Stability of Photocathodes: A Review on Principles, Design, and Strategies

Nanoengineered Kesterite Photocathodes: Enhancing Photoelectrochemical Performance for Water Splitting and Beyond

Harnessing solar energy for the production of storable and transportable chemicals via photoelectrochemical (PEC) reactions offers a promising solution to overcome the intermittence of solar irradiation. Kesterites have been known as cost-efficient, environmentally friendly, and efficient semiconductor photoelectrode materials for PEC solar fuel production. While significant progress has been made in water splitting, there is increasing attention paid to extending applications to CO2 reduction, ammonia synthesis, and more. However, when efficient kesterite-based photoelectrodes are designed for water splitting and beyond, it is crucial to comprehensively consider both photoelectrode activity and reaction selectivity. This review elaborates on strategies for rationally designing kesterite-based photoelectrodes by optimizing photoactivity in terms of photogenerated charge migration and regulating the surface catalytic sites through nanoscale engineering. More importantly, it discusses optical management and system integration to advance PEC device design for future scalable applications. The perspectives and challenges are also proposed for future solar fuel applications.

The Influence of Laser Cleaning Treatment on the Quantum Efficiency of the Most Used Metallic Photocathodes: An Overview

This paper presents a comprehensive investigation into the quantum efficiency (QE) of metallic photocathodes used in modern high-performance radio frequency (RF) and superconducting radio frequency (SRF) guns. The study specifically examines how laser cleaning treatment impacts the QE of these photocathodes, providing detailed insights into their performance and potential improvements for accelerator applications, and assesses the chemical and environmental factors affecting the surface composition of metallic laser-photocathodes used in modern high-performance radio frequency (RF) and superconducting radio frequency (SRF) electron guns. This paper overviews the photocathode rejuvenation effects of laser cleaning treatment. Laser cleaning removes the oxides and hydrides responsible for the deterioration of photocathodes, increases the photoelectron emission quantum efficiency (QE) and extends the operational lifetime of high-brightness electron injectors. QE enhancement is analyzed with the aim of parametric cleaning process optimization. This study excludes semiconductor and thermionic cathodes, focusing solely on the widely used bulk and thin-film photocathodes of Cu, Mg, Y, Pb and Nb. Laser cleaning enhancement of QE in Cu from 5 × 10-5 to 1.2 × 10-4, in Mg from 5.0 × 10-4 to 1.8 × 10-3, in Y from 10-5 to 3.3 × 10-4, in Pb from 3 × 10-5 to 8 × 10-5, and in Nb from 2.1 × 10-7 to 2.5 × 10-5 is demonstrated. The analysis concludes with a specialized practical guide for improving photocathode efficacy and lifetime in RF and SRF guns.

Reductive Photocatalytic Proton-Coupled Electron Transfer by a Zirconium-Based Molecular Platform

Reductive proton-coupled electron transfer (PCET) has important energetic implications in numerous synthetic and natural redox processes. The development of catalytic systems that can mediate such transformations has become an attractive target, especially when light is used to generate the reactive species towards solar-to-chemicals conversion. However, such approach becomes challenged by kinetic competition with H2 evolution. Here we describe the excited state reactivity of a molecular Zr-based platform under visible light irradiation for the efficient reduction of multiple bonds. Mechanistic investigations shine light on a charge separation process that colocalizes an excited electron and an acidic proton to promote selective PCET. We further leveraged this reactivity for the photocatalytic reduction of a variety of organic substrates. Our results demonstrate the promise of this molecular platform to design strong photocatalytic PCET mediators for reductive transformations. More broadly, we also show the potential relevance of PCET mechanisms in the (photo)redox chemistry of Zr-based molecular materials.

Advances of nanostructured metal oxide as photoanode in photoelectrochemical (PEC) water splitting application

Water splitting via photoelectrochemical (PEC) cells offers a promising route to generate hydrogen fuel using solar energy. Nanostructured metal oxides have emerged as leading candidates as photoelectrodes in photocatalytic H2 production due to their photo-electrochemical stability, large surface area, earth abundance, and suitable band gap energies. This review reports the recent advancements of nanostructured metal oxide as photoanodes in photoelectrochemical (PEC) water-splitting applications. This review focuses on recent advancements in metal oxide photoanodes, their synthesis methods, modification strategies, and performance in PEC water splitting. Critical materials such as TiO2, Fe2O3, WO3, and BiVO4 are discussed in detail, highlighting their strengths, limitations, and future research directions to enhance efficiency and stability. This review will give clear insight into the trends and the critical factors for efficient metal oxide photoelectrode to improve the photocatalytic effectiveness in generating hydrogen fuel as an alternative energy source in the future. Finally, this study emphasises the potential of incorporating machine learning methods into experimental workflows to accelerate the optimisation of electrocatalysis performance, representing a significant advancement in developing efficient and sustainable hydrogen production technologies.

A Zero-Gap Gas Phase Photoelectrolyzer for CO2 Reduction with Porous Carbon Supported Photocathodes

A modified Metal-Organic Framework UiO-66-NH2-based photocathode in a zero-gap gas phase photoelectrolyzer was applied for CO2 reduction. Four types of porous carbon fiber layers with different wettability were employed to tailor the local environment of the cathodic surface reactions, optimizing activity and selectivity towards formate, methanol, and ethanol. Results are explained by mass transport through the different type and arrangement of carbon fiber support layers in the photocathodes and the resulting local environment at the UiO-66-NH2 catalyst. The highest energy-to-fuel conversion efficiency of 1.06 % towards hydrocarbons was achieved with the most hydrophobic carbon fiber (H23C2). The results are a step further in understanding how the design and composition of the photoelectrodes in photoelectrochemical electrolyzers can impact the CO2 reduction efficiency and selectivity.

Influence of solution pH dependency on structure, optical with photoelectrochemical characteristics of SILAR deposited copper oxide thin films

Photoelectrochemical (PEC) technology is a promising approach for converting solar energy into chemical energy, offering significant potential for renewable energy applications. In this work, the CuO thin film was fabricated with different pH value in between 8.5 ± 0.1 and 10.5 ± 0.1 via Successive Ionic Layer Adsorption and Reaction (SILAR) method. The Effect of pH on thickness, structural, morphological, elemental composition and optical properties were investigated by using stylus profilometry, XRD, SEM, TEM, EDX, UV-vis and PL. The XRD results showed that as the pH increased, the crystallite size increased from 19.24 nm to 25.62 nm, with a monoclinic phase along the (111) direction. The CuO film deposited at pH value 10.5 ± 0.1 exhibit well defined identical particle with its size in the range between 200 and 300 nm was confirmed by SEM and TEM analysis. As the pH increased from 8.5 ± 0.1 to 10.5 ± 0.1, the CuO film bandgap (Eg) value reduced from 1.52 eV to 1.42 eV with indirect transition. The CuO photocathode deposited at pH 10.5 ± 0.1 shows maximum photocurrent density of 1.45 mA/cm2 at -0.1 V vs. RHE in 0.5 M Na2SO4 solution. Furthermore, the Electrochemical Impedance Spectroscopy (EIS) analysis shows, the CuO (pH 10.5 ± 0.1) electrode have higher conductivity value of 0.6862 S/cm compared CuO at pH 8.5 ± 0.1 (0.2779 S/cm) and CuO at pH 9.5 ± 0.1 (0.4646 S/cm) electrodes.

Solar-Driven Sustainability: III-V Semiconductor for Green Energy Production Technologies

Long-term societal prosperity depends on addressing the world's energy and environmental problems, and photocatalysis has emerged as a viable remedy. Improving the efficiency of photocatalytic processes is fundamentally achieved by optimizing the effective utilization of solar energy and enhancing the efficient separation of photogenerated charges. It has been demonstrated that the fabrication of III-V semiconductor-based photocatalysts is effective in increasing solar light absorption, long-term stability, large-scale production and promoting charge transfer. This focused review explores on the current developments in III-V semiconductor materials for solar-powered photocatalytic systems. The review explores on various subjects, including the advancement of III-V semiconductors, photocatalytic mechanisms, and their uses in H2 conversion, CO2 reduction, environmental remediation, and photocatalytic oxidation and reduction reactions. In order to design heterostructures, the review delves into basic concepts including solar light absorption and effective charge separation. It also highlights significant advancements in green energy systems for water splitting, emphasizing the significance of establishing eco-friendly systems for CO2 reduction and hydrogen production. The main purpose is to produce hydrogen through sustainable and ecologically friendly energy conversion. The review intends to foster the development of greener and more sustainable energy source by encouraging researchers and developers to focus on practical applications and advancements in solar-powered photocatalysis.

Stability and degradation of (oxy)nitride photocatalysts for solar water splitting

Advancing towards alternative technologies for the sustainable production of hydrogen is a necessity for the successful integration of this potentially green fuel in the future. Photocatalytic and photoelectrochemical water splitting are promising concepts in this context. Over the past decades, researchers have successfully explored several materials classes, such as oxides, nitrides, and oxynitrides, in their quest for suitable photocatalysts with a focus on reaching higher efficiencies. However, to pave the way towards practicability, understanding degradation processes and reaching stability is essential, a domain where research has been scarcer. This perspective aims at providing an overview on recent progress concerning stability and degradation with a focus on (oxy)nitride photocatalysts and at providing insights into the opportunities and challenges coming along with the investigation of degradation processes and the attempts to improve the stability of photocatalysts.

3D Hierarchical Sunflower-Shaped MoS2/SnO2 Photocathodes for Photo-Rechargeable Zinc Ion Batteries

Photo-rechargeable zinc-ion batteries (PRZIBs) have attracted much attention in the field of energy storage due to their high safety and dexterity compared with currently integrated lithium-ion batteries and solar cells. However, challenges remain toward their practical applications, originating from the unsatisfactory structural design of photocathodes, which results in low photoelectric conversion efficiency (PCE). Herein, a flexible MoS2/SnO2-based photocathode is developed via constructing a sunflower-shaped light-trapping nanostructure with 3D hierarchical and self-supporting properties, enabled by the hierarchical embellishment of MoS2 nanosheets and SnO2 quantum dots on carbon cloth (MoS2/SnO2 QDs@CC). This structural design provides a favorable pathway for the effective separation of photogenerated electron-hole pairs and the efficient storage of Zn2+ on photocathodes. Consequently, the PRZIB assembled with MoS2/SnO2 QDs@CC delivers a desirable capacity of 366 mAh g-1 under a light intensity of 100 mW cm-2, and achieves an ultra-high PCE of 2.7% at a current density of 0.125 mA cm-2. In practice, an integrated battery system consisting of four series-connected quasi-solid-state PRZIBs is successfully applied as a wearable wristband of smartwatches, which opens a new door for the application of PRZIBs in next-generation flexible energy storage devices.

Optical and Electrical Modulation Strategies of Photoelectrodes for Photoelectrochemical Water Splitting

When constructing efficient, cost-effective, and stable photoelectrodes for photoelectrochemical (PEC) systems, the solar-driven photo-to-chemical conversion efficiency of semiconductors is limited by several factors, including the surface catalytic activity, light absorption range, carrier separation, and transfer efficiency. Accordingly, various modulation strategies, such as modifying the light propagation behavior and regulating the absorption range of incident light based on optics and constructing and regulating the built-in electric field of semiconductors based on carrier behaviors in semiconductors, are implemented to improve the PEC performance. Herein, the mechanism and research advancements of optical and electrical modulation strategies for photoelectrodes are reviewed. First, parameters and methods for characterizing the performance and mechanism of photoelectrodes are introduced to reveal the principle and significance of modulation strategies. Then, plasmon and photonic crystal structures and mechanisms are summarized from the perspective of controlling the propagation behavior of incident light. Subsequently, the design of an electrical polarization material, polar surface, and heterojunction structure is elaborated to construct an internal electric field, which serves as the driving force to facilitate the separation and transfer of photogenerated electron-hole pairs. Finally, the challenges and opportunities for developing optical and electrical modulation strategies for photoelectrodes are discussed.

High-Photovoltage Silicon Nanowire for Biological Cofactor Production

Photocathodic conversion of NAD+ to NADH cofactor is a promising platform for activating redox biological catalysts and enzymatic synthesis using renewable solar energy. However, many photocathodes suffer from low photovoltage, consequently requiring a high cathodic bias for NADH production. Here, we report an n+p-type silicon nanowire (n+p-SiNW) photocathode having a photovoltage of 435 mV to drive energy-efficient NADH production. The enhanced band bending at the n+/p interface accounts for the high photovoltage, which conduces to a benchmark onset potential [0.393 V vs the reversible hydrogen electrode (VRHE)] for SiNW-based photocathodic NADH generation. In addition, the n+p-SiNW nanomaterial exhibits a Faradaic efficiency of 84.7% and a conversion rate of 1.63 μmol h-1 cm-1 at 0.2 VRHE, which is the lowest cathodic potential to achieve the maximum productivity among SiNW-sensitized cofactor production.