Rational Design and Synthesis of Nickel–Cobalt Sulfide Core–Shell Nanospheres as Efficient Electrocatalysts for Sustainable CO 2 Reduction

In this work, the synthesis of the core–shell nickel–cobalt sulfide (NCS) catalyst was carried out via a direct sulfidation of nickel–cobalt glycerate (NC-g) through a template-free, facile, and cost-effective solvothermal method and applied for electrochemical CO2RR. The NCS material was thoroughly characterized using various analytical and spectroscopic techniques. The NCS catalyst exhibited distinctive structural and electronic properties, such as a narrow band gap, high conductivity, induced sulfur-defect-rich sites, and core–shell ball-in-ball nanospheres morphology. The electrocatalytic performance of the as-synthesized NCS was systematically compared with NC-g, NCO, NiS, and CoS catalysts. Among these, the NCS catalyst depicted superior electrocatalytic results, displaying a higher peak current density of −54.15 mA/cm2 and a low Tafel slope of 45.9 mV dec–1. The NCS catalyst demonstrated excellent selectivity with a Faradaic efficiency (FE %) for HCOOH production, reaching over 86.9% at an optimum potential of −0.5 V vs RHE. Comprehensive analyses of the reaction kinetics using Tafel slopes and electrochemical impedance spectroscopy measurements further designated rapid electron transfer kinetics and a low charge transfer resistance (Rct), enabling efficient CO2 conversion and long-term stability over a prolonged period of 24 h. The synergistic effect of Ni, Co, and S in the NCS matrix mutually enriched the eCO2RR performance by promoting H* formation, CO2 adsorption, and stabilization of HCOO* as an intermediate, supporting HCOOH formation. The microstructural and crystallographic analyses further supported the electrochemical results. These findings highlight that conductivity alone is inadequate for efficient CO2RR and that surface defect engineering and retention of the CO2-philic sites are acute. This work not only establishes NCS via a direct sulfidation of NC-g as an effective strategy through an anion-dependent effect (S2– vs O2– vs glycerate) for designing an efficient catalyst for CO2 to HCOOH conversion but also contributes to advancing scalable strategies for electrochemical CO2 reduction toward value-added products.