Elucidation of the oxidation and reduction mechanism of Raney nickel by sorption experiments

Abstract

As electrocatalyst for hydrogen anodes in alkaline fuel cells for civilian applications one prefers Raney nickel, introduced into electrochemistry by Justi et al. in 1953 [1]. However, the catalytic activity of Raney nickel is considerably diminished as soon as the Raney nickel is partially oxidized to nickel(II) hydroxide Ni(OH)9. at a potential of 170 mV/Ha rev [2]. Such an oxidation of the catalyst can also be caused by insufficient gas supply or by overloading the fuel cell. For reactivation of the Raney nickel the nickel-hydroxide must be reduced; this happens at room temperature to about 40 per cent only and a complete reduction is possible only above 60°C [3]. Fuel cells are commonly operated in the temperature range of 60-80°C; if oxidation of Raney nickel has taken place at these temperatures a complete reactivation of the oxidized catalyst has no chances. This behaviour of the hydroxide formed on the Raney nickel has been explained in former publications as an ageing process of the nickel hydroxide, i.e. one has supposed that the hydroxide is partially transformed into a more stable, less reducible form called aged nickel-hydroxide [2, 3]. In the meantime an ageing by dehydration could be excluded by our X-ray structure investigations; in contrary we have confirmed that on the Raney nickel is always exclusively formed crystal water containing a-3Ni(OH)~. 2H20, which otherwise is not stable in 6N KOH according to Bode et aL [4]. At the same time our X-ray experiments have shown that nickel hydroxide formed at 80°C and being nearly irreducible has a higher degree of crystallinity, i.e. consists of larger crystals [5]. These results have suggested that the ageing process of nickel(II) hydroxide may be a kind of recrystallization. The geometric arrangement of nickel hydroxide may be responsible that its reduction is more difficult than its oxidation. To check this assumption and to get detailed information of its oxidation-reduction mechanism we have investigated the pore structures and surfaces of differently oxidized or reduced Raney nickel specimens by sorption measurements.