Photoelectrochemical Water Splitting Research Articles

Defect Chemistry of Titanium Dioxide (Rutile). Progress Toward Sustainable Energy.

This work, which overviews defect chemistry of TiO2 (rutile), is focused on atomic-size structural defects that are thermodynamically reversible. Here it is shown that thermodynamics can be used in defect engineering of TiO2-based energy materials, such as photoelectrodes and photocatalysts. We show that surface segregation of defects leads to the building-up of new surface structures that are responsible for reactivity. Since rational design of surface properties requires in situ surface characterization in operational conditions, expansion of bulk defect chemistry to surface defect chemistry requires a defect-related surface-sensitive tool for in situ monitoring of defect-related properties at elevated temperatures corresponding to defect equilibria and in a controlled gas-phase environment. Here we show that the high-temperature electron probe is a defect-related surface-sensitive tool that is uniquely positioned to aid surface defect engineering and determine unequivocal surface properties. The related applied aspects are considered for photoelectrochemical water splitting and the performance of solid oxide fuel cells. Here we report that trail-blazing studies on in situ surface monitoring of TiO2 during gas/solid equilibration, along with in situ characterization of surface semiconducting properties, leads to the discovery of a segregation-induced low-dimensional surface structure that is responsible for stable performance of oxide semiconductors, such as TiO2, in operational conditions.

Exploiting the Bragg Mirror Effect of TiO2 Nanotube Photonic Crystals for Promoting Photoelectrochemical Water Splitting.

Exploiting the Bragg mirror effect of photonic crystal photoelectrode is desperately desired for photoelectrochemical water splitting. Herein, a novel TiO2 nanotube photonic crystal bi-layer structure consisting of a top nanotube layer and a bottom nanotube photonic crystal layer is presented. In this architecture, the photonic bandgap of bottom TiO2 nanotube photonic crystals can be precisely adjusted by modulating the anodization parameters. When the photonic bandgap of bottom TiO2 nanotube photonic crystals overlaps with the electronic bandgap of TiO2, the bottom TiO2 nanotube photonic crystal layer will act as a Bragg mirror, leading to the boosted ultraviolet light absorption of the top TiO2 nanotube layer. Benefiting from the promoted UV light absorption, the TiO2 NT-115-NTPC yields a photocurrent density of 1.4 mA/cm2 at 0.22 V vs. Ag/AgCl with a Faradic efficiency of 100%, nearly two times higher than that of conventional TiO2 nanotube arrays. Furthermore, incident photon-to-current conversion efficiency is also promoted within ultraviolet light region. This research offers an effective strategy for improving the performance of photoelectrochemical water splitting through intensifying the light-matter interaction.

Aliovalent metal cation doping of La5Ti2AgO7S5 particles for improved photocatalytic and photoelectrochemical water splitting.

The doping of photocatalytic La5Ti2AgO7S5 particles with aliovalent metal cations was investigated. The incorporation of lower-valence Al3+ cations at Ti4+ sites improved the photocatalytic hydrogen evolution activity. In addition, the anodic photocurrent during oxygen evolution was increased upon adding higher-valence Ta5+ ions. The effect of doping on the carrier density in the photocatalytic particles was also examined in this work.

Facile one-step synthesis of a WO3/ZnWO4 heterojunction modified using ZnFe LDH enhances the PEC water splitting efficiency.

Photoelectrochemical water splitting represents a promising approach for directly converting solar energy into green hydrogen, offering a potential solution to the challenges of energy shortages and environmental pollution. In this work, a WO3/ZnWO4 binary heterojunction was synthesised by a simple and effective one-step drop casting method to enhance the charge separation efficiency; ZnFe LDH was deposited on the surface of the heterojunction with the aim of accelerating water oxidation and synergising with the heterojunction to enhance the photoelectrochemical performance of the photoanode. The photocurrent density of the WO3/ZnWO4/ZnFe LDH electrode can reach 2.1 mA cm-2 at 1.23 V (vs. RHE). This value is approximately 4 times greater than that observed for pure WO3 (0.53 mA cm-2). The IPCE and ABPE were able to improve by 3.1 times and 6 times, respectively.

Photoelectrochemical Hydrogen Production from Water Using Copper‐based Chalcopyrite Thin Films

AbstractCopper (Cu)‐based chalcopyrite compounds are promising photoabsorber materials not only for solar cells but also for photoelectrochemical (PEC) systems for conversion of sunlight energy into chemical energy. PEC water splitting to generate hydrogen (H2) is one of the most advanced technologies in a PEC system for the use of Cu‐based chalcopyrite compounds. In this review, we firstly introduce crystallographic/energetic structures of Cu‐based chalcopyrite compounds in view of their applications to PEC water splitting. Explanations for the operation of PEC water splitting using semiconductor materials are then overviewed. Based on these backgrounds, studies on PEC H2 evolution over photocathodes based on CuInS2 and CuGaSe2 thin films that we have developed are reviewed in detail. For realizing efficient PEC H2 evolution over these thin films, surface modifications with an n‐type layer such as CdS and a catalytic site such as Pt deposit were found to be indispensable. Precise controls of p‐n heterointerfaces formed by introducing an n‐type layer should also be required to enhance PEC performance. Although PEC water splitting has not reached the required efficiency to be useful, effective combinations of appropriate surface and interface modifications should lead to further improvements of properties to be close to practical applications.

Surface Epitaxial Growth of 2D/2D Bi2O2S/CdS Heterojunction Photoanodes and Their Photoelectrochemical Properties.

Constructing high catalytic activity heterojunctions to compensate for the shortcomings of single catalysts has promoted the development of semiconductor catalysts in photoelectrochemical (PEC) water splitting. In this case, the 2D/2D Bi2O2S/CdS composite was successfully constructed by an in situ surface epitaxial growth method. At 1.23 V vs RHE, the catalytic activity of Bi2O2S/CdS with a 2D/2D heterojunction is the highest, and the current density of the Bi2O2S/CdS photoanode is 3.46 mA/cm2. Compared with the Bi2O2S photoanode (0.59 mA/cm2), the performance has been improved by 5.86 times. In electrochemical impedance spectroscopy testing, the arc radius of 2D/2D Bi2O2S/CdS is smaller than that of Bi2O2S, indicating faster charge-transfer kinetics. The data show that the 2D/2D heterojunction with surface-surface contact successfully enhances the catalytic activity of Bi2O2S, greatly elevating the efficiency of charge separation and migration. This study provides a method to enhance the PEC activity in type-I heterojunction photoelectrodes, promoting the application of Bi2O2S-based materials in photoelectrochemistry.

Homo-Nuclear Hetero-Atomic Conjugated Reticular Oligomers for Heterojunction: A Novel "Electron Medium" for Panel Photoelectrocatalysis.

A substantial challenge in employing covalent organic frameworks (COFs) for photoelectrochemical (PEC) water splitting lies in improving their solution-processability while concurrently facilitating the transfer of charges and mass to the catalytic sites. Herein, we synthesize a solution-processable conjugated reticular oligomers (CROs), and further embed ruthenium (Ru) into the CRO, forming a CRO-Ru with homo-nuclear hetero-atomic. Thereafter, CRO and CRO-Ru construct an organic-organic heterojunction membrane at the nanoscale. Thisdesign achieves perfect lattice matching, significantly reducing the energy barrier of mass transfer, and effectively lowering the recombination rate of charge carriers. The optimized photocathode, CuI/CRO-Bpy:CRO-Bpy-Ru-1:1+P3HT/SnO2/Pt, exhibits an efficiency of 111.0µAcm-2 at 0.4V versus a reversible hydrogen electrode (RHE). Compared with the original bulk COFs and CROs, the efficiency is significantly improved. The apparent improvements in charge carrier separation and transfer are responsible for the high PEC activity. In the heterojunction, the incorporation of CRO-Bpy-Ru with a longer excited-state lifetime and a substantial built-in electric field has effectively accelerated the photo-induced electron transfer from the conduction band (CB) of CRO-Bpy to the valence band (VB) of CRO-Bpy-Ru, effectively suppressing the recombination of charges. These findings offer significant guidance for the design and optimization of high-performance photoelectrochemical catalysts.

Co-optimization of CuBi2O4 photocathode by heterojunction and hole-selective layer for efficient photoelectrochemical water splitting

Co-optimization of CuBi2O4 photocathode by heterojunction and hole-selective layer for efficient photoelectrochemical water splitting

Modification of solar energy and photoelectrochemical water splitting properties: using FTO/Er-TiO2/MAPbI3/CuPc/Au as modified photoelectrode

In the past decades, converting solar energy into hydrogen through photoelectrochemical water splitting has been attracted. Here, we designed a new FTO/Er-TiO2/MAPbI3/CuPc/Au photoanode with n-i-p structure for using in solar water splitting. Accourding to obtained PEC results, the FTO/Er-TiO2/MAPbI3/CuPc/Au photoanode has a photocurrent density of ∼9.8 mA cm−2 ( under a visible lamp with intensity of 100 mW cm−2 irradiation) compared to FTO/Er-TiO2/Au and FTO/Er-TiO2/MAPbI3/Au photoanodes with photocurrent density of 0.7 and 6.12 mA cm−2, respectively. This important enhancement is refered to the light absorption feature of Er-TiO2/MAPbI3 and a fast electron and hole transfer process in the Er-TiO2 and CuPc, respectively. The photoelectrochemical action of the photoelectrode is increased using erbium doping due to creation of oxygen vacancies and titanium (III) ions. Furthermore, in this research, we studided the power conversion efficiency (PCE) of FTO/Er-TiO2/MAPbI3/Au and FTO/Er-TiO2/MAPbI3/CuPc/Au solar cells. The results shown that the significant enhancement in photovoltaic properties can be observed by utilizing spin-coated CuPc nanoparticles (as hole transport layer) at least by 1.39% compared to the FTO/Er-TiO2/MAPbI3 perovskite solar cell.

Hierarchical Bi2S3 nanothorn on ZnO core-branch photoelectrode: A promising heterostructure for enhanced photoelectrochemical water splitting

The zinc oxide semiconductor associated with defects and their recombination effect restricts the development of photoelectrode for hydrogen evolution. The combination of semiconductor hetero-junction and hierarchical interface of ZnO/Bi2S3 photoelectrode fabricated. In this study, heterostructure ZnO/Bi2S3 were studied as a photoanode with their impact of oxygen vacancy in ZnO nano rods. The trace of the Bi2S3 on the ZnO was studied and compared with pristine and oxygen annealed ZnO nano rods. The photon-luminescence studies reveal that shallow donor and acceptor defect in ZnO and restricted by Bi2S3 heterostructure. The less defect contemplations in the photoanodes accelerates up electron and hole migration leading to significant built-in potential and photocurrent generation. The appropriate method has been followed to architype less interfacial defect in ZnO(300)/Bi2S3 photoanode and boosted the photo-redox reactions for efficient hydrogen evolution. The photoanode exhibits substantial properties of photocurrent density 0.33 mA/cm2, charge transfer resistance of 700 Ω cm2 and higher inbuilt potential of −0.88V vs Ag/AgCl with 0.17 % applied bias photon to electron conversion efficiency and 0.11 % solar to hydrogen conversion efficiency.

Investigating the effect of Cu underlayer and FTO etching towards photoelectrochemical performance enhancement of Cu2O photoelectrode

Cuprous oxide (Cu2O) exhibits potential as a photoactive material for photoelectrochemical water splitting, owing to its appropriate bandgap, efficient charge carrier separation, and ability to enhance solar-driven hydrogen production. This study investigates the influence of substrate etching, Cu underlayer and Cu2O electrodeposition time, and annealing time on enhancing the photoelectrochemical (PEC) performance. Electrodeposition and thermal oxidation techniques were used to fabricate the Cu2O/Cu/FTOe-A photocathode. It has been observed that FTO etching improves adhesion, light transmission, and efficiency. A Cu underlayer also impacts the PEC performance, wherein an ideal thickness of Cu leads to enhanced PEC performance. This study also focuses on the annealing time that leads to CuO layers and nanowires forming on the Cu2O surface. The structural and chemical changes before and after annealing are confirmed via XRD, XPS, AFM and FESEM analyses. UV–Vis analysis also reveals that the presence of Cu underlayer, FTO etching, and the annealing process affect the electrical properties and light absorption capacities of the Cu2O photoelectrode. Electrochemical impedance analysis (EIS) and Mott-Schottky analysis have provided insights into the enhanced charge transfer properties and band bending in the Cu2O/Cu/FTOe-A, resulting in enhanced PEC performance. Overall, this study provides significant insights into the understanding and enhancement of Cu2O/Cu/FTOe-A photocathodes for potential use in PEC water splitting applications.

Enhanced Photoelectrochemical Water Splitting of In2S3 Photoanodes by Surface Modulation with 2D MoS2 Nanosheets.

Photoanodes with ample visible-light absorption and efficient photogenerated charge carrier dynamics expedite the actualization of high-efficiency photoelectrochemical water splitting (PEC-WS). Herein, we fabricated the heterojunction nanostructures of In2S3/MoS2 on indium-doped tin oxide glass substrates by indium sputtering and sulfurization, followed by the metal-organic chemical vapor deposition of 2D MoS2 nanosheets (NSs). The photocurrent density of In2S3/MoS2 was substantially enhanced and higher than those of pristine In2S3 and MoS2 NSs. This improvement is due to the MoS2 NSs extending the visible-light absorption range and the type-II heterojunction enhancing the separation and transfer of photogenerated electron-hole pairs. This work offers a promising avenue toward the development of an efficient photoanode for solar-driven PEC-WS.

Coupling multifunctional ZnCoAl-layered double hydroxides on Ti-Fe2O3 photoanode for efficient photoelectrochemical water oxidation

The efficiency of photoelectrochemical (PEC) water splitting is hindered by the slow kinetics of the oxygen evolution reaction (OER). This study developed a composite photoanode for water oxidation by incorporating ternary LDHs (ZnCoAl-LDH) onto Ti-Fe2O3 as a cocatalyst. The ZnCoAl-LDH/Ti–Fe2O3 photoanode achieved a photocurrent density of 3.51 mA/cm2 at 1.23 V vs. RHE, which is 9.8 times higher than that of bare Ti-Fe2O3. Through a series of characterizations, the synergistic effects among the three metals were revealed. Furthermore, the addition of Zn can induce the formation of more high-valent Co, increasing the conductivity of CoAl-LDH and significantly reducing the surface charge transfer resistance. These advantages significantly enhance the injection efficiency of ZnCoAl-LDH/Ti-Fe2O3 (82 %), thereby accelerating the OER kinetics of Ti-Fe2O3. Our work introduces new approaches for selecting photoelectrochemical cocatalysts and designing high-performance photoanodes for water splitting.

N-ZrS3/p-ZrOS Photoanodes with NiOOH/FeOOH Oxygen Evolution Catalysts for Photoelectrochemical Water Oxidation.

Photoelectrochemical water splitting offers a promising approach for carbon neutrality, but its commercial prospects are still hampered by a lack of efficient and stable photoelectrodes with earth-abundant materials. Here, we report a strategy to construct an efficient photoanode with a coaxial nanobelt structure, comprising a buried-ZrS3/ZrOS n-p junction, for photoelectrochemical water splitting. The p-type ZrOS layer, formed on the surface of the n-type ZrS3 nanobelt through a pulsed-ozone-treatment method, acts as a hole collection layer for hole extraction and a protective layer to shield the photoanode from photocorrosion. The resulting ZrS3/ZrOS photoanode exhibits light harvesting with good photo-to-current efficiencies across the whole visible region to over 650 nm. By further employing NiOOH/FeOOH as the oxygen evolution reaction cocatalyst, the ZrS3/ZrOS/NiOOH/FeOOH photoanode yields a photocurrent density of ~9.3 mA cm-2 at 1.23 V versus the reversible hydrogen electrode with an applied bias photon-to-current efficiency of ~3.2% under simulated sunlight irradiation in an alkaline solution (pH = 13.6). The conformal ZrOS layer enables ZrS3/ZrOS/NiOOH/FeOOH photoanode operation over 1000 hours in an alkaline solution without obvious performance degradation. This study, offering a promising approach to fabricate efficient and durable photoelectrodes with earth-abundant materials, advances the frontiers of photoelectrochemical water splitting.

Tuning the photocatalytic performance of halide perovskites for efficient solar hydrogen production: A DFT study of CsGeCl3-xXx (X= Br, I)

Photoelectrochemical water splitting holds immense potential for sustainable hydrogen production to address mounting energy demands. However, the development of efficient and cost-effective photocatalysts remains a significant challenge. Perovskites, recognized for their cost-effectiveness and tunable characteristics, show potential as viable candidates in this context. Our investigation is the first to mark the photocatalytic efficacy of CsGeCl3. The findings reveal a modest solar-to-hydrogen (STH) efficiency of 0.74 % due to its wide bandgap energy. Halide mixing with iodine and bromine significantly improves the STH efficiencies, reaching approximately 8.47 %. In addition, applying uniaxial compressive stress further boosts the efficiency to 14.6 %. In this study, we employed the density functional theory (DFT) through the WIEN2k software to scrutinize the structural, electronic, optical, photocatalytic, and thermoelectric properties of all the studied structures. Furthermore, the study evaluated the compounds' capacity for CO2 photoreduction, susceptibility to degradation, and the influence of pH on their photocatalytic performance. The insights gained from this work contribute to the development of efficient and cost-effective perovskite-based photocatalysts for renewable hydrogen production.

Band Gap Engineering of Spin-Coated <i>α</i>-Fe<sub>2</sub>O<sub>3</sub> for Solar Water Splitting Utilizing a Facile Zinc Doping Route

Iron oxide (hematite, α-Fe2O3) is an earth-abundant material having a favorable band gap for solar energy harvesting by producing Hydrogen (H2) by photoelectrochemical (PEC) water splitting. Although Fe2O3 has a valence band maximum (VBM) more positive than the water oxidation potential in the normalized hydrogen energy scale (NHE), it can not be used as a photocathode because of the more positive conduction band minima (CBM) than the reduction potential of the electrons required for the hydrogen evolution reaction (HER). This paper reports the band gap engineering of Fe2O3 by doping it with Zinc (Zn) through a facile way to modify its CBM and VBM to enable it to work as an ideal and efficient photocatalyst. One-pot synthesis of α-Fe2O3 doped with Zn was achieved by spin-coating a precursor solution prepared with FeCl2 and Zinc acetate (Zn(CH3COO)2.2H2O) on fluorine-doped tin oxide (FTO) or glass substrates. The films were then annealed in air at around 450 °C. The surface morphology and the crystal structure of the Fe2O3 were studied using SEM (Scanning Electron Microscope) and XRD (X-ray diffractometer). The UV-VIS spectrophotometry and spectroelectrochemistry (SEC) determined the band energies of α-Fe2O3 and Zn-doped α-Fe2O3. The band gap and CBM of pristine α-Fe2O3 were found to be -2.09 eV and -5.31 eV vs. EVAC, whereas the band gap and CBM of Zn-doped α-Fe2O3 were -1.96 eV and -4.3 eV vs. EVAC respectively. The outcome of the photoelectrochemical study shows that α-Fe2O3 doped with Zn can have a reduced band gap with more negative CBM than the H+/H2 reduction potential that efficiently ensures both oxygen and hydrogen evolution reaction.

Photoelectrochemical Ethylene Glycol Oxidization Coupled with Hydrogen Generation Using Metal Oxide Photoelectrodes.

Photoelectrochemical (PEC) water splitting represents a promising approach for harnessing solar energy and transforming it into storable hydrogen. However, the complicated 4-electron transfer process of water oxidation reaction imposes kinetic limitations on the overall efficiency. Herein, we proposed a strategy by substituting water oxidation with the oxidation of ethylene glycol (EG), which is a hydrolysis byproduct of polyethylene terephthalate (PET) plastic waste. To achieve this, we developed and synthesized BiVO4/NiCo-LDH photoanodes capable of achieving a high Faradaic efficiency (FE) exceeding 85% for the oxidation of EG to formate in a strongly alkaline environment. The reaction mechanism was further elucidated using in-situ FTIR spectroscopy. Additionally, we successfully constructed an unassisted PEC device for EG oxidation and hydrogen generation by pairing the translucent Mo:BiVO4/NiCo-LDH photoanode with a state-of-the-art Cu2O photocathode, resulting in an approximate photocurrent density of 2.3 mA/cm2. Our research not only offers a PEC pathway for converting PET plastics into valuable chemicals but also enables simultaneous hydrogen production.

Photoelectrochemical Ethylene Glycol Oxidization Coupled with Hydrogen Generation Using Metal Oxide Photoelectrodes

Photoelectrochemical (PEC) water splitting represents a promising approach for harnessing solar energy and transforming it into storable hydrogen. However, the complicated 4‐electron transfer process of water oxidation reaction imposes kinetic limitations on the overall efficiency. Herein, we proposed a strategy by substituting water oxidation with the oxidation of ethylene glycol (EG), which is a hydrolysis byproduct of polyethylene terephthalate (PET) plastic waste. To achieve this, we developed and synthesized BiVO4/NiCo‐LDH photoanodes capable of achieving a high Faradaic efficiency (FE) exceeding 85% for the oxidation of EG to formate in a strongly alkaline environment. The reaction mechanism was further elucidated using in‐situ FTIR spectroscopy. Additionally, we successfully constructed an unassisted PEC device for EG oxidation and hydrogen generation by pairing the translucent Mo:BiVO4/NiCo‐LDH photoanode with a state‐of‐the‐art Cu2O photocathode, resulting in an approximate photocurrent density of 2.3 mA/cm2. Our research not only offers a PEC pathway for converting PET plastics into valuable chemicals but also enables simultaneous hydrogen production.

Characterization of titania self-aligned nanotubes prepared under various conditions for the photoelectrochemical water splitting

TiO2 nanotubes were grown by anodization of Ti sheets in an ethylene glycol solution. The then prepared nanotubes were tested with and without heat treatment at 450 ºC, for the water splitting process. The heat treatment produced increased photocurrent from 4.9 µA cm−2 to 56.8 µA cm−2 under illumination at 0.7 V vs. SHE. Modification of the nanotubes was carried out by electroreduction on both heat treated and untreated samples and its photoactivity evaluated. The spectral response showed increased photocurrent for the reduced material (black titania), although no appreciable change was detected in the absorbed spectra for the visible wavelengths. Finally, high-purity (> 99.6%) and commercially pure (99%) Ti sheets were used to grow TiO2 nanotubes with no clear difference in photocurrents (61.5 µA cm−2 vs. 65.4 µA cm−2 while illuminated at 0.7 V vs. SHE) showing the potential for cheap commercialization.

NiO@GaN nanorods-based core-shell heterostructure for enhanced photoelectrochemical water splitting via efficient charge separation

Remarkable properties of III-V semiconductors, particularly GaN nanostructured based photoelectrodes offers a potential attention in the field of photoelectrochemical water splitting (PEC-WS) for clean and sustainable hydrogen production, due to its wide bandgap, magnificent optoelectrical properties. However, the presence of inevitable surface states in GaN nanorods (NRs) leads to low solar-to-hydrogen (STH) conversion efficiency with poor stability, thereby severely limiting their practical application in PEC-WS, which can be effectively alleviated by constructing a hybrid heterostructures. Herein, we present the development of interfacial engineering of a type-II core-shell heterostructure based on p-NiO nanoparticles (NPs) loaded on n-GaN NRs photoelectrodes for PEC-WS. We assessed the impact of the NiO NPs shell density on core GaN NRs, finding that the optimized NiO@GaN NRs photoelectrode achieved a photocurrent density (Jph) of 1.38 mA/cm² at 1.23 V versus RHE and an excellent applied bias photo-to-current conversion efficiency (ABPE) of ∼0.39 %, which was 3.8 (Jph) and 4.8 (ABPE)-fold times higher than the pristine GaN NRs photoanode under 1-Sun illumination. The type-II p-n heterojunction band alignment between core-shell NiO@GaN NRs photoelectrode effectively reduced the photogenerated carrier recombination rate through surface states passivation and boost the light absorption and harvesting capacity. This facilitates a significant charge separation and transfer at the photoanode/electrolyte interface leading to enhanced redox reactions, resulting in improved PEC-WS and STH performances. These findings offer a promising strategy to design and fabricate highly efficient III-V hybrid heterostructure photoelectrodes for futuristic PEC-WS-based green energy applications.