Determining the atomic hydrogen surface coverage on iron and nickel electrodes under water treatment conditions

Reductive methods for removing, detoxifying, or inactivating contaminants in water often involve reactions with atomic hydrogen produced from water reduction. Knowledge of how the solution pH value and electrode potential affect the concentration of atomic hydrogen on the reactive surface will be useful for evaluating possible reaction mechanisms and in optimizing treatment schemes. Presently, there are no simple methods for determining the atomic hydrogen surface coverage on the base metals that are typically used as cathodes or sacrificial reactants in water treatment operations. This research develops and evaluates an iterative, coulometric method for determining the fractional atomic hydrogen surface coverage (θ_H) on iron and nickel electrodes under water treatment conditions. The method is applicable at pH values and potentials where proton discharge is the rate-limiting step for the hydrogen evolution reaction (HER), and is valid under conditions where the metals are covered by oxide layers that lower the apparent electron transfer coefficients by up to 40% as compared to oxide-free conditions at low pH values. The method is also able to determine the exchange current density and the rate constants for the Volmer discharge and Tafel recombination steps of the HER.