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 degrees 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 degrees 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 degrees 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 degrees C for periods up to 200 h, without any evidence of a decrease in their catalytic activity. (C) 2000 cm International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.