Hierarchical Polymorphic Nickel-Tin Sulfides/MWCNT for Permselective Electroreduction of CO ₂ to Formate: RRDE-Assisted Product Validation

Electrochemical CO 2 reduction (eCO 2 RR) to value added chemicals such as formate is a promising tool toward carbon neutrality, yet achieving high selectivity and current density, simultaneously remains a significant challenge. Most of the catalysts suffer from competing hydrogen evolution reaction (HER) and poor electronic conductivity. In this study, we developed a polymorphic nickel-tin-sulfides (NTS) hybrid catalyst integrated with multi-walled carbon nanotubes (MWCNT) via a facile solvothermal method (NTS/MWCNT). Structural characterizations have confirmed the polymorphic structures that promote the selective CO 2 reduction to formate. The optimized NTS/MWCNT catalyst exhibited exceptional performance for CO 2 reduction and achieved maximum faradaic efficiency (FE) of 93.4% for formate generation with a partial current density of 30.4 mA cm −2 . To provide deeper insight into liquid and gaseous product distribution and catalytic activity during eCO 2 RR, we employed 1 H NMR, gas chromatography (GC) and hydrodynamic voltammetry using rotating ring-disk electrode (RRDE) studies. Accordingly, this work highlights the existence of synergistic interaction between polymorphic NTS, such as γ-Sn 2 S 3, NiS, and Ni 3 S 2 phases and MWCNT, which is not only selective towards formate generation but also robust in terms of structural stability. Thus, NTS/MWCNT acts as an efficient catalyst for eCO 2 RR, and RRDE study demonstrates as a powerful tool for in situ product monitoring during electrolysis with ease at oxidative and reductive cycles, offering valuable perceptions for rational designing of next-generation eCO 2 RR electrocatalysts.