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Electrochemistry Communications 8 (2006) 1358–1362

High-power H2/Br2 fuel cell

V. Livshits, A. Ulus, E. Peled *

School of Chemistry, Tel-Aviv University, Tel Aviv 69978, Israel

Received 25 April 2006; received in revised form 15 June 2006; accepted 19 June 2006Available online 26 July 2006

Abstract

We report on the properties and performance of a hydrogen-bromine fuel cell based on a nanoporous proton-conducting membrane(NP-PCM), operated with hydrobromic acid. The use of the NP-PCM that consists of a ceramic nano-powder and PVDF is a majoradvantage in H2/Br2 FC, since the fuel cell itself produces an acid (HBr) that serves as an ionic carrier. Maximum power densities ofabove 1.5 W/cm2 have been achieved in a 5 cm2 NP-PCM-based H2/Br2 fuel cell under dry-hydrogen feed at 80 �C. The energy-conver-sion efficiency of this cell is very high – close to 90% (about twice that of the hydrogen/air cell). Thus, in view of the fact that bromine issustained in a closed cycle and with the appropriate safety measures, the H2/Br2 (NP-PCM)-based FC is a promising candidate for bothhigh-power RFC, and combined electricity and HBr production.� 2006 Elsevier B.V. All rights reserved.

Keywords: Hydrogen–bromine fuel cell; HBr production; Direct-oxidation fuel cell

1. Introduction

The hydrogen–oxygen regenerative fuel cell (RFC) sys-tem offers high energy density, long life and high efficiency– a combination of properties that could ideally suit bothterrestrial applications and low-orbit missions [1]. How-ever, they are prone to a higher degradation rate of themembrane electrode assembly (MEA). The causes of mem-brane failure have been discussed in recent reviews [2,3],and have been ascribed to the creation of peroxide radicals,stimulated by high concentrations of oxygen at the cath-ode. These radicals attack the membrane and thus shortenthe life of the fuel cell. Another disadvantage in using oxy-gen is the need for heavy, high-pressure storage tanks. Thishas led scientists to look for other oxidizing agents, such asbromine, as suitable substitutes for oxygen [1,4,5]. Interestin H2/Br2 fuel cells has somewhat decreased in the last dec-ade for several reasons, mainly the high degree of toxicityas well as environmental hazards related to the use of bro-mine and hydrobromic acid.

1388-2481/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2006.06.021

* Corresponding author.E-mail address: peled@post.tau.ac.il (E. Peled).

However, in some RFC applications [1,4,5], brominecan be used as an oxidizing agent since it is sustained inclosed cycle and, with the appropriate safety precautions,the environmental hazards can be minimized.

Furthermore, when hydrogen and bromine are recom-bined in a fuel cell, up to 90% of the chemical energy storedin the reactants can be converted to electricity, as opposedto only about 50% for state-of-the-art hydrogen/air fuelcells. This leads to an electric-to-electric efficiencyapproaching 80% for the system compared to 40% forhydrogen/air and 35% for most fossil-fuel-fired power gen-erators [6].

Fuel-cell studies [4] were carried out with the use of 1 MBr2/6.9 M HBr electrolyte and �1 mgPt/cm2 loading at theanode. A 6.5 cm2 cell was run for a very long time undercurrent density of 310 mA/cm2. The initial voltage was0.67–0.68 V (210 mW/cm2). After 10,000 h, the voltagehad fallen to �0.63 V (a 6–8% loss in fuel-cell perfor-mance). Some theoretical models of H2/Br2 FC were madeby Savinell et al. [7–9]. Savinell derived theoretical expres-sions to estimate the OCV of fuel cell. In these expressions,temperature, hydrogen pressure, and bromine and hydro-bromic acid concentrations were taken into consideration.

Fig. 1. A 5 cm2 H2/Br2 FC in the synthetic graphite blocks (Poco Inc.) (inwhich flow fields were engraved) and the copper current collectorshousing.

Fig. 2. Schematic representation of the fuel cell components.

V. Livshits et al. / Electrochemistry Communications 8 (2006) 1358–1362 1359

They also considered the effects of the Nafion� membraneand the various bromide complex species.

In the presence of bromide bromine forms the tribro-mide ion ðBr�3 Þ. The species are in equilibrium accordingto Eq. (1):

Br2ðaqÞ þ Br�ðaqÞ $ Br�3 ðaqÞ ð1Þ

The equilibrium constant for this reaction at 25 �C is 1.24.Thus, there will always be some tribromide in the solution,and two cathode reactions may take place simultaneously:

ðaÞ Br2ðaqÞ þ 2e� $ 2Br�ðaqÞ E0 ¼ 1:098 V

ðbÞ Br�3 ðaqÞ þ 2e� $ 3Br�ðaqÞ E0 ¼ 1:062 V

Another potential application for the H2/Br2 fuel cell is acombined power and HBr-production system. Currently,HBr is produced by a thermal process via a direct reac-tion at very high temperature. This is an energy-consum-ing, unsafe and environmentally unfriendly process. Inaddition to the waste of chemical energy, it is necessaryto cool the reactor, a process which also consumes muchenergy. At a production rate of 4 tons of HBr per hour, itis possible to produce one megawatt of electrical power.In addition it is a much ‘‘greener’’ and safer process thanthe thermal reaction.

Our interest in the H2/Br2 fuel cell was triggered by thedevelopment in our group of a new nanoporous proton-conducting membrane (NP-PCM) [10–14] for a directmethanol fuel cell (DMFC), and other direct-oxidation fuelcells.

The use of the NP-PCM in the DMFC offers severaladvantages over the Nafion-based DMFC: much lowermembrane cost, higher conductivity and lower fuel cross-over. Its hydraulic water permeation is more than ten timesthat of Nafion 117 [10–14]. Therefore, the water fluxthrough the NP-PCM is much smaller, leading to water-neutral operating conditions, to reduced cathode-catalystflooding and to low relative humidity in the cathode flowfield, thus to a high oxygen partial pressure and toimproved cell performance [14–16]. In addition, the useof the NP-PCM significantly reduces the need for oxidanthumidification. The effect of these advantages is expressedin a DMFC of very high power (0.5 W/cm2) [16].

In the H2/Br2 FC, a membrane with high water perme-ability under hydraulic pressure leads to an increase in thehydrogen humidity at the anode, which is necessary for lowanode/membrane interfacial resistance. This, in turn, elim-inates the need to humidify the hydrogen feed and there-fore simplifies the balance of plant (BOP).

In this work we characterized an H2/Br2 FC containingan MEA made of our NP-PCM and demonstrated itsafore-mentioned advantages over other types of fuel cellsthat use Nafion� We present very encouraging preliminaryresults (above 1.5 W/cm2), which call for further develop-ment of a cell that produces both HBr and power. Thechoice of PVDF as the binding polymer of our membraneturned out to be successful, since Br2 and PVDF are inert

to each other, at least over the operating-temperature rangeof the fuel cell (which is limited by the boiling point of Br2).

2. Experimental

The FC setup has been described previously [10–13].The test vehicle was a 5 cm2 FC (Figs. 1 and 2) operatingat 0–2 atm (g) dry-hydrogen pressure at the anode and0.3–0.9 M Br2 and 1 or 5 M HBr solution at the cathode.The fuel-cell housing was built from synthetic graphiteblocks (Poco Inc.), in which flow fields were engraved.The composition of the 100l-thick membrane was 30%(v/v) Poly-vinylidene-difluoride (PVDF), 10% SiO2, and60% pore volume (that is filled with the same solution asthat flowing through the cathode compartment). It wasproduced on a semi-industrial coater (Dixon) at a rate of15 m2/h. The cathode ink was prepared from 60% Pt–Ru(1:1 atomic) or 60% Pt on XC72 (Johnson Matthey) andspread over un-teflonated Toray paper. In both cases, thecathode loading was 1.5 mg Pt/cm2. An E-TEK electrodewith 1 mg Pt/cm2 (30% Pt on XC72) was used as the anode.The membrane electrode assembly (MEA) was hot-pressedat 100 �C and at a pressure of 24 kg/cm2.

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Fig. 3. Effect of temperature and bromine concentration on the perfor-mance of Pt–Ru cathode-catalyst-based H2/Br2 FC. Ambient H2 pressure,no H2 humidification, stoich H2 = 2; 1 M HBr; 100l NP-PCM. Anode:1 mgPt/cm2 (E-Tek, 30% Pt on XC72), homemade cathode: 1.5 mgPt/cm2

(JM, 60% Pt–Ru on XC72).

1360 V. Livshits et al. / Electrochemistry Communications 8 (2006) 1358–1362

A solution of 0.3–0.9 M Br2 and 1 or 5 M HBr was cir-culated over the cathode at �200 ml/min, and dry hydro-gen was fed through the anode. Cell resistance wasmeasured with a Solartron 1260. Cells were tested with aMaccor model series 2000 battery tester.

3. Results and discussion

Our NP-PCM-based fuel cells employ soluble acid as theelectrolyte and, since in the hydrogen/bromine cell HBr isproduced, there is no need to add another acid. On theother hand, it is well known that bromine is a catalytic poi-son for the platinum electrode at least in the hydrogen/oxy-gen fuel cell. A very limited number of papers has beenpublished on this system in the open literature. In the mostdetailed paper [4], which demonstrated stable FC operationfor thousands of hours, it is implied that iridium-based cat-alyst was used at the cathode. Electrode stability is a crucialpart of fuel cell performance. We used two types of cathodecatalyst: platinum, which, according to Pourbaix diagrams[17], supposed to be even more stable than iridium-basedone, and platinum–ruthenium. Ruthenium may be proneto dissolution at 0.7–1.0 V vs. NHE in acidic conditions[17]. However the Pt–Ru alloy may be more stable as inthe case of Pt–Ni and Pt–Co catalysts [18]. Platinum cata-lyst was used at the anode in both cases. Worth mentioningthat no evidence for any type of instability of platinum orplatinum–ruthenium was noticed at the cathode during theshort tests we run. In case of catalyst corrosion, a strategyused in hydrogen PEM FC [19] can be used here: to keepthe cell always under load, at voltage, lower than the cor-rosion voltage.

Regardless of the type of cathode catalyst used, it is nec-essary to keep the anode potential close to zero (vs. RHE)by flushing the anode with hydrogen before feeding thecathode with the hydrogen bromide/bromine solution. Ifthis is not done, the bromide ions diffuse through the mem-brane to the anode side and reversibly poison the platinumcatalyst, sometimes so seriously that the OCV of the cellclose to zero.

The unique property of NP-PCM – the high water per-meability under hydraulic pressure allows the excess waterat the cathode to return back to the anode, which elimi-nates the need to humidify the hydrogen feed. However,on the other hand, it may cause extra bromine, tribromideand bromide crossover to the anode. Therefore, flushingthe anode with hydrogen, mentioned above, was notstopped throughout entire test. The extremely high powerreported bellow for the NP-PCM-based H2/Br2 fuel cellat 1 M HBr shows that bromine, tribromide and bromidecrossover has only a small influence on cell performanceunder continuous hydrogen feed conditions.

First we studied the effect of temperature and bromineconcentration on cell performance. Fig. 3 presents the V/I and power density/current density curves measured atthree different temperatures �25, 50 and 80 �C in a Pt/Pt–Ru based cell. The hydrogen stoich number was only

2. Bromine concentration was 0.3, 0.6 and 0.9 M at 25,50 and 80 �C respectively. In order to obtain high power,a high bromine concentration is needed; however, brominecrossover to the anode side and its reduction by the hydro-gen leads to a drop in the OCV, the cell-operating voltageand the energy-conversion efficiency. The OCV of the celldecreases from 1.01 V to about 0.95 V in going from25 �C and 0.3 M Br2 to 80 �C and 0.9 M Br2. These OCVvalues are close to the theoretical E0 value of the cell(1.098 V at 25 �C [20]). At a current density of 1 A/cm2,the cell voltage rises from 0.675 V at 25 �C to about0.8 V at 50 �C and 80 �C. The maximum power densityof the cell increases from 0.68 W/cm2 at 25 �C to 1.1 W/cm2 at 50 �C and to 1.5 W/cm2 at 80 �C. The voltage atmaximum power is about 0.6–0.65 V at all three tempera-tures. These power values are up to more than twice thosemeasured for the hydrogen/air fuel cell operating undersimilar conditions. The maximum power of the hydro-gen/air fuel cell at 80 �C at a total pressure of 150 kPaabs

is only 0.7 W/cm2 (at 0.65 V) [21].The cell containing the Pt cathode catalyst produced an

even higher OCV (1.06 vs. 1.01 V at 25 �C and 1.01 vs.0.97 V at 50 �C), higher potentials under load (0.81 vs.0.675 V at 1 A/cm2 at 25 �C and 0.82 vs. 0.80 V at 1 A/cm2 at 50 �C) and a higher maximum power (1.14 W/cm2

vs. 0.68 W/cm2 at 25 �C and 1.36 W/cm2 vs. 1.1 W/cm2 at50 �C) (Fig. 4). No significant difference in performancebetween the two types of cell was observed at 80 �C.

The voltage drop across the membrane is relativelysmall, as the cell resistance (for both cells) is 0.13, 0.12and 0.08 X cm2 at 25, 50 and 80 �C respectively. This is aunique feature of the NP-PCM, since for PEMs the resis-tance of the membrane rises with temperature [22].

The effect on cell performance of increasing the hydro-gen feed rate was studied for the cell containing the Pt–Ru cathode catalyst and is shown in Fig. 5. At 25 �C theOCV is higher by about 30 mV while at 80 �C it is lowerby about 40 mV, the latter probably due to drying of the

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Fig. 4. Effect of temperature on the performance of Pt cathode-catalyst-based H2/Br2 FC. Ambient H2 pressure, no H2 humidification, stoichH2 = 2; 0.6 M Br2, 1 M HBr; 100l NP-PCM. Anode: 1 mgPt/cm2 (E-Tek,30% Pt on XC72), homemade cathode: 1.5 mgPt/cm2 (JM, 60% Pt onXC72).

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Fig. 5. Performance of Pt–Ru cathode-catalyst-based H2/Br2 FC at highhydrogen feed rate. Ambient H2 pressure, no H2 humidification, high H2

stoich (about 10); 1 M HBr; 100l NP-PCM. Anode: 1 mgPt/cm2 (E-Tek,30% Pt on XC72), homemade cathode: 1.5 mgPt/cm2 (JM, 60% Pt–Ru onXC72).

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Fig. 6. Performance of Pt–Ru cathode-catalyst-based H2/Br2 FC operatedwith 5 M HBr electrolyte solution. Ambient H2 pressure, no H2

humidification, stoich H2 = 2; 100l NP-PCM. Anode: 1 mgPt/cm2 (E-Tek, 30% Pt on XC72), homemade cathode: 1.5 mgPt/cm2 (JM, 60%Pt–Ru on XC72).

V. Livshits et al. / Electrochemistry Communications 8 (2006) 1358–1362 1361

membrane. The increase in hydrogen stoich number causesan increase of both cell voltage and cell power, especially at25–50 �C. At a current density of 1 A/cm2, the cell voltagerises from 0.675 V to 0.79 V at 25 �C and from 0.8 V to0.83 V at 50 �C. Maximum cell power rises from 0.68 W/cm2 to 0.99 W/cm2 at 25 �C and from 1.1 W/cm2 to about1.35 W/cm2 at 50 �C, while at 80 �C there is no change.This can be explained by partial drying out of the mem-brane caused by a combination of high hydrogen flow rateand elevated operating temperature, leading to a highanode/membrane interfacial resistance. The next step wasto study the effect of hydrogen bromide concentration oncell performance. Fig. 6 presents the V/I and power-densitycurves where the bromide concentration was increased to5 M and the cell was tested at low stoich (2.0) of hydrogen.In comparison to Fig. 3 it can be seen that the OCVdropped by about 150 mV. This voltage drop is explainedby both the Nernst equation, since HBr is the product of

the reaction (41 mV and 49 mV at 25 �C and 80 �C, respec-tively), and the rest by poisoning of the Pt catalysts by bro-mide ions. The maximum cell power fell from 1.5 W/cm2 to0.7 W/cm2 at 80 �C and from 0.68 W/cm2 to 0.55 W/cm2 at25 �C. This deterioration in cell performance can also bepartially explained by the poisoning effect of bromide onthe platinum catalyst. Thus, with respect to power genera-tion, it is preferable to use the lowest HBr concentrationthat still provides adequate conductivity. This cell has, onthe one hand, a lower E0 than does the hydrogen/oxygencell and on the other, higher operating voltages. Thisresults in much higher efficiency of energy conversion. At0.9 V the energy-conversion efficiency is close to 90% ascompared to 50% (0.7/1.4) for the hydrogen/oxygen cell.In addition, much less heat must be dissipated when thehydrogen/bromine cell is used.

4. Summary

In this paper, we report on the development of the H2/Br2 (NP-PCM)-based fuel cell and we present preliminaryresults regarding its properties and performance. Theimportance of the use of a proton-conducting membranewith high water permeability, which eliminates the needto humidify the hydrogen anode and utilizes the productof the reaction (HBr) as an ionic carrier, is demonstrated.Maximum power densities of 1.14 W/cm2, 1.28 W/cm2,1.36 W/cm2 and 1.51 W/cm2 have been achieved in a5 cm2 H2/Br2 FC, with a 100l-thick NP-PCM, underdry-hydrogen feed at 25, 30, 50 and 80 �C respectively.These values are much higher than that measured for thehydrogen/air fuel cell. Such a powerful H2/Br2 FC shouldstimulate further development of a combined system thatproduces both power and HBr at a very high efficiency.

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