Design of Nickel-based Electrocatalysts for Water Splitting:Thin Films, 3D Electrodes, and Activation Mechanisms

Green hydrogen is increasingly recognized as an impactful solution in transitioning from fossil fuels toward a sustainable energy landscape. Among the various production methods, alkaline water electrolysis is considered a mature technology suitable for large-scale hydrogen production. However, their widespread implementation is still challenged by the limited efficiency and long-term stability of non-precious metal electrocatalysts for the oxygen and hydrogen evolution reactions (OER and HER, respectively). In this context, nickel-based electro-catalysts have gained significant attention due to their relatively low cost and favorable electrochemical properties under alkaline conditions. This dissertation investigates the design, synthesis, and electrochemical evaluation of nickel-based catalysts, combining gas phase thin-film deposition and 3D electrode engineering, with the aim of unravelling relationships between film properties and electrochemical performance. Two materials synthesis methods, atomic layer deposition (ALD) and gas-phase sulphurization, are exploited in the dissertation. In this regard, nickel oxide (NiO) thin films are investigated for the oxygen evolution reaction (OER), while nickel sulphides (NixSy) are tailored for the hydrogen evolution reaction (HER). Chapter 1 outlines the scientific motivation and key research questions while introducing the experimental strategies designed to address current knowledge gaps in the development and mechanistic understanding of nickel-based electrocatalysts for alkaline water splitting. In addition, Chapter 2 provides the theoretical foundation by discussing the water splitting in alkaline media, the role of nickel oxides and sulphides, and catalytic activation pathways, including Fe incorporation, surface reconstruction, and phase transformation. Moreover, Chapter 3 details the experimental methodologies, covering substrate preparation, ALD/sulphurization processes, electrochemical cell setups, and film characterization techniques. Chapters 4 and 5 focus on the application of ALD to design and engineer NiO electrocatalysts with tunable properties in alkaline conditions. Using both thermal and plasma-assisted ALD methods, ~10 nm-thick NiO thin films were deposited on model 2D substrates (FTO-glass and planar Ni) and commercial 3D-structured Ni electrodes to systematically study the relationship between surface/bulk characteristics and electrochemical activation. The ALD NiO alters the catalytic properties of Ni electrodes, leading to a notable enhancement of the OER performance, decreasing the overpotential at 500 mA·cm-2 from 640 mV (pristine Ni Veco) to 470 mV and 560 mV following deposition of thermal and plasma-assisted ALD NiO films, respectively. This improvement in OER performance is attributed to modifications in surface chemistry and surface energy of the electrodes, thereby facilitating the formation of higher concentrations of Ni3+ species, widely recognized as the catalytically active sites for OER in alkaline media. Moreover, the electrochemical activation process promoted surface restructuring into active nanostructured features, thereby increasing the electrochemically active surface area (ECSA). This structural evolution, combined with the incorporation of trace Fe species from the electrolyte during activation, contributed to enhanced catalytic activity by modifying the surface/bulk properties and promoting faster charge transfer kinetics. In parallel, the potential of nickel sulphides for HER was explored via gas-phase sulphurization of Ni electrodes. Chapter 6 focuses on how key sulphurization parameters, including temperature, H2S flow rate, and time, influence the resulting crystallographic phase and chemical composition as well as the electrochemical surface area (ECSA), ultimately identifying an optimal Ni3S2/Ni3S4 phase mixture that exhibits high HER activity, outperforming pristine Ni and approaching Pt-coated 3D Ni electrodes. Building on these findings, Chapter 7 investigates the structural and electrochemical evolution of various nickel sulphide catalysts during prolonged HER operation, revealing the transition toward the more active Ni3S2 phase. This electrochemical activation, primarily driven by sulphur leaching, enhances HER kinetics by increasing surface area, improving charge transfer, and shifting the rate-determining step, with structural stabilization observed after the initial activation phase. In summary, these studies provide new insights into the roles of the electrocatalyst surface and bulk properties, the electrode’s architecture, and the electrochemical activation of nickel-based catalysts for water splitting. The findings contribute to the design of high-performance electrodes for green hydrogen production.