Page 81 - MODERN ASPECTS OF ELECTROCHEMISTRY

DirectAMethanolAFuelACellsA
challenge.= One option for a fuel cell-powered vehicle to attain high power 65
and energy levels on a gravimetric and volumetric basis is to develop fuel
cell/batteryhybrid vehicles as in the United States PNGV program. The
competition in the case of hybrid vehicles is from diesel engines.= Even
though gas turbines are also being considered as candidates for hybrid
electric vehicle applications in the PNGV= program, they are not in as
advanced a state as fuel cells. On the other hand, the prospects are very
low for battery-powered vehicles to meet both the performance and cost
goals.

II. DIRECT METHANOL FUEL CELLSA

1. Historical Development of DMFCs 3
Since methanol is a relatively inexpensive and reactive fuel for DMFCs,
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its electrooxidation has been studied for more than three decades. One
of the first investigations was in 1954 byPavela, who demonstrated
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interest in methanol as a soluble fuel in aqueous electrolytes. Methanol
and other small organic fuels are more reactive in alkaline media than in
acid media, so that the first attempts to build a DMFC used concentrated
NaOH or KOH as electrolytes which also contained the fuel. One of the
first such systems, described in 1955 by Justi and Winsel, used DSK nickel
electrodes, porous nickel for the methanol anode, or porous nickel-silver
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for the oxygen cathode. A similar unit constructed by Wynn in 1960
consisted of a platinized porous carbon anode and a porous carbon cathode
impregnated with silve–cobalt–aluminum mixed oxides. Using air as
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2
the cathodic reactant, this cell yielded 8 mA/cm at 0.35 V. During this
period, the first methanol-fed PEMFC was conceived byHunger, who
built a cell with an anion exchange membrane on both sides of which
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porous catalytic electrodes were pressed. However, the performance was
2
quite low: 1 mA/cm at 0.25 Vat room temperature with methanol and air
as the reactants.=
The first DMFC power source operating at a reasonablyhigh power
level and using an alkaline electrolyte (5 M KOH) was constructed in 1963
by Murray and Grimes at Allis-Chalmers Manufacturing Co. It consisted
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o
of a 40-cell stack operating at 50 C.=Each cell in the stack consisted of a
nickel sheet anode on which was deposited a platinum-palladium electro-
catalyst and a porous nickel sheet cathode impregnated with silver.= The
open circuit voltage was 22 V and the maximum power output was 750 W