Early-d-metal Janus MXenes as near-thermoneutral hydrogen evolution electrocatalysts: role of anion identity and d-band asymmetry

Abstract

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 (ΔG_H* = −0.03 eV) and VNbNO₂ (−0.11 eV) both satisfy |ΔG_H*| ≤ 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 ε_d correlates strongly with ΔG_H* 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 ΔG_H* 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.