Progress on ternary oxide-based photoanodes for use in photoelectrochemical cells for solar water splitting.

Solar water splitting using photoelectrochemical cells (PECs) has emerged as one of the most promising routes to produce hydrogen as a clean and renewable fuel source. Among various semiconductors that have been considered as photoelectrodes for use in PECs, oxide-based photoanodes are particularly attractive because of their stability in aqueous media in addition to inexpensive and facile processing compared to other types of semiconductors. However, they typically suffer from poor charge carrier separation and transport. In the past few years, there has been tremendous progress in developing ternary oxide-based photoelectrodes, specifically, photoanodes. The use of ternary oxides provides more opportunities to tune the composition and electronic structure of the photoelectrode compared to binary oxides, thus providing more freedom to tune the photoelectrochemical properties. In this article, we outline the important characteristics to analyze when evaluating photoanodes and review the major recent progress made on the development of ternary oxide-based photoanodes. For each system, we highlight the favorable and unfavorable features and summarize the strategies utilized to address the challenges associated with each material. Finally, by combining our analyses of all the photoanodes surveyed in this review, we provide possible future research directions for each compound and an outlook for constructing more efficient oxide-based PECs. Overall, this review will provide a critical overview of current ternary oxide-based photoanodes and will serve as a platform for the design of future oxide-based PECs.

Perspectives on the Development of Oxide-Based Photocathodes for Solar Fuel Production.

Photoelectrochemical cells (PECs), which use semiconductor electrodes (photoelectrodes) to absorb solar energy and perform chemical reactions, constitute one of the most attractive strategies to produce chemical fuels using renewable energy sources. Oxide-based photoelectrodes specifically have been intensively investigated for the construction of PECs due to their relatively inexpensive processing costs and better stability in aqueous media compared with other types of photoelectrodes. Although there have been many advancements in the development of oxide-based photoanodes, our understanding of oxide-based photocathodes remains limited. The goal of this Perspective is to examine the recent progress made in the field of oxide-based photocathodes and discuss future research directions. The photocathode systems considered here include binary and ternary Cu-based photocathodes and ternary Fe-based photocathodes. We assessed the characteristics and major advantages and drawbacks of each system and identified the most critical research gaps. The insights and discussions provided in this Perspective will serve as useful resources for the design of future studies, leading to the development of more efficient and practical PECs.

Elucidating Structure-Composition-Property Relationships of Ni-Based Prussian Blue Analogues for Electrochemical Seawater Desalination.

Nickel hexacyanoferrate (NiHCF), a type of Prussian blue analogue (PBA), has recently emerged as one of the most promising Na-storage electrodes for use in electrochemical desalination. Previous studies have revealed that NiHCF can be prepared with both cubic and rhombohedral symmetries depending on the oxidation state of Fe (FeII vs FeIII) and the related A-site occupancy. However, our understanding of the effects of the lattice-type of the as-prepared samples on their electrochemical performances, structural transitions that occur during sodiation/desodiation, cyclability, and rate capabilities is presently lacking. Additionally, the optimum structural and compositional features required to prepare high-performing NiHCF electrodes have not yet been clearly established. In this work, we report the synthesis of two sets of cubic and rhombohedral NiHCF samples with different particle sizes, crystallinities, and compositions. Using these samples, we systematically elucidated the structure-composition-property relationships of NiHCF to develop rational design principles to prepare high-performing PBAs. Our results show that high crystallinity, a low number of Fe(CN)6 vacancies, and a large unit cell size to allow for consistent structural changes during cycling are critical factors to produce NiHCF with a high capacity, good cycling stability, and good rate capabilities, and these factors are considerably affected by the synthesis conditions. One of the samples prepared in this study with optimum structural features demonstrates the best performance and stability among any PBA electrode tested in neutral saline solutions to date.

MnO Conversion in Li-Ion Batteries: In Situ Studies and the Role of Mesostructuring.

Complex manganese oxides have been extensively studied as intercalation Li-ion battery electrodes. The simple oxide MnO has been proposed as a conversion anode material with a theoretical capacity of 756 mAh g(-1) for full reduction to the metal. We report the reaction of MnO with Li using in situ X-ray diffraction and find no sign of crystalline products upon either discharge or charge. However, the absence of reflections, paired with electrochemical impedance spectroscopy, suggests disordered discharge products. We also examine composite electrodes with porous particles of MnO as the active component, with pores generated through the reductive heating of Mn3O4. We compare the behavior of these with more dense MnO powders, including studies of the electrode morphologies pre- and postcyling. We find differences in the first discharge relevant to the utility of such mesostructuring in conversion reaction materials. Specifically, we find this type of mesostructure, which gives advantage in intercalation and pseudocapacitive storage, does not yield the same benefits for conversion reaction systems.