Nanocrystalline Ni₅P₄: A hydrogen evolution electrocatalyst of exceptional efficiency in both alkaline and acidic media

Producing hydrogen (H₂) by splitting water with fossil-free electricity is considered a grand challenge for developing sustainable energy systems and a carbon dioxide free source of renewable H₂. Renewable H₂ may be produced from water by electrolysis with either low efficiency alkaline electrolyzers that suffer 50-65% losses, or by more efficient acidic electrolyzers with rare platinum group metal catalysts (Pt). Consequently, research has focused on developing alternative, cheap, and robust catalysts made from earth-abundant elements. Here, we show that crystalline Ni₅P₄ evolves H₂ with geometric electrical to chemical conversion efficiency on par with Pt in strong acid (33 mV dec^-1 Tafel slope and -62 mV overpotential at -100 mA cm^-2 in 1 M H₂SO₄). The conductivity of Ni₅P₄ microparticles is sufficient to allow fabrication of electrodes without conducting binders by pressing pellets. Significantly, no catalyst degradation is seen in short term studies at current densities of -10 mA cm^-2, equivalent to ∼10% solar photoelectrical conversion efficiency. The realization of a noble metal-free catalyst performing on par with Pt in both strong acid and base offers a key step towards industrially relevant electrolyzers competing with conventional H₂ sources.