Probing Terra Incognita of Ni–P Catalysts: <i>Operando</i> Explorations during Hydrogen Evolution Reaction

We have developed two Ni phosphide preparation methods allowing <i>operando</i> XAS surface-sensitive studies of well-defined bulk systems. For Ni <i>K</i>-edge XAS, a Ni₂P phase-pure powder was sintered into a high-density pellet and polished for grazing incidence XAS. Ni sites were mildly affected by the acidic electrolyte prior to the HER, while the applied cathodic potential caused the reduction of Ni surface states beyond the states of as-prepared Ni₂P. The computed fully H-covered Ni₂P [0001] model describes the difference in the <i>operando</i> Ni <i>K</i>-edge GIXAS spectrum well. Upon turning the applied bias off, the Ni sites became immediately oxidized, forming NiO on the surface. Thus, the active phase during the HER is covalent Ni⁰ close to that in the intermetallic phosphides, and Ni²⁺ oxides formed after, and not during, the HER. For P <i>K</i>-edge XAS, Ni foam was phosphorized to form a thin Ni₃P layer while preserving its high surface area. Upon immersion in the acidic electrolyte, the P sites underwent removal of P⁵⁺ phosphates and formed new P coordination, possibly due to the adsorption of protons from the electrolyte. These new P surface states were not affected by turning the cathodic current on and off as soon as the sample was immersed in the acidic electrolyte. However, the removal of the sample from the electrochemical cell and drying in air resulted in substantial depletion and oxidation of surface P. Echoing the observed Ni site chemistry during HER, XAS and XPS suggest that the <i>in situ</i> active P sites are different from the oxidized P states observed under <i>ex situ</i> conditions.