Enhanced Long-Term Stability of Crystalline Nickel-Boride (Ni₂₃B₆) Electrocatalyst by Encapsulation with Hexagonal...

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.