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Identifying earth-abundant alternatives to platinum for the hydrogen evolution reaction (HER) remains a central challenge in electrocatalysis. Janus MXenes—two-dimensional carbides or nitrides with chemically distinct metal sublattices—break the mirror symmetry of conventional MXenes and create an electronic non-equivalence between the two oxygen-terminated surfaces that is inaccessible in symmetric structures. Here we exploit this asymmetry through a spin-polarised GGA + U density functional theory screening of six O-terminated Janus MXenes M1M2XO2 (M1/M2 = Ti/Zr, V/Nb, Cr/Mo; X = C, N), constructed as a 3 × 2 factorial series that disentangles d-electron count, 3d/4d orbital extent, and anion electronegativity. Among the thermodynamically stable candidates, only the group-5 VNb pair achieves near-thermoneutral hydrogen binding: VNbCO2 (ΔGH* = −0.03 eV) and VNbNO2 (−0.11 eV) both satisfy |ΔGH*| ≤ 0.3 eV, whereas the group-4 TiZr systems under-bind and the group-6 CrMo nitride exhibits anomalous under-binding driven by exchange splitting of the Cr d-states. A site-resolved d-band centre analysis reveals that the 3d-metal εd correlates strongly with ΔGH* across the non-magnetic systems (R = −0.91) but loses predictive power when spin polarisation is significant. The anion sublattice acts as a secondary compositional lever—shifting ΔGH* by 0.08 eV within the VNb pair while triggering a 0.67 eV destabilising swing in the magnetic CrMo pair—demonstrating that its tuning capacity is contingent on the metal pair having first placed the system near the volcano apex. These results establish that optimal HER performance in early-d-metal Janus MXenes requires a non-magnetic, spatially asymmetric 3d/4d pair at intermediate d-filling, and identify the VNb combination as a computationally promising candidate for noble-metal-free electrocatalysis.

Background Hydrogen evolution reaction (HER) is an electrochemical process for achieving sustainable energy by environment friendly …

Hydrogen production via the Hydrogen Evolution Reaction (HER) is crucial for sustainable energy, but its reliance on expensive Pt-based …

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