Cation-exchange-induced structural and chemical modulation of transition metal spinel sulfides to enhance their oxygen evolution performance

Abstract

Transition metal spinels are promising oxygen evolution reaction (OER) catalysts; however, their rational design has been impeded by their inherent elemental and structural complexities. To alleviate these issues, this paper describes the role of cation exchange in developing active OER catalysts whose chemical and structural features are tailored while their morphology and crystal structure are maintained for enabling a systematic comparison. Through cation exchange, pre-synthesized Co3S4 spinel nanoparticles (NPs) transform into (NixCo3−x)S4 NPs with compositions ranging from Co-rich to Ni-rich phases. Apart from stoichiometry, the atomic disorder and chemical state of the metal cations are amended through excessive interdiffusion and interstitial site preference of the cations, evidently helping boost the catalytic activity. Specifically, (Ni1.2Co1.8)S4—which exhibits an optimal Ni/Co ratio, high structural disorder, and distinct surface chemical states—shows excellent OER performance (overpotential at 10 mA/cm2: 298 mV). In-depth analyses including in situ Raman spectroscopy and DFT calculations indicate that (Ni1.2Co1.8)S4 gradually transforms into OER-active Co/NiOOH—which has a unique electronic structure with strong reaction-intermediate-adsorbing characteristics—during the OER cycles. Overall, the study findings underscore the potency of cation exchange as a tool for controlling structural and chemical features, thereby helping explore complex catalyst materials while enhancing their activity. © 2024