(406b) Bifunctional NixFeySz Thin Film Catalysts for Electrochemical Water Splitting

Green Hydrogen (H₂) synthesized via water electrolysis is crucial for the transition towards a future sustainable energy economy but still requires optimization. The catalysts that drive the hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER), are thereby of critical importance. They need to be based on low-cost, abundant elements, synthesizable in an economic manner in large quantities, while at the same time showing high activity, selectivity and durability, to allow for efficient long-term operation. Transition metal sulfides of Ni, Co, and Fe have shown promise for both the OER and HER in alkaline media, thus allowing for application as bifunctional catalysts, potentially allowing for cost reduction.^[1] Some applications additionally require very thin, homogeneous catalyst films that necessitate novel synthesis strategies. One example is photoelectrochemistry, where thick layers can result in extensive parasitic light absorption.

Herein we present a simple and fast synthesis of crystalline Ni-Fe-sulfide (Ni_xFe_yS_z) thin films via physical vapor deposition followed by sulfidation. Sulfidation temperatures of ≥250 °C are sufficient for the formation of crystalline Ni_xFe_yS_z. The low synthesis temperatures increase the scope of possible substrates, including many semiconductor photoabsorbers. At lower temperatures the pentlandite crystal structure is formed for a variety of different Ni:Fe stoichiometries, while at slightly higher temperatures phase-pure thiospinels are obtained. The sulfides were tested for alkaline HER and OER, allowing for a comparison of activity and stability depending on the Ni:Fe ratio, phase composition, crystallinity and morphology. Electrochemical performance metrics from cyclic voltammetry (CV) and impedance measurements were supported by measurements of dissolution using inductive coupled plasma mass spectrometry (ICP-MS), and correlated to material properties. We additionally evaluate material transformations, which helps identifying the active species and decisive factors driving stability and durability.

[1] M. Wang, L. Zhang, Y. He, H. Zhu, J. Mater. Chem. A 2021, 9, 5320–5363.