Probing Terra Incognita of Ni–P Catalysts: Operando Explorations during Hydrogen Evolution Reaction

We have developed
two Ni phosphide preparation methods allowing operando XAS surface-sensitive studies of well-defined bulk
systems. For Ni K-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 operando Ni K-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 K-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 in situ active P sites are different
from the oxidized P states observed under ex situ conditions.