Oxygen Layers on Nickel in Alkaline Electrolytes

The preceding considerations may also be applied to the discussion of the formation and reduction of oxygen layers on Pt, Rh, and Ir in alkaline electrolytes since the respective i—U curves display similar properties in acid and alkaline solutions. The extent of phase hydroxides and oxides on platinum metals is small in the potential region which is of interest for the anodic oxidation of fuels and for the oxygen electrode in fuel cells. The formation of thicker oxide layers at more positive potentials will not be considered here. 

The formation of oxygen layers on nickel electrodes in alkaline solutions has been investigated [71—90] with different techniques. Anodic charging curves taken with small currents on electrodeposited [75] or smooth [74, 85] nickel from a potential of hydrogen evolution display two well defined arrests before the oxygen evolution starts. Fine details are also observable in the i — U curves measured during anodic potential sweeps. The curves in Fig. 31 resulted [83] during the first sweep 

Since only the potential region between 0 and 1.2 V is of interest for fuel cells, a discussion of the phenomena occurring above 1.2 V is not given here. 

The capacitance Cp in an analog parallel circuit at 100 Hz during the first anodic sweep is shown [84] as a function of U for a polycrystalline electrode in Fig. 32. The sweep started at —0.3 V. The sweep rate was 3mV/sec. The Cp—U curve consists of at least two overlapping waves between —0.3 V and 0.4 V. It shows a minimum of 22pF/cm between 

6 V and 0.8 V. This Cp value was taken [84] as the double layer capacitance. Above 0.6 V the capacitance rises again with U, A similar shape of the Cp — C curve had already been found [73] by measurements made point by point from —0.2 V or 0 to positive potentials on polished nickel electrodes. The capacitance at 100 Hz decreased [73] from 40pF/cm at 0.2 V to 7pF/cm at 0.7 V and achieved a value of 45pF/cm at 1.2 V in this case. The first wave of the Cp—U curve between — 0.3 V and 0 was attributed [84] as a faradaic capacitance to the Volmer reaction. The second wave was ascribed to reaction 7. However, it is conceivable that 

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