Influence of deposition current density on the composition and properties of electrodeposited Ni + RuO2 and Ni + IrO2 composites

Ni + IrO2 composites were deposited from suspensions of fine powders of the noble metal oxide in two Ni2+ containing electrolytes (a mildly acid NiCl2, NH4Cl solution and a basic NiSO4, (NH4)2SO4, NH4OH solution) using current densities varying from −5 mA cm−2 to −2.5 A cm−2. The composition, morphology and surface roughness of the deposits were studied as a function of the deposition current density (jDEP) and compared to that of Ni + RuO2 composites deposited from the chloride electrolyte. For all systems, the volume fraction of the dispersed phases (αIrO2αIrO2 or αRuO2αRuO2) decreased as jDEP increased. At the high jDEP values usually employed to deposit porous Ni, αIrO2αIrO2 was very low, irrespective of the deposition bath, whereas αRuO2αRuO2 remained as high as 0.2. At the same jDEP values, the surface roughness of the Ni + RuO2 composites was ca. four times higher than that of the Ni + IrO2 composites, which was comparable to that of porous Ni devoid of dispersed particles. Since composites simultaneously possessing large surface roughness and large concentration of catalytic sites were obtained only when RuO2 was used as dispersed phase, the Ni + RuO2 cathodes had much higher catalytic activity in the hydrogen evolution reaction than Ni + IrO2 cathodes, the latter being only slightly more active than porous Ni cathodes.