Platinum Metals Rev., 1969, 13, (1), 8
Economics of Platinum Catalysts in Fuel Cells
The advantages of employing platinum metal electrocatalysts in fuel cells continue to be debated, but a paper by K. R. Williams, of “Shell” Research, presented at the Sixth International Power Source Symposium at Brighton, gives a useful guide to the economics of their use in certain types of systems.
Starting from the basis of using methanol or hydrocarbons in a low temperature system in the range 1 to 5 kW, the author compared the costs and performance of cells fuelled by pure hydrogen, impure hydrogen and methanol. Pure hydrogen may be obtained by steam-reforming methanol in a fluidised bed reactor and extracting the hydrogen by means of a silver-palladium diffusion cell. This system produces a very pure hydrogen fuel. Methanol can also be reformed to yield impure hydrogen; the cost of a diffusion cell and associated equipment would be saved, but the system weight would scarcely be reduced since more reforming catalyst is required to achieve an adequately low carbon monoxide concentration. Obtaining hydrogen by hydrocarbon reforming necessitates the use of a two-stage system – a reforming stage in the temperature range 500 to 700°C, followed by a shift reaction at lower temperatures to increase the hydrogen content of the gas.
Ruthenium-platinum electrocatalysts applied to electrodes comprising microporous plastics coated with a conductive metallic layer have been demonstrated to show a number of advantages in this type of system. In 6N KOH systems at platinum loadings of 1 mg/cm2 power outputs of 100 mW/cm2 can be achieved with hydrogen fuel, compared with only 40 mW/cm2 with less expensive nickel catalysts. The ruthenium-platinum catalysts, moreover, can deal effectively with impure hydrogen, as well as with methanol directly, at a small cost of slightly increased electrode polarisation. When total system costs are compared for fuel cells operating with ruthenium-platinum and nickel catalysts, a 1 kW battery would cost around £60 in the former case (with two-thirds of this sum being attributed to the platinum) and about £50 in the latter instance. There is thus only a slight saving when non-platinum catalysts are used, due to the increased construction costs that almost completely offset the savings in electro-catalyst. The weight of such a system, moreover, is increased significantly.
In acid electrolytes the catalyst loading has a very marked effect on cell performance. Ruthenium-platinum electrocatalysts operate best around 70°C when impure hydrogen is used as fuel, but—in general—construction costs are much higher when acid electrolytes are employed due to corrosion problems.
From an analysis of costs and performances of the systems reviewed, the author concludes that although the least expensive system at the 5 kW level is that using a methanol reforming system giving pure hydrogen and non-platinum catalysts in an alkaline electrolyte, the additional cost of using platinum catalysts is minimal and bestows significant advantages in increased performance/weight parameters. The system using impure hydrogen is about times as expensive; most expensive is that using methanol directly in an acid medium.