Enhanced Long‐Term Stability of Crystalline Nickel–Boride (Ni₂₃B₆) Electrocatalyst by Encapsulation with Hexagonal Boron Nitride

Abstract Nickel boride catalysts show great potential as low‐cost and efficient alternatives to noble‐metal catalysts in acidic media; however, synthesizing and isolating a specific phase and composition of nickel boride is nontrivial, and issues persist in their long‐term stability as electrocatalysts. Here, a single‐crystal nickel boride, Ni₂₃B₆, is reported which exhibits high electrocatalytic activity for the hydrogen evolution reaction (HER) in an acidic solution, and that its poor long‐term stability can be overcome via encapsulation by single‐crystal trilayer hexagonal boron nitride (hBN) film. Interestingly, hBN‐covered Ni₂₃B₆ on a Ni substrate shows an identical overpotential of 52 mV at a current density of 10 mA cm⁻² to that of bare Ni₂₃B₆. This phenomenon indicates that the single‐crystalline hBN layer is catalytically transparent and does not obstruct HER activation. The hBN/Ni₂₃B₆/Ni has remarkable long‐term stability with negligible changes to its polarization curves for 2000 cycles, whereas the Ni₂₃B₆/Ni shows significant degradation after 650 cycles. Furthermore, chronoamperometric measurements indicate that stability is preserved for >20 h. Long‐term stability tests also reveal that the surface morphology and chemical structure of the hBN/Ni₂₃B₆/Ni electrode remain preserved. This work provides a model for the practical design of robust and durable electrochemical catalysts through the use of hBN encapsulation.