Formation of Highly Active NiO(OH) Thin Films from Electrochemically Deposited Ni(OH)2 by a Simple Thermal Treatment at a Moderate Temperature: A Combined Electrochemical and Surface Science Investigation

Electrodeposited alpha-Ni(OH)(2) thin films on Ti substrates were investigated as electrocatalysts for cost-effective water-splitting devices. A facile annealing of the alpha-Ni(OH)(2) films at 150 degrees C for 1 h is of extreme importance to enhance their electrocatalytic activity for the oxygen evolution reaction (OER). Voltammetric analysis combined with X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and scanning electron microscopy (SEM) reveals that the induced structural evolution of the alpha-Ni(OH)(2) results in a strong improvement of electrical conductivity, which is a crucial factor for the highly enhanced activity. Moreover, the annealing accelerates the generation of active sites, which can even be further improved easily via electrochemical galvanostatic conditioning. Thus, the alpha-Ni(OH)(2) thin films exhibit an overpotential of 310 mV (at j = 10 mA cm(-2)) with a Tafel slope of 42.6 mV dec(-1) in 1.0 M KOH, which is much better than that for a commercial RuO2/Ti electrode. The active component at the applied electrode potential of 10 mA cm(-2) is characterized to be gamma-(NiO0.63)-O-2-O-.-O-63(OH)(1.)(37) showing an "apparent oxidation state" of +2.63 for the Ni. In addition, the annealed electrocatalyst exhibits only a negligible overpotential increase after 26 h of measurement. The overpotential even decreases to 292 mV in the first 3 h, demonstrating outstanding electrocatalytic activity and long-term stability.