Smoothing Surface Trapping States in 3D Coral-Like CoOOH-Wrapped-BiVO4 for Efficient Photoelectrochemical Water Oxidation

Selective Photoelectrochemical Oxidation of Glycerol to Glyceric Acid on (002) Facets Exposed WO3 Nanosheets

Glycerol is a byproduct of biodiesel production. Selective photoelectrochemical oxidation of glycerol to high value-added chemicals offers an economical and sustainable approach to transform renewable feedstock as well as store green energy at the same time. In this work, we synthesized monoclinic WO3 nanosheets with exposed (002) facets, which could selectively oxidize glycerol to glyceric acid (GLYA) with a photocurrent density of 1.7 mA cm-2 , a 73 % GLYA selectivity and a 39 % GLYA Faradaic efficiency at 0.9 V vs. reversible hydrogen electrode (RHE) under AM 1.5G illumination (100 mW cm-2 ). Compared to (200) facets exposed WO3 , a combination of experiments and theoretical calculations indicates that the superior performance of selective glycerol oxidation mainly originates from the better charge separation and prolonged carrier lifetime resulted from the plenty of surface trapping states, lower energy barrier of the glycerol-to-GLYA reaction pathway, more abundant active sites and stronger oxidative ability of photogenerated holes on the (002) facets exposed WO3 . Our findings show great potential to significantly contribute to the sustainable and environmentally friendly chemical processes via designing high performance photoelectrochemical cell via facet engineering for renewable feedstock transformation.

Highly Selective Ammonia Oxidation on BiVO4 Photoanodes Co-catalyzed by Trace Amounts of Copper Ions

High-efficient photoelectrocatalytic direct ammonia oxidation reaction (AOR) conducted on semiconductor photoanodes remains a substantial challenge. Herein, we develop a strategy of simply introducing ppm levels of Cu ions (0.5-10 mg/L) into NH3 solutions to significantly improve the AOR photocurrent of bare BiVO4 photoanodes from 3.4 to 6.3 mA cm-2 at 1.23 VRHE , being close to the theoretical maximum photocurrent of BiVO4 (7.5 mA cm-2 ). The surface charge-separation efficiency has reached 90 % under a low bias of 0.8 VRHE . This AOR exhibits a high Faradaic efficiency (FE) of 93.8 % with the water oxidation reaction (WOR) being greatly suppressed. N2 is the main AOR product with FEs of 71.1 % in aqueous solutions and FEs of 100 % in non-aqueous solutions. Through mechanistic studies, we find that the formation of Cu-NH3 complexes possesses preferential adsorption on BiVO4 surfaces and efficiently competes with WOR. Meanwhile, the cooperation of BiVO4 surface effect and Cu-induced coordination effect activates N-H bonds and accelerates the first rate-limiting proton-coupled electron transfer for AOR. This simple strategy is further extended to other photoanodes and electrocatalysts.

NiFe MOF modified BiVO4 photoanode with strong π-π conjugation enhances built-in electric field for boasting photoelectrochemical water oxidation

The photoelectrochemical (PEC) performance ofBiVO4 is limited by sluggish kinetics and poor stability. In this work, a novel high-performance BiVO4/NiFe MOF(BPDC) photoanode is constructed by loading NiFe MOF with biphenyl-4,4'-dicarboxylic acid (BPDC) as an organic ligand on BiVO4 by a simple one-step hydrothermal method. The XPS, OCP, UPS, and KPFM show that the enhanced π-π conjugation effect causes more electrons transfer from the BiVO4 to the MOFs and affects the magnitude of the work function, leading to a strong built-in electric field to drive carrier separation and migration. Therefore, the BiVO4/NiFe MOF(BPDC) has a strong hole extraction and carrier separation capability to enhance photoelectrochemical water oxidation and improve photostability. The BiVO4/NiFe MOF(BPDC) photoanode has an enhanced photocurrent density of 4.16 mA cm-2 at 1.23 VRHE, which is 4.33 times higher than that of the pure BiVO4 (0.96 mA cm-2) photoanode with a negative shift of 376 mV in the onset potential plot, exhibiting excellent photostability of 7 h at 1.23 VRHE. This work demonstrates that the composite photoanodes constructed by BiVO4 and the MOFs with strong π-π conjugation are promising, which provides an effective strategy for the preparation of efficient and stable photoanodes.

Cocatalysts-Photoanode Interface in Photoelectrochemical Water Splitting: Understanding and Insights

Sluggish oxygen evolution reactions on photoanode surfaces severely limit the application of photoelectrochemical (PEC) water splitting. The loading of cocatalysts on photoanodes has been recognized as the simplest and most efficient optimization scheme, which can reduce the surface barrier, provide more active sites, and accelerate the surface catalytic reaction kinetics. Nevertheless, the introduction of cocatalysts inevitably generates interfaces between photoanodes and oxygen evolution cocatalysts (Ph/OEC), which causes severe interfacial recombination and hinders the carrier transfer. Recently, many researchers have focused on cocatalyst engineering, while few have investigated the effect of the Ph/OEC interface. Hence, to maximize the advantages of cocatalysts, interfacial problems for designing efficient cocatalysts are systematically introduced. In this review, the interrelationship between the Ph/OEC and PEC performance is classified and some methods for characterizing Ph/OEC interfaces are investigated. Additionally, common interfacial optimization strategies are summarized. This review details cocatalyst-design-based interfacial problems, provides ideas for designing efficient cocatalysts, and offers references for solving interfacial problems.

Synergy Effect of the Enhanced Local Electric Field and Built-In Electric Field of CoS/Mo-Doped BiVO4 for Photoelectrochemical Water Oxidation

Bismuth vanadate is a promising material for photoelectrochemical water oxidation. However, it suffers from a low quantum efficiency, poor stability, and slow water oxidation kinetics. Here, we developed a novel photoanode of CoS/Mo-BiVO4 with excellent photoelectrochemical water oxidation performance. It achieved a photocurrent density of 4.5 mA cm-2 at 1.23 V versus the reversible hydrogen electrode, ∼4 times that of BiVO4. The CoS/Mo-BiVO4 photoanode also exhibited good stability, and the photocurrent density generated by the CoS/Mo-BiVO4 photoanode did not significantly decrease after light irradiation for 2 h. Upon replacement of part of the V with Mo doping in BiVO4, the local electric field around the Mo-O bond was enhanced, thus promoting carrier separation in BiVO4. The CoS was deposited on the surface of Mo-BiVO4, forming a built-in electric field at the interface. Under the action of the bias electric field and the built-in electric field, the carriers of CoS/Mo-BiVO4 were efficiently separated in the direction of the inverse type II heterojunction. In addition, CoS improved the light absorption and charge injection efficiency of the CoS/Mo-BiVO4 photoanode.

Engineering Unsymmetrically Coordinated Fe Sites via Heteroatom Pairs Synergetic Contribution for Efficient Oxygen Reduction

Single-atom Fe catalysts are considered as the promising catalysts for oxygen reduction reaction (ORR). However, the high electronegativity of the symmetrical coordination N atoms around Fe site generally results in too strong adsorption of *OOH intermediates on the active site, severely limiting the catalytic performance. Herein, a "heteroatom pair synergetic modulation" strategy is proposed to tailor the coordination environment and spin state of Fe sites, enabling breaking the shackles of unsuitable adsorption of intermediate products on the active centers toward a more efficient ORR pathway. The unsymmetrically Co and B heteroatomic coordinated Fe single sites supported on an N-doped carbon (Fe─B─Co/NC) catalyst perform excellent ORR activity with high half-wave potential (E1/2 ) of 0.891 V and a large kinetic current density (Jk ) of 60.6 mA cm-2 , which is several times better than those of commercial Pt/C catalysts. By virtue of in situ electrochemical impedance and synchrotron infrared spectroscopy, it is observed that the optimized Fe sites can effectively accelerate the evolution of O2 into the *O intermediate, overcoming the sluggish O─O bond cleavage of the *OOH intermediate, which is responsible for fast four-electron reaction kinetics.

Rapid Synthesis of Ultrathin Ni:FeOOH with In Situ-Induced Oxygen Vacancies for Enhanced Water Oxidation Activity and Stability of BiVO4 Photoanodes

The coupling of oxygen evolution reaction (OER) catalysts with photoanodes is a promising strategy for enhancing the photoelectrochemical (PEC) performance by passivating photoanode's surface defect states and facilitating charge transfer at the photoanode/electrolyte interface. However, a serious interface recombination issue caused by poor interface and OER catalysts coating quality often limits further performance improvement of photoanodes. Herein, a rapid Fenton-like reaction method is demonstrated to produce ultrathin amorphous Ni:FeOOH catalysts with in situ-induced oxygen vacancies (Vo) to improve the water oxidation activity and stability of BiVO₄ photoanodes. The combined physical characterizations, PEC studies, and density functional theory calculations revealed that the reductive environment in a Fenton-like reaction in situ produces abundant Vo in Ni:FeOOH catalysts, which significantly improves charge separation and charge transfer efficiency of BiVO₄ while also offering abundant active sites and a reduced energy barrier for OER. As a result, Ni:FeOOH-Vo catalysts yielded a more than 2-fold increased photocurrent density in the BiVO₄ photoanode (from 1.54 to 4.15 mA cm^-2 at 1.23 V_RHE), accompanied by high stability for 5 h. This work not only highlights the significance of abundant Vo in catalysts but also provides new insights into the rational design and fabrication of efficient and stable solar water-splitting systems.

A novel sustainable N recycling process: Upcycling ammonia to ammonium fertilizer from dilute wastewater and simultaneously realizing phenol degradation via a visible solar-driven PECMA system with efficient Ag2S-BiVO4 photoanodes

The selective recovery of NH₄⁺ as N fertilizers from dilution wastewater is a promising but challenging topic. Herein, a novel visible-light driven photo-electrochemical membrane stripping cell (designated "PECMA") with Ag₂S-BiVO₄ heterojunction photoanode was proposed to recover ammonium from dilute wastewater, which comprised an anode chamber for organics treatment, intermediate chamber for separating ammonium, cathode chamber for upcycling NH₄⁺ into NH₃, and recovery chamber for converting NH₃ into (NH₄)₂SO₄. The NH₄⁺ is concentrated by 21.5 times and recovered as (NH₄)₂SO₄ with a concentration of 7103 mg L^-1 after 10 cycles. At a current density of 3.86 A m^-2, PECMA system achieves excellent NH₄⁺ removal and recovery rates of 97.5 and 37.2 g N m^-2 d^-1 in 100 mg_N L^-1 wastewater. Moreover, PECMA degrades refractory organic pollutants through ClO· generated by Ag₂S-BiVO₄ photoanode, which effectively decompose phenol to CO₂ with a degradation rate of 93 %. Although tested as a proof-of-concept, the hybrid system opens up a novel field involving a sunlight-water-energy nexus, promising high efficiency NH₄⁺ recovery and wastewater remediation.

Manipulating the surface states of BiVO4 through electrochemical reduction for enhanced PEC water oxidation

Bismuth vanadate (BiVO₄) is a prospective candidate for photoelectrochemical (PEC) water oxidation, but its commercial application is limited due to the serious surface charge recombination. In this work, we propose a novel and effective electrochemical reduction strategy combined with co-catalyst modification to manipulate the surface states of the BiVO₄ photoanode. Specifically, an ultrathin amorphous structure is formed on the surface of BiVO₄ after electrochemical reduction ascribed to the breaking of the surface metal-O bonds. Photoelectrochemical measurements and first-principles calculation show that the electrochemical reduction treatment can effectively reduce the surface energy, thereby passivating the recombined surface states (r-ss) and increasing the mobility of photogenerated holes. In addition, the FeOOH co-catalyst layer further increases the intermediate surface states (i-ss) of BiVO₄, stabilizes the surface structure and enhances its PEC performance. Benefiting from the superior charge transfer efficiency and the excellent water oxidation kinetics, the -0.8/BVO/Fe photoanode achieves 2.02 mA cm^-2 photocurrent at 1.23 V_RHE (2.4 times that of the original BiVO₄); meanwhile, the onset potential shifts 90 mV to the cathode. These results provide a new surface engineering tactic to modify the surface states of semiconductor photoanodes for high-efficiency PEC water oxidation.

Hematite decorated with nanodot-like cobalt (oxy)hydroxides for boosted photoelectrochemical water oxidation

Photoelectrochemical (PEC) water splitting has been considered as an alternative process to produce green hydrogen. However, the energy conversion efficiency of PEC systems was still limited by the inefficient photoanode. Cocatalysts decoration is regarded as an efficient strategy for improving PEC performance of photoanode. In this work, nanodot-like cobalt (oxy)hydroxides was rationally decorated on hematite to fabricate CoOOH/Fe2O3 photoanode. The resulted CoOOH/Fe2O3 exhibits a high photocurrent density of 1.92 mA cm-2 at 1.23 V vs. RHE, which is 2.6 times than that of bare Fe2O3. In addition, the onset potential displays a cathodic shift of ca. 110 mV, indicating that CoOOH can efficiently accelerate water oxidation kinetics over Fe2O3. The comprehensive PEC and electrochemical characterizations reveal that CoOOH could not only provide abundant accessible Co active sites for water oxidation, but also could passivate the surface states of Fe2O3, thus increase the carrier density and decrease the interfacial resistance. As a result, the PEC water oxidation performance over Fe2O3 was significantly boosted. This work supports that the roles of CoOOH cocatalyst is generic and such CoOOH could be used for other semiconductor-based photoanodes for outstanding PEC water splitting performance.

Boosting the Performance of BiVO4 Photoanodes by the Simultaneous Introduction of Oxygen Vacancies and Cocatalyst via Photoelectrodeposition

Photoelectrochemical (PEC) water splitting is a promising way to convert solar energy into hydrogen energy, but the efficiency is limited by severe charge recombination especially in photoanodes. Herein, to reduce the charge recombination in the bulk phase and at the surface of the BiVO₄ photoanodes, oxygen vacancy introduction and cocatalyst loading were realized simultaneously by one-step photocathode deposition. A unique re-BiVO₄/FeOOH photoanode was obtained by the photocathodic reduction of BiVO₄ in an electrolyte containing Fe³⁺, where the oxygen vacancies were introduced during the reduction process and the deposition of the FeOOH cocatalyst on the surface was induced by the generated OH^-. When used for PEC water oxidation, the obtained re-BiVO₄/FeOOH photoanode achieved an excellent PEC performance with a photocurrent density of 5.35 mA/cm² at 1.23 V versus RHE under AM 1.5G illumination, which was considerably higher than those for the pristine BiVO₄ photoanode (2.88 mA/cm²) and the re-BiVO₄ photoanode obtained by photocathodic reduction without Fe³⁺ (4.32 mA/cm²). After further modification with a cobalt silicate (Co-Sil) cocatalyst, the resultant re-BiVO₄/FeOOH/Co-Sil photoanode exhibited a photocurrent density as high as 6.10 mA/cm² at 1.23 V versus RHE and a remarkable applied bias photon-to-current efficiency of 2.25%. The outstanding performance of the re-BiVO₄/FeOOH/Co-Sil photoanode could be attributed to the coexistence of plenty of oxygen vacancies in BiVO₄ reducing recombination of photogenerated carriers, the FeOOH cocatalyst interlayer as a hole-transport layer, and the outer Co-Sil cocatalyst with a high activity toward oxygen evolution. This work may open a new avenue toward multifunctional modifications of photoanode systems for efficient solar conversion.

Rational construction of S-doped FeOOH onto Fe2O3 nanorods for enhanced water oxidation

Hematite-based photoanode (α-Fe₂O₃) is considered the promising candidate for photoelectrochemical (PEC) water splitting due to its relatively small optical bandgap. However, severe charge recombination in the bulk and poor surface water oxidation kinetics have limited the PEC performance of Fe₂O₃ photoelectrodes, which is far below the theoretical value. Herein, a new catalyst, S-doped FeOOH (S-FeOOH), has been immobilized onto the surface of the Fe₂O₃ nanorod (NR) array by a facile chemical bath deposition incorporated thermal sulfuration process. The grown S-FeOOH layer acts not only as an efficient catalyst layer to accelerate the water oxidation on the surface of photoelectrode but also constructs a heterojunction with the light absorption layer to facilitate the interface charge carrier separation and transfer. As expected, the modified S-FeOOH@Fe₂O₃ photoanode achieves a remarkable increase in PEC performance of 2.30 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode (V_RHE) andan apparent negative shifted onset potential of 250 mV in comparison with pristine Fe₂O₃ (0.95 mA cm^-2 at 1.23 V_RHE). These results provide a simple and effective strategy to coupling oxygen evolution catalysts with photoanodes for practically high-performance PEC applications.

Emerging Surface, Bulk, and Interface Engineering Strategies on BiVO4 for Photoelectrochemical Water Splitting

The photoelectrochemical (PEC) cell that collects and stores abundant sunlight to hydrogen fuel promises a clean and renewable pathway for future energy needs and challenges. Monoclinic bismuth vanadate (BiVO₄ ), having an earth-abundancy, nontoxicity, suitable optical absorption, and an ideal n-type band position, has been in the limelight for decades. BiVO₄ is a potential photoanode candidate due to its favorable outstanding features like moderate bandgap, visible light activity, better chemical stability, and cost-effective synthesis methods. However, BiVO₄ suffers from rapid recombination of photogenerated charge carriers that have impeded further improvements of its PEC performances and stability. This review presents a close look at the emerging surface, bulk, and interface engineering strategies on BiVO₄ photoanode. First, an effective approach of surface functionalization via different cocatalysts to improve the surface kinetics of BiVO₄ is discussed. Second, state-of-the-art methodologies such as nanostructuring, defect engineering, and doping to further enhance light absorption and photogenerated charge transport in bulk BiVO₄ are reviewed. Third, interface engineering via heterostructuring to improve charge separation is introduced. Lastly, perspectives on the foremost challenges and some motivating outlooks to encourage the future research progress in this emerging frontier are offered.

Suppressing photoinduced charge recombination at the BiVO4||NiOOH junction by sandwiching an oxygen vacancy layer for efficient photoelectrochemical water oxidation

Nickel oxyhydroxide (NiOOH) is regarded as one of the promising cocatalysts to enhance the catalytic performance of photoanodes but suffers from serious interfacial charge-carrier recombination at the photoanode||NiOOH interface. In this work, surface-engineered BiVO₄ photoanodes are fabricated by sandwiching an oxygen vacancy (O_vac) interlayer between BiVO₄ and NiOOH. The surface O_vac interlayer is introduced on BiVO₄ by a chemical reduction treatment using a mild reducing agent, sodium hypophosphite. The induced O_vac can alleviate the interfacial charge-carrier recombination at the BiVO4||NiOOH junction, resulting in efficient charge separation and transfer efficiencies, while an outer NiOOH layer is coated to prevent the O_vac layer from degradation. As a result, the as-prepared NiOOH-P-BiVO₄ photoanode exhibits a high photocurrent density of 3.2 mA cm^-2 at 1.23 V vs. RHE under the irradiation of 100 mW/cm² AM 1.5G simulated sunlight, in comparison to those of bare BiVO₄, P-BiVO_4, and NiOOH-BiVO₄ photoanodes (1.1, 2.1 and 2.3 mA cm^-2, respectively). In addition to the superior photoactivity, the 5-h amperometric measurements illustrate improved stability of the surface-engineered NiOOH-P-BiVO₄ photoanode. Our work showcases the feasibility of combining cocatalysts with O_vac, for improved photoactivity and stability of photoelectrodes.

Fabricating BiVO4/FeOOH/ZnFe-LDH hierarchical core–shell nanorod arrays for visible-light-driven photoelectrochemical water oxidation

In this paper, porous BiVO4 nanorod arrays were encapsulated with FeOOH and ZnFe layered double hydroxide (ZnFe LDH) for the construction of hierarchical structures to achieve excellent transfer and separation of photogenerated electron–hole pairs.

Surface Reconstruction of Cobalt Species on Amorphous Cobalt Silicate-Coated Fluorine-Doped Hematite for Efficient Photoelectrochemical Water Oxidation

The slow kinetics of photoelectrochemical (PEC) water oxidation reaction is the bottleneck of PEC water splitting. Here, we report a comprehensive method to improve the PEC water oxidation performance of a hematite (α-Fe₂O₃) photoanode, that is, fluorine doping and an ultrathin amorphous cobalt silicate (Co-Sil) oxygen evolution reaction (OER) cocatalyst by photo-assisted electrophoretic deposition (PEPD). Detailed investigations reveal that fluorine doping can reduce the interfacial transfer resistance of charge and increase the carrier density to improve the conductivity of hematite. Also, simultaneously, the Co-Sil is used as an excellent OER cocatalyst to accelerate OER kinetics. Specifically, surface reconstruction of cobalt species occurred, and its average oxidation state increased significantly, which was more conducive to water oxidation. In addition, the presence of silicate groups could reduce the OOH* adsorption free energy. The synergistic effect of these efforts significantly reduced the onset potential and overpotential and enhanced the charge separation of the α-Fe₂O₃ photoanode, resulting in an excellent photocurrent density around 2.61 mA cm^-2 at 1.23 V vs RHE (4.75 times higher than the primitive α-Fe₂O₃). This work provides a feasible strategy for the construction and development of a potential hematite photoanode.

F- regulate the preparation of polyhedral BiVO4 enclosed by High-Index facet and enhance its photocatalytic activity

The selective exposure of high-index facets at the surface of nanocrystals is an important and challenging research topic. Herein, polyhedral bismuth vanadate (BiVO₄) crystals predominantly surrounded by {2 1 3} and {1 2 1} high-index facets were fabricated through the engineering of high-index surfaces by fluorinion (F^-) mediated hydrothermal process. The as-prepared BiVO₄-0.2F (the feeding amount of NaF was 0.2 g) catalyst exhibited high apparent quantum efficiency of 17.7% under 420 nm light irradiation and 9.3 fold enhancement of O₂ evolution relative to its low-index counterparts. Moreover, the growth of high-index facets results in significant enhancement of hydroxyl radical (•OH) production, photocatalytic degradation of Rhodamine B (RhB) and photoelectrochemical (PEC) properties by the BiVO₄ polyhedron, relative to its low-index counterparts. The enhanced photoreactivity is the result of the synergistic effect of F^- on the surface of the BiVO₄ crystals and exposed high-index facets. For one thing, F^- on the surface of the BiVO₄ facilitate the separation and transport of photo-induced charge carriers. For another, the exposed high-index facets on polyhedral BiVO₄ provided much more reactive sites for photocatalytic reactions. Hopefully, this F^- mediated method will be a useful guideline for designing and synthesizing novel high-index faceted micro-/nanostructures for overcoming the practical energy and environment problems.

Boosting Hole Transfer in the Fluorine-Doped Hematite Photoanode by Depositing Ultrathin Amorphous FeOOH/CoOOH Cocatalysts

The charge transfer is a key issue in the development of efficient photoelectrodes. Here, we report a method using F-doping and dual-layer ultrathin amorphous FeOOH/CoOOH cocatalysts coupling to enable the inactive α-Fe₂O₃ photoanode to become highly vibrant for the oxygen evolution reaction (OER). Fluorine doping is revealed to increase the charge density and improve the conductivity of α-Fe₂O₃ for rapid charge transfer. Furthermore, ultrathin FeOOH was deposited on F-Fe₂O₃ to extract photogenerated holes and passivate the surface states for accelerated charge carrier transfer. Moreover, CoOOH as an excellent cocatalyst was coated onto FeOOH/F-Fe₂O₃ with the photoassisted electrodeposition method remarkably expediting OER kinetics through an optional pathway of holes utilized by Co species. Ultimately, the CoOOH/FeOOH/F-Fe₂O₃ photoanode exhibits a satisfactory photocurrent density (3.3-fold higher than pristine α-Fe₂O₃) and a negatively shifted onset potential of 80 mV. This work showcases an appealing maneuver to activate the water oxidation performance of the α-Fe₂O₃ photoanode by an integration strategy of heteroatom doping and cocatalyst coupling.

Cl- modification for effective promotion of photoelectrochemical water oxidation over BiVO4

Postsynthetic treatment is an attractive method to enhance photoelectrochemical water splitting. The facile Cl^- modification approach developed in this work remarkably promotes the photocurrent density of BiVO₄ up to 2.7 mA cm^-2 by facilitating carrier transfer in addition to a charge carrier separation efficiency enhancement.

Unveiling the Effects of Nanostructures and Core Materials on Charge-Transport Dynamics in Heterojunction Electrodes for Photoelectrochemical Water Splitting

Understanding the photogenerated charge-transport dynamics of metal oxide electrodes is the key to providing a strategy for practical improvement in the photoelectrochemical reaction activity. Here, we analyze the electron transport of a 3D bicontinuous SnO₂/BiVO₄ nanostructured photoelectrode by intensity-modulated photocurrent spectroscopy. We compare this electrode with 3D WO₃/BiVO₄ and planar-type bilayer SnO₂/BiVO₄ electrodes. In the results, we observe an order of magnitude faster electron transport in the 3D electrodes relative to the bilayer electrode. Moreover, we observe trap-limited transport on widely applied WO₃/BiVO₄ electrodes but confirm rapid trap-free transport on 3D SnO₂/BiVO₄. We also characterize the effect of electron transport on the water-splitting reaction. The electron-transport rate is directly related to the charge-separation efficiency in the water-splitting reaction. The fast transport time of the 3D SnO₂/BiVO₄ leads to the achievement of a significantly higher charge separation efficiency of 94%.

Efficient BiVO4 Photoanodes by Postsynthetic Treatment: Remarkable Improvements in Photoelectrochemical Performance from Facile Borate Modification

Water-splitting photoanodes based on semiconductor materials typically require a dopant in the structure and co-catalysts on the surface to overcome the problems of charge recombination and high catalytic barrier. Unlike these conventional strategies, a simple treatment is reported that involves soaking a sample of pristine BiVO4 in a borate buffer solution. This modifies the catalytic local environment of BiVO4 by the introduction of a borate moiety at the molecular level. The self-anchored borate plays the role of a passivator in reducing the surface charge recombination as well as that of a ligand in modifying the catalytic site to facilitate faster water oxidation. The modified BiVO4 photoanode, without typical doping or catalyst modification, achieved a photocurrent density of 3.5 mA cm-2 at 1.23 V and a cathodically shifted onset potential of 250 mV. This work provides an extremely simple method to improve the intrinsic photoelectrochemical performance of BiVO4 photoanodes.

Interlayer Photoelectron Transfer Boosted by Bridged RuIV Atoms in GaS Nanosheets for Efficient Water Splitting

Photocatalytic water splitting over layered nanosheet (NS) catalysts has caught a lot of attention for renewable hydrogen fuel production. However, the weak van der Waals interlayer interactions make it a great challenge to realize an effective dissociation of photogenerated excitons and efficient charge transfer across the interior of layered catalysts during the photocatalysis process. Here, we propose an intercalation strategy of high-valence Ru^IV atoms to render two-dimensional GaS NS photocatalysts with rapid electron-hole dissociation and long photocarrier lifetime in visible-light-driven water splitting. Experimental and theoretical results unravel that the intercalated single-site Ru, confined in interlayer of GaS NSs, with a hexagonal structural configuration of "Ru₁-S₆", can serve as an electron-trapped high-speed channel toward simultaneously accelerating electron-hole pairs dissociation and promoting photoelectron transportation through the van der Waals interlayer. Consequently, the as-developed Ru-intercalated GaS NSs can give a notable H₂ production rate of 340 μmol g^-1 h^-1 under visible-light irradiation and an apparent yield of 7% at 420 nm, 38 times that of pure GaS NSs. This study opens up a feasible way for a new design of highly active layered photocatalysts toward high-efficiency solar energy conversion.

Subnano Amorphous Fe-Based Clusters with High Mass Activity for Efficient Electrocatalytic Oxygen Reduction Reaction

The development of cost-effective and efficient oxygen-relative electrocatalysts with high mass activity is extremely critical for modern sustainable fuel cells. Here, we present a new type of subnano amorphous transition-metal clusters supported on a hierarchical carbon framework as a promising oxygen reduction reaction (ORR) electrocatalyst, synthesized by a novel "amino-induced spatial confinement" strategy. This developed Fe subnano-cluster/3D-C could deliver outstanding ORR performance with a large mass activity of ∼8600 A g_Fe^-1 at a half-wave potential of 0.92 V, ∼10 times that of the benchmarking Pt/C electrocatalyst. The atomic characterizations and theoretical calculations jointly reveal the robust surface-covalent Fe-N bonds, and the synergistic effect of hetero Fe^2+/0 species is essentially beneficial for the adsorption of *O₂ and the formation of key *O intermediate during the ORR process, contributing to high oxygen-relative electrocatalytic activity for subnano amorphous Fe clusters.

Tailoring the Porosity in Iron Phosphosulfide Nanosheets to Improve the Performance of Photocatalytic Hydrogen Evolution

Metal sulfide photocatalysts are typically required during water splitting to produce hydrogen. However, the rapid recombination of photogenerated electron-hole pairs in these highly unstable photocatalysts has restricted hydrogen production to small-scale batch reactions. In this work, porous transition-metal thiophosphites were used to enable continuous long-term hydrogen production through photocatalysis. A wide bandgap (2.04 eV) was essential for generating hydrogen at a rate of 305.6 μmol h^-1  g^-1 , 180 % faster than nonporous FePS₃ nanosheets. More importantly, the high in-plane stiffness of these approximately 7 nm thick porous FePS₃ nanosheets ensured structural stability during 56 h of continuous photocatalysis reactions. The reaction results with D₂ O instead of H₂ O indicated that hydrogen mainly came from H₂ O. Furthermore, a sacrificial reagent (triethylamine) was photodegraded into diethylamine and acetaldehyde through a monoelectronic oxidation process, as indicated by HPLC and LC-MS. This synthesis strategy reported for FePS₃ porous nanosheets paves a new pathway for designing other dianion-based inorganic nanocrystals for hydrogen energy applications.

Strategies of Anode Materials Design towards Improved Photoelectrochemical Water Splitting Efficiency

This review presents the latest processes for designing anode materials to improve the efficiency of water photolysis. Based on different contributions towards the solar-to-hydrogen efficiency, we mainly review the strategies to enhance the light absorption, facilitate the charge separation, and enhance the surface charge injection. Although great achievements have been obtained, the challenges faced in the development of anode materials for solar energy to make water splitting remain significant. In this review, the major challenges to improve the conversion efficiency of photoelectrochemical water splitting reactions are presented. We hope that this review helps researchers in or coming to the field to better appreciate the state-of-the-art, and to make a better choice when they embark on new research in photocatalytic water splitting.

Rational Design and Construction of Cocatalysts for Semiconductor-Based Photo-Electrochemical Oxygen Evolution: A Comprehensive Review

Photo-electrochemical (PEC) water splitting, as an essential and indispensable research branch of solar energy applications, has achieved increasing attention in the past decades. Between the two photoelectrodes, the photoanodes for PEC water oxidation are mostly studied for the facile selection of n-type semiconductors. Initially, the efficiency of the PEC process is rather limited, which mainly results from the existing drawbacks of photoanodes such as instability and serious charge-carrier recombination. To improve PEC performances, researchers gradually focus on exploring many strategies, among which engineering photoelectrodes with suitable cocatalysts is one of the most feasible and promising methods to lower reaction obstacles and boost PEC water splitting ability. Here, the basic principles, modules of the PEC system, evaluation parameters in PEC water oxidation reactions occurring on the surface of photoanodes, and the basic functions of cocatalysts on the promotion of PEC performance are demonstrated. Then, the key progress of cocatalyst design and construction applied to photoanodes for PEC oxygen evolution is emphatically introduced and the influences of different kinds of water oxidation cocatalysts are elucidated in detail. Finally, the outlook of highly active cocatalysts for the photosynthesis process is also included.

Highly conformal deposition of ultrathin cobalt acetate on a bismuth vanadate nanostructure for solar water splitting

In this work, the effect of photochemically modifying nanoporous bismuth vanadate in Co²⁺ solution in acetate buffer (abbreviated as Co–Ac) on water oxidation was thoroughly studied.

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.