Covalent Surface\nFunctionalization of Exfoliated MoS<sub>2</sub> Nanosheets for Improved\nElectrocatalysis

Transition metal dichalcogenides, particularly the metallic\nphase\nof molybdenum disulfide (1T’-MoS<sub>2</sub>), have attracted\nsignificant attention as promising nonprecious metal electrocatalysts\nfor the hydrogen evolution reaction (HER) due to their appealing electrochemical\nproperties and optimal hydrogen adsorption free energy (Δ<i>G</i><sub>H</sub>). In this study, we used a combined theoretical\nand experimental approach to investigate the covalent functionalization\nof MoS<sub>2</sub> with −CH<sub>2</sub>COOH groups. Notably,\nthe presence of Mo oxidized species(MoO<sub><i>x</i></sub>) was significantly diminished in the functionalized material, suggesting\na potential protective role of the functional groups against MoS<sub>2</sub> oxidation. Additionally, our findings show a decrease in\nthe proportion of the metallic 1T’ phase upon functionalization\n(58% for MoS<sub>2</sub>/CH<sub>2</sub>COOH compared to 74% for Exf-MoS<sub>2</sub>). However, electrochemical measurements unveil a remarkable\nenhancement in electrocatalytic activity upon the integration of −CH<sub>2</sub>COOH groups, as evidenced by observations from Linear Sweep\nVoltammetry (LSV) and Electrochemical Impedance Spectroscopy (EIS).\nIn LSV analysis, a substantial 53% reduction in the Tafel slope is\nobserved, declining from −145 mV dec<sup>–1</sup> (for\nExf-MoS<sub>2</sub>) to −77 mV dec<sup>–1</sup> (for\nMoS<sub>2</sub>/CH<sub>2</sub>COOH). Additionally, EIS measurements\nindicate a significant decrease in charge transfer resistance, dropping\nfrom 82.03 ± 0.11 kΩ/cm<sup>2</sup> (for Exf-MoS<sub>2</sub>) to 12.41 ± 0.01 kΩ/cm<sup>2</sup> (for MoS<sub>2</sub>/CH<sub>2</sub>COOH). The integration of these functional groups\nonto the MoS<sub>2</sub> surface demonstrates remarkable effectiveness\nin inhibiting oxidation while concurrently leading to a noteworthy\nenhancement of catalytic activity for the HER.