Impact of Complex Ni Surface Chemistry on HER In Alkaline Electrolytes

Ni-based materials have long served as cathodes in industrial alkaline water and chlor-alkali electrolysis, offering an effective balance of affordability, durability, and catalytic performance. However, progress in improving their intrinsic activity has been limited for decades, largely due to the lack of fundamental understanding of how the complex surface chemistry of Ni governs catalytic activity at the atomic/molecular level. Anyone working with Ni-based electrodes in the pursuit of improved hydrogen evolution reaction (HER) catalysts has likely faced serious reproducibility challenges. Even those who achieve internally consistent results ultimately still fall within the broad spread shown in Figure 1a, which compiles HER activities of metallic Ni and its oxide- or hydroxide-modified variants from numerous published studies. This variability does not result from poor reproducibility, the common practice of reporting geometric current densities instead of ECSA-normalized activities, or intrinsic reactivity. Rather, it stems from the uncontrolled diversity of Ni surface states, combined with variations in electrode and electrolyte histories and measurement protocols across the field. In this work, we do not aim to introduce yet another “high-performing” catalyst, but to resolve the fundamental origins of this scatter. Using well-defined, extended Ni surfaces prepared through rigorously controlled protocols, we identify the distinct roles of native Ni species in alkaline HER. Specifically, NiO often believed to participate in HER, acts as a passive spectator; NiH x inhibits HER; and Ni(OH) 2 promotes HER by facilitating water dissociation and generating new, highly active sites in its vicinity on metallic Ni (Figure 1b) [1]. These sites differ inherently from those composed solely of Ni atoms. The behavior of these species is interpreted in the context of two major factors controlling HER rates on Ni-based catalysts in alkaline solutions: the availability of active sites on the Ni electrode surface (the 1−θ ad term) and the energetics of the activated water complex (the Δ G 0# (H2O) term). Taken together, this study provides a unified framework for understanding the intrinsic activity of Ni-based catalysts, resolving discrepancies in the literature and offering guidance for the design of the next-generation alkaline water electrolyzers. Figure 1. (a) Summary of reported HER activities for metallic Ni (black symbol) and for Ni surfaces modified by oxide (red symbol) or hydroxide (green symbol), compiled from numerous published studies. Dashed horizontal lines mark the activity range measured in this work, from the least to the most active surface. (b) Representative polarization curves illustrating the distinct HER behavior induced by different native Ni surface species. Figures adapted from the authors’ own preprint/work. Reference: [1] D.K. Kozlica, M. Finšgar, P.F.B.D. Martins, M. Huš, B. Genorio, J.G. Connell, M. Martins, M. Hývl, T. Žibert, K.A. Varda, A. Osmić, M. Bele, B. Likozar, D. Strmčnik, The role of individual nickel surface species in the hydrogen evolution reaction on nickel in alkaline electrolytes, PREPRINT (Version 1) available at Research Square (doi.org/10.21203/rs.3.rs-7285662/v1), (22 August 2025). ___________________ The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan. http://energy.gov/downloads/doe-public-access-plan Figure 1