In situ determination of the 'real area factor' in H-2 evolution electrocatalysis at porous Ni-Fe composite electrodes

Composite cathode materials, such as electrodeposited Ni-Fe, Ni-Mo or Ni-Mo-Cd, are of considerable practical interest as possible electrocatalysts for H-2 production in water electrolysers and for cathodes in the chlor-alkali process. The present work extends the examination of the real electrode surface area factor in electrocatalysis at materials having porous or irregular surfaces in relation to evaluation of the role of true electrocatalytic behaviour of a 65 at.% Fe + 35 at.% Ni electrodeposited material. The results also enable the behaviour of the adsorbed H intermediate in the cathodic H-2 evolution reaction to be examined. Two distinct regions of steady state behaviour, exhibiting different Tafel slopes over the temperature range 298 to 348 K, are observed. The real to apparent electrode surface area ratio was evaluated from the double-layer capacitance values that were determined by means of a recently published method based on a curve-fitting extrapolation procedure applied to the initial, short-time, regions of potential-relaxation transients recorded after current interruptions. The double-layer capacitance values are derived under conditions of sufficient electrolyte conductivity that the effects of distributed ohmic resistance in the pores of the electrode matrix are minimized. The adsorption behaviour of the H species was also studied by means of a.c. impedance and potential-relaxation measurements.