MOFs in photoelectrochemical water splitting: New horizons and challenges

Growing energy consumption with the augmentation in universal population to more than nine billion by 2050 and exhausting fossil fuel reserves necessitates a harsh revolution from non-renewable energy reservoirs to renewable energy reservoirs with zero carbon emission. In the present scenario, solar energy prompted photoelectrochemical (PEC) water splitting or “Artificial Photosynthesis” via light gripping semiconductor material, originates out as the most promising methodology in accomplishing the global energy crisis. Recent studies have amply demonstrated the potential of metal-organic frameworks (MOF) towards PEC applications. They are porous crystalline coordination polymers assembled through an appropriate choice of metal ions and multidentate organic ligands. Owing to their structural regularity and synthetic tunability, MOFs integration with PEC is considered in terms of enhancing and broadening light absorption, providing active sites and directing charge transfer dynamics. Here, we have explored MOFs role in PEC and classified them into different categories such as photosensitizers, co-catalysts, counter electrode, template and also for imparting additional stability to the electrode system. MOFs mediated PEC water splitting is promising but is still rare and in its infancy. Therefore, it is pertinent and timely to take stock of the advancements made and develop insight on the use of MOFs, as an emerging solution for the problems encountered in PEC. This review covers the basics of MOF & mainly describes various case studies done during last 10 years and providing adequate impetus to researchers for critically assessing the recent advances and challenges that are faced by scientists and researchers at large.     

A current perspective for photocatalysis towards the hydrogen production from biomass-derived organic substances and water

Recently, an increasing interest has been devoted to produce chemical energy – hydrogen (H₂) by converting sustainable sunlight energy via water splitting and reforming of renewable biomass-derived organic substances. These photocatalytic processes are very promising, sustainable, economic, and environment-friendly. Herein, this article gives a concise overview of photocatalysis to produce H₂ as solar fuel via two approaches: water splitting and reforming of biomass-derived organic substances. For the first approach – photocatalytic water splitting, there are two reaction types have been used, including photoelectrochemical (PEC) and photochemical (PC) cell reactions. For the second approach, biomass-derived oxygenated substrates could undergo selective photocatalytic reforming under renewable solar irradiation. Significant efforts to date have been made for photocatalysts design at the molecular level that can efficiently utilize solar energy and optimize the reaction conditions, including light irradiation, type of sacrificial reagents. Critical challenges, prospects, and the requirement to give more attention to photocatalysis for producing H₂ are also highlighted.     

Recent trends and outlooks on engineering of BiVO4 photoanodes toward efficient photoelectrochemical water splitting and CO2 reduction: A comprehensive review

Photoelectrochemical (PEC) water splitting, and carbon dioxide (CO₂) utilization devices have attracted immense attention as sustainable technologies for the generation of hydrogen (H₂) fuel and value-added chemicals feedstock. Among numerous semiconductors, bismuth vanadate (BiVO₄) has emerged as a promising photoanode owing to its fascinating features such as high chemical stability, straddling band alignment with water redox levels, eco-friendly, and cost-effectiveness. However, sluggish oxidation kinetics, photo-corrosive nature, low electronic conductivity, and short carrier diffusion length limit its commercialization on the PEC horizon. To mitigate these inadequacies, several strategies have emerged such as novel heterojunctions, doping with unique materials, interface modulation, morphology, facet orientation, co-catalyst loading for surface engineering, etc. to realize the outstanding cost-to-efficiency ratios and long-term stability of PEC devices. The review highlights the recent advancement in BiVO₄-based photoanodes in last five years (2018–2022) and their utilization in the single absorber and unexplored tandem PEC systems towards boosted water splitting and CO₂ reduction. A discussion on theoretical studies of BiVO₄-based PEC systems elucidates the microscopic mechanism of promotion effect of the bulk/interface/surface strategies on surface catalysis as well as interfacial charge transfer in boosting oxidation kinetics. Moreover, this review addresses the versatility of the BiVO₄-based photoanode for the novel yet commercially viable PEC applications. This review will provide a broad avenue in designing highly durable, and scalable BiVO₄-based systems toward various PEC energy conversion devices.     

Theoretical investigation on new materials for raising efficiency of energy conversion in PEC solar cells

In order to raise the conversion efficiency of solar energy into electric or chemical energy in solar cells, we have to developed new types of composite semiconductor materials. A composite structure with three layers has been designed. The design of the composite photoanode for water splitting in a photoelectrochemical (PEC) solar cell can be considered a breakthrough in principle. The energy correlations in the dark and under illumination have been analysed.     

Recent progress in photocatalysts for overall water splitting

Photocatalytic splitting of water with solar energy is considered as the most promising approach for the production of hydrogen fuel. However, its solar to hydrogen conversion efficiency is much below the industrial requirement (10%). This situation has stimulated intensive efforts to improve photocatalytic overall water splitting (namely, simultaneously providing unassisted oxidation and reduction of water), leading to the invention of novel catalysts in the recent years. The evaluation of these recent progresses constitutes this review article, with emphasis on the strategies employed for the development of catalysts. The catalysts were deeply reviewed and were classified into four types: (a) perovskite compounds, (b) metal oxides (sulfides and nitrides), (c) Bi‐ and In‐based materials, and (d) multicomponent catalysts. Furthermore, the challenges that remain with the process and catalysts and the potential advances were discussed as an outlook for future research.     

Recent trends in development of hematite (α-Fe2O3) as an efficient photoanode for enhancement of photoelectrochemical hydrogen production by solar water splitting

Solar-assisted water splitting using photoelectrochemical (PEC) cell is an environmentally benign technology for the generation of hydrogen fuel. However, several limitations of the materials used in fabrication of PEC cell have considerably hindered its efficiency. Extensive efforts have been made to enhance the efficiency and reduce the hydrogen generation cost using PEC cells. Photoelectrodes that are stable, efficient and made of cost-effective materials with simple synthesizing methods are essential for commercially viable solar water splitting through PEC technology. To this end, hematite (α-Fe₂O₃) has been explored as an excellent photoanode material to be used in the application of PEC water oxidation owing to its suitable bandgap of 2.1 eV that can utilize almost 40% of the visible light. In this study, we have summarized the recent progress of α-Fe₂O₃ nanostructured thin films for improving the water oxidation. Strategic modifications of α-Fe₂O₃ photoanodes comprising nanostructuring, heterojunctions, surface treatment, elemental doping, and nanocomposites are highlighted and discussed. Some prospects related to the challenges and research in this innovative research area are also provided as a guiding layout in building design principles for the improvement of α-Fe₂O₃ photoanodes in photoelectrochemical water oxidation to solve the increasing environmental issues and energy crises.     

Two-step thermochemical electrolysis: An approach for green hydrogen production

Electrolysis and thermochemical water splitting are approaches to produce green hydrogen that use either an electrical potential (electrolysis) or a chemical potential (thermochemical water splitting) to split water. Electrolysis is technologically mature when applied at low temperatures, but it requires large quantities of electrical energy. In contrast to electrolysis, thermochemical water splitting uses thermal energy, as thermal energy can typically be supplied at a lower unit cost than electrical energy using concentrating solar power. Thermochemical water splitting, however, typically suffers from high thermal losses at the extremely high process temperatures required, substantially increasing the total energy required. We show how, by combining electrical and chemical potentials, a novel and cost-efficient water splitting process can be envisioned that overcomes some of the challenges faced by conventional electrolysis and thermochemical water splitting. It uses a mixed ionic and electronic conducting perovskite with temperature-dependent oxygen non-stoichiometry as an anode in an electrolyzer. If solar energy is used as the primary source of all energy required in the process, the cost of the energy required to produce hydrogen could be lower than in high-temperature electrolysis by up to 7%.     

Surface and bulk modification for advanced electrode design in photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a prominent strategy for harnessing solar energy in the production of sustainable hydrogen fuel from water. Over the past few decades, extensive efforts have been devoted to develop advanced electrodes for efficient PEC water splitting. This review presents the recent progress in the development of efficient photoanodes through two major approaches: surface modification, including co-catalyst-loading, passivation, and defect engineering; and bulk modification, including hybridization, dopant engineering, and structural control. By virtue of bulk and surface modification a considerable improvement in PEC activity has been obtained so far. Photocurrent response of various anodes observed in the range of 0.063 mA cm⁻² – 8.5 mA cm⁻² (as listed in Table 1) require further improvement to upgrade the overall performance efficiency of PEC cells.This review also provides a systematic overview of the fundamentals of PEC water splitting, as well as the key challenges and notable achievements made so far in terms of electrode design and material modification. Finally, future research perspectives that will further advance this field are discussed. The contribution of this paper is to provide fundamental information about bulk and surface modifications, which will aid in the design of advanced electrodes for high-performance PEC cells.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

Rational design and fabrication of surface tailored low dimensional Indium Gallium Nitride for photoelectrochemical water cleavage

Currently several type of energy sources exist in the modern world. The energy makes people's life more comfortable, easy, time savings, fast transformation of information and various modes of transmission. Because of large demand of energy, efforts on production of energy increases day by day which subsequently increase serious environmental concerns such as pollution and lack of existing natural resources. In this respect, several attempts have been proposed for new type of renewable and chemical energy systems to overcome the economic burden, global warming and environmental problems caused by the use of conventional fossil fuels. Hydrogen production via water splitting is a promising and ideal route for renewable energy using the most abundant resources of solar light and water. Cost effective photocatalyst for Photoelectrochemical (PEC) water splitting using semiconductor materials as light absorbers have been extensively studied due to their stability and simplicity. Over the past few decades, various metal oxide photocatalysts for water splitting have been developed and their photocatalytic application was studied under UV irradiation. Alternative semiconductor photocatalyst should harness solar energy in the visible light, one such semiconductor material is indium gallium nitride (InGaN), owing to its suitable and tunable energy band-gap, chemical resistance and notable photoelectrocatalytic activity. This review article is initiated with the brief introduction about the origin and methods of production of hydrogen gas from both renewable and nonrenewable energy sources. Multi-functional properties and applications of InGaN are described along with past and recent efforts of InGaN materials for hydrogen evolution by several investigators are provided in detail. In addition, future prospects and ways to improve the PEC performance of InGaN are presented at the end of this review.     

Performance assessment study of photo-electro-chemical water-splitting reactor designs for hydrogen production

Solar water splitting is considered a greatly promising technique for producing clean hydrogen fuel. However, limited studies have paid attention to the designs of photo-electrochemical (PEC) reactors. In this regard, two different designs of PEC reactor are proposed and studied numerically in the present paper. The effects of important design parameters on the system performance are also investigated. The PEC governing equations of transport phenomena related to water splitting reactor are developed and numerically solved. According to the current results, the rate of the hydrogen volume production and the solar - to - hydrogen conversion efficiency increase as an applied solar incident flux increases for both proposed designs. The solar - to - hydrogen conversion efficiencies are calculated to be 12.65% for design 1 and 12.48% for design 2. The hydrogen volume production rate is performed to achieve 78.3 L/m² h by design 1, and 74.8 L/m² h by design 2.     

Membrane electrode assembly-based photoelectrochemical cell for hydrogen generation

A novel photoelectrochemical cell (PEC) for generation of hydrogen via photocatalytic water splitting is proposed and investigated. At the heart of the PEC is a membrane electrode assembly (MEA) integrated with Degussa P25 TiO₂ powder as a model photocatalyst for the photoanode and Pt catalyst powder for the dark cathode, respectively. It serves as a compact photocatalytic reactor for water splitting as well as an effective separator for the generated hydrogen and oxygen. The unique characteristic of the MEA-based PEC is that the use of co-catalyst, sacrificial reagent and supporting electrolyte in the cell is totally not required. The novel PEC can be operated without addition of water in the cathode compartment resulting in improved photo conversion efficiency. In addition, the application of a Degussa P25/BiVO₄ mixed photocatalyst was found to significantly enhance the hydrogen generation. Further improvements for the MEA-based PEC utilizing solar energy are also proposed.     

Photoelectrochemical performance of bismuth vanadate photoanode for water splitting under concentrated light irradiation

Photoelectrochemical (PEC) water splitting is a promising way to convert solar energy into hydrogen energy. It is typically carried out at room temperature (RT) and 1 sun illumination. The PEC water splitting under concentrated light is expected to be an effective route to improve PEC performance, but there are few studies on it. Herein, CoPi/Mo:BiVO₄ photoanode was selected to investigate the effect of concentrated light and the reaction temperature on its PEC performance. It was revealed that CoPi/Mo:BiVO₄ showed enhanced PEC performance under concentrated light. The photocurrent density was enhanced with increased light intensity and increased reaction temperature. At a high temperature (60 °C), the normalized photocurrent density (3.31 mA cm⁻² at 1.23 V vs. RHE) was found to be optimal at 4 suns, which was attributed to the synergistic effect of concentrating light and heating. It is proved that concentrated light can effectively improve the PEC performance, which has important guiding significance to realize the low-cost and efficient PEC water splitting.     

Recent progresses in porphyrin assisted hydrogen evolution

The indiscriminate exploitation of fossil fuels over a period of two centuries has eventually led us to a juncture where search for alternate energy sources and sustainable development has become inevitable. Solar energy remains the most reliable renewable energy source, efficient harnessing of which can serve to meet the future energy demands. Photo-assisted water splitting to generate hydrogen, a potential clean fuel has been the focus of current research in this field. Design and development of suitable materials for efficient solar energy conversion remains the major challenge to be tackled in this aspect. A cost-effective technology for conversion of solar energy is still a distant dream. The present paper attempts a general overview of the basic principles of water splitting with special focus on porphyrin-based systems as promising water splitting systems.     

Tailoring morphological characteristics of zinc oxide using a one-step hydrothermal method for photoelectrochemical water splitting application

In the present paper describe the zinc oxide (ZnO) with various morphologies have been synthesized using the one-step hydrothermal method, in which the growth of ZnO nanostructures are significantly tailored by adjusting the pH level between 9 and 12 using 0.1 M Sodium hydroxide (NaOH). Significant results reveal the morphological properties of ZnO nanostructures varied with different pH values with the formation of ZnO nanostructures have different morphological such as a baton, star, flower, and rod-like structures. The present results show the rod-like structure of ZnO nanostructures exhibits the highest photocurrent density of 746.61 μAcm⁻² (at 1.23 V vs RHE) under simulated solar AM 1.5G illumination in Potassium hydroxide (KOH) medium, also the other morphologies. The dependent of the photoelectrochemical (PEC) water splitting properties on the different morphological of ZnO nanostructures are studied. Achieving the morphological evolution mechanism has become one of the method to obtain the production of the hydrogen growth regime used for solar energy conversion and their applied storage potentials. The application of the ZnO nanostructures for PEC water splitting was proposed with the adoption of screen-printed carbon electrodes (SPCEs). These are to quantify the best degree of the highest photocurrent density with one-step tailoring with an ideal modeling system to enhance PEC water splitting performances. Thus, the screen-printed carbon electrodes (SPEs) has been used as an alternative method for fabrication and photoelectrodes testings.     

PEC electrode of ZnO nanorods sensitized by CdS with different size and its photoelectric properties

The gradient PEC electrodes of ZnO nanorods sensitized by CdS with different size were fabricated via successive ionic layer adsorption and reaction (SILAR) method and applied in photochemical water splitting. The concentrations of reaction solution and SILAR cycles were investigated in the synthetic process and the working mechanism of the gradient PEC electrode was suggested. The results showed that the hydrogen generation efficiency of 4.88% was achieved for the ZnO/CdS gradient PEC electrode constructed by decreasing of the CdS quantum dots size on ZnO nanorods due to the improved absorption and appropriate energy gap structure, which was confirmed by enhanced absorption spectrum. The expected products have potential application in photoelectrochemical water splitting.     

Active and robust novel bilayer photoanode architectures for hydrogen generation via direct non-electric bias induced photo-electrochemical water splitting

Photo-electrochemical (PEC) water splitting is a promising and environmentally benign approach for generation of hydrogen using solar energy with minimum greenhouse gas emissions. The development of semiconductor materials for photoanode with superior optoelectronic properties combined with excellent photoelectrochemical activity and stability is vital for the realization of viable commercial development of PEC water splitting systems. Herein, we report for the very first time, the study of nanoscale bilayer architecture of WO₃ and Nb and N co-doped SnO₂ nanotubes (NTs), wherein WO₃ NTs are coated with (Sn_0.95Nb_0.05)O₂:N-600 (annealed in NH₃ at 600 °C) layer of different thicknesses, as a potential semiconductor photoanode material for PEC water splitting. An excellent long term photoelectrochemical stability under illumination in the acidic electrolyte solution combined with a solar-to-hydrogen efficiency (STH) of ～3.83% (under zero applied potential) is obtained for the bilayer NTs, which is the highest STH obtained thus far, to the best of our knowledge compared to the other well studied semiconductor materials, such as TiO₂, ZnO and Fe₂O₃. These promising results demonstrate the excellent potential of bilayer NTs as a viable and promising photoanode in PEC water splitting.     

Remarkable improvement of photoelectrochemical water splitting in pristine and black anodic TiO2 nanotubes by enhancing microstructural ordering and uniformity

Efficient photoelectrochemical (PEC) water splitting is crucial for future energy and sustainable world. We here report on the improvement of PEC activity of anodic TiO₂ nanotubes (TNTs) by enhancing tube ordering and subsequent electrochemical reduction. TNTs were prepared by two-step anodic oxidisation from an organic electrolyte containing fluoride ions. The effects of first-step anodisation time on the ordering of TNTs and subsequent electrolytic reduction were investigated on the PEC performance under simulated solar light spectrum. The photocurrent densities of TNTs anodised for 1 h, 4 h and subsequently reduced are about 25.12 μA cm⁻², 51.76 μA cm⁻² and 126.89 μA cm⁻², respectively, at 1.23 V vs RHE and their conversion efficiency of light to electrical energy achieved are about 0.016%, 0.04% and 0.08% respectively. Electrochemical impedance spectroscopy (ESI) curves revealed the improved PEC water splitting confirmed by sharper charge carrier separation and enhanced charge transfer in highly ordered pristine and black TNTs. This improvement of PEC in dopant-free TNT is at the first instance interpreted by enhancing TNT ordering and uniformity achieved by prolonging of the first-step anodisation time and its effect on the electronic band structure of TNTs. This significant effect on PEC performance of pristine TNT under visible light absorption takes place due to the induced surface defects and slower recombination rates of hole and electron. This demonstrates an efficient economic materials production appraoch for PEC hydrogen production.     

Mixed-metal organic framework-coated ZnO nanowires array for efficient photoelectrochemical water oxidation

Designing of high-performance photoanodes is essential for efficient solar energy conversion in photoelectrochemical (PEC) water splitting. Herein, we report an effective approach to synthesize three dimensional (3D) mixed-metal organic framework-coated ZnO nanowires array (ZnNi MOF@ZnO) for the effective PEC performance. The ZnO nanowires act as photon absorber as well as rapid charge transporter; whilst the ZnNi MOF provides the active sites for PEC process by lowering the energy barrier of water oxidation and suppressing electron-hole recombination. The 3D nanostructure of ZnNi MOF@ZnO nanowires array provides intimate interfacial contact through covalent interactions between the ZnNi MOF and ZnO nanowires which facilitates the rapid charge transfer during photocatalytic oxygen evolution reactions. As a result, the ZnNi MOF@ZnO nanowires array exhibited excellent photoelectrochemical water oxidation with very low onset potential (0.31 V vs. RHE) and high photocurrent density (1.40 mA/cm²) as compared to the Zn MOF @ZnO and ZnO nanowires array. This facile strategy provides a promising direction towards high performance photoanode design for adequate solar energy conversion.