Water Oxidation Cocatalyst Research Articles

Dual Cocatalysts on Covalent Organic Framework as Charge Transfer Mediator Facilitating Photocatalytic Water Oxidation

AbstractCovalent organic frameworks (COFs) have been regarded as promising photocatalytic materials. However, the sluggish diffusion of photoinduced charges in a single‐component COF remains the obstacles that have to be overcome. Here, we demonstrate a synthesis of a COF bearing dual cocatalysts Co3O4 and Re complex as photocatalyst for water oxidation. The dual cocatalysts are coordinated on the COF, and enable the spatial separation of photoinduced charges. The generated electrons were attracted to Re moiety, while the holes were localized on Co3O4 cocatalyst for water oxidation to produce O2 product. The synergy of dual cocatalysts of COF promote the charge separation and redox reaction. Moreover, hydroxyl radicals generated from the hydration of COF contribute to O−O bond formation, which is the key step of O2 formation. Accordingly, an excellent O2 production rate of 400 μmol g−1 h−1 under visible light irradiation was obtained. This work may pave the way to the development of highly efficient COF photocatalysts for solar energy conversion.

Simultaneous enhancement of charge transfer and surface catalysis through a polymetallic oxide cocatalyst on BiVO4 photoanodes for highly efficient and stable water oxidation

The rate-determining oxygen evolution reaction (OER) always limits the high-efficient conversion of solar energy to green hydrogen fuels through photoelectrocatalytic or photocatalytic water splitting. The high catalytic overpotential and the instability of catalytic center are commonly regarded as the primary factors contributing to the low rate of OER, remaining to be a challenge in the field of water splitting. Herein, a polymetallic oxide cocatalyst (Mo-MnOy/FeCoNiOx) with well-defined electronic and catalytic properties is designed on BiVO4 photoelectrode for highly efficient and stable photoelectrocatalytic water oxidation. The experimental characterization demonstrates that the dual-layer design of Mo-MnOy/FeCoNiOx can significantly optimizes the electronic property of MnOy and FeCoNiOx components, boosting the photogenerated charge transfer between Mo-MnOy/FeCoNiOx cocatalyst and BiVO4 photoelectrode. The density functional theory (DFT) simulation reveals that the Mo sites in Mo-MnOy layer can activate the neighboring surface Mn sites instead of directly serving as the catalytic center, thereby establishing these Mn sites as primary active centers for achieving stable OER. The developed Mo-MnOy/FeCoNiOx/BiVO4 photoelectrode exhibits a current density of 6.18 mA cm−2 with an excellent stability for 30 h at 1.23 VRHE under 1 sun irradiation, exhibiting the excellent activity and durability. This work sheds light on design of high-performance multiple-component water-oxidation cocatalyst on photoanode.

Supramolecular catalyst with [FeCl4] unit boosting photoelectrochemical seawater splitting via water nucleophilic attack pathway

Propelled by the structure of water oxidation co-catalysts in natural photosynthesis, molecular co-catalysts have long been believed to possess the developable potential in artificial photosynthesis. However, the interfacial complexity between light absorber and molecular co-catalyst limits its structural stability and charge transfer efficiency. To overcome the challenge, a supramolecular scaffold with the [FeCl4] catalytic units is reported, which undergo a water-nucleophilic attack of the water oxidation reaction, while the supramolecular matrix can be in-situ grown on the surface of photoelectrode through a simple chemical polymerization to be a strongly coupled interface. A well-defined BiVO4 photoanode hybridized with [FeCl4] units in polythiophene reaches 4.72 mA cm−2 at 1.23 VRHE, which also exhibits great stability for photoelectrochemical seawater splitting due to the restraint on chlorine evolution reaction by [FeCl4] units and polythiophene. This work provides a novel solution to the challenge of the interface charge transfer of molecular co-catalyst hybridized photoelectrode.

High-performance BiVO4 photoanodes: elucidating the combined effects of Mo-doping and modification with cobalt polyoxometalate

Significant water oxidation reactivity enhancement in BiVO4 photoanodes is achieved by simultaneous bulk doping with molybdenum and surface-modification with a polyoxometalate water oxidation co-catalyst.

MoO3/BiVO4 heterojunction modified with cobalt-manganese layered double hydroxide cocatalyst for photoelectrochemical water oxidation

To improve the charge recombination and poor oxygen evolution reaction (OER) kinetics of BiVO4, in this paper, a high-performance CoMn-LDH/MoO3/BiVO4 photoanode was synthesized by a simple electrodeposition and calcination method. CoMn-LDH/MoO3/BiVO4 shows a photocurrent density of 3.78 mA cm−2 at 1.23 V vs. RHE, which is 3.78 times higher than that of pure BiVO4 with negative shift onset potentials of 330 mV. The maximum applied bias photon-to-current efficiency (ABPE) and incident photon-to-current conversion efficiency (IPCE) are 1.24 % and 55.5 %, respectively, which are 13.8 and 12.5 times higher than those of pure BiVO4. In-depth studies demonstrate that the MoO3/BiVO4 heterojunction can better drive the transfer of photogenerated carriers and suppress bulk charge recombination, CoMn-LDH improves the OER kinetics and broadens the absorption of visible light, their synergistic effect significantly enhances charge separation and light harvesting of BiVO4. This work provides a facile electrodeposition method to fabricate highly efficient photoanodes for solar-driven water splitting.

Amorphous FeOOH-mediated directional charges transfer in light-switchable oxygen vacancies BiOBr for boosting photocatalytic oxygen evolution

Directing the transfer of photoinduced charges to various reaction sites through catalyst design plays crucial roles in enhancing photocatalytic oxygen evolution. Herein, we used a facile alcoholysis method to synthesize amorphous FeOOH modified light-switchable oxygen vacancies BiOBr, which exhibited outstanding photocatalytic oxygen evolution activity. Density functional theory calculations, photoelectronchemical tests and kelvin probe force microscopy revealed that FeOOH overlayer acts as the hole storage layer, while oxygen vacancies (OVs) provided a number of electron trap centers. Benefiting from this synergistic effect, the photogenerated charges showed an outstanding directional transfer performance, which greatly prolong the lifetime of the photo-induced charge carriers. Moreover, as a cocatalyst for water oxidation, FeOOH can effectively lower the kinetics barriers and provide more active sites for oxygen evolution reaction. Therefore, under simulated sunlight irradiation, the 2%FeOOH/BiOBr photocatalyst demonstrated remarkable photocatalytic activity towards oxygen evolution, achieving a rate of 284.34 μmol·g−1·h−1, which is around 1.71 folds than pure BiOBr. Finally, the possible reaction mechanism has also been discussed. This work has provided valuable insights and inspiration for the effective design and optimization of BiOBr-based catalysts, which play a critical role in facilitating efficient directional transfer of photogenerated charges.

Embedding cobalt polyoxometalate in polypyrrole shell for improved photoelectrochemical performance of BiVO4 core

Bismuth vanadate (BiVO4), as an appropriate semiconducting material with a narrow band gap and high light-harvesting, suffers from a high charge recombination rate and low photoactivity. To address these issues, in this work, the embedded cobalt polyoxometalate (POM) within the network of polypyrrole (PPy) coated on BiVO4 has been proposed to promote charge injection and separation. The photoanode based on BiVO4 was fabricated by electrophoretic deposition (EPD) under the optimized conditions. Conducting a comprehensive and fundamental study, we also implemented experiments to achieve the best conditions for BiVO4 preparation and the creation and optimization of PPy heterojunction with and without POM. The photoelectrochemical and physical analysis results revealed that the simultaneous incorporation of PPy and POM makes BiVO4 experience an impressive current enhancement, a 900% increase relative to bare BiVO4. Moreover, the hole extraction efficiency was enhanced to 93.7% from 15.12% (BiVO4), and charge separation yielded 33.83% from 24.47% (BiVO4). This study inspires promising studies on heterojunction formation through inorganic/organic semiconductor hybridization, achieved by molecular water oxidation co-catalysts like POM.

N-Si/SiOx /CoOx -Mo Photoanode for Efficient Photoelectrochemical Water Oxidation.

Green hydrogen is considered to be the key for solving the emerging energy and environmental issues. The photoelectrochemical (PEC) process for the production of green hydrogen has been widely investigated because solar power is clean and renewable. However, mass production in this way is still far away from reality. Here, a Si photoanode is reported with CoOx as co-catalyst for efficient water oxidation. It is found that a high photovoltage of 350mV can be achieved in 1.0m K3 BO3 . Importantly, the photovoltage can be further increased to 650mV and the fill factor of 0.62 is obtained in 1.0m K3 BO3 by incorporating Mo into CoOx . The Mo-incorporated photoanode is also highly stable. It is shown that the incorporation of Mo can reduce the particle size of co-catalyst on the Si surface, improve the particle-distribution uniformity, and increase the density of particles, which can effectively enhance the light absorption and the electrochemical active surface area. Importantly, the Mo-incorporation results in high energy barrier in the heterojunction. All of these factors are attributed to improved the PEC performance. These findings may provide new strategies to maximize the solar-to-fuel efficiency by tuning the co-catalysts on the Si surface.

Modulating Oxygen Vacancies in Lead Chromate forPhotoelectrocatalytic Water Splitting.

Although modulating oxygen vacancies in semiconductors has attracted broad interest in photocatalysis and photoelectrocatalysis, identifying the intrinsic roles of oxygen vacancies on photoelectrocatalytic properties is often elusive. In this work, the oxygen vacancies in a typical semiconductor lead chromate (PbCrO4 ) are regulated via controlling the oxygen chemical potentials of O-poor and O-rich post-annealing atmospheres. Oxygen vacancies identified in PbCrO4 can introduce electronically shallow energy levels and deep energy levels owing to the symmetry difference of oxygen atoms in the structure. A higher population of deep energy levels created under O-poor atmosphere induces the formation of more surface trapped states, resulting in a higher photovoltage for charge separation. Meanwhile, the existence of surface trapped states can significantly improve the charge injection efficiency of the PbCrO4 photoanode and enhance the water oxidation activity. By modulating oxygen vacancies in the PbCrO4 photoanode, a photocurrent density of 3.43mA cm-2 at 1.23V vs reversible hydrogen electrode (RHE) under simulated AM1.5G is acheived. Further passivation of surface trapped states and introducing the water oxidation cocatalyst CoPi lead to a record applied bias photon-to-current efficiency (ABPE) of 1.12%. This work provides a guide to understand the mechanism of oxygen vacancies in oxide-based semiconductor photocatalysis and photoelectrocatalysis.

Graphene Mediates Charge Transfer between Lead Chromate and a Cobalt Cubane Cocatalyst for Photocatalytic Water Oxidation.

The interfacial barrier of charge transfer from semiconductors to cocatalysts means that the photogenerated charges cannot be fully utilized, especially for the challenging water oxidation reaction. Using cobalt cubane molecules (Co4 O4 ) as water oxidation cocatalysts, we rationally assembled partially oxidized graphene (pGO), acting as a charge-transfer mediator, on the hole-accumulating {-101} facets of lead chromate (PbCrO4 ) crystal. The assembled pGO enables preferable immobilization of Co4 O4 molecules on the {-101} facets of the PbCrO4 crystal, which is favorable for the photogenerated holes transferring from PbCrO4 to Co4 O4 molecules. The surface charge-transfer efficiency of PbCrO4 was boosted by selective assembly of pGO between PbCrO4 and Co4 O4 molecules. An apparent quantum efficiency for photocatalytic water oxidation on the Co4 O4 /pGO/PbCrO4 photocatalyst exceeded 10 % at 500 nm. This strategy of rationally assembling charge-transfer mediator provides a feasible method for acceleration of charge transfer and utilization in semiconductor photocatalysis.

Graphene Mediates Charge Transfer between Lead Chromate and a Cobalt Cubane Cocatalyst for Photocatalytic Water Oxidation

AbstractThe interfacial barrier of charge transfer from semiconductors to cocatalysts means that the photogenerated charges cannot be fully utilized, especially for the challenging water oxidation reaction. Using cobalt cubane molecules (Co4O4) as water oxidation cocatalysts, we rationally assembled partially oxidized graphene (pGO), acting as a charge‐transfer mediator, on the hole‐accumulating {−101} facets of lead chromate (PbCrO4) crystal. The assembled pGO enables preferable immobilization of Co4O4 molecules on the {−101} facets of the PbCrO4 crystal, which is favorable for the photogenerated holes transferring from PbCrO4 to Co4O4 molecules. The surface charge‐transfer efficiency of PbCrO4 was boosted by selective assembly of pGO between PbCrO4 and Co4O4 molecules. An apparent quantum efficiency for photocatalytic water oxidation on the Co4O4/pGO/PbCrO4 photocatalyst exceeded 10 % at 500 nm. This strategy of rationally assembling charge‐transfer mediator provides a feasible method for acceleration of charge transfer and utilization in semiconductor photocatalysis.

Interfacial band alignment and photoelectrochemical properties of all-sputtered BiVO4/FeNiOx and BiVO4/FeMnOx p–n heterojunctions

This work introduces FeMnOx as a novel effective cocatalyst for PEC water oxidation and provides simple tools for comprehending how interfacial effects influence the PEC performance of all-sputtered BiVO4/FeNiOx and BiVO4/FeMnOx p–n heterojunctions.

Unveiling the activity and stability of BiVO4 photoanodes with cocatalyst for water oxidation

Developing low-cost and efficient cocatalyst is highly desirable for photoelectrochemical (PEC) water oxidation. FeOOH is one of the most typical cocatalysts, the active sites of FeOOH in PEC remain unclear. Herein, the FeOOH was deposited on BiVO4 photoanodes through electrodeposition. A series of experimental evidence shows that the Bi–O–Fe bond is beneficial to the improvement of water oxidation activity, while the FeOOH can not restrain the dissolution of V5+ and improve the stability of the photoelectrode. In order to further improve the activity and stability of BiVO4/FeOOH photoanode, a low-cost metal-organic coordination composed of tannic acid coordinated with Ni ions (TANi) was assembled on the BiVO4/FeOOH (denoted as BiVO4/FeOOH/TANi). Under AM 1.5 G illumination, an impressive current density of 4.6 mA cm−2 (1.23 V vs. RHE) was obtained for BiVO4/FeOOH/TANi photoanode. Systematic studies reveal that the enhanced PEC activity comes from the protection of BiVO4 and enhancement of charge separation efficiency. This work provides a strategy for designing cocatalysts to construct efficient and stable PEC water oxidation.

Hybrid Ce-Fe2O3/ZIF-67 Photoanode with Efficient Photoelectrochemical Water Oxidation Performance.

Surface states and slow water oxidation kinetics greatly limit the photoelectrochemical (PEC) water oxidation performance of Fe2O3. To solve the above problems, coupling Fe2O3 with a passivation layer and an oxygen evolution cocatalyst, respectively, is the common method. Though this method may improve its PEC performance, this also results in a low charge-transfer efficiency caused by the interface resistance between Fe2O3 and the modification materials (passivation layer and oxygen evolution cocatalyst). Therefore, it is important to identify a multifunctional modifier material to reduce the interfacial resistance due to the presence of multiple different materials. In this work, we introduced a thin cobalt-based metal-organic framework layer (ZIF-67) as a dual-functional material that acted as both a passivation layer and a water oxidation cocatalyst for a photoanode based on Ce-Fe2O3 nanorod arrays. The ZIF-67 layer inhibited charge carrier recombination by passivating the surface states. The PEC performance was improved due to the rich Co2+ photogenerated hole-capture sites, which facilitated charge transfer and separation. As expected, the Ce-Fe2O3/ZIF-67 photoanode exhibited superior water oxidation performance, with a photocurrent of 2.07 mA cm-2 at 1.23 VRHE, which is 1.74 times higher than that of the Ce-Fe2O3 photoanode. The onset potential was negatively shifted by 71 mV. This study provides basic insights and a strategy for reducing interfacial resistance in hybrid materials.

Wide-pH-compatible MoSx co-catalyst layer on TiO2 nanowire arrays photoanode for simultaneous acceleration of charge carrier separation and catalytic reactions

The water oxidation reaction is a key reaction for solar water splitting, which requires excellent oxygen evolution co-catalyst to accelerate charges separation efficiency and high catalytic reaction sites for O2 releasing. So far, many earth-abundant co-catalysts can only play roles at a single pH (only in neutral or alkaline electrolyte), but the development of wide-pH compatible co-catalyst with low price are rarely obtained. Herein, amorphous MoSx layer was conformally coated on TiO2 nanowire arrays (NAs) as an active co-catalyst for multi-pH water photo-oxidation. The experimental results showed that the MoSx greatly enhanced the photoelectrochemical (PEC) performance of TiO2 NAs with high photocurrent densities of 1.95, 1.67, and 1.55 mA/cm2 at 1.23 VRHE in acidic, alkaline, and neutral electrolyte, respectively, significantly higher than bare TiO2 and most TiO2/co-catalyst literature values. Density functional theory (DFT) calculations revealed that the MoSx on TiO2 improves electronic interaction at their interface and forms a type-II band structure for more efficient separation of charge carriers. It is also found that high-density active sites provided by MoSx can reduce the Gibbs free energy and overpotential of potential-determining step for OER, which further enhance the adsorption activity of reaction intermediates. Our work contributes to the in-depth understanding of amorphous MoSx as efficient water oxidation co-catalyst for multi-pH PEC water splitting.

Immobilization of a [CoIIICoII(H2O)W11O39]7- Polyoxoanion for the Photocatalytic Oxygen Evolution Reaction.

The ongoing transition to renewable energy sources and the implementation of artificial photosynthetic setups call for an efficient and stable water oxidation catalyst (WOC). Here, we heterogenize a molecular all-inorganic [CoIIICoII(H2O)W11O39]7– ({CoIIICoIIW11}) Keggin-type polyoxometalate (POM) onto a model TiO2 surface, employing a 3-aminopropyltriethoxysilane (APTES) linker to form a novel heterogeneous photosystem for light-driven water oxidation. The {CoIIICoIIW11}-APTES-TiO2 hybrid is characterized using a set of spectroscopic and microscopic techniques to reveal the POM integrity and dispersion to elucidate the POM/APTES and APTES/TiO2 binding modes as well as to visualize the attachment of individual clusters. We conduct photocatalytic studies under heterogeneous and homogeneous conditions and show that {CoIIICoIIW11}-APTES-TiO2 performs as an active light-driven WOC, wherein {CoIIICoIIW11} acts as a stable co-catalyst for water oxidation. In contrast to the homogeneous WOC performance of this POM, the heterogenized photosystem yields a constant WOC rate for at least 10 h without any apparent deactivation, demonstrating that TiO2 not only stabilizes the POM but also acts as a photosensitizer. Complementary studies using photoluminescence (PL) emission spectroscopy elucidate the charge transfer mechanism and enhanced WOC activity. The {CoIIICoIIW11}-APTES-TiO2 photocatalyst serves as a prime example of a hybrid homogeneous–heterogeneous photosystem that combines the advantages of solid-state absorbers and well-defined molecular co-catalysts, which will be of interest to both scientific communities and applications in photoelectrocatalysis and CO2 reduction.

Ti-Fe2O3/perylene-3,4,9,10-tetracarboxylic acid heterojunction modified with Co(OH)2 as cocatalyst for photoelectrochemical water oxidation

Interface charge modulation by integrating the extraction layer and cocatalyst with semiconductor is closely linked with the photoelectrochemical (PEC) water oxidation performance, which is aimed at the directed transfer of photogenerated holes from semiconductors to electrolyte. Herein, a hybrid photoanode is elaborately designed for PEC water oxidation, which is fabricated by loading perylene-3,4,9,10-tetracarboxylic acid (PTCA) as a extraction layer on Ti-Fe2O3 for Ti-Fe2O3/PTCA heterojunction followed by the deposition of Co(OH)2 as the water oxidation cocatalyst. On the one hand, the band structure at the Ti-Fe2O3/PTCA interface and the PTCA/Co(OH)2 interface is in favor of the formation of successive interfacial electric filed in Co(OH)2/PTCA/Ti-Fe2O3, which in turn accelerates the hole extraction from Ti-Fe2O3 to Co(OH)2. In addition, the passivating role of Co(OH)2 also has contribution to the effective hole extraction. On the other hand, the in situ formed CoOOH from Co(OH)2 motivates the quick hole injection at the photoanode/electrolyte interface. To be brief, the vectorial transfer of photogenerated holes (Ti-Fe2O3→PTCA→Co(OH)2→electrolyte) is achieved by the co-deposition of PTCA and Co(OH)2. As such, Co(OH)2/PTCA/Ti-Fe2O3 exhibits a photocurrent density of 2.48 mA/cm2 at 1.23 V vs. RHE, which is much superior to Ti-Fe2O3.

Bi-Based Metal-Organic Framework Decorated BiVO4 Photoelectrode for Photoelectrochemical Water Splitting

Decoration of metal-organic frameworks (MOFs) is emerging as attractive co-catalysts for effective photoelectrochemical (PEC) water oxidation. In this work, we report Bi-based MOF nanoparticle decorated BiVO 4 thin film via a multi-step immersion process for efficient PEC water oxidation. Bi-MOF on the surface improves active sites and promotes surface charge transport under PEC reaction. The PEC performance of our Bi-MOF/BiVO 4 electrode is 2 times higher than that of the bare BiVO4 sample. In addition, the operation stability was significantly improved and its value is retained even after 24 h. These results reveal that Bi-based MOF decoration is an attractive strategy to improve the surface kinetics and stability as a co-catalyst and passivation layer for efficient water oxidation.

Facet-dependent spatial charge separation with rational cocatalyst deposition on BiVO4

Rational assembly of cocatalyst on the photocatalyst with exposed functional crystal facets has been demonstrated to be viable in enhancing the photocatalytic activity. However, greater attention is paid to the property of the loaded cocatalyst while the effect of varied functional facet exposure is relatively less discussed. Herein, dual-faceted BiVO4 with well-defined {010} and {110} facets but with varied {010}/{110} exposure extent ratios are coupled with CoOx which is known as a cocatalyst for water oxidation. The enhancement induced by the cocatalyst loading depends on both its deposit location and the ratio of {010}/{110} facet exposure. When selectively deposited on oxidation {110} facet, CoOx generates over two-fold enhancement on BiVO4 in photocatalytic oxygen evolution as compared to its randomly deposited analogue. Meanwhile, the {010}-dominant BiVO4 is more obviously benefited from the cocatalyst loading to both photocatalytic water oxidation and hydrogen peroxide generation reactions with respect to the {110}-dominant BiVO4. Further investigation reveals that CoOx functions more effectively in PL quenching and generating greater band bending on {010}-dominant BiVO4, substantiating the better enhancement in reducing charge recombination and forming larger space charge region width for charge separation, which deliver greater photocatalytic activity enhancement on {010}-dominant BiVO4.

A Semiconductor-Mediator-Catalyst Artificial Photosynthetic System for Photoelectrochemical Water Oxidation.

In fabricating an artificial photosynthesis (AP) electrode for water oxidation, we have devised a semiconductor-mediator-catalyst structure that mimics photosystem II (PSII). It is based on a surface layer of vertically grown nanorods of Fe2 O3 on fluorine doped tin oxide (FTO) electrodes with a carbazole mediator base and a Ru(II) carbene complex on a nanolayer of TiO2 as a water oxidation co-catalyst. The resulting hybrid assembly, FTO|Fe2 O3 |-carbazole|TiO2 |-Ru(carbene), demonstrates an enhanced photoelectrochemical (PEC) water oxidation performance compared to an electrode without the added carbaozle base with an increase in photocurrent density of 2.2-fold at 0.95 V vs. NHE and a negatively shifted onset potential of 500 mV. The enhanced PEC performance is attributable to carbazole mediator accelerated interfacial hole transfer from Fe2 O3 to the Ru(II) carbene co-catalyst, with an improved effective surface area for the water oxidation reaction and reduced charge transfer resistance.