Structure-Activity Correlation in NiCo2X4 (X = S, Se, and Te) Systems for Electrocatalytic Glycerol Oxidation

The overall efficiency of electrocatalytic water splitting is hampered by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode. This issue can be resolved by replacing the OER with a more thermodynamically favorable glycerol oxidation reaction. Herein, we have synthesized a series of nickel-cobalt-based spinels (NiCo2X4; X = S, Se, and Te) as potent electrocatalysts for the glycerol electrochemical oxidation reaction (GEOR). The trend observed for their catalytic activity toward GEOR is: NiCo2Se4 > NiCo2S4 > NiCo2Te4. A high selectivity of 75.37% and Faradaic efficiency of 83.61% toward formic acid, along with 64.14% conversion of glycerol, is achieved at 1.625 V vs RHE after 24 h using NiCo2Se4. Post-electrocatalysis revealed that NiCo2Se4 acts as a precatalyst and undergoes surface reconstruction into NiCoOOH during glycerol oxidation. Upon integrating the NiCo2Se4 electrocatalyst as both cathode and anode, a two-electrode system for hybrid water splitting (HWS) (HER||GEOR) requires 1.497 V to generate a current density of 10 mA cm–2, which is 470 mV lower than the conventional water splitting (HER||OER). There is a 15.7% reduction in energy consumption in HWS compared to the overall water splitting. Theoretical calculations revealed that the inherent structural asymmetry of NiCo2Se4 is responsible for enhanced activity toward glycerol oxidation, as reflected by a higher interfacial electrostatic potential difference value upon glycerol adsorption than in NiCo2S4 and NiCo2Te4.