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.

We realize the modulation of nanoarray morphology from nanorod to nanosheet and improve the catalytic activity for water splitting by simply changing the hydrothermal temperature.

The paired electrolysis via anodic organic oxidation and cathodic hydrogen evolution has been an emerging hydrogen production technology with high efficiency and high values. Nickel oxyhydroxide is a special electrocatalyst that is capable of selectively oxidizing various alcohols, aldehydes, and amines to the corresponding carboxylates and nitriles at more favorable potentials compared to the oxygen evolution reaction. However, its detailed molecular-level mechanism is still in debate, especially for the potential-dependent behavior of some organic substrates. In this study, we revealed that the nickel oxyhydroxide can be dissected into two functional regions with a more facilely oxidized surface for facilitated substrate adsorption and a relatively inert bulk phase for accelerated electron transfer via probe-assisted kinetics and in situ surface-enhanced Raman spectroscopy. The prerequisite of the two regions conceals a universal potential-dependent oxidation behavior for almost all organic substrates. Further combining with the computational investigation unravels the origin of this potential dependence from the exothermic O/N-centered adsorption process on the surface and the rate-limiting proton-coupled electron-transfer (PCET) steps for the C−H bond breaking that demands bulk electron conductivity. This provides a rationale for designing more conductive underlayers to break the intrinsic limitations of nickel oxyhydroxide toward more efficient organic electrooxidation processes.

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