Electrocatalytic oxygen evolution reaction (OER) is a core reaction responsible for converting renewable electricity into storable fuels; yet, it is kinetically challenging, because of the complex proton-coupled multielectron transfer process. Transition-metal-based electrocatalysts, which provide the possibility for the realization of low-cost, high-activity, and stable OER in alkaline solution, therefore have attracted significant research interest in recent years. A fundamental understanding of composition–structure–activity relationships for these electrocatalysts is essential to guide the design of practical electrocatalysts for industrial applications. With more advanced ex situ and in situ techniques to determine the active sites, there has been increasing evidence revealing the critical role of Fe in the high performance of Fe-containing transition metal-based electrocatalysts. Here, we present a critical review of recent progress in Fe-containing electrocatalysts for OER, highlighting the significant role of Fe in enhancing the OER activity. We outline the historical development of the Fe-containing electrocatalysts, summarize the conflicting viewpoints on catalytic active sites, and offer guidelines for more rigorous identification. The synthesis techniques and the major challenges in improving the intrinsic catalytic activity and stability are discussed. Finally, a perspective regarding emerging issues yet to be explored for developing OER electrocatalysts for practical applications are also provided.

Graphene and two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant interest due to their unique properties that cannot be obtained in their bulk counterparts. These atomically thin 2D materials have demonstrated strong light-matter interactions, tunable optical bandgap structures and unique structural and electrical properties, rendering possible the high conversion efficiency of solar energy with a minimal amount of active absorber material. The isolated 2D monolayer can be stacked into arbitrary van der Waals (vdWs) heterostructures without the need to consider lattice matching. Several combinations of 2D/3D and 2D/2D materials have been assembled to create vdWs heterojunctions for photovoltaic (PV) and photoelectrochemical (PEC) energy conversion. However, the complex, less-constrained, and more environmentally vulnerable interface in a vdWs heterojunction is different from that of a conventional, epitaxially grown heterojunction, engendering new challenges for surface and interface engineering. In this review, the physics of band alignment, the chemistry of surface modification and the behavior of photoexcited charge transfer at the interface during PV and PEC processes will be discussed. We will present a survey of the recent progress and challenges of 2D/3D and 2D/2D vdWs heterojunctions, with emphasis on their applicability to PV and PEC devices. Finally, we will discuss emerging issues yet to be explored for 2D materials to achieve high solar energy conversion efficiency and possible strategies to improve their performance.

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Since the first report of all-inorganic perovskite solar cells (PSCs) in 2014, more than 200 research articles have been published on this topic, reporting the enhancement in the stabilized power conversion efficiency (PCE) up to 18.4%. Allinorganic PSCs have become one of the most astonishing research domains in the field of perovskite-based photovoltaics. In this Review, significant improvements in all-inorganic PSCs are analytically reviewed, with some insight into the kinetics of intrinsic phase, light, and thermal stability of all-inorganic perovskites. Theoretical calculations specify that there is still a large capacity for further enhancement of device parameters. The critical challenges and the possible elucidations concerning improving the performance of all-inorganic PSCs are also discussed. Our focus is on the assessment of all-inorganic perovskite materials' properties and the recapitulation of the latest approaches of improving the PCE of corresponding devices in order to introduce new horizons toward commercialization.

Photoelectrochemical (PEC) solar-fuel conversion is a promising approach to converting energy from sunlight into storable chemical fuels. The development of low cost, highly efficient, and stable semiconductor-based photoelectrodes is a...

Interface engineering is a proven strategy to improve the efficiency of thin film semiconductor based solar energy conversion devices. Ta3N5 thin film photoanode is a promising candidate for photoelectrochemical (PEC) water splitting. Yet, a concerted effort to engineer both the bottom and top interfaces of Ta3N5 thin film photoanode is still lacking. Here, we employ n-type In:GaN and p-type Mg:GaN to modify the bottom and top interfaces of Ta3N5 thin film photoanode, respectively. The obtained In:GaN/Ta3N5/Mg:GaN heterojunction photoanode shows enhanced bulk carrier separation capability and better injection efficiency at photoanode/electrolyte interface, which lead to a record-high applied bias photon-to-current efficiency of 3.46% for Ta3N5-based photoanode. Furthermore, the roles of the In:GaN and Mg:GaN layers are distinguished through mechanistic studies. While the In:GaN layer contributes mainly to the enhanced bulk charge separation efficiency, the Mg:GaN layer improves the surface charge inject efficiency. This work demonstrates the crucial role of proper interface engineering for thin film-based photoanode in achieving efficient PEC water splitting.

While self-healing is considered a promising strategy to achieve long-term stability for oxygen evolution reaction (OER) catalysts, this strategy remains a challenge for OER catalysts working in highly alkaline conditions. The self-healing of the OER-active nickel iron layered double hydroxides (NiFe-LDH) has not been successful due to irreversible leaching of Fe catalytic centers. Here, we investigate the introduction of cobalt (Co) into the NiFe-LDH as a promoter for in situ Fe redeposition. An active borate-intercalated NiCoFe-LDH catalyst is synthesized using electrodeposition and shows no degradation after OER tests at 10 mA cm−2 at pH 14 for 1000 h, demonstrating its self-healing ability under harsh OER conditions. Importantly, the presence of both ferrous ions and borate ions in the electrolyte is found to be crucial to the catalyst’s self-healing. Furthermore, the implementation of this catalyst in photoelectrochemical devices is demonstrated with an integrated silicon photoanode. The self-healing mechanism leads to a self-limiting catalyst thickness, which is ideal for integration with photoelectrodes since redeposition is not accompanied by increased parasitic light absorption.

Ta3N5 is a promising semiconductor for solar-driven photocatalytic or photoelectrochemical (PEC) water splitting. However, the lack of an in-depth understanding of its intrinsic defect properties limits further improvement of its performance. In this study, comprehensive spectroscopic characterizations are combined with theoretical calculations to investigate the defect properties of Ta3N5. The obtained electronic structure of Ta3N5 reveals that oxygen impurities are shallow donors, while nitrogen vacancies and reduced Ta centers (Ta3+) are deep traps. The Ta3+ defects are identified to be most detrimental to the water splitting performance because their energetic position lies below the water reduction potential. Based on these findings, a simple H2O2 pretreatment method is employed to improve the PEC performance of the Ta3N5 photoanode by reducing the concentration of Ta3+ defects, resulting in a high solar-to-hydrogen conversion efficiency of 2.25%. The fundamental knowledge about the defect properties of Ta3N5 could serve as a guideline for developing more efficient photoanodes and photocatalysts.