Failure mechanism analysis and emerging strategies for enhancing the photoelectrochemical stability of photoanodes.

The development of efficient and stable photoanode materials is essential for driving the possible practical application of photoelectrochemical water splitting. This article begins with a basic understanding of the fundamentals of photoelectrochemical devices and photoanodes. State-of-the-art strategies for designing photoanodes with long-term stability are highlighted, including insertion of hole transport layers, construction of protective/passivation layers, loading of co-catalysts, construction of heterojunctions, and modification of the electrolyte. Based on the insights gained from these effective strategies, we present an outlook for addressing key aspects of the challenges of stabilizing photoanodes development in the future work. Widespread adoption of stability assessment criteria will facilitate reliable comparisons of results from different laboratories. In addition, deactivation of photoanode is defined as a 50% reduction in productivity. An in-depth understanding of the deactivation mechanism is essential for the design and development of efficient and stable photoanodes. This work will provide insights into the stability assessment of photoanode and facilitate the production of practical solar fuels.

Directional regulation of reactive oxygen species in titanium dioxide boosting the photocatalytic degradation performance of azo dyes.

It has been widely accepted that the generation of reactive oxygen species such as superoxide radical, hydroxyl radical, and hydrogen peroxide during photocatalysis is responsible for the degradation of azo dyes. However, it is unclear which reactive oxygen species primarily contributes to the degradation efficiency of azo dyes. Here, we demonstrate that the directional regulation of reactive oxygen species in titanium dioxide (TiO2) to form superoxide radicals by ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) can significantly improve the degradation performance of methyl orange. The optimized addition of EDTA-2Na can completely degrade azo dyes such as methyl orange, acid orange and alkaline orange at a concentration of 10 mg/L in about 20 min, which is not only higher than that achieved by pristine TiO2 under Xe lamp light but also far superior to the reported degradation efficiency of modified TiO2. Even under natural sunlight, this strategy can also effectively decompose azo dyes, demonstrating the great potential for practical water treatment using low-cost TiO2 photocatalysts.

Induced dipole moments in amorphous ZnCdS catalysts facilitate photocatalytic H2 evolution.

Amorphous semiconductors without perfect crystalline lattice structures are usually considered to be unfavorable for photocatalysis due to the presence of enriched trap states and defects. Here we demonstrate that breaking long-range atomic order in an amorphous ZnCdS photocatalyst can induce dipole moments and generate strong electric fields within the particles which facilitates charge separation and transfer. Loading 1 wt.% of low-cost Co-MoSx cocatalysts to the ZnCdS material increases the H2 evolution rate to 70.13 mmol g-1 h-1, which is over 5 times higher than its crystalline counterpart and is stable over the long-term up to 160 h. A flexible 20 cm × 20 cm Co-MoSx/ZnCdS film is prepared by a facile blade-coating technique and can generate numerous observable H2 bubbles under natural sunlight, exhibiting potential for scale-up solar H2 production.

Imperfect makes perfect: defect engineering of photoelectrodes towards efficient photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting for hydrogen evolution has been considered as a promising technology to solve the energy and environmental issues. However, the solar-to-hydrogen (STH) conversion efficiencies of current PEC systems are far from meeting the commercial demand (10%) due to the lack of efficient photoelectrode materials. The recent rapid development of defect engineering of photoelectrodes has significantly improved the PEC performance, which is expected to break through the bottleneck of low STH efficiency. In this review, the category and the construction methods of different defects in photoelectrode materials are summarized. Based on the in-depth summary and analysis of existing reports, the PEC performance enhancement mechanism of defect engineering is critically discussed in terms of light absorption, carrier separation and transport, and surface redox reactions. Finally, the application prospects and challenges of defect engineering for PEC water splitting are presented, and the future research directions in this field are also proposed.

A design of MnO-CNT@C3N4 cathodes for high-performance aqueous zinc-ion batteries.

Manganese oxides have been regarded as one of the most promising candidates in rechargeable aqueous zinc ion batteries due to their high specific capacity, high operating voltage, low cost and no-toxicity. Nevertheless, the grievous dissolution of manganese and the sluggish Zn2+ ions diffusion kinetics deteriorate the long cycling stability and the rate performance. Herein, we propose a combination of hydrothermal and thermal treatment strategy to design a MnO-CNT@C3N4 composite cathode material where MnO cubes are coated by carbon nanotubes (CNTs) and C3N4. Owing to the enhanced conductivity by CNTs and the alleviation of the dissolution of Mn2+ from the active material by C3N4, the optimized MnO-CNT@C3N4 exhibits an excellent rate performance (101 mAh g-1 at a large current density of 3 A g-1) and a high capacity (209 mAh g-1 at a current density of 0.8 A g-1), which is much better than its MnO counterpart. The energy storge mechanism of MnO-CNT@C3N4 is confirmed to be the co-insertion of H+/Zn2+. The present work provides a viable strategy for the design of advanced cathodes for high-performance zinc ion batteries.

A BiVO4 Photoanode with a VOx Layer Bearing Oxygen Vacancies Offers Improved Charge Transfer and Oxygen Evolution Kinetics in Photoelectrochemical Water Splitting.

Sluggish oxygen evolution kinetics are one of the key limitations of bismuth vanadate (BiVO4 ) photoanodes for efficient photoelectrochemical (PEC) water splitting. To address this issue, we report a vanadium oxide (VOx ) with enriched oxygen vacancies conformally grown on BiVO4 photoanodes by a simple photo-assisted electrodeposition process. The optimized BiVO4 /VOx photoanode exhibits a photocurrent density of 6.29 mA cm-2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which is ca. 385 % as high as that of its pristine counterpart. A high charge-transfer efficiency of 96 % is achieved and stable PEC water splitting is realized, with a photocurrent retention rate of 88.3 % upon 40 h of testing. The excellent PEC performance is attributed to the presence of oxygen vacancies in VOx that forms undercoordinated sites, which strengthen the adsorption of water molecules onto the active sites and promote charge transfer during the oxygen evolution reaction. This work demonstrates the potential of vanadium-based catalysts for PEC water oxidation.

Boosting the photocatalytic hydrogen production performance of graphitic carbon nitride nanosheets by tailoring the cyano groups.

Graphitic carbon nitride (g-C3N4) is a promising visible light responsive photocatalyst for solar hydrogen production. However, pristine g-C3N4 suffers from severe charge recombination, resulting in a poor photocatalytic activity. Herein, a facile KOH-assisted sealed heating process is designed to tailor the electronic structure of g-C3N4, leading to a significantly enhanced and stable photocatalytic hydrogen production rate of 225.1 µmol h-1 using only 50 mg of the photocatalyst. An excellent apparent quantum efficiency of 16.82% is achieved at 420 nm. Systematic studies reveal that KOH-assisted sealed heating can generate more cyano groups onto the framework of g-C3N4, which can increase the charge carrier density and reduce the surface charge transfer resistance, promoting charge separation and transfer. The new findings demonstrated in this work provide a facile strategy for the design of low-cost and efficient photocatalyst for solar fuel production.

Vacancy defect engineering of BiVO4 photoanodes for photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting has been regarded as a promising technology for sustainable hydrogen production. The development of efficient photoelectrode materials is the key to improve the solar-to-hydrogen (STH) conversion efficiency towards practical application. Bismuth vanadate (BiVO4) is one of the most promising photoanode materials with the advantages of visible light absorption, good chemical stability, nontoxic feature, and low cost. However, the PEC performance of BiVO4 photoanodes is limited by the relatively short hole diffusion length and poor electron transport properties. The recent rapid development of vacancy defect engineering has significantly improved the PEC performance of BiVO4. In this review article, the fundamental properties of BiVO4 are presented, followed by an overview of the methods for creating different kinds of vacancy defects in BiVO4 photoanodes. Then, the roles of vacancy defects in tuning the electronic structure, promoting charge separation, and increasing surface photoreaction kinetics of BiVO4 photoanodes are critically discussed. Finally, the major challenges and some encouraging perspectives for future research on vacancy defect engineering of BiVO4 photoanodes are presented, providing guidelines for the design of efficient BiVO4 photoanodes for solar fuel production.

Bifunctional photoelectrochemical process for humic acid degradation and hydrogen production using multi-layered p-type Cu2O photoelectrodes with plasmonic Au@TiO2.

Bifunctional photoelectrochemical (PEC) process for simultaneous hydrogen production and mineralisation of humic acid in water using TiO2-1 wt% Au@TiO2/Al2O3/Cu2O multi-layered p-type photoelectrodes is demonstrated. The newly designed bifunctional PEC system leads to a high degradation efficiency of dissolved humic compounds, the target pollutant, by up to 87% during 2 h reaction time. Simultaneously, humic acid is also served as a sacrificial electron donor in the proposed system, contributing to a high photocurrent density of the multi-layered p-type Cu2O photoelectrodes up to -6.32 mA cm-2 at 0 V vs. Reversible Hydrogen Electrode (RHE) under the AM 1.5 simulated 1-Sun solar illumination. The Z-scheme feature of this bifunctional PEC devices exhibiting a short-circuit photocurrent density of -0.45 mA cm-2 and solar-to-hydrogen conversion (STH) of 0.5 % in the presence of humic acid sheds light on the new bias-free artificial photosynthesis PEC system.

In Situ Formation of Oxygen Vacancies Achieving Near-Complete Charge Separation in Planar BiVO4 Photoanodes.

Despite a suitable bandgap of bismuth vanadate (BiVO4 ) for visible light absorption, most of the photogenerated holes in BiVO4 photoanodes are vanished before reaching the surfaces for oxygen evolution reaction due to the poor charge separation efficiency in the bulk. Herein, a new sulfur oxidation strategy is developed to prepare planar BiVO4 photoanodes with in situ formed oxygen vacancies, which increases the majority charge carrier density and photovoltage, leading to a record charge separation efficiency of 98.2% among the reported BiVO4 photoanodes. Upon loading NiFeOx as an oxygen evolution cocatalyst, a stable photocurrent density of 5.54 mA cm-2 is achieved at 1.23 V versus the reversible hydrogen electrode (RHE) under AM 1.5 G illumination. Remarkably, a dual-photoanode configuration further enhances the photocurrent density up to 6.24 mA cm-2 , achieving an excellent applied bias photon-to-current efficiency of 2.76%. This work demonstrates a simple thermal treatment approach to generate oxygen vacancies for the design of efficient planar photoanodes for solar hydrogen production.

Molten-Salt-Mediated Synthesis of an Atomic Nickel Co-catalyst on TiO2 for Improved Photocatalytic H2 Evolution.

Atomic co-catalysts offer high potential to improve the photocatalytic performance, of which the preparation with earth-abundant elements is challenging. Here, a new molten salt method (MSM) is designed to prepare atomic Ni co-catalyst on widely studied TiO2 nanoparticles. The liquid environment and space confinement effect of the molten salt leads to atomic dispersion of Ni ions on TiO2 , while the strong polarizing force provided by the molten salt promotes formation of strong Ni-O bonds. Interestingly, Ni atoms are found to facilitate the formation of oxygen vacancies (OV) on TiO2 during the MSM process, which benefits the charge transfer and hydrogen evolution reaction. The synergy of atomic Ni co-catalyst and OV results in 4-time increase in H2 evolution rate compared to that of the Ni co-catalyst on TiO2 prepared by an impregnation method. This work provides a new strategy of controlling atomic co-catalyst together with defects for efficient photocatalytic water splitting.

Efficient sequential harvesting of solar light by heterogeneous hollow shells with hierarchical pores.

In nature, sequential harvesting of light widely exists in the old life entity, i.e. cyanobacteria, to maximize the light absorption and enhance the photosynthesis efficiency. Inspired by nature, we propose a brand new concept of temporally-spatially sequential harvesting of light in one single particle, which has purpose-designed heterogeneous hollow multi-shelled structures (HoMSs) with porous shells composed of nanoparticle subunits. Structurally, HoMSs consist of different band-gap materials outside-in, thus realizing the efficient harvesting of light with different wavelengths. Moreover, introducing oxygen vacancies into each nanoparticle subunit can also enhance the light absorption. With the benefit of sequential harvesting of light in HoMSs, the quantum efficiency at wavelength of 400 nm is enhanced by six times compared with the corresponding nanoparticles. Impressively, using these aforementioned materials as photocatalysts, highly efficient photocatalytic water splitting is realized, which cannot be achieved by using the nanoparticle counterparts. This new concept of temporally-spatially sequential harvesting of solar light paves the way for solving the ever-growing energy demand.

Crystal Facet Engineering of Photoelectrodes for Photoelectrochemical Water Splitting.

Photoelectrochemical (PEC) water splitting is a promising approach for solar-driven hydrogen production with zero emissions, and it has been intensively studied over the past decades. However, the solar-to-hydrogen (STH) efficiencies of the current PEC systems are still far from the 10% target needed for practical application. The development of efficient photoelectrodes in PEC systems holds the key to achieving high STH efficiencies. In recent years, crystal facet engineering has emerged as an important strategy in designing efficient photoelectrodes for PEC water splitting, which has yet to be comprehensively reviewed and is the main focus of this article. After the Introduction, the second section of this review concisely introduces the mechanisms of crystal facet engineering. The subsequent section provides a snapshot of the unique facet-dependent properties of some semiconductor crystals including surface electronic structures, redox reaction sites, surface built-in electric fields, molecular adsorption, photoreaction activity, photocorrosion resistance, and electrical conductivity. Then, the methods for fabricating photoelectrodes with faceted semiconductor crystals are reviewed, with a focus on the preparation processes. In addition, the notable advantages of the crystal facet engineering of photoelectrodes in terms of light harvesting, charge separation and transfer, and surface reactions are critically discussed. This is followed by a systematic overview of the modification strategies of faceted photoelectrodes to further enhance the PEC performance. The last section summarizes the major challenges and some invigorating perspectives for future research on crystal facet engineered photoelectrodes, which are believed to play a vital role in promoting the development of this important research field.

Polyethylenimine Expanded Graphite Oxide Enables High Sulfur Loading and Long-Term Stability of Lithium-Sulfur Batteries.

To realize practical lithium-sulfur batteries (LSBs) with long cycling life, designing cathode hosts with a high specific surface area (SSA) is recognized as an efficient way to trap the soluble polysulfides. However, it is also blamed for diminishing the volumetric energy density and being susceptible to side reactions. Herein, polyethylenimine intercalated graphite oxide (PEI-GO) with a low SSA of 4.6 m2 g-1 and enlarged interlayer spacing of 13 Å is proposed as a superior sulfur host, which enables homogeneous distribution of high sulfur content (73%) and facilitates Li+ transfer in thick sulfur electrode. LSBs with a moderate sulfur loading (3.4 mg S cm-2 ) achieve an initial capacity of 1157 and 668 mAh g-1 after 500 cycles at 0.5 C. Even when the sulfur loading is increased to 7.3 mg cm-2 , the electrode still delivers a high areal capacity of 4.7 mAh cm-2 (641 mAh g-1 ) after 200 cycles at 0.2 C. The excellent electrochemical properties of PEI-GO are mainly attributed to the homogeneous distribution of sulfur in PEI-GO and the strong chemical interactions between polysulfides and amine groups, which can mitigate the loss of active phases and contribute to the better cycling stability.

Understanding the Roles of Oxygen Vacancies in Hematite-Based Photoelectrochemical Processes.

Oxygen vacancy (VO ) engineering is an effective method to tune the photoelectrochemical (PEC) performance, but the influence of VO on photoelectrodes is not well understood. Using hematite as a prototype, we herein report that VO functions in a more complicated way in PEC process than previously reported. Through a comprehensive analysis of the key charge transfer and surface reaction steps in PEC processes on a hematite photoanode, we clarify that VO can facilitate surface electrocatalytic processes while leading to severe interfacial recombination at the semiconductor/electrolyte (S-E) interface, in addition to the well-reported improvements in bulk conductivity. The improved bulk conductivity and surface catalysis are beneficial for bulk charge transfer and surface charge consumption while interfacial charge transfer deteriorates because of recombination through VO -induced trap states at the S-E interface.

Hollow Nanostructures for Photocatalysis: Advantages and Challenges.

Photocatalysis for solar-driven reactions promises a bright future in addressing energy and environmental challenges. The performance of photocatalysis is highly dependent on the design of photocatalysts, which can be rationally tailored to achieve efficient light harvesting, promoted charge separation and transport, and accelerated surface reactions. Due to its unique feature, semiconductors with hollow structure offer many advantages in photocatalyst design including improved light scattering and harvesting, reduced distance for charge migration and directed charge separation, and abundant surface reactive sites of the shells. Herein, the relationship between hollow nanostructures and their photocatalytic performance are discussed. The advantages of hollow nanostructures are summarized as: 1) enhancement in the light harvesting through light scattering and slow photon effects; 2) suppression of charge recombination by reducing charge transfer distance and directing separation of charge carriers; and 3) acceleration of the surface reactions by increasing accessible surface areas for separating the redox reactions spatially. Toward the end of the review, some insights into the key challenges and perspectives of hollow structured photocatalysts are also discussed, with a good hope to shed light on further promoting the rapid progress of this dynamic research field.

An Integrated Strategy towards Enhanced Performance of the Lithium-Sulfur Battery and its Fading Mechanism.

To fulfil the potential of Li-S batteries (LSBs) with high energy density and low cost, multiple challenges need to be addressed simultaneously. Most research in LSBs has been focused on the sulfur cathode design, although the performance is also known to be sensitive to other parameters such as binder, current collector, separator, lithium anode, and electrolyte. Here, an integrated LSB system based on the understanding of the different roles of binder, current collector, and separator is developed. By using the cross-linked carboxymethyl cellulose-citric acid (CMC-CA) binder, Toray carbon paper current collector, and reduced graphene oxide (rGO) coated separator, LSBs achieve a high capacity of 960 mAh g-1 after 200 cycles (2.5 mg cm-2 ) and 930 mAh g-1 after 50 cycles (5 mg cm-2 ) at 0.1 C. Moreover, the failure mechanism at a high sulfur loading with characteristics of fast capacity decay and infinite charging is discussed. This work highlights the synergistic effect of different components and the challenges towards more reliable LSBs with high sulfur loading.

Energy loss analysis in photoelectrochemical water splitting: a case study of hematite photoanodes.

Insightful understanding of the energy loss mechanism during photoelectrochemical (PEC) processes is of vital importance for efficient solar fuel production. Potential-current features under light illumination are typically used to evaluate the effectiveness of the PEC processes. However, energy loss that leads to various shapes of measured photocurrent-potential (j-E) curves is still not well understood. Herein, based on hematite photoanodes, we systematically studied the photocurrent-potential-light intensity (j-E-I) relationships to acquire quantitative understanding of the loss mechanism during the PEC process by decoupling it into a photovoltaic (PV) and an electrocatalytic (EC) process. Both numerical simulation and experimental results have confirmed the reasonability of this analysis method. It provides comprehensive understanding of the energy loss at the semiconductor-electrolyte junction and the surface electrocatalysis process for further optimizing the PEC solar energy conversion process.

Mesoporous ZnFe2 O4 Photoanodes with Template-Tailored Mesopores and Temperature-Dependent Photocurrents.

Mesoporous ZnFe2 O4 photoanodes have been prepared via dip-coating utilizing the evaporation-induced self-assembly of two different block-copolymer templates to investigate the influence of pore geometry on the photoelectrochemcial performance of those earth-abundant photoelectrodes. The use of commercial block copolymers, triblock copolymer Pluronic® F127 and the diblock copolymer PIB3000 as templates, leads to different pore morphologies under identical preparation conditions due to different polymer stabilities. Interestingly, pore morphology in mesoporous ZnFe2 O4 turned out to be less important for the photoelectrochemical performance. Contrary, sufficiently developed crystalline domains gained through optimized temperature treatment resulted in maximum photoelectrochemical performance among the investigated samples. This disproves the necessity of expensive, tailor-made polymer soft templates to synthesize high-performing mesoporous ZnFe2 O4 photoanodes.

New BiVO4 Dual Photoanodes with Enriched Oxygen Vacancies for Efficient Solar-Driven Water Splitting.

Bismuth vanadate (BiVO4 ) is a promising photoanode material for photoelectrochemical (PEC) water splitting. However, owing to the short carrier diffusion length, the trade-off between sufficient light absorption and efficient charge separation often leads to poor PEC performance. Herein, a new electrodeposition process is developed to prepare bismuth oxide precursor films, which can be converted to transparent BiVO4 films with well-controlled oxygen vacancies via a mild thermal treatment process. The optimized BiVO4 film exhibits an excellent back illumination charge separation efficiency mainly due to the presence of enriched oxygen vacancies which act as shallow donors. By loading FeOOH/NiOOH as the cocatalysts, the BiVO4 dual photoanodes exhibit a remarkable and highly stable photocurrent density of 5.87 mA cm-2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination. An artificial leaf composed of the BiVO4 /FeOOH/NiOOH dual photoanodes and a single sealed perovskite solar cell delivers a solar-to-hydrogen conversion efficiency as high as 6.5% for unbiased water splitting.