A Stand‐Alone Si‐Based Porous Photoelectrochemical Cell

Wireless photoelectrochemical (PEC) devices promise easy device fabrication as well as reduced losses. Here, the design and fabrication of a stand‐alone ion exchange material‐embedded, Si membrane‐based, photoelectrochemical cell architecture with micron‐sized pores is shown, to overcome the i) pH gradient formation due to long‐distance ion transport, ii) product crossover, and iii) parasitic light absorption by application of a patterned catalyst. The membrane‐embedded PEC cell with micropores utilizes a triple Si junction cell as the light absorber, and Pt and IrO_x as electrocatalysts for the hydrogen evolution reactions and oxygen evolution reactions, respectively. The solar‐to‐hydrogen efficiency of 7% at steady‐state operation, as compared to an unpatterned η_PV of 10.8%, is mainly attributed to absorption losses by the incorporation of the micropores and catalyst microdots. The introduction of the Nafion ion exchange material ensures an intrinsically safe PEC cell, by reducing the total gas crossover to <0.1%, while without a cation exchange membrane, a crossover of >6% is observed. Only in a pure electrolyte of 1 m H₂SO₄, a pH gradient‐free system is observed thus completely avoiding the build‐up of a counteracting potential.     

Boosting Photoelectrochemical Water Splitting by TENG‐Charged Li‐Ion Battery

The need for cost‐effective and sustainable power supplies has spurred a growing interest in hybrid energy harvesting systems, and the most elementary energy production process relies on intermittent solar power. Here, it is shown how the ambient mechanical energy leads to water splitting in a photoelectrochemical (PEC) cell boosted by a triboelectric nanogenerator (TENG). In this strategy, a flexible TENG collects and transforms mechanical energy into electric current, which boosts the PEC water splitting via the charged Li‐ion battery. Au nanoparticles are deposited on TiO₂ nanoarrays for extending the available light spectrum to visible part by surface plasmon resonance effect, which yields a photocurrent density of 1.32 mA cm^?2 under AM 1.5 G illumination and 0.12 mA cm^?2 under visible light with a bias of 0.5 V. The TENG‐charged battery boosts the water splitting performance through coupling electrolysis and enhanced electron–hole separation efficiency. The hybrid cell exhibits an instantaneous current more than 9 mA with a working electrode area of 0.3 cm², suggesting a simple but efficient route for simultaneously converting solar radiation and mechanical energy into hydrogen.     

Hybrid Heterojunction and Solid‐State Photoelectrochemical Solar Cells

A hybrid heterojunction and solid‐state photoelectrochemical solar cell based on graphene woven fabrics (GWFs) and silicon is designed and fabricated. The GWFs are transferred onto n‐Si to form a Schottky junction with an embedded polyvinyl alcohol based solid electrolyte. In the hybrid solar cell, solid electrolyte serves three purposes simutaneously; it is an anti‐reflection layer, a chemical modification carrier, and a photoelectrochemical channel. The open‐circuit voltage, short‐circuit current density, and fill factor are all significantly improved, achieving an impressive power conversion efficiency of 11%. Solar cell models are constructed to confirm the hybrid working mechanism, with the heterojunction junction and photoelectrochemical effect functioning synergistically.     

All‐Surface‐Atomic‐Metal Chalcogenide Sheets for High‐Efficiency Visible‐Light Photoelectrochemical Water Splitting

Artificial all‐surface‐atomic 2D sheets can trigger breakthroughs in tailoring the physical and chemical properties of advanced functional materials. Here, the conceptually new all‐surface‐atomic semiconductors of SnS and SnSe freestanding sheets are realized using a scalable strategy. As an example, all‐surface‐atomic SnS sheets undergo surface atomic elongation and structural disordering, which is revealed by X‐ray absorption fine structure spectroscopy and first‐principles calculations, endowing them with high structural stability and an increased density of states at the valence band edge. These exotic atomic and electronic structures make the all‐surface‐atomic SnS sheet‐based photoelectrode exhibit an incident photon‐to‐current conversion efficiency of 67.1% at 490 nm, much higher than the efficiencies of other visible‐light‐driven water splitting. A photocurrent density of 5.27 mA cm^‐2, which is two orders of magnitude higher than that of the bulk counterpart, is also achieved for the all‐surface‐atomic SnS sheets‐based photoelectrode. This will allow the manipulation of the basic properties of advanced materials on the atomic scale, thus paving the way for innovative applications.     

New Insights into Defect‐Mediated Heterostructures for Photoelectrochemical Water Splitting

Understanding the interfacial electronic structures of heterojunctions, a challenging undertaking, is extremely important to the design of photoelectrodes for efficient water splitting. The heterostructured interfaces in terms of crystal defects at the atomic‐level exemplified by TiO₂/BiVO₄ are studied. Results from both experimental observations and theoretical calculations clearly confirm the spontaneous formation of defective interfaces in the heterostructures. TiO₂/BiVO₄ junction with engineered interfacial defects can efficiently increase the carrier density and extend the lifetime of electrons. The inherent phenomenon of defective electronic structures in different heterostructures creates a significant impact on their photoelectrochemical performance. The synergetic effect between defect‐mediated mechanism and organic quantum dots sensitization yields significantly increased photoconversion efficiency, which is even superior to that of common metal sulfide sensitized ones. This result demonstrates an approach worthy for the design and fabrication of defect‐mediated heterostructures for water splitting, without utilizing harmful metal sulfides. Moreover, new insights into the influence of intrinsic defects on the interfacial charge transfer process between two different semiconductors for energy‐related applications have also been provided.     

Photoelectrochemical Behavior of Planar and Microwire‐Array Si|GaP Electrodes

Gallium phosphide exhibits a short diffusion length relative to its optical absorption length, and is thus a candidate for use in wire array geometries that allow light absorption to be decoupled from minority carrier collection. Herein is reported the photoanodic performance of heteroepitaxially grown gallium phosphide on planar and microwire‐array Si substrates. The n‐GaP|n‐Si heterojunction results in a favorable conduction band alignment for electron collection in the silicon. A conformal electrochemical contact to the outer GaP layer is produced using the ferrocenium/ferrocene (Fc⁺/Fc) redox couple in acetonitrile. Photovoltages of ～750 mV under 1 sun illumination are observed and are attributed to the barrier formed at the (Fc⁺/Fc)|n‐GaP junction. The short‐circuit current densities of the composite microwire‐arrays are similar to those observed using single‐crystal n‐GaP photoelectrodes. Spectral response measurements along with a finite‐difference‐time‐domain optical model indicate that the minority carrier diffusion length in the GaP is ～80 nm. Solid‐state current–voltage measurements show that shunting occurs through thin GaP layers that are present near the base of the microwire‐arrays. The results provide guidance for further studies of 3D multi‐junction photoelectrochemical cells.     

Surface‐Plasmon‐Assisted Photoelectrochemical Reduction of CO2 and NO3− on Nanostructured Silver Electrodes

Electrochemical reduction of carbon dioxide (CO₂) typically suffers from low selectivity and poor reaction rates that necessitate high overpotentials, which impede its possible application for CO₂ capture, sequestration, or carbon‐based fuel production. New strategies to address these issues include the utilization of photoexcited charge carriers to overcome activation barriers for reactions that produce desirable products. This study demonstrates surface‐plasmon‐enhanced photoelectrochemical reduction of CO₂ and nitrate (NO₃^?) on silver nanostructured electrodes. The observed photocurrent likely originates from a resonant charge transfer between the photogenerated plasmonic hot electrons and the lowest unoccupied molecular orbital (MO) acceptor energy levels of adsorbed CO₂, NO₃^?, or their reductive intermediates. The observed differences in the resonant effects at the Ag electrode with respect to electrode potential and photon energy for CO₂ versus NO₃^? reduction suggest that plasmonic hot‐carriers interact selectively with specific MO acceptor energy levels of adsorbed surface species such as CO₂, NO₃^?, or their reductive intermediates. This unique plasmon‐assisted charge generation and transfer mechanism can be used to increase yield, efficiency, and selectivity of various photoelectrochemical processes.     

Retarded Charge–Carrier Recombination in Photoelectrochemical Cells from Plasmon‐Induced Resonance Energy Transfer

N‐type metal oxides such as hematite (α‐Fe₂O₃) and bismuth vanadate (BiVO₄) are promising candidate materials for efficient photoelectrochemical water splitting; however, their short minority carrier diffusion length and restricted carrier lifetime result in undesired rapid charge recombination. Herein, a 2D arranged globular Au nanosphere (NS) monolayer array with a highly ordered hexagonal hole pattern (hereafter, Au array) is introduced onto the surface of photoanodes comprised of metal oxide films via a facile drying and transfer‐printing process. Through plasmon‐induced resonance energy transfer, the Au array provides a strong electromagnetic field in the near‐surface area of the metal oxide film. The near‐field coupling interaction and amplification of the electromagnetic field suppress the charge recombination with long‐lived photogenerated holes and simultaneously enhance the light harvesting and charge transfer efficiencies. Consequently, an over 3.3‐fold higher photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) is achieved for the Au array/α‐Fe₂O₃. Furthermore, the high versatility of this transfer printing of Au arrays is demonstrated by introducing it on the molybdenum‐doped BiVO₄ film, resulting in 1.5‐fold higher photocurrent density at 1.23 V versus RHE. The tailored metal film design can provide a potential strategy for the versatile application in various light‐mediated energy conversion and optoelectronic devices.     

Recent Advancement of p‐ and d‐Block Elements,Single Atoms,and Graphene‐Based Photoelectrochemical Electrodes for Water Splitting

Solar‐assisted photoelectrochemical (PEC) water splitting to produce hydrogen energy is considered the most promising solution for clean, green, and renewable sources of energy. For scaled production of hydrogen and oxygen, highly active, robust, and cost‐effective PEC electrodes are required. However, most of the available semiconductors as a PEC electrodes have poor light absorption, material degradation, charge separation, and transportability, which result in very low efficiency for photo‐water splitting. Generally, a promising photoelectrode is obtained when the surface of the semiconductor is modified/decorated with a suitable co‐catalyst because it increases the light absorbance spectrum and prevents electron–hole recombination during photoelectrode reactions. In this regard, numerous p‐ and d‐block elements, single atoms, and graphene‐based PEC electrodes have been widely used as semiconductor/co‐catalyst junctions to boost the performances of PEC overall water splitting. This review enumerates the recent progress and applications of p‐ and d‐block elements, single atoms, and graphene‐based PEC electrodes for water splitting. The focus is placed on fundamental mechanism, efficiency, cells design, and various aspects that contribute to the large‐scale prototype device. Finally, future perspectives, summary, challenges, and outlook for improving the activity of PEC photoelectrodes toward whole‐cell water splitting are addressed.     

Moisture‐Assisted Preparation of Compact GaN:ZnO Photoanode Toward Efficient Photoelectrochemical Water Oxidation

Developing strategies that can promote charge transportation in photodevices is crucial for achieving high solar energy conversion efficiency. Herein a moisture‐assisted nitridation approach is presented for the fabrication of efficient gallium‐zinc oxynitride (GaN:ZnO) photoanode with compact structure to facilitate the charge transportation. With moisture‐assisted nitridation, the charge separation efficiency and injection efficiency obtained on GaN:ZnO photoanode are significantly enhanced. Correspondingly, the photocurrent at 1.23 V vs reversible hydrogen electrode (RHE) has 18 folds improvement compared with that prepared without moisture assistance. Furthermore, via treating with HCl acid and modification with cobalt phosphate (CoPi) as a cocatalyst, state‐of‐the‐art photocurrent over 2.0 mA cm^?2 is achieved on independent GaN:ZnO photoanode when bias is higher than 1.4 V vs RHE. To the best of our knowledge, this is the first paradigm of moisture‐assisted preparing oxynitride‐based photoanode. The participation of moisture is found to improve the interconnection between adjacent GaN:ZnO nanoparticles as well as that between the GaN:ZnO film and the underlying substrate. Moreover, the volatilization of Zn can be substantially suppressed due to the modulation of reaction pathway by moisture. These two factors are confirmed to be the main reasons for the enhanced charge transportation and PEC performance obtained on GaN:ZnO photoanode.     

3D‐Printed Conical Arrays of TiO2 Electrodes for Enhanced Photoelectrochemical Water Splitting

Control over the topography of semiconducting materials can lead to enhanced performances in photoelectrochemical related applications. One means of implementing this is through direct patterning of metal‐based substrates, though this is inadequately developed. Conventional techniques for patterned fabrication commonly involve technologically demanding and tedious processes. 3D printing, a form of additive fabrication, enables creation of a 3D object by deposition of successive layers of material via computer control. In this work, the feasibility of fabricating metal‐based 3D printed photoelectrodes is explored. Electrodes comprised of conical arrays are fabricated and the performance for photoelectrochemical water splitting is further enhanced by the direct growth of TiO2 nanotubes on this platform. 3D metal printing provides a flexible and versatile approach for the design and fabrication of novel electrode structures.     

A Delaminated Defect‐Rich ZrO2 Hierarchical Nanowire Photocathode for Efficient Photoelectrochemical Hydrogen Evolution

An efficient way to combat the energy crisis and the greenhouse gas effect of fossil fuels is the production of hydrogen fuel from solar‐driven water splitting reaction. Here, this study presents a p‐type ZrO₂ nanoplate‐decorated ZrO₂ nanowire photocathode with a high photoconversion efficiency that makes it potentially viable for commercial solar H₂ production. The composition of oxygen vacancy defects, low charge carrier transport property, and high specific surface area of these as‐grown hierarchical nanowires are further improved by an hydrofluoric acid (HF) treatment, which causes partial delamination and produces a thin amorphous ZrO₂ layer on the surface of the as‐grown nanostructured film. The presence of different types of oxygen vacancies (neutral, singly charged, and doubly charged defects) and their compositional correlation to the Zr^x+ oxidation states (4 > x > 2) are found to affect the charge transfer process, the p‐type conductivity, and the photocatalytic activity of the ZrO₂ nanostructured film. The resulting photocathode provides the highest overall photocurrent (?42.3 mA cm^?2 at 0 V vs reversible hydrogen electrode (RHE)) among all the photocathodes reported to date, and an outstanding 3.1% half‐cell solar‐to‐hydrogen conversion efficiency with a Faradaic efficiency of 97.8%. Even more remarkable is that the majority of the photocurrent (69%) is produced in the visible light region.     

Tailored TiO2 Protection Layer Enabled Efficient and Stable Microdome Structured p‐GaAs Photoelectrochemical Cathodes

Group III–V compound semiconductors are a promising group of materials for photoelectrochemical (PEC) applications. In this work, a metal assisted wet etching approach is adapted to acquiring a large‐area patterned microdome structure on p‐GaAs surface. In addition, atomic layer deposition is used to deposit a TiO₂ protection layer with controlled thickness and crystallinity. Based on a PEC photocathode design, the optimal configuration achieves a photocurrent of ?5 mA cm^?2 under ?0.8 V versus Ag/AgCl in a neutral pH electrolyte. The TiO₂ coating with a particular degree of crystallization deposited via controlled temperature demonstrates a superior stability over amorphous coating, enabling a remarkably stable operation, for as long as 60 h. The enhanced charge separation induced by favorable band alignment between GaAs and TiO₂ contributes simultaneously to the elevated solar conversion efficiency. This approach provides a promising solution to further development of group III–V compounds and other photoelectrodes with high efficiency and excellent durability for solar fuel generation.     

Barium Bismuth Niobate Double Perovskite/Tungsten Oxide Nanosheet Photoanode for High‐Performance Photoelectrochemical Water Splitting

Recently, a new method to effectively engineer the bandgap of barium bismuth niobate (BBNO) double perovskite was reported. However, the planar electrodes based on BBNO thin films show low photocurrent densities for water oxidation owing to their poor electrical conductivity. Here, it is reported that the photoelectrochemical (PEC) activity of BBNO‐based electrodes can be dramatically enhanced by coating thin BBNO layers on tungsten oxide (WO₃) nanosheets to solve the poor conductivity issue while maintaining strong light absorption. The PEC activity of BBNO/WO₃ nanosheet photoanodes can be further enhanced by applying Co_0.8Mn_0.2O_x nanoparticles as a co‐catalyst. A photocurrent density of 6.02 mA cm^?2 at 1.23 V (vs reversible hydrogen electrode (RHE)) is obtained using three optically stacked, but electrically parallel, BBNO/WO₃ nanosheet photoanodes. The BBNO/WO₃ nanosheet photoanodes also exhibit excellent stability in a high‐pH alkaline solution; the photoanodes demonstrate negligible photocurrent density decay while under continuous PEC operation for more than 7 h. This work suggests a viable approach to improve the PEC performance of BBNO absorber‐based devices.     

A Sandwich‐Like Organolead Halide Perovskite Photocathode for Efficient and Durable Photoelectrochemical Hydrogen Evolution in Water

Organolead halide perovskite materials have demonstrated great potential in the solar cells field owing to their excellent optoelectronic properties. However, the instability issue of the perovskites impedes the translation of their attractive features for the solar fuel production such as photoelectrochemical H₂ production from water splitting. Herein, CH₃NH₃PbI₃ a photocathode with a sandwich‐like structure is fabricated with a general and scalable approach toward addressing this issue. The photocathode exhibits an onset potential at 0.95 V versus reversible hydrogen electrode (RHE) and a photocurrent density of ?18 mA cm^?2 at 0 V versus RHE with an impressive ideal ratiometric power‐saved efficiency of 7.63%. More impressively, the photocathode retains good stability under 12 h continuous illumination in water at wide pH range. This performance is much superior to that of the best perovskite‐based photoelectrode ever reported.     

1D Co‐Pi Modified BiVO4/ZnO Junction Cascade for Efficient Photoelectrochemical Water Cleavage

The most important factors dominating solar hydrogen synthesis efficiency include light absorption, charge separation and transport, and surface chemical reactions (charge utilization). In order to tackle these factors, an ordered 1D junction cascade photoelectrode for water splitting, grown via a simple low‐cost solution‐based process and consisting of nanoparticulate BiVO₄ on 1D ZnO rods with cobalt phosphate (Co‐Pi) on the surface is synthesized. Flat‐band measurements reveal the feasibility of charge transfer from BiVO₄ to ZnO, supported by PL measurements and photocurrent observation in the presence of an efficient hole scavenger, which demonstrate that quenching of luminescence of BiVO₄ and enhanced current are caused by electron transfer from BiVO₄ to ZnO. A dramatic cathodic shift in onset potential under both visible and full arc irradiation, coupled with a 12‐fold increase in photocurrent (ca. 3 mA cm^‐2) are observed compared to BiVO₄, resulting in ≈47% IPCE at 410 nm (4% for BiVO₄) with high solar energy conversion efficiency (0.88%). The reasons for these enhancements stem from enhanced light absorption and trapping, in situ rectifying electron transfer from BiVO₄ to ZnO, hole transfer to Co‐Pi for water oxidation, and facilitating electron transport along 1D ZnO.     

Photoelectrochemical Cycle of Bacteriorhodopsin

The scheme of the bacteriorhodopsin photocycle associated with a transmembrane proton transfer and electrogenesis is considered. The role of conformational changes in the polypeptide chain during the proton transport is discussed.     

Novel 3‐Oxo‐ and 3,24‐Dinor‐2,4‐secooleanane‐Type Triterpenes from Terminalia ivorensis A. Chev.

Two new oleanane‐type triterpenes named ivorengenin A (=3‐oxo‐2α,19α,24‐trihydroxyolean‐12‐en‐28‐oic acid; 1 ) and ivorengenin B (=4‐oxo‐19α‐hydroxy‐3,24‐dinor‐2,4‐secoolean‐12‐ene‐2,28‐dioic acid; 2 ), together with five known compounds, arjungenin, arjunic acid, betulinic acid, sericic acid, and oleanolic acid, were isolated from the barks of Terminalia ivorensis A. Chev . (Combretaceae). Their structures were established on the basis of 1D‐ and 2D‐NMR data, and mass spectrometry. A biogenetic pathway to the formation of these compounds from sericic acid, isolated as the major compound from this plant, was proposed. The antioxidant activities of different compounds were investigated by means of the 2,2‐azinobis(3‐ethylbenzothiazoline‐6‐sulfonic acid) (ABTS) and 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) assays, and IC₅₀ values were calculated and compared with Trolox activity. Antiproliferative activities of the isolated compounds were also evaluated against MDA‐MB‐231, PC3, HCT116, and T98G human cancer cell lines, against which the compounds showed significant cytotoxic activities.     

Band Edge Engineering of Oxide Photoanodes for Photoelectrochemical Water Splitting: Integration of Subsurface Dipoles with Atomic‐Scale Control

One of the crucial parameters dictating the efficiency of photoelectrochemical water‐splitting is the semiconductor band edge alignment with respect to hydrogen and oxygen redox potentials. Despite the importance of metal oxides in their use as photoelectrodes, studies to control the band edge alignment in aqueous solution have been limited predominantly to compound semiconductors with modulation ranges limited to a few hundred mV. The ability to modulate the flat band potential of oxide photoanodes by as much as 1.3 V, using the insertion of subsurface electrostatic dipoles near a Nb‐doped SrTiO₃/aqueous electrolyte interface is reported. The tunable range achieved far exceeds previous reports in any semiconductor/aqueous electrolyte system and suggests a general design strategy for highly efficient oxide photoelectrodes.