Anion-defect engineering of Mo2CO2 MXenes for site-selective hydrogen binding in hydrogen evolution

MXenes are a novel family of 2D materials that have garnered significant interest in electrocatalytic applications owing to their high conductivity, stability, and tuneable surface chemistry. The present study explores the electrocatalytic properties of molybdenum-based MXenes (Mo 2 CO 2 ) modified with sulfur (S), phosphorus (P), and oxygen vacancies (V O ) to optimise catalytic performance and promote hydrogen evolution reaction (HER) activity. Using first-principles calculations, we investigated the electronic structures and hydrogen adsorption behaviour of these modified MXenes and benchmarked them against MoS 2 and Mo 3 P. Our findings reveal that substituted S weakens H binding at the S site while tuning neighbouring O sites, achieving near-total thermoneutrality (ΔG H⁎ = +0.03 eV). In contrast, P draws electron density from neighbouring Mo atoms on the modified surface, with the P Mo bridge configuration providing a more moderate ΔG H⁎ than the P-top site, suggesting a site-dependent contribution to H binding. This study highlights the role of anion engineering in tailoring the electrocatalytic properties of MXenes for sustainable energy applications.