New insights into the hydrogen evolution reaction kinetics using advanced electrochemical techniques

This work investigates hydrogen evolution reaction (HER) kinetics on pure Ni in electrolytes with a wide pH range using multiple electrochemical techniques, including potentiodynamic/potentiostatic polarization, rotating disk electrode (RDE), electrochemical mass spectrometry (ECMS) and electrochemical impedance spectroscopy (EIS). Polarization curves reveal that HER at low overpotentials is predominantly controlled by the Volmer and Heyrovsky reaction steps in all electrolytes. EIS corroborates that Heyrovsky-related adsorption resistance dominates the kinetics and shows increasing capacitance with overpotential related to greater coverage by adsorbed hydrogen atoms. RDE results indicate that while the dissolved oxygen transport influences the current at low overpotentials, mass transport has little effect on the overall HER reaction rate. X-ray photoelectron spectroscopy (XPS) analysis reveals only partial reduction of surface oxide/hydroxide films despite cathodic cleaning. Consequently, the evaluated HER kinetics correspond to Ni hydroxide rather than pure Ni in neutral/alkaline electrolytes. ECMS results verify the HER efficiency on the Ni hydroxide surface, which has little impact on HER activity. The novel combination of techniques employed in this work yields valuable insights into HER kinetics, facilitating more accurate predictions of hydrogen absorption in cathodically polarized metals.