Electrochemical water oxidation is considered as one of the most important reactions for a sustainable future. However, in heterogeneous water oxidation, atomistic understanding of this fourelectron reaction is still elusive. In particular, the mechanism of O− O bond formation, the coupled transfer of proton and electron, and the hopping of high-valent metal-oxo intermediates are challenging issues. To date, binding energy related descriptors have been used successfully to predict the catalytic activities by matching the experimental data and quantitatively comparing the performances. In this Perspective, we attempt to emphasize the significance of entropic contribution, like enthalpy, by highlighting recent literature and calculating the entropy from the available data. We envision that this Perspective can suggest a new research direction toward the catalyst design to control the entropy during water oxidation and be technologically useful for characterizing the temperature effects.

Although NiFeO x H y has attracted enormous attention as an alkaline water electrolyzer owing to its high performance in the oxygen evolution reaction (OER), it still requires further improvement in terms of catalytic activity, durability, and an underlying understanding of the mechanism. Here we report the fabrication of Zn-doped NiFeO x H y (Zn;NiFeO x H y ) that has advanced electrocatalytic activity and stability in comparison to pure NiFeO x H y in the OER via simple chemical bath deposition and electrochemical activation. The improvement with Zn;NiFeO x H y is due to the Zn2+ dopant, a strong Lewis acid that modulates its electronic property for better intermediate binding for the electrochemical process, resulting in the stabilization of the OER-active phase. This work opens up an opportunity to elucidate the detailed mechanism of NiFeO x H y -based electrocatalysts and improves our understanding of redox tuning through inductive effects, thereby leading to advanced OER catalyst developments.

There is an urgent need to develop metal-free, low cost, durable, and highly efficient catalysts for industrially important oxygen evolution reactions. Inspired by natural geodes, unique melamine nanogeodes are successfully synthesized using hydrothermal process. Sulfur-modified graphitic carbon nitride (S-modified g-CN ) electrocatalysts are obtained by annealing these melamine nanogeodes in situ with sulfur. The sulfur modification in the g-CN structure leads to excellent oxygen evolution reaction activity by lowering the overpotential. Compared with the previously reported nonmetallic systems and well-established metallic catalysts, the S-modified g-CN nanostructures show superior performance, requiring a lower overpotential (290 mV) to achieve a current density of 10 mA cm and a Tafel slope of 120 mV dec with long-term durability of 91.2% retention for 18 h. These inexpensive, environmentally friendly, and easy-to-synthesize catalysts with extraordinary performance will have a high impact in the field of oxygen evolution reaction electrocatalysis.

The effects of exchange current density, Tafel slope, system resistance, electrode area, light intensity, and solar cell efficiency were systematically decoupled at the converter-assisted photovoltaic–water electrolysis system. This allows key determinants of overall efficiency to be identified. On the basis of this model, 26.5% single-junction GaAs solar cell was combined with a membrane-electrode-assembled electrolysis cell (EC) using the dc/dc converting technology. As a result, we have achieved a solar-to-hydrogen conversion efficiency of 20.6% on a prototype scale and demonstrated light intensity tracking optimization to maintain high efficiency. We believe that this study will provide design principles for combining solar cells, ECs, and new catalysts and can be generalized to other solar conversion chemical devices while minimizing their power loss during the conversion of electrical energy into fuel.

The reaction mechanism of electrochemical chloride oxidation at neutral pH is different from that at acidic pH, in which a commercial chlor-alkali process has been developed. Different proton concentrations and accelerated hydrolysis of the generated chlorine into hypochlorous acid at high pH can change the electrokinetics and stability of reaction intermediates. We have investigated a unique reaction mechanism of Co 3 O 4 nanoparticles for chloride oxidation at neutral pH. In contrast with water oxidation, the valency of cobalt was not changed during chloride oxidation. Interestingly, a new intermediate of Co−Cl was captured spectroscopically, distinct from the reaction intermediate at acidic pH. In addition, Co 3 O 4 nanoparticles exhibited high selectivity for active chlorine generation at neutral pH, comparable to commercially available RuO 2 -based catalysts. We believe that this study provides insight into designing efficient electrocatalysts for active chlorine generation at neutral pH, which can be practically applied to electrochemical water treatment coupled to hydrogen production.