Metal boride better than Pt: HCP Pd2B as a superactive hydrogen evolution reaction catalyst

In the search for a highly active bulk material for the hydrogen evolution reaction (HER) under acidic conditions, we developed a simple solvothermal approach to synthesize Pd2B nanosheets supported on carbon, which achieves a low overpotential for the HER, 15.3 mV at 10 mA cm(-2), a small Tafel slope of 22.5 mV dec(-1) and a high exchange current density (j(0)) of 2.84 mA cm(-2). The atomic structure evolution from Pd to Pd2B catalyst during synthesis is analyzed in detail via experimental and theoretical calculations, which shows that the slow insertion of B is assisted by the layer-by-layer fcc-to-hcp phase transition. Theoretical calculations further revealed that both the subsurface B and the lattice expansion after the hcp lattice formation play a key role to boost the HER activity. Since the Pd2B crystal is the global minimum in the Pd-B alloy, the success in the synthesis and demonstration of high HER activity paves the way towards further exploration of the catalytic performance for this stable metal boride material.