Activity and Stability Boosting of an Oxygen‐Vacancy‐Rich BiVO 4 Photoanode by NiFe‐MOFs Thin Layer for Water Oxidation

Bimetallic Fe/Ni metal organic framework-based hypoxanthine biosensor for early monitoring of freshness changes of aquatic products

The timely detection of freshness changes of aquatic products is crucial. In this study, we have developed a reliable, cost-effective, and user-friendly method for rapidly detecting hypoxanthine using a xanthine oxidase (XOD)/nanozyme enzymatic cascade system. The nanozyme, derived from the Fe7/Ni3 metal-organic framework (Fe7Ni3MOF), exhibited good peroxidase-mimetic activity and stability. Our proposed XOD/Fe7Ni3MOF enzymatic cascade system demonstrated a linear response to hypoxanthine in the range of 3-70 μM, with a low detection limit of 1.39 μM. We also analyzed hypoxanthine in actual aquatic products, achieving spiked recoveries ranging from 90.04 % to 107.37 %. The correlation coefficient between our developed colorimetric method and the HPLC method was 0.98. Importantly, our proposed method holds several advantages over alternative techniques, particularly in terms of cost-effectiveness, precision, and speed. Consequently, this methodology shows great promise for the early detection of freshness changes in aquatic samples.

Unveiling the effect of the structural transformation of CoZn-MOF on BiVO4 photoanode for efficient photoelectrochemical water oxidation

Photoelectrochemical (PEC) water splitting has been widely investigated for solar-to-hydrogen conversion. However, issues like high charge recombination rate and slow surface water oxidation kinetics severely hinder its (PEC) conversion efficiency. Herein, we constructed MOF-derived CoOOH cocatalyst on BiVO4 photoanode, using a feasible electrochemical activation strategy. The BiVO4-based photoanode obtained shows a high photocurrent density of 3.15 mA/cm2 at 1.23 VRHE and low onset potential. Detailed experiments and theoretical calculations show that during the activation of CoZn-MOFs, there was a partial breakage of 2-methylimidazole (mIM) linker, an increase in the oxidation state of Cobalt ion (Co), and increased O2-. The high PEC performance is mainly attributed to the MOF-derived CoOOH, which provides rich active sites for hole extraction and reduces the overpotential for oxygen evolution reaction. Furthermore, when CoZnNiFe-LDHs were decorated on BiVO4 using the ions exchange method, the photocurrent density of BiVO4/CoZnNiFe-LDHs photoanode got to 4.0 mA/cm2 at 1.23 VRHE, accompanied with high stability. This study provides insights into understanding the key role played by the structural transformation of MOF cocatalyst in PEC water splitting processes.

Advancing BiVO4 Photoanode Activity for Ethylene Glycol Oxidation via Strategic pH Control

The photoelectrochemical (PEC) conversion of organic small molecules offers a dual benefit of synthesizing value-added chemicals and concurrently producing hydrogen (H2). Ethylene glycol, with its dual hydroxyl groups, stands out as a versatile organic substrate capable of yielding various C1 and C2 chemicals. In this study, we demonstrate that pH modulation markedly enhances the photocurrent of BiVO4 photoanodes, thus facilitating the efficient oxidation of ethylene glycol while simultaneously generating H2. Our findings reveal that in a pH = 1 ethylene glycol solution, the photocurrent density at 1.23 V vs. RHE can attain an impressive 7.1 mA cm-2, significantly surpassing the outputs in neutral and highly alkaline environments. The increase in photocurrent is attributed to the augmented adsorption of ethylene glycol on BiVO4 under acidic conditions, which in turn elevates the activity of the oxidation reaction, culminating in the maximal production of formic acid. This investigation sheds light on the pivotal role of electrolyte pH in the PEC oxidation process and underscores the potential of the PEC strategy for biomass valorization into value-added products alongside H2 fuel generation.

Enhancing Iron(III) Oxide Photoelectrochemical Water Splitting Performance Using Defect Engineering and Heterostructure Construction

Fe2O3 is a promising semiconductor for photoelectrochemical (PEC) water decomposition. However, severe charge recombination problems limit its applications. In this study, a F-Fe2O3-x/MoS2 nanorod array photoanode was designed and prepared to facilitate charge separation. Detailed characterization and experimental results showed that F doping in Fe2O3 regulated the electronic structure to improve the conductivity of Fe2O3 and induced abundant oxygen vacancies to increase the carrier concentration and promote charge separation in bulk. In addition, the internal electric field between F-Fe2O3-x and MoS2 facilitated the qualitative transfer of the photogenerated charge, thus inhibiting their recombination. The synergistic effect between the oxygen vacancy and F-Fe2O3-x/MoS2 heterojunction significantly enhanced the PEC performance of Fe2O3. This study provides a universal strategy for designing other photoanode materials with high-efficiency charge separation.

High-performance bismuth vanadate photoanodes cocatalyzed with nitrogen, sulphur co-doped ferrocobalt-metal organic frameworks thin layer for photoelectrochemical water splitting

In this study, we prepare a highly efficient BiVO4 photoanode co-catalyzed with an ultrathin layer of N, S co-doped FeCo-Metal Organic Frameworks (MOFs) for photoelectrochemical water splitting. The introduction of N and S into FeCo-MOFs enhances electron and mass transfer, exposing more catalytic active sites and significantly improving the catalytic performance of N, S co-doped FeCo-based MOFs in water oxidation. The optimized BiVO4/NS-FeCo-MOFs photoanode exhibits impressive results, with a photocurrent density of 5.23 mA cm-2 at 1.23 V vs. Reversible Hydrogen Electrode (RHE) and an incident photon-to-charge conversion efficiency (IPCE) of 74.4 % at 450 nm in a 0.1 M phosphate buffered solution (pH = 7). These values are 4.84 times and 6.2 times higher than those of the original BiVO4 photoanode, respectively. Furthermore, the optimized BiVO4/NS-FeCo-MOFs photoanode demonstrates exceptional long-term stability, maintaining 96 % of the initial current after five hours.

The route for commercial photoelectrochemical water splitting: a review of large-area devices and key upscaling challenges

Green-hydrogen is considered a "key player" in the energy market for the upcoming decades. Among currently available hydrogen (H2) production processes, photoelectrochemical (PEC) water splitting has one of the lowest environmental impacts. However, it still presents prohibitively high production costs compared to more mature technologies, such as steam methane reforming. Therefore, the competitiveness of PEC water splitting must rely on its environmental and functional advantages, which are strongly linked to the reactor design, to the intrinsic properties of its components, and to their successful upscaling. This review gives special attention to the engineering aspects and categorizes PEC devices into four main types, according to the configuration of electrodes and strategies for gas separation: wired back-to-back, wireless back-to-back, wired side-by-side, and wired separated electrode membrane-free. Independently of the device architecture, the use of concentrated sunlight was found to be mandatory for achieving competitive green-H2 production. Additionally, feasible strategies for upscaling the key components of PEC devices, especially photoelectrodes, are urgently needed. In a pragmatic context, the way to move forward is to accept that PEC devices will operate close to their thermodynamic limits at large-scale, which requires a solid convergence between academics and industry. Research efforts must be redirected to: (i) build and demonstrate modular devices with a low-cost and highly recyclable embodiment; (ii) optimize thermal and power management; (iii) reduce ohmic losses; (iv) enhance the chemical stability towards a thousand hours; (v) couple solar concentrators with PEC devices; (vi) boost PEC-H2 production through the use of organic compounds; and (vii) reach consensual standardized methods for evaluating PEC devices, at both environmental and techno-economic levels. If these targets are not met in the next few years, the feasibility of PEC-H2 production and its acceptance by industry and by the general public will be seriously compromised.

A novel pencil graphite electrode modified with an iron-based conductive metal-organic framework exhibited good ability in simultaneous sensing bisphenol A and bisphenol S

Bisphenol A (BPA) and its substitute bisphenol S (BPS) are desirable materials widely used in manufacturing plastic products but can pose carcinogenic risks to humans. A new conductive iron-based metal-organic framework (Fe-HHTP)-modified pencil graphite electrode (PGE) for electrochemically sensing BPA and BPS was prepared and fully characterized by SEM, TEM, FT-IR, XRD, and XPS. Results showed that the optimal conditions for preparing Fe-HHTP/PGE were a pH of 6.5, a Fe-HHTP concentration of 2 mg·mL-1, a deposition potential of 0 V, and a deposition time of 100 s. The Fe-HHTP/PGE prepared under such conditions harbored a significant electrocatalytic activity with a detection limit of 0.8 nM for BPA and 1.7 nM for BPS (S/N = 3). Correspondingly, the electrochemical response current was linearly correlated to BPA and BPS, ranging from 0.01 to 100 μM. Fe-HHTP/PGE also obtained satisfactory recoveries by 93.8-102.1% and 96.0-101.3% for detecting BPA and BPS in plastic food packaging samples. Our work has provided a novel electrochemical tool to simultaneously detect BPA and BPS in food packaging samples and environmental matrixes.

A Comprehensive Review on the Boosted Effects of Anion Vacancy in the Heterogeneous Photocatalytic Degradation, Part II: Focus on Oxygen Vacancy

Environmental problems, including the increasingly polluted water and the energy crisis, have led to a need to propose novel strategies/methodologies to contribute to sustainable progress and enhance human well-being. For these goals, heterogeneous semiconducting-based photocatalysis is introduced as a green, eco-friendly, cost-effective, and effective strategy. The introduction of anion vacancies in semiconductors has been well-known as an effective strategy for considerably enhancing the photocatalytic activity of such photocatalytic systems, giving them the advantages of promoting light harvesting, facilitating photogenerated electron-hole pair separation, optimizing the electronic structure, and enhancing the yield of reactive radicals. This Review will introduce the effects of anion vacancy-dominated photodegradation systems. Then, their mechanism will illustrate how an anion vacancy changes the photodegradation pathway to enhance the degradation efficiency toward pollutants and the overall photocatalytic performance. Specifically, the vacancy defect types and the methods of tailoring vacancies will be briefly illustrated, and this part of the Review will focus on the oxygen vacancy (OV) and its recent advances. The challenges and development issues for engineered vacancy defects in photocatalysts will also be discussed for practical applications and to provide a promising research direction. Finally, some prospects for this emerging field will be proposed and suggested. All permission numbers for adopted figures from the literature are summarized in a separate file for the Editor.

Heterostructured Bimetallic MOF-on-MOF Architectures for Efficient Oxygen Evolution Reaction

Electron modulation presents a captivating approach to fabricate efficient electrocatalysts for the oxygen evolution reaction (OER), yet it remains a challenging undertaking. In this study, an effective strategy is proposed to regulate the electronic structure of metal-organic frameworks (MOFs) by the construction of MOF-on-MOF heterogeneous architectures. As a representative heterogeneous architectures, MOF-74 on MOF-274 hybrids are in situ prepared on 3D metal substrates (NiFe alloy foam (NFF)) via a two-step self-assembly method, resulting in MOF-(74 + 274)@NFF. Through a combination of spectroscopic and theory calculation, the successful modulation of the electronic property of MOF-(74 + 274)@NFF is unveiled. This modulation arises from the phase conjugation of the two MOFs and the synergistic effect of the multimetallic centers (Ni and Fe). Consequently, MOF-(74 + 274)@NFF exhibits excellent OER activity, displaying ultralow overpotentials of 198 and 223 mV at a current density of 10 mA cm-2 in the 1.0 and 0.1 M KOH solutions, respectively. This work paves the way for manipulating the electronic structure of electrocatalysts to enhance their catalytic activity.

BiVO4 Photoanodes Enhanced with Metal Phosphide Co-Catalysts: Relevant Properties to Boost Photoanode Performance

Achieving highly performant photoanodes for oxygen evolution is key to developing photoelectrochemical devices for solar water splitting. In this work, BiVO4 photoanodes are enhanced with a series of core-shell structured bimetallic nickel-cobalt phosphides (MPs), and key insights into the role of co-catalysts are provided. The best BiVO4 /Ni1.5 Co0.5 P and BiVO4 /Ni0.5 Co1.5 P photoanodes achieve a 3.5-fold increase in photocurrent compared with bare BiVO4 . It is discovered that this enhanced performance arises from a synergy between work function, catalytic activity, and capacitive ability of the MPs. Distribution of relaxation times analysis reveals that the contact between the MPs, BiVO4 , and the electrolyte gives rise to three routes for hole injection into the electrolyte, all of which are significantly improved by the presence of a second metal cation in the co-catalyst. Kinetic studies demonstrate that the significantly improved interfacial charge injection is due to a lower charge-transfer resistance, enhanced oxygen-evolution reaction kinetics, and larger surface hole concentrations, providing deeper insights into the carrier dynamics in these photoanode/co-catalyst systems for their rational design.

Nanoconfinement-triggered oligomerization pathway for efficient removal of phenolic pollutants via a Fenton-like reaction

Heterogeneous Fenton reaction represents one of the most reliable technologies to ensure water safety, but is currently challenged by the sluggish Fe(III) reduction, excessive input of chemicals for organic mineralization, and undesirable carbon emission. Current endeavors to improve the catalytic performance of Fenton reaction are mostly focused on how to accelerate Fe(III) reduction, while the pollutant degradation step is habitually overlooked. Here, we report a nanoconfinement strategy by using graphene aerogel (GA) to support UiO-66-NH2-(Zr) binding atomic Fe(III), which alters the carbon transfer route during phenol removal from kinetically favored ring-opening route to thermodynamically favored oligomerization route. GA nanoconfinement favors the Fe(III) reduction by enriching the reductive intermediates and allows much faster phenol removal than the unconfined analog (by 208 times in terms of first-order rate constant) and highly efficient removal of total organic carbon, i.e., 92.2 ± 3.7% versus 3.6 ± 0.3% in 60 min. Moreover, this oligomerization route reduces the oxidant consumption for phenol removal by more than 95% and carbon emission by 77.9%, compared to the mineralization route in homogeneous Fe2++H2O2 system. Our findings may upgrade the regulatory toolkit for Fenton reactions and provide an alternative carbon transfer route for the removal of aqueous pollutants.

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.

Prompt Hole Extraction Suppresses V5+ Dissolution and Sustains Large-Area BiVO4 Photoanodes for Over 2100 h Water Oxidation

Nanostructured bismuth vanadate (BiVO4) is at the forefront of emerging photoanodes in photoelectrochemical tandem devices for solar water splitting owing to the suitable band edge position and efficient charge separation capability. However, the (photo)chemical corrosion involving V5+ dissolution limits the long-term stability of BiVO4. Herein, guided by DFT calculations, we introduce an ALD-derived NiOx catalyst layer on BiVO4 to stabilize the surface Bi-O bonds, facilitate hole extraction, and thus suppress the V5+ dissolution. At the same time, the ALD NiOx catalyst layer could efficiently suppress the surface recombination and accelerate the surface OER kinetics, boosting the half-cell applied bias photon-to-current efficiency of BiVO4 to 2.05%, as well as a fill factor of 47.1%. By adding trace NaVO3 to the electrolyte, the NiOx/BiVO4 photoanode with an illumination area of 10.5 cm2 shows a record operational stability of more than 2100 h.

Schiff Base Complex Cocatalyst with Coordinatively Unsaturated Cobalt Sites for Photoelectrochemical Water Oxidation

Integrating inorganic oxygen evolution cocatalysts (OECs) with photoanodes is regarded as an available strategy to increase the photogenerated charge utilization for accelerated water oxidation kinetics. Nevertheless, most widely used transition metal (oxyhydr)oxides OECs suffer from inevitable charge recombination at photoanode/OECs interfaces and underabundant catalytic active sites. Herein, a cobalt-organic complex with microflower-like features (denoted as MF) was constructed by coordination of Schiff base ligands and Co2+ metal ions and then decorated on porous BiVO4 employed as photoanodes for photoelectrochemical (PEC) water oxidation. The as-synthesized BiVO4/MF photoanode achieves a photocurrent density of 4.38 mA cm-2 and at 1.23 VRHE in 0.5 M Na2SO4 electrolyte under simulated 1 sun illumination, over approximately 5.48 times larger than that of BiVO4 counterpart, and exhibits a 120 mV cathodic shift of onset potential with outstanding photostability. Systematic characterizations reveal that the improved PEC efficiency is mainly attributed to the well-designed coordinatively unsaturated Co2+ sites, which not only serve as powerful photohole extraction engines along reversed interfacial Co-O-Bi bonds to promote charge transfer across the BiVO4/complex interface but also act as reaction active centers by accelerating surface water oxidation kinetics. This work provides new insights for designing highly effective OECs for PEC water oxidation.

Enhanced Active Sites and Stability in Nano-MOFs for Electrochemical Energy Storage through Dual Regulation by Tannic Acid

The limited active sites and poor acid-alkaline solution stability of metal-organic frameworks (MOFs), significantly limit their wider application. In this study, the acid property of tannic acid (TA) was used as an etchant to etch the surface-active sites. Subsequently, the further chelation of the protonated TA with the exposed metal active site can effectively protect the metal ions. Meanwhile, the TA provided a large amount of phenolic hydroxyl groups, which can greatly improve the stability of imidazolate-coordinated MOFs. The electrochemical test results indicated that the MOFs composite materials synthesized using this scheme had high specific capacitance and stability. And the mechanism of its electrochemical reaction process was explored through in situ X-ray diffraction (XRD) and theoretical calculations. In addition, the same treatment was carried out through a series of carboxyl-coordinated MOFs, which further confirmed the principle of this scheme to obtain a higher active site and stability. This paper explains the mechanism of functionalization of nano-MOFs by polyphenolic compounds, providing new ideas for the research of nano-MOFs.

Spin-State Modulation on Metal-Organic Frameworks for Electrocatalytic Oxygen Evolution

Electrochemical oxygen evolution reaction (OER) kinetics are heavily correlated with hybridization of the transition metal d-orbital and oxygen intermediate p-orbital, which dictates the barriers of intermediate adsorption/desorption on the active sites of catalysts. Herein, a strategy is developed involving strain engineering and coordination regulation to enhance the hybridization of Ni 3d and O 2p orbitals, and the as-synthesized Ni-2,6-naphthalenedicarboxylic acid metal-organic framework (DD-Ni-NDA) nanosheets deliver a low OER overpotential of 260 mV to reach 10 mA cm-2 . By integrating an alkaline anion exchange membrane electrolyzer and Pt/C electrode, 200 and 500 mA cm-2 current densities are reached with cell voltages of 1.6 and 2.1 V, respectively. When loaded on a BiVO4 photoanode, the nanosheet enables highly active solar-driven water oxygen. Structural characterizations together with theoretical calculations reveal that the spin state of the centre Ni atoms is regulated by the tensile strain and unsaturated coordination defects in DD-Ni-NDA, and such spin regulation facilitates spin-dependent charge transfer of the OER. Molecular orbital hybridization analysis reveals the mechanism of OH* and OOH* adsorption energy regulation by changes in the DD-Ni-NDA spin state, which provides a deeper understanding of the electronic structure design of catalysts for the OER.

Ni-Rh-based bimetallic conductive MOF as a high-performance electrocatalyst for the oxygen evolution reaction

Metal-organic frameworks (MOFs) have recently been considered the promising catalysts due to their merits of abundant metal sites, versatile coordination groups, and tunable porous structure. However, low electronic conductivity of most MOFs obstructs their direct application in electrocatalysis. In this work, we fabricate an Ni-Rh bimetallic conductive MOF ([Ni2.85Rh0.15(HHTP)2]n/CC) grown in situ on carbon cloth. Abundant nanopores in the conductive MOFs expose additional catalytic active sites, and the advantageous 2D π-conjugated structure helps accelerate charge transfer. Owing to the introduction of Rh, [Ni2.85Rh0.15(HHTP)2]n/CC exhibited substantially improved oxygen evolution reaction (OER) activity and exhibited only an overpotential of 320 mV to achieve the current density of 20 mA cm-2. The remarkable OER performance confirmed by the electrochemical tests could be ascribed to the synergistic effect caused by the doped Rh together with Ni in [Ni2.85Rh0.15(HHTP)2]n/CC, thereby exhibiting outstanding electrocatalytic performance.

Highly Durable and Efficient Seawater Electrolysis Enabled by Defective Graphene-Confined Nanoreactor

Direct seawater electrolysis is a promising technology for massive green hydrogen production but is limited by the lack of durable and efficient electrocatalysts toward the oxygen evolution reaction (OER). Herein, we develop a core-shell nanoreactor as a high-performance OER catalyst consisting of NiFe alloys encapsulated within defective graphene layers (NiFe@DG) by a facile microwave shocking strategy. This catalyst needs overpotentials of merely 218 and 276 mV in alkalized seawater to deliver current densities of 10 and 100 mA cm-2, respectively, and operates continuously for 2000 h with negligible activity decay (1.0%), making it one of the best OER catalysts reported to date. Detailed experimental and theoretical analyses reveal that the excellent durability of NiFe@DG originates from the formation of the built-in electric field triggered by the defective graphene coating against chloride ions at the electrode/electrolyte interface, thus protecting the active NiFe alloys at the core from dissolution and aggregation under harsh operation conditions. Further, a highly stable and efficient seawater electrolyzer is assembled with the NiFe@DG anode and the Pt/C cathode to demonstrate the practicability of the catalysts.

Introducing Bidirectional Axial Coordination into BiVO4 @Metal Phthalocyanine Core-Shell Photoanodes for Efficient Water Oxidation

Core-shell photoanodes have shown great potential for photoelectrochemical (PEC) water oxidation. However, the construction of a high-quality interface between the core and shell, as well as a highly catalytic surface, remains a challenge. Herein, guided by computation, we present a BiVO4 photoanode coated with ZnCoFe polyphthalocyanine using pyrazine as a coordination agent. The bidirectional axial coordination of pyrazine plays a dual role by facilitating intimate interfacial contact between BiVO4 and ZnCoFe polyphthalocyanine, as well as regulating the electron density and spin configuration of metal sites in ZnCoFe phthalocyanine, thereby promoting the potential-limiting step of *OOH desorption. The resulting photoanode displayed a high photocurrent density of 5.7±0.1 mA cm-2 at 1.23 VRHE . This study introduces a new approach for constructing core-shell photoanodes, and uncovers the key role of pyrazine axial coordination in modulating the catalytic activity of metal phthalocyanine.

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.

Surface Oxygen Species in Metal Oxide Photoanodes for Solar Energy Conversion

Converting and storing solar energy directly as chemical energy through photoelectrochemical devices are promising strategies to replace fossil fuels. Metal oxides are commonly used as photoanode materials, but they still encounter challenges such as limited light absorption, inefficient charge separation, sluggish surface reactions, and insufficient stability. The regulation of surface oxygen species on metal oxide photoanodes has emerged as a critical strategy to modulate molecular and charge dynamics at the reaction interface. However, the precise role of surface oxygen species in metal oxide photoanodes remains ambiguous. The review focuses on elucidating the formation and regulation mechanisms of various surface oxygen species in metal oxides, their advantages and disadvantages in photoelectrochemical reactions, and the characterization methods employed to investigate them. Additionally, the article discusses emerging opportunities and potential hurdles in the regulation of surface oxygen species. By shedding light on the significance of surface oxygen species, this review aims to advance our understanding of their impact on metal oxide photoanodes, paving the way for the design of more efficient and stable photoelectrochemical devices.

Photo-Electrochemical Glycerol Conversion over a Mie Scattering Effect Enhanced Porous BiVO4 Photoanode

The photo-electrochemical (PEC) oxidation of glycerol (GLY) to high-value-added dihydroxyacetone (DHA) can be achieved over a BiVO₄ photoanode, while the PEC performance of most BiVO₄ photoanodes is impeded due to the upper limits of the photocurrent density. Here, an enhanced Mie scattering effect of the well-documented porous BiVO₄ photoanode is obtained with less effort by a simple annealing process, which significantly reduces the reflectivity to near zero. The great light absorbability increases the basic photocurrent density by 1.77 times. The selective oxidation of GLY over the BiVO₄ photoanode results in a photocurrent density of 6.04 mA cm^-2 and a DHA production rate of 325.2 mmol m^-2 h^-1 that exceeds all reported values. This work addresses the poor ability of nanostructured BiVO₄ to harvest light, paving the way for further improvements in charge transport and transfer to realize highly efficient PEC conversion.

Synchronous coupling of defects and a heteroatom-doped carbon constraint layer on cobalt sulfides toward boosted oxide electrolysis activities for highly energy-efficient micro-zinc-air batteries

The sluggish kinetics of oxygen electrocatalysis reactions on cathodes significantly suppresses the energy efficiency of zinc-air batteries (ZABs). Herein, by coupling in situ generated CoS nanoparticles rich in cobalt vacancies (V_Co) with a dual-heteroatom-doped layered carbon framework, a hybrid Co-based catalyst (Co_1-xS@N/S-C) is designed and synthesized from Co-MOF precursor. Experimental analyses, together with density functional theory (DFT)-based calculations, demonstrate that the facilitated ion diffusion enabled by the introduced V_Co, together with the enhanced electron transport benefiting from the well-designed dual-heteroatom-doped laminated carbon framework, synergistically boost the bifunctional electrocatalytic activity of Co_1-xS@N/S-C (ΔE = 0.76 V), which is much superior to that of CoS@N/S-C without V_Co (ΔE = 0.89 V), CoS without V_Co (ΔE = 1.23 V), and the dual-heteroatom-doped laminated carbon framework. As expected, the further assembled ZAB employing Co_1-xS@N/S-C as the cathode electrocatalyst exhibits enhanced energy efficiency in terms of better cycling stability (510 cycles/170 hours) and a higher specific capacity (807 mA h g^-1). Finally, a flexible/stretched solid state micro-ZAB (F/SmZAB) with Co_1-xS@N/S-C as the cathode electrocatalyst and a wave-shaped GaIn-Ni-based liquid metal as the electronic circuit is further designed, which can display excellent electrical properties and long elongation. This work provides a new defect and structure coupling strategy for boosting the oxide electrolysis activities of Co-based catalysts. Furthermore, F/SmZAB represents a promising solution for a compatible micropower source in wearable microelectronics.

Photochemical and electrochemical co-regulation of the BiVO4 photoanode for water splitting

A novel pretreatment strategy that can regulate the amount of oxygen vacancies (O_vac) across the wormlike-BiVO₄ photoanode by photochemical and electrochemical co-processing. Upon decorating NiFeO_x as an oxygen evolution cocatalyst for promoting the surface oxidation kinetics, a record-high photocurrent density of 6.42 mA cm^-2 is obtained at 1.23 vs. RHE (100 mW cm^-2).

A Conformal Carbon Nanolayer Coated Fe2 O3 Cocatalyst for the Promoted Activity of Plasma-Sputtered BiVO4 Photoanode

To simultaneously improve the hole extraction ability of the BiVO₄ photoanode and accelerate the surface reaction kinetics, herein, a carbon nanolayer conformally coated Fe₂ O₃ (C-Fe₂ O₃ ) as oxygen evolution catalyst (OEC) is loaded on the H₂ plasma treated nanoporous BiVO₄ (BVO(H₂ )) surface by a hydrothermal reaction. It is found that the H₂ plasma induced vacancies in BVO remarkably increases the conductivity, and the C-Fe₂ O₃ enables hole extraction from the bulk to the surface as well as efficient hole injection to the electrolyte. As a result, the C-Fe₂ O₃ /BVO(H₂ ) photoanode achieves a photocurrent density of 4.4 mA/cm² at 1.23 V vs. reversible hydrogen electrode (RHE) and an ABPE value of 1.5 % at 0.68 V vs. RHE, which are 4.8-fold and 13-fold higher than that of BVO photoanode, respectively.

Selective Valorization of Glycerol to Formic Acid on a BiVO4 Photoanode through NiFe Phenolic Networks

The integration of the glycerol oxidation reaction (GOR) with the hydrogen evolution reaction in photoelectrochemical (PEC) cells is a desirable alternative to PEC water splitting since a large quantity of glycerol is easily accessible as the byproduct from the biodiesel industry. However, the PEC valorization of glycerol to the value-added products suffers from low Faradaic efficiency and selectivity, especially in acidic conditions, which is beneficial for hydrogen production. Herein, by loading bismuth vanadate (BVO) with a robust catalyst composed of phenolic ligands (tannic acid) coordinated with Ni and Fe ions (TANF), we demonstrate a modified BVO/TANF photoanode for the GOR with a remarkable Faradaic efficiency of over 94% to value-added molecules in a 0.1 M Na₂SO₄/H₂SO₄ (pH = 2) electrolyte. The BVO/TANF photoanode achieved a high photocurrent of 5.26 mA·cm^-2 at 1.23 V versus reversible hydrogen electrode under 100 mW/cm² white light irradiation for formic acid production with 85% selectivity, equivalent to 573 mmol/(m²·h). Transient photocurrent and transient photovoltage techniques and electrochemical impedance spectroscopy along with intensity-modulated photocurrent spectroscopy indicated that the TANF catalyst could accelerate hole transfer kinetics and suppress charge recombination. Comprehensive mechanistic investigations reveal that the GOR is initiated by the photogenerated holes of BVO, while the high selectivity to formic acid is attributed to the selective adsorption of primary hydroxyl groups in glycerol on TANF. This study provides a promising avenue for highly efficient and selective formic acid generation from biomass in acid media via PEC cells.

Unconventional Substitution for BiVO4 to Enhance Photoelectrocatalytic Performance by Accelerating Polaron Hopping

Bismuth vanadate (BiVO4) as a fascinating semiconductor for photoelectrocatalytic (PEC) water oxidation with suitable band gap (Eg) has been limited by the issue of poor separation and transportation of charge carriers. Herein, we propose an unconventional substitution of V5+ sites by Ti4+ in BiVO4 (Ti:BiVO4) for the similar ionic radii and accelerated polaron hopping. Ti:BiVO4 increased the photocurrent density 1.90 times up to 2.51 mA cm-2 at 1.23 V vs RHE and increased the charge carrier density 1.81 times to 5.86 × 1018 cm-3. Compared with bare BiVO4, Ti:BiVO4 improves the bulk separation efficiency to 88.3% at 1.23 V vs RHE. The DFT calculations have illustrated that Ti-doping modification could decrease the polaron hopping energy barrier, narrow the Eg, and decrease the overpotential of the oxygen evolution reaction (OER) concurrently. With further spin-coated FeOOH cocatalyst, the photoanode has a photocurrent density of 3.99 mA cm-2 at 1.23 V vs RHE. The excellent PEC performance of FeOOH/Ti:BiVO4 is attributed to the synergistic effect of the FeOOH layer and Ti doping, which could promote charge carrier separation and transfer by expediting polaron migration.

Structural Engineering of BiVO4 /CoFe MOF Heterostructures Boosting Charge Transfer for Efficient Photoelectrochemical Water Splitting

Boosting charge separation and transfer of photoanodes is crucial for providing high viability of photoelectrochemical hydrogen (H₂ ) generation. Here, a structural engineering strategy is designed and synthesized for uniformly coating an ultrathin CoFe bimetal-organic framework (CoFe MOF) layer over a BiVO₄ photoanode for boosted charge separation and transfer. The photocurrent density of the optimized BiVO₄ /CoFe MOF(NA) photoanode reaches a value of 3.92 mA cm^-2 at 1.23 V versus reversible hydrogen electrode (RHE), up to 6.03 times that of pristine BiVO₄ , due to the greatly increased efficiency of charge transfer and separation. In addition, this photoanode records one onset potential that is considerably shifted negatively when compared to BiVO₄ . Transient absorption spectroscopy reveals that the CoFe MOF(NA) prolongs charge recombination lifetime by blocking the hole-transfer pathway from the BiVO₄ to its surface trap states. This work sheds light on boosting charge separation and transfer through structural engineering to enhance the photocurrent of photoanodes for solar H₂ production.

Subsurface Engineering Induced Fermi Level De-pinning in Metal Oxide Semiconductors for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is a promising approach for renewable solar light conversion. However, surface Fermi level pinning (FLP), caused by surface trap states, severely restricts the PEC activities. Theoretical calculations indicate subsurface oxygen vacancy (sub-O_v ) could release the FLP and retain the active structure. A series of metal oxide semiconductors with sub-O_v were prepared through precisely regulated spin-coating and calcination. Etching X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and electron energy loss spectra (EELS) demonstrated O_v located at sub ∼2-5 nm region. Mott-Schottky and open circuit photovoltage results confirmed the surface trap states elimination and Fermi level de-pinning. Thus, superior PEC performances of 5.1, 3.4, and 2.1 mA cm^-2 at 1.23 V vs. RHE were achieved on BiVO₄ , Bi₂ O₃ , TiO₂ with outstanding stability for 72 h, outperforming most reported works under the identical conditions.

Construction of Frustrated Lewis Pair Sites in CeO2-C/BiVO4 for Photoelectrochemical Nitrate Reduction

Photoelectrochemical nitrate reduction reaction (PEC NIRR) could convert the harmful pollutant nitrate (NO₃^-) to high-value-added ammonia (NH₃) under mild conditions. However, the catalysts are currently hindered by the low catalytic activity and slow kinetics. Here, we reported a heterostructure composed of CeO₂ and BiVO₄, and the "frustrated Lewis pairs (FLPs)" concept was introduced for understanding the role of Lewis acids and Lewis bases on PEC NIRR. The electron density difference maps indicated that FLPs were significantly active for the adsorption and activation of NO₃^-. Furthermore, carbon (C) improved the carrier transport ability and kinetics, contributing to the NH₃ yield of 21.81 μg h^-1 cm^-2. The conversion process of NO₃^- to NH₃ was tracked by ¹⁵NO₃^- and ¹⁴NO₃^- isotopic labeling. Therefore, this study demonstrated the potential of CeO₂-C/BiVO₄ for efficient PEC NIRR and provided a unique mechanism for the adsorption and activation of NO₃^- over FLPs.

Design and preparation of highly active TiO2 photocatalysts by modulating their band structure

Titanium dioxide photocatalysts with high reduction potential and visible light response hold great promise in photochemical conversion. Here, we used a simple glycine doping method to synthesize novel N-TiO2@C photocatalysts with upward shifted conduction bands and narrowed band gaps as well as inhibited recombination of photoinduced electron-hole pairs. The N-TiO2@C photocatalysts exhibited higher visible light response and remarkably enhanced photocatalytic activity in the production of nicotinamide adenine dinucleotide (NADH) by photomediated reduction of NAD+ without any electron mediator. The yield of NADH was up to 70.3 % far greater than that of the undoped TiO2 (11.3 %), and it stabilized at ca. 60 % after 10 cycles. The viability of coupling NADH regeneration with enzymatic reaction (alcohol dehydrogenase) was established in aldehyde reduction where formaldehyde was specifically reduced to methanol. These findings shed new light on the modulation of the band structure of semiconductors and develop an electron mediator free strategy for NADH-dependent artificial photosynthesis through coupled photocatalytic and enzymatic approaches.

Engineering the Near-Surface Structure of WO3 by an Amorphous Layer with Trivalent Ni and Self-Adapting Oxygen Vacancies for Efficient Photocatalytic and Photoelectrochemical Acidic Oxygen Evolution Reaction

Exploiting an effective strategy to tailor the construction, composition, and local electronic structure of the photocatalyst surface is pivotal to photocatalytic activity, but remains challenging. Transition metal elements can boost the oxygen evolution reaction activity especially one like Ni in high oxidation states, whereas it is uneasy to prepare Ni³⁺ under mild conditions or play to their strengths in acidic conditions. In this article, we report a facile "etch and dope" synthesis of Ni³⁺-doped WO₃ nanosheets with oxygen vacancies. Through detailed experimental and theoretical studies, it is established that the abundant oxygen vacancies and the doped Ni³⁺ ions in the near-surface amorphous layer can synergistically optimize the surface electronic structure of WO₃ and the adsorption and desorption of intermediates. Impressively, the etched WO₃ nanosheets coupled with Ni³⁺ offer a greatly promoted photocatalytic performance of 1.78 mmol g^-1 h^-1, and the photoanode achieves a photocurrent density of 2.11 mA cm^-2 at 1.23 V versus reversible hydrogen electrode (V_RHE). This work provides a new inspiration for rational manufacture of defects and high-valence metal ions in catalysts for photocatalytic and photoelectrochemical reactions.

Comprehension of the Synergistic Effect between m&t-BiVO4/TiO2-NTAs Nano-Heterostructures and Oxygen Vacancy for Elevated Charge Transfer and Enhanced Photoelectrochemical Performances

Through the utilization of a facile procedure combined with anodization and hydrothermal synthesis, highly ordered alignment TiO2 nanotube arrays (TiO2-NTAs) were decorated with BiVO4 with distinctive crystallization phases of monoclinic scheelite (m-BiVO4) and tetragonal zircon (t-BiVO4), favorably constructing different molar ratios and concentrations of oxygen vacancies (Vo) for m&t-BiVO4/TiO2-NTAs heterostructured nanohybrids. Simultaneously, the m&t-BiVO4/TiO2-NTAs nanocomposites significantly promoted photoelectrochemical (PEC) activity, tested under UV-visible light irradiation, through photocurrent density testing and electrochemical impedance spectra, which were derived from the positive synergistic effect between nanohetero-interfaces and Vo defects induced energetic charge transfer (CT). In addition, a proposed self-consistent interfacial CT mechanism and a convincing quantitative dynamic process (i.e., rate constant of CT) for m&t-BiVO4/TiO2-NTAs nanoheterojunctions are supported by time-resolved photoluminescence and nanosecond time-resolved transient photoluminescence spectra, respectively. Based on the scheme, the m&t-BiVO4/TiO2-NTAs-10 nanohybrids exhibited a photodegradation rate of 97% toward degradation of methyl orange irradiated by UV-visible light, 1.14- and 1.04-fold that of m&t-BiVO4/TiO2-NTAs-5 and m&t-BiVO4/TiO2-NTAs-20, respectively. Furthermore, the m&t-BiVO4/TiO2-NTAs-10 nanohybrids showed excellent PEC biosensing performance with a detection limit of 2.6 μM and a sensitivity of 960 mA cm-2 M-1 for the detection of glutathione. Additionally, the gas-sensing performance of m&t-BiVO4/TiO2-NTAs-10 is distinctly superior to that of m&t-BiVO4/TiO2-NTAs-5 and m&t-BiVO4/TiO2-NTAs-20 in terms of sensitivity and response speed.

Structural and Electronic Modulation of Iron-Based Bimetallic Metal-Organic Framework Bifunctional Electrocatalysts for Efficient Overall Water Splitting in Alkaline and Seawater Environment

Metal-organic frameworks (MOFs) are considered potential electrocatalysts for efficient water splitting. However, the structure-activity relationship of most MOFs is not systematically analyzed for electrocatalysis for anodes and cathodes. In this paper, we provide a strategy to modulate the electronic microstructure of iron-based bimetallic MOFs (MFe-BDC (M: Mg, Zn, Cd)) grown on the nickel foam (NF) as bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The optimal bimetallic CdFe-BDC via modulating appropriate metal cations of IIA and IIB possesses excellent OER and HER performance with the lowest overpotentials of 290 mV at 100 mA cm^-2 and 148 mV at 10 mA cm^-2, respectively. The overall water splitting performance of the as-prepared CdFe-BDC requires 1.68 V to achieve a current density of 10 mA cm^-2 in the real seawater media, and it exhibits the competitive H₂ and O₂ production rates of 6.4 and 3.1 μL s^-1, respectively, in ambient alkaline conditions, suggesting its potential practical applications. Density functional theory (DFT) calculations demonstrate the relationship between microstructure and electrocatalytic performance of bimetallic MFe-BDC. This work emphasizes the significance of tailoring the electronic microstructure of bimetallic MOFs for efficient overall water splitting in alkaline and seawater environment.

Exceptional catalytic activity of oxygen evolution reaction via two-dimensional graphene multilayer confined metal-organic frameworks

Oxygen evolution reaction (OER) plays a key role in many renewable energy technologies such as water splitting and metal-air batteries. Metal-organic frameworks (MOFs) are appealing to design efficient OER electrocatalysts, however, their intrinsic poor conductivity strongly hinders the activity. Here, we show a strategy to boost the OER activity of poor-conductive MOFs by confining them between graphene multilayers. The resultant NiFe-MOF//G gives a record-low overpotential of 106 mV to reach 10 mA cm⁻² and retains the activity over 150 h, which is in significant contrast to 399 mV of the pristine NiFe-MOF. We use X-ray absorption spectroscopy (XAS) and computations to demonstrate that the nanoconfinement from graphene multilayers not only forms highly reactive NiO₆-FeO₅ distorted octahedral species in MOF structure but also lowers limiting potential for water oxidation reaction. We also demonstrate that the strategy is applicable to other MOFs of different structures to largely enhance their electrocatalytic activities.

While metal-organic frameworks offer a diverse array of structural motifs for electrocatalysis, poor conductivity and mass permeability limit performances. Here, authors confine low-conductivity metal-organic frameworks between graphene multilayers to enhance oxygen evolution performances.

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.

Porous BiVO4/Boron-Doped Diamond Heterojunction Photoanode with Enhanced Photoelectrochemical Activity

Constructing heterojunction is an attractive strategy for promoting photoelectrochemical (PEC) performance in water splitting and organic pollutant degradation. Herein, a novel porous BiVO4/Boron-doped Diamond (BiVO4/BDD) heterojunction photoanode containing masses of ultra-micro electrodes was successfully fabricated with an n-type BiVO4 film coated on a p-type BDD substrate by magnetron sputtering (MS). The surface structures of BiVO4 could be adjusted by changing the duration of deposition (Td). The morphologies, phase structures, electronic structures, and chemical compositions of the photoanodes were systematically characterized and analyzed. The best PEC activity with the highest current density of 1.8 mA/cm2 at 1.23 VRHE was achieved when Td was 30 min, and the sample showed the highest degradation efficiency towards tetracycline hydrochloride degradation (TCH) as well. The enhanced PEC performance was ascribed to the excellent charge transport efficiency as well as a lower carrier recombination rate, which benefited from the formation of BiVO4/BDD ultra-micro p-n heterojunction photoelectrodes and the porous structures of BiVO4. These novel photoanodes were expected to be employed in the practical PEC applications of energy regeneration and environmental management in the future.

Activating oxygen deficient TiO2 in the visible region by Bi2MoO6 for CO2 photoreduction to methanol

Fast photogenerated charge recombination and inappropriate bandgap for visible light driven charge generation hinders the performance of TiO₂. In this study, TiO₂ was activated for visible light driven CO₂ reduction in the presence of Bi₂MoO₆ as an electron donor. Furthermore, the introduction of oxygen vacancies resulted in enhanced CO₂ adsorption and conversion. The best catalyst gives 27.1 μmol g^-1 h^-1 methanol formation. DRIFTS was used to explain the methanol formation mechanism on oxygen deficient TiO₂.