"Activation of porous Ni electrodes by (co)deposition of noble metals or noble metal oxides"

Various strategies for the activation of porous Ni electrodes towards the hydrogen evolution reaction are compared. Porous Ni electrodes can be obtained by electrodeposition, at large cathodic current densities, from NiCl2 + NH4Cl solutions. Two main routes for the activation of these electrodes, respectively involving one or two preparation steps, were explored: (i) co-deposition of Ni and noble metal oxide particles and (ii) deposition of noble metal nuclei onto the pre-formed porous Ni layers. Porous Ni+RuO2 composites were obtained by electrolyzing suspensions of RuO2 in the NiCl2 + NH4Cl electrolyte. The co-deposition of RuO2 particles somewhat depressed the current efficiency of Ni deposition, but did not prevent the formation of deposits with a large surface roughness. The preparation of Ni+IrO2 composites was significantly more difficult at the high current densities required to promote the formation of porous Ni. Spontaneous deposition of either Ru or Ir onto porous Ni was achieved by immersion of the Ni deposits in deaerated solutions containing Ru(III) or Ir(IV) chloride complexes. Noble metal deposition and Ni dissolution occurred simultaneously, at open circuit, causing a further increase in the initial large area of the Ni deposits. The spontaneous deposition of Ru was faster than that of Ir. The preparation time of Ir-modified electrodes could be shortened by electrodepositing the noble metal nuclei. Both routes led to active cathodes for hydrogen evolution, mainly studied in basic solution. Typical kinetic parameters were: exchange current density around 10 mA cm-2 and Tafel slope in the range 40 to 50 mV. The best stability, both under hydrogen evolution conditions and upon polarity inversion, was shown by the porous Ni+RuO2 composites.