Characterization of Vanadium Deposit Formation at a Hydrogen Evolving Electrode in Alkaline Media

Although nickel exhibits a high electrocatalytic activity toward the hydrogen evolution reaction, it undergoes extensive deactivation as a cathode during alkaline water electrolysis. The addition of dissolved to the electrolyte results in partial reactivation of nickel cathodes in 8 mol/L KOH at 70°C by formation of a vanadium-rich deposit. Various analytical techniques were employed to characterize the deposit in terms of surface morphology, phase analysis, and chemical composition. The deposit on nickel had a smooth and compact surface with a thickness of 1-2 μm. X-ray diffraction results indicated an amorphous structure with a Scherrer's length less than 2 nm. Chemical and thermal analyses led to an empirical compound formula of That formula implied a mixed-valence vanadium compound, later confirmed by X-ray photoelectron spectroscopy. The mechanism of deposit formation was then investigated by cyclic voltammetry on a mercury electrode. A cathodic peak representing the reduction of vanadium species was apparent near the onset of hydrogen evolution on mercury. The behavior of the cathodic peak with concentration and sweep rate revealed key steps in the mechanisms of deposit formation. Deposit formation was modeled as an irreversible precipitation/polymerization reaction following a charge transfer step, to produce a large, mixed-valence vanadium compound. © 2001 The Electrochemical Society. All rights reserved.