C. R. Acad. Sci. Paris, Serie IIc, Chimie / Chemistry 3 (2000) 573–575© 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS. All rights reservedS1387-1609(00)00152-3/FLA

Surface chemistry and catalysis / Chimie des surfaces et catalyse

The generation of hydrogen for the solid polymermembrane fuel cellRobert J. Farrauto*

Engelhard Corporation, 101 Wood Avenue, Iselin, New Jersey 08830, USA

Received 5 January 2000, accepted 13 March 2000

Presented by Francois Mathey

Abstract – The basic principles of the solid polymer membrane fuel cell with special emphasis on hydrogen generation arepresented. The role of catalysts and the improvements needed are discussed. © 2000 Academie des sciences / Editionsscientifiques et medicales Elsevier SAS

fuel cell / PEM / H2 generation / catalysts

Generation d’hydrogene dans les piles a combustible a membrane polymere solide. Les principes de base des pilesa combustible a membrane polymere solide sont presentes, plus particulierement sous l’angle de la generation d’hydrogene.Le role des catalyseurs et les ameliorations necessaires sont discutees. © 2000 Academie des sciences / Editions scientifiqueset medicales Elsevier SAS

piles a combustible / membrane echangeuse de protons / generation de H2 / catalyseurs

1. Introduction

Considerable progress has been made in the de-velopment of the solid polymer fuel cell system, amajor effort is being undertaken for clean powergeneration for stationary and mobile source applica-tions [1]. The fuel cell directly converts chemicalenergy into electricity, thereby eliminating the me-chanical process steps that limit thermodynamic effi-ciency. The fuel cell can be two to three times asefficient as the internal combustion engine with little,if any, emission of primary pollutants; carbonmonoxide, hydrocarbons and nitric oxides. Also, be-cause of its enhanced fuel efficiency, it generates lesscarbon dioxide (green house gas). With current fuelcell systems hydrogen is the only fuel which can beelectrochemically oxidized at the anode. Therefore,the problem of on-site generation of hydrogen be-

comes a key issue that can be addressed by catalystsand catalytic engineering.

2. General principles of the fuel cell

The principle of operation is simple; hydrogen gasis electrocatalytically oxidized to hydrogen ions atthe anode composed of Pt deposited on a conduc-tive carbon. The protons pass through a membraneof a fluoropolymer of sulfonic acid called a protonexchange membrane or PEM [2]. At the Pt on carboncathode O2 from air is electrocatalytically reducedand combines with the protons producing H2O. Theelectrons flow through the external circuit and workas schematically shown in the figure 1. The cells arestacked in series to generate higher voltages.

* Correspondence and reprintsE-mail address: bob.farrauto@engelhard.com (R.J. Farrauto).

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Research will continue to focus on greater reduc-tions of the precious metals in the electrocatalysts. Atarget is less than 0.05 mg·cm–2 of Pt. Current Ptcontents are about five times the target. Promoterssuch as Ru are used to improve the tolerance to COpoisoning [1].

3. Hydrogen generation

One major issue is the source of H2 for the anodefuel because hydrocarbons cannot be directly con-verted. Therefore, it is necessary to convert readilyavailable fuels such as natural gas (stationary appli-cations) and gasoline (mobile applications) to H2.

Producing H2 from hydrocarbons is currently prac-ticed in the chemical industry [3] for production ofammonia and alcohol under steady state conditionswith carefully controlled catalytic unit operations.The process to remove sulfur from hydrocarbons iscatalytic (Co, Mo) hydrodesulfurization, as seen inreaction (1). Obviously this step is not required formethanol but would be necessary for petroleumbased fuels.

HC–S+H2 � H2S+HC (1)

The H2S produced is removed by adsorption on ZnO(2). Sulfur removal, according to the reaction (2):

H2S+ZnO � ZnS+H2O (2)

is necessary, due to the sensitivity of downstreamcatalysts to sulfur poisoning.

The next step is nickel (Ni) catalyzed steam re-forming (3) which is highly endothermic and re-quires high energy input (inlet temperatures exceed800 °C) depending on the fuel. Equilibrium limitsconversion so a partial oxidation reaction

HC+H2O X CO+H2 (3)

or secondary reformer (4), also using a Ni containingcatalyst, is used to generate more heat and more H2.

HC+ limO2 � CO+H2+CO2+H2O (4)

The exit from the secondary reformer contains about10–12 % CO which is fed to a high temperaturewater gas shift (WGS) reactor, reaction (5), using anFe, Cr containing catalyst at about 350 °C. This fur-ther increases the H2 content and

CO+H2O X H2+CO2 (5)

decreases the CO to about 2 % as governed by thethermodynamics and kinetics of the exothermicreaction.

The product gas is fed to a low temperature Cu,ZnO, Al2O3 WGS catalyst at about 200 °C. The CO isdecreased to less than about 0.5 %. The remainingCO, which poisons down stream ammonia ormethanol synthesis catalysts, is removed by pressureswing adsorption or methanation (6) over Ni or Rubased catalysts at about 250 °C.

CO+3 H2 � CH4+H2O (6)

Weight, size and transit operations are not critical forH2 plants but are important for fuel cell systems.Therefore, this technology will have to be modifiedsignificantly to meet the demands of the fuel cell.This will entail new catalysts and process conditionsalong with considerable catalytic engineering.

Desulfurization is required in order to protect theNi steam reforming catalyst and downstream cata-lysts from being poisoned. The traditional hy-drodesulfurization process is practiced at pressuresin excess of 1 000 psig, so alternative methods ofsulfur removal must be developed. Adsorbents arethe primary candidates for desulfurizing natural gas,however, gasoline or other liquid fuels which con-tain large molecular weight organo-sulfur com-pounds, will require alternative materials and/orprocesses. Furthermore, methods of regeneration ofthe adsorbents need to be developed in order toestablish reasonable maintenance and material re-placement schedules. Naturally this problem doesnot exist for sulfur free methanol fuels, however, thelack of an adequate distribution or infrastructure andpossible safety issues may limit the widespread useof methanol.

Another key issue is the spontaneous nature ofreduced Ni catalysts reacting with oxygen in theevent of accidental exposure to air generating largequantities of heat. Thus, non-pyrophoric catalysts ornon-self-heating catalysts need to be developed. Fi-Figure 1. Schematic of a single cell PEM fuel cell.

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nally the steam reforming reaction is highly en-dothermic and thus heat integration must be de-signed into the process to maintain high conversionefficiencies.

Water gas shift catalysts are also pyrophoric orself-heating and therefore new catalysts are needed.Furthermore, catalysts with considerably higher ac-tivity must be developed to reduce the size andweight of the hydrogen generating system. Extensiveresearch is in progress but the development of anon-pyrophoric (non-self-heating) water gas shiftcatalyst is proving to be very challenging.

The anode electro-kinetics are greatly hindered bytraces of CO present from the hydrocarbon steamreforming and water gas shift processes. For thisreason a more CO tolerant Pt/Ru anode was devel-oped but still the CO must be decreased to less than5 ppm. For large-scale production of H2 for theammonia industry pressure swing adsorption is usedto reduce the CO content of the H2. This technologyis not practical for fuel cell applications because ofexcessive size and cost related to compressors. Cata-lytic methanation is not an acceptable choice be-cause of the highly exothermic hydrogenation of theCO2 present. An alternative method is to oxidize theCO to CO2. Ideally the catalyst must selectively oxi-dize about 0.5 % (5 000 ppm) CO to less than 5 ppmwithout oxidizing any of the 40–70 % H2 present[4–6]. Large amounts of water and carbon dioxidefrom the upstream hydrocarbon reforming reactionsare also present and can inhibit the catalystperformance.

4. Commercial activities

Dailmer-Benz Ballard is aggressively developingtechnology for automotive applications (model year

2004) using methanol as the source of H2 [7]. PlugPower [8] is forecasting production of residential fuelcells powered by natural gas by the year 2001.

Other automobile companies such as General Mo-tors, Ford, Toyota, Honda, and Nissan are planningto produce fuel cells powered by on board methanolor other liquid fuels to H2 by 2004 [9]. DaimlerChrysler executives estimate the current fuel cellcosts to be about $30 000, while a cost-effectivesystem must be about $3 000 [10]. The US initiativecalled the Partnership for a New Generation Vehicles(PNGV) is a partnership between US industry, uni-versities, and government with the goal of develop-ing technologies that can be used to create costeffective full size sedans capable of obtaining 80miles per gallon of fuel. The fuel cell is the predom-inant technology under consideration.

The most highly desired technology is the directelectrochemical oxidation of hydrocarbon fuels,thereby eliminating the necessity to reform the hy-drocarbon. Research will continue in this area butbreakthroughs in electrocatalysts and membranes arenecessary.

References

[1] Acres G., Frost J., Hards G., Potter R., Ralph T., ThompsonD., Burstein G., Hutchins G., Catalysis Today 38 (1997) 393.

[2] Voss D., Science 285 (1999) 683.[3] Farrauto R., Bartholomew C., Fundamentals of Industrial

Catalytic Processes, Kluwer Academic Publishers, 1997, Chap-ter 7.

[4] Brown M., Green A., US Patent 3 088 919, 7 May 1963.[5] Cohn J.G., US Patent 3 216 782, 9 November 1965.[6] Korotkikh O., Farrauto R.J., Am. Chem. Soc., Division of Fuel

Chemistry 44 (4) (1999) 987.[7] Industrial Management and Technology, 30 March 1998,

p. 22.[8] Plug Power Press Release, 23 December 1998.[9] Burch S., Automotive Engineering International 4 (1999) 42.

[10] New York Times, Friday 2 April 1999, F1.

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