Reactivation of nickel cathodes by vanadium species during alkaline water electrolysis

Recent advances in the hydrogen fuel cell for zero-emission transportation vehicles have triggered a renewed interest in alkaline water electrolysis as a competitive method for pure hydrogen supply. In industrial water electrolyzers, nickel (or some form of it) is used as electrode material, and potassium hydroxide as the electrolyte at a concentration of 6–8 mol/L and a temperature of 60–80°C. The applied current density is typically 150–250 mA/cm2. The energy efficiency of electrolysis is partly related to the hydrogen evolution reaction (HER) at the cathode. Nickel exhibits a high initial electrocatalytic activity towards the HER in alkaline solutions, but it undergoes extensive deactivation during long-term electrolysis. The deactivation is indicated by an increase in the hydrogen overpotential at a constant applied current. The addition of a soluble oxide of vanadium (V ₂O₅) to the electrolyte has been found to result in reactivation of nickel and formation of a vanadium-bearing deposit on the cathode surface.; The deactivation and reactivation phenomena were investigated in the present work using various electrochemical and analytical techniques. The kinetic parameters (Tafel slope and exchange current density), hydrogen permeation rate and impedance behaviour were monitored during the course of cathode deactivation and reactivation at 100 mA/cm2. The deactivation was ascribed to the absorption of hydrogen into the metal lattice and subsequent formation of a nickel hydride phase. The reactivation was attributed mostly to the decomposition of the hydride phase formed prior to vanadium addition, and only partly to an increase in surface area upon deposition. The electrocatalytic activity of the vanadium-modified nickel was not higher than that for fresh nickel (no synergetic effects); however, the activity was stable after reactivation. The magnitude of reactivation was independent of dissolved vanadium species concentration. The deposit was found to have a smooth and compact surface with a film thickness of 1–2 μm and a largely amorphous structure. The average stoichiometry of the deposit was K₂H₂V ₁₀O₂₆·4H₂O. The mechanism of deposition involved a charge transfer step (reduction of V(+5) species), followed by a polymerization reaction (precipitation of the generated V (+4) with V(+5) species from solution) to produce a mixed-valence vanadium-based deposit.