Photoanodes For Water Splitting Research Articles

NiCo3-LDH catalysts decorated with p-n heterojunction CuCoO2/BiVO4 photoanodes for enhanced photoelectrochemical water oxidation

Due to the suitable bandgap and valence band for photoelectrochemical (PEC) water oxidation, BiVO4 has been regarded as a promising photoanode material. However, poor charge-separation and -transport behavior as well as sluggish surface oxygen evolution reaction (OER) kinetics caused the BiVO4 photoanode for water splitting to be significantly lower than its theoretical value. In this study, the synergistic effect of constructing an n-BiVO4/p-CuCoO2 heterojunction and NiCo3-LDH as an OER cocatalyst was designed to enhance the PEC performance of BiVO4 photoanode. The resultant BiVO4/CuCoO2/NiCo3-LDH demonstrates a maximum photocurrent density of 6.95 mA/cm2 at 1.8 V vs. RHE, which is approximately 24.8 times that of BiVO4 (0.28 mA/cm2) and 5.5 times that of BiVO4/CuCoO2 (1.27 mA/cm2) with good stability (70 % remained after 2 h of operation). This work broadens the construction strategy of photoanode heterojunctions to enhance PEC performance.

Modelling TiO2 photoanodes for PEC water splitting: Decoupling the influence of intrinsic material properties and film thickness

Semiconductor metal oxides are intensively studied in electrodes for photoelectrochemical (PEC) water splitting. On a series of nanoparticulate TiO2 photoanodes, we analyze specific fabrication variables by means of data fitting. First, the experimental outcome is gathered using PEC characterization techniques, mostly cyclic voltammetry and transient photocurrent measurements. Subsequently, we apply models to gain insights into the involved charge trapping and transfer phenomena. We find that capacitance coefficients and the switch-on transient kinetics depend on the TiO2 layer thickness, respectively indicating surface mechanisms and stationary regimes that are mediated by light accessibility. On the contrary, exponential factors of capacitance are independent of thickness, but reflect changes in the density of electron states with different sintering atmospheres. Also, the transfer resistance in the electrolyte side is indirectly influenced by sintering. Through meticulous quantitative analysis of trends, we stablish simple mathematical relationships that connect thickness-dependent parameters. This knowledge delves into fundamental mechanisms governing the TiO2 photoelectrode behaviour, and aims to facilitate further improvements in the efficiency of materials and electrodes for green hydrogen production.

Investigation of charge dynamics in dinuclear cobalt phthalocyanine ammonium sulfonate (PDS) modified Ti-Fe2O3 photoanodes for photoelectrochemical water oxidation

The kinetic competition between water oxidation/electron extraction processes and recombination behaviors is a key consideration in the development of efficient photoanodes for solar-driven water splitting. Investigating the photogenerated charge behaviors could guide the construction of high-efficiency photoanodes. In this study, the charge carrier kinetics involved in photoelectrochemical water oxidation of PDS/Ti-Fe2O3 were analyzed using surface photovoltage (SPV), transient photovoltage (TPV), short-pulse transient photocurrent (TPC) and photoelectrochemical impedance spectra (PEIS). The TPC results indicate the interfacial electric field introduced by the PDS loading increases the electron extraction and suppresses the bulk recombination, enhancing the spatial separation of photogenerated charges, which is consistent with the SPV and TPV results. Besides, the surface recombination of the back electron (BER) is also attenuated, which enhances the long-lived holes at the surface of PDS/Ti-Fe2O3 photoanode. Similarly, as obtained by PEIS fitting, the loading of PDS accelerates holes transfer at the photoanode/electrolyte interface, and increases the utilization of long-lived holes. In other word, the recombination behaviors of photogenerated charges are restrained both in the bulk and surface of the photoanode after the deposition of PDS, leading to enhanced PEC performance. These findings highlight the importance of understanding charge carrier dynamics in the design of high-efficient photoanodes.

A Synergic Effect of Bi-metallic Layered Hydro-Oxide Cocatalyst on 1-D TiO2 Driven Photoelectrochemical Water Splitting

ABSTRACT Photoelectrochemical (PEC) water splitting is one of the most sustainable approaches for converting solar energy into hydrogen fuel. Affordable and robust photoelectrodes are crucial for the commercialization of PEC technologies. Recently, transition metal-based co-catalysts, especially Ni- and Fe-based catalysts, have attracted much interest owing to their exceptional OER characteristics. Given this, we here proposed the decoration of a Fe-Ni-based cocatalyst on the surface of the TiO2 photoanode for PEC water splitting. The TiO2 photoanode was hydrothermally synthesized and then decorated by Fe-Ni hydroxide catalyst using photo-assisted electrodeposition. The optimized TiO2/FeNiOOH photoanode exhibited the maximum photocurrent density value of 1.36 mA cm−2, which is almost twice the value obtained for bare TiO2, at 1.23 V vs RHE under the AM 1.5 G illumination. Due to the enhanced light absorption in the UV region, the optimized photoanode exhibited remarkable IPCE and photoconversion efficiency of 87.8% and 0.93%, respectively. Furthermore, excellent faradaic efficiencies of ∼90% for H2 and ∼70% for O2 generations were obtained. Predominantly, the enhancement in the photocurrent potentials was explained in detail. Our study shows the roles and benefits of using bimetallic catalysts with TiO2 photoanodes for sustainable water-splitting applications.

Nanostructured AlFeO3 thin films as a novel photoanode for photoelectrochemical water splitting

Nanostructured AlFeO3 thin films as a novel photoanode for photoelectrochemical water splitting

Rugged Forest Morphology of Magnetoplasmonic Nanorods that Collect Maximum Light for Photoelectrochemical Water Splitting.

A feasible nanoscale framework of heterogeneous plasmonic materials and proper surface engineering can enhance photoelectrochemical (PEC) water-splitting performance owing to increased light absorbance, efficient bulk carrier transport, and interfacial charge transfer. This article introduces a new magnetoplasmonic (MagPlas) Ni-doped Au@Fex Oy nanorods (NRs) based material as a novel photoanode for PEC water-splitting. A two stage procedure produces core-shell Ni/Au@Fex Oy MagPlas NRs. The first-step is a one-pot solvothermal synthesis of Au@Fex Oy . The hollow Fex Oy nanotubes (NTs) are a hybrid of Fe2 O3 and Fe3 O4 , and the second-step is a sequential hydrothermal treatment for Ni doping. Then, a transverse magnetic field-induced assembly is adopted to decorate Ni/Au@Fex Oy on FTO glass to be an artificially roughened morphologic surface called a rugged forest, allowing more light absorption and active electrochemical sites. Then, to characterize its optical and surface properties, COMSOL Multiphysics simulations are carried out. The core-shell Ni/Au@Fex Oy MagPlas NRs increase photoanode interface charge transfer to 2.73 mAcm-2 at 1.23V RHE. This improvement is made possible by the rugged morphology of the NRs, which provide more active sites and oxygen vacancies as the hole transfer medium. The recent finding may provide light on plasmonic photocatalytic hybrids and surface morphology for effective PEC photoanodes.

WO3@Fe2O3 Core-Shell Heterojunction Photoanodes for Efficient Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) hydrogen production from water splitting is a green technology to convert solar energy into renewable hydrogen fuel. The construction of host/guest architecture in semiconductor photoanodes has been proven to be an effective strategy to improve solar-to-fuel conversion efficiency. In this study, WO3@Fe2O3 core-shell nanoarray heterojunction photoanodes are synthesized from the in-situ decomposition of WO3@Prussian blue (WO3@PB) and then used as host/guest photoanodes for photoelectrochemical water splitting, during which Fe2O3 serves as guest material to absorb visible solar light and WO3 can act as host scaffolds to collect electrons at the contact. The prepared WO3@Fe2O3 shows the enhanced photocurrent density of 1.26 mA cm−2 (under visible light) at 1.23 V. vs RHE and a superior IPEC of 24.4% at 350 nm, which is higher than that of WO3@PB and pure WO3 (0.43 mA/cm−2 and 16.3%, 0.18 mA/cm−2 and 11.5%) respectively, owing to the efficient light-harvesting from Fe2O3 and the enhanced electron-hole pairs separation from the formation of type-II heterojunctions, and the direct and ordered charge transport channels from the one-dimensional (1D) WO3 nanoarray nanostructures. Therefore, this work provides an alternative insight into the construction of sustainable and cost-effective photoanodes to enhance the efficiency of the solar-driven water splitting.

Photoelectrochemical water oxidation properties of Mo-doped CuWO4: Effect of p-type sulfide loading and annealing

Photoelectrochemical water splitting has received much attention from the perspective of the production of hydrogen as a next-generation source of energy. Herein, we focus on CuWO4 as a photoanode for water splitting because it is highly responsive to visible light. Mo doping led to longer wavelength shift of absorption edge in UV–vis absorption spectrum. Linear sweep voltammetry revealed that Mo-doped CuWO4 showed a 47-times higher photo-current density than pristine CuWO4 due to enhanced visible light absorption efficiency. The photo-current density of the Mo-doped sample also doubled when loaded with p-type MoS2.

Electronic Structure of Orthorhombic Bi4V2O11 and Role of Gap States in Photoelectrochemical Water Splitting

A group of bismuth vanadium oxides shows promise as an excellent material for photoanodes in water splitting. However, the efficiency of Bi4V2O11 in water splitting is not satisfactory compared to BiVO4. In this study, we investigated the electronic structure of orthorhombic Bi4V2O11 using density functional theory (DFT) and synchrotron soft X-ray spectroscopy techniques to clarify the causes of inefficiency. Our DFT calculations of the two-fold superlattice, as confirmed by near-edge X-ray absorption fine structure measurements, revealed its indirect-band-gap nature. An angle-resolved photoelectron spectroscopy study identified the states within the band gap, and the resonant photoemission technique and DFT simulation clarified that the origin of the gap states was anisotropic localization of electrons around V–O tetrahedra. Water absorption enhances the gap states, which dramatically impedes the kinetics of water oxidation reactions. Our results reveal the role of gap states in water oxidation reactions and offer new insights into manipulating gap states in 3d transition metal oxides.

Long-and short-range structure of SnO2 nanoparticles: Synthesis and photo(electro)catalytic activity

We report herein a detailed study on the influence of hydrothermal treatment temperature on both long- and short-range structures of SnO2 nanoparticles (NPs) applied as photocatalysts for the discoloration of organic pollutants and as photoanodes for water splitting. Synchrotron X-ray diffraction and X-ray absorption near-edge spectroscopy measurements confirmed the enhancement of the structural order of SnO2 NPs as a function of hydrothermal temperature. Fourier transform infrared spectroscopy revealed that the hydrothermal treatment increased the amount of hydroxyl groups on the SnO2 NPs surface. Regarding the photocatalytic activity, the NPs were able to promote the discoloration of different dyes that can act as potential organic pollutants. The photoelectrocatalytic performance of the samples depended on the hydrothermal treatment temperature, with the degree of crystallinity and surface hydroxyl groups playing a significant role in their performance as photoanodes. In particular, the NPs treated at a higher temperature presented a better degree of crystallinity, in addition to many hydroxyls on their surface, leading to increased mobility of the photogenerated charge carriers and improving the interaction between the molecules degraded and the material surface. The results demonstrated that the hydroxyls adsorbed on the SnO2 surface favor the formation of hydroxyl radicals, a species that indirectly participate in the photocatalytic oxidation of rhodamine B dye. The photoelectrocatalytic tests showed that the NPs treated at 200 °C increased oxygen evolution reaction performance.

Construction of CdS/Ti-Nb-O composite photoanode with favorable optical absorption and charge transfer for dramatically boosted photoelectrochemical water splitting

Rational design and construction of highly effective Ti-based photoanodes with advantageous structures are crucial to enhance photoelectrochemical (PEC) water splitting. In this work, we reported a CdS/Ti-Nb-O composite photoanode with satisfactory charge transfer and optical absorption for dramatically boosted PEC performance, which was constructed by bulk-phase Nb-doping synergistically with CdS quantum dots (QDs) sensitization of Ti-based oxide nanostructures. The composite system exhibited a remarkable photocurrent density of 16.11 mA cm-2, which was 34 times higher than pure TiO2 (0.47 mA cm-2). The electron lifetime was substantially increased and the solar-to-hydrogen (STH) conversion efficiency could be as high as 9.45%. Bulk-phase Nb-doping improved the conductivity of Ti-Nb-O, which enabled speedy and sustained charge separation in the composite-electrolyte solid-liquid interface. Meanwhile, CdS sensitization extended light absorption from 410 nm to 640 nm, thereby generating more photogenerated carriers. Simultaneously, Nb-doping negatively shifted the valence band (VB) and conduction band (CB) of Ti-Nb-O, which made the energy band more compatible with CdS. And the well-matched energy band structures of CdS and Ti-Nb-O not only improved separation-transfer properties of photogenerated electrons and holes but also increased overall optical absorption property of the composite system. This work involved dual-regulation charge separation strategy through bulk-phase doping synergetically with QDs sensitization may contribute to constructing high-efficient Ti-based photoanodes for PEC water splitting.

Synthesis and photoelectrochemical performance of Co doped SrTiO3 nanostructures photoanode

AbstractIt is pertinent to realize that scientific research indicates that the most promising method for producing H2 is photo electrochemical water splitting through a photo anode. Cobalt‐doped SrTiO3 (Co‐SrTiO3) composite nanostructures were created in this study via hydrothermal synthesis. The impact of cobalt concentration change on Co‐SrTiO3 has been identified using morphological, structural, and photo electrochemical research. Surface morphology of pure SrTiO3 nanoparticles using SEM and TEM reveals that the particles are intermittently agglomerated. The inclusion of Cobalt lowered the particle size of the nanostructures to 23 nm than pure SrTiO3 (41 nm). In addition, the peak profile has been influenced by cubic phase also identified from the x‐ray diffraction analysis. The purity and composition of the materials were revealed by XPS analysis. The Co‐SrTiO3 composite's produced the best charge transfer and recombination capabilities at 3% Co doping, according to electrochemical chemical impedance (EIS) spectroscopy. At 0.2 V applied potential, the obtained 3% Co‐doped SrTiO3 photoanode system displays a photocurrent density of around 3.45 mA/cm2. The outcomes show that a promising application for the Co‐doped SrTiO3 photoanode in photoelectrochemical water splitting.

Utilizing a Siloxane-Modified Organic Semiconductor for Photoelectrochemical Water Splitting

We explore the potential of employing diketopyrrolopyrrole (DPP) based π-conjugated OSs as a hole transport layer material in heteroatom-doped hematite (Ti–Fe2O3/Ge–Fe2O3) photoanodes for efficient photoelectrochemical water splitting. The siloxane-modified π-conjugated polymer (PSi) with a high carrier mobility and crystallinity revealed great potential to extract holes by forming a built-in potential with hematite photoanodes while showing high stability in an alkaline electrolyte for photoelectrochemical water oxidation. Because of the easy hole extraction and subsequent fast hole transport property of the PSi interlayer between NiFe(OH)x and Ge-doped porous Fe2O3(Ge-PH), NiFe(OH)x/PSi/Ge-PH showed a 1.8-fold increase in photocurrent density (4.57 mA cm–2 at 1.23 VRHE) with a cathodic shift of the onset potential (0.735 VRHE) and good stability for 65 h compared to Ge-PH. This study demonstrates the successful use of inherently unstable π-conjugated OSs as a hole extracting/transport medium in a photoanode, addressing the intrinsic recombination issues of hematite for efficient and stable water splitting.

Three-dimensional WO3 nanorod arrays on Si as photoanodes for efficient photoelectrochemical water splitting

Arrays of WO3–Si forest hierarchical structures have been successfully synthesized by growing WO3 nanorod on the surface of vertically aligned n-Si micropillar arrays using a chemical vapor deposition technique. The three-dimensional (3D) WO3–Si nanorod forest improves photoelectrochemical performances; it exhibits higher photocurrent density and a more negative onset potential of 0.5 V vs. reversible hydrogen electrode compared with the WO3 nanorod array on planar Si substrate. More importantly, this directly grown 3D hierarchical structure exhibits extraordinary stability for water oxidation with negligible losses of photoactivity for 24 h in a neutral pH solution. The enhanced photostability is attributed not only to the formation of O vacancies in WO3, which is resistive to photocorrosion but also the multiple internal reflection in 3D hierarchical structure. Our work demonstrates the importance of structural innovation in improving solar water efficiency.

Loading PVDF polymer on BiFeO3 films as photoelectrodes for efficient piezoelectric-photoelectrochemical water splitting

Controlling carrier separation by regulating polarization intensity is of great significance for built-in electric field assisted photoelectrochemical (PEC) water splitting. Here, we synthesize BiFeO3/PVDF composite photoelectrodes by loading a certain amount of PVDF on BiFeO3 films. The strong piezoelectric effect of PVDF can improve their piezoelectric built-in electric field to drive carrier separation, and the composite photoelectrode after alkali etching also has an improvement in hydrophily, thus together improving the piezoelectric PEC water splitting. The maximum photocurrent density of BiFeO3/PVDF composite photoelectrodes can be up to 2.03 mA/cm2 at 1.23 V vs RHE, which is significantly enhanced over the photocurrent of bare BiFeO3. In addition, the photocurrent density of BiFeO3/PVDF photoelectrode after alkali etching reaches 2.51 mA/cm2 at 1.23 V vs RHE. The influence of polarization intensity and hydrophilicity on PEC water splitting performance is studied systematically through BiFeO3/PVDF composite systems. This work provides a strategy for designing high activity photoanodes for PEC water splitting in organic/inorganic composites.

Multi-walled carbon nanotubes decorated polyaniline- α-Fe2O3 nanocomposite as an efficient photoanode for photoelectrochemical water splitting and photoelectrochemical cathodic protection of mild steel

In this research, polyaniline nanocomposites with iron (III) oxide and multi-walled carbon nanotubes (PANi-Fe and PANi-Fe-CNT) were synthesized and successfully characterized by various techniques such as X ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, Field emission scanning electron microscopy (FESEM), and X-ray energy dispersive spectroscopy (EDS). The photoelectrochemical (PEC) performance of PANi, α-Fe2O3 nanoparticles, PANi-Fe and PANi-Fe-CNT on indium tin oxide (ITO) substrate was evaluated by linear sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) in the dark and under simulated sunlight (100 mW/cm2). The results showed that ITO/PANi-Fe-CNT photoanode had higher photocurrent density (1.05 mA/cm2), more significant applied bias photo-to-current efficiency percentage (0.16%), and smaller charge transfer resistance (1008 Ωcm2) than other photoanodes under light irradiation. To study the photoelectrochemical cathodic protection (PEC cathodic protection) properties, the pastes were prepared from synthesized compounds and doctor-bladed on the pre-treated titanium (Ti) substrates and used as the photoanodes for the protection of mild steel. The performance of the as-prepared photoanodes in photoelectrochemical cathodic protection of mild steel was investigated using electrochemical impedance spectroscopy (EIS) and Tafel polarization. The corrosion potential (Ecorr) of mild steel decreased from −706 mV (without connection to the photoanode) to −864 mV in the state of connection to Ti/PANi-Fe-CNT photoanode under light irradiation, which indicated its cathodic protection.

Promoting the BiVO4 Photoanode Solar Water Oxidation Performance by Coating Hole-Blocking and Oxygen Evolution Layers

Bismuth vanadate (BiVO4)-based photoelectrochemical (PEC) water-splitting is a propitious technique for green fuel production, while the high surface charge carrier recombination and sluggish oxygen production kinetics limits its practical applications. Herein, we discuss the favorable impacts of grafting a hole-blocking SnO2 layer as an inner layer onto the fluorine-doped tin oxide and an oxygen evolution N-FeCoOx layer on BiVO4 photoanodes. Interestingly, the optimized SnO2@BiVO4/N-FeCoOx photoanode demonstrates an exceptional photocurrent density of 4.6 mA cm–2 at 1.23 V versus reversible hydrogen electrode under AM 1.5 G illumination, accompanied by the long-term stability up to 10 h. More specifically, the synergistic effect of both hole-blocking as well as oxygen evolution layers promotes the interfacial charge separation, transport, and PEC stability. Altogether, this work offers a simple and effective approach for constructing highly efficient photoanodes for PEC water splitting.

Rational design and fabrication of Cu2O film as photoelectrode for water splitting

Cu2O can be n-type and p-type depending on the oxygen and copper vacancies as either photocathode or photoanode for water splitting. However, the synthesis of n-type Cu2O is still a challenge. At the condition of pH= 6 and ΔU= −0.05 V, n-type Cu2O films can be electrodeposited at varied temperatures. The bandgap of n-type Cu2O films is in the range of 1.6–2.0 eV with the impurity level of 1.57 eV. Among them, n-type Cu2O films deposited at 60 °C achieved the highest photocurrent in water splitting (pH=7) as the light on due to the less impurity concentration and more highly texture structure. The n-type Cu2O photoelectrode shows a stable LSV cycling performance in Na2SO4 solution as compared to the buffer acetate and phosphate solution which may be due to the formation of Cu(OH)2 layer. This work can benefit the electrodeposition of n-type Cu2O for future water splitting applications.

Direct Growth of Polymeric Carbon Nitride Nanosheet Photoanode for Greatly Efficient Photoelectrochemical Water-Splitting.

A general method for the direct synthesis of highly homogeneous and dense polymerized carbon nitride (PCN) nanosheet films on F: SnO2 (FTO) is developed. Detailed photoelectrochemical (PEC) water-splitting studies reveal that the as-synthesized PCN films exhibit outstanding performance as photoanode for PEC water-splitting. The optimal PCN photoanode exhibits excellent photocurrent density of 650µA cm-2 , and monochromatic incident photon-to-electron conversion efficiency (IPCE) value up to 30.55% (λ= 400nm) and 25.97% (λ= 420nm) at 1.23 VRHE in 0.1m KOH electrolyte. More importantly, the PCN photoanode has an excellent hole extraction efficiency of up to 70 ± 3% due to the abundance of active sites provided by the PCN photoanode nanosheet, which promotes the transport rates of OER-relevant species. These PCN films provide a new benchmark for PCN photoanode materials.

CuS–CdS@TiO2 multi-heterostructure-based photoelectrode for highly efficient photoelectrochemical water splitting

A three-dimensional hierarchical CuS–CdS@TiO2 multi-heterostructure has been fabricated as a highly promising photoanode for PEC water splitting. The CdS@TiO2 structure was synthesized by a two-step hydrothermal method, therein TiO2 nanorods (TiO2 NRs) were used as the template to grow the CdS branches (CdS BRs). Then the surface of the heterostructure was sequentially coated with a p-type CuS semiconductor via a simple SILAR method. The optimized CuS–CdS@TiO2 photoelectrode exhibits a remarkably high photocurrent density of 12.6 mA cm−2 at 1.23 V vs RHE under standard AM 1.5 light illumination. The enhancement in the PEC performance could attribute to the wide range of solar spectrum absorption in CdS BRs, which can generate more photo charge carriers. Meanwhile, the high conductivity of TiO2 NRs serves as an efficient pathway to transfer photogenerated electrons from the CdS BRs to the FTO substrate. In addition, the p-n junction formed between CuS and CdS@TiO2 heterostructure could improve the separation of electron-hole on the surface of the photoanode. This study demonstrates an efficient pathway to improve the photoelectrochemical performance of TiO2-based photoanodes.