Photoelectrochemical reaction for the efficient production of hydrogen and high-value-added oxidation reagents

Recent Developments in Nickel-Based Layered Double Hydroxides for Photo(-/)electrocatalytic Water Oxidation

Layered double hydroxides (LDHs), especially those containing nickel (Ni), are increasingly recognized for their potential in photo(-/)electrocatalytic water oxidation due to the abundant availability of Ni, their corrosion resistance, and their minimal toxicity. This review provides a comprehensive examination of Ni-based LDHs in electrocatalytic (EC), photocatalytic (PC), and photoelectrocatalytic (PEC) water oxidation processes. The review delves into the operational principles, highlighting similarities and distinctions as well as the benefits and limitations associated with each method of water oxidation. It includes a detailed discussion on the synthesis of monolayer, ultrathin, and bulk Ni-based LDHs, focusing on the merits and drawbacks inherent to each synthesis approach. Regarding the EC oxygen evolution reaction (OER), strategies to improve catalytic performance and insights into the structural evolution of Ni-based LDHs during the electrocatalytic process are summarized. Furthermore, the review extensively covers the advancements in Ni-based LDHs for PEC OER, including an analysis of semiconductors paired with Ni-based LDHs to form photoanodes, with a focus on their enhanced activity, stability, and underlying mechanisms facilitated by LDHs. The review concludes by addressing the challenges and prospects in the development of innovative Ni-based LDH catalysts for practical applications. The comprehensive insights provided in this paper will not only stimulate further research but also engage the scientific community, thus driving the field of photo(-/)electrocatalytic water oxidation forward.

HClO-Mediated Photoelectrochemical Epoxidation of Alkenes with Near 100 % Conversion Rate and Selectivity by Regulating Lattice Chlorine Cycle

Directional organic transformation via a green, sustainable catalytic reaction has attracted a lot of attention. Herein, we report a photoelectrochemical approach for highly selective epoxidation of alkenes in a salt solution using Co2 (OH)3 Cl (CoOCl) as a bridge of photo-generated charge, where the lattice Cl- of CoOCl can be oxidized to generate HClO by the photo-generated holes of BiVO4 photoanode and be spontaneously recovered by Cl- of a salt solution, which then oxidizes the alkenes into the corresponding epoxides. As a result, a series of water-soluble alkenes, including 4-vinylbenzenesulfonic acid sodium, 2-methyl-2-propene-1-sulfonic acid sodium, and 3-methyl-3-buten-1-ol can be epoxidized with near 100 % conversion rate and selectivity. Through further inserting a MoOx protection layer between BiVO4 and CoOCl, the stability of CoOCl-MoOx /BiVO4 can be maintained for at least 120 hours. This work opens an avenue for solar-driven organic epoxidation with a possibility of on-site reaction around the abundant ocean.

Tandem cells for unbiased photoelectrochemical water splitting

Hydrogen is an essential energy carrier which will address the challenges posed by the energy crisis and climate change. Photoelectrochemical water splitting (PEC) is an important method for producing solar-powered hydrogen. The PEC tandem configuration harnesses sunlight as the exclusive energy source to drive both the hydrogen (HER) and oxygen evolution reactions (OER), simultaneously. Therefore, PEC tandem cells have been developed and gained tremendous interest in recent decades. This review describes the current status of the development of tandem cells for unbiased photoelectrochemical water splitting. The basic principles and prerequisites for constructing PEC tandem cells are introduced first. We then review various single photoelectrodes for use in water reduction or oxidation, and highlight the current state-of-the-art discoveries. Second, a close look into recent developments of PEC tandem cells in water splitting is provided. Finally, a perspective on the key challenges and prospects for the development of tandem cells for unbiased PEC water splitting are given.

Solar-to-Pharmaceutical Raw Material Production: Photoelectrochemical Naphthoquinone Formation Using Stabilized BiVO4 Photoanodes in Acid Media

In the quest for efficient use of solar energy to produce high-value-added chemicals, we first achieved the photoelectrochemical (PEC) diketonization of naphthalene, using a BiVO₄/WO₃ photoanode, to obtain naphthoquinone, an important pharmaceutical raw material with excellent efficiency by solar energy conversion. In the electrochemical (EC) reaction using F-doped SnO₂ (FTO) substrates and a 0.5 M H₂SO₄ H₂O-acetone (60 vol %) mixed solution containing 5 mM naphthalene, we produced a small amount of naphthoquinone evolution in the dark. However, naphthoquinone (η_NQ)'s Faradic efficiency and its evolution rate at 1.7 V_Ag/AgCl were only 28.5% and 0.48 μmol·cm^-2·h^-1, respectively. The PEC reaction using a WO₃ photoanode had very low efficiency for naphthalene diketonization, with low η_NQ and evolution rate values at 1.1 V_Ag/AgCl of 0.3% and 0.039 μmol·cm^-2·h^-1, respectively. In contrast, the BiVO₄/WO₃ photoanode strongly enhanced the PEC reaction, and the η_NQ and evolution rates at 1.1 V_Ag/AgCl were boosted up to 37.5% and 4.7 μmol·cm^-2·h^-1, respectively. The evolution rate of the PEC reaction in the BiVO₄/WO₃ photoanode was 10 times higher than that of the EC reaction with the FTO substrate regardless of the very low bias voltage. This result suggests that the BiVO₄-based photoanode was very efficient for the selective oxidation of naphthalene even in acid media because of the acetone-mixed electrolyte's anti-photocorrosion effect and the multilayering of WO₃ and BiVO₄. At a naphthalene concentration of 20 mM, the naphthoquinone evolution rate reached its maximum value of 7.1 μmol·cm^-2·h^-1. Although η_NQ tended to decrease with the increase in the electric charge, it reached 100% at a low bias voltage of 0.7 V_Ag/AgCl. An intensity-modulated photocurrent spectroscopy analysis indicated the rate constant of charge transfer at the photoanode surface to the naphthalene molecules was strongly enhanced at a low bias voltage of 0.7-1.1 V_Ag/AgCl, resulting in the high η_NQ value. The acid-resistant BiVO₄/WO₃ photoanode functioned in acetone-mixed electrolytes enabled the realization of a new PEC oxidation reaction driven by solar energy to produce high-value-added pharmaceutical raw materials.

Photoelectrochemical Oxidation in Ambient Conditions Using Earth-Abundant Hematite Anode: A Green Route for the Synthesis of Biobased Polymer Building Blocks

This study demonstrates the use of a photoelectrochemical device comprising earth-abundant hematite photoanode for the oxidation of 5-hydroxymethylfurfural (5-HMF), a versatile bio-based platform chemical, under ambient conditions in the presence of an electron mediator. The results obtained in this study showed that the hematite photoanode, upon doping with fluorine, can oxidize water even at lower pH (4.5 and 9.0). For 5-HMF oxidation, three different pH conditions were investigated, and complete oxidation to 2,5-furandicarboxylic acid (FDCA) via 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) was achieved at pH above 12. At lower pH, the oxidation followed another route via 2,5-diformylfuran (DFF), yielding 5-formyl-2-furancarboxylic acid (FFCA) as the main product. Using the oxidized intermediates as substrates showed DFF to be most efficiently oxidized to FDCA. We also show that, at pH 4.5, the addition of the laccase enzyme promoted the oxidation of 5-HMF to FFCA.

The design of high performance photoanode of CQDs/TiO2/WO3 based on DFT alignment of lattice parameter and energy band, and charge distribution

Photoanode is the key issue for photoelectrocatalytic (PEC) water splitting and organics degradation. However, it always faces several restrictions including severe photocorrosion, low charge separation and transfer efficiencies, poor visible light harvesting, and sluggish interfacial reaction kinetics, which often required a variety of modifications with only low improvements achieved. Herein, a high performance CQDs/TiO₂/WO₃ photoanode was designed on the basis of density function theory (DFT) alignment of lattice parameters and energy band, and charge distribution. The TiO₂/WO₃ heterojunction can abate photocorrosion through the hetero-epitaxial growth of TiO₂ (001) on WO₃ (002) for the lattice mismatch <3% eliminating dangling bonds, with high corrosion resistance and photostability of TiO₂. As the built-in field constructed by a staggered band alignment structure with the valence band offset (VBO) of 0.51 eV, the photogenerated carriers transfer and separation are promoted dramatically. Through the DFT calculations, the sunlight absorption wavelength can be extended, and the interfacial reaction kinetics can be expedited with the modification of carbon quantum dots (CQDs) on TiO₂/WO₃, due to the narrower bandgap (E_g) and the accumulation of electrons at TiO₂ side. The DFT designed CQDs/TiO₂/WO₃ photoanode significantly increase photocurrent density from 0.90 to 2.03 mA cm^-2 at 1.23 V, charge separation efficiency from 56.3 to 79.2% and charge injection efficiency from 51.2 to 70.4%, and extend light absorption edge from 455 to 463 nm over pristine WO₃, with better photostability and lower holes-to-water resistance.

Acid-Resistant BiVO4 Photoanodes: Insolubility Control by Solvents and Weak W Diffusion in the Lattice

We have revealed for the first time that BiVO₄ photoanodes can be used even in strong acid media by mixing organic solvents into the electrolyte and depositing multilayers with a WO₃ bottom layer. In general, the BiVO₄ photoanodes are photocorrosive, especially in acid solutions. However, this shortcoming has been overcome using a combination of the two aforementioned modifications. We deduced that the contribution of each mixing organic solvent for the anti-photocorrosion of BiVO₄ in sulfuric acid solutions can be evaluated on the basis of a new empirical indicator that incorporates molecular density, the Hansen solubility parameter, and molecular polarizability. Acetone and tert-butyl alcohol were especially promising solvents for stabilizing BiVO₄ in acid media. We confirmed that the mixed organic solvents stabilized surface-emergent Bi oxide species as a passivation layer, which was generated via multilayering with a WO₃ bottom layer. During heat treatment in the fabrication process, W weakly diffused into the BiVO₄ layer and a Bi oxide layer was formed on the outermost surface because of the Bi segregation that arose from the charge compensation between W⁶⁺ and V⁵⁺ in the BiVO₄ lattice. The surface Bi oxide layer, which was protected by the mixed organic solvents, steadily served as a passivation layer for anti-photocorrosion of the underlying BiVO₄ layer. We have confirmed that the BiVO₄/WO₃ photoanodes in acetone-mixed aqueous sulfuric acid solution reliably functioned for a photoelectrochemical reaction under simulated sunlight illumination, and photoelectrochemical production of S₂O₈^2- ions was confirmed under light irradiation at λ > 480 nm. These results suggest that the BiVO₄-based photoanodes have significant potential for use in acid media in conjunction with very straightforward modifications.

Improvement of photoelectrochemical HClO production under visible light irradiation by loading cobalt oxide onto a BiVO4 photoanode

Loading CoO_x species onto BiVO₄/WO₃ photoanodes was effective to improve the oxidative HClO production from aqueous NaCl solution under simulated solar-light irradiation.

Emerging Concepts in Carbon Nitride Organic Photocatalysis

Carbon nitrides encompass a class of transition-metal-free materials possessing numerous advantages such as low cost (few Euros per gram), high chemical stability, broad tunability of redox potentials and optical bandgap, recyclability, and a high absorption coefficient (>10⁵  cm^-1 ), which make them highly attractive for application in photoredox catalysis. In this Review, we classify carbon nitrides based on their unique properties, structure, and redox potentials. We summarize recently emerging concepts in heterogeneous carbon nitride photocatalysis, with an emphasis on the synthesis of organic compounds: 1) Illumination-Driven Electron Accumulation in Semiconductors and Exploitation (IDEASE); 2) singlet-triplet intersystem crossing in carbon nitride excited states and related energy transfer; 3) architectures of flow photoreactors; and 4) dual metal/carbon nitride photocatalysis. The objective of this Review is to provide a detailed overview regarding innovative research in carbon nitride photocatalysis focusing on these topics.

Insight into the Kinetic Influence of Oxygen Vacancies on the WO3 Photoanodes for Solar Water Oxidation

Improvements to solar water oxidation performance for WO₃ photoanodes due to oxygen vacancies have in general been ascribed to thermodynamic effects. Detailed insights into the water oxidation kinetics for WO₃ photoanodes with oxygen vacancies are still lacking. Here, our experimental and computational investigations revealed that the water oxidation pathway on WO₃ photoanodes with oxygen vacancies is more inclined to follow the four-hole pathway. This finding reasonably explained the common observations of higher faradaic efficiency for oxygen evolution, better stability, and faster kinetics for water oxidation usually achieved on the WO₃ photoanodes with oxygen vacancies.

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.

Solar-light-driven photocatalytic production of peroxydisulfate over noble-metal loaded WO3

The high-value-added chemical reagent peroxydisulfate (S2O82-) was produced photocatalytically over noble-metal loaded WO3 powder suspensions in aqueous H2SO4 under flowing O2 and simulated solar light irradiation. Pt cocatalyst showed the highest photocatalytic activity for S2O82- formation of the studied metals (Au, Pd, Rh, and Ru). Further study indicated that continuous accumulation of S2O82- was achieved only over the Pt/WO3 photocatalyst.

Two-step electrodeposition to fabricate the p–n heterojunction of a Cu2O/BiVO4 photoanode for the enhancement of photoelectrochemical water splitting

A p-Cu₂O/n-BiVO₄ heterojunction photoanode fabricated by a two-step electrodeposition method has been demonstrated to enhance photoelectrochemical water splitting effectively.

WO3/BiVO4 photoanode coated with mesoporous Al2O3 layer for oxidative production of hydrogen peroxide from water with high selectivity

A mesoporous and amorphous Al₂O₃ layer coated on WO₃/BiVO₄ photoanode significantly facilitated inhibiting the oxidative degradation of generated H₂O₂ into O₂ on the photoanode, resulting in the achieving excellent H₂O₂ selectivity.

Highly Enhanced Photoelectrochemical Water Oxidation Efficiency Based on Triadic Quantum Dot/Layered Double Hydroxide/BiVO4 Photoanodes

The water oxidation half-reaction is considered to be a bottleneck for achieving highly efficient solar-driven water splitting due to its multiproton-coupled four-electron process and sluggish kinetics. Herein, a triadic photoanode consisting of dual-sized CdTe quantum dots (QDs), Co-based layered double hydroxide (LDH) nanosheets, and BiVO4 particles, that is, QD@LDH@BiVO4, was designed. Two sets of consecutive Type-II band alignments were constructed to improve photogenerated electron-hole separation in the triadic structure. The efficient charge separation resulted in a 2-fold enhancement of the photocurrent of the QD@LDH@BiVO4 photoanode. A significantly enhanced oxidation efficiency reaching above 90% in the low bias region (i.e., E < 0.8 V vs RHE) could be critical in determining the overall performance of a complete photoelectrochemical cell. The faradaic efficiency for water oxidation was almost 90%. The conduction band energy of QDs is ∼1.0 V more negative than that of LDH, favorable for the electron injection to LDH and enabling a more efficient hole separation. The enhanced photon-to-current conversion efficiency and improved water oxidation efficiency of the triadic structure may result from the non-negligible contribution of hot electrons or holes generated in QDs. Such a band-matching and multidimensional triadic architecture could be a promising strategy for achieving high-efficiency photoanodes by sufficiently utilizing and maximizing the functionalities of QDs.

Efficient oxidative hydrogen peroxide production and accumulation in photoelectrochemical water splitting using a tungsten trioxide/bismuth vanadate photoanode

H₂O₂was produced on a WO₃/BiVO₄photoanode using hydrogen carbonate as an electrolyte in H₂production from water under solar light.