Climbing the Volcano of Electrocatalytic Activity while Avoiding Catalyst Corrosion: Ni₃P, a Hydrogen Evolution Electrocatalyst Stable in Both Acid and Alkali

We report microcrystalline Ni₃P as a noble-metal-free electrocatalyst for the H₂ evolution reaction (HER) with high activity just below those of Ni₅P₄ and Pt, the two most efficient HER catalysts known. Ni₃P has previously been dismissed for the HER, owing to its anticipated corrosion and its low activity when formed as an impurity in amorphous alloys. We observe higher activity of single-phase Ni₃P crystallites than for other nickel phosphides (except Ni₅P₄) in acid, high corrosion tolerance in acid, and zero corrosion in alkali. We compare its electrocatalytic performance, corrosion stability, and intrinsic turnover rate to those of different transition-metal phosphides. Electrochemical studies reveal that poisoning of surface Ni sites does not block the HER, indicating P as the active site. Using density functional theory (DFT), we analyze the thermodynamic stability of Ni₃P and compare it to experiments. DFT calculations predict that surface reconstruction of Ni₃P (001) strongly favors P enrichment of the Ni₄P₄ termination and that the H adsorption energy depends strongly on the surface reconstruction, thus revealing a potential synthetic lever for tuning HER catalytic activity. A particular P-enriched reconstructed surface on Ni₃P(001) is predicted to be the most stable surface termination at intermediate P content, as well as providing the most active surface site at low overpotentials. The P adatoms present on this reconstructed surface are more active for HER at low overpotentials in comparison to any of the sites investigated on other terminations of Ni₃P(001), as they possess nearly thermoneutral H adsorption. To our knowledge this is the first time reconstructed surfaces of transition-metal phosphides have been identified as having the most active surface site, with such good agreement with the experimentally observed catalytic current onset and Tafel slope. The active site geometry achieved through reconstruction identified in this work shows great similarity to that reported for Ni₂P(0001) and Ni₅P₄(0001) facets, serving as a general design principle for the future development of even more active transition-metal phosphide catalysts and further climbing the volcano plot.