Water splitting is the essential chemical reaction to enable the storage of intermittent energies such as solar and wind in the form of hydrogen fuel. The oxygen evolution reaction (OER) is often considered as the bottleneck in water splitting. Though metal oxides had been reported as OER electrocatalysts more than half a century ago, the recent interest in renewable energy storage has spurred a renaissance of the studies of transition metal oxides as Earth-abundant and nonprecious OER catalysts. This Perspective presents major progress in several key areas of the field such as theoretical understanding, activity trend, in situ and operando characterization, active site determination, and novel materials. A personal overview of the past achievements and future challenges is also provided.

The oxygen evolution reaction (OER) is the performance-limiting half reaction of water splitting, which can be used to produce hydrogen fuel using renewable energies. Whereas a number of transition metal oxides and oxyhydroxides have been developed as promising OER catalysts in alkaline medium, the mechanisms of OER on these catalysts are not well understood. Here we combine electrochemical and in situ spectroscopic methods, particularly operando X-ray absorption and Raman spectroscopy, to study the mechanism of OER on cobalt oxyhydroxide (CoOOH), an archetypical unary OER catalyst. We find the dominating resting state of the catalyst as a Co(IV) species CoO 2 . Through oxygen isotope exchange experiments, we discover a cobalt superoxide species as an active intermediate in the OER. This intermediate is formed concurrently to the oxidation of CoOOH to CoO 2 . Combing spectroscopic and electrokinetic data, we identify the rate-determining step of the OER as the release of dioxygen from the superoxide intermediate. The work provides important experimental fingerprints and new mechanistic perspectives for OER catalysts.

In recent years, electrochemical reduction of carbon dioxide (CO2) has received a great deal of attention due to the potential that this process can mitigate the atmospheric CO2 concentration and produce valuable organic compounds. In particular, Cu and Cu-based catalysts have exhibited the capability of converting CO2 into multicarbon fuels and chemicals in significant quantities. Here, we report a facile and cheap fabrication method for the development of an Ag-incorporated cuprous oxide (Ag-Cu2O) electrode enabling selective synthesis of ethanol via electrochemical CO2 reduction and reveal the key factor improving the ethanol (C2H5OH) selectivity. The incorporation of Ag into Cu2O leads to the suppression of hydrogen (H2) evolution, and furthermore, by varying the elemental arrangement (phase-separated and phase-blended) of Ag and Cu, we observe that C2H5OH selectivity can be controlled. Consequently, the Faradaic efficiency for C2H5OH on phase-blended Ag-Cu2O (Ag-Cu2OPB) is 3 times higher than that of the Cu2O without Ag dopant. We propose that the electrochemical reaction behavior is not solely associated with a role of Ag dopant, carbon monoxide (CO) leading to an ethanol formation pathway over ethylene, but also the doping pattern related population of Ag-Cu biphasic boundaries relatively suppresses the H2 evolution reaction and encourages the reaction of mobile CO generated on Ag to a residual intermediate on a Cu site.

Nickel iron oxyhydroxide is the benchmark catalyst for the oxygen evolution reaction (OER) in alkaline medium. Whereas the presence of Fe ions is essential to the high activity, the functions of Fe are currently under debate. Using oxygen isotope labeling and operando Raman spectroscopic experiments, we obtain turnover frequencies (TOFs) of both Ni and Fe sites for a series of Ni and NiFe layered double hydroxides (LDHs), which are structurally defined samples of the corresponding oxyhydroxides. The Fe sites have TOFs 20–200 times higher than the Ni sites such that at an Fe content of 4.7 % and above the Fe sites dominate the catalysis. Higher Fe contents lead to larger structural disorder of the NiOOH host. A volcano‐type correlation was found between the TOFs of Fe sites and the structural disorder of NiOOH. Our work elucidates the origin of the Fe‐dependent activity of NiFe LDH, and suggests structural ordering as a strategy to improve OER catalysts.

Electrocatalytic conversion of carbon dioxide (CO2) has recently received considerable attention as one of the most feasible CO2 utilization techniques. In particular, copper and copper-derived catalysts have exhibited the ability to produce a number of organic molecules from CO2. Herein, we report a chloride (Cl)-induced bi-phasic cuprous oxide (Cu2O) and metallic copper (Cu) electrode (Cu2OCl) as an efficient catalyst for the formation of high-carbon organic molecules by CO2 conversion, and identify the origin of electroselectivity toward the formation of high-carbon organic compounds. The Cu2OCl electrocatalyst results in the preferential formation of multi-carbon fuels, including n-propanol and n-butane C3-C4 compounds. We propose that the remarkable electrocatalytic conversion behavior is due to the favorable affinity between the reaction intermediates and the catalytic surface.