Dimensionality-Tailored\nFerromagnetism in Quasi-Two-Dimensional\nMnSe<sub>2</sub> for the Magnetoelectrochemical Hydrogen Evolution\nReaction in Alkaline Media

The application of an external magnetic field to the\ncathode shows\ngreat promise in facilitating the hydrogen evolution reaction (HER)\nvia water electrolysis. However, the criteria for designing such cathodes\nare still under investigation. Among various aspects, understanding\nthe effect of different magnetic states of the cathode material is\ncrucial, especially for the HER in alkaline conditions, which possesses\ndifferent reaction steps compared to that in acidic conditions. Herein,\nwe present MnSe<sub>2</sub> as a cathode material for the magneto-electrocatalytic\nHER in alkaline media, utilizing its dimension-dependent magnetic\nphase transition. By tailoring its dimensionality, we have achieved\nroom-temperature ferromagnetism in its quasi-two-dimensional (2D)\nform, whereas its bulk counterpart exhibits paramagnetism. Upon being\nsubjected to a low external magnetic field of 0.4 T at −182\nmV (vs RHE) overpotential, quasi-2D MnSe<sub>2</sub> exhibited a 120%\nimprovement in current density compared to itself at zero magnetic\nfield, while negligible changes were observed in the bulk material.\nThis performance enhancement under a magnetic field could originate\nfrom the higher spin polarization of the ferromagnetic catalyst. This\nwork signifies a conceptual advancement of the catalyst’s spin\nstate in magnetically enhanced electrocatalytic reaction kinetics.