An evaluation of some non-noble metal fuel cell catalysts

Abstract

The platinum group metals are frequently used in low temperature fuel cells because of their stability and high catalytic activity [1-3]. Unfortunately the high cost and scarcity of these metals have limited fuel cells to military and space applications [4]. A more widespread application of fuel cells would probably result if the amount of noble metal catalyst required for a given power could be reduced, or if an inexpensive non-noble metal catalyst could be developed. Recently, by the use of a catalyst support, some success has been achieved in reducing the amount of noble metals required. With carbon, the catalyst loading for high performance hydrogen and oxygen electrodes has been reduced from 10 mg/cm 2 [5] to 0.2-2 mg/cm 2 [6], while for propane oxidation it has been reduced from 35 mg/cm 2 [7] to 10 mg/cm 2 [8]. Similar reductions in the catalyst loading for propane oxidation have been obtained with a boron carbide support [9]. This material is preferred to carbon because it is more inert, and because platinum has less tendency to sinter on it. For carbon monoxide oxidation a smaller amount of platinum has been used by diluting it with various transition metal oxides [10]. In another approach [11] the amount of noble metal required for hydrocarbon oxidation has been lowered by using a compound of platinum with phosphorus [PtP2]. Several non-noble metal materials have also been examined for electrocatalytic activity. For the oxygen electrode cobalt phthalocyanine [12], boron carbide [13], titanium nitride [14], tungsten bronze [15], nickel boride [16], and cobalt boride [16] have been shown to have some activity. Of these only tungsten bronze appears as though it might replace platinum in acid solutions. For the fuel electrode, Raney nickel [17], nickel boride [18, 19, 20], cobalt boride [21], hydrides of titanium, zirconium, and hafnium [22], cobalt molybdate [23], and manganese molybdate [24] have been reported. Raney nickel and nickel boride, apparently, are almost as good as platinum for the electrooxidation of hydrogen in alkaline solutions, while cobalt and manganese molybdates are claimed to be as good as platinum for the electro-oxidation of hydrocarbons. In the work reported here the electrocatalytic properties of cobalt and manganese molybdates have been re-examined.