FUEL CELLS – EXPLORATORY FUEL CELLS | Hydrogen–Bromine Fuel Cells

FUEL CELLS – EXPLORATORY FUEL CELLS | Hydrogen–Bromine Fuel Cells

Hydrogen–Bromine Fuel Cells E Peled, A Blum, and M Goor, Tel Aviv University, Tel Aviv, Israel & 2009 Elsevier B.V. All rights reserved. Introduction...

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Hydrogen–Bromine Fuel Cells E Peled, A Blum, and M Goor, Tel Aviv University, Tel Aviv, Israel & 2009 Elsevier B.V. All rights reserved.

Introduction Hydrogen–bromine (HBr) fuel cells and electrolyzers consist of a bromine electrode and a hydrogen electrode with a proton-conducting membrane between them. There are two major applications of HBr fuel cells and electrolyzers: electrical energy storage systems (ESSs) and hydrogen production. The development of a hydrogen economy and the expectation that hydrogen will be one of the candidates for fueling future cars led to extensive work on technologies for hydrogen production. Many thermochemical cycles for hydrogen production are possible, with the two leading candidates for hybrid cycles using a HBr electrolyzer. The second promising application of the HBr fuel cell and electrolyzer is the large-scale ESS. This electrochemical technology was originally developed in 1979 by General Electric Co. and National Aeronautics and Space Administration (NASA). These fuel cells contain Nafion membranes. The HBr system is attractive for energy-storage applications because of the reversibility of both hydrogen and bromine electrodes, which allows a single electrochemical unit to be used in both fuel cell and electrolyzer modes. The need for large electrical ESSs can be seen in two major areas: renewable power sources and load-leveling renewable power sources such as wind and solar power are unpredictable by nature, and tend to increase the net variation in an electric grid. This is especially true for wind power, which is the fastest-growing renewable energy source. Thus, wind power is typically limited to o20% of the total grid power. Large-scale ESS will be used as a buffer between the renewable power source and the grid, and will turn it into a dispatchable power source. Additionally, it may be used to store renewable energy during periods of low demand, for use during periods of peak demand. The hydrogen oxidation catalyst with the highest activity is platinum (or its alloys) and it is used in most hydrogen–oxygen proton exchange membrane (PEM) fuel cells. However, there is a continuing effort to find better and lower cost catalysts. The platinum group metals are typified by the underpotential deposition (UPD) of hydrogen and thus have become the focus of attention regarding their surface reactivity in the metal hydride (MH) region. In HBr fuel cells and electrolyzers, the membrane does not completely prevent the crossover of bromides or bromine species. Therefore, these species tend to poison the

182

hydrogen electrode catalyst, and thereby reduce the catalyst activity. Because the most common catalyst for the hydrogen electrode is platinum and its alloys, the focus is on bromide adsorption on platinum.

The Bromine Electrode Hydrogen–bromine fuel cells and electrolyzers consist of a bromine electrode and a hydrogen electrode with a proton-conducting membrane between them. All cell components and especially the electrodes must be capable of resisting corrosion by bromine and hydrobromic acid. The cell reaction is given by H2 þ 2Br2 $2HBr2

½I

From the reversible cell voltage given by the Nernst equation as shown in eqn [1], it can be seen that the HBr cell voltage decreases with increasing hydrogen bromide activity and increases with hydrogen pressure and bromine activity: E ¼ Eo þ

RT RT RT ln aBr þ ln PH2  ln aHþ aBr 2F 2F 2F

½1

where Eo is, in fact, the standard potential of the Br2/Br electrode (1.088 V vs NHE). The formation of bromine complexes reduces Eo by o0.1 V. The experimental open circuit voltage (OCV) values at room temperature for a fully charged regenerative HBr fuel cell based on nanoporous protonconducting membrane (NP-PCM) (details are provided later) containing 3–7 mol L1 hydrogen bromide are about 1 V. Bromine is highly soluble in aqueous electrolytes. The solubility of bromine in water increases in the presence of bromides because of the formation of complex ions such as Br3  and Br5  . For example, the solubility of bromine in 1 mol L1 hydrogen bromide at 25 1C is 1.495 mol L1, whereas in 3.1 mol L1 sodium bromide it is 6.83 mol L1 (partly because of the formation of higher complexes such as Br5  ). The color of the solution is yellow at low bromine (or tribromide) concentrations and deep red at high bromine (or tribromide) concentrations. The molar absorptivity of bromine gas at 405 nm is 162 and that of aqueous bromine solution at 393 nm is 164.

Fuel Cells – Exploratory Fuel Cells | Hydrogen–Bromine Fuel Cells

The formation of tribromide ion in the presence of bromine and bromide is a fast reaction given by Kf

Br2 þ Br $ þ Br3  Kb

Br2 þ 2e

reduction

$

oxidation reduction

$

oxidation

3Br 2Br

Platinum Group Metals as Hydrogen Electrode Catalysts in Hydrogen Bromide Solutions

½II

The equilibrium constant for this reaction at 25 1C is 17. As a result, in practical fuel cells and electrolyzers containing 3–7 mol L1 hydrogen bromide, most of the bromine is present as tribromide ions (and some as pentabromide ions) and the concentration of free bromine is low. For example, at 25 1C in a solution of 3 mol L1 hydrogen bromide and 1 mol L1 bromine, the concentrations of Br3  and Br2 (ignoring the formation of pentabromide ions, which further reduces the bromine concentration) are 0.97 and 0.03 mol L1, respectively. In the HBr fuel cell, there are two major parallel reactions at the bromine electrode: Br3  þ 2e

183

½III

½IV

Because in practical fuel cells with high hydrogen bromide concentration, the concentration of free bromine is much lower than that of the tribromide ion, it is expected that reaction [III] prevails. The mechanisms of these reactions depend on the catalyst used. Where platinum is used as a catalyst, the platinum surface is saturated at all times with covalently bound bromide (PtBrx), where X varies from 0.42 to 0.65, depending on test conditions (details are provided later). At high bromide concentrations, desorption of bromide takes place only at potentials negative to NHE; this is not the case in bromine and tribromide solutions. Thus, the following redox reactions are suggested: PtBr þ 2e þ Br3  $PtBr þ 3Br

½V

PtBr þ 2e þ Br2 $PtBr þ 2Br

½VI

and

These reactions may consist of two steps: A tribromide ion (or bromine molecule) is adsorbed on two platinum bromide (PtBr) surface sites (bridge-type adsorption) and this leads to its dissociation, releasing bromide ion followed by reduction of the bromine molecule adsorbed on the two PtBr sites (eqns [VII] and [VIII]). There is no agreement as to the rate-determining step of this process. However, in some reports, it was suggested that the kinetics of Br3  reduction is controlled by the surface dissociation of the bromine molecules. 2PtBr þ Br3  $2PtBr2 þ Br

½VII

2PtBr2 þ 2e$2PtBr þ 2Br

½VIII

The platinum group metals are typified by the underpotential deposition (UPD) of hydrogen and thus have become the focus of attention regarding their surface reactivity at the MH region. The hydrogen oxidation catalyst with the highest activity is platinum (or its alloys), and it is being used in most hydrogen–oxygen PEM fuel cells. However, there is continuing effort to find better and lower cost catalysts. A computational screening procedure was used in the search for new metal and alloy catalysts. Several electrocatalysts for the hydrogen evolution reaction (HER) were identified by the use of density functional theory (DFT) calculations (Figure 1). The theoretical analysis was used to compute the free energy of adsorption of hydrogen on the surface of the catalysts. Figure 1 shows the volcano curve for catalytic material activity for hydrogen evolution as a function of the free energy of hydrogen adsorption, DGH, along with experimental data. Maximum activity was found for DGH ¼ 0 and it decreases as the value of DGH increases or becomes negative. Good agreement was obtained between theoretical calculations and experimental data. These theoretical calculations predict that BiPt and palladium overlayers on platinum and on PtRu have higher activity than pure platinum. The original data on the activity of HER catalysts were collected under conditions in which the catalyst surface was free of adsorption impurities. This is not the case for HBr fuel cells and electrolyzers, where electrode poisoning by the adsorption of bromide anions is a major issue. In HBr fuel cells and electrolyzers, the membrane does not completely prevent the crossover of bromides or bromine species from the bromine electrode to the hydrogen electrode. Therefore, these species tend to poison the hydrogen electrode catalyst, and thereby reduce the catalyst activity. The adsorption of bromine species on platinum group metals depends on their concentration, the electrode potential, pH, catalyst crystal facet, surface defects, and temperature. Thus, not all catalysts can be used for both hydrogen evolution (in electrolyzers) and hydrogen oxidation (in fuel cells). What is sought are lower cost and stable catalysts for both hydrogen and bromine electrodes. Graphite is a cheap and resistant material, but it also has too high a hydrogen overvoltage (too small exchange current density) and thus cannot be used in high-power cells. For a material to be suited for practical use as a hydrogen catalyst, it must be cheap, must have a sufficiently low hydrogen overvoltage (high exchange current density), and must be resistant to poisoning by bromine species.

184

Fuel Cells – Exploratory Fuel Cells | Hydrogen–Bromine Fuel Cells 0 Pd overlayers ( )

−1

−3

Pd*/PtRu Polycrystalline Pd*/Pt pure metals ( ) Re Pd Pd*/Au

Single-crystal Pt pure metals ( ) Rh Pd*/lr Pd*/Ru lr Pd*/Re P*/Rh

−4 Co Ni

−5

−8 −0.8

Au

W Nb

Au Mo

1.0

α

=

0. 5

−6 −7

Ag

Cu

α=

Log(l0 (A cm−2))

−2

−0.6

−0.4

Ag −0.2

0 ΔGH(eV)

0.2

0.4

0.6

Bi 0.8

Figure 1 Hydrogen evolution reaction (HER) exchange current densities against DGH for various pure metals and metal overlayers. Reproduced with permission from Greeley J, Jaramillo TF, Bonde J, Chorkendorff I, and Nørskov JK (2006) Computational highthroughput screening of electrocatalytic materials for hydrogen evolution. Nature Materials 5: 909–913; Nature & 2006 Publishing Group.

The Adsorption of Bromine Species and Hydrogen Kinetics on the Platinum Electrode As previously mentioned, in HBr fuel cells and electrolyzers, the membrane does not completely prevent the crossover of bromides or bromine species. Therefore, these species tend to poison the hydrogen electrode catalyst, and thereby reduce the catalyst activity. Because the most common catalyst for the hydrogen electrode is platinum and its alloys, bromide adsorption on platinum is considered here (as do most of the publications in this field). It is expected that the adsorption of bromide on other platinum group metals will show some similarity to the behavior on platinum. The adsorption of bromine species on platinum group metals is a complex process, not well understood. It is affected by the metal, the type of dosing (from bromine gas or in bromide solution), the crystal facet, the surface defects, the concentrations of the bromine species, the pH, and the temperature. Most of the publications focus on platinum at low (o1 mmol L1) bromide concentrations, whereas for practical applications such as fuel cells and electrolyzers, adsorption at higher (41 mol L1) concentrations is the most important. Bromine adsorption on platinum from the gas phase and from aqueous solutions was studied, and contradictory results were obtained. To avoid damage to the surface, adsorption from the gas phase must be studied at low temperatures. The reported maximum values of the bromide coverage on platinum (yBr) vary from 0.42 to 0.66. Some recent publications show that the saturated value of yBr formed in dilute bromide aqueous solution is equal to 0.42–0.44.

Adsorption from the gas phase results in a more strongly adsorbed layer than does adsorption from aqueous solutions containing low concentrations of bromide. In both the cases, the bromide overlayer is sufficiently stable so that the electrode can be removed from the solution (or the bromine gas chamber) and studied in air. When bromine is adsorbed from the gas phase on Pt(1 1 0), the Br–Br atomic distance is 0.8 nm, whereas that of Pt–Pt is about half of that –0.39 nm. The surface structure of bromide adsorbed on Pt(1 1 1) from concentrated bromide solution is a (3  3) overlayer, which corresponds to yBr ¼ 0.44. Under atmospheric conditions, exposing a Pt(1 1 1) surface to bromine vapor leads to coverage from 0.44 to 0.50. In comparison, the same (3  3) bromide structure on Pt(1 1 1) is attained in ultrahigh vacuum (UHV) by dosing with either hydrogen bromide or bromine. This results in quite an irreversible adsorption, and cleaning the platinum surface requires several voltammetric sweeps at low voltage. Cyclic voltammetry has been used to measure the amount of hydrogen and oxygen adsorbed on the electrode in order to determine the effect of bromide ion adsorption on the electrocatalyst surface. The results have been reported in several publications. The addition of bromides and tribromides to acid solutions changed the voltammetric properties of polycrystalline platinum over the range of 0–1.2 V (Figure 2). Raising the Br and Br3  concentrations from 0 to 100 mmol L1 in 1 mol L1 perchloric acid (HClO4) has an increasing effect on the voltammetric behavior: (1) both Pt–O adsorption and desorption waves are reduced and (2) the peak of strongly adsorbed hydrogen decreases and that of weakly adsorbed hydrogen increases. In the higher concentration range (150 mmol L1 to –1 mmol L1), additional properties

400 300 200 100 0

−100 −200

−250 −300

−300

−350 −0.2 (a)

185

500

250 200 150 100 50 0 −50 −100 −150 −200

i (μA cm−2)

i (μA cm−2)

Fuel Cells – Exploratory Fuel Cells | Hydrogen–Bromine Fuel Cells

0.0

0.2

0.4

0.6

0.8

1.0

1.2

−0.2

1.4

E (SCE)(V)

(b)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

E (SCE)(V)

Figure 2 Cyclic voltammetric curves recorded in 1 mol L1 HClO4, with increasing amounts of Br/Br3 in solution: (a) from 4 to 100 mmol L1; and (b) from 150 to 1000 mmol L1. Scan rate 0.1 V s1. Arrows indicate the evolution of the profile as the Br-related species in solution is increased. Reproduced from Ferro S and De Battisti A (2004) The bromine electrode. Part I: Adsorption phenomena at polycrystalline platinum electrodes. Journal of Applied Electrochemistry 34: 981–987.

were measured. Among them is the development of redox Br =Br3  peaks at about 0.944 V versus SCE and another peak at 1.29 V versus SCE, corresponding to the BrO/ Br2 redox couple. The height of these peaks grows as the Br and Br3  concentrations are increased. In addition, the oxide reduction peak becomes smaller but does not disappear. The adsorption/desorption processes of Br on platinum at low bromide concentrations appear to be highly reversible, so at 0 V (on the NHE scale) bromide coverage (yBr  ) is zero. In general (on the assumption that maximum coverage for bromide is 0.44), the reactions taking place on platinum in acid bromide solutions, over the potential range 0–0.5 V (NHE), can be written as follows: PtBr0:44 þ ðx þ yÞe þ xHþ $PtHx Brð0:44yÞ þ yBr

½IX

where the values of x and y vary with potential. For a fully reversible reaction, it is expected that, at 0.5 V, y ¼ 0 and x ¼ 0, whereas at 0 V, x ¼ 1 and y ¼ 0.44. Thus, x þ y ¼ 1.44 and the total capacity (Qt) associated with these reactions is expected to be more than 300 mC cm2 (210 mC cm2  1.44). However, even at low bromide concentrations, Qt was found to be lower than this (similar to the results published earlier for smooth platinum (see later)). So although at 0 V, yBr is practically zero and yH is smaller than 1. The rotating ring disk electrode technique with a Pt(1 1 1) single crystal in the disk position was used to establish both the bromide adsorption isotherm and its electrosorption valency on Pt(1 1 1). In 0.1 mol L1 perchloric acid and 0.8  104 mol L1 Br, bromide adsorption begins at about –0.15 V (SCE) and reaches a coverage of 0.42 (160 mC cm2) at 0.5 V; the electrosorption valency is essentially unity (the adsorbed

bromine species has practically zero charge, so it is covalently bound to the platinum). The major peak in the voltammogram (Figure 3) at about –0.1 V is the simultaneous desorption/adsorption of hydrogen and adsorption/desorption of bromide (eqn [IX]). The hydrogen adsorption/desorption processes in the presence of bromide were deconvoluted (Figure 4) by subtracting the bromide flux (measured at the ring) from the Pt(1 1 1) disk current. At the negative potential limit, no bromide is adsorbed on the Pt(1 1 1) disk, so that a saturated monolayer of H(ads) is expected to exist, similar to that on Pt(1 1 1) in bromide-free electrolyte. As the voltage is swept in a positive direction, hydrogen desorbs as bromide gets adsorbed, finally resulting in saturation coverage by bromide above 0.5 V. At this potential, QBr is 100 mC cm2. More importantly, the difference between the total capacity (Qt) and the QBr amounts to 160 mC cm2 (inset in Figure 4). This is smaller than that for a complete monolayer of adsorbed hydrogen on Pt(1 1 1) in bromidefree solutions (240 mC cm2). Therefore, saturated coverage of H(ads) on Pt(1 1 1) corresponds to the value associated with ‘weakly’ adsorbed hydrogen on Pt(1 1 1) in the absence of bromide. V. S. Bagotzky and coworkers made an extensive study of bromine adsorption on smooth platinum electrodes for a broad range of bromide concentrations (from 10 mmol L1 to 1 mol L1) in 0.5 mol L1 sulfuric acid solutions. A three-step potentiodynamic pulse method was used. The first step, at 1.1–1.2 V (NHE), served to free the surface from impurities, then a step at 0 V to remove the anions previously adsorbed from low (o0.1 mmol L1) concentrations of bromide, and finally the adsorption step, at the voltage at which bromide adsorption was studied. At bromide concentrations higher than 0.1 mmol L1, adsorption is strong and, in

186

Fuel Cells – Exploratory Fuel Cells | Hydrogen–Bromine Fuel Cells

Pt(111) + 8 × 10−5 mol L−1 Br− (900 rpm: 50 mVs−1: 0.1 mol L−1 HClO4)

Disk current (μA)

(900 rpm: 50mVs−1: 0.1 mol L−1 HClO4)

Current (μA)

Pt(111) + 8× 10−5M Br−

H adsorption/desorption on Pt(111)/Br CV for Pt(111)/Br Base CV

0

0 0.4 0.3 0.2 0.1

(a)

0 Ring current (μA)

(c) Br −

10 −0.2 0.0

+1.08V

0.2

0.4 E/V

ir∞

−0.2

0.0

0.2

QdBr 100

0

0.4

E(V)

Figure 3 Potentiodynamic ring-shielding experiment with an RRDPt(1 1 1)E in 0.1 mol L1 HClO4 and 8  105 mol L1 Br at 50 mV s1 and 900 rpm. (a) Cyclic voltammogram on the Pt(1 1 1)-disk; (- - -) base voltammogram without bromide in solution under otherwise identical conditions for comparison. (b) Ring-shielding currents at a ring potential of 1.08 V. (c) Bromide adsorption isotherm in monolayers, evaluated for the negative sweep. Reprinted with permission from Gasteiger HA, Markovic´ NM, and Ross Jr PN (1996) Bromide adsorption on Pt(1 1 1): Adsorption isotherm and electrosorption valency deduced from RRDPt(1 1 1)E measurements. Langmuir 12: 1414–1418. Copyright (1996) American Chemical Society.

−0.2

0.0

Q dt

200

½Br2 + e−

1

(b)

Q [C cm−2]

θBr (ML)

10

−0.2

0.2

0.2

0.4

0.4

E(V)

E(V)

Figure 4 Deconvolution of potentiodynamic hydrogen adsorption/desorption currents from bromide desorption/ adsorption on Pt(1 1 1) in 0.1 mol L1 HClO4 with 8  105 mol L1 Br at 50 mV s1. ( . . . ) Pt(1 1 1) in the presence of Br; (- - -) voltammogram of Pt(1 1 1) without Br. Reprinted with permission from Figure 3; (—) potentiodynamic hydrogen adsorption/desorption in the presence of Br. Inset: Qdt is the disk charging curve in the presence of Br (without double-layer correction); QdBr is the charge contribution to the disk because of bromide adsorption/desorption. Reproduced from Gasteiger HA, Markovic´ NM, and Ross Jr PN (1996) Bromide adsorption on Pt(1 1 1): Adsorption isotherm and electrosorption valency deduced from RRDPt(1 1 1)E measurements. Langmuir 12: 1414– 1418. Copyright (1996) American Chemical Society.

values. The adsorption of bromide on a smooth platinum electrode obeys the Temkin logarithmic isotherm: yBr ¼ AðjÞ þ B ln CBr

order to remove the adsorbed bromide, negative potentials (on the hydrogen scale) were required. At bromide concentrations lower than 0.01 mol L1, there are changes in the potential and size of the platinum hydride (PtH) peaks. The peak height for strongly bound hydrogen (at about 0.3 V) decreases and the peak moves to lower potentials, whereas that for weakly bound hydrogen (at about 0.1 V) increases. The maximum coverage of hydrogen (measured at 0 V) decreases with the increase in bromide concentrations, by up to 30% at 1 mol L1 bromide. This may be caused by the shift of the adsorption/desorption platinum hydride peaks to lower potentials, so that at 0 V a complete monolayer of hydrogen is not obtained (yH o 1). The saturated adsorption of bromide on smooth platinum at room temperature is reported to be about 0.6; however, as mentioned above, recent publications report lower

0.0

½2

The slope B of the isotherm for 0.35 o j o 1 V is about 15. The parameter A depends on the potential j, and as the potential is increased, the isotherms shift toward lower concentrations. For example, at 0.15 V, adsorption saturation is attained at 0.003 mol L1 bromide, whereas at 0 V, it is attained only at 1 mol L1 bromide. Thus, in solutions with high bromide concentrations, yBr– 4 0 even at negative potentials (vs NHE). In practical fuel cells and electrolyzers containing high concentrations of bromide, the platinum catalyst is at least partially covered by bromide, especially in the fuel cell mode and, as a result, the activity of the catalyst is expected to be lower. The adsorption and desorption rates of bromide ions on platinum and other catalysts are important for practical HBr fuel cells and electrolyzers in which the

Fuel Cells – Exploratory Fuel Cells | Hydrogen–Bromine Fuel Cells

hydrogen electrode potential changes with time. The adsorption rate of bromide on smooth platinum in 0.5 mol L1 sulfuric acid and at bromide concentrations lower than 1 mmol L1 was found to be determined by kinetics rather than by diffusion. The rate increases with bromide concentration and, to a lesser extent, with electrode potential. It increases linearly with concentration at low coverage and drops exponentially with increasing surface coverage. For example, at 0.5 V and for 1 and 0.01 mmol L1 bromide, about 10 and 1000 s, respectively, are required to attain surface saturation. So it is expected that, in practical cells with bromide concentrations larger than 1 mol L1, it will take a few milliseconds to obtain saturation. Desorption of bromide occurs at potentials close to or less than 0 V, depending on bromide concentration. The rate of desorption increases slowly with the decrease in electrode potential. At þ 0.05 to –0.05 V, in 0.5 mol L1 sulfuric acid and 1 mmol L1 bromide, the coverage decreases with the log of desorption time. For example, in this solution, at –0.05 V, about 20 s are required to reduce yBr on smooth platinum to close to zero. At potentials more positive than the Br/Br2 equilibrium potential and at bromide concentrations lower than 0.1 mol L1, the coverage falls because of the oxidation of the adsorbed bromide species (PtBr) to bromine or BrO species. However, at higher bromide concentrations (namely, those relevant to practical applications), no noticeable decrease in yBr– was found at potentials as high as 1.8 V. So at high bromide concentrations, the only possible way to clean the platinum hydrogen electrode, at least partially, is to apply negative potentials. Hydrogen oxidation at a gas diffusion platinum black catalyst fuel cell electrode is kinetically controlled in 48% hydrogen bromide, but is diffusion-limited in 4 mol L1 hydrogen sulfuric acid. If it is assumed that hydrogen dissociation is the rate-determining step in the oxidation process, then the kinetics is a function of potential because of the potential-dependent coverage of hydrogen on the platinum surface. The results of cyclic voltammetry indicate that hydrogen adsorption on platinum is weakened by the presence of Br (cathodic shift of the Pt–H peak). Therefore, the kinetics may be slower because of this weakening of the Pt–H bond, in addition to the fact that a part of the surface is blocked by the adsorbed bromide.

Electrolyzer and Fuel Cell Stacks for the Hydrogen Bromide System

187

compatible materials for system components and storage tanks are glass, glass-lined metals, tantalum, Teflons, Kalrezs, Halars, Kynars (trademarks of well-known fluoropolymers), graphite, and carbon materials such as graphite felts and RVC. For 48% (wt/wt) hydrogen bromide solutions, acceptable materials for pipes, pumps, and valves at ambient temperatures are FRP-Derakane 470 or 411 and carbon steel-lined with Teflon, Kynar, high-density polyethylene or polypropylene. Gaskets and hoses made of Vitons, Kalrez, ethylene propylene diene monomer (EPDM), neoprene, high-density polyethylene and polypropylene, and fluoropolymers such as Teflon, polyvinylidene fluoride (PVDF), and Garlock Gylons are satisfactory. The only metals with satisfactory resistance to 48% (wt/wt) hydrogen bromide solutions are tantalum and niobium. Graphite and composite graphite are the most common materials for the bipolar plates in PEM-type fuel cells. When composite-type structural materials are used, the compatibility of the binder employed must be confirmed. Membranes The typical membrane used in HBr electrolyzers and fuel cells is a PEM such as Nafions. These membranes consist of hydrophobic and hydrophilic microphases. The acid groups attached to the backbone of the polymer are located in the hydrophilic microphases, the typical diameter of which is about 3–5 nm. The PEM must contain water in order to conduct protons. The concentration of the acid groups in the hydrophilic microphases is about 1 mol L1, and in common hydrogen–air fuel cells no additional electrolyte is added. When PEMs are used in HBr fuel cells or electrolyzers, the soluble ions (protons, bromide, and tribromide) enter the hydrophilic microphases. When high concentrations of hydrogen bromide are used, this phenomenon reduces the conductivity of the membrane. A different type of membrane (not a PEM type) was used for the H2/Br2 fuel cell by E. Peled. This is a composite membrane, called the NP-PCM, and it consists of nanosized silica powder and PVDF binder (which is one of the most stable materials for HBr–Br2 solutions). The typical pore size of the NP-PCM is 1.5 nm, and it contains no acid groups attached to the polymer. The conductivity is achieved by the electrolyte, which fills the pores. The NP-PCM is more hydrophilic than Nafion (even in the dry state) and its conductivity is higher, especially when a solution consisting of high hydrogen bromide concentration is used.

Construction Materials

Catalysts

Hydrogen–bromine and bromine are very corrosive substances. Only a very limited number of materials can be used for such systems. In the case of wet bromine, the

Graphite is a practical catalyst material for the bromine electrode because of its resistance to attack by bromine dissolved in hydrobromic acid. Several types of graphite

Fuel Cells – Exploratory Fuel Cells | Hydrogen–Bromine Fuel Cells

Durability of the Hydrogen-Bromide Fuel Cell If a system based on HBr fuel cells and electrolyzers is to compete with other systems, it must achieve thousands of hours of reliable operation.

Catalysts and loadings for various systems

Table 1

HBr Electrode

H2 Electrode

Catalyst type

Loading

Catalyst type

Loading

– RuO2 30% Pt E-tek Pt, PtIr, IrO2

– 2.0 mg cm2 1.0 mg cm2 of Pt

Pt RuO2 60% Pt or 60% Pt-Ru PtIr

2.5–5 mg cm  2 2.0 mg cm2 1.5 mg cm2 of Pt

0.8 Open circuit

0.7

2 AMP load

0.6

10 000

9000

8000

7000

6000

5000

4000

0

3000

0.4

2000

0.5 1000

bipolar plates underwent 4000 h of operation in an electrolyzer at 373 K in 40% (wt/wt) hydrogen bromide and 10% (wt/wt) bromine with no sign of corrosion. However, significant attack on some graphite bipolar plates was found in an electrolyzer operated at very high current densities at 353 K in a solution containing 16% bromine and 35% hydrogen bromide. This is probably due to the formation of bromine–oxygen species at potentials higher than that of the Br2/Br couple. Some experiments indicate that graphite is more stable in solutions containing more than 40% hydrogen bromide than at low hydrogen bromide concentration, where oxygen is formed at the electrode. Platinum is less stable than graphite – in a concentrated hydrogen bromide solution containing 10% (wt/wt) bromine at 373 K, a corrosion rate of 8.07  104 mg cm2 was measured. Finely divided iridium and platinum powders, obtained by electrodeposition, corrode too rapidly in these solutions, to be suitable for technical applications at elevated temperatures. No corrosion was found for iridium and iridium oxide in 1 mol L1Br2/6.9 mol L1 HBr solution at 80 1C after more than 4500 h of tests. Various catalysts electrodeposited on graphite were investigated for the HER, with the use of 47.5% hydrogen bromide at temperatures of 353 and 373 K. These included platinum, palladium, ruthenium, iridium, osmium, rhenium, and gold. Platinum and palladium were found to have the lowest overvoltages at current densities from 100 to 500 mA cm2. The required amount of deposited palladium and platinum for the maximum activity is over 20 and 2 mg cm2, respectively. Moreover, the deposited platinum layer is more stable than the palladium layer in concentrated hydrogen bromide. In concentrated (8.9 mol L1) hydrogen bromide solution, thin films (o10 nm sputtered on silicon) of iridium, and platinum–iridium (7:3) alloy, were found to have similar hydrogen evolution activity (and much better than that for pure iridium). They were also found to be very stable in 1 mol L1 Br2/6.9 mol L1 HBr solution at 80 1C at the hydrogen evolution potential. No corrosion was found at this temperature after more than 4500 h of tests. The corrosion rate of pure platinum was found to be 100 times that of the Pt/Ir (7:3) alloy. Iridium oxide decomposes rapidly in this solution under hydrogen evolution conditions. Table 1 summarizes the catalysts and loadings used in various systems. The most common catalyst is platinum and its alloys.

Volts (V)

188

Running time (h)

Figure 5 A Br2/H2 fuel cell (1 inch2; 1 inch ¼ 0.025 4 m) operated for 10 000 h. Reproduced from Barna GG, Frank SN, Teherani TH, and Weedon LD (1984) Lifetime studies in H2/Br2 fuel cell. Journal of the Electrochemical Society 131(9): 1973– 1980.

Small (1 and 10 inch2) laboratory HBr fuel cells were developed by Texas Instruments and operated for up to 10 000 h. The cells were discharged at close to room temperature and at a current density of 310 mA cm2 (Figure 5). The cell consists of a platinum–hydrogen electrode coated with a thin layer of silicon carbide (SiC) powder, an appropriate (unspecified) membrane, and a porous carbon (or graphite) bromine electrode. The electrolyte is 1 mol L1 Br2/6.9 mol L1 HBr, and its concentration is kept constant with the use of an electrolyzer in series with the electrolyte loop of the fuel cell. As can be seen, the voltage is relatively stable under constant current, with some degradation in voltage appearing after 7000 h of operation. At all times, dry hydrogen gas is fed to the hydrogen electrode in order to minimize platinum corrosion. The main reason for the moderate degradation in performance is hydrogen electrode flooding because of changes in hydrophobicity over time. In most tests, no recrystallization of platinum was found, and this was attributed to the low operating temperature of the cell (30 1C). Potentially, platinum could dissolve; if the hydrogen flow was stopped, bromine (or tribromide) could cross the membrane and attack the catalyst. As long as hydrogen is fed to the electrode, any bromine or tribromide that crosses the membrane is immediately reduced. This process reduces the current efficiency in regenerative HBr fuel cells and must be minimized by choosing an appropriate selective membrane.

Fuel Cells – Exploratory Fuel Cells | Hydrogen–Bromine Fuel Cells

Applications of Hydrogen-Bromine Fuel Cells and Electrolyzers Hydrogen–bromine fuel cells and electrolyzers have two major applications: electrical ESSs and hydrogen production. The development of the hydrogen economy and the expectation that hydrogen will be one of the candidates for fueling future cars have led to extensive work on technologies for hydrogen production. Many thermochemical cycles for hydrogen production are possible, with the two most promising involving a HBr electrolyzer. These consist of a series of chemical reactions at temperatures lower than 800 1C, as opposed to those involving direct thermal dissociation of water, which occurs at much higher temperatures (generally above 2500 1C). The two leading candidates for water splitting are a sulfur-based cycle and a calcium bromide–based cycle. The advantages of hybrid thermochemical cycles are small energy losses in the electrolyzer and the option of using a ‘green’ thermal source such as sunlight. The overpotential of the HBr electrolyzer is less than 0.1 V, as opposed to 0.5 V in a water electrolyzer. The sulfur-based thermochemical cycle for hydrogen and oxygen production is a closed loop (reactions [X]– [XIII]). It consists of the following steps: (1) aqueous HBr solution is electrolyzed to form bromine; (2) bromine reacts with sulfur dioxide at 293–373 K to form hydrogen bromide gas and sulfuric acid; and (3) sulfuric acid is thermally decomposed at 923–1123 K to form water, sulfur dioxide, and oxygen. The net reaction is the decomposition of water to hydrogen and oxygen [XIII] 2HBrðaqÞ-H2 þ Br2 ðthe electrolysis stageÞ

½X

Br2 þ SO2 þ 2H2 O-2HBrðgÞ þ H2 SO4 ðaqÞ

½XI

H2 SO4 -H2 O þ SO2 þ 12 O2

½XII

H2 O-H2 þ 12 O2

½XIII

The calcium bromide–based system utilizes a PEM-based HBr electrolyzer. The procedure consists (Figure 6) of the following steps: calcium oxide (CaO) and bromine are converted to calcium bromide (CaBr2) and oxygen at 550 1C, and calcium bromide (CaBr2) is converted back to calcium oxide (CaO) and hydrogen bromide at 730 1C; in the second step, HBr is electrolyzed to form hydrogen and bromine. The membrane– electrode assembly (MEA) consists of Nafion 105 and both anode and cathode catalysts are 2 mg cm2 ruthenium oxide (RuO2). The electrolysis takes place at 80 1C and preheated deionized water is fed into the hydrogen side of the electrolyzer. Another promising application of the HBr fuel cell and electrolyzer is the large-scale ESS. This

H2O

189

HBr CaBr2

730°C heat

Electrolysis

CaO O2

Br2

H2

550°C heat

Figure 6 The Ca–Br cycle for hydrogen production. Reproduced from Sivasubramanian P, Ramasamy RP, Freire FJ, Holland CE, and Weidner JW (2007) Electrochemical hydrogen production from thermochemical cycles using a proton exchange membrane electrolyzer. International Journal of Hydrogen Energy 32: 463–468.

Hydrogen

Regenerative fuel cell

Aqueous solution containing HBr, HBr3, and some Br2

Figure 7 Energy storage system (ESS) based on HBr. Red lines indicate charging and green lines discharging. Reproduced from Peled et al., 2007.

electrochemical technology was originally developed in 1979 by General Electric Co. and NASA. These fuel cells contained Nafion membranes. The HBr system is attractive for energy storage applications because of the reversibility of both hydrogen and bromine electrodes, which allows a single electrochemical unit to be used in both fuel cell and electrolyzer modes. The need for large electrical ESSs can be seen in two major areas: A. Renewable power: Renewable power sources such as wind and solar power are unpredictable in nature and tend to increase the net variation in an electrical grid. This is especially true for wind power, which is the fastest-growing renewable energy source. Thus, wind

190

Fuel Cells – Exploratory Fuel Cells | Hydrogen–Bromine Fuel Cells 1.2

Cell voltage (V)

1.0 0.8

Charge/discharge at 0.2Acm−2 40 °C, ambient pressure, 3 mol L−1 HBr Current efficiency 92% Voltage efficiency 86% 79% Energy efficiency

0.6 0.4 0.2 0.0 0

20

40

60

80

100

120

140

160

Test time (min)

Figure 8 Performance of hydrogen–tribromide 5 cm2 regenerative fuel cell. Reproduced from Peled et al., 2007.

power is typically limited to o20% of the total grid power. Large-scale ESS will be used as a buffer between the renewable power source and the grid, and turn it into a dispatchable power source. In addition, it can be used to store renewable energy during periods of low-demand, for use during periods of peak demand. B. Load leveling: Power-grid load-leveling systems enable better utilization of generation infrastructure, by storing energy produced at off-peak hours for use during peak hours. In this way, grids can be designed to meet average rather than peak demand. Additionally, ESS may be used as an immediate backup in case of unexpected failure of a major generation station, or an unexpected surge in demand. The HBr ESS has definite advantages over other battery systems: (1) The cost of bromine is very low, much lower than any other material used for electrochemical storage (except for water). (2) Both hydrogen and bromine electrodes are highly reversible allowing very high electrical energy conversion efficiencies. (3) The cell can operate at high power density in both modes, resulting in smaller stacks and lower capital costs. (4) Both reactants are stored separately from the cell, which makes it cost effective for both peaking and load leveling. A typical ESS based on regenerative HBr fuel cell can be seen in Figure 7. When electrical energy is supplied to the regenerative fuel cell (red arrows), hydrogen bromide is electrolyzed, producing hydrogen gas and tribromide. The hydrogen is stored in devices such as cylinders or as MHs, and the tribromide is dissolved in the aqueous hydrogen bromide solution. When energy is generated (green arrows),

hydrogen and tribromide solution are supplied to the fuel cell and hydrogen bromide is formed. At all times, hydrogen is present in the hydrogen electrode compartments and the aqueous HBr–tribromide solution circulates through the bromine electrode compartments. The performance of a small (5 cm2) regenerative fuel cell based on an NP-PCM is shown in Figure 8. The initial concentration of hydrogen bromide was 3 mol L1. The cell operated at 40 1C at 0.2 A cm2. The turnover current efficiency was 92% and the turnover voltage efficiency was 86%, giving a turnover energy efficiency of 79%. This type of regenerative fuel cell can be scaled up into 50–100 kW stacks, which will be used to build megawatt storage systems.

Nomenclature Symbols and Units A B E Eo Io ir Kf Kh Qt Q Br d Q

t d

R T DGH /

parameter dependent on potential j slope of the isotherm cell voltage standard electrode potential exchange current density Ring-shielding current forward reaction rate backward reaction rate total capacity capacity associated with adsorbed bromide the total capacity associated with all adsorbed species universal gas constant absolute temperature free energy of hydrogen adsorption electrode potential

Abbreviations and Acronyms AMP CV DFT

ampere cyclic voltammetry density functional theory

Fuel Cells – Exploratory Fuel Cells | Hydrogen–Bromine Fuel Cells EPDM ESS HER MEA MH ML NASA NHE NP-PCM OCV PEM PVDF rpm RRD RVC SCE UHV UPD

ethylene propylene diene monomer energy storage system hydrogen evolution reaction membrane–electrode assembly metal hydride monolayer National Aeronautics and Space Administration normal hydrogen electrode nanoporous proton-conducting membrane open circuit voltage proton exchange membrane polyvinylidene fluoride revolutions per minute rotating ring disk a type of carbon felt material saturated calomel electrode ultrahigh vacuum underpotential deposition

See also: Electrochemical Theory: Hydrogen Evolution; Energy: Energy Storage; Hydrogen Economy; Fuel Cells – Exploratory Fuel Cells: Regenerative Fuel Cells.

Further Reading Adanuvor PK and White RE (1987) The effect of the tribromide complex reaction on the oxidation/reduction current of the Br2/Br electrode. Journal of the Electrochemical Society 134(6): 1450--1454. Bagotzky VS, Vassilyev YB, Weber J, and Pirtskhalava JN (1970) Adsorption of anions on smooth platinum electrodes. Journal of Electroanalytical Chemistry 27: 31--46. Barna GG, Frank SN, and Teherani TH (1982) Oxidation of H2 at gas diffusion electrodes in H2SO4 and HBr. Journal of the Electrochemical Society 129: 2464--2468. Barna GG, Frank SN, Teherani TH, and Weedon LD (1984) Lifetime studies in H2/Br2 fuel cell. Journal of the Electrochemical Society 131(9): 1973--1980. Conway BE, Phillips Y, and Qian SY (1995) Surface electrochemistry and kinetics of anodic bromine formation at platinum. Journal of Chemical Society, Faraday Transactions 91(2): 283--293. Feess H, Ko¨ster K, and Schu¨tz GH (1981) The influence of noble metal catalysts on the electro-chemical decomposition of concentrated hydrobromic acid at graphite electrodes. Journal of Hydrogen Energy 6(4): 377--388. Ferro S and De Battisti A (2004) The bromine electrode. Part I: Adsorption phenomena at polycrystalline platinum electrodes. Journal of Applied Electrochemistry 34: 981--987.

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Ferro S, Orsan C, and De Battisti A (2005) The bromine electrode. Part II: Reaction kinetics at polycrystalline Pt. Journal of Applied Electrochemistry 35: 273--278. Fritts SD and Savinell RF (1986) The open circuit voltage of a hydrogen– bromine fuel cell. Proceedings/The Electrochemical Society 86: 75--85. Garcia-Araez N, Lukien JJ, Koper MTM, and Feliu JM (2006) Competitive adsorption of hydrogen and bromide on Pt(100): Meanfield approximation vs. Monte Carlo simulations. Journal of Electroanalytical Chemistry 588: 1--14. Gasteiger HA, Markovic´ NM, and Ross PN, Jr. (1996) Bromide adsorption on Pt(1 1 1): Adsorption isotherm and electrosorption valency deduced from RRDPt(1 1 1)E measurements. Langmuir 12: 1414--1418. Greeley J, Jaramillo TF, Bonde J, Chorkendorff I, and Nørskov JK (2006) Computational high-throughput screening of electrocatalytic materials for hydrogen evolution. Nature Materials 5: 909--913. Greeley J, Nørskov JK, Kibler LA, El-Aziz AM, and Kolb DM (2006) Hydrogen evolution over bimetallic systems: Understanding the trends. ChemPhysChem 7: 1032--1035. Hala´sz D, Visy C, Szu¨cs A, and Nova´k M (1992) Bromide ion oxidation on various Pt surfaces. Reaction Kinetics and Catalysis Letters 48(1): 177--188. Jolles ZE (ed.) (1966) Bromine and Its Compounds. Ernest Benn Limited. Kosek JA and Laconti AB (1988) Advanced hydrogen electrode for hydrogen–bromine battery. Journal of Power Sources 22: 293--300. Livshits V, Ulus A, and Peled E (2006) High-power H2/Br2 fuel cell. Electrochemistry Communications 8: 1358--1362. Luttmer JD, Konrad D, and Trachtenberg I (1985) Electrode materials for hydrobromic acid electrolysis in Texas Instruments’ solar chemical converter. Journal of the Electrochemical Society 135(5): 1054--1058. Orts JM, Go´mez R, and Feliu JM (1999) Bromine monolayer adsorption on Pt(1 1 0) surfaces. Journal of Electroanalytical Chemistry 467: 14--19. Peled E, Duvdevani T, and Melman A (1998) A novel proton-conducting membrane. Electrochemical and Solid-State Letters 1: 210--211. Rosen A (1995) Energy and exergy analysis of electrolytic hydrogen production. Journal of Hydrogen Energy 20(7): 547--553. Salita GN, Stern DA, Lu F, et al. (1986) Structure and composition of a platinum(111) surface as a function of pH and electrode potential in aqueous bromide solutions. Langmuir 2: 828--835. Savinell RF and Fritts SD (1988) Theoretical performance of a hydrogen–bromine rechargeable SPE fuel cell. Journal of Power Sources 22: 423--440. Schuetz GH (1977) Hydrogen producing cycles using electricity and heat-hydrogen halide cycles: Electrolysis of HBr. Journal of Hydrogen Energy 1: 379--388. Schuetz GH and Fibelmann PJ (1980) Electrolysis of hydrobromic acid. Journal of Hydrogen Energy 5: 305--316. Sivasubramanian P, Ramasamy RP, Freire FJ, Holland CE, and Weidner JW (2007) Electrochemical hydrogen production from thermochemical cycles using a proton exchange membrane electrolyzer. International Journal of Hydrogen Energy 32: 463--468. Stickney JL, Rosasco SD, Salaita GN, and Hubbard AT (1984) Ordered ionic layers formed on Pt(111) from aqueous solutions. Langmuir 1: 66--71. Will FG (1979) Bromine diffusion through Nafion perfluorinated ion exchange membranes. Journal of the Electrochemical Society 126(1): 36--43.