Photoelectrochemical Water-splitting Performance Research Articles

Shape-Controlled First-Row Transition Metal Vanadates for Electrochemical and Photoelectrochemical Water Splitting.

Transition metal vanadates (MVs) possess abundant electroactive sites, short ion diffusion pathways, and optical properties that make them suitable for various electrochemical (EC) and photoelectrochemical (PEC) applications. While these materials are commonly used in energy storage devices like batteries and capacitors, their shape-controlled 1D and 2D morphologies have gained equal popularity in water splitting (WS) technology in recent times. This review focuses on recent progress made on various first-row (3d, 4 s) transition metal vanadates (t-MVs) having controlled one-dimensional (fiber, wire, or rod) and two-dimensional (layered or sheet) morphologies with a specific emphasis on copper vanadates (CuV), cobalt vanadates (CoV), iron vanadates (FeV), and nickel vanadates (NiV). The review covers different aspects of shape-controlled 1D and 2D t-MVs including optoelectrical properties, wet chemistry synthesis, and electrochemical (EC-WS) and photoelectrochemical water splitting (PEC-WS) performance in terms of onset potential, overpotential, and long-term stability or high cyclic performance. The review concludes by providing some possible thoughts on how to promote the water-splitting attributes of shape-controlled t-MVs more effectively.

Highly effective direct decomposition of organic pollutants via Ag–Zn co-doped In2S3/rGO photocatalyst

Currently, novel photocatalysts have attracted increasing attention to effectively utilizing abundant solar energy to meet the energy demands of humans and mitigate environmental burdens. In this work, we developed a novel and highly efficient photocatalyst consisting of In2S3 doped with two elements (Ag and Zn) and decorated with reduced graphene oxide (rGO) sheets. The crystal structure, morphology, electrical properties, and optical properties of the prepared materials were studied using various analytical techniques, and their photocatalytic activity was thoroughly investigated. It was confirmed that within 10 min, over 97% decomposition of organic dyes was achieved by using Ag–Zn co-doped In2S3/rGO catalyst, while only 50 and 60% decompositions were achieved by conventional pure In2S3 and In2S3/rGO nanocomposite, respectively. Its photoelectrochemical (PEC) water-splitting performance was also significantly improved (∼120%) compared with pure In2S3 nanoparticles. This study provides a new vision of using Ag–Zn:In2S3 decorated on rGO sheets as an efficient photocatalyst under solar light irradiation for environmental remediation and hydrogen production.

Two-Dimensional Sb Modified TiO2 Nanorod Arrays as Photoanodes for Efficient Solar Water Splitting.

As one of the widely studied semiconductor materials, titanium dioxide (TiO2) exhibits high photoelectrochemical (PEC) water-splitting performance as well as high chemical and photo stability. However, limited by a wide band gap and fast electron-hole recombination rate, the low solar-to-hydrogen conversion efficiency remains a bottleneck for the practical application of TiO2-based photoelectrodes. To improve the charge separation and water oxidation efficiency of TiO2 photoanodes, antimonene, a two-dimensional (2D) material obtained by liquid-phase exfoliation, was assembled onto TiO2 nanorod arrays (TNRAs) by a simple drop-coating assembly process. PEC measurements showed that the resulting 2D Sb/TiO2 photoelectrode displayed an enhanced photocurrent density of about 1.32 mA cm-2 in 1.0 M KOH at 0.3 V vs. Hg/HgO, which is ~1.65 times higher than that of the pristine TNRAs. Through UV-Vis absorption and electrochemical impedance spectroscopy measurements, it was possible to ascribe the enhanced PEC performances of the 2D Sb/TiO2 photoanode to increased absorption intensity in the visible light region, and improved interfacial charge-transfer kinetics in the 2D Sb/TiO2 heterojunction, which promotes electron-hole separation, transfer, and collection.

Role of active area on photoelectrochemical water-splitting performance of inverse opal CuBi2O4 photocathodes

Copper bismuth oxide(CuBi2O4), a ternary metal oxide is considered as one of the best photocathodes for photoelectrochemical water splitting. However, to date, the majority of research activities have been aimed at developing photocathodes at laboratory scale (≤1 cm2 electrochemical active area) and the issues that arise with scaling up have scarcely been addressed. Herein, we demonstrate a layered self-assembly method for fabricating uniform anddifferent electrochemical active area (1–15 cm2) inverse opal structured CuBi2O4 photocathodes, intending to validate it for photoelectrochemical hydrogen evolution under solar irradiation. After scaling up, it was found that CuBi2O4 with an active area of 5 cm2 exhibits the best photocurrent density (3.24 mA/cm2 at 0.6 V vs. RHE) and Faradaic efficiency (98.06 %). The scaling-up strategy and low-cost method employed for the fabrication of photocathodes could be expanded to large-scale applications associated with the manufacturing of highly efficient photoelectrochemical hydrogen production devices.

TiO2/black phosphorus heterojunction modified by Ag nanoparticles for efficient photoelectrochemical water splitting

How to improve photon absorption, photoelectric conversion efficiency, and carrier separation and transfer efficiency is a huge challenge for promoting the efficiency of photoelectrochemical (PEC) water splitting. Herein, the black phosphorus nanosheets (BPNs) was assembled on the surface of TiO2 nanorods arrays (TiO2 NAs) through electrostatic attraction to form TiO2/BP heterojunction material. Meanwhile, Ag nanoparticles were successfully loaded on the surface of TiO2/BP heterojunction material to form Ag nanoparticles modified TiO2/BP heterojunction composite material. The TiO2/BP heterojunction modified by Ag nanoparticles nanostructures exhibited a remarkable enhancement in the PEC water splitting in comparison with the pristine TiO2. Photoelectrochemical performance at 1.23 V vs. RHE bias-potential under 100 mW cm−2 solar light illumination shows that synthesized Ag–TiO2/BP photoanode displaying the highest photocurrent density of 1.74 mA cm−2, which is practically 3.34 times higher than that of pristine TiO2 (0.52 mA cm−2), presenting a significantly improved PEC performance for water splitting. After 18 months, the photocurrent of synthesized Ag–TiO2/BP photoanode can still reach 1.63 mA cm−2, and the performance is stable. It demonstrated that the formation of heterojunction by electrostatic attraction of TiO2 and BP and then modification with Ag nanoparticles is an effective strategy for improving the PEC water splitting.

Photo-electrochemical water splitting and electrochemical performance of silicon nanowire arrays

Silicon nanowires (SiNWs) were prepared using n-Si(100) by a simple two-step metal-assisted chemical etching (MACE) approach with different durations of 15 and 30 min. Surface morphology, structural, and optical properties of prepared SiNWs were investigated using Scanning Electron Microscope (SEM), x-ray diffraction (XRD) and UV–vis absorption, respectively. Under visible light, a photoelectrochemical cell (PEC) was used to measure the properties of a photoanode device that was fabricated based on n-SiNWs for splitting water. At 0.78 V, the SiNWs that were prepared in 30 min had a photocurrent density of 3.72 mA.cm−2 and a photoconversion efficiency (η) of 1.37%. Cyclic voltammetry (CV) measurements showed that both the n-Si(100) wafer and the n-SiNWs that were made with etching times of 15 and 30 min showed faradaic behavior with redox peaks. Electrochemical impedance spectroscopy (EIS) showed that the SiNWs photoanode prepared with 30 min of etching time had a charge transfer resistance of 3112.3. This is low enough to make it easy for charge to move across the interface. The Mott-Schottky (M-S) analysis revealed a high concentration of carriers of 4.77 × 1021 cm−3 at the working electrode/electrolyte interface,

Design of Ti-Pt Co-doped α-Fe2O3 photoanodes for enhanced performance of photoelectrochemical water splitting

This study demonstrates Ti and Pt co-doping can synergistically improve the PEC performance of the α-Fe2O3 photoanode. By varying the doping methods, the sample with in-situ Ti ex-situ Pt doping (Tii-Pte) exhibits the best performance. It demonstrates that Ti doping in bulk facilities charge separation and Pt doping on the surface further accelerates charge transfer. In contrast, Ti doping on the surface inhibits charge separation, and Pt doping in bulk hinders charge separation and transfer. HCl treatment is used to minimize the onset potential further, while it is favorable for the ex-situ doped α-Fe2O3, which is more efficient on Tie than the Pte-doped ones. On the ex-situ Ti-doped α-Fe2O3 after HCl treatment, anatase TiO2 is probed, suggesting that Ti-O bonds accumulate when Fe-O bonds are partly removed, which enhances the charge transfer in surface states. Unfortunately, HCl treatment also induces lattice defects that are adverse to charge transport, inhibiting the performance of in-situ doped α-Fe2O3 and excessively treated ex-situ doped ones. Coupled with methanol solvothermal treatment and NiOOH/FeOOH cocatalysts loading, the optimized Ti-Pt/Fe2O3 photoanode exhibits an impressive photocurrent density of 2.81 mA cm−2 at 1.23 V vs. RHE and a low onset potential of 0.60 V vs. RHE.

Solution-Processed Cu2S Nanostructures for Solar Hydrogen Production.

Cu2S is a promising solar energy conversion material due to its suitable optical properties, high elemental earth abundance, and nontoxicity. In addition to the challenge of multiple stable secondary phases, the short minority carrier diffusion length poses an obstacle to its practical application. This work addresses the issue by synthesizing nanostructured Cu2S thin films, which enables increased charge carrier collection. A simple solution-processing method involving the preparation of CuCl and CuCl2 molecular inks in a thiol-amine solvent mixture followed by spin coating and low-temperature annealing was used to obtain phase-pure nanostructured (nanoplate and nanoparticle) Cu2S thin films. The photocathode based on the nanoplate Cu2S (FTO/Au/Cu2S/CdS/TiO2/RuO x ) reveals enhanced charge carrier collection and improved photoelectrochemical water-splitting performance compared to the photocathode based on the non-nanostructured Cu2S thin film reported previously. A photocurrent density of 3.0 mA cm-2 at -0.2 versus a reversible hydrogen electrode (V RHE) with only 100 nm thickness of a nanoplate Cu2S layer and an onset potential of 0.43 V RHE were obtained. This work provides a simple, cost-effective, and high-throughput method to prepare phase-pure nanostructured Cu2S thin films for scalable solar hydrogen production.

Magnetic-thermal external field activate the pyro-magnetic effect of pyroelectric crystal (NaNbO3) to build a promising multi-field coupling-assisted photoelectrochemical water splitting system

Rationalizing the external field advantage and manipulating more carrier separation is of great significance for expanding the research of external field-assisted photoelectrochemical (PEC) water splitting. Hence, we choose NaNbO3 material with pyroelectric properties for the realization of an efficient multi-field coupled-assisted PEC water splitting system using magnetic and temperature fields for the first time. Detailed experimental results show that the enhancement of photoelectric performance by magnetic fields seems to be limited. However, it is amazing that the coupling effect of magnetic and thermal field triggers the generation of pyro-magnetic effect, which breaks the limitation of non-radiative recombination of magnetic field manipulated carriers, and significantly improves the performance of PEC water splitting. The multi-field coupling drive the current density of the photoanode to span to 0.45 mA at 1.23 V vs. RHE (3 times higher than photocurrent). The first principle density functional theory (DFT) calculation is combined with experimental tests to fundamentally understand the interrelationship between crystal structure changes, charge density distribution and external fields, and the effect on catalytic performance. This research provides insights into the interaction mechanism between external fields and the radiative/non-radiative recombination of manipulating carriers. Insights are provided for the establishment of a multi-field coupling-assisted PEC water splitting system.

Surface Passivation Engineering for Photoelectrochemical Water Splitting

Surface passivation engineering is an imperative way to improve photoelectrode performance for photoelectrochemical (PEC) water splitting. To the best of our knowledge, it has never been systematically reviewed in a feature article. In this review, we summarize various passivation materials and their preparation, characterizations by PEC measurements and some related spectral technologies. We highlight the features of the passivation effect that separate it from other modifications, such as cocatalyst decoration, and we demonstrate significant progress in combining surface passivation engineering with other interfacial modification strategies for the rational design of photoelectrodes. Ideas for future research on surface passivation modification for improving the performance of photoelectrodes are also proposed.

Investigation of in situ sulfide/nitride/phosphide treatments of hematite photoanodes for improved solar water oxidation.

Surface catalyst engineering can effectively improve the photoelectrochemical water splitting (PEC-WS) performance of semiconductor photoelectrodes. In situ surface functional treatments can effectively reduce interface defects and improve photogenerated carrier transport. In this study, FTO/Sn@α-Fe2O3/FeOOH photoanodes were modified with in situ sulfide/nitride/phosphide treatments to improve their PEC-WS performance. Compared with the pure α-Fe2O3 photoanode, the photocurrent densities of FTO/Sn@α-Fe2O3/FeOOH photoanodes after sulfide/nitride/phosphide treatments increased from 0.88 to 3.38 mA cm-2 at 1.23 VRHE. The onset potential showed a cathode shift of 0.1 V. Photoelectrochemical analyses and theoretical calculation demonstrated that the surface engineering by sulfide/nitride/phosphide treatments can significantly reduce surface defects, enhance electrical conductivity and promote photogenerated carrier separation and transfer efficiency by regulating interface charge transfer, binding energy and internal electric field. The formation of an FeSx catalyst and N/P coordination complexes in the sulfide/nitride/phosphide processes on the surface of α-Fe2O3 photoanodes can effectively reduce photogenerated carrier recombination. This work provides experimental and theoretical support for surface structure design and improved photoelectric conversion performance of semiconductor photoelectrode materials.

Optimizing the performance of Auy/Nix/TiO2NTs photoanodes for photoelectrochemical water splitting.

Water splitting using photoelectrochemical (PEC) techniques is thought to be a potential method for creating green hydrogen as a sustainable energy source. How to create extremely effective electrode materials is a pressing concern in this area. In this work, a series of Nix/TiO2 anodized nanotubes (NTs) and Auy/Nix/TiO2NTs photoanodes were prepared by electrodeposition via cyclic voltammetry and UV-photoreduction, respectively. The photoanodes were characterized by several structural, morphological, and optical techniques and their performance in PEC water-splitting for oxygen evolution reaction (OER) under simulated solar light was investigated. The obtained results revealed the nanotubular structure of TiO2NTs was preserved after deposition of NiO and Au nanoparticles while the band gap energy was reduced allowing for effective utilization of solar light with lower charge recombination rate. The PEC performance was monitored and it was found that the photocurrent densities of Ni20/TiO2NTs and Au30/Ni20/TiO2NTs were 1.75-fold and 3.25-fold that of pristine TiO2NTs, respectively. It was confirmed that the performance of the photoanodes depends on the number of electrodeposition cycles and duration of photoreduction of gold salt solution. The observed enhanced OER activity of Au30/Ni20/TiO2NTs could be attributed to the synergism between the local surface plasmon resonance (LSPR) effect of nanometric gold which increased solar light harvesting and the p-n heterojunction formed at the NiO/TiO2 interface which led to better charge separation and transportation suggesting its potential application as an efficient and stable photoanode in PEC water splitting for H2 production.

Interfacial engineering of CuWO4/WO3 thin films precisely fabricated by ultrasonic spray pyrolysis for improved solar water splitting

Heterostructural CuWO4/WO3 thin films on FTO glass substrate with accurately controlled thickness and composition were fabricated by ultrasonic spray pyrolysis technique.

Advanced oxygen evolution reaction catalysts for solar-driven photoelectrochemical water splitting

Herein, we summarize the recent developments in structure optimizations of oxygen evolution reaction catalysts for promoting photoelectrochemical water splitting performances.

Investigation of charge carrier dynamics in beaded ZnO nanowire decorated with SnS2/IrOx cocatalysts for enhanced photoelectrochemical water splitting

We report the synthesis of shape-modulated ZnO nanowires (NWs) and their role to achieve competent photoelectrochemical water splitting (PEC-WS) performance. The carrier dynamics of smooth (s-) and beaded ZnO NW (b-ZnO NW) photoanodes were investigated using time-resolved, temperature-controlled (room-temperature and low-temperature) photoluminescence spectroscopy. The enhanced PEC-WS performance obtained with the b-ZnO NWs was related to the reduced density of surface recombination states, superior crystallinity, and enhanced carrier lifetime, which improved carrier separation. Furthermore, an effective type-II aligned structure with b-ZnO NWs encapsulated in SnS2 and IrOx co-catalyst for enhanced PEC-WS performance and viability have been demonstrated. The optimized IrOx/15 nm SnS2/b-ZnO NW photoanodes demonstrated higher PEC-WS performance (0.58 mA/cm2 at 1.23 V vs RHE) with feasible photostability retention. We believe that our structural modulation approach, in-depth understanding of the carrier dynamics, and demonstration of a co-catalyst-enabled core–shell structure will be beneficial for obtaining highly efficient PEC-WS performances in ZnO-based photoanodes.

Surface dependent photoelectrochemical water-splitting performance of zinc tin oxide films

The process of splitting water into hydrogen (H2) and oxygen (O2) gases via the photoelectrochemical (PEC) cell route has recently been emerged as a promising approach, which requires the development of novel and high performance photoanode materials with good stability. Zinc-tin oxide (ZTO) is a promising photoanode material for PEC cell application because of its high chemical stability. Herein, ZTO films were deposited using the commercial co-sputtering process at various substrate temperatures (Tsub). At 200 °C of Tsub, the ZTO films changed from amorphous to crystalline nature. The ZTO films had the band gap in the range of 3.40–3.70 eV and resistivity in the range of 0.69–0.10 Ω·cm. The high photocurrent of 0.22 mA/cm2 was achieved for the ZTO films sputtered at a Tsub of 100 °C and thickness of 1000 nm due to the high surface roughness of 7.55 nm. The electrochemical impedance spectroscopy (EIS) result of ZTO film with 1000 nm thickness showed smallest semicircle, indicating lowest charge transfer resistance. Moreover, a detailed explanation of the effects of the Tsub on the properties of films and the performance of photoanode has been presented.

Enhanced photoelectrochemical water splitting and photocatalytic degradation performance of visible light active ZnIn2S4/PANI nanocomposite

In our work, zinc indium sulfide coupled polyaniline (ZnIn2S4/PANI) nanocomposites were synthesized using chemisorption method. The substantial enhancement in the photocatalytic and photoelectrochemical (PEC) water splitting activity under visible light illumination was demonstrated. ZnIn2S4/PANI nanocomposites were shown ∼3 times improvement in the photodegradation of methylene blue (MB) dye as compared to bare ZnIn2S4 (ZIS) nanoflakes. In addition, the ZIS/PANI nanocomposite photoelectrode was exhibited ∼3 times enhancement in photocurrent density (∼260 μA/cm2 at 0.8 V vs. Ag/AgCl) as compared to ZnIn2S4 photoelectrode. However, the ZIS/PANI photoanode was demonstrated ∼5 fold enhancement in the incident photon to current conversion efficiency (IPCE) than bare ZIS. The enhancement in the photodegradation and PEC water splitting activity of ZIS/PANI nanocomposite is ascribed to extended visible light absorption, and synergetic effect of PANI along with type-II heterojunction formation, which is beneficial for separation of charge and faster transfer efficiency of photogenerated charge carriers. This study demonstrates that establishing a heterostructure system by coupling a ternary chalcogenide semiconductor with a conducting polymer is effective for photocatalytic wastewater treatment and PEC water splitting applications.

Molecular sieving of semiconductive NTU-9 coatings on titanium dioxide nanowire arrays: Augmented yet selective photoelectrochemical response enabled by boosting charge separation and transfer in confined space

Radial heterostructures with titanium dioxide (TiO2) nanowire as core and NTU-9 as shell are synthesized via a surfactant-free approach based on the favorable bonding of linkers with TiO2 nanowire. Relative to the traditional growth strategy of surface modification, the thin NTU-9 shell with ordered arrangement of two-dimensional networks is uniformly formed on the sidewalls of TiO2 nanowire through the orientational growth process. Using the core–shell nanowire arrays as photoanodes, wide-range light absorption and high charge carrier separation efficiency are achieved due to the conjugation of NTU-9, leading to enhanced water oxidation performance in photoelectrochemical (PEC) water splitting. Under appropriately low applied potentials, the photogenerated holes are preferable to accumulate at TiO2/NTU-9/electrolyte three-phase interface and thus the long-range ordered NTU-9 shell can also serve as a size-exclusion filter to improve selectivity toward molecules of different sizes. Consequently, the TiO2/NTU-9 core–shell nanowire array exhibits an augmented and selective PEC response for small size molecules (e.g.,H2O2) at a very low potential (–0.25 V vs Ag/AgCl), outperforming the pure TiO2 nanowire array and the counterpart with a grain-boundary-rich NTU-9 shell that is prepared by pretreatment of the TiO2 nanowires with PVP functionalization.

Versatile TiO2 bandgap modification with metal, non-metal, noble metal, carbon material, and semiconductor for the photoelectrochemical water splitting and photocatalytic dye degradation performance

TiO2 is a flexible material for photocatalytic applications owing to its convenient redox potential, eco-friendliness, and abundance. However, TiO2 requires minor adjustments to improve its absorption ability and to suppress charge carrier recombination. Hence, simple chemical procedures have been used to modify TiO2 by using metals (e.g., Ni), non-metals (e.g., N), noble metals (e.g., Ag), carbon materials (e.g., carbon nanofibers), and semiconductors (Fe2O3) as dopants or by forming composites. The photoelectrochemical (PEC) water splitting and photocatalytic dye degradation performance of the synthesized materials were compared. Fe2O3-TiO2 composite performed admirably in both reactions owing to its unique absorption strength and the formation of the heterojunction interface. Fe2O3-TiO2 demonstrated azo dye degradation efficiency of 94.4%. The Fe2O3-TiO2 photoelectrode in the PEC system exhibited a photocurrent response of 0.244 amp/cm2. This outstanding performance is due to the formation of the heterojunction interface that is attributable to the p-type and n-type semiconductor characteristics of the composite. Moreover, the noticeable absorption strength of Fe2O3-TiO2 enables the generation of additional charge carriers and effectively reduces the recombination rate owing to the formation of the heterojunction interface. Characterization techniques including X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet–visible diffuse reflectance spectroscopy, fluorescence spectroscopy, scanning electron microscopy, and transmission electron microscopy, were used to investigate the photocatalytic characteristics. This study promotes the development of new heterojunction semiconductors and dopants for photocatalytic applications.

Anodic nanoporous WO3 modified with Bi2S3 quantum dots as a photoanode for photoelectrochemical water splitting

Although anodic nanoporous (ANP) WO3 has gained a lot of attention for photoelectrochemical water splitting (PEC-WS), there is still a lack of efficient WO3-based photoanodes with sufficient light absorption and good e−/h+ separation and transfer. The decoration of ANP WO3 with narrow bandgap semiconductor quantum dots (QDs) can enhance charge carrier transfer while reducing their recombination, resulting in a high PEC efficiency. In this study, ANP WO3 was synthesized via an anodic oxidation process and then modified with Bi2S3 QDs via successive ionic layer adsorption and reaction (SILAR) process and examined as a photoanode for PEC-WS under ultraviolet–visible illumination. The ANP WO3 photoanode modified with ten cycles of Bi2S3 QDs demonstrated the highest current density of 16.28 mA cm−2 at 0.95 V vs RHE, which is approximately 19 times that of pure ANP WO3 (0.85 mA cm−2). Furthermore, ANP WO3/Bi2S3 QDs (10) photoanode demonstrated the highest photoconversion efficiency of 4.1 % at 0.66 V vs RHE, whereas pure ANP WO3 demonstrated 0.3 % at 0.85 V vs RHE. This can be attributed to the proper number of Bi2S3 QDs significantly enhancing the visible light absorption, construction of type-II band alignment with WO3, and improved charge separation and migration. The modification of ANP WO3 with nontoxic Bi2S3 QDs as a prospective metal chalcogenide for enhancing visible light absorption and PEC-WS performance has not yet been investigated. Consequently, this study paves the path for a facile technique of designing effective photoelectrodes for PEC-WS.