Covalent Surface Functionalization of Exfoliated MoS₂ Nanosheets for Improved Electrocatalysis

Transition metal dichalcogenides, particularly the metallic phase of molybdenum disulfide (1T’-MoS2), have attracted significant attention as promising nonprecious metal electrocatalysts for the hydrogen evolution reaction (HER) due to their appealing electrochemical properties and optimal hydrogen adsorption free energy (ΔGH). In this study, we used a combined theoretical and experimental approach to investigate the covalent functionalization of MoS2 with −CH2COOH groups. Notably, the presence of Mo oxidized species(MoOx) was significantly diminished in the functionalized material, suggesting a potential protective role of the functional groups against MoS2 oxidation. Additionally, our findings show a decrease in the proportion of the metallic 1T’ phase upon functionalization (58% for MoS2/CH2COOH compared to 74% for Exf-MoS2). However, electrochemical measurements unveil a remarkable enhancement in electrocatalytic activity upon the integration of −CH2COOH groups, as evidenced by observations from Linear Sweep Voltammetry (LSV) and Electrochemical Impedance Spectroscopy (EIS). In LSV analysis, a substantial 53% reduction in the Tafel slope is observed, declining from −145 mV dec–1 (for Exf-MoS2) to −77 mV dec–1 (for MoS2/CH2COOH). Additionally, EIS measurements indicate a significant decrease in charge transfer resistance, dropping from 82.03 ± 0.11 kΩ/cm2 (for Exf-MoS2) to 12.41 ± 0.01 kΩ/cm2 (for MoS2/CH2COOH). The integration of these functional groups onto the MoS2 surface demonstrates remarkable effectiveness in inhibiting oxidation while concurrently leading to a noteworthy enhancement of catalytic activity for the HER.