Solar-driven Photoelectrochemical Water Splitting Research Articles

Fabrication of Flower-Shaped Sb2S3/Fe2O3 Heterostructures for Enhanced Photoelectrochemical Performance.

Antimony sulfide (Sb2S3) has been recognized as a catalytic material for splitting water by solar energy because of its suitable narrow band gap, high absorption coefficient, and abundance of elements. However, many deep-level defects in Sb2S3 result in a significant recombination of photoexcited electron-hole pairs, weakening its photoelectrochemical performance. Here, by using a simple hydrothermal and spin-coating method, we fabricated a step-scheme heterojunction of Sb2S3/α-Fe2O3 to improve the photoelectrochemical performance of pure Sb2S3. Our Sb2S3/α-Fe2O3 photoanode has a photocurrent density of 1.18 mA/cm2 at 1.23 V vs reversible hydrogen electrode, 1.39 times higher than that of Sb2S3 (0.84 mA/cm2). In addition, our heterojunction has a lower onset potential, a higher absorbance intensity, a higher incident photon-to-current conversion efficiency, a higher applied bias photon-to-current efficiency, and a lower charge transfer resistance compared to pure Sb2S3. Based on ultraviolet photoelectron spectroscopy, we constructed a step-scheme band structure of Sb2S3/α-Fe2O3 to explain its photoelectrochemical enhancement. This work offers a promising strategy to optimize the performance of Sb2S3 photoelectrodes for solar-driven photoelectrochemical water splitting.

Organic-inorganic p-type PEDOT:PSS/CuO/MoS2 photocathode with in-built antipodal photogenerated holes and electrons transfer pathways for efficient solar-driven photoelectrochemical water splitting

Organic PEDOT:PSS thin film and n-type MoS2 flakes were incorporated on CuO photocathode as the underlying layer and surface co-catalysts, respectively, establishing a rationally-designed novel integrated structure of PEDOT:PSS/CuO/MoS2. The organic-inorganic interface of PEDOT:PSS/CuO effectively shuttles photogenerated holes towards the FTO/PEDOT:PSS junction. Furthermore, the surface MoS2 co-catalysts form a p-n junction with CuO, which accelerates the transport of photogenerated electrons towards the photocathode/electrolyte junction, in addition to providing more reactive sites for the H2 evolution reaction. These synergistically formed antipodal charge transfer pathways have led to improved charge separation, rapid interfacial charge transfer kinetics and impeded electron-hole recombination in the PEDOT:PSS/CuO/MoS2 photocathode, which can achieve a photocurrent density of – 2.26 mA/cm2 at −0.6 V vs Ag/AgCl (2.1 times higher than that of the bare CuO photocathode). Analytical characterisations, electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS), also provide supporting evidence for its dramatically enhanced PEC water splitting performance.

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.

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.

A sustainable molybdenum oxysulphide-cobalt phosphate photocatalyst for effectual solar-driven water splitting

IntroductionHydrogen is considered as a clean alternative green energy future fuel. Since the Honda-Fujishima effect for photoelectrochemical water splitting is known, there has been a substantial boost in this field. Numerous photocatalysts based on metals, semiconductors, and organic-inorganic hybrid-systems have been proposed. Several factors limit their efficiency, e.g., a stable PEC-WS setup, absorbing visible light, well-aligned band energy for charge transfer, electrons and holes, and their separation to avoid recombination and limited water redox reactions. Metallic doping and impregnation of stable and efficient co-catalysts such as Pt, Ag, and Au showed enhanced PEC-WS. We used Cobalt-based co-catalyst with molybdenum oxysulfide photocatalyst for effectual solar-driven water splitting. ObjectivesTo develop photocatalysts for efficient PEC processes capable of absorbing sufficient visible light, good band energy for effective charge transfer, inexpensive, significant solar-to-chemical energy conversion efficiencies. Above all, it is developing such PEC-WS systems that will be commercially viable for renewable energy resources. MethodsWe prepared Molybdenum oxysulphide-cobalt phosphate photocatalyst for PEC-WS through a facile hydrothermal route using ammonium heptamolybdate, thiourea, and metallic Cobalt precursors. ResultsAn effectual photocatalyst is produced for solar-driven water splitting. The conformal morphology of MoOxSy-CoPi nanoflowers is a significant feature, as observed under FE-SEM and HR-TEM. XRD confirmed the degree of purity and orthorhombic crystal structure of MoOxSy-CoPi. EDX and XPS identify the elemental compositions and corresponding oxidation states of each atom. A 2.44 eV band-gap energy is calculated for MoOxSy-CoPi from the diffused reflectance spectrum. Photo- Electrochemical Studies (PEC) under 1-SUN solar irradiation revealed 7-8 folds enhanced photocurrent (∼ 3.5 mA/cm2) generated from MoOxSy-CoPi/FTO in comparison to Co-PI/FTO (∼ 0.5 mA/cm2) and MoOxSy-/FTO respectively, within 0.5 M Na2SO4 electrolyte (@pH=7) and standard three electrodes electrochemical cell. ConclusionOur results showed MoOxSy-CoPi as promising photocatalyst material for improved solar-driven photoelectrochemical water splitting system.

Effect of noble metal ions dopants on solar photoelectrochemical water splitting and electrochemical supercapacitive performance of BiVO4 hollow tubes

Owing to their poor electrochemical properties, photoelectrochemical (PEC) solar water splitting and electrochemical energy storage activity of BiVO4 are presently limited in their applications. One of the effective methods for improving electrochemical activity is the doping of noble-metal ions into the host lattice. The doping of noble-metal ions can improve absorption in visible light and support the direct transfer of energy via hot electrons. In this study, the synthesis of Au-doped BiVO4 nanostructured catalysts with different concentrations of Au ions was prepared via a template-free ultrasonication technique, and it's both solar-driven PEC water splitting and electrochemical storage properties were examined. The optimal doped photoelectrode exhibited a lower resistance to charge transfer than other photoelectrodes, including a significant enhancement in photocurrent density (0.102 mAcm−2), which is approximately 4.1 times superior over undoped photoanode. Furthermore, the optimal doped electrode exhibited an electrochemical capacitance that is 2 times greater than that of the pure electrode. The excellent electrochemical behavior and PEC properties of the optimal doped electrode were attributed to the increase in surface area and suitable band gap. Consequently, enables the dispersion of electron at the interface of electrode/electrolyte. Solar PEC solar water splitting and supercapacitor performance of the optimal doped electrodes were significantly improved by the proper incorporation of Au ions.

Excellent stability of molecular catalyst/BiVO4 photoanode in borate buffer solution

Solar-driven photoelectrochemical water splitting is a promising solution to energy problem. However, not-yet-available long-term stability of photoelectrodes inhibits their practical application. Introducing structure tunable, highly active and easily prepared molecular catalysts is an efficient approach to improve their stability and has recently attracted great attention. Nonetheless, the long-term stability is still challenged due to regeneration of molecular catalyst during stability test. Herein, tannic acid coordinated with Ni and Fe ions (TANF) molecular catalyst is immobilized on BiVO4 photoanode for oxygen evolution reaction (OER). Taking advantage of the regeneration of molecular catalyst, effects of buffer species on activity, stability and regeneration of TANF/BiVO4 photoanodes are investigated. In an optimal borate electrolyte, the TANF/BiVO4 photoanode shows nearly no-decay after operating at harsh conditions (at 1.2 VRHE with continuous light illumination) for 62 h. The long-term stability profits from the regenerated catalyst, which is composed of a carbon conductive network with metal oxides/hydroxides as OER-active sites. This excellent stability of the integrated photoanode surpasses most of existing OEC/BiVO4 photoanodes, and the stabilized catalyst is comparable with the state-of-the-art FeOOH/NiOOH dual-cocatalyst and ferrihydrite catalyst.

Transferred monolayer MoS2 onto GaN for heterostructure photoanode: Toward stable and efficient photoelectrochemical water splitting

Solar-driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes is considered a promising solution for sustainable, renewable, clean, safe and alternative energy sources such as hydrogen. Here, we report the synthesis and characterization of a novel heterostructure MoS2/GaN to be used as a photoanode for PEC-WS. The heterostructure was synthesized by metal-organic chemical vapor deposition of single crystalline GaN onto a c-plane sapphire substrate, followed by the deposition of a visible light responding MoS2 monolayer (Eg = 1.9 eV) formed by a Mo-sulfurization technique. Our experimental results reveal that MoS2/GaN photoanode achieved efficient light harvesting with photocurrent density of 5.2 mA cm−2 at 0 V vs Ag/AgCl, which is 2.6 times higher than pristine GaN. Interestingly, MoS2/GaN exhibited a significantly enhanced applied-bias-photon-to-current conversion efficiency of 0.91%, whereas reference GaN yielded an efficiency of 0.32%. The superior PEC performance of the MoS2/GaN photoelectrode is mainly related to the enhanced light absorption due to excellent photocatalytic behavior of MoS2, which reduces charge transfer resistance between the semiconductor and electrolyte interface, and the improvement of charge separation and transport. This result gives a new perspective on the importance of MoS2 as a cocatalyst coated onto GaN to synthesize photoelectrodes for efficient solar energy conversion devices.

Tuning of reduced graphene oxide thin film as an efficient electron conductive interlayer in a proven heterojunction photoanode for solar-driven photoelectrochemical water splitting

Although bismuth vanadate (BiVO4) has shown excellent photoelectrochemical (PEC) properties and is a good candidate of photoanode materials, the solar-driven PEC water splitting performance is still remained below its full potential due to the fast recombination and sluggish charge mobility of photogenerated charge carriers. Previously, we have communicated a proven Type II staggered vanadium pentoxide (V2O5)/BiVO4 heterojunction photoanode that could improve the photocurrent density. This study aimed to examine the effect of introducing an rGO thin film as an efficient electron conductive interlayer in a proven V2O5/BiVO4 heterojunction photoanode, and subsequently tuning the rGO film thickness in achieving the optimum PEC performance. The resultant ternary photoanode structure of V2O5/rGO/BiVO4 was characterised by using field emission-scanning electron microscopy (FE-SEM), high resolution-transmission electron microscopy (HR-TEM), UV–vis spectroscopy, X-ray diffractometer (XRD), Raman spectroscopy and photoluminescence (PL) measurements. Results showed that the interlayer rGO thin film arising from the sequential drop cast and electrochemical reduction of 320 μL ultrasonicated GO solution resulted in the optimal photocurrent density of 2.1 mA/cm2 at 1.5 V vs. Ag/AgCl. Furthermore, the chemical physics surrounding the photogenerated charge carrier transfer for heterojunction V2O5/BiVO4 was validated for the structure with and without the rGO interlayer. In particular, the electrochemical impedance spectroscopy (EIS) was used to measure multiple resistances at the FTO/semiconductor, semiconductor/semiconductor and semiconductor/electrolyte interfaces. Additionally, the charge transfer (Kt) and recombination (Kr) rate constants for the heterojunction V2O5/BiVO4 with the rGO interlayer were quantified using intensity modulated photocurrent spectroscopy (IMPS). Finally, the PEC H2 evolution rate from the ternary V2O5/rGO/BiVO4 photoanode was measured to be 32.7 μ mol/hr, which was about 3-fold higher than the bare V2O5/BiVO4 heterojunction photoanode.

Synergistic effects of dual-electrocatalyst FeOOH/NiOOH thin films as effective surface photogenerated hole extractors on a novel hierarchical heterojunction photoanode structure for solar-driven photoelectrochemical water splitting

Herein, we report the rational design of a novel hierarchical V2O5/BiVO4 heterojunction photoanode structure with rGO interlayer that functionalises as photogenerated electron collector, and dual electrocatalyst thin films of FeOOH and NiOOH as photogenerated hole extractors for solar-driven PEC water splitting. Results showed that the novel hierarchical FTO/V2O5/rGO/BiVO4/FeOOH/NiOOH photoanode exhibited an unprecedented and stable photocurrent density of 3.06 mA/cm2 at 1.5 V vs Ag/AgCl, and an apparent cathodic onset potential shift down to 0.2 V under AM 1.5 G simulated solar light illumination. The significant enhancement in PEC performance is ascribed to band potentials matching between V2O5 and BiVO4 in forming a Type II staggered heterojunction alignment, and further coupling with rGO interlayer and dual-electrocatalyst thin films as photogenerated electron collector and photogenerated hole extractors, respectively. Three different configurations of the novel hierarchical FTO/V2O5/rGO/BiVO4 photoanodes without electrocatalyst, with mono- and dual-electrocatalyst thin films were systematically examined. It was proven though EIS and IMPS measurements that the dual-electrocatalyst configuration photoanode exhibited the shortest transit time (τ) of 31.8 ms for the diffusion of photogenerated electrons to the counter electrode, and the lowest charge transfer resistance across the interface of electrode/electrolyte as estimated using the Randles-Ershel model. We believe that the proof-of-principle work described here not only provides an in-depth understanding on the roles of electrocatalyst thin films but also provides a design guide over the incorporation of electrocatalyst materials for further improving the photogenerated charge carrier dynamics in photoanodes used in solar-driven PEC water splitting.

Type-II ZnO/ZnS core-shell nanowires: Earth-abundant photoanode for solar-driven photoelectrochemical water splitting.

A core-shell structure, formed in a nanostructured photoanode, is an effective strategy to achieve high solar-to-hydrogen conversion efficiency. In this study, we present a facile and simple synthesis of a unique vertically aligned ZnO/ZnS core-shell heterostructure nanowires (NWs) on a Si substrate. Well-aligned ZnO NWs were grown on Si (100) substrates on a low-temperature ZnO buffer layer by metal-organic chemical vapor deposition. The ZnO NWs were then coated with various thicknesses of ZnS shell layers using atomic layer deposition. The structural characterizations exhibit the well-developed ZnO/ZnS core-shell NWs heterostructure. The as-prepared ZnO/ZnS core-shell NWs was applied as photoanode for photoelectrochemical (PEC) water splitting. This unique ZnO/ZnS core-shell NWs photoanode shows photocurrent density of 1.21 mA cm-2, which is 8.5 times higher than bare ZnO NWs. The PEC performance and the applied-bias-photon-to-current conversion efficiency of ZnO/ZnS core-shell NWs photoanode are further improved with the optimized ZnS shell. The type-II band alignment of the heterostructure photoanode is the key factor for their excellent PEC performance. Importantly, this type of core-shell NWs heterostructure provides useful insights into novel electrode design and fabrication based on earth abundant materials for low-cost solar fuel generation.

Enhanced photoelectrochemical water splitting performance of α-Fe2O3 nanostructures modified with Sb2S3 and cobalt phosphate

In this paper, α-Fe2O3 photoanodes with different morphologies have been prepared on FTO by hydrothermal method. The α-Fe2O3/Sb2S3 heterojunction and α-Fe2O3/Sb2S3/Co-Pi nanocomposites are firstly developed as photoanodes applied in photoelectrochemical (PEC) water splitting, which possesses low onset potential and high photocurrent density. In detail, α-Fe2O3 films with morphologies of nanoparticals and nanorods have been compared and the morphological effect on the photoelectrocatalytic activity has been discussed. To mitigate the limitation of charge recombination of pure α-Fe2O3, the antimony sulfide (Sb2S3) and cobalt-phosphate (Co-Pi) are subsequently incorporated onto α-Fe2O3 with different morphologies. Among them, α-Fe2O3/Sb2S3/Co-Pi photoelectrocatalyst based on α-Fe2O3 nanorods achieved a high photocurrent of 1.14 mA cm−2 at 1.23 V vs RHE and a low onset potential of 0.8 V vs. RHE as compared to those of other photoanodes. The superior PEC behaviors benefit from the improved charge separation and transfer efficiency caused by the modification of Sb2S3 due to the better visible response and the appropriate CB/VB designs as well as the coupling with Co-Pi that provides an alternative pathway for water splitting. This work emphasizes that composite photoanode strategies provide a conceptual blueprint to develop sustainable, environmental, and efficient photoelectrodes for solar-driven photoelectrochemical water splitting.

Influence of design and operating conditions on the performance of tandem photoelectrochemical reactors

Solar-driven photoelectrochemical water splitting technology is a promising avenue for a sustainable hydrogen production. In this work, a comprehensive 2-dimensional model is developed and numerically simulated with hematite (α-Fe2O3) as the principal photoelectrode. The model evaluates light absorption, charge transport and electrochemical reactions to elucidate the effects of light transmitting materials, electrolyte height and electrolyte velocity on hydrogen and oxygen gas production. Results indicated that major losses in photocurrents are attributed to the transparent conducting oxide while losses due to the electrolyte increase with its height. Gas concentrations increase with increasing photocurrent densities and also in the direction of the flow. Gas bubbles however decrease with increasing electrolyte velocity. From these results, light reception in the reactor is uneven and poses a bigger challenge due to the bias in gas bubble distribution. Prospects of upscaling tandem schemes hence not only lie in the semiconductor material combinations but rather in the proper integration of system components and operating conditions.

Understanding and Mitigating Degradation Phenomena in Photoelectrochemical Devices

Photoelectrochemical (PEC) water splitting is an attractive and clean method for the production of solar hydrogen. The main challenges for industrialization and large-scale implementation of PEC devices are [1]: the efficiency, the cost, the sustainability, and the durability. During the last decades, researches have been mainly focused on the development of high-performing, low cost, and earth abundant materials for solar hydrogen generation [2]. However, limited efforts are conducted to understand the degradation mechanisms occurring in the device or to predict approaches to increase the device’s lifetime, robustness and reliability. Here, we review the degradation of the PEC device’s components (semiconductor, catalyst, and solid and liquid electrolyte). For the semiconductor, the different degradations mechanisms considered include chemical corrosion, electrochemical corrosion, photoelectrochemical corrosion, chemical destabilization, protonation, and hydroxylation. For the electrocatalyst, the presented degradation mechanisms include dissolution, agglomeration, deactivation, and catalyst support corrosion. For solid electrolytes, chemical attack, poisoning by foreign ions, mechanical degradation, phototendering, photolysis, and embrittlement are considered. For the liquid electrolyte, the different degradations mechanisms include pollution, salt deposits, and saturation. Figure 1 summarizes the different degradation mechanisms occurring in the different components and interfaces, and the interplay between the degradation effects of the different components. In Figure 1.a, degradation mechanisms are organized according to the components and interfaces suffering from it. The intrinsic degradations of the components are indicated with red circles, while the degradations arising from the degradation of the other components are circled in green. In Figure 1.b, the interplay between the degradation effects of the different components is presented. At the center of the semiconductor-electrocatalyst-electrolyte triangle is the solar radiation. The direct interaction between the photons and the different components is responsible for all degradation mechanisms specific to photo-driven applications. We then discuss the impact of the PEC device design and operating conditions on degradation and performance loss. A bottom-up modeling approach, using degradation rates of the device components reported in literature, was used to predict the degradation-related performance loss in three different, illustrative PEC device design examples under different operating conditions [3]. The reference designs are: i) a monolithic design (wireless), ii) a wired design with two separated electrodes in a liquid electrolyte, and iii) a wired design with a separated membrane-electrode assembly. The operating conditions considered include: i) no irradiation concentration and no current dilution (reference condition), ii) irradiation concentration and no current dilution, iii) current dilution and no irradiation concentration, and iv) irradiation concentration and current dilution. The impact of the PEC device design and operating conditions on the salience of different degradations mechanisms and on the overall lifetime of the device is discussed. It is highlighted how heat and water management, device maintenance, and component replacement can help in mitigating degradation and the corresponding effects on device performance. [1] M. Dumortier, S. Y. Tembhurne and S. Haussener. Holistic design guidelines for solar hydrogen production by photo-electrochemical routes, in Energy Environmental Science, vol. 8, p. 3614 - 3628, 2015. [2] J. W. Ager, M. R. Shaner, K. A. Walczak, I. D. Sharp, and S. Ardo, Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting. Energy Environ. Sci.8:2811–2824, 2015 [3] F. Nandjou and S. Haussener. Degradation in photoelectrochemical devices: Review with an illustrative case study. Journal of Physics D, Special issue on Solar Fuels(Accepted with minor revision) Figure 1

Printed assemblies of GaAs photoelectrodes with decoupled optical and reactive interfaces for unassisted solar water splitting

Despite their excellent photophysical properties and record-high solar-to-hydrogen conversion efficiency, the high cost and limited stability of III–V compound semiconductors prohibit their practical application in solar-driven photoelectrochemical water splitting. Here we present a strategy for III–V photocatalysis that can circumvent these difficulties via printed assemblies of epitaxially grown compound semiconductors. A thin film stack of GaAs-based epitaxial materials is released from the growth wafer and printed onto a non-native transparent substrate to form an integrated photocatalytic electrode for solar hydrogen generation. The heterogeneously integrated electrode configuration together with specialized epitaxial design serve to decouple the material interfaces for illumination and electrocatalysis. Subsequently, this allows independent control and optimization of light absorption, carrier transport, charge transfer, and material stability. Using this approach, we construct a series-connected wireless tandem system of GaAs photoelectrodes and demonstrate 13.1% solar-to-hydrogen conversion efficiency of unassisted-mode water splitting. Photoelectrochemical devices based on III–V semiconductors have high performance potential but their cost and stability inhibit their wide application. Kang et al. make printed assemblies of GaAs-based photoelectrodes with separate optical and reactive interfaces, demonstrating water-splitting efficiency up to 13.1%.

A facile one-step strategy for in-situ fabrication of WO3-BiVO4 nanoarrays for solar-driven photoelectrochemical water splitting applications

This paper presents a facile single step strategy for fabrication of tungsten, bismuth and vanadium mixed metal oxide nanoarrays. WO3-BiVO4 heteronanostructure was obtained hydrothermally with reaction time of two hrs at low temperature 110°C. The morphology of as prepared WO3-BiVO4 heterostructure revealed uniform and prominent nanorods like architectures under FE-SEM. These heteronanostructures were of variable sizes i.e., width ⩽100nm and length 200–400nm respectively. The energy dispersive X-ray analysis (EDX) and elemental mapping of heteronanostructure further confirmed W, Bi, V and O entities in good elemental composition. The purity and crystalline nature of as synthesized WO3-BiVO4 were confirmed from X-ray crystallographic (XRD) analysis. UV–Visible spectroscopy and Raman analysis were also carried out to investigate optical properties of WO3-BiVO4. The band gap energy of WO3-BiVO4 calculated from UV–Visible absorption and diffused reflectance spectroscopy’s was observed to be 2.1eV respectively. The photoelectrochemical (PEC) studies of FTO coated WO3-BiVO4 showed a stable and repeatable photocurrent response under 1 SUN solar irradiation source. The linear sweep voltammetry (LSV) and Cyclic Voltammetry (CV) also corroborated substantial photocurrents at different oxidation and reduction potentials. Consequently, it is envisioned that this one-step strategy for in-situ fabrication of WO3-BiVO4 heteronanostructures have potential applications in solar-driven photoelectrochemical water splitting reactions.

Amorphous Cobalt-Iron Hydroxide Nanosheet Array Electrocatalyst for Efficient Electrochemical and Photoelectrochemical Oxygen Evolution

Water splitting by electrolysis or solar-driven photoelectrochemical (PEC) reaction has been broadly recognized as one of the most promising chemical method for sustainable energy conversion and storage systems. However, the overall efficiency of water splitting is largely limited by the kinetically sluggish oxygen evolution reaction (OER). Efficient electrocatalysts are required to enhance OER by lowering the overpotential and accelerating the kinetics. Herein, we report amorphous cobalt-iron hydroxide (CoFe-H) nanosheet array synthesized by a facile electrodeposition method as a highly efficient OER catalyst for both electrochemical and photoelectrochemical water splitting. The nanosheet array maximized the electrochemically active surface area, meanwhile the amorphous features with abundant defects offered rich active sites for OER. Therefore, the amorphous CoFe-H nanosheet array exhibited superior electrochemical activity toward OER. More importantly, we designed CoFe-H/BiVO4 core-shell architecture for PEC-OER system. Due to the high efficient OER activity of amorphous CoFe-H nanosheets, CoFe-H/BiVO4 photoanodes also exhibited superior PEC-OER performance. Our work provides a promising approach to fabricate highly efficient electrocatalysts for electrochemical and solar-driven photoelectrochemical water splitting.

Sonochemical Assisted Solvothermal Synthesis of Gallium Oxynitride Nanosheets and their Solar-Driven Photoelectrochemical Water-Splitting Applications.

Gallium oxynitride (GaON) nanosheets for photoelectrochemical (PEC) analysis are synthesized via direct solvothermal approach. Their FE-SEM revealed nanosheets morphology of GaON prepared at a reaction time of 24 hours at 180 °C. The elemental composition and mapping of Ga, O and N are carried out through electron dispersive X-ray spectroscopy (EDX). The cubic structure of GaON nanosheets is elucidated by X-ray diffraction (XRD)analysis. The X-ray Photoelectron Spectroscopy (XPS) further confirms Ga, O and N in their respective ratios and states. The optical properties of GaON nanosheets are evaluated via UV-Visible, Photoluminescence (PL) and Raman spectroscopy’s. The band gap energy of ~1.9 eV is calculated from both absorption and diffused reflectance spectroscopy’s which showed stronger p-d repulsions in the Ga (3d) and N (2p) orbitals. This effect and chemical nitridation caused upward shift of valence band and band gap reduction. The GaON nanosheets are investigated for PEC studies in a standard three electrode system under 1 Sun irradiation in 0.5 M Na2SO4. The photocurrent generation, oxidation and reduction reactions during the measurements are observed by Chronoampereometry, linear sweep Voltametry (LSV) and Cyclic Voltametry (CV) respectively. Henceforward, these GaON nanosheets can be used as potential photocatalyts for solar water splitting.

Opportunities to improve the net energy performance of photoelectrochemical water-splitting technology

The hydrogen energy provided by solar-driven photoelectrochemical water splitting must be greater than the energy used to produce and operate the technology, to enable energetic benefits to society.

Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting

Laboratory demonstrations of spontaneous photoelectrochemical solar water splitting cells are reviewed. Reported solar-to-hydrogen conversion efficiencies are as high as 18%. Reported operational lifetimes are relatively short, with few demonstrations exceeding one week.