Cobalt Hexacyanoferrate on BiVO4 Photoanodes for Robust Water Splitting

Exploring suitability of solid 3D substrates for designing self-supported electrocatalysts for water splitting

The choice of solid 3D substrates to design electrocatalysts significantly impacts the efficiency and effectiveness of self-supported electrocatalysts used in water splitting. These substrates are pivotal in significantly boosting the performance by providing structural support, facilitating electron transport, and increasing the active surface area. This improvement leads to higher catalytic performance and better stability, ultimately optimizing the electrocatalytic process. The interaction between the substrate and the electrocatalyst can also affect the intrinsic properties of the catalyst, further influencing its performance. Therefore, understanding and optimizing the use of solid 3D substrates is vital for advancing water-splitting technologies. This article explores the critical role of solid 3D substrates in enhancing the activity of self-supported materials for water splitting. By examining recent developments and research in this region, we target to showcase a comprehensive understanding of how different 3D substrates influence electrocatalyst properties and performance and to highlight future directions for optimizing these systems in water-splitting applications.

NC Meets CN: Porous Photoanodes with Polymeric Carbon Nitride/ZnSe Nanocrystal Heterojunctions for Photoelectrochemical Applications

The utilization of photoelectrochemical cells (PEC) for converting solar energy into fuels (e.g., hydrogen) is a promising method for sustainable energy generation. We demonstrate a strategy to enhance the performance of PEC devices by integrating surface-functionalized zinc selenide (ZnSe) semiconductor nanocrystals (NCs) into porous polymeric carbon nitride (CN) matrices to form a uniformly distributed blend of NCs within the CN layer via electrophoretic deposition (EPD). The achieved type II heterojunction at the CN/NC interface exhibits intimate contact between the NCs and the CN backbone since it does not contain insulating binders. This configuration promotes efficient charge separation and suppresses carrier recombination. The reported CN/NC composite structure serves as a photoanode, demonstrating a photocurrent density of 160 ± 8 μA cm-2 at 1.23 V vs a reversible hydrogen electrode (RHE), 75% higher compared with a CN-based photoelectrode, for approximately 12 h. Spectral and photoelectrochemical analyses reveal extended photoresponse, reduced charge recombination, and successful charge transfer at the formed heterojunction; these properties result in enhanced PEC oxygen production activity with a Faradaic efficiency of 87%. The methodology allows the integration of high-quality colloidal NCs within porous CN-based photoelectrodes and provides numerous knobs for tuning the functionality of the composite systems, thus showing promise for achieving enhanced solar fuel production using PEC.

Electrochemical versus Photoelectrochemical Water Oxidation Kinetics on Bismuth Vanadate (Photo)anodes

This study reports a comparison of the kinetics of electrochemical (EC) versus photoelectrochemical (PEC) water oxidation on bismuth vanadate (BiVO4) photoanodes. Plots of current density versus surface hole density, determined from operando optical absorption analyses under EC and PEC conditions, are found to be indistinguishable. We thus conclude that EC water oxidation is driven by the Zener effect tunneling electrons from the valence to conduction band under strong bias, with the kinetics of both EC and PEC water oxidation being determined by the density of accumulated surface valence band holes. We further demonstrate that our combined optical absorption/current density analyses enable an operando quantification of the BiVO4 photovoltage as a function of light intensity.

Tunable Photocatalytic Activity of CoFe Prussian Blue Analogue Modified SrTiO3 Core-Shell Structures for Solar-Driven Water Oxidation

This study presents a pioneering semiconductor-catalyst core-shell architecture designed to enhance photocatalytic water oxidation activity significantly. This innovative assembly involves the in situ deposition of CoFe Prussian blue analogue (PBA) particles onto SrTiO3 (STO) and blue SrTiO3 (bSTO) nanocubes, effectively establishing a robust p-n junction, as demonstrated by Mott-Schottky analysis. Of notable significance, the STO/PB core-shell catalyst displayed remarkable photocatalytic performance, achieving an oxygen evolution rate of 129.6 μmol g-1 h-1, with stability over an extended 9-h in the presence of S2O82- as an electron scavenger. Thorough characterization unequivocally verified the precise alignment of the band energies within the STO/PB core-shell assembly. Our research underscores the critical role of tailored semiconductor-catalyst interfaces in advancing the realm of photocatalysis and its broader applications in renewable energy technologies.

Complementary probes for the electrochemical interface

The functions of electrochemical energy conversion and storage devices rely on the dynamic junction between a solid and a fluid: the electrochemical interface (EI). Many experimental techniques have been developed to probe the EI, but they provide only a partial picture. Building a full mechanistic understanding requires combining multiple probes, either successively or simultaneously. However, such combinations lead to important technical and theoretical challenges. In this Review, we focus on complementary optoelectronic probes and modelling to address the EI across different timescales and spatial scales - including mapping surface reconstruction, reactants and reaction modulators during operation. We discuss how combining these probes can facilitate a predictive design of the EI when closely integrated with theory.

Lattice-Stabilized Chromium Atoms on Ceria for N2O Synthesis

The development of selective catalysts for direct conversion of ammonia into nitrous oxide, N2O, will circumvent the conventional five-step manufacturing process and enable its wider utilization in oxidation catalysis. Deviating from commonly accepted catalyst design principles for this reaction, reliant on manganese oxide, we herein report an efficient system comprised of isolated chromium atoms (1 wt %) stabilized in the ceria lattice by coprecipitation. The latter, in contrast to a simple impregnation approach, ensures firm metal anchoring and results in stable and selective N2O production over 100 h on stream up to 79% N2O selectivity at full NH3 conversion. Raman, electron paramagnetic resonance, and in situ UV-vis spectroscopies reveal that chromium incorporation enhances the density of oxygen vacancies and the rate of their generation and healing. Accordingly, temporal analysis of products, kinetic studies, and atomistic simulations show lattice oxygen of ceria to directly participate in the reaction, establishing the cocatalytic role of the carrier. Coupled with the dynamic restructuring of chromium sites to stabilize intermediates of N2O formation, these factors enable catalytic performance on par with or exceeding benchmark systems. These findings demonstrate how nanoscale engineering can elevate a previously overlooked metal into a highly competitive catalyst for selective ammonia oxidation to N2O, paving the way toward industrial implementation.

Prussian blue analog with separated active sites to catalyze water driven enhanced catalytic treatments

Chemodynamic therapy (CDT) uses the Fenton or Fenton-like reaction to yield toxic ‧OH following H2O2 → ‧OH for tumoral therapy. Unfortunately, H2O2 is often taken from the limited endogenous supply of H2O2 in cancer cells. A water oxidation CoFe Prussian blue (CFPB) nanoframes is presented to provide sustained, external energy-free self-supply of ‧OH from H2O to process CDT and/or photothermal therapy (PTT). Unexpectedly, the as-prepared CFPB nanocubes with no near-infrared (NIR) absorption is transformed into CFPB nanoframes with NIR absorption due to the increased Fe3+-N ≡ C-Fe2+ composition through the proposed proton-induced metal replacement reactions. Surprisingly, both the CFPB nanocubes and nanoframes provide for the self-supply of O2, H2O2, and ‧OH from H2O, with the nanoframe outperforming in the production of ‧OH. Simulation analysis indicates separated active sites in catalyzation of water oxidation, oxygen reduction, and Fenton-like reactions from CFPB. The liposome-covered CFPB nanoframes prepared for controllable water-driven CDT for male tumoral mice treatments.

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.

Building an Iron Chromophore Incorporating Prussian Blue Analogue for Photoelectrochemical Water Oxidation

The replacement of traditional ruthenium-based photosensitizers with low-cost and abundant iron analogs is a key step for the advancement of scalable and sustainable dye-sensitized water splitting cells. In this proof-of-concept study, a pyridinium ligand coordinated pentacyanoferrate(II) chromophore is used to construct a cyanide-based CoFe extended bulk framework, in which the iron photosensitizer units are connected to cobalt water oxidation catalytic sites through cyanide linkers. The iron-sensitized photoanode exhibits exceptional stability for at least 5 h at pH 7 and features its photosensitizing ability with an incident photon-to-current conversion capacity up to 500 nm with nanosecond scale excited state lifetime. Ultrafast transient absorption and computational studies reveal that iron and cobalt sites mutually support each other for charge separation via short bridging cyanide groups and for injection to the semiconductor in our proof-of-concept photoelectrochemical device. The reorganization of the excited states due to the mixing of electronic states of metal-based orbitals subsequently tailor the electron transfer cascade during the photoelectrochemical process. This breakthrough in chromophore-catalyst assemblies will spark interest in dye-sensitization with robust bulk systems for photoconversion applications.

Deposition of zinc cobaltite nanoparticles onto bismuth vanadate for enhanced photoelectrochemical water splitting

During the past few decades, photoelectrochemical (PEC) water splitting has attracted significant attention because of the reduced production cost of hydrogen obtained by utilizing solar energy. Significant efforts have been invested by the scientific community to produce stable ternary metal oxide semiconductors, which can enhance the stability and increase the overall production of oxygen. Herein, we present the ternary metal oxide deposition of ZnCo₂O₄ as a route to obtain a novel photocatalyst layer on BiVO₄ to form BiVO₄/ZnCo₂O₄ a novel composite photoanode for PEC water splitting. The structural, topographical, and optical analyses were performed using field emission scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy, and UV-Vis spectroscopy to confirm the structure of the ZnCo₂O₄ grafted over BiVO₄. A remarkable 4.4-fold enhancement of the photocurrent was observed for the BiVO₄/ZnCo₂O₄ composite compared with bare BiVO₄ under visible illumination. The optimum loading of ZnCo₂O₄ over BiVO₄ yields unprecedented stable photocurrent density with an apparent cathodic shift of 0.46 V under 1.5 AM simulated light illumination. This is also evidenced by the flat-band potential change through Mott-Schottky analysis, which reveals the formation of p-ZnCo₂O₄ on n-BiVO₄. The improvement in the PEC performance of the composite with respect to bare BiVO₄ is ascribed to the formation of thin passivating layer of p-ZnCo₂O₄ on n-BiVO₄ which improves the kinetics of interfacial charge transfer. Based on our study, we have gained an in-depth understanding of the BiVO₄/ZnCo₂O₄ composite as high potential in efficient PEC water splitting devices.

Water oxidation kinetics of nanoporous BiVO4 photoanodes functionalised with nickel/iron oxyhydroxide electrocatalysts

In this work, spectroelectrochemical techniques are employed to analyse the catalytic water oxidation performance of a series of three nickel/iron oxyhydroxide electrocatalysts deposited on FTO and BiVO₄, at neutral pH. Similar electrochemical water oxidation performance is observed for each of the FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts studied, which is found to result from a balance between degree of charge accumulation and rate of water oxidation. Once added onto BiVO₄ photoanodes, a large enhancement in the water oxidation photoelectrochemical performance is observed in comparison to the un-modified BiVO₄. To understand the origin of this enhancement, the films were evaluated through time-resolved optical spectroscopic techniques, allowing comparisons between electrochemical and photoelectrochemical water oxidation. For all three catalysts, fast hole transfer from BiVO₄ to the catalyst is observed in the transient absorption data. Using operando photoinduced absorption measurements, we find that water oxidation is driven by oxidised states within the catalyst layer, following hole transfer from BiVO₄. This charge transfer is correlated with a suppression of recombination losses which result in remarkably enhanced water oxidation performance relative to un-modified BiVO₄. Moreover, despite similar electrocatalytic behaviour of all three electrocatalysts, we show that variations in water oxidation performance observed among the BiVO₄/MOOH photoanodes stem from differences in photoelectrochemical and electrochemical charge accumulation in the catalyst layers. Under illumination, the amount of accumulated charge in the catalyst is driven by the injection of photogenerated holes from BiVO₄, which is further affected by the recombination loss at the BiVO₄/MOOH interface, and thus leads to deviations from their behaviour as standalone electrocatalysts.

Elucidating the role of charge accumulation and reaction kinetics in governing the performance of Ni/Fe oxyhydroxides as electrocatalysts and as co-catalysts on BiVO₄ photoanodes water oxidation.

How to Build Prussian Blue Based Water Oxidation Catalytic Assemblies: Common Trends and Strategies

Prussian blue (PB) and its analogues (PBAs) have at least a three-century-long history in coordination chemistry. Recently, cobalt-based PBAs have been acknowledged as efficient and robust water oxidation catalysts. Given the flexibility in their synthesis, the structure and morphology of cobalt-based PBAs have been modified for enhanced catalytic activity under electrochemical (EC), photocatalytic (PC), and photoelectrochemical (PEC) conditions. Here, in this review, the work on cobalt-based PBAs is presented in four sections: i) electrocatalytic water oxidation with bare PBAs, ii) photocatalytic processes in the presence of a photosensitizer (PS), iii) photoelectrochemical water oxidation by coupling PBAs to proper semiconductors (SCs), and iv) the utilization of PBA-PS assemblies coated on SCs for the dye-sensitized photoelectrochemical water oxidation. This review will guide readers through the structure and catalytic activity relationship in cobalt-based PBAs by describing the role of each structural component. Furthermore, this review aims to provide insight into common strategies to enhance the catalytic activity of PBAs.

Precious Metal-Free Photocatalytic Water Oxidation by a Layered Double Hydroxide-Prussian Blue Analogue Hybrid Assembly

The development of earth-abundant photocatalytic assemblies has been one of the bottlenecks for the advancement of scalable water splitting cells. In this study, a ZnCr layered double hydroxide and a CoFe Prussian blue analogue are combined to afford an earth-abundant photocatalytic assembly involving a visible light-absorbing semiconductor (SC) and a water oxidation catalyst (WOC). Compared to bare ZnCr-LDH, the SC-WOC hybrid assembly exhibits a threefold enhancement in photocatalytic activity, which is maintained for 6 h under photocatalytic conditions at pH 7. The band energy diagram was extracted from optical and electrochemical studies to elucidate the origin of the enhanced photocatalytic performance. This study marks a straightforward pathway to develop low-cost and precious metal-free assemblies for visible light-driven water oxidation.

Photocatalytic water oxidation with a Prussian blue modified brown TiO2

A recently emerging visible light-absorbing semiconductor, brown TiO₂ (b-TiO₂), was coupled with a CoFe Prussian blue (PB) analogue to prepare an entirely earth-abundant semiconductor/water oxidation catalyst hybrid assembly. PB/b-TiO₂ exhibits a sevenfold higher photocatalytic water oxidation activity compared to b-TiO₂. An elegant band alignment unified with the optical absorption of b-TiO₂ and excellent electronic dynamics of PB yield a high-performance photocatalytic system.

Defects as catalytic sites for the oxygen evolution reaction in Earth-abundant MOF-74 revealed by DFT

The oxygen evolution reaction (OER) is the bottleneck of hydrogen production via water splitting and understanding electrocatalysts at atomic level becomes paramount to enhance the efficiency of this process.

Electrochemical transformation of Prussian blue analogues into ultrathin layered double hydroxide nanosheets for water splitting

Herein, we demonstrate a template directed route for the synthesis of self-supported cobalt-iron based Prussian blue analogues (PBAs). The PBAs are electrochemically transformed into layered double hydroxides to produce excellent water oxidation and hydrogen evolution activity, while the overall water splitting is attained at a cell voltage of 1.58 V to reach 20 mA cm-2 current density.

Strong Light-Matter Interactions in Au Plasmonic Nanoantennas Coupled with Prussian Blue Catalyst on BiVO4 for Photoelectrochemical Water Splitting

A facial and large-scale compatible fabrication route is established, affording a high-performance heterogeneous plasmonic-based photoelectrode for water oxidation that incorporates a CoFe-Prussian blue analog (PBA) structure as the water oxidation catalytic center. For this purpose, an angled deposition of gold (Au) was used to selectively coat the tips of the bismuth vanadate (BiVO₄ ) nanostructures, yielding Au-capped BiVO₄ (Au-BiVO₄ ). The formation of multiple size/dimension Au capping islands provides strong light-matter interactions at nanoscale dimensions. These plasmonic particles not only enhance light absorption in the bulk BiVO₄ (through the excitation of Fabry-Perot (FP) modes) but also contribute to photocurrent generation through the injection of sub-band-gap hot electrons. To substantiate the activity of the photoanodes, the interfacial electron dynamics are significantly improved by using a PBA water oxidation catalyst (WOC) resulting in an Au-BiVO₄ /PBA assembly. At 1.23 V (vs. RHE), the photocurrent value for a bare BiVO₄ photoanode was obtained as 190 μA cm^-2 , whereas it was boosted to 295 μA cm^-2 and 1800 μA cm^-2 for Au-BiVO₄ and Au-BiVO₄ /PBA, respectively. Our results suggest that this simple and facial synthetic approach paves the way for plasmonic-based solar water splitting, in which a variety of common metals and semiconductors can be employed in conjunction with catalyst designs.

Effects of Structure and Constituent of Prussian Blue Analogs on Their Application in Oxygen Evolution Reaction

The importance of advanced energy-conversion devices such as water electrolysis has manifested dramatically over the past few decades because it is the current mainstay for the generation of green energy. Anodic oxygen evolution reaction (OER) in water splitting is one of the biggest obstacles because of its extremely high kinetic barrier. Conventional OER catalysts are mainly noble-metal oxides represented by IrO2 and RuO2, but these compounds tend to have poor sustainability. The attention on Prussian blue (PB) and its analogs (PBA) in the field of energy conversion systems was concentrated on their open-framework structure, as well as its varied composition comprised of Earth-abundant elements. The unique electronic structure of PBA enables its promising catalytic potential, and it can also be converted into many other talented compounds or structures as a precursor. This undoubtedly provides a new approach for the design of green OER catalysts. This article reviews the recent progress of the application of PBA and its derivatives in OER based on in-depth studies of characterization techniques. The structural design, synthetic strategy, and enhanced electrochemical properties are summarized to provide an outlook for its application in the field of OER. Moreover, due to the similarity of the reaction process of photo-driven electrolysis of water and the former one, the application of PBA in photoelectrolysis is also discussed.

Quasi-degenerate states and their dynamics in oxygen deficient reducible metal oxides

The physical and chemical properties of oxides are defined by the presence of oxygen vacancies. Experimentally, non-defective structures are almost impossible to achieve due to synthetic constraints. Therefore, it is crucial to account for vacancies when evaluating the characteristics of these materials. The electronic structure of oxygen-depleted oxides deeply differs from that of the native forms, in particular, of reducible metal oxides, where excess electrons can localize in various distinct positions. In this perspective, we present recent developments from our group describing the complexity of these defective materials that highlight the need for an accurate description of (i) intrinsic vacancies in polar terminations, (ii) multiple geometries and complex electronic structures with several states attainable at typical working conditions, and (iii) the associated dynamics for both vacancy diffusion and the coexistence of more than one electronic structure. All these aspects widen our current understanding of defects in oxides and need to be adequately introduced in emerging high-throughput screening methodologies.

The Positive Effect of Iron Doping in the Electrocatalytic Activity of Cobalt Hexacyanoferrate

The lack of an earth-abundant, robust, and fast electrocatalyst for the oxygen evolution reaction (water oxidation) is a major bottleneck for the development of an scalable scheme towards the production of electrolytic hydrogen and other synthetic fuels from renewable energy and natural feedstocks. While many transition metal oxides work reasonably well in basic media, very few alternatives are available in neutral or acidic media. One promising candidate comes from the Prussian blue family, cobalt hexacyanoferrate. This electrocatalyst offers robust activity in a large pH range ( 0 < pH < 13 ), although current densities are limited due to slow charge transfer kinetics. Herein, we report how the partial substitution of catalytically active Co centres by additional Fe boosts current densities, reaching over 100 mA/cm 2 , more than double the performance of the parent Co 2 [Fe(CN) 6 ]. Those new results clearly increase the opportunity for this catalyst to become relevant in industrial-ready electrolyser architectures.

Enhanced Photoelectrocatalytical Performance of Inorganic-Inorganic Hybrid Consisting BiVO4, V2O5, and Cobalt Hexacyanocobaltate as a Perspective Photoanode for Water Splitting

Thin layers of BiVO4/V2O5 were prepared on FTO substrates using pulsed laser deposition technique. The method of cobalt hexacyanocobaltate (Cohcc) synthesis on the BiVO4/V2O5 photoanodes consists of cobalt deposition followed by electrochemical oxidation of metallic Co in K3[Co(CN)6] aqueous electrolyte. The modified electrodes were tested as photoanodes for water oxidation under simulated sunlight irradiation. Deposited films were characterized using UV-Vis spectroscopy, Raman spectroscopy, and scanning electron microscopy. Since the V2O5 is characterized by a narrower energy bandgap than BiVO4, the presence of V2O5 shifts absorption edge (ΔE = ~0.25 eV) of modified films towards lower energies enabling the conversion of a wider range of solar radiation. The formation of heterojunction increases photocurrent of water oxidation measured at 1.2 V vs Ag/AgCl (3 M KCl) to over 1 mA cm-2, while bare BiVO4 and V2O5 exhibit 0.37 and 0.08 mA cm-2, respectively. On the other hand, the modification of obtained layers with Cohcc shifts onset potential of photocurrent generation into a cathodic direction. As a result, the photocurrent enhancement at a wide range of applied potential was achieved.

Insight into the PEC and interfacial charge transfer kinetics at the Mo doped BiVO4 photoanodes

BiVO4 is a promising photoanode material for the photoelectrochemical (PEC) oxidation of water; however, its poor charge transfer, transport, and slow surface catalytic activity limit the expected theoretical efficiency. Herein, we have investigated the effect of Mo doping on SnO2 buffer layer coated BiVO4 for PEC water splitting. SnO2 and Mo doped BiVO4 layers are coated with layer by layer deposition through a precursor solution based spin coating technique followed by annealing. At 5% doping of Mo, the sample (SBM5) shows a maximum current density of 1.65 mA cm-2 at 1.64 V vs. RHEl in 0.1 M phosphate buffer solution under AM 1.5 G solar simulator, which is about 154% improvement over the sample without Mo (SBM0). The significant improvement in the photocurrent upon Mo doping is due to the improvement of various bulk and interfacial properties in the materials as measured by UV-vis spectroscopy, electrochemical impedance spectroscopy (EIS), Mott-Schottky analysis, and open-circuit photovoltage (OCPV). The charge transfer kinetics at the BiVO4/electrolyte interface are investigated to simulate the oxygen evolution process in photoelectrochemical water oxidation in the feedback mode of scanning electrochemical microscopy (SECM) using 2 mM [Fe(CN)6]3- as the redox couple. SECM investigation reveals a significant improvement in effective hole transfer rate constant from 2.18 cm s-1 to 7.56 cm s-1 for the hole transfer reaction from the valence band of BiVO4 to [Fe(CN)6]4- to oxidize into [Fe(CN)6]3- with the Mo doping in BiVO4. Results suggest that Mo6+ doping facilitates the hole transfer and suppresses the back reaction. The synergistic effect of fast forward and backward conversion of Mo6+ to Mo5+ expected to facilitate the V5+ to V4+ which has an important step to improve the photocurrent.

Versatile Nature of Oxygen Vacancies in Bismuth Vanadate Bulk and (001) Surface

Bismuth vanadate (BiVO₄) has emerged as one of the most promising photoanode materials for solar fuel production. Oxygen vacancies play a pivotal role in the photoelectrochemical efficiency, yet their electronic nature and contribution to  n-type conductivity are still under debate. Using first-principles calculations, we show that oxygen vacancies in BiVO₄ have two distinguishable geometric configurations characterized by either undercoordinated, reduced V^IVO₃ and Bi^IIO₇ subunits or a V^IV-O-V^IV/V bridge (split vacancy), quenching the oxygen vacancy site. While both configurations have similar energies in the bulk, the (001) subsurface acts like an energetic sink that stabilizes the split oxygen vacancy by ∼1 eV. The barrierless creation of a bridging V₂O₇ unit allows for partial electron delocalization throughout the near-surface region, consistent with recent experimental observations indicating that BiVO₄(001) is an electron-rich surface.

Computational Approaches to Photoelectrode Design through Molecular Functionalization for Enhanced Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is a promising carbon-free approach to produce hydrogen from water. A photoelectrochemical cell consists of a semiconductor photoelectrode in contact with an aqueous electrolyte. Its performance is sensitive to properties of the photoelectrode/electrolyte interface, which may be tuned through functionalization of the photoelectrode surface with organic molecules. This can lead to improvements in the photoelectrode's properties. This Minireview summarizes key computational investigations on using molecular functionalization to modify photoelectrode stability, barrier height, and catalytic activity. It is discussed how first-principles density functional theory, first-principles molecular dynamics, and device modeling simulations can provide predictive insights and complement experimental investigations of functionalized photoelectrodes. Challenges and future directions in the computational modeling of functionalized photoelectrode/electrolyte interfaces within the context of experimental studies are also highlighted.

Modified Electrodeposited Cobalt Foam Coatings as Sensors for Detection of Free Chlorine in Water

Metal foams offer a substantial specific surface area and sturdy frame, which makes them great candidates for various applications such as catalysts, sensors, heat sinks, etc. Cobalt and its various compounds are being considered as a cheaper alternative for precious and rare metal catalysts. The cobalt foams have been electrodeposited under galvanostatic and current pulse modes; the porous surface was created using a dynamic hydrogen bubble template. In order to obtain the highest porosity, four different solutions were tested, as well as a wide current density window (0.6–2.5 A/cm²), in addition many different combinations of pulse durations were applied. The effects of surfactant (isopropanol) on porosity were also investigated. The morphology of obtained foams was examined by SEM coupled with EDS, and XRD spectroscopy. True surface area was estimated based on the values of a double electric layer capacitance that was extracted from EIS data. Cobalt foams were modified using K3[Fe(CN)6] solution and cyclic voltammetry to form a cobalt hexacyanoferrate complex on the foam surface. In order to find optimal modification conditions, various potential scan rates and numbers of cycles were tested as well. Free chlorine sensing capabilities were evaluated using chronoamperometry.

Elaborately Modified BiVO4 Photoanodes for Solar Water Splitting

Photoelectrochemical (PEC) cells for solar-energy conversion have received immense interest as a promising technology for renewable hydrogen production. Their similarity to natural photosynthesis, utilizing sunlight and water, has provoked intense research for over half a century. Among many potential photocatalysts, BiVO₄ , with a bandgap of 2.4-2.5 eV, has emerged as a highly promising photoanode material with a good chemical stability, environmental inertness, and low cost. Unfortunately, its charge transport properties are modest, at most a hole diffusion length (L_p ) of ≈70 nm. However, recent rapid developments in multiple modification strategies have elevated it to a position as the most promising metal oxide photoanode material. This review summarizes developments in BiVO₄ photoanodes in the past 10 years, in which time it has continuously broken its own performance records for PEC water oxidation. Effective modification techniques are discussed, including synthesis of nanostructures/nanopores, external/internal doping, heterojunction fabrication, surface passivation, and cocatalysts. Tandem systems for unassisted solar water splitting and PEC production of value-added chemicals are also discussed.

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.

Electrochemical water oxidation by cobalt-Prussian blue coordination polymer and theoretical studies of the electronic structure of the active species

The search for earth-abundant metal-based catalysts for the oxygen evolution reaction (OER) that operates in neutral conditions is a challenge in the field of sustainable energy.

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.

Visible light-driven water oxidation with a ruthenium sensitizer and a cobalt-based catalyst connected with a polymeric platform

A novel PS–WOC dyad which incorporates a ruthenium-based photosensitizer (PS) connected to a Prussian blue type water oxidation catalyst (WOC) through a P4VP platform is presented.

Hierarchical organization of perylene bisimides and polyoxometalates for photo-assisted water oxidation

The oxygen in Earth's atmosphere is there primarily because of water oxidation performed by photosynthetic organisms using solar light and one specialized protein complex, photosystem II (PSII). High-resolution imaging of the PSII 'core' complex shows the ideal co-localization of multi-chromophore light-harvesting antennas with the functional reaction centre. Man-made systems are still far from replicating the complexity of PSII, as the majority of PSII mimetics have been limited to photocatalytic dyads based on a 1:1 ratio of a light absorber, generally a Ru-polypyridine complex, with a water oxidation catalyst. Here we report the self-assembly of multi-perylene-bisimide chromophores (PBI) shaped to function by interaction with a polyoxometalate water-oxidation catalyst (Ru₄POM). The resulting [PBI]₅Ru₄POM complex shows a robust amphiphilic structure and dynamic aggregation into large two-dimensional paracrystalline domains, a redshifted light-harvesting efficiency of >40% and favourable exciton accumulation, with a peak quantum efficiency using 'green' photons (λ > 500 nm). The modularity of the building blocks and the simplicity of the non-covalent chemistry offer opportunities for innovation in artificial photosynthesis.

Bismuth Vanadate with Electrostatically Anchored 3D Carbon Nitride Nano-networks as Efficient Photoanodes for Water Oxidation

In this study, we report a photoanode consisting of a polymeric/inorganic nanojunction between novel nanostructured 3D C₃ N₄ nano-networks and BiVO₄ substrate. This nanojunction is formed such that 3D C₃ N₄ nano-networks with a positively charged surface are efficiently anchored on the BiVO₄ photoanode with a negatively charged surface. This electrostatic self-assembly can initiate and contribute to an intimate contact at the interfaces, leading to an enhanced photoelectrochemical activity and stability compared with that fabricated by non-electrostatic assembly. The C₃ N₄ nano-network/BiVO₄ photoanode achieved a remarkable photocurrent density of 4.87 mA cm^-2 for water oxidation at 1.23 V (vs. reversible hydrogen electrode) after depositing FeOOH/NiOOH as oxygen-evolution co-catalyst, which is among the highest photocurrent densities reported so far for BiVO₄ -based photoanodes.