Study of a Decomposing Hydride Phase at Nickel Cathodes by Measurement of Open‐Circuit Potential Decay

At Ni or Raney Ni Al‐type electrodes subjected to cathodic polarization with evolution in alkaline solutions, characteristic time dependent currents are observed when measurements are made in the direction of increasing or decreasing potentials. Also, after polarization to high cathodic overvoltage, appreciable quantities of continue to be evolved after interruption of the current or an anodic current is observed at low cathodic overpotentials as the potential is reduced, that is at potentials still negative to the reversible potential. These observations suggest that a three‐dimensional hydride is formed which undergoes decomposition at low overpotentials. The kinetics of decomposition of such a surface or thin layer metal hydride were evaluated by means of open‐circuit potential decay measurements. A mixed "corrosion"‐type mechanism is considered with anodic hydride decomposition coupled concurrently with cathodic evolution through the steps and Evidence shows that the anodic process [1] is not Al dissolution. Kinetic treatment of the potential decay on open circuit is given in terms of the above mechanism involving adsorbed H and decomposition of a bulk hydride. Certain conditions are shown to correspond with the observed experimental behavior. Similar behavior with respect to potential decay and evolution is found with electroplated Ni‐Mo‐Cd‐coated cathode preparations.