral solution and in the presence of platinum or cobalt(II) chloride catalyst reacts according to BH4  þ 2H2 O-BO2  þ 4H2

½IV

Equation [IV] provides the basis for the IBFCs; borohydride supplies the hydrogen gas, which 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 five decades by a large number of academic and industrial research groups: 2H2 þ O2 -2H2 O Ecell ¼ 1:18 V vs SHE

½V

The hydrogen reactant for this fuel cell can be obtained from hydrogen generators consisting of a reactor containing borohydride in the presence of a catalyst to carry out reaction [IV]. 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 even small amounts of carbon monoxide (o0.1%) and sulfur. Hydrogen needs to be purified, which increases the number of reactors used.

193

Electrolysis is another common technology to produce high-purity hydrogen, but 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 this process, however, depends on location and climate changes. In the DBFC, the hydrolysis reaction [IV] must be suppressed in order to take advantage of the large 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 are considered.

Catalytic Materials Electrode Materials for Direct Borohydride Fuel Cells The electrochemical oxidation of borohydride was considered for fuel cell 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 catalyst materials studied during the 1960s for borohydride oxidation in strong alkali solutions were nickel, platinum, palladium, and gold. High open-circuit potentials and current densities of E0.1 A cm2 indicated the possibility of achieving reasonable energy and power density outputs; however, the number of transferred electrons was low (4 at Ni and o4 at Pt electrode). High-surface-area electrodes, such as Pt and Pd on Ni and C substrates, have been reported with negative opencircuit potentials at the Pt surfaces. The oxidation of borohydride yielded a six-electron process on platinum deposited on a sintered porous Ni plate at 50–200 mA cm2 current densities. Direct oxidation of borohydride ion requires selective anodes with low catalytic activity for the decomposition of borohydride to hydrogen and borates (eqn [IV]). More recently, several electrode materials for the direct oxidation of borohydride have been evaluated; these include 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 eightelectron transfer stated in the theoretical oxidation reaction (eqn [I]). Transfer of less than eight electrons indicates partial oxidation or high rates of the hydrolysis of borohydride through reaction [IV]. Figure 1(a) shows the cell voltage for different electrode materials in a borohydride fuel cell versus the current density, while Figure 1(b) shows the change in

194

Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

1.0

Cell voltage, Ecell (V)

0.8

0.6

0.4

0.2

0.0

0

20

40 60 80 100 120 Current density, j (mA cm−2)

(a)

140

160

High-surface-area electrodes

Power density (mW cm−2)

60

40

20

0 0 (b)

the power density with the current density. Nickel electrodes show a higher performance than Au/Pt, Pt inks, and MnO2 electrodes. More detailed investigations of the oxidation of borohydride have been carried out on gold ultramicroelectrodes. These studies concluded that eight electrons were transferred during the oxidation of borohydride and that the reaction becomes mass transport controlled at high overpotentials. Other studies on gold and platinum electrodes have shown 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 the performance. A rather negative open-circuit voltage could be obtained on nickel electrodes, and acceptable overpotentials were observed at several hundreds of milliamperes per square centimeter, especially at elevated temperatures. However, the number of electrons remained low at four, indicating that only half the theoretical energy value could be obtained with nickel anodes.

50

100

150

200

250

Current density, j (mA cm−2)

Figure 1 Performance parameters of different anodes in a borohydride fuel cell: (a) cell potential vs current density; (b) power density vs current density. () 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% NaOH (Amendola et al., 1999); (J) Ni þ 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 NaOH (Liu et al., 2005); (’) cathode and anode inks prepared using unsupported Pt black and Nafion, 0.5 mol dm3 NaBH4 in 6 mol dm3 NaOH (Kim, 2004); (&) manganese dioxide cathode and standard Pt/Ni anode (Electro-Chem-Technic, UK) with 1 mol dm3 NaBH4 in 3 mol dm3 KOH (with no membrane; Verma and Basu, 2005b); (,) MmNi3.35Co0.75Mn0.4Al0.3 (Mm, misch metal alloy) anode and 2 mg cm2 MnO2 cathode in an electrolyte consisted of 0.4 g of KBH4 in 200 cm3 of 6 mol dm3 KOH (Wang and Xia, 2006). All experiments were reported at room temperature.

High-surface-area electrodes containing gold or gold alloy (Au 97% þ Pt 3%) catalysts have been used for the oxidation of borohydride. The number of electrons transferred at 0.2 mA cm2 current density and 343 K approaches 7. Platinum high-surface-area electrodes could achieve current densities in the range of 10–60 mA cm2, with approximately five electrons being 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 of the nickel electrode is lower than that of palladium and platinum electrodes. Table 2 gives the comparison of different anode materials for borohydride oxidation. As is evident from the table, gold- or nickel-based materials are the best choices for the oxidation of borohydride; nickel gives the most negative potential for borohydride oxidation, but with a low number of electrons being transferred, whereas gold requires a higher overpotential but gives a better fuel efficiency. Further work on 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 to be stored within the lattice of the anode alloy. 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

Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

Table 2

195

Summary of the performance of electrode materials for the oxidation of borohydride

Anode material

z value

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 NA NA 7, 8 2–4 6 5, 6 NA NA 4 7.5

Open-circuit potential vs SHE (V)

 1.03  1.03  1.02  0.99  1.0  1.00,  0.91  0.91  0.99  1.07  1.15  1.12

Performance Cell voltage (V)

Current density (A cm2)

0.7  0.6 NA NA  0.6  0.92

0.6 NA 40.1 possible 0.7 0.1 o0.1 NA 0.1 0.3 0.1

NA  0.99  0.7 0.58

Mm, misch metal alloy; NA, not given in the reference; SHE, standard hydrogen electrode; z, the number of electrons transferred during the oxidation reaction.

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 the hydrolysis of borohydride, while the other metal oxidizes the generated hydrogen. Catalytic Materials for Indirect Borohydride Fuel Cells The generation of hydrogen from the hydrolysis reaction 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 IBFCs the concentrated borohydride solution in aqueous alkali is converted to hydrogen gas in a catalytic generator, and the hydrogen gas is then used for a PEMFC or any other hydrogen-fueled cell. Such systems have been recommended 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 according to the following reaction, where an aqueous solution of sodium borohydride is pumped through a catalyst bed: NaBH4 þ 2H2 O

catalyst

4H2 þ NaBO2 þ heat

½VI

A schematic diagram of the hydrogen generator system is shown in Figure 2. In alkaline solutions, the reaction without the catalytic metal is slow. The advantages of the in

High-purity H2 to the fuel cell Gas/liquid separator

Reactor

Catalyst bed

Alkaline borohydride solution

By-product

Pump Recycle unit

Figure 2 The generation of hydrogen from sodium borohydride solution.

situ hydrogen generation for the PEMFCs are as follows: (1) 50% of the hydrogen generated is provided by the water in the electrolyte; (2) platinum anodes can be used in the PEMFC, as only high-purity hydrogen is supplied; (3) controlled supply of humidified H2 gas; (4) continuous generation of H2 by addition of NaBH4 into the hydrolysis reactor; and (5) water recirculation from the fuel cell back to the hydrolysis reactor. Some challenges for the development of this system are (1) cheap catalyst with high conversion and optimum reaction rate, (2) tolerance of the catalyst to the borate product, (3) water reuse, (4) optimal connection to the PEMFC, and (5) optimal reactor design. One of the most commonly used 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 that of the polycrystalline Co, in 1 wt% NaOH þ 10 wt% NaBH4 solution at 203 K. Unlike carbon reformers, hydrogen generators using borohydride solutions do not need to be preheated; the

196 Table 3

Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

Comparison of selected catalysts for the hydrolysis of NaBH4

Catalyst

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 g NixB cat heat-treated at 423 K in vacuum 0.3 g NixB cat heat-treated at 353 K in air Co–P electroplated on Cu at 0.01 A cm2 Co electroplated on Cu at 0.01 A cm2 1.4 m mol dm3 water dispersed Ru(0) nano clusters 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

Hydrogen generation rate (L min1)

Conditions NaBH4

NaOH

T (K )

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

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 – – – – – 298 – – 303 303 298 293 293 293 323 283 293 293 293 293 293

reactor 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, as it causes blocking of the active sites of the catalysts and clogging; further work in heat and water management is required. Table 3 gives 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 the lowest.

Direct Oxidation Two mechanisms are generally recognized: Mechanism (A): The first step is an electron transfer to form the BH4 d radical: ½VII

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 transfers. Mechanism (B): The first step is the electron transfer, reaction [VII], followed by predissociation at surface sites of the catalyst: 2M þ BH4 d -M  H þ M  BH3 

combination as a secondary reaction to produce hydrogen gas. Metallic surfaces that are able to support predissociation are generally good catalysts, and the adsorption of hydrogen atoms leads normally to the formation of hydrogen gas. Since gold does not support coverage by adsorbed hydrogen atoms, mechanism (A) would be expected. In nickel, platinum, and palladium, hydrogen gets adsorbed and mechanism (B) would predominate. It is likely that the reaction M þ H2 O þ e -M  H þ OH

Mechanism of Borohydride Oxidation

BH4   e -BH4 d

20 mL at 10 wt% 20 mL at 10 wt% 20 mL at 10 wt% 20 mL at 10 wt% 20 mL at 10 wt% 20 mL at 10 wt% 20 mL at 10 wt% 100 mL at 10 wt% 100 mL 10 wt% 100 mL at 10 wt%

½VIII

This is followed by a series of steps that involve electron transfer, surface reaction, and hydrogen atom

½IX

will occur at the open-circuit potential for borohydride oxidation because the formal potential of borohydride oxidation (see eqn [I]) is negative compared to the formal potential of the H2O–H2 couple. As a result, a mixed potential would be observed in 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 because of the additional hydrogen evolution via reaction [IX]. 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 ion, via reaction [I], should be used, but as most catalytic materials also catalyze hydrolysis of the borohydride ion (eqn [IV]), 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

Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

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 via reaction [I], could restrict the eight-electron transfer involved in the oxidation of borohydride:

3 wt% Ru/C extruded, and 0.5 wt% Ru/C granules. Most catalysts disintegrated during the experiments due to the evolution of hydrogen except for the Ru/C extruded catalyst. The hydrolysis reaction was assumed to occur in two steps; first, the BH4  ion gets adsorbed on the surface of the catalyst: M þ BH4  -M  BH4 

2H2 O þ 2e -H2 þ 2OH 



½X

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 of BH3OH, which is formed during the partial hydrolysis reaction of BH4  ions: BH4  þ H2 O-BH3 OH þ H2

½XI

The BH3OH ion will have the effect of increasing the open-circuit potential because it is more readily oxidized than BH4  . Indirect Oxidation The heterogeneous oxidation of borohydride for the generation of hydrogen from alkaline solutions is preferred to the homogeneous 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 owing to the fast proton production during its reduction by BH4  ions. Works on the investigation of the mechanism of hydrogen production from alkaline sodium borohydride show that the reaction kinetics is of the first order when the molar ratio [BH4  ]/[Pd] 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% was 2.4  104 s1, whereas in the absence of the catalyst at pH 13 it was 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 depends 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,

197

½XII

and second, the adsorbed species react to form adsorbed hydrogen: M  BH4  þ 2H2 O-MBO2  þ 4H2  M

½XIII

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). Thus, the importance of water management for the effective operation of a hydrogen generator system needs to be highlighted. It has also been shown 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 d½NaBH4  ¼ k½NaBH4 0:41 ½NaOH0:13 ½H2 O0:68  wdt

½XIV

where w is the mass of sodium borohydride and k 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 mass 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 catalytically active materials are used, e.g., acids, or by raising the temperature. The use of heterogeneous catalysts at different temperatures is the most commonly used 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 values of 75 kJ mol1 for Co, 71 kJ mol1 for Ni, and 63 kJ mol1 for Raney Ni. In 2004 Kojima and colleagues investigated the hydrolysis of NaBH4 in various metal oxides covered with Pt. They found that Pt–LiCoO2 catalyst worked as

Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

an excellent H2 generator from NaBH4 solutions. The catalyst was prepared from a honeycomb cordierite monolith, which was immersed in a 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 of calcination at 720 K. The final concentration of Pt in the monolith was 1.5 wt%.

Load



+ −

OH 6H2O − BO2

8e−

12OH



8e−



8OH − BH4

(a)



OH



2O2 4H2O

Cathode

OH

Cation permeable membrane

Direct Oxidation

Load −

+ −

OH 8e− Anode



OH

6H2O − BO2

12OH



2O2 4H2O

8OH − BH4



8e−



8OH



OH



Cathode

OH

Anion permeable membrane

(b)

Figure 3 Borohydride fuel cells with (a) a cation-permeable membrane and (b) an anion-permeable membrane, drawn to emphasize the chemical balance. 35 1.0 30 0.8 Cell voltage, Ecell (V)

Most borohydride fuel cells consist of two electrodes separated by a membrane. Both cationic and anionic membranes have been in use, and each produces different chemical characteristics within the cell. Cation membranes lead to a chemical imbalance; the oxidation of 1 mol of BH4  transfers 8 mol of Na þ ions across the membrane, increasing the concentration of NaOH in the catholyte and decreasing it in the anolyte. Long operation tests could cause problems because BH4  ions are stable only in solutions with strong alkali concentration. Hence, a procedure to return NaOH from the catholyte to the 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 needs to be replenished. Figure 3 shows sketches of the cell with each type of membrane from a cell with sodium electrolyte. It is assumed that the electrode reaction (eqn [I]) is occurring without the competing hydrolysis of borohydride, and chemical changes during energy generation are emphasized, though 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. Nafions 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 Tosohs is no longer manufactured because of the very high cost of production. At present, the commercially available anionpermeable 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 E5%, whereas higher concentrations are required to ensure the stability of sodium borohydride solution. A comparison of a Nafion N115 cationic membrane and two anionic membranes, Asahi Kasei A-501SB and CU1, in a borohydride fuel cell is shown in Figure 4 in terms of cell voltage and power density versus current



8Na+ Anode

Fuel Cell Configurations

OH

25 20

0.6

15 0.4 10 0.2

Power density (mW cm−2)

198

5

0.0

0 0

20

40 60 80 Current density, j (mA cm−2)

100

Figure 4 Comparison of cell voltage and power density vs current density for a borohydride fuel cell using three different (one cationic and two anionic) membranes: cationic (J) Nafion 115; anionic (&) Asahi Kasei A-501SB membrane at 298 K; Ni þ PTFE powders pressed on Ni foam as anode and air cathode made of Pt or Ag supported on carbon black (Liu et al., 2005). (m) CU1 membrane area 4 cm2; anode, 2 mg cm2 Au/C; cathode, 2 mg cm2 Pt/C. Cell: parallel flow field with 1.32 mol dm3 NaBH4 in 2.5 mol dm3 NaOH at 10 cm3 min1 and O2 at 200 cm3 min1 at 298 K (Cheng et al., 2007).

density. The thickness of the membranes is E130 mm. The open-circuit potentials of the systems are similar, but the fuel cell operating with the cationic membrane presents higher cell voltage than the systems with the

Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

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 systems operating 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 as they help to avoid interactions between the reactants and the products at both the electrodes. Nevertheless, an undivided borohydride–oxygen fuel cell might be possible, as both the reactants do not react in a homogeneous solution. This will require a cathode catalyst inactive toward borohydride ions and their 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 voltage approached E0.6 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 and with MnO2/C/Ni cathode and Pt black on a Ni mesh anode. Current densities of 35 mA cm2 at a cell voltage of 0.8 V in 1 mol dm3 NaBH4 and 5 mol dm3 NaOH have been achieved. 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 a strong concentration gradient and the crossover of borohydride ions across the membrane, particularly in anion membranes at open circuit. In 2006 Chatenet and colleagues studied the reduction of oxygen on carbonsupported 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 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 SHE while the Pt/C electrode disaggregates due to the hydrogen evolution from the hydrolysis of borohydride, and its open-circuit potential falls to E  0.8 V

199

versus SHE (E0.2 V versus SHE without BH4). This potential reflects the mixed potential value of the cathodes that tend to catalyze the oxidation of borohydride ions rather than reduce oxygen. In the case of manganese oxide electrode, the open-circuit potential changes from 0.075 to 0.05 V versus SHE in the 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, wherein 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 the anions and cations to the anode and cathode, respectively. If an anion membrane is used, the borohydride ions are blocked from the catholyte compartment, and the electrical field created during operation prevents their migration from the anolyte to the 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 electrolyte charges balanced, and if hydroxide is more concentrated in the catholyte, it will be favorable. In 2006 Lakeman and colleagues 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 on one side of the membrane and 30 wt% NaOH on the other. The crossover measurements were taken during the 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, UK) showed the highest crossover value of 4.6  106 mol cm2 s1, while the lowest borohydride transport of 0.4  106 mol cm2 s1 was observed in a Nafion 117 membrane. 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 that 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.

Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

The physical properties of borohydride solutions such as specific gravity, viscosity, and melting point as functions 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 have shown that the open-circuit potential at Zn–Ni alloy anode electrode is not sensible when the concentration of borohydride ion is more than E5 wt%.

1.2

Cell voltage, Ecell (V)

200

1.0

0.8

0.6

Temperature

Indirect Borohydride Oxidation The hydrolysis of borohydride to produce hydrogen gas is normally carried out in aqueous NaOH, KOH, or neutral solutions. The efficiency of hydrogen generation increases from E92% up to E99% as the concentration of alkali increases, and is also affected by the concentration of NaBH4, which is the most common source of hydrogen employed. Other aspects of the generation of hydrogen from NaBH4 include reuse of the catalyst and the use of ethylene glycol instead of an aqueous solution to dissolve the NaBH4. This reduces the temperature of the hydrolysis and allows the regeneration of the reaction products back to borohydride ion in one step.

Fuel Cell Performance Direct Borohydride Oxidation A comparison of two borohydride fuel cells operating at 333 and 353 K with two methanol fuel cells operating at 363 and 383 K is shown in Figures 6(a) and 6(b), respectively. The open-circuit cell voltage for methanol and borohydride fuel cells is E0.83 and 1.24 V, respectively. The IR (ohmic) drop is the dominant effect obscuring the activation polarization overpotential in both the borohydride fuel cells. In the methanol fuel cells, the activation polarization overpotential is E0.22 V, which is followed by a gradual decrease in voltage due to IR drop, but at a rate lower than that of the borohydride fuel cell operated at 358 K. At a current density of 400 mA cm2,

0.4 0

100 200 300 Current density, j (mA cm−2)

400

0

100 200 300 Current density, j (mA cm−2)

400

(a)

200

150 Power density (mW cm−2)

Unlike methanol fuel cells, which operate at between 343 and 373 K, the borohydride fuel cells provide similar energy levels at room temperature, E298 K. The energy and cell voltages of the DBFC are higher at elevated temperatures (E373 K), but the stability of the solutions decreases, and so does the availability of fuel. A clear indication of the better performance of the borohydride fuel cell is presented in Figure 5 in the form of cell voltage (Figure 5(a)) and power density (Figure 5(b)) at different temperatures. The highest cell voltage and power density occur at 258 K.

100

50

0 (b)

Figure 5 (a) Cell voltage vs current density and (b) power density vs current density of a borohydride fuel cell at different temperatures. (J) 298 K, anode: Ni þ 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 (Liu et al., 2005); (&) 323 K; (.) 358 K. Anode: AB2 (Zr0.9Ti0.1Mn0.6V0.2Co0.1Ni1.1) fed with 10 wt% NaBH4 in 20 wt% NaOH at a flow rate of 0.2 L min1. Cathode: Pt/C fed with humidified O2 at 0.2 L min1 at 1 atm and Nafion membrane as electrolyte (Li et al., 2003).

the cells show cell voltages of E0.4 V, except for the borohydride fuel cell operating at 333 K, which shows a cell voltage of 0.65 V. The maximum power density of the methanol fuel cells is E160 and E180 mW cm2 occurring at E400 mA cm2 when they operated at 90 and 383 K, respectively. The borohydride fuel cells show maximum power densities of E190 mW cm2 at

Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

1.2

Cell potential, Ecell (V)

1.0

0.8

0.6

0.4

0.2

0.0 0

100

(a)

200 300 400 500 Current density, j (mA cm−2)

600

300

Power density (mW cm−2)

250

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 is difficult to be compared; not always the complete picture of the experiments is presented, and the time scale of the experimental runs is generally omitted in the works reported in the literature. 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 densities of 0.1 A cm2 can be observed at E0.7 V cell voltage 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 direct borohydride fuel cells

The reduction of hydrogen peroxide in acidic media is

200

4H2 O2 þ 8Hþ þ 8e -8H2 O E - ¼ 1:77 V vs SHE

½XV

150

100

50

0 0

(b)

201

100

200

300

400

500

600

Current density, j (mA cm−2)

Figure 6 (a) Cell voltage vs current density and (b) power density vs current density for two borohydride fuel cells and two methanol fuel cells. Borohydride: (&), 10 wt% NaBH4 in 20 wt% NaOH, 0.15 mL min1 at 333 K (Li et al., 2005); (’), 10 wt% NaBH4 in 20 wt% NaOH at a flow rate of 0.2 L min1 at 358 K; anode, AB2 (Zr0.9Ti0.1Mn0.6V0.2Co0.1Ni1.1) alloy; and cathode, Pt/ C fed with humidified O2 at 0.2 L min1 at 1 atm and Nafion NE424 membrane as ionic separator (Li et al., 2003). Methanol: (J), 1 mol dm3 CH3OH; anode, 85% Pt/Ru in Vulcan XC carbon; cathode, air feed 85% Pt on Vulcan, at 383 K (Buttin, 2001); (), anode Pt–Ru/C and cathode 1.37 mg cm2 Pt–Fe/C at 363 K (Shukla and Raman, 2003).

E300 mA cm2 and E290 mW cm2 at E600 mA cm2 for the cells operated at 333 and 358 K, respectively. It should be noted that the borohydride fuel cells operated at lower temperatures show results comparable with those of the direct methanol fuel cells. Typical performance data for the DBFC are presented in Table 4. Different catalysts and separators have been

Using this reaction in the borohydride fuel cell will increase the pH gradient across the membrane as well as contribute to a higher cell voltage. The coupling of the oxidation of borohydride and the acid reduction of hydrogen peroxide results in a theoretical cell voltage of E3 V. If alkaline hydrogen peroxide is used, the cell voltage decreases to E2 V. Hydrogen peroxide coupled with borohydride has been proposed for underwater/deep sea applications as was demonstrated by Raman and colleagues in 2004; dispersed Pt/C and AB5 metal hydrogen storage alloy were used as cathode and anode, 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 voltages of 1.6 and 1.2 V at 0.1 A cm2 were observed at 343 K and at room temperature, respectively. 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 343 K 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 air–borohydride fuel cell is below

Ag on Ni Pt/C

Pt/C Pt/C

Pt/C

MnO2 MnO2/ C/C 2 mg MnO2

Ni2B alloy 97%Au þ 3%Pt particles on carbon cloth ZrCr0.8Ni1.2 Zr0.9Ti0.1Mn0.6V0.2Co0.1Ni1.1

Pt/C

Au Pt/Ni

NA, not given in the reference.

MmNi3.35Co0.75Mn0.4Al0.3 hydrogen storage alloy

Cathode

Anode catalyst

(Undivided)

Asbestos Anion Pall RAI No. 2259-60 NA Cation Nafion NE-424 Cation, 5% Nafion binder solution (Undivided) (Undivided)

Membrane

6

6 1–5

6

6 5

6.2 6

[OH ] (mol dm3)



Conditions

0.04

1 2

0.5

0.05 2.5

0.4 5

[BH4 ] (mol dm3)



298

298 298

298

NA 358

298 343

T (K)

0.94

0.8 0.8

1.05

NA 1.26

0.92 0.95

Open-circuit voltage (V)

0–0.25

0.001–0.005 0.35

0.01–0.1

0.12 0.02–0.3

0.01–0.06 0.01–0.3

Typical current density (A cm2)

Fuel cell performance

Some typical performance data for borohydride fuel cells at a current density of B50 mA cm2

Cell components

Table 4

0.57

NA NA

0.7

0.7 0.95

0.73 0.6

Cell voltage at 0.1 A cm2

NA

NA NA

2.8

0.420 NA

NA 0.184

Specific energy density (kWh kg1)

0.07

NA 0.019

0.04

0.09 0.18

NA 0.06

Maximum power output (W cm2)

202 Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

160 140

1.0

120 0.8 100 0.6

80 60

0.4

40 0.2 0.0

Power density (mW cm−2)

Cell potential, Ecell (V)

1.2

20 0

100

200 300 400 500 Current density, j (mA cm−2)

0 600

Figure 7 Comparison of two borohydride fuel cells operating at 343 K 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 (Mm, misch metal alloy; 5 mg cm2) in contact with 10% aqueous NaBH4 in 20 wt% NaOH (Choudhury et al., 2005); (&) anode, highly dispersed 97% Au þ 3% Pt–air on carbon silk (Amendola et al., 1999).

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 a hydrogen peroxide fuel cell is better than that of a cell with an air-breathing cathode. Indirect Oxidation On-site generators of hydrogen gas using NaBH4 for PEMFCs are designed for applications different from those of the DBFCs. The competitiveness of the DBFCs and IBFCs can be analyzed in terms of cost; the conclusion is that borohydride crossover makes DBFCs uncompetitive compared to IBFCs. If the crossover was resolved, the cost of a six-electron transfer in a DBFC would be equivalent to that of an IBFC, although the volume consumption of NaBH4 would be 1.7 times larger. If an eight-electron transfer borohydride oxidation can be achieved, the DBFC would be more competitive. Another study showed that a hydrogen generator can produce 120 mL min1 of hydrogen, which can be fed into a PEMFC. The cell operates at a cell potential of 0.7 V and would generate 12 kW.

Commercial Ventures and Potential Future Developments The promising results obtained in the laboratory have not been sufficient to stimulate a large number of

203

commercial activities using this technology. Much work has been done in the study of hydrogen generation for IBFCs in the United States, where hydrogen generators and PEMFCs 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 have been designed. In the United Kingdom, a direct borohydride microfuel cell containing a gold anode for special applications has been tested. In Israel, commercialization of DBFCs has begun, targeting the portable electronics market and some military applications. Direct borohydride fuel cells show a promising performance. In comparison with the extended development of methanol and hydrogen fuel cells over the last 25 years, DBFCs have developed rapidly, over the last few years, as an alternative source of electrical energy from a liquid fuel.

Challenges Borohydride fuel cells face a number of challenges that need to be addressed. These include oxygen cathodes tolerant to borohydride ions in alkali • solutions performing similarly to cathodes in acidic

• • • • • •

media, low overpotential anodes able to withdraw eight electrons from the borohydride ion, 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 longterm operations, metaborate removal and water management, and if cation membranes are used, a strategy to return alkali solutions from catholyte to anolyte.

Summary Fuel cells utilizing the direct and indirect oxidation of borohydride ions have been described. Direct borohydride fuel cells are inherently more efficient than IBFCs; the DBFCs can produce energy densities comparable to those of methanol and oxygen–hydrogen fuel cells. The DBFC still presents a number of technical challenges such as borohydride ion crossover and the need for more selective anode materials and improved anionic membranes. The IBFCs use the well-studied MEA that is fed with high-purity hydrogen produced from the efficient hydrolysis of sodium borohydride. The main problems in this cell are removal of borohydride oxidation products, reuse of catalyst, and heat and water management.

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Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

Overall, the use of the borohydride ion as an alternative source of energy is promising, but more fundamental and long-term data are necessary.

Nomenclature Symbols and Units E/Volts Ecell/Volts EO/Volts I/A J/mA cm  2 k/cm s  1 R/ohm T/1C w/mol z

voltage cell voltage equilibrium voltage current current density rate constant electrical resistance temperature mass number of electrons in the reaction

Abbreviations and Acronyms DBFC IBFC IRA Lm MEA Mm PEMFC PTFE SHE

direct borohydride fuel cell indirect borohydride fuel cell trade mark for ion exchange membrane lanthanum-rich misch metal membrane–electrode assembly Misch metal alloy proton-exchange membrane fuel cell polytetrafluoroethene standard hydrogen electrode

Further Reading Amendola SC, Onnerud P, Kelly PT, Petillo PJ, Sharp-Goldman SL, and Binder M (1999) A novel high power density borohydride-air cell. Journal of Power Sources 84(1): 130--133. Amendola SC, Sharp-Goldman SL, Janjua MS, et al. (2000) Safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. International Journal of Hydrogen Energy 25(10): 969--975. Buttin D, Dupont M, Straumann M, et al. (2001) Development and operation of a 150 W air-feed direct methanol fuel cell stack. Journal of Applied Electrochemistry 31(3): 275--279. Chatenet M, Micoud F, Roche I, Chainet E, and Vondrak J (2006a) Kinetics of sodium borohydride direct oxidation and oxygen reduction in sodium hydroxide electrolyte. Part II. O2 reduction. Electrochimica Acta 51: 5452--5458. Chatenet M, Micoud F, Roche I, Chainet E, and Vondrak J (2006b) Kinetics of sodium borohydride direct oxidation and oxygen reduction in sodium hydroxide electrolyte. Part I. BH4 electrooxidation on Au and Ag catalysts. Electrochimica Acta 51: 5449--5467. Cheng H and Scott K (2006) Investigation of non-platinum cathode catalysts for direct borohydride fuel cells. Journal of Electroanalytical Chemistry 596: 117--123. Cheng H, Scott K, Lovell KV, Horsfall JA, and Waring SC (2007) Evaluation of new ion exchange membranes for direct borohydride fuel cells. Journal of Membrane Science 288(1–2): 168--174. Cho KW and Kwon HS (2007) Effects of electrodeposited Co and Co–P catalyst on the hydrogen generation properties from hydrolysis of alkaline sodium borohydride. Catalyst Today 120(3–4): 336--340.

Choudhury NA, Raman RK, Sampath S, and Shukla AK (2005) An alkaline direct borohydride fuel cell with hydrogen peroxide as oxidant. Journal of Power Sources 143: 1--8. Feng RX, Dong H, Wang YD, Ai XP, Cao YL, and Yang HX (2005) A simple and high efficient direct borohydride fuel cell with MnO2catalyzed cathode. Electrochemistry Communications 7(4): 449--452. Hoogers G (ed.) (2003) Fuel Cell Technology Handbook. Boca Raton, FL: CRC Press. Hua D, Hanxi Y, Xinping A, and Chuansin C (2003) Hydrogen production from catalytic hydrolysis of sodium borohydride solution using nickel boride catalyst. International Journal of Hydrogen Energy 28(10): 1095--1100. Indig ME and Snyder RN (1962) Sodium borohydride, an interesting fuel. Journal of the Electrochemical Society 109(11): 1104--1106. Jasinski R (1965) Fuel cell oxidation of alkali borohydrides. Electrochemical Technology 3(1–2): 40--43. Kim J-H, Kim H-S, Kang Y-M, et al. (2004) Carbon-supported and unsupported Pt anodes for direct borohydride liquid fuel cells. Journal of the Electrochemical Society 151(7): A1039--A1043. Kojima Y, Suzuki K-I, Fukumoto K, et al. (2002) Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide. International Journal of Hydrogen Energy 27(10): 1029--1034. Kojima Y, Susuki K, Fukumoto K, et al. (2005) Development of 10 kWscale hydrogen generator using chemical hydride. Journal of Power Sources 125(1): 22--26. Kubokawa M, Yamashita M, and Abe K (1968) Fuel cells III anodic reduction of dissolved hydrogen at porous flow through electrodes. Denki Kagaku 36(11): 784--788. Lakeman JB, Rose A, Pointon KD, et al. (2006) The direct borohydride fuel cell for UUV propulsion power. Journal of Power Sources 162(2): 765--772. Lee S-M, Kim J-H, Lee H-H, Lee PS, and Lee J-Y (2002) The characterization of an alkaline fuel cell that uses hydrogen storage alloys. Journal of the Electrochemical Society 149(5): A603--A606. Li ZP, Liu BH, Arai K, and Suda S (2003) A fuel cell development for using borohydrides as the fuel. Journal of the Electrochemical Society 150(7): A868--A872. Li ZP, Liu BH, Arai K, and Suda S (2005) Development of the direct borohydride fuel cell. Journal of Alloys and Compounds 404–406: 648--652. Liu BH, Li ZP, Arai K, and Suda S (2005) Performance improvement of a micro borohydride fuel cell operating at ambient conditions. Electrochimica Acta 50(18): 3719--3725. Liu BH, Li ZP, and Suda S (2003) Anodic oxidation of alkali borohydrides catalyzed by nickel. Journal of the Electrochemical Society 150(3): A398. Liu BH, Li ZP, and Suda S (2004) Electrocatalysts for the anodic oxidation of borohydrides. Electrochimica Acta 49(19): 3097--3105. Liu BH, Li ZP, and Suda S (2006) Nickel- and cobalt-based catalysts for hydrogen generation by hydrolysis of borohydride. Journal of Alloys and Compounds 415(1–2): 288--293. Mirkin MV and Bard AJ (1991) Voltammetric method for the determination of borohydride concentration in alkaline aqueous solutions. Analytical Chemistry 63(5): 532--533. O¨zkar S and Zahmakiran M (2005) Hydrogen generation from hydrolysis of sodium borohydride using Ru(0) nanoclusters as catalyst. Journal of Alloys and Compounds 404–406: 728--731. Ponce-de-Leo´n C, Walsh FC, Pletcher D, Browning DJ, and Lakeman JB (2006) Direct borohydride fuel cells. Journal of Power Sources 155(2): 172--181. Ponce de Leo´n C, Walsh FC, Rose A, Lakeman JB, Browning DJ, and Reeve RW (2007) A direct borohydride-acid peroxide fuel cell. Journal of Power Sources 164(2): 441--448. Raman RK, Choudhury NA, and Shukla AK (2004) A high output voltage direct borohydride fuel cell. Electrochemical and Solid-State Letters 7(12): A488--A491. Shang Q, Wu Y, Sun X, and Ortega J (2007) Kinetic of catalytic hydrolysis of stabilized sodium borohydride solutions. Industrial & Engineering Chemistry Research 46(4): 1120--1124.

Fuel Cells – Exploratory Fuel Cells | Sodium Borohydride Fuel Cells

Shukla AK and Raman RK (2003) Methanol-resistant oxygen-reduction catalysts for direct methanol fuel cells. Annual Review of Materials Research 33: 155--168. Verma A and Basu S (2005) Direct use of alcohols and sodium borohydride as fuel in an alkaline fuel cell. Journal of Power Sources 145(2): 282--285. Verma A, Jha AK, and Basu S (2005) Manganese dioxide as a cathode for a direct alcohol or sodium borohydride fuel cell with a flowing alkaline electrolyte. Journal of Power Sources 141(1): 30--34. Vielstich W, Lamm A, and Gasteiger HA (eds.) (2003) Handbook of Fuel Cells – Fundamentals, Technology and Applications, Vols. 1–4, Chichester, England: Wiley.

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Wang LB, Ma CN, Sun YM, and Suda S (2005) AB5-type hydrogen storage alloy used as anodic materials in borohydride fuel cell. Journal of Alloys and Compounds 391(1–2): 318--392. Wang Y-G and Xia Y-Y (2006) A direct borohydride fuel cell using MnO2catalyzed cathode and hydrogen storage alloy anode. Electrochemistry Communications 8: 1775--1778. Wee J-H (2006) A comparison of sodium borohydride as a fuel for proton exchange membrane fuel cells and for direct borohydride fuel cells. Journal of Power Sources 155: 329--339. Wu C, Zhang H, and Yi B (2004) Hydrogen generation from catalytic hydrolysis of sodium borohydride for proton exchange membrane fuel cells. Catalysis Today 93–95: 477--483.