Hydrogen peroxide (H2O2), as a green fuel and oxidant, has drawn increasing attention in the energy and environmental research. Compared with the traditional anthraquinone process, the ...electrochemical (EC) and photoelectrochemical (PEC) syntheses of H2O2 are cost‐effective and environmentally friendly. In order to construct membraneless EC/PEC devices for the full H2O2 synthesis, anodic H2O2 production by water oxidation, which is less developed than cathodic H2O2 generation, is highly desirable. Here, we review recent developments for the EC/PEC H2O2 production by water oxidation, including fundamental aspects, benchmarking activity evaluation, material/catalyst selection, and strategies for increasing selectivity, efficiency, and accumulation. Furthermore, we discuss the challenges and outlook of water oxidation for H2O2 production, especially device‐level development, accumulation and stability, and industrial applications. Our review is intended to stimulate studies further improving EC/PEC H2O2 production.
The electrochemical and photoelectrochemical generation of H2O2 is a promising approach for the large‐scale production of this important green fuel and oxidant. Recent developments in H2O2 production on the anode side, including property evaluation and material/catalyst selection, and strategies for increasing the selectivity, efficiency, and accumulation, are comprehensively discussed in this Minireview.
Abstract
Artificial photosynthesis of H
2
O
2
using earth-abundant water and oxygen is a promising approach to achieve scalable and cost-effective solar fuel production. Recent studies on this topic ...have made significant progress, yet are mainly focused on using organic polymers. This set of photocatalysts is susceptible to potent oxidants (e.g. hydroxyl radical) that are inevitably formed during H
2
O
2
generation. Here, we report an inorganic Mo-doped faceted BiVO
4
(Mo:BiVO
4
) system that is resistant to radical oxidation and exhibits a high overall H
2
O
2
photosynthesis efficiency among inorganic photocatalysts, with an apparent quantum yield of 1.2% and a solar-to-chemical conversion efficiency of 0.29% at full spectrum, as well as an apparent quantum yield of 5.8% at 420 nm. The surface-reaction kinetics and selectivity of Mo:BiVO
4
were tuned by precisely loading CoO
x
and Pd on {110} and {010} facets, respectively. Time-resolved spectroscopic investigations of photocarriers suggest that depositing select cocatalysts on distinct facet tailored the interfacial energetics between {110} and {010} facets and enhanced charge separation in Mo:BiVO
4
, therefore overcoming a key challenge in developing efficient inorganic photocatalysts. The promising H
2
O
2
generation efficiency achieved by delicate design of catalyst spatial and electronic structures sheds light on applying robust inorganic particulate photocatalysts to artificial photosynthesis of H
2
O
2
.
The global adoption of efficient sustainable energy sources is a crucial step toward meeting energy demands while achieving carbon emission reduction targets. Solar energy has become a clean and ...cost-competitive alternative to traditional fossil fuels, but the intermittent nature of sunlight results in challenges associated with energy storage and transport. Photocatalytic carbon dioxide reduction intends to mimic natural photosynthesis for utilizing sunlight to chemically convert water and CO
2
into fuels. In this process, the solar energy is captured and stored in fuels, so-called solar fuels, for widespread on-demand use. Heterogeneous solar fuel production systems are multi-component, comprising light-harvesting (photosensitizer) and catalytic (cocatalyst) units. Cocatalysts are indispensable for photocatalytic CO
2
reduction systems, which promote charge carrier separation and transport, reduce the reaction activation energy, and alter the reaction route, thereby enhancing the activity and selectivity of the photocatalytic reactions. This review presents a comprehensive summary of the recent advancements in cocatalysts for photocatalytic CO
2
reduction reaction (CO
2
RR), with the purpose of providing new insights and guidance to the field with regard to research directions and best practices. We summarize how various cocatalysts including inorganic nanoparticles, metal complexes, enzymes, and bacteria can be combined with semiconductor photosensitizer for light-driven photocatalytic CO
2
RR. Side-by-side comparisons reveal the strengths and limitations of each kind of cocatalysts and how lessons extracted from studying natural photosynthetic systems can be applied to investigations of artificial photosynthesis, presenting an outlook discussing possible future concepts for a more effective photocatalytic CO
2
reduction process.
Abstract
Oxynitride photocatalysts hold promise for renewable solar hydrogen production via water splitting owing to their intense visible light absorption. Cocatalyst loading is essential for ...activation of such oxynitride photocatalysts. However, cocatalyst nanoparticles form aggregates and exhibit weak interaction with photocatalysts, which prevents eliciting their intrinsic photocatalytic performance. Here, we demonstrate efficient utilization of photoexcited electrons in a single-crystalline particulate BaTaO
2
N photocatalyst prepared with the assistance of RbCl flux for H
2
evolution reactions via sequential decoration of Pt cocatalyst by impregnation-reduction followed by site-selective photodeposition. The Pt-loaded BaTaO
2
N photocatalyst evolves H
2
over 100 times more efficiently than before, with an apparent quantum yield of 6.8% at the wavelength of 420 nm, from a methanol aqueous solution, and a solar-to-hydrogen energy conversion efficiency of 0.24% in Z-scheme water splitting. Enabling uniform dispersion and intimate contact of cocatalyst nanoparticles on single-crystalline narrow-bandgap particulate photocatalysts is a key to efficient solar-to-chemical energy conversion.
Oxysulfide semiconductors have narrow bandgaps suitable for water splitting under visible-light irradiation, because the electronegative sulfide ions negatively shift the valence band edges of the ...corresponding oxides
. However, the instability of sulfide ions during the water oxidation is a critical obstacle to simultaneous evolution of hydrogen and oxygen
. Here, we demonstrate the activation and stabilization of Y
Ti
O
S
, with a bandgap of 1.9 eV, as a photocatalyst for overall water splitting. On loading of IrO
and Rh/Cr
O
as oxygen and hydrogen evolution co-catalysts, respectively, and fine-tuning of the reaction conditions, simultaneous production of stoichiometric amounts of hydrogen and oxygen was achieved on Y
Ti
O
S
during a 20 h reaction. The discovery of the overall water splitting capabilities of Y
Ti
O
S
extends the range of promising materials for solar hydrogen production.
Photoelectrochemical water oxidation over bare hematite (α-Fe2O3) involved physical–chemical processes through surface states. It is found that the surface state with a higher oxidative energy (S1) ...served as a reaction intermediate for water oxidation, while the other surface state with a lower oxidative energy (S2) was basically catalytically inactive and induced charge recombination. Since a cocatalyst is commonly loaded on α-Fe2O3 to enhance water oxidation, the physical–chemical processes between the surface states and the cocatalyst are supposedly important but remain unclear. In the present study, we elucidated such processes by employing photoelectrochemical impedance spectroscopy. We found that, in a CoPi-loaded α-Fe2O3, S1 no longer served as an intermediate for water oxidation and was mostly passivated. At the same time, S2 became the key intermediate for water oxidation by serving as a hole reservoir, prolonging the hole lifetime, and finally transferring holes to CoPi. This study reveals the surface chemistry for optimizing cocatalyst-loaded α-Fe2O3, which is different from that for bare α-Fe2O3.
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•Analytical data of photoanodes directly predict PEC performance.•Various machine learning method was used for the prediction.•The category of the JV curves were also predicted.•About ...a hundred samples of BiVO4 photoanodes were used.•Physical/chemical origins were identified.
Bismuth vanadate (BiVO4) is one of the most promising materials for photoanodes in photoelectrochemical (PEC) water splitting. The PEC performance (measured by the current–voltage curve) is generally improved by the trial-and-error of the fabrication procedure; however the origin of the structural and physical properties is not always clear, especially in the developing stage. To clarify the physical origins for determination of the PEC performance, we used two types of the machine learning (ML) calculations for a combination of various analytical data of BiVO4 electrodes. First, the photocurrent density of the BiVO4 electrodes was predicted by the descriptors in the analytical data and the dominant factors were identified. The selected descriptors could provide a prediction of photocurrent values with the determination coefficient, ∼0.8. Secondly, we could predict two different types of the current–voltage curve by the ML categorization functions. Three dominant descriptors were identified and its accuracy was 100 %. The analysis of the dominant factors provided us useful information of the structural and physical properties to understand the photocurrent behavior without any prior knowledge.
Development of sunlight-driven water splitting systems with high efficiency, scalability, and cost-competitiveness is a central issue for mass production of solar hydrogen as a renewable and storable ...energy carrier. Photocatalyst sheets comprising a particulate hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) embedded in a conductive thin film can realize efficient and scalable solar hydrogen production using Z-scheme water splitting. However, the use of expensive precious metal thin films that also promote reverse reactions is a major obstacle to developing a cost-effective process at ambient pressure. In this study, we present a standalone particulate photocatalyst sheet based on an earth-abundant, relatively inert, and conductive carbon film for efficient Z-scheme water splitting at ambient pressure. A SrTiO3:La,Rh/C/BiVO4:Mo sheet is shown to achieve unassisted pure-water (pH 6.8) splitting with a solar-to-hydrogen energy conversion efficiency (STH) of 1.2% at 331 K and 10 kPa, while retaining 80% of this efficiency at 91 kPa. The STH value of 1.0% is the highest among Z-scheme pure water splitting operating at ambient pressure. The working mechanism of the photocatalyst sheet is discussed on the basis of band diagram simulation. In addition, the photocatalyst sheet split pure water more efficiently than conventional powder suspension systems and photoelectrochemical parallel cells because H+ and OH– concentration overpotentials and an IR drop between the HEP and OEP were effectively suppressed. The proposed carbon-based photocatalyst sheet, which can be used at ambient pressure, is an important alternative to (photo)electrochemical systems for practical solar hydrogen production.
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•We designed a phase-junction configuration of α and γ-Fe2O3 by inserting maghemite (γ-Fe2O3) particles in between a hematite (α-Fe2O3) layer and FTO substrate.•We prepared the ...micelle templated γ-Fe2O3 nanoparticles to suppress the phase transition from γ-Fe2O3 to α-Fe2O3.•The photo-anode consisting of the phase-junction configuration of α and γ-Fe2O3 showed a drastic improvement of the current density.•We could demonstrate using a time-resolved spectroscopic method that the enhancement of the performance originated from the retardation of the recombination, resulting from the improvement of the charge separation efficiency.
To enhance the photo-electrochemical performance of hematite photo-electrode, we designed a phase-junction configuration of α and γ-Fe2O3. By inserting maghemite (γ-Fe2O3) particles synthesized by reverse micelle deposition, which is for suppressing the phase transition from γ-Fe2O3 to α-Fe2O3, in between a hematite (α-Fe2O3) layer and fluorine-doped tin oxide (FTO) substrate, we could achieve a drastic improvement of the current density. We successfully confirmed using Raman spectroscopy that γ-Fe2O3 still remained after the annealing process performed at high temperature and the phase-junction configuration of α/γ-Fe2O3 was constructed. In addition, we could demonstrate using a time-resolved spectroscopic method and an electro-impedance spectroscopy measurement that the enhancement of the performance originated from the retardation of the recombination, resulting from the improvement of the charge separation efficiency.
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