Advancements made in the development of ternary oxide-based photoanodes for use in water splitting photoelectrochemical cells (PECs) are reviewed.

In the pursuit of high-capacity electrochemical energy storage, a promising domain of research involves conversion reaction schemes, wherein electrode materials are fully transformed during charge and discharge. There are, however, numerous difficulties in realizing theoretical capacity and high rate capability in many conversion schemes. Here we employ operando studies to understand the conversion material FeS 2 , focusing on the local structure evolution of this relatively reversible material. X-ray absorption spectroscopy, pair distribution function analysis, and first-principles calculations of intermediate structures shed light on the mechanism of charge storage in the Li−FeS 2 system, with some general principles emerging for charge storage in chalcogenide materials. Focusing on second and later charge/discharge cycles, we find small, disordered domains that locally resemble Fe and Li 2 S at the end of the first discharge. Upon charge, this is converted to a Li−Fe−S composition whose local structure reveals tetrahedrally coordinated Fe. With continued charge, this ternary composition displays insertion−extraction behavior at higher potentials and lower Li content. The finding of hybrid modes of charge storage, rather than simple conversion, points to the important role of intermediates that appear to store charge by mechanisms that more closely resemble intercalation.

Redox active electrode materials derived from organic precursors are of interest for use as alternative cath--odes in Li batteries due to the potential for their sustainable production from renewable resources. Here, a series of or--ganic networks that either contain triazine units, or are derived from triazine--containing precursors are evaluated as cathodes versus Li metal anodes as possible active materials in Li batteries. The role of the molecular structure on the electrochemical performance is studied by comparing several materials prepared across a range of conditions allowing control over functionality and long range order. Well--defined structures in which the triazine unit persists in the final material exhibit very low capacities at voltages relevant for cathode materials (<10 mA.h g -1 ). Relatively high, reversible capacity (around 150 mA.h g -1 ), is in fact displayed by amorphous materials with little evidence of triazine functionality. This result directly contradicts previous suggestions that the triazine unit is responsible for charge storage in this family of materials. While the gently sloping discharge and charge profiles suggest a capacitive--type mechanism - further con--firmed by the trend of increasing capacity with increasing surface area - electron paramagnetic resonance spectroscopy studies show that the materials exhibiting higher capacities also display larger signals, potentially implicating unpaired spins in a charge storage mechanism that could involve charge transfer.

A desalination battery is an attractive route for seawater desalination because it couples ion removal with energy storage. In this work, we paired Cu 3 [Fe(CN) 6 ] 2 •nH 2 O as the Na-storage electrode with Bi as the Cl-storage electrode to construct a novel desalination battery that enables membrane-free desalination. Most current desalination technologies, with the exception of thermal distillation, rely on the use of membranes. Eliminating the need for a membrane can significantly simplify the construction and maintenance of desalination systems. After carefully examining the sodiation/desodiation reactions and cycle performance of Cu 3 [Fe(CN) 6 ] 2 •nH 2 O in both acidic and neutral saline solutions (0.6 M NaCl), we combined Cu 3 [Fe(CN) 6 ] 2 •nH 2 O with Bi, which was previously identified as a promising Cl-storage electrode, to construct a Cu 3 [Fe(CN) 6 ] 2 •nH 2 O/Bi desalination battery. The Cu 3 [Fe(CN) 6 ] 2 •nH 2 O/Bi desalination battery generates an electrical energy output during desalination, which is equivalent to discharging, and requires an electrical energy input during salination, which is equivalent to charging. We investigated optimum pH conditions to perform salination to minimize the energy necessary for charging so that the desalination/salination cycle could be achieved with a minimum overall energy input. The results obtained in this study suggest that with further optimization the Cu 3 [Fe(CN) 6 ] 2 •nH 2 O/Bi desalination battery will offer new possibilities for practical seawater desalination.

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 Febased 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.