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

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.

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.

Ni-Doped BiVO4 photoanode for efficient photoelectrochemical water splitting

BiVO₄ (BVO) based photoanode is one of the most mega-potential materials for solar water splitting while suffers from poor charge transfer and separation efficiency limit its practical application. Herein, FeOOH/Ni-BiVO₄ photoanode synthesized by the facile wet chemical method were investigated for improved charge transport and separation efficiency. The photoelectrochemical (PEC) measurements demonstrate that the water oxidation photocurrent density can reach as high as 3.02 mA cm^-2 at 1.23 V vs RHE, and the surface separation efficiency can be boosted to 73.3 %, which increases around 4 times comparing with that of pure sample. Further depth studies showed that the Ni doping can effectively promote hole transport/trapping and introduce more active sites for the oxidation of water, while FeOOH co-catalyst could passivate the Ni-BiVO₄ photoanode surface. This work provides a model for the design of BiVO₄-based photoanodes with combined thermodynamic and kinetic advantages.

Functionalized MOF-Based Photocatalysts for CO2 Reduction

Metal-organic frameworks (MOFs) materials have become a research forefront in the field of photocatalytic CO₂ reduction attributed to their ultra-high specific surface area, adjustable structure, and abundant catalytic active sites. Particularly, MOFs can be facilely tuned to match CO₂ photoreduction by utilizing post-modification of metal nodes, functionalization of organic linkers, and combination with other active materials. Herein, the recent advances in the construction strategy of MOF-based photocatalysts materials for CO₂ reduction are highlighted. Some systematic modification strategies on MOF-based photocatalysts are also discussed, such as modification of metal sites and organic ligands, construction of heterojunction, introduction of single/dual-atom, and strain engineering. Finally, the future development directions of MOF-based photocatalysts in the field of CO₂ reduction are presented.

Photoelectrochemical water oxidation by a MOF/semiconductor composite

Herein, we report the development of a MOF-semiconductor composite film active for water oxidation at a thermodynamic underpotential.

A Potential Roadmap to Integrated Metal Organic Framework Artificial Photosynthetic Arrays

Metal organic frameworks (MOFs), a class of coordination polymers, gained popularity in the late 1990s with the efforts of Omar Yaghi, Richard Robson, Susumu Kitagawa, and others. The intrinsic porosity of MOFs made them a clear platform for gas storage and separation. Indeed, these applications have dominated the vast literature in MOF synthesis, characterization, and applications. However, even in those early years, there were hints to more advanced applications in light-MOF interactions and catalysis. This perspective focuses on the combination of both light-MOF interactions and catalysis: MOF artificial photosynthetic assemblies. Light absorption, charge transport, H2O oxidation, and CO2 reduction have all been previously observed in MOFs; however, work toward a fully MOF-based approach to artificial photosynthesis remains out of reach. Discussed here are the current limitations with MOF-based approaches: diffusion through the framework, selectivity toward high value products, lack of integrated studies, and stability. These topics provide a roadmap for the future development of fully integrated MOF-based assemblies for artificial photosynthesis.

High-performance BiVO4 photoanodes cocatalyzed with bilayer metal-organic frameworks for photoelectrochemical application

In this work, we modified a BiVO₄ photoanode with bilayer Fe-MOF and Ni-MOF as cocatalysts for the first time and obtained a highly efficient BiVO₄ composite photoanode whose photocurrent density was increased by 2.7 times. The optimized BiVO₄/Fe-MOF/Ni-MOF photoanode demonstrated a photocurrent density of 1.80 mA/cm² at 1.23 V vs. a reversible hydrogen electrode (RHE). The onset potential of the BiVO₄/Fe-MOF/Ni-MOF photoanode markedly decreased from 0.9 V to 0.69 V in comparison with the pure BiVO₄ photoanode. It is speculated that Fe-MOF and Ni-MOF led to more reactive oxygen evolution sites and that the bilayer cocatalysts synergistically promoted the separation of photogenerated electron-hole pairs, which may be the influencing factor for the photoelectrochemical performance of the BiVO₄/Fe-MOF/Ni-MOF photoanode being distinctively enhanced. Thus, this work sheds some interesting new light on the construction of a high-efficiency photoanode for photoelectrochemical applications.

Carbon matrix of biochar from biomass modeling components facilitates electron transfer from zero-valent iron to Cr(VI)

Biochar-harbored zero-valent iron (ZVI/BC) has been extensively used to detoxify hexavalent chromium (Cr(VI)). However, the role played by biochar in promoting electron transfer of ZVI and Cr(VI) reduction was not fully uncovered. Herein, three biomass modeling components (cellulose, hemicellulose, and lignin) and their blends were utilized to synthesize ZVI/BC via co-pyrolysis with hematite. X-ray diffraction analysis showed that hematite was successfully reduced to ZVI in nitrogen ambience. Batch sorption experiment showed that mass ratio (hematite to lignocellulosic component) of 1:20 is most optimal for reduction of Cr(VI) by ZVI/BCs. ZVI supported by BC derived from cellulose, hemicellulose, and their binary mixture demonstrated better Cr(VI) removal capacity (23.8-38.3 mg g-1) owing to higher ordered and graphitic carbon structure as revealed by Raman spectrum. In addition, lower Tafel corrosion potentials and smaller electrochemical impedance arc radiuses were observed based on electrochemical analysis, suggesting their higher electrical conductivity and faster electron transfer, whereas the BCs derived from lignin and lignin-containing hybrids were not conducive to electron transfer of ZVI due to lower degree of graphitization, thus compromising Cr(VI) removal by ZVI/BC (7.7-17.7 mg g-1). As per X-ray photoelectron spectroscopy analysis, reduction, complexation, and co-precipitation were the main mechanisms for Cr(VI) removal. The present study provided a scientific evidence for screening plant-derived biomass feedstock with high contents of cellulose and hemicellulose and low lignin content to fabricate ZVI/BC to achieve high Cr(VI) removal.

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

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

2D MOF-derived porous NiCoSe nanosheet arrays on Ni foam for overall water splitting

A novel 2D porous NiCoSe nanosheet arrays were grown on Ni foam using ZIF-67 as precursors, which exhibited outstanding bifunctional electrocatalytic activity and superior durability for overall water splitting.

Porous framework-based hybrid materials for solar-to-chemical energy conversion: from powder photocatalysts to photoelectrodes

Porous framework materials such as metal organic frameworks (MOFs) and covalent organic frameworks (COFs) can be considered promising materials for solar-to-chemical energy conversion.

Conductive MOFs with Photophysical Properties: Applications and Thin-Film Fabrication

Metal-organic frameworks (MOFs) are a class of hybrid materials with many promising applications. In recent years, lots of investigations have been oriented toward applications of MOFs in electronic and photoelectronic devices. While many high-quality reviews have focused on synthesis and mechanisms of electrically conductive MOFs, few of them focus on their photophysical properties. Herein, we provide an in-depth review on photoconductive and photoluminescent properties of conductive MOFs together with their corresponding applications in solar cells, luminescent sensing, light emitting, and so forth. For integration of MOFs with practical devices, recent advances in fabrication of photoactive MOF thin films are also summarized.

Growth of NiMn layered double hydroxides on nanopyramidal BiVO4 photoanode for enhanced photoelectrochemical performance

Photoelectrochemical water oxidation for hydrogen generation via utilizing sunlight is considered a very promising pathway for generating sustainable energy in an environmental manner. Here, a composite photoanode, consisting of nanopyramidal BiVO₄ arrays and one layered double hydroxide (NiMn-LDH) was designed and fabricated via a facile route. The obtained BiVO₄/NiMn-LDH composite photoelectrode presented a significant enhancement in the photoelectrochemical (PEC) current density, conversion efficiency and stability for solar water oxidation. With 2D NiMn-LDH decoration, an obvious cathodic shift of ∼480 mV in the onset potential can be observed, and more than two times enhancement in photocurrent performance is achieved. The improvement in photoelectrochemical activity for BiVO₄/NiMn-LDH composite photoanode can be attributed to the enhanced water-oxidation kinetics leading to the efficient separation, transfer and collection of charge carriers at the photoanode/electrolyte interface. The result demonstrates NiMn-LDH represents one of the active oxygen evolution catalysts (OECs) to improve the PEC activity of metal oxide photoanode.

Green Synthetic Fuels: Renewable Routes for the Conversion of Non-Fossil Feedstocks into Gaseous Fuels and Their End Uses

Innovative renewable routes are potentially able to sustain the transition to a decarbonized energy economy. Green synthetic fuels, including hydrogen and natural gas, are considered viable alternatives to fossil fuels. Indeed, they play a fundamental role in those sectors that are difficult to electrify (e.g., road mobility or high-heat industrial processes), are capable of mitigating problems related to flexibility and instantaneous balance of the electric grid, are suitable for large-size and long-term storage and can be transported through the gas network. This article is an overview of the overall supply chain, including production, transport, storage and end uses. Available fuel conversion technologies use renewable energy for the catalytic conversion of non-fossil feedstocks into hydrogen and syngas. We will show how relevant technologies involve thermochemical, electrochemical and photochemical processes. The syngas quality can be improved by catalytic CO and CO2 methanation reactions for the generation of synthetic natural gas. Finally, the produced gaseous fuels could follow several pathways for transport and lead to different final uses. Therefore, storage alternatives and gas interchangeability requirements for the safe injection of green fuels in the natural gas network and fuel cells are outlined. Nevertheless, the effects of gas quality on combustion emissions and safety are considered.

The role of photoinduced charge transfer for photocatalysis, photoelectrocatalysis and luminescence sensing in metal–organic frameworks

Understanding PCT taking place within MOFs is crucial for designing porous photo/electrocatalysts and luminescent sensors. Unique features of PCT in MOFs and recent progress along with state-of-the-art characterization methods are discussed in the context of its applications.

Strategies of Anode Materials Design towards Improved Photoelectrochemical Water Splitting Efficiency

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

Mixed-Ligand-Architected 2D Co(II)-MOF Expressing a Novel Topology for an Efficient Photoanode for Water Oxidation Using Visible Light

The structural diversity of Co(II) metal centers is known to influence their physicochemical properties. A novel two-dimensional (2D) Co(II)-MOF {[Co₅(HL)₄(dpp)₂(H₂O)₂(μ-OH)₂]·21H₂O} n has been designed and synthesized by adopting a mixed-ligand strategy, using 1,3-di(4-pyridyl)propane (dpp) colinker with a flexible spacer H₃L (H₃L: 5-(2 carboxybenzyloxy)isophthalic acid). Co(II)-MOF features a 2D network, which is further interpenetrated among the equivalent sets and therefore results in a 3D supramolecular network. Topologically, the entire network can be viewed as a (3,4,8)-connected three-nodal net with the extended point symbol of {4.5.7}4{4¹².5².7¹⁰.9⁴}{5².8.9².10}2, duly assigned to the novel topological type smm2. The functional utility of Co(II)-MOF is demonstrated by employing it toward oxygen evolution reaction (OER) in a photoelectrochemical cell, exhibiting appreciable photocurrents of up to 5.89 mA/cm² when used as an anode in a photoelectrochemical cell, while also displaying encouraging electrocatalytic currents of 9.32 mA/cm² (at 2.01 V vs RHE) for the OER. Moreover, detailed electrochemical impedance spectroscopy studies confirm enhanced charge-transfer kinetics and improved conductivity under illumination with minimal effect of interfacial phenomena. This work provides a reference for the expanding field of research into applications of MOF materials and establishes MOF materials as favorable candidates for sustainable and efficient design of electrodes for water splitting.

In situ synthesis of FeP-decorated Ti–Fe2O3: an effective strategy to improve the interfacial charge transfer in the photoelectrochemical water oxidation reaction

The unprecedented FeP/Ti–Fe₂O₃ possesses the advantages of efficient charge transfer in the bulk photoanode and at the interface of the photoanode and the electrolyte.