Tailoring sulfurization conditions in NiFe-PBA-derived NiSx@FeSx electrocatalysts for enhanced bifunctional water splitting

Designing efficient, cost-effective electrocatalysts for electrochemical splitting of water is essential towards developing clean hydrogen energy devices. Here, we report a set of NiS_x@FeS_x nanocomposites from NiFe-based Prussian Blue Analogue (PBA) materials, along with a detailed analysis of how sulfurization temperature in a range of 400–650 °C impacts structure evolution, surface composition, and electrochemical performance. The structure determination of NiS_x@FeS_x nanocomposites confirmed mixed-metal sulfide phases of NiS₂ as well as FeS₂, along with heterogeneous interfaces and favorable Ni³⁺ and Fe³⁺ oxidation states, both of which are significant for catalytic activity. Amongst all, NiS_x@FeS_x nanocomposite sulfurized at 500 °C displayed maximal electrocatalytic performance. Its higher catalytic activity was attributed to a synergistic coupling of heterogeneous, interconnected interfaces along with increased ionic conductivity, allowing for more effective charge transfer as well as enhanced reaction kinetics. The electrochemical data affirmed that NiS_x@FeS_x-500 needed a low overpotential of 276 mV towards oxygen evolution reaction (OER) at 50 mA cm⁻², coupled with a resultant Tafel slope of 91 mV dec⁻¹. Towards hydrogen evolution reaction (HER), it demonstrated a low overpotential of 179 mV at 10 mA cm⁻² as well as a Tafel slope of 81 mV dec⁻¹, reflecting effective reaction kinetics. Additionally, the two-electrode electrolyzer constructed using this material both as anode and cathode needed merely 1.706 V to reach 10 mA cm⁻², while operating stably over a period of 10 h. These observations point towards potential applications of NiFe-PBA-derived NiS_x@FeS_x-500 nano composite as a stable, cost-effective bifunctional electrocatalyst towards overall water splitting technologies.

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