  NiFe oxyhydroxide is one of the most promising oxygen evolution reaction
(OER) catalysts for renewable hydrogen production, and deciphering the identity
and reactivity of the oxygen intermediates on its surface is a key challenge
but is critical to understanding the OER mechanism as well as designing
water-splitting catalysts with higher efficiencies. Here, we screened and
utilized in situ reactive probes that can selectively target specific oxygen
intermediates with high rates to investigate the OER intermediates and pathway
on NiFe oxyhydroxide. Most importantly, the oxygen atom transfer (OAT) probes
(e.g. 4-(Diphenylphosphino) benzoic acid) could efficiently inhibit the OER
kinetics by scavenging the OER intermediates, exhibiting lower OER currents,
larger Tafel slopes and larger kinetic isotope effect values, while probes with
other reactivities demonstrated much smaller effects. Combining the OAT
reactivity with electrochemical kinetic and operando Raman spectroscopic
techniques, we identified a resting Fe=O intermediate in the Ni-O scaffold and
a rate-limiting O-O chemical coupling step between a Fe=O moiety and a vicinal
bridging O. DFT calculation further revealed a longer Fe=O bond formed on the
surface and a large kinetic energy barrier of the O-O chemical step,
corroborating the experimental results. These results point to a new direction
of liberating lattice O and expediting O-O coupling for optimizing NiFe-based
OER electrocatalyst.
