Efficient Photoelectrochemical Water Research Articles

Unraveling Photothermal-Enhanced bulk charge transport and surface oxygen reactions in TiO2 photoanodes for highly efficient photoelectrochemical water oxidation

Titanium dioxide (TiO2) has been extensively used as an important photoanode material for photoelectrochemical (PEC) water splitting, but its activity is greatly hindered by the low charge transport and sluggish oxygen evolution reaction (OER) kinetics. Herein, we demonstrate spinel oxide NiCo2O4 (NCO) acted as both photothermal material and oxygen evolution cocatalyst could significantly promote the PEC performance of TiO2 photoanodes, and a high photocurrent of 2.34 mA cm−2 at 1.23 V vs. reversible hydrogen electrode has been achieved by NCO/TiO2 photoanodes assisted by the photothermal effect. Importantly, a mechanism is proposed to explain the enhanced bulk charge transport and surface OER activity by an integrated various operando characterizations and density functional theory (DFT) study. Intensity-modulated photocurrent spectroscopy (IMPS) analysis and DFT calculations imply the relatively high temperature induced by the photothermal effect could accelerate small polaron hopping in the bulk of TiO2, which is verified by the electron diffusion coefficient variation of TiO2 with oxygen vacancies at different temperatures. Operando cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) reveal that photothermal conversion facilitates the oxidation of Ni2+ at lower potential and promotes the adsorption of *OH and deprotonation of *OOH intermediates. This work provides deep insight into the photothermal-enhanced PEC performance by observing the bulk small polaron hopping and surface dynamic evolution during the PEC process, which may underpin future advances in promoting PEC and photocatalytic performances of small-polaron-type semiconductors.

Modulation of Lewis and Brønsted Acidic Sites to Enhance the Redox Ability of Nb2O5 Photoanodes for Efficient Photoelectrochemical Performance.

Accelerated surface redox reaction and regulated carrier separation are the crux to the development of highly reactive oxide semiconductors for efficient photoelectrochemical water splitting. Here, we have selected Nb2O5 materials that combine unique surface acidity and semiconductor properties, and first used surface phosphorylation to change its surface acidic sites (Lewis and Brønsted acidic sites) to achieve efficient photoelectrochemical water splitting. The resulting photoanode born from this strategy exhibits a high photocurrent density of 0.348 mA/cm2 at 1.23 VRHE, which is about 2-fold higher than that of the bare Nb2O5, and a cathodic shift of 60 mV. Detailed experimental results show that the large increase in the Lewis acidic site can effectively modulate the electronic structure of the active sites involved in catalysis in [NbO5] polyhedra and promote the activation of lattice oxygen. As a result, higher redox properties and the ability to inhibit carrier recombination are exhibited. In addition, the weakening of the Brønsted acidic site drives the reduction of protons in the oxygen evolution reaction (OER) and accelerates the reaction kinetics. This work advances the development of efficient photoelectrochemical water splitting on photoanodes driven by the effective use of surface acidity and provides a strategy for enhancing redox capacity to achieve highly active photoanodes.

Ni-Doped BiVO4 photoanode for efficient photoelectrochemical water splitting

BiVO4 (BVO) based photoanode is one of the most mega-potential materials for solar water splitting while suffers from poor charge transfer and separation efficiency limit its practical application. Herein, FeOOH/Ni-BiVO4 photoanode synthesized by the facile wet chemical method were investigated for improved charge transport and separation efficiency. The photoelectrochemical (PEC) measurements demonstrate that the water oxidation photocurrent density can reach as high as 3.02 mA cm−2 at 1.23 V vs RHE, and the surface separation efficiency can be boosted to 73.3 %, which increases around 4 times comparing with that of pure sample. Further depth studies showed that the Ni doping can effectively promote hole transport/trapping and introduce more active sites for the oxidation of water, while FeOOH co-catalyst could passivate the Ni-BiVO4 photoanode surface. This work provides a model for the design of BiVO4-based photoanodes with combined thermodynamic and kinetic advantages.

Boosting Charge Transfer Efficiency by Nanofragment MXene for Efficient Photoelectrochemical Water Splitting of NiFe(OH)x Co-Catalyzed Hematite.

The use of oxygen evolution co-catalysts (OECs) with hematite photoanodes has received much attention because of the potential to reduce surface charge recombination. However, the low surface charge transfer and bulk charge separation rate of hematite are not improved by decorating with OECs, and the intrinsic drawbacks of hematite still limit efficient photoelectrochemical (PEC) water splitting. Here, we successfully overcame the sluggish oxygen evolution reaction performance of hematite for water splitting by inserting zero-dimensional (0D) nanofragmented MXene (NFMX) as a hole transport material between the hematite and the OEC. The 0D NFMX was fabricated from two-dimensional (2D) MXene sheets and deposited onto the surface of a three-dimensional (3D) hematite photoanode via a centrifuge-assisted method without altering the inherent performance of the 2D MXene sheets. Among many OECs, NiFe(OH)x was selected as the OEC to improve hematite PEC performance in our system because of its efficient charge transport behavior and high stability. Because of the great synergy between NFMX and NiFe(OH)x, NiFe(OH)x/NFMX/Fe2O3 achieved a maximum photocurrent density of 3.09 mA cm-2 at 1.23 VRHE, which is 2.78-fold higher than that of α-Fe2O3 (1.11 mA cm-2). Furthermore, the poor stability of MXene in an aqueous solution for water splitting was resolved by uniformly coating it with NiFe(OH)x, after which it showed outstanding stability for 60 h at 1.23 VRHE. This study demonstrates the successful use of NFMX as a hole transport material combined with an OEC for highly efficient water splitting.

Atomically dispersed iridium catalysts on silicon photoanode for efficient photoelectrochemical water splitting

Stabilizing atomically dispersed single atoms (SAs) on silicon photoanodes for photoelectrochemical-oxygen evolution reaction is still challenging due to the scarcity of anchoring sites. Here, we elaborately demonstrate the decoration of iridium SAs on silicon photoanodes and assess the role of SAs on the separation and transfer of photogenerated charge carriers. NiO/Ni thin film, an active and highly stable catalyst, is capable of embedding the iridium SAs in its lattices by locally modifying the electronic structure. The isolated iridium SAs enable the effective photogenerated charge transport by suppressing the charge recombination and lower the thermodynamic energy barrier in the potential-determining step. The Ir SAs/NiO/Ni/ZrO2/n-Si photoanode exhibits a benchmarking photoelectrochemical performance with a high photocurrent density of 27.7 mA cm−2 at 1.23 V vs. reversible hydrogen electrode and 130 h stability. This study proposes the rational design of SAs on silicon photoelectrodes and reveals the potential of the iridium SAs to boost photogenerated charge carrier kinetics.

A BiVO4Photoanode with a VOxLayer Bearing Oxygen Vacancies Offers Improved Charge Transfer and Oxygen Evolution Kinetics in Photoelectrochemical Water Splitting

AbstractSluggish oxygen evolution kinetics are one of the key limitations of bismuth vanadate (BiVO4) photoanodes for efficient photoelectrochemical (PEC) water splitting. To address this issue, we report a vanadium oxide (VOx) with enriched oxygen vacancies conformally grown on BiVO4photoanodes by a simple photo‐assisted electrodeposition process. The optimized BiVO4/VOxphotoanode exhibits a photocurrent density of 6.29 mA cm−2at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which is ca. 385 % as high as that of its pristine counterpart. A high charge‐transfer efficiency of 96 % is achieved and stable PEC water splitting is realized, with a photocurrent retention rate of 88.3 % upon 40 h of testing. The excellent PEC performance is attributed to the presence of oxygen vacancies in VOxthat forms undercoordinated sites, which strengthen the adsorption of water molecules onto the active sites and promote charge transfer during the oxygen evolution reaction. This work demonstrates the potential of vanadium‐based catalysts for PEC water oxidation.

A BiVO4 Photoanode with a VOx Layer Bearing Oxygen Vacancies Offers Improved Charge Transfer and Oxygen Evolution Kinetics in Photoelectrochemical Water Splitting.

Sluggish oxygen evolution kinetics are one of the key limitations of bismuth vanadate (BiVO4 ) photoanodes for efficient photoelectrochemical (PEC) water splitting. To address this issue, we report a vanadium oxide (VOx ) with enriched oxygen vacancies conformally grown on BiVO4 photoanodes by a simple photo-assisted electrodeposition process. The optimized BiVO4 /VOx photoanode exhibits a photocurrent density of 6.29 mA cm-2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which is ca. 385 % as high as that of its pristine counterpart. A high charge-transfer efficiency of 96 % is achieved and stable PEC water splitting is realized, with a photocurrent retention rate of 88.3 % upon 40 h of testing. The excellent PEC performance is attributed to the presence of oxygen vacancies in VOx that forms undercoordinated sites, which strengthen the adsorption of water molecules onto the active sites and promote charge transfer during the oxygen evolution reaction. This work demonstrates the potential of vanadium-based catalysts for PEC water oxidation.

A novel BiVO4/DLC heterojunction for efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is a promising technology to produce renewable hydrogen, which still faces problems of low photoelectric efficiency and instability. This work introduces a novel Type II heterojunction consisting of BiVO4 thin film and diamond-like carbon (DLC) to the PEC system as a photoanode and achieves an increased PEC water-splitting activity. Compared to the pure BiVO4, the BiVO4/DLC heterojunction has an increased photocurrent density from 0.79 mA cm−2 to 2.39 mA cm−2 at 1.23 V vs RHE, and the IPCE from 4.8 % to 23.4 %. After 120 min (180 cycles) of PEC water splitting, the BiVO4/DLC heterojunction retains up to 82.3 % of the initial photocurrent density value, while the pure BiVO4 has only 33.99 % remaining under the same conditions. Furthermore, the PEC system of BiVO4/DLC reaches the evolution rate of hydrogen of over 0.32 mmol h−1 cm−2, which is much higher than that of its counterparts. We ascribe the enhancement in PEC performance to the Type II heterojunction photoanodes that greatly promote carrier mobility and reduce the recombination rate of electron-hole pairs. We also identify that the band bending and depletion layer are the main reasons for the decrease in charge transfer and interface resistance. It is demonstrated that the sp2 carbon phase in DLC acts as a conductive channel to improve the separation and migration efficiency of free carriers, while the sp3 carbon phase provides stable support and passivation effect to the photoanode, thereby leading to an enhancement for both PEC water splitting performance and the long-time stability. This work provides an effective way to improve the BiVO4 photoanode-based PEC water-splitting systems.

Surface passivation decoration of amorphous cobalt silicate on molybdenum-doped bismuth vanadate for efficient photoelectrochemical water oxidation

Such a subtly strategy greatly promoted the surface holes consumption rate and made composite photoanode dramatically achieve in 2.82 mA /cm2 at 1.23 VRHE (4.7 - fold higher than BiVO4) with a negative onset potential offset of 207 mV. The amorphous Co - species decoration tremendously enhanced surface charge injection efficiency to almost 90 % at 1.23 VRHE. And the biggest photovoltage and lowest charge recombination kinetics of composite photoanode also presented that the amorphous Co species had the potential ability to be passivation layer which were characterized by OCP and IMPS test.

Hybrid Reduced Graphene Oxide/GaN Nanocolumns on Flexible Niobium Foils for Efficient Photoelectrochemical Water Splitting

We present the photoelectrochemical (PEC) water-splitting properties of pristine and reduced graphene oxide (rGO)-coated GaN nanocolumns (NCs) on flexible niobium (Nb) metal foils. The structural, optical, and electronic structure analyses of rGO-coated GaN-NCs on Nb foils revealed the formation of a rGO/GaN-NCs hybrid structure. Further, the valence-band studies of pure GaN-NCs shows valence-band maxima at ∼3.0 eV below the Fermi level, which decreased to ∼2.8 eV for rGO/GaN-NCs. The PEC measurement performed on pristine GaN-NCs under standard (1 Sun) conditions shows an effective photocatalytic nature with a photocurrent density of ∼60 μA/cm2 at 0.8 V (vs Ag/AgCl) in a 1 M oxalic acid electrolyte, which increases to ∼110 μA/cm2 for rGO/GaN-NCs. The efficient PEC characteristics of rGO/GaN-NCs are attributed to the effective charge separation/transport of photogenerated carriers. Furthermore, transient photocurrent measurement reveals that hybrid and pristine GaN-NCs on flexible metal foil have a fast and stable photoresponse in an aqueous solution. The development of a hybrid nitride nanostructure-based PEC device on flexible metal foils paves the way toward developing scalable PEC devices for hydrogen production applications.

Photochemically Etching BiVO4 to Construct Asymmetric Heterojunction of BiVO4 /BiOx Showing Efficient Photoelectrochemical Water Splitting.

BiVO4 as a promising semiconductor candidate of the photoanode for solar driven water oxidation always suffers from poor charge carrier transport property and photo-induced self-corrosion. Herein, by intentionally taking advantage of the photo-induced self-corrosion process, a controllable photochemical etching method is developed to rationally construct a photoanode of BiVO4 /BiOx asymmetric heterojunction from faceted BiVO4 crystal arrays. Compared with the BiVO4 photoanode, the resulting BiVO4 /BiOx photoanode gains over three times enhancement in short-circuit photocurrent density (≈3.2mAcm-2 ) and ≈75mV negative shift of photocurrent onset potential. This is due to the formation of the strong interacted homologous heterojunction, which promotes photo-carrier separation and enlarges photovoltage across the interface. Remarkably, the photocurrent density can remain at ≈2.0mAcm-2 even after 12h consecutive operation, while only ≈0.1mAcm-2 is left for the control photoanode of BiVO4 . Moreover, the Faraday efficiency for water splitting is determined to be nearly 100% for the BiVO4 /BiOx photoanode. The controllable photochemical etching process may shed light on the construction of homologous heterojunction on other photoelectrode materials that have similar properties to BiVO4 .

Tube length optimization of titania nanotube array for efficient photoelectrochemical water splitting

Anodic TiO2 nanotube arrays (TNTAs) have attracted much attention due to their excellent photoelectrochemical (PEC) properties. In this work, the tube length of TNTAs was optimized for efficient PEC water splitting under two different conditions, in which very few or a massive amount of gas bubbles were generated on the electrodes. As a result, relatively longer TNTAs were found to be preferable for higher PEC performance when a larger number of bubbles were generated. This suggests that the mass transport in the electrolyte is assisted by the generated bubbles, so that the electrode surfaces are more easily exposed to the fresh electrolyte, leading to the higher PEC performance.

Correction: An organometal halide perovskite photocathode integrated with a MoS2 catalyst for efficient and stable photoelectrochemical water splitting

Correction for ‘An organometal halide perovskite photocathode integrated with a MoS2 catalyst for efficient and stable photoelectrochemical water splitting’ by Hojoong Choi et al., J. Mater. Chem. A, 2021, 9, 22291–22300, https://doi.org/10.1039/D1TA05377A.

Construction of WO3 quantum dots/TiO2 nanowire arrays type II heterojunction via electrostatic self-assembly for efficient solar-driven photoelectrochemical water splitting.

Construction of a heterojunction between quantum dots and TiO2 nanowire arrays via electrostatic self-assembly is rarely reported. In this work, mercury lamp irradiation was used to change the surface potential of WO3 quantum dots and TiO2 nanowire arrays, resulting in WO3 quantum dots tightly attached on the surface of TiO2 nanowire through electrostatic self-assembly. Photoelectrochemical measurements showed that the WO3 quantum dots formed a type II heterojunction with the TiO2 nanowire arrays rather than serving as carrier-trapping sites. In the self-assembly system, the TiO2 nanowire arrays provide a charge-transfer channel for the WO3 quantum dots, greatly improving the contribution of the WO3 quantum dots to the photocurrent. Quantitative calculations showed that the improvement of the bulk carrier-separation efficiency was the reason for the enhanced photoelectrochemical performance of the self-assembled system. The photocurrent density of the optical self-assembled system at 1.23 V (vs. RHE) was ∼5.5 times as high as that of the TiO2 nanowire arrays. More importantly, the self-assembled system exhibited excellent photoelectrochemical stability.

Core–shell InN/PM6 Z-scheme heterojunction photoanodes for efficient and stable photoelectrochemical water splitting

This Z-scheme heterostructure forms strong In–O bonds under light, driving the electrons of InN to combine with the holes of PM6, thus inhibiting charge recombination. The optimized device exhibits excellent PEC performance and photochemical stability.

Nanoporous silicon photocathodes with [Co] molecular catalyst for enhanced solar-driven hydrogen generation

Efficient photoelectrochemical water splitting by nanoporous Si photocathode using Co(dmgH)2(py)Cl as H2-evolution catalyst under illumination of simulated sunlight.

Angle-independent solar radiation capture by 3D printed lattice structures for efficient photoelectrochemical water splitting.

Photoelectrochemical water splitting is one of the sustainable routes to renewable hydrogen production. One of the challenges to deploying photoelectrochemical (PEC) based electrolyzers is the difficulty in the effective capture of solar radiation as the illumination angle changes throughout the day. Herein, we demonstrate a method for the angle-independent capture of solar irradiation by using transparent 3 dimensional (3D) lattice structures as the photoanode in PEC water splitting. The transparent 3D lattice structures were fabricated by 3D printing a silica sol-gel followed by aging and sintering. These transparent 3D lattice structures were coated with a conductive indium tin oxide (ITO) thin film and a Mo-doped BiVO4 photoanode thin film by dip coating. The sheet resistance of the conductive lattice structures can reach as low as 340 Ohms per sq for ∼82% optical transmission. The 3D lattice structures furnished large volumetric current densities of 1.39 mA cm-3 which is about 2.4 times higher than a flat glass substrate (0.58 mA cm-3) at 1.23 V and 1.5 G illumination. Further, the 3D lattice structures showed no significant loss in performance due to a change in the angle of illumination, whereas the performance of the flat glass substrate was significantly affected. This work opens a new paradigm for more effective capture of solar radiation that will increase the solar to energy conversion efficiency.

Boosted charge transport through Au-modified NiFe layered double hydroxide on silicon for efficient photoelectrochemical water oxidation

Atomically doped Au in NiFe-LDH effectively passivates defect sites, substantially enhancing overall PEC water oxidation properties.

A large size BiVO4photoanode with high stability for efficient water oxidation and wastewater treatment coupled with H2evolution

A large size BiVO4photoanode with high stability for efficient photoelectrochemical water oxidation and TCH degradation coupled with H2evolution.

Dual functional WO3/BiVO4 heterostructures for efficient photoelectrochemical water splitting and glycerol degradation.

Dual functional heterojunctions of tungsten oxide and bismuth vanadate (WO3/BiVO4) photoanodes are developed and their applications in photoelectrochemical (PEC) water splitting and mineralization of glycerol are demonstrated. The thin-film WO3/BiVO4 photoelectrode was fabricated by a facile hydrothermal method. The morphology, chemical composition, crystalline structure, chemical state, and optical absorption properties of the WO3/BiVO4 photoelectrodes were characterized systematically. The WO3/BiVO4 photoelectrode exhibits a good distribution of elements and a well-crystalline monoclinic WO3 and monoclinic scheelite BiVO4. The light-absorption spectrum of the WO3/BiVO4 photoelectrodes reveals a broad absorption band in the visible light region with a maximum absorption of around 520 nm. The dual functional WO3/BiVO4 photoelectrodes achieved a high photocurrent density of 6.85 mA cm-2, which is 2.8 times higher than that of the pristine WO3 photoelectrode in the presence of a mixture of 0.5 M Na2SO4 and 0.5 M glycerol electrolyte under AM 1.5 G (100 mW cm-2) illumination. The superior PEC performance of the WO3/BiVO4 photoelectrode was attributed to the synergistic effects of the superior crystal structure, light absorption, and efficient charge separation. Simultaneously, glycerol plays an essential role in increasing the efficiency of hydrogen production by suppressing charge recombination in the water redox reaction. Moreover, the WO3/BiVO4 photoelectrode shows the total organic carbon (TOC) removal efficiency of glycerol at about 82% at 120 min. Notably, the WO3/BiVO4 photoelectrode can be a promising photoelectrode for simultaneous hydrogen production and mineralization of glycerol with a simple, economical, and environmentally friendly approach.