Recent developments in electrode materials for water electrolysis

This work reports improvements and alternatives for the preparation of high area Ni and Ni–Co coatings as well as the deposition of a highly active Ni–Fe layer on mild steel substrates. It also describes the formation of spinel NiCo2O4 layers on several substrates using the Sol-Gel method. High area Ni and multilayer Ni–Co alloy coatings have been obtained with a new electrodeposition methodology that allows the complete removal of Zn. This generates highly porous surfaces with roughness factors of 2200 for Ni and 4400 for Ni–Co. The hydrogen overpotential measured for these coatings at 135 mA cm−2 and 70°C were 0.1 and 0.09 V, respectively. Rough Ni–Fe coatings having 40 at % of Ni were deposited from an acetate bath and were activated by anodic polarization in HCl. Analyses by SEM and dissolution voltammetry suggest that the activation removes a passivating layer on the surface, revealing an active Ni–Fe phase. Polarization curves obtained at different temperatures presented Tafel slopes of 63–80 for 25–80°C and an apparent activation energy of 55 kJ mol−1. Long term operation in 4 M NaOH at 135 mA cm−2 showed a hydrogen overpotential of 0.16 V at 70°C, a value considerably lower than 0.35 V found for smooth Ni or 0.49 V for an equivalent Ni–Fe metallurgical alloy. Active anodes were prepared by the synthesis of NiCo2O4 spinel oxides using the Sol-Gel method on Ni and Ti substrates. The coatings showed an oxygen overpotential of nearly 0.3 V, approximately 0.6 V less than that obtained for pure Ni oxide anodes, under the experimental conditions presented above. These anode materials were tested in long term operations at 135 mA cm−2 and 70°C for periods up to 200 h, without any evidence of a decrease in their catalytic activity.