Electrochemical voltage spectroscopy for analysis of nickel electrodes

Electrochemical voltage spectroscopy (EVS) is a technique that directly measures the density of electrochemically active states in an electrode as a function of the applied voltage. In EVS measurements, the voltage of an electrode is scanned at a rate that is slow enough to maintain the electrode close to thermodynamic equilibrium, over a potential range where electroactive species may be oxidized or reduced. The density of reactive sites is obtained from the Coulombs of charge passed through the electrode per voltage increment, which is essentially differential capacitance. For most electrodes, interest is primarily in the Faradaic components of the EVS spectra, which exhibit sharp peaks at the electrochemical redox potentials, although non-Faradaic components (such as double-layer or surface capacitance) can also be measured. For nickel electrodes, EVS provides an extremely useful method for probing the phase composition of the active material based on subtle differences in redox potentials. Alternatively, EVS can detect trace levels of electroactive contaminants in nickel-hydrogen cells or nickel electrodes by scanning the potential over the redox range for the contaminant of interest. We discuss the use of nickel electrode EVS signatures to indicate cobalt additive levels, sinter corrosion, surface changes, double-layer capacitance, electrode swelling, and other factors influencing the performance of the nickel electrode.