d-Band Engineering of Layered (Fe_1-xNi_x)₃GaTe₂ for Enhanced Alkaline Hydrogen Evolution by Ni-Substitutional Doping

Tuning the d-band electronic structure of non-noble-metal catalysts is a central strategy for an alkaline hydrogen evolution reaction (HER), yet how composition controls the d orbital in multi-Wyckoff-site layered systems remains insufficiently understood. Here, layered (Fe_1-xNi_x)₃GaTe₂ single crystals (x = 0.2-1.0) were synthesized by the self-flux method as a platform to address this question. Single-crystal XRD and EDS confirm that Ni is uniformly incorporated into the parent P6₃/mmc framework while inducing a composition-dependent lattice evolution. Electrochemical measurements in 1.0 M KOH reveal a clear volcano-shaped composition dependence, peaking at x = 0.6, where the lowest overpotential, the smallest Tafel slope (94 mV dec^-1), the lowest charge-transfer resistance and the largest double-layer capacitance are simultaneously reached. First-principles calculations show that Ni doping reshapes the Fe-site d orbital strongly composition-dependent rate: the Fe d-band center upshifts rapidly by ~0.5 eV between x = 0.4 and x = 0.6, while the Ni d-band center stays nearly fixed in the same composition range. The maximum of HER activity therefore aligns with a steep upshift of the Fe d-band center rather than with the Ni content itself. Charge-density mapping of (Fe_0.4Ni_0.6)₃GaTe₂ further demonstrates that the electron-enriched regions are located on the Fe and interlayer Ni3 sublattices that dominate the d states near E_F.