In-situ monitoring of hydrogen absorption into Ni thin film electrodes during alkaline water electrolysis

Abstract

Although hydrogen production by water electrolysis has already been studied intensively, research efforts continue to be directed towards further improving the electrochemical kinetics and the overall cell efficiency. The objective of the current work is to address an often overlooked parasitic reaction step during the hydrogen evolution reaction (HER), namely the diffusion and absorption of hydrogen adatoms from the electrode surface into the bulk. Chrono-amperometric measurements have been realised in a homemade electrochemical cell on Ni thin film samples. The latter were coupled to a multi-beam optical sensor to obtain high resolution in-situ curvature measurements during the HER. In the thin film geometry, hydrogen absorption into the electrode results in a constrained volume expansion. This in turn leads to internal stress generation, which can be monitored in-situ by means of curvature measurements. Our results on Ni thin film electrodes during alkaline water electrolysis show that such in-situ curvature measurements allow to study both the kinetics and the amount of hydrogen absorption in great detail. In particular, we observed that a higher fraction of hydrogen is being absorbed into the electrode at low overpotential, and that the rate of absorption increases significantly with increasing overpotential. We also discuss the impact of these observations on the identification of the rate-determining steps during a typical HER polarisation curve.