Photoelectrochemistry (PEC) holds potential as a direct route for solar energy storage. Its performance is governed by how efficiently photoexcited charges are separated and how fast the charges are transferred to the solution, both of which are highly sensitive to the photoelectrode surfaces near the electrolyte. While other aspects of a PEC system, such as the light-absorbing materials and the catalysts that facilitate charge transfer, have been extensively examined in the past, an underwhelming amount of attention has been paid to the energetics at the photoelectrode/electrolyte interface. The lack of understanding of this interface is an important reason why many photoelectrode materials fail to deliver the expected performance in harvesting solar energy in a PEC system. Using hematite (α-Fe2O3) as a material platform, we present in this Perspective how surface modifications can alter the energetics and the resulting consequences on the overall PEC performance. It has been shown that a detailed understanding of the photoelectrode/eletrolyte interfaces can contribute significantly to improving the performance of hematite, which enabled unassisted solar water splitting when combined with an amorphous Si photocathode.

Ammonia is traditionally an essential chemical for fertilizers and other nitrogencontaining products that have been supporting most of the world population for over a century. Recently, ammonia is receiving renascent attentions as a potential hydrogen storage medium and carbon-free fuel, due to its advantages of easy liquefication to achieve a higher volumetric energy density and more facile transportation as compared with other gas-based fuels. [1] Conventionally, the industry-scale production of ammonia, based on the Habor-Bosch method through a nitrogen reduction reaction (NRR), is high-cost, energy-intensive, and environmentally unfriendly, as it not only consumes a large amount of fossil energy but also is associated with the release of Electrochemical nitrogen reduction reaction (NRR) over nonprecious-metal and single-atom catalysts has received increasing attention as a sustainable strategy to synthesize ammonia. However, the atomic-scale regulation of such active sites for NRR catalysis remains challenging because of the large distance between them, which significantly weakens their cooperation. Herein, the utilization of regular surface cavities with unique microenvironment on graphitic carbon nitride as "subnano reactors" to precisely confine multiple Fe and Cu atoms for NRR electrocatalysis is reported. The synergy of Fe and Cu atoms in such confined subnano space provides significantly enhanced NRR performance, with nearly doubles ammonia yield and 54%-increased Faradic efficiency up to 34%, comparing with the single-metal counterparts. First principle simulation reveals this synergistic effect originates from the unique Fe-Cu coordination, which effectively modifies the N 2 absorption, improves electron transfer, and offers extra redox couples for NRR. This work thus provides new strategies of manipulating catalysts active centers at the sub-nanometer scale.

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