Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles**

Nanoparticles of cobalt phosphide, CoP, have been prepared and evaluated as electrocatalysts for the hydrogen evolution reaction (HER) under strongly acidic conditions (0.50M H2SO4, pH0.3). Uniform, multi-faceted CoP nanoparticles were synthesized by reacting Co nanoparticles with trioctylphosphine. Electrodes comprised of CoP nanoparticles on a Ti support (2mgcm(-2) mass loading) produced a cathodic current density of 20mAcm(-2) at an overpotential of -85mV. The CoP/Ti electrodes were stable over 24h of sustained hydrogen production in 0.50M H2SO4. The activity was essentially unchanged after 400 cyclic voltammetric sweeps, suggesting long-term viability under operating conditions. CoP is therefore amongst the most active, acid-stable, earth-abundant HER electrocatalysts reported to date.