Photoelectrochemical Water Reduction Research Articles

Molecular and Heterojunction Device Engineering of Solution-Processed Conjugated Reticular Oligomers: Enhanced Photoelectrochemical Hydrogen Evolution through High-Effective Exciton Separation.

Covalent organic frameworks (COFs) face limited processability challenges as photoelectrodes in photoelectrochemical water reduction. Herein, sub-10nm benzothiazole-based colloidal conjugated reticular oligomers (CROs) are synthesized using an aqueous nanoreactor approach, and the end-capping molecular strategy to engineer electron-deficient units onto the periphery of a CRO nanocrystalline lattices (named CROs-Cg). This results in stable and processable "electronic inks" for flexible photoelectrodes. CRO-BtzTp-Cg and CRO-TtzTp-Cg expand the absorption spectrum into the infrared region and improve fluorescence lifetimes. Heterojunction device engineering is used to develop interlayer heterojunction and bulk heterojunction (BHJ) photoelectrodes with a hole transport layer, electron transport layer, and the main active layers, using a CROs/CROs-Cg or one-dimensional (1D) electron-donating polymer HP18 mixed solution via spinning coating. The ITO/CuI/CRO-TtzTp-Cg-HP18/SnO2/Pt photoelectrode shows a photocurrent of 94.9µA cm‒2 at 0.4V versus reversible hydrogen electrode (RHE),which is 47.5 times higher than that of ITO/Bulk-TtzTp. Density functional theory calculations show reduced energy barriers for generating adsorbed H* intermediates and increased electron affinity in CROs-Cg. Mott-Schottky and charge density difference analyses indicate enhanced charge carrier densities and accelerated charge transfer kinetics in BHJ devices. This study lays the groundwork for large-scale production of COF nanomembranes and heterojunction structures, offering the potential for cost-effective, printable energy systems.

Solution-processed asymmetry conjugated reticular oligomers for large-area panel photoelectrodes in photoelectrochemical devices

Covalent organic frameworks (COFs) show potential in photoelectrochemical (PEC) water reduction. Herein, we synthesize solution-processed benzoxazole-based conjugated reticular oligomers (BBO-CROs) in micelle nanoreactors, acting as an “electronic paint” for fabricating consecutive panel-type photoelectrodes for PEC water reduction. The small-molecule BBO-CROPDA, BBO-CRODHTA, and BBO-CROBPY exhibit a narrower energy bandgap and prolong the fluorescence lifetimes of photoinduced charge carriers compared to bulk BBO-COFs. Colloidal COFs in PEC water reduction are illustrated through the creation of flexible full devices via spinning coating. The optimized photoelectrode CuI/BBO-CROBPY+P27/SnO2/Pt result in ΔJ of 86.5 μA cm−2 at 0.4 V vs. RHE, which is 86.5 times higher than that of bulk BBO-COFBPY. The CuI/BBO-CROBPY+P27/SnO2/Pt photoelectrode with an area of 100 cm² displays an impressive ΔJ of 82.0 μA cm−2 with good durability. A flat photoelectrode surface and the generation of photogenerated charge carriers under light are observed using scanning electrochemical microscopy (SECM) as an in-situ technique. Density Functional Theory (DFT) calculation indicates that the asymmetry of the BBO unit prompts electrons to preferentially move in a defined direction upon light excitation in the small-molecular framework, thereby suppressing electron backflow. This work overcomes COF processability limitations in PEC water reduction and paves the way for using flexible devices.

Facilitating light utilization and electron transfer of TiO2 nanotube arrays for catalyzing photoelectrochemical water reduction via acid etching and copper doping

Photoelectrochemical catalytic capability mainly relies on light absorption and charge transfer efficiency. TiO2 nanotube array (TNA) having suitable band edges and one-dimensional hollow structure is regarded as one of efficient photoelectrocatalysts toward water reduction. In this study, two strategies are firstly applied to modify TNA as the photocatalyst of water reduction. Ti foils are etched by acid to produce multiple light reflections planes for improving light absorption, and Cu is doped in TNA to enhance charge transfer efficiency. The optimal Cu-doped TNA electrode on the acid-etched Ti foil presents a much smaller overpotential of 91 mV at 10 mA/cm2 compared with that of the undoped TNA (524 mV). A smaller Tafel slope of 134 mV/dec is obtained for the optimal Cu-doped TNA electrode than that of the undoped TNA (167 mV/dec). After measuring the photoelectrochemical performance for 1000 cycles, overpotential and charge-transfer resistance of modified TNA are largely reduced owing to activation. Current retention of 96.7% is achieved after continuously illuminating for 10 h. Highly improved photoelectrochemical catalytic ability of TNA modified by acid etching and Cu doping provides new pages for material modifications on substrate and active material.

Ferri-hydrite: A Novel Electron-Selective Contact Layer for InP Photovoltaic and Photoelectrochemical Cells.

Solar energy conversion devices with charge-selective contacts are attracting significant research interest as a cost-effective alternative to homojunction counterparts. This study presents a novel approach for fabricating high-performance solar cells based on InP heterojunctions using a solution-processed ferri-hydrite (Fh) electron-selective contact (ESC). The champion cell efficiency of 16.6% is achieved, which is a significant improvement over those from previous studies using other solution-processed ESC materials. X-ray photoelectron spectroscopy measurements showed that the low conduction band offset at the Fh-InP interface facilitated selective transport of photogenerated electrons from InP. Moreover, the Fh electron-selective contact layer provided an excellent photoelectrochemical half-cell water reduction efficiency of 8.4%. The Fh layer not only selectively extracts photogenerated electrons from InP but also simultaneously serves as a surface protection layer, improving the cell's long-term stability. These results demonstrate the potential of Fh as a low-cost and easily fabricated material for use in high-efficiency photovoltaic and photoelectrochemical devices. Our findings pave the way for further improvements in the efficiency of InP heterojunction solar cells by addressing the losses incurred in the cells.

Crystalline-face-dependent photoelectrochemical properties of single crystalline CuGaSe2 photocathodes for hydrogen evolution under sunlight radiation

Photoelectrochemical (PEC) water splitting by using a electrode based on a single crystalline compound provide a good opportunity for studying on fundamental aspects of net semiconductor bulk properties related to PEC performance. In this study, single crystalline CuGaSe2 wafers exposed (321), (312), and (112) facets were used as sources of photocathodes for investigating PEC properties for water reduction into hydrogen under sunlight radiation related to their surface properties. A CuGaSe2 single crystalline ingot grown by using the zone melting method was cut along with different directions to obtain wafers with different crystal orientations. The electrical and electron-energy properties of wafers after annealing in an Se atmosphere indicated the formation of a Cu-deficient component on their surfaces when the wafers oriented (321) and (312) directions were used, whereas formation of a component was not observed on the wafer oriented in the (112) direction. Since the Cu-deficient component acted as a hole blocking layer to suppress the recombination of photoexcited carries at the heterojunction, photocathodes based on wafers having (321) and (312) orientations showed enhanced PEC performance for water reduction compared to the PEC performance of a photocathode based on a wafer having the (112) orientation. Highly active photocathodes for water reduction based on polycrystalline thin films usually have preferential (112) orientation, whereas those of densely packed powder forms would not be probable for controlling crystalline orientations. Therefore, present results would be a guide to achieve further improvements of PEC water reduction using such conventional systems.

Bandgap Reduction and Enhanced Photoelectrochemical Water Electrolysis of Sulfur-doped CuBi2O4 Photocathode

As interest in hydrogen energy grows, eco-friendly methods of producing hydrogen are being explored. CuBi2O4 is one of the p-type semiconductor cathode materials that can be used for photoelectrochemical hydrogen production via environment-friendly water electrolysis. CuBi2O4 has a bandgap of 1.5 – 1.8 eV which allows it to photogenerate electrons and holes from the absorption of visible light. This study investigated the effect of sulfur doping on the bandgap and photoelectrochemical water reduction properties of CuBi2O4. Sulfur-doped CuBi2O4 thin films were electrochemically synthesized using a nitrate-based precursor solution with thiourea. This was followed by two-step annealing in an Ar atmosphere, which effectively prevented the oxidation of sulfur. Sulfur doping up to 0.1 at% led to the expansion of the lattice volume of the CuBi2O4. The bandgap was reduced from 1.9 eV to 1.5 eV with increasing doping concentration, which resulted in the enhancement of photoelectrochemical current density by ~240%. X-ray photoelectron spectroscopy showed that sulfur-doping reduced oxygen vacancies with increasing doping concentration, confirming that the enhanced photoelectrochemical properties resulted from the reduction in bandgap, not from any extrinsic factor such as oxygen vacancies. Further studies of sulfur-doped CuBi2O4 to improve surface coverage are expected to lead to a more promising photoelectrochemical cathode material.

Effect of sintering optimization on the photoelectrochemical performance of Cu2ZnSnS4 nanocrystal films

AbstractBACKGROUNDSemiconductor‐based photoelectrochemical (PEC) water splitting represents one attractive route to generate clean hydrogen. Cu2ZnSnS4 (CZTS) is considered a promising photoelectric material for PEC water splitting due to its high absorption coefficient, small bandgap, nontoxicity, and earth‐abundant elements. Only a few critical parameters limit the performance of CZTS nanocrystal films. Of those, heat treatment is essential for producing high‐performance films.RESULTSIn this work, CZTS nanocrystals were produced by a facile method and applied in making CZTS films for PEC water reduction. Therefore, the influences of heat treatment parameters on the performance of films were investigated in the low‐temperature heat treatment range. The surfactant polyvinylpyrrolidone (PVP) in the films could not be completely decomposed at low heat treatment temperatures, leading to film resistance that was harmful to the PEC process. Meanwhile, SnSx impurities would arise at temperatures over 400 °C, thus reducing the PEC performance.CONCLUSIONThe best activity for PEC water reduction was obtained at 385 °C for 10 min. Furthermore, the PEC performance of CZTS films generated by fast heating (10 °C s−1) was superior to that of films generated by slow heating (10 °C min−1) due to the inhibition from the development of SnSx impurities. © 2022 Society of Chemical Industry (SCI).

Ferroelectric polarization controlled surface potential and charge transport across ITO-Nd-doped BiFeO3 polycrystalline ceramic bulk for photoelectrochemical solar water splitting

The spontaneous polarization in ferroelectrics provides an effective way to engineer interfacial charge distribution at the ferroelectric heterojunctions, thereby depletion width in p-n junction and energy band structures. Here, we integrated p-type multiferroic 7% neodymium-doped bismuth ferrite Bi0.93Nd0.07FeO3 (BFO7Nd) polycrystalline ceramic bulk with n-type transparent conductive indium tin oxide (ITO) to demonstrate the influence of ferroelectric polarization on surface potential and its effect on photoelectrochemical (PEC) water reduction. Kelvin probe force microscopy (KPFM) results reveal the surface potential change during ferroelectric polarization switching, suggesting that the polarization bound charges may have an effect on space charge region between ITO-BFO7Nd, thus dominating surface potential at the electrode/electrolyte interface. This implies that the interface barrier between ITO and BFO7Nd can be easily modulated by polarization bound charges. The electron lifetime of upward-polarization (P-up) ITO-BFO7Nd is estimated 29.30 ms, about 15 times longer than unpoled ITO-BFO7Nd. The photocurrent density of P-up ITO-BFO7Nd is ∼1.2 × 10−6 A/cm2 (at 0 V vs RHE), which is one-order higher compared to that of the unpoled and P-down states under illumination with an intensity of 100 mWcm−2. This work shows that ferroelectric polarization combined with photogenerated carriers is an effective approach for effective charge transfer in multiferroic ceramic to boost PEC solar water splitting.

Hydrogen-substituted graphdiyne encapsulated cuprous oxide photocathode for efficient and stable photoelectrochemical water reduction

Photoelectrochemical (PEC) water splitting is an appealing approach for “green” hydrogen generation. The natural p-type semiconductor of Cu2O is one of the most promising photocathode candidates for direct hydrogen generation. However, the Cu2O-based photocathodes still suffer severe self-photo-corrosion and fast surface electron-hole recombination issues. Herein, we propose a facile in-situ encapsulation strategy to protect Cu2O with hydrogen-substituted graphdiyne (HsGDY) and promote water reduction performance. The HsGDY encapsulated Cu2O nanowires (HsGDY@Cu2O NWs) photocathode demonstrates a high photocurrent density of −12.88 mA cm−2 at 0 V versus the reversible hydrogen electrode under 1 sun illumination, approaching to the theoretical value of Cu2O. The HsGDY@Cu2O NWs photocathode as well as presents excellent stability and contributes an impressive hydrogen generation rate of 218.2 ± 11.3 μmol h−1cm−2, which value has been further magnified to 861.1 ± 24.8 μmol h−1cm−2 under illumination of concentrated solar light. The in-situ encapsulation strategy opens an avenue for rational design photocathodes for efficient and stable PEC water reduction.

VS2 wrapped Si nanowires as core-shell heterostructure photocathode for highly efficient photoelectrochemical water reduction performance.

Interfacing an electrocatalyst with photoactive semiconductor surfaces is an emerging strategy to enhance the photocathode performance for the solar water reduction reaction. Herein, a core-shell heterostructure photocathode consisting of vanadium disulfide (VS2) as a 2D layered electrocatalyst directly deposited on silicon nanowire (Si NWs) surface is realized via single-step chemical vapor deposition towards efficient hydrogen evolution under solar irradiation. In an electrochemical study, 2D VS2/Si NWs photocathode exhibits a saturated photocurrent density (17mAcm-2) with a maximal photoconversion efficiency of 10.8% at -0.53V vs. RHE in neutral electrolyte condition (pH∼7). Under stimulated irradiation, the heterostructure photocathode produces a hydrogen gas evolution around 23μmolcm-2 h-1 (at 0V vs. RHE). Further, electrochemical impedance spectroscopy (EIS) analysis reveals that the high performance of the core-shell photocathode is associated with the generation of the high density of electron-hole pairs and the separation of photocarriers with an extended lifetime. Density functional theory calculations substantiate that core-shell photocathodes are active at very low Gibbs free energy (ΔGH*) with abundant hydrogen evolution reaction (HER) active sulphur sites. The charge density difference plot with Bader analysis of heterostructure reveals the accumulation of electrons on the sulphur sites via modulating the electronic band structure near the interface. Thus, facilitates the barrier-free charge transport owing to the synergistic effect of Si NWs@2D-VS2 core-shell hybrid photocatalyst for enhanced solar water reduction performance.

(Invited, Digital Presentation) Photo-Electrochemical Water and CO2 Reduction Devices Operating Under Concentrated Radiation

Achieving high current densities while maintaining a high energy conversion efficiency is one of the main challenges for enhancing the economic competitiveness of solar fuel producing photo-electrochemical devices [1]. I will discuss two device implementations utilizing concentrated irradiation to achieve high current density operation. The water-splitting device is utilizing thermal integration to sustain high performance while dealing with high current density and the corresponding overpotentials [2]. I will quantify the theoretical increase in the maximum efficiencies at given current densities of photoelectrochemical devices resulting from thermal synergies. I will then discuss device implementation of such an approach and show how more realistic device models (multi-dimensional, multi-scale, multi-physics) can be used to support the device implementation and its operational understanding [3]. I will then show how the design principles developed for water splitting can be translated to CO2 reduction devices and discuss a corresponding device implementation.[1] M. Dumortier, S. Tembhurne, S. Haussener, Energy Environmental Science, 8: 3614-3628, 2015.[2] S. Tembhurne, F. Nandjou, S. Haussener, Nature Energy, 4: 399-407, 2019.[3] S. Tembhurne, S. Haussener, Journal of The Electrochemical Society, 163: H1008-H1018, 2016.

Detachable all-carbon-linked 3D covalent organic framework films for semiconductor/COF heterojunctions by continuous flow synthesis

Detachable all-carbon-linked 3D covalent organic framework films for semiconductor/COF heterojunctions by continuous flow synthesis

Solar-Driven Syngas Production Using Al-Doped ZnTe Nanorod Photocathodes.

Syngas, traditionally produced from fossil fuels and natural gases at high temperatures and pressures, is an essential precursor for chemicals utilized in industry. Solar-driven syngas production can provide an ideal pathway for reducing energy consumption through simultaneous photoelectrochemical CO2 and water reduction at ambient temperatures and pressures. This study performs photoelectrochemical syngas production using highly developed Al-doped ZnTe nanorod photocathodes (Al:ZnTe), prepared via an all-solution process. The facile photo-generated electrons are transferred by substitutional Al doping on Zn sites in one-dimensional arrays to increase the photocurrent density to −1.1 mA/cm2 at −0.11 VRHE, which is 3.5 times higher than that for the pristine ZnTe. The Al:ZnTe produces a minor CO (FE ≈ 12%) product by CO2 reduction and a major product of H2 (FE ≈ 60%) by water reduction at −0.11 VRHE. Furthermore, the product distribution is perfectly switched by simple modification of Au deposition on photocathodes. The Au coupled Al:ZnTe exhibits dominant CO production (FE ≈ 60%), suppressing H2 evolution (FE ≈ 15%). The strategies developed in this study, nanostructuring, doping, and surface modification of photoelectrodes, can be applied to drive significant developments in solar-driven fuel production.

Theoretical Insights into Photoelectrochemical Water Reduction on Delafossite CuRhO2

Theoretical Insights into Photoelectrochemical Water Reduction on Delafossite CuRhO₂

Yb3+, Er3+ co-doped NaGdF4/BiVO4 embedded Cu2O photocathodes for photoelectrochemical water reduction with near infrared light

Upconverting near infrared light to photoelectrode absorbable visible light is desirable for enhancing photoelectrochemical hydrogen production. In this work, photocathodes were fabricated by embedding β-NaGdF4:Yb3+,Er3+ nanocrystals and BiVO4 microcrystals into Cu2O matrix, where β-NaGdF4:Yb3+, Er3+ nanocrystals were used for upconversion luminescence and the BiVO4 microcrystals were employed to form abundant hetero-junctions inside the Cu2O matrix. In comparison with the pristine Cu2O thin film, the NaGdF4:Yb3+,Er3+/BiVO4 embedded Cu2O photocathodes illustrated good response to near infrared light. Photocurrent produced by 980 nm light was enhanced by a factor of 40 to about 1 mA cm−2. The high photocurrent can be attributed to the widely distributed internal electric fields introduced by the BiVO4 in the bulk of the Cu2O, which inhibits the recombination of the electrons and holes excited by the NaGdF4:Yb3+,Er3+ emitted visible light in the vicinity. Furthermore, the 980 nm light induced temperature increment exhibits significant synergistic effect on the enhanced photocurrent.

Highly active titanium oxide photocathode for photoelectrochemical water reduction in alkaline solution

Developing low-cost, efficient and stable metal oxide cathode catalyst for hydrogen evolution reaction (HER) is of great importance to overcome the major challenge in photoelectrochemical water splitting (PECWS) cell. Anodic titanium dioxide nanotube (TNT) as the electrode has shown promising for its flexibility, economy and efficiency. Herein, a highly active and stable photocathode for HER was achieved with a technique of stepwise cathodic reduction of anodic TNT without loading any cocatalyst. It exhibits a self-improving PECWS performance after the five-step cathodic reduction process. The onset voltage displays an anodic shift from −0.79 to 0.19 V versus the reversible hydrogen electrode (RHE) and the overall current shows an enhancement from −110.06 to −210.66 mA at −1.0 VRHE in 1 M NaOH aqueous solution (pH = 13.6). The photocathode also withstands a voltage @ −1.0 VRHE and a current @ −100 mA in the WS cell for more than 12,000 s sequentially. This work demonstrates a feasible strategy for forming a multijunction composite for efficient HER with the anodic TNT as the precursor.

The strong interfacial coupling effect of Nafion between LaFeO3/electrolyte for efficient photoelectrochemical water reduction

Insufficient reduction capability and scanty active substance limit the application of LaFeO3 (LFO) in the field of photoelectrochemical (PEC) water splitting. This work demonstrates a judicious combination of LFO/Nafion composite to improve the PEC performance by a unique dip-coating method on the FTO. The photocurrent density of the LFO electrode coated with two layers Nafion increased to −23.9 μA cm−2 at 0.47 V versus RHE, which is 4.1 times that of the pristine LFO. Based on the experimental data and theoretical analysis, the improvement of the PEC properties is attributed to the construction of organic/inorganic units, which would enable strong electronic coupling and favor interfacial charge transfer, resulting in a 30 mV downward shift of its flat band potential. Thus, the conduction band gets closer to the proton reduction potential of H+ to H2 after decoration with Nafion, resulting in a stronger photogenerated electron reduction ability. Our study provides insights that organic materials modify semiconductor photoelectrodes for accelerating charge kinetics.

Electropolymerization process dependent poly(1,4-di(2-thienyl)benzene) based full spectrum activated photocathodes for efficient photoelectrochemical hydrogen evolution

In this work, with the help of different electrochemical polymerization methods, the surface morphology of poly-1,4-bis(2-thienyl)benzene (PDTB) photocathode and S oxidation state were modulated. Thus, PDTB photocathodes with strong light absorption (310–2500 nm) and strong charge separation ability were successfully prepared. • Controllable composition, morphology and performance of PDTB photocathodes. • A novel strategy for design photocathodes with low band gap. • The enhanced photoelectrochemical performance was confirmed by detailed experiments. Electrochemical surfaces are essential for photoelectrocatalytic processes occuring on semiconductor electrodes, which influence light harvesting capability and charge separation of photoelectrochemical (PEC) hydrogen evolution process. In this work, electrochemical processes through cyclic voltammetry (CV), chronoamperometry (CA) and amperometry ( i-t ) show different abilities to electropolymerization. It results in poly-1,4-bis(2-thienyl)benzene (PDTB) photoelectordes with various microporous structures and regulated S oxidation states. Benifiting from these effect factors, the light absorption ability and charge separation of PDTB photoelectordes are expanded and promoted. In particular, PDTB photoelectrodes prepared by CA with 1.23 eV band gap and strong light absorption (310–2500 nm) show the best photoelectrochemical activity with a charge transfer resistance of 7 Ω, a photocurrent of 0.9 mA cm −2 and increases the onset hydrogen production potential to 0.92 V. The incident photon to current efficiency (IPCE) can reach 14.16 % at 850 nm. It suggests that photoelectrodes from electropolymerization may offer a simple and efficient strategy by adjusting the degree of polymerization to get a promising low band gap semiconductor for efficient PEC water reduction.

Cover Feature: Towards Highly Efficient Chalcopyrite Photocathodes for Water Splitting: The Use of Cocatalysts beyond Pt (ChemSusChem 21/2021)

The Cover Feature shows non-noble metal-based alternative cocatalysts beyond Pt, for CIGS and CIS photocathodes applied in the photoelectrochemical water reduction such as transition metal alloys and chalcogenides, and inorganic carbon-based nanostructures. More information can be found in the Minireview by A. Corrado Salomao et al.

Fabrication of TiOx/Sb2Se3/p-NiOx photocathode for efficient photoelectrochemical water reduction

Photoelectrochemical (PEC) water splitting directly converts solar energy into clean and sustainable hydrogen energy, representing a promising and effective technique for solving the world's energy crisis. A hole transfer layer (HTL) plays a vital role for extracting holes from a semiconductor layer to a back electrode in PEC cells for water splitting. In this work, a TiOx/Sb2Se3/NiOx photocathode was fabricated by using p-NiOx as HTL. We show that the electrical properties of NiOx films can be well adjusted by the pulsed laser deposition technique. By systematically adjusting the electrical properties of the NiOx HTL and passivating the surface defects through coating with the TiOx layer, we achieved a high photocurrent density of up to −18.08 mA cm−2 at 0 V vs a reversible hydrogen electrode (RHE), an onset potential of 0.43 V vs RHE, and a half-cell solar to hydrogen conversion efficiency of 1.01% for the TiOx/Sb2Se3/NiOx photocathode. This work demonstrated NiOx as a potential hole transport material for a cost-effective Sb2Se3 PEC cell.