(Invited) Renewable Hydrogen Evolution on Nickel Phosphide Electrocatalysts: A Comparative Study of Efficiency and Tolerance to Corrosion

The intermittent nature of renewable energy sources like photovoltaics and wind mills requires storage of large quantities of energy. Renewable hydrogen produced by electrochemically water splitting into its elements pose an unlimited resource for energy storage. Gaseous hydrogen is not only a carbon-free energy carrier which could replace fossil fuels, but also an important bulk chemical of which ~10⁸ ton/year is produced from natural gas. Platinum and nickel (or alloys thereof) are the electrocatalysts used today for hydrogen evolution in acidic and alkaline solution, respectively.  However, the scarcity of platinum and the low efficiency of nickel, have greatly restricted the development of renewable hydrogen from water splitting. Since 2013 the number of reports on the use of transition metal phosphides as a replacement for Pt for hydrogen evolution have virtually exploded. Ni₃P and Ni₂P were the first phases shown to be highly electrochemically active for hydrogen evolution. Ni₂P was unfortunately was shown to deactivate in acid and alkali. We recently reported, that another crystalline phase of the nickel phosphide family, Ni₅P₄, outlperforms Ni₂P and Ni₃P and is comparable to platinum on a geometric basis in its electrical efficiency at hydrogen evolution (kinetics and intrinsic overpotential). Moreover, unlike Ni₂P this catalyst is corrosion resistant, being stable in both strong acid and alkali during electrocatalysis. Here, we will present our most recent work on multiple nickel phosphides as well a structural basis for the unusually high catalytic activity and corrosion resistance. The work was supported by AFOSR, the NSF and NATCO Pharma.