Effect of Electrochemical Surface Oxidation on Intrinsic Activity Towards the Hydrogen Evolution Reaction of Ultra-Smooth Nanocrystalline Ni Thin Films

Abstract

Green hydrogen has emerged as a leading contender in the pursuit of a sustainable energy transition, driving the need for optimized catalysts in hydrogen production. Therefore, considerable effort is taken to replace the highly active, yet expensive Pt-group metals with non-noble alternatives such as Ni. Understanding the hydrogen evolution reaction (HER) mechanism on well-performing Ni surfaces is essential for gaining fundamental knowledge and implementing this in practical applications. While high surface area structures are typically designed for enhanced efficiency, there is limited research available on the fundamental aspects of thin films and smooth surfaces, which are crucial for benchmarking intrinsic catalytic activity and elucidating catalytic mechanisms. To address this gap, we perform a combined microstructural and electrochemical evaluation of smooth Ni thin film electrodes to analyze HER in alkaline media.This study aimed to evaluate the intrinsic catalytic properties of nanocrystalline Ni thin films and the effect of electrochemical oxidation on the catalytic behavior, while precisely monitoring the evolution of the film surface state and roughness. In addition to experimental work, Density Functional Theory (DFT) calculations and Kinetic Monte Carlo (KMC) simulations are used to understand HER on oxide-modified Ni surfaces and the role of different elementary processes involved in the HER mechanism.Nanocrystalline low-roughness Ni electrodes for HER were produced via magnetron sputter deposition to minimize the difference between geometrical and electrochemical surface area. This means we find intrinsic material properties directly, bypassing the need for additional measurements for real surface area determination. The analysis of catalyst activities towards HER as well as the oxidation process [1] was performed via cyclic voltammetry, using a three-electrode setup. Within the conducted studies, we can report a significant lowering of the overpotential for HER upon in-situ oxidation of the electrodes due to the existence of NiO, and Ni(OH)2 on the Ni catalyst surface [2], leading also to a limitation of the deactivation processes. Further analysis including topography evaluation (AFM), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) in combination with selected area diffraction (SAED) of the as-deposited and oxidized electrodes confirmed that the observed effect can be assigned to the availability of oxides on the surface assuredly (Figure 1). Additionally, theoretical calculations and simulations (DFT, KMC) support the experimental data and derived theories.In conclusion, the beneficial effect of NiO and Ni(OH)2 presence on Ni surfaces was examined and successfully proven by a combination of electrochemical measurements, material characterization, theoretical calculations, and a careful experimental design that minimizes the influence of interfering factors, such as surface roughness.