Photoelectrochemical Water Oxidation Performance Research Articles

Photoelectrochemical water oxidation performance promoted by a cupric oxide-hematite heterojunction photoanode

Hematite is a promising material for photoelectrochemical (PEC) water oxidation due to its narrow bandgap and chemical stability in alkaline electrolytes. However, the PEC performance of hematite is known to be inhibited by a short carrier diffusion length and slow kinetics for the oxygen evolution process. The rational control of morphology, crystallinity, and interfacial charge transfer plays an important role in tuning the PEC performance of α-Fe2O3 photoelectrodes. In this study, different iron oxide nano/microstructures are synthesized by the sintering of iron sheets in air at different temperatures. The synergetic effect of the surface morphology and crystallinity on the photocurrent and onset potential of the photoanode is investigated. The coral-like structure with high index (104) facets prepared at a high temperature shows a decreased onset potential, suggesting a strong correlation between the onset potential and crystalline orientation. To further improve the photocurrent density of this hematite photoanode, cupric oxide-hematite heterostructures are explored. An obvious improvement in the photocurrent density is observed from 1.1 to 1.6 V. The resulting α-Fe2O3/CuO photoanode exhibits a photocurrent density of 1.5 mA cm−2 at 1.6 V, 25% higher than that of the pristine hematite. The Mott-Schottky plot and EIS measurement indicate that the α-Fe2O3/CuO heterojunction facilitates the extraction of the accumulated holes in the hematite through a type-II band alignment and the CuO can serve as a catalyst for promoting the oxygen evolution reaction.

(0 2 0)-Textured tungsten trioxide nanostructure with enhanced photoelectrochemical activity

Texturing, i.e., preferentially oriented deposition of a film with a specific crystallographic direction, enables the manipulation of the charge transport properties and surface reactivity of photoelectrodes for solar water-splitting. The advancement of solar water-splitting systems under neutral conditions is a vital strategy to reduce the economic and ecological traits of the prevailing strong acid or alkaline-based solar water-splitting systems. However, the photoelectrodes have to endure cumulative barriers in neutral media to convalesce the performance of the neutral solar water-splitting system. The implication of texturing in materials enforces the synergistic effect that is essential to confine the barriers to improve the performance of the photoelectrodes in eco-friendly neutral pH conditions. Here, we synthesized tungsten trioxide (WO3) films to achieve a columnar-type nanostructure with (020) texture, through a laser ablation deposition. Specifically, we modulated both deposition temperature and working pressure, enabling the (020) textured deposition of films, as well as the fine-tuning of the surface morphology. With optimized fabrication conditions, the (020)-textured WO3 film (thickness: 3.6 µm) showed improved photoelectrochemical water-oxidation performance, and the photocurrent density was ~3 mA/cm2 at 1.23 V versus reversible hydrogen electrode in an economic and ecological neutral condition. The WO3 films were further characterized using various methods, namely a UV–Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and Hall Effect measurements. Based on the measured film characteristics, we attributed enhanced charge transport and transfer characteristics to the (020)-texturing, and the formation of the optimal amount of oxygen vacancies.

Synergy of porous structure and cation doping in Ta3N5 photoanode towards improved photoelectrochemical water oxidation

Herein, a cross-linked porous Ta 3 N 5 film was prepared via a simple solution combustion route followed by a high-temperature nitridation process for photoelectrochemical (PEC) water oxidation. Meanwhile, the metal cations (Mg 2+ and Zr 4+ ) were incorporated into the porous Ta 3 N 5 to enhance the PEC performance. The porous Mg/Zr co-doped Ta 3 N 5 photoanode yielded a photocurrent density of 1.40 mA cm −2 at 1.23 V vs RHE, which is 5.6 times higher than that of the dense Ta 3 N 5 photoanode. The enhanced performance should be ascribed to the synergistic effect of porous structure and cation doping, which can enlarge the electrochemical active surface area and accelerate the charge transfer by introducing O N substitution defects. Subsequently, Co(OH) 2 cocatalyst was loaded on the Mg/Zr-Ta 3 N 5 photoanode to negatively shift the onset potential to 0.45 V vs RHE and further improve the photocurrent density to 3.5 mA cm −2 at 1.23 V vs. RHE, with a maximum half-cell solar to hydrogen efficiency of 0.45%. The present study provides a new strategy to design efficient Ta 3 N 5 photoelectrodes via the simultaneous control of the morphology and composition. A cross-linked porous Ta 3 N 5 film co-doped with Mg 2+ and Zr 4+ cations showed a significantly improved photoelectrochemical performance for water oxidation due to enlarged electrochemical active surface area, enhanced charge transfer ability, and accelerated surface reaction.

Selective Removal of Excess V2O5 Via Electrochemical Etching for Reproducible Photoelectrochemical Water Oxidation Performance of BiVO4 Photoanodes

Monoclinic bismuth vanadate (BiVO4) is a promising candidate for photoanodes of photoelectrochemical (PEC) water oxidation due to its ability of visible light absorption, suitable valance band edge position and chemical stability under neutral electrolyte conditions. Among various fabrication methods for BiVO4, two-step process consisting of electrodeposition of Bi-based film followed by drop casting of V containing precursor solution to form to BiVO4 has been shown to produce BiVO4 photoanodes with high photoelectrochemical water oxidation performance. However, a careful removal of excess V2O5 is necessary in this fabrication method. The most common method of removing excess V2O5 is chemical etching by NaOH solution. However, in often cases, unwanted dissolution of BiVO4 occurs along with the etching of V2O5, which results in poor reproducibility of the PEC performance. Here, we developed a simple yet very selective etching method for the removal of excess V2O5 without any damage to BiVO4, which is based on electrochemical etching. BiVO4 photoanodes prepared by our electrochemical etching resulted in very reproducible PEC performance. Physical and chemical analyses of BiVO4 films after different etching methods (chemical vs. electrochemical) revealed that the loss of surface vanadium is the source of the fluctuation in PEC water oxidation performance. Details of material characterizations and electrochemical analyses will be presented.

Fabrication of Heterostructured Fe2TiO5–TiO2 Nanocages with Enhanced Photoelectrochemical Performance for Solar Energy Conversion

AbstractPhotocatalysts with well‐designed compositions and structures are desirable for achieving highly efficient solar‐to‐chemical energy conversion. Heterostructured semiconductor photocatalysts with advanced hollow structures possess beneficial features for promoting the activity towards photocatalytic reactions. Here we develop a facile synthetic strategy for the fabrication of Fe2TiO5–TiO2 nanocages (NCs) as anode materials in photoelectrochemical (PEC) water splitting cells. A hydrothermal reaction is performed to transform MIL‐125(Ti) nanodisks (NDs) to Ti–Fe–O NCs, which are further converted to Fe2TiO5–TiO2 NCs through a post annealing process. Owing to the compositional and structural advantages, the heterostructured Fe2TiO5–TiO2 NCs show enhanced performance for PEC water oxidation compared with TiO2 NDs, Fe2TiO5 nanoparticles (NPs) and Fe2TiO5–TiO2 NPs.

Fabrication of Heterostructured Fe2 TiO5 -TiO2 Nanocages with Enhanced Photoelectrochemical Performance for Solar Energy Conversion.

Photocatalysts with well-designed compositions and structures are desirable for achieving highly efficient solar-to-chemical energy conversion. Heterostructured semiconductor photocatalysts with advanced hollow structures possess beneficial features for promoting the activity towards photocatalytic reactions. Here we develop a facile synthetic strategy for the fabrication of Fe2 TiO5 -TiO2 nanocages (NCs) as anode materials in photoelectrochemical (PEC) water splitting cells. A hydrothermal reaction is performed to transform MIL-125(Ti) nanodisks (NDs) to Ti-Fe-O NCs, which are further converted to Fe2 TiO5 -TiO2 NCs through a post annealing process. Owing to the compositional and structural advantages, the heterostructured Fe2 TiO5 -TiO2 NCs show enhanced performance for PEC water oxidation compared with TiO2 NDs, Fe2 TiO5 nanoparticles (NPs) and Fe2 TiO5 -TiO2 NPs.

Influence of light intensity on the steady-state kinetics in tungsten trioxide particulate photoanode studied by intensity-modulated photocurrent spectroscopy

Charge-carrier dynamics during photoelectrochemical (PEC) water splitting over WO3 particulate photoanode deposited on titanium sheet (Ti/WO3) was studied using intensity-modulated photocurrent spectroscopy (IMPS). The WO3 particulate film calcined at 650 °C showed high PEC performance for water oxidation to evolve O2. In linear sweep voltammetry, the photocurrent density proportionally increased with incident light intensity (I0). This means that the incident photon-to-current conversion efficiency (IPCE) of Ti/WO3 was not dependent on I0 in voltammetry. In contrast, there was no proportional relation between I0 and the photocurrent density in chronoamperometry at a constant potential, indicating that the IPCE at steady state decreased with an increase of I0. To elucidate the effect of light intensity on the steady-state IPCE, we investigated the IMPS response of Ti/WO3 photoanodes by applying a simple branching model with respect to the photogenerated holes (h+). It was found that the recombination rate constant (krec) rather than the charge transfer rate constant (kct) significantly depends on I0 at the steady-state condition. The pseudo-first-order krec increased with an increase of I0, suggesting that recombination is increased by the accumulation of photogenerated carriers during continuous PEC water oxidation. This is the reason for the decrease in the steady-state IPCE of Ti/WO3 under high-intensity light irradiation. The effects of applied potentials on the kinetic parameters (kct and krec) were also evaluated for Ti/WO3 under different I0.

Copper-doped ZrO2 nanoparticles as high-performance catalysts for efficient removal of toxic organic pollutants and stable solar water oxidation

Doping effect on the photoelectrochemical (PEC) water splitting efficiency and photocatalytic activities of ZrO2 under visible light are reported. The XRD analysis revealed that pure, 0.1 and 0.3 mol% doped samples showed mixed crystal phases (tetragonal and monoclinic) and 0.5 mol% doped sample showed a pure tetragonal phase. Under visible light, 90% of methyl orange dye degradation was achieved with in 100 min. Moreover, the optimal doped sample showed a significant degradation rate constant over other samples. The doped photoelectrodes display a better PEC water oxidation performance over pure photoelectrode. Furthermore, the optimal doped (0.3 mol %) electrode shows 0.644 mAcm−2 photocurrent density, corresponding to an approximate 50-fold enhancement over pure electrode (0.013 mAcm−2). The optimized doped sample achieved 98% degradation of methyl orange within 100 min of light irradiation. The superior PEC water oxidation and photocatalytic activity of optimal doped samples under visible light are credited to suitable doping content, crystalline size, greater surface area, suitable bandgap, a lower charge carrying resistance, surface properties and the ability for decreasing the charge carrier's recombination rate.

Synergistic Effect of High-Performance N,S–TiO2/N,S–RGO Nanocomposites for Photoelectrochemical Water Oxidation

Nitrogen and sulfur codoped mesoporous anatase–brookite biphasic TiO2 anchored on surface of nitrogen and sulfur codoped reduced graphite oxide nanocomposites (N,S–TiO2/N,S–RGO) were easily prepared via a simple hydrothermal reaction process. XRD and Raman revealed anatase and brookite mixed phases. Also, the selected area electron diffraction (SAED) and the fast Fourier transform (FFT), from HRTEM measurements, indicated the presence of the anatase and brookite two phases. XPS showed the incorporation of N and S in both TiO2 matrix and carbon framework structure of RGO. The N,S–TiO2/N,S–RGO photoanodes exhibited an excellent photoelectrochemical (PEC) water oxidation performance. The highest photocurrent density reached 1.1 mA cm−2 for the N,S–TiO2/N,S–RGO(0.1) nanocomposite photoanode which is 6 times enhancement greater than bare TiO2 and 5 times enhancement than the N–TiO2/N–RGO(0.1) (the same GO ratio) at the same potential. These are considered significant promising results in comparison with our previous reported S–TiO2/S–RGO nanocomposite which revealed only 3 times enhancement in photocurrent density than bare TiO2.

Improvement of the water oxidation performance of Ti, F co-modified hematite by surface modification with a Co(salen) molecular cocatalyst

A new system by combining Ti-doping and F-treatment in hematite modified with Co(salen) as a cocatalyst exhibits enhanced photoelectrochemical water oxidation performance.

A graphene oxide–molecular Cu porphyrin-integrated BiVO4 photoanode for improved photoelectrochemical water oxidation performance

Monoclinic bismuth vanadate (BiVO4), as a rising star in light-catching materials, has been researched in many fields, such as photoelectrochemical water splitting.

Halogen bonding induced aqueously stable CsPbBr3@MOFs-Derived Co3O4/N-doped-C heterostructure for high-performance photoelectrochemical water oxidation

Recently, inorganic perovskite shows great potential in photocatalysis, while intrinsic instability against moisture seriously hinders its application in water splitting. Herein, a facile co-precipitation-calcination strategy is applied to prepare novel aqueous-stable CsPbBr3@MOFs-derived Co3O4/N-doped-C heterostructures (CPB@Co3O4/NC) with excellent photoelectrochemical water oxidation performance. Unique N-Br halogen bonds between Pyrrodic-N from NC frameworks and Br-atoms from CPB chemically immobilize CPB on NC layers, enhancing the aqueous-stability of CPB@Co3O4/NC by prohibiting CPB decomposition. Furthermore, benefitting from the type-II heterojunction at CPB and Co3O4/NC interface, system carrier-separation performance can be significantly improved. Meanwhile, acting as water oxidation center, Co3O4/NC can obviously enhance the interfacial carrier-injection efficiency of CPB. The rationally designed CPB@Co3O4/NC achieves boosted photocurrent-density and carrier migration of 2.50 mA cm−2 and 41.40 %, which are 9.61 and 12.24 times larger than pristine CPB. This work sets up a new strategy to synthesize CPB-based material with excellent aqueous-stability, for promising catalysis and other optoelectronic devices.

Ultrathin g-C3N4/Mo:BiVO4 photoanode for enhanced photoelectrochemical water oxidation

A g-C3N4/Mo:BiVO4 (CMB) heterojunction photoanode is constructed with enhanced photoelectrochemical (PEC) water oxidation performance, in which ultrathin g-C3N4 is coated on Mo-doped BiVO4. CMB shows a remarkable water oxidation photocurrent of 3.11 mA cm−2 at 1.23 V vs. RHE, which is 3.21 times higher than pristine BiVO4. The maximal incident photon-to-current efficiency (IPCE) reaches 45.5% at 430 nm and the applied bias photo-to-current efficiency (ABPE) reaches 0.74% at 0.78 V vs. RHE, which are 2.62 and 5.76 times compared with pristine BiVO4, respectively. The amounts of hydrogen and oxygen generated by CMB are 18.58 and 9.32 μmol within 1 h, which are 4.66 and 5.51 times higher than pristine BiVO4. The significant enhancements are attributed to the improvement of charge separation and acceleration of oxygen evolution reaction (OER) kinetics. Mo-dopant enhances charge separation due to its excellent electron transfer capability. Ultrathin g-C3N4 also boosts charge separation via forming a heterojunction with Mo:BiVO4 and promotes OER kinetics by accelerating the transfer of holes to the photoelectrode surface. The work testifies the promise of combing metal-doping with constructing heterojunctions using ultrathin g-C3N4 to enhance water oxidation performance, and provides an excellent reference for designing and constructing efficient photoanodes for PEC water oxidation.

Cubic SiC Photoanode Coupling with Ni:FeOOH Oxygen‐Evolution Cocatalyst for Sustainable Photoelectrochemical Water Oxidation

As an efficient water oxidation cocatalyst, the Earth‐abundant nickel‐iron oxyhydroxide (Ni:FeOOH) is introduced to coat on the cubic silicon carbide (3C‐SiC) photoanode surface for improving the photoelectrochemical (PEC) water oxidation performance. The FeOOH is prepared on the 3C‐SiC photoanode surface by hydrothermal deposition, followed by a photoassisted electrodeposition of NiOOH. It is shown that the Ni:FeOOH layer is composed of the β‐FeOOH nanorods with a conformal coating of the amorphous NiOOH. Under AM1.5G 100 mW cm−2 illumination, the 3C‐SiC/Ni:FeOOH photoanode exhibits a very low onset potential of 0.2 V versus reversible hydrogen electrode (VRHE) and a high photocurrent density of 1.15 mA cm−2 at 1.23 VRHE, distinctly outperforming the 3C‐SiC and the 3C‐SiC/FeOOH counterparts. Open‐circuit potential and impedance spectroscopy results demonstrate that the nanostructured Ni:FeOOH layer on the 3C‐SiC surface increases the photovoltage and promotes the charge transfer toward the electrolyte, thus significantly improving the PEC water‐splitting performance. These results provide new insights for the development of photoanodes toward efficient solar‐fuel generation.

A review on tungsten-trioxide-based photoanodes for water oxidation

Photoelectrochemical (PEC) water splitting capable of reducing and oxidizing water into hydrogen and oxygen in a generation mode of spatial separation has gained extensive popularity. In order to effectively produce hydrogen at the photocathode of a PEC cell, the photoanode, where the oxygen evolution reaction occurs, should be systematically developed on priority. In particular, WO3 has been identified as one of the most promising photoanode materials owing to its narrow band gap and high valence band position. Its practical implementation, however, is still limited by excessive electron–hole recombination and poor water oxidation kinetics. This review presents the various strategies that have been studied for enhancing the PEC water oxidation performance of WO3, such as controlling the morphology, introducing defects, constructing a heterojunction, loading a cocatalyst, and exploiting the plasmonic effect. In addition, the possible future research directions are presented.

Photodeposited CoOx as highly active phases to boost water oxidation on BiVO4/WO3 photoanode

Surface decoration of photoanodes with oxygen evolution cocatalysts is an efficient approach to improve the photoelectrochemical water splitting performance. Herein, ultrafine CoOx was selectively immobilized on the surface of BiVO4/WO3 photoanode by using the photogenerated holes to in-situ oxidize Co4O4 cubane. The composited photoanode (CoOx/BiVO4/WO3) displayed an enhanced photoelectrochemical (PEC) water oxidation performance, with a photocurrent density of 2.3 mA/cm2 at 1.23 VRHE under the simulated sunlight irradiation, which was 2 times higher than that of bare BiVO4/WO3. The characterization results for the morphological, optical and electrochemical properties of the photoelectrodes revealed that, the enhanced PEC performances could be attributed to the improved charge carrier separation/transport behaviors and the promoted water oxidation kinetics when the photoelectrodes were loaded with CoOx.

Interfacial Coupling Effect on Electron Transport in Hierarchical TaON/Au/ZnCo-LDH Photoanode with Enhanced Photoelectrochemical Water Oxidation.

Rational design and construction of interface heterostructures, which can simultaneously accelerate the photoinduced carrier separation and enhance the surface water oxidation kinetics, is of great necessity for photoelectrochemical (PEC) water oxidation. Herein, we report a new strategy for boosting the PEC water oxidation by introducing Schottky junction and semiconductor/water oxidation cocatalysts (SC/WOCs) junction into the TaON photocatalyst. Compared with pristine TaON photoanode, the hierarchical TaON/Au/ZnCo-LDH (LDH = layered double hydroxide) photoanode reveals a cathodic shift of 156 mV for the onset potential and 17.3-fold photocurrent density enhancement at 1.23 V vs RHE, as well as improved long-term stability. Diagnostic efficiencies of the TaON/Au/ZnCo-LDH photoanode demonstrate that the enhanced PEC performance is not dominated by surface electrochemical water oxidation kinetics but largely contributed by the improved charge separation and transfer, indicative of synergistic effects of Au and ZnCo-LDH. Theoretical calculations further reveal that the midgap states introduced by Au and ZnCo-LDH in TaON electronic structures bring about photoexcited electrons concentrated on TaON, while holes accumulated on ZnCo-LDH to achieve efficiently spatial charge separation, which is responsible for the boosted PEC water oxidation performance. The present work highlights the importance and elucidates the mechanism of interface heterojunction in PEC water oxidation, which can provide an efficient approach to design and fabricate a new structural photoanode.

Facile preparation of BiVO4/FeVO4 heterostructure for efficient water-splitting applications

In this paper, a BiVO4/FeVO4 heterostructure photoanode was synthesized by electrospray technique, and its photoelectrochemical water oxidation performance was investigated. The maximum photocurrent density of 0.4 mA cm−2 at 1.23 VRHE was 6 times higher than that of pristine BiVO4 films (0.06 mA cm−2). Through the analysis of the electrochemical impedance spectroscopy (EIS) results, the improvement of photoelectrochemical performance could be attributed to the formation of heterostructure at the two-phase interface, which led to the effective separation of electron-hole pairs. This work offers a new effective strategy to construct semiconductor nanocomposites for efficient photoelectrochemical water oxidation.

The IV period transition metal modified carbon@TiO2 nanoflower with high photo-electrochemical water oxidation performance under solar irradiation

Improving the photo-electrochemical water oxidation performance (PEWOP) of pristine TiO2 has been seriously hampered by its poor carrier transport efficiency and sluggish surface water oxidation kinetics. In this work, we prepared a novel IV period transition metal modified carbon layer-coated TiO2 (M-C@TiO2) nanoflower via a facile hydrothermal process followed by annealing at 800 °C. The as-prepared M-C@TiO2 nanoflower is uniform and stable with the size < 600 nm. The PEWOP of different IV period transition metal Zn, Cu, Ni, Co, Fe, Mn and Cr modified M-C@TiO2 nanoflowers are systematically investigated. Among them, the Cu, Ni, and Mn metals show high sensitivity to the current response in the modified nanoflowers. The current density of as-prepared Cu0.1-C@TiO2, Ni0.5-C@TiO2, and Mn1.0-C@TiO2 nanoflowers are increased to 4.6, 6.7 and 6.2 times higher than that of pristine TiO2, respectively, implying a significant enhancement to the PEWOP of pristine TiO2. This work provides an effective way to significantly enhance the PEWOP of pristine TiO2. Furthermore, it may arouse new interest and insight for the designing and preparing of M-C@TiO2 nanostructures for large-scale and practical water oxidation applications.

Enhanced interface charge transfer via n-n WO3/Ti–Fe2O3 heterojunction formation for water splitting

Inevitable drawbacks on α-Fe2O3 photoanodes, such as poor conductivity, short hole diffusion length, high electron-hole recombination rate, limit their photoelectrochemical (PEC) performance for water oxidation. The construction of heterojunction is an effective approach to improve the PEC performance of photoanodes. Herein, we report the design of the WO3/Ti–Fe2O3 heterojunction photoanodes, which are synthesized by two simple hydrothermal method. The optimized WO3/Ti–Fe2O3 photoanode shows remarkably improved photocurrent of 2.15 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE) without additional cocatalyst, which is higher than that of previous literatures. The improvement benefits from reduced charge transfer resistance in the bulk of Ti–Fe2O3 photoanode and improved charge separation efficiency, which can been further confirmed by electrochemical impedance spectroscopy (EIS), the transient photovoltage (TPV) and the work function (WF) measurement. The present work also provide new opportunities in developing high performance photoanodes for PEC water splitting.