An Electrochemically Treated BiVO4 Photoanode for Efficient Photoelectrochemical Water Splitting

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.

Manipulation of Charge Transport by Metallic V13 O16 Decorated on Bismuth Vanadate Photoelectrochemical Catalyst

Conductive metal oxides represent a new category of functional material with vital importance for many modern applications. The present work introduces a new conductive metal oxide V₁₃ O₁₆ , which is synthesized via a simplified photoelectrochemical procedure and decorated onto the semiconducting photocatalyst BiVO₄ in controlled mass percentages ranging from 25% to 37%. Owing to its excellent conductivity and good compatibility with oxide materials, the metallic V₁₃ O₁₆ -decorated BiVO₄ hybrid catalyst shows a high photocurrent density of 2.2 ± 0.2 mA cm^-2 at 1.23 V versus reversible hydrogen electrode (RHE). Both experimental characterization and density functional theory calculations indicate that the superior photocurrent derives from enhanced charge separation and transfer, resulting from ohmic contact at the interface of mixed phases and superior electrical conductivity from V₁₃ O₁₆ . A Co-Pi coating on BiVO₄ -V₁₃ O₁₆ further increases the photocurrent to 5.0 ± 0.5 mA cm^-2 at 1.23 V versus RHE, which is among the highest reported for BiVO₄ -based photoelectrodes. Surface photovoltage and transient photocurrent measurements suggest a charge-transfer model in which photocurrents are enhanced by improved surface passivation, although the barrier at the Co-Pi/electrolyte interface limits the charge transfer.

Giant Defect-Induced Effects on Nanoscale Charge Separation in Semiconductor Photocatalysts

Defects can markedly impact the performance of semiconductor-based photocatalysts, where the spatial separation of photogenerated charges is required for converting solar energy into fuels. However, understanding exactly how defects affect photogenerated charge separation at nanometer scale remains quite challenging. Here, using time- and space-resolved surface photovoltage approaches, we demonstrate that the distribution of surface photogenerated charges and the direction of photogenerated charge separation are determined by the defects distributed within a 100 nm surface region of a photocatalytic Cu₂O particle. This is enabled by the defect-induced charge separation process, arising from the trapping of electrons at the near-surface defect states and the accumulation of holes at the surface states. More importantly, the driving force for defect-induced charge separation is greater than 4.2 kV/cm and can be used to drive photocatalytic reactions. These findings highlight the importance of near-surface defect engineering in promoting photogenerated charge separation and manipulating surface photogenerated charges; further, they open up a powerful avenue for improving photocatalytic charge separation and solar energy conversion efficiency.

Acetylenic carbon-rich frameworks on copper foam as conjugated polymer photocathodes for efficient and stable water reduction

A poly(1,3,5-triethynylbenzene) (PTEB) nanofiber is synthesized on a copper foam surface and presents a 100 times increased record-high photocathodic current density for efficient water reduction.

Confined growth of Co–Pi co-catalyst by organic semiconductor polymer for boosting the photoelectrochemical performance of BiVO4

The significant recombination of carriers and low OER kinetics depress the solar to chemical energy conversion efficiency over BiVO₄.

Efficient Water Splitting Cascade Photoanodes with Ligand-Engineered MnO Cocatalysts

The band edge positions of semiconductors determine functionality in solar water splitting. While ligand exchange is known to enable modification of the band structure, its crucial role in water splitting efficiency is not yet fully understood. Here, ligand-engineered manganese oxide cocatalyst nanoparticles (MnO NPs) on bismuth vanadate (BiVO4) anodes are first demonstrated, and a remarkably enhanced photocurrent density of 6.25 mA cm-2 is achieved. It is close to 85% of the theoretical photocurrent density (≈7.5 mA cm-2) of BiVO4. Improved photoactivity is closely related to the substantial shifts in band edge energies that originate from both the induced dipole at the ligand/MnO interface and the intrinsic dipole of the ligand. Combined spectroscopic analysis and electrochemical study reveal the clear relationship between the surface modification and the band edge positions for water oxidation. The proposed concept has considerable potential to explore new, efficient solar water splitting systems.

Ultrathin Fe-NiO nanosheets as catalytic charge reservoirs for a planar Mo-doped BiVO4 photoanode

Charge accumulation at the interface reflects the charge separation and recombination kinetics, and will strongly contribute to the photoelectrochemical reactions.

The energy conversion efficiency of a photoelectrochemical system is intimately connected to a number of processes, including light absorption, charge excitation, separation and transfer processes. Of these processes, the charge transfer rate at the electrode|electrolyte interface is the slowest and, hence, the rate-limiting step causing charge accumulation. Such an understanding underpins efforts focused on applying highly active electrocatalysts, which may contribute to the overall performance by augmenting surface charge accumulation, prolonging charge lifetime or facilitating charge transfer. How the overall effect depends on these individual possible mechanisms has been difficult to study previously. Aiming at advancing knowledge about this important interface, we applied first-order serial reactions to elucidate the charge excitation, separation and recombination kinetics on the semiconductor|electrocatalyst interfaces in air. The study platform for the present work was prepared using a two-step Mo-doped BiVO₄ film modified with an ultrathin Fe-doped NiO nanosheet, which was derived from an Fe-doped α-Ni(OH)₂ nanosheet by a convenient precipitation and ion-exchange method. The simulation results of the transient surface photovoltage (TSPV) data showed that the surface charge accumulation was significantly enhanced, even at an extremely low coverage (0.12–120 ppm) using ultra-thin Fe-NiO nanosheets. Interestingly, no improvement in the charge separation rate constants or reduction of recombination rate constants was observed under our experimental conditions. Instead, the ultra-thin Fe-NiO nanosheets served as a charge storage layer to facilitate the catalytic process for enhanced performance.

A Cobalt-Based Metal-Organic Framework as Cocatalyst on BiVO4 Photoanode for Enhanced Photoelectrochemical Water Oxidation

A metal-organic framework (MOF)-modified bismuth vanadate (BiVO₄ ) photoanode is fabricated by an ultrathin sheet-induced growth strategy, where ultrathin cobalt oxide sheets act as a metal source for the in situ synthesis of Co-based MOF poly[Co₂ (benzimidazole)₄ ] (denoted [Co₂ (bim)₄ ]) nanoparticles on the surface of BiVO₄ . [Co₂ (bim)₄ ] with small particle size and high dispersion can serve as a promising cocatalyst to accept holes transferred from BiVO₄ and boost surface reaction kinetics for photoelectrochemical (PEC) water oxidation. The photocurrent density of a [Co₂ (bim)₄ ]-modified BiVO₄ photoanode can achieve 3.1 mA cm^-2 under AM 1.5G illumination at 1.23 V versus the reversible hydrogen electrode (RHE), which is better than those of pristine and cobalt-based inorganic materials-modified BiVO₄ photoanodes. [Co₂ (bim)₄ ], with porosity and abundant metal sites, exhibits a high surface charge-separation efficiency (83 % at 1.2 V versus RHE), leading to the enhanced PEC activity. This work will bring new insight into the development of MOF materials as competent cocatalysts for PEC water splitting applications.

Photoelectrochemical Device Designs toward Practical Solar Water Splitting: A Review on the Recent Progress of BiVO4 and BiFeO3 Photoanodes

Solar-driven water splitting technology is considered to be a promising solution for the global energy challenge as it is capable of generating clean chemical fuel from solar energy. Various strategies and catalytic materials have been explored in order to improve the efficiency of the water splitting reaction. Although significant progress has been made, there are many intriguing fundamental phenomena that need to be understood. Herein, we review recent experimental efforts to demonstrate enhancement strategies for efficient solar water splitting, especially for the light absorption, charge carrier separation, and water oxidation kinetics. We also focus on the state of the art of photoelectrochemical (PEC) device designs such as application of facet engineering and the development of a ferroelectric-coupled PEC device. Based on these experimental achievements, future challenges, and directions in solar water splitting technology will be discussed.

In-situ electrochemical activation designed hybrid electrocatalysts for water electrolysis

Developing transition metal-based electrocatalysts with rich active sites for water electrolysis plays important roles in renewable energy fields. So far, some strategies including designing nanostructures, incorporating conductive support or foreign elements have been adopted to develop efficient electrocatalysts. Herein, we summarize recent progresses and propose in-situ electrochemical activation as a new pretreating technique for enhanced catalytic performances. The activation techniques mainly comprise facile electrochemical processes such as anodic oxidation, cathodic reduction, etching, lithium-assisted tuning and counter electrode electro-dissolution. During these electrochemical treatments, the catalyst surfaces are modified from bulk phase, which can tune local electronic structures, create more active species, enlarge surface area and thus improve the catalytic performances. Meanwhile, this technique can couple the atomic, electronic structures with electrocatalysis mechanisms for water splitting. Compared to traditional chemical treatment, the in-situ electrochemical activation techniques have superior advantages such as facile operation, mild environment, variable control, high efficiency and flexibility. This review may provide guidance for improving water electrolysis efficiencies and hold promising for application in many other energy-conversion fields such as supercapacitors, fuel cells and batteries.

NiO Nanoparticles Anchored on Phosphorus-Doped α-Fe2 O3 Nanoarrays: An Efficient Hole Extraction p-n Heterojunction Photoanode for Water Oxidation

The photoelectrochemical (PEC) water-splitting efficiency of a hematite-based photoanode is still far from the theoretical value due to its poor surface reaction kinetics and high density of surface trapping states. To solve these drawbacks, a photoanode consisting of NiO nanoparticles anchored on a gradient phosphorus-doped α-Fe₂ O₃ nanorod (NR) array (NiO/P-α-Fe₂ O₃ ) was fabricated to achieve optimal light absorption and charge separation, as well as rapid surface reaction kinetics. Specifically, a photoanode with the NR array structure allowed a high mass-transport rate to be achieved, while phosphorus doping effectively decreased the number of surface trapping sites and improved the electrical conductivity of α-Fe₂ O₃ . Furthermore, the p-n junction that forms between NiO and P-α-Fe₂ O₃ can further improve the PEC performance due to efficient hole extraction and the water oxidization catalytic activity of NiO. Consequently, the NiO/P-α-Fe₂ O₃ NR photoanode produced a high photocurrent density of 2.08 mA cm^-2 at 1.23 V versus a reversible hydrogen electrode and a 110 mV cathodic shift of the onset potential. This rational design of structure offers a new perspective in exploring high-performance PEC 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.

Rapid Formation of a Disordered Layer on Monoclinic BiVO4 : Co-Catalyst-Free Photoelectrochemical Solar Water Splitting

A surface disordered layer is a plausible approach to improve the photoelectrochemical performance of TiO₂ . However, the formation of a crystalline disordered layer in BiVO₄ and its effectiveness towards photoelectrochemical water splitting has remained a big challenge. Here, we report a rapid solution process (within 5 s) that is able to form a disordered layer of a few nanometers thick on the surface of BiVO₄ nanoparticles using a specific solution with a controllable reducing power. The disordered layer on BiVO₄ alleviates charge recombination at the electrode-electrolyte interface and reduces the onset potential greatly, which in turn results in a photocurrent density of approximately 2.3 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode (RHE). This value is 2.1 times higher than that of bare BiVO₄ . The enhanced photoactivity is attributed to the increased charge separation and transfer efficiencies, which resolve the intrinsic drawbacks of bare BiVO₄ such as the short hole diffusion length of around 100 nm and poor surface oxygen evolution reactivity.

Mimicking the Key Functions of Photosystem II in Artificial Photosynthesis for Photoelectrocatalytic Water Splitting

It has been anticipated that learning from nature photosynthesis is a rational and effective way to develop artificial photosynthesis system, but it is still a great challenge. Here, we assembled a photoelectrocatalytic system by mimicking the functions of photosystem II (PSII) with BiVO₄ semiconductor as a light harvester protected by a layered double hydroxide (NiFeLDH) as a hole storage layer, a partially oxidized graphene (pGO) as biomimetic tyrosine for charge transfer, and molecular Co cubane as oxygen evolution complex. The integrated system exhibited an unprecedentedly low onset potential (0.17 V) and a high photocurrent (4.45 mA cm^-2), with a 2.0% solar to hydrogen efficiency. Spectroscopic studies revealed that this photoelectrocatalytic system exhibited superiority in charge separation and transfer by benefiting from mimicking the key functions of PSII. The success of the biomimetic strategy opened up new ways for the rational design and assembly of artificial photosynthesis systems for efficient solar-to-fuel conversion.

Highly Efficient Photoelectrochemical Water Splitting from Hierarchical WO3/BiVO4 Nanoporous Sphere Arrays

Nanoarchitecture of bismuth vanadate (BiVO₄) photoanodes for effectively increasing light harvesting efficiency and simultaneously achieving high charge separation efficiency is the key to approaching their theoretic performance of solar-driven water splitting. Here, we developed hierarchical BiVO₄ nanoporous sphere arrays, which are composed of small nanoparticles and sufficient voids for offering high capability of charge separation. Significantly, multiple light scattering in the sphere arrays and voids along with the large effective thickness of the BiVO₄ photoanode induce efficient light harvesting. In addition, attributed to ultrathin two-dimensional Bi₂WO₆ nanosheets as the precursor, the synergy of various enhancement strategies including WO₃/BiVO₄ nanojunction formation, W-doping, and oxygen vacancy creation can be directly incorporated into such a unique hierarchical architecture during the one-step synthesis of BiVO₄ without complex pre- or post-treatment. The as-obtained photoanode exhibits a water splitting photocurrent of 5.5 mA cm^-2 at 1.23 V versus RHE under 1-sun illumination, among the best values reported up-to-date in the field.