Photoanode In Photoelectrochemical Cell Research Articles

Minute‐Scale High‐Temperature Synthesis of Polymeric Carbon Nitride Photoanodes

Polymeric carbon nitride (CN) has emerged as a promising photoanodic material in water‐splitting photoelectrochemical cells (PEC). However, the current deposition methods of CN layers on substrates usually include a long heating process at 500−550 °C, which might cause sublimation or decomposition of the CN monomers and destruction of the substrate, leading to a nonuniform CN film. Herein, a simple, fast, and scalable energy‐economic procedure to synthesize homogenous CN films is introduced. The predesigned CN monomers film is subjected for several minutes to higher temperatures than the standard calcination procedure. The short heating process allows the formation of a uniform CN layer, with excellent contact with the substrate and good activity as a photoanode in PEC. The optimal CN photoanode reaches photocurrent densities of ≈200 μA cm−2 at 1.23 versus reversible hydrogen electrode in neutral and acidic solutions and 120 μA cm−2 in a basic solution.

Physicochemical Properties of Tungsten Trioxide Photoanodes Fabricated by Wet Coating of Soluble, Particulate, and Mixed Precursors

Advanced oxidation processes are emerging technologies for the decomposition of organic pollutants in various types of water by harnessing solar energy. The purpose of this study is to examine the physicochemical characteristics of tungsten(VI) oxide (WO3) photoanodes, with the aim of enhancing oxidation processes in the treatment of water. The fabrication of WO3 coatings on conductive fluorine-doped tin oxide (FTO) substrates was achieved through a wet coating process that utilized three different liquid formulations: a dispersion of finely milled WO3 particles, a fully soluble WO3 precursor (acetylated peroxo tungstic acid), and a combination of both (applying a brick-and-mortar strategy). Upon subjecting the WO3 coatings to firing at a temperature of 450 °C, it was observed that their properties exhibited marked variations. The fabricated photoanodes are examined using a range of analytical techniques, including profilometry, thermo-gravimetric analysis (TGA), X-ray diffraction (XRD), and voltammetry. The experimental data suggest that the layers generated through the combination of particulate ink and soluble precursor (referred to as the brick-and-mortar building approach) display advantageous physicochemical properties, rendering them suitable for use as photoanodes in photoelectrochemical cells.

An anthraquinone-based bismuth-iron metal-organic framework as an efficient photoanode in photoelectrochemical cells.

Metal-organic frameworks (MOFs) are appealing candidate materials to design new photoelectrodes for use in solar energy conversion because of their modular nature and chemical versatility. However, to date there are few examples of MOFs that can be directly used as photoelectrodes, for which they must be able to afford charge separation upon light absorption, and promote the catalytic dissociation of water molecules, while maintaining structural integrity. Here, we have explored the use of the organic linker anthraquinone-2, 6-disulfonate (2, 6-AQDS) for the preparation of MOFs to be used as photoanodes. Thus, the reaction of 2, 6-AQDS with Bi(iii) or a combination of Bi(iii) and Fe(iii) resulted in two new MOFs, BiPF-10 and BiFePF-15, respectively. They display similar structural features, where the metal elements are disposed in inorganic-layer building units, which are pillared by the organic linkers by coordination bonds through the sulfonic acid groups. We show that the introduction of iron in the structure plays a crucial role for the practical use of the MOFs as a robust photoelectrode in a photoelectrochemical cell, producing as much as 1.23 mmol H2 cm-2 with the use of BiFePF-15 as photoanode. By means of time-resolved and electrochemical impedance spectroscopic studies we have been able to unravel the charge transfer mechanism, which involves the formation of a radical intermediate species, exhibiting a longer-lived lifetime by the presence of the iron-oxo clusters in BiFePF-15 to reduce the charge transfer resistance.

The Design of Supramolecular Assemblies with Metal Salt as Precursors Enables The Growth of Stable Polymeric Carbon Nitride Photoanodes

AbstractPolymeric carbon nitrides (CN) have gained significant interest as photoanodes in photoelectrochemical cells (PEC). A widely researched approach for synthesizing CN films with controlled optical and photoelectrochemical properties relies on using supramolecular assemblies as the precursors for thermal polymerization over a transparent conductive substrate. However, the formation of supramolecular assemblies is highly dependent on the temperature and solubility in a given solvent, limiting the full potential of this method. Moreover, the intercalation of metal ions is challenging due to the use of polar solvents. Here, this study shows a new way of synthesizing supramolecular assemblies with metal ions using a solvothermal approach. The solvent, monomer composition, salt quantity, reaction temperature, and film thickness are varied in this study. As a result, well‐attached, uniform CN films with good optoelectronic properties are achieved. The synthesized photoactive CN films exhibit very low onset potentials and reach ≈0.13, ≈0.15, and 0.30 ± 0.01 mA cm−2 photocurrents in 0.1 m phosphate buffer (neutral) solution, 0.1 m KOH(aq) (basic) solution, and 0.1 m KOH solution containing 10 vol.% triethanolamine as the hole scavenger, respectively.

Novel gamma-ray enhanced TiO2 nanoparticles photoanode for efficient photoelectrochemical (PEC) water splitting

The photocatalytic activity of TiO2 nanoparticles (TiO2NPs) for Hydrogen Evolution Reduction (HER) was significantly enhanced through a multi-step process involving oxygen-doping with γ-ray irradiation treatment (ranging from 10 kGy to 100 kGy), methanolic dispersion, and post-annealing temperatures. Remarkably, γ rays induced oxygen-doping, leading to improved electronic properties and chemical bonding, as demonstrated in the XPS section, which ultimately contributed to the exceptional stability of the photoanode. The resulting higher crystallinity and larger crystallite sizes, evident in Raman and XRD spectra, further enhanced the structure of the TiO2NPs. Upon γ irradiation, the deposited TiO2NPs exhibited enlargement and agglomeration, which promoted enhanced surface area, catalytic sites, and light absorption when used as a photoanode in PEC cells. The post-irradiation conditions caused a reduction in the energy band gap, resulting in a quenching effect from 3.25 eV to 3.18 eV. Intriguingly, PL analysis showed that the radiated photoanode displayed a remarkable reduction in the energetic separation of photo-generated electron-hole pairs, accompanied by a simultaneous decrease in carrier recombination. Overall, the 70 kGy TiO2NP photoanode demonstrated exceptional photostability and significantly outperformed the pure TiO2NP counterpart by increasing the photocurrent density by over 300%, reaching approximately 100.12 μA cm−2 at 1.23 vs. RHE, compared to 36.42 μA cm−2 for the pure TiO2NP. These findings underscore the significance of gamma irradiation in the field of nanomaterials and its promising potential for photoelectrochemical (PEC) solar water splitting applications.

Coupling surface modification with cocatalyst deposition on BiVO4 photoanode to enhance charge transfer for efficient solar-driven water splitting

Bismuth vanadate (BiVO4) has attracted wide attention as photoanode in water-splitting photoelectrochemical (PEC) devices. However, its catalytic performance is greatly limited by the serious charge recombination on the surface states. Herein, borate modified BiVO4 plus additional CoPi cocatalyst (B-BVO/CoPi) is developed as photoanode for PEC cell via a simple soaking and electrodeposition process. Based on the electrochemistry tests and material characteristics, the borate groups [B(OH)4] – is able to effectively prevent the charge recombination, while the outer CoPi cocatalyst enhance the activity toward oxygen evolution reaction. Therefore, the designed B-BVO/CoPi photoanode could obtain a high current density of 2.67 mA cm−2 at 1.23 VRHE with an onset potential of 0.29 VRHE, much higher than pristine BiVO4 (0.78 mA cm−2, 0.52 VRHE). This work may provide a new insight for constructing multifunctional modified photoanodes for efficient solar conversion.

Developing extended visible light responsive polymeric carbon nitrides for photocatalytic and photoelectrocatalytic applications.

Polymeric carbon nitride (CN) has emerged as an attractive material for photocatalysis and photoelectronic devices. However, the synthesis of porous CNs with controlled structural and optical properties remains a challenge, and processable CN precursors are still highly sought after for fabricating homogenous CN layers strongly bound to a given substrate. Here, we report a general method to synthesize highly dispersed porous CN materials that show excellent photocatalytic activity for the hydrogen evolution reaction and good performance as photoanodes in photoelectrochemical cells (PEC): first, supramolecular assemblies of melem and melamine in ethylene glycol and water are prepared using a hydrothermal process. These precursors are then calcined to yield a water-dispersible CN photocatalyst that exhibits beneficial charge separation under illumination, extended visible-light response attributed to carbon doping, and a large number of free amine groups that act as preferential sites for a Pt cocatalyst. The optimized CN exhibits state-of-the-art HER rates up to 23.1 mmol h-1 g-1, with an AQE of 19.2% at 395 nm. This unique synthetic route enables the formation of a homogeneous precursor paste for substrate casting; consequently, the CN photoanode exhibits a low onset potential, a high photocurrent density and good stability after calcination.

Evolution of surface properties of titanium oxide thin films

Titanium oxide thin films were deposited by rf magnetron sputtering of Ti metallic target in flow-controlled O2 + Ar atmosphere. Comprehensive investigations of crystallographic structure, phase composition, morphology and optical properties were performed by means of X-ray diffraction, X-ray reflectivity (XRR), scanning electron microscopy, optical spectrophotometry, and X-ray photoemission spectroscopy. It was found that as the O2/(O2 + Ar) flow ratio increased, the microstructure of thin films changed from TiO, Ti2O3, to rutile TiO2, rutile-anatase mixture and, finally, to pure anatase. Special emphasis was given to soft X-ray absorption spectroscopy in surface-sensitive total electron yield mode, which provided simultaneous information on electronic occupancy and local atomic structure. Studies of titanium L23 edge revealed systematic changes in crystal field strength, increasing upon rutile formation and decreasing when anatase became the dominant phase in sputtered films. Diffuse reflectance and XRR were employed to investigate the light scattering effects related to the surface roughness. The highest value (5.3 nm) of XRR roughness was observed for pure rutile sample. Performance of films as photoanodes in photoelectrochemical cells was assessed by photocurrent vs potential and time measurements. Results revealed that crystallized, stoichiometric films containing a mixture of anatase and rutile were promising photoanode materials for green hydrogen generation.

Recent research progress on operational stability of metal oxide/sulfide photoanodes in photoelectrochemical cells

Photoelectrochemical (PEC) water splitting can directly convert solar energy into hydrogen energy for storage, effectively ending the energy crisis and solving environmental problems. With their modification by many researchers, photoanodes have rapidly improved in PEC performance. Nevertheless, the poor stability of PEC water-splitting devices has not been effectively corrected, seriously hindering their practical application and large-scale commercialization. In this review, we provide a detailed introduction to the photocorrosion mechanism of photoanodes and characterizations of stability, summarizing the current research progress on the stability of metal oxide/sulfide photoanode materials. According to the specificity of each semiconductor, the corrosion mechanism and modification strategy of each photoanode are discussed in detail. Finally, we summarize the deficiencies in the current stability research and propose influencing factors and possible solutions that need to be considered in the photocorrosion research field of photoanodes. This review can provide a reference for the stability research of photoanodes based on metal oxides and sulfides, especially for the design of efficient and stable metal sulfide-based photoanodes.

ZnS/CZTS QDs modification for escalating photoelectrochemical properties of α-Fe2O3 thin film

Present study investigates the use of visible light active green CZTS quantum dots for boosting the photo-activity of hematite thin film towards efficient hydrogen generation. A photoelectrode has been prepared by depositing a passivating layer of ZnS on Cu2ZnSnS4 (CZTS) QDs sensitized hematite thin film (H-CZTS QDs/ZnS) for solar hydrogen production. These films were implemented as photoanode in photoelectrochemical cell for photoresponse measurements. XRD, HRTEM, FESEM and UV–vis diffuse reflectance spectroscopy (DRS) techniques were used to characterize these thin films for deeper analysis. The modified hematite film has showed 1.48 times more photocurrent density as compared to pristine counterpart. An increase in photocurrent could be attributed to better light absorption ability and reduced charge transfer resistance after modification. Applied bias to photon conversion efficiency was also calculated and found higher for modified sample.

Stacked titanium dioxide nanotubes photoanode facilitates unbiased hydrogen production in a solar-driven photoelectrochemical cell powered with a microbial fuel cell treating animal manure wastewater

Photoelectrochemical water splitting is a green, sustainable technology for harnessing solar energy to generate H2; however, the energy demand needed to drive this non-spontaneous reaction limits the technology's competitiveness. In addition, the poor efficiency of photoanodes in photoelectrochemical cells (PECs) has been one of the factors governing the overall solar conversion efficiency of PECs. Here, a novel stacking approach of n-type titanium dioxide nanotubes (TiO2 NTs) photoanodes was proposed to improve their light-harvesting and charge transfer properties. Interestingly, the stacked photoanodes exhibited a much higher photocurrent of ∼ 0.79 mA/cm2 at 1 V (vs. SHE) than single sheet TiO2 NTs photoanode (i.e., ∼ 0.07 mA/cm2) at the same potential, implying that TiO2 NTs stacking approach resulted in effective light absorbance and management and much lower charge transfer resistance across the interface of photoanode and electrolyte. In addition, a self-biased, integrated solar-microbial system was developed, in which a microbial fuel cell (MFC) fed with real animal manure wastewater was used as a power source to drive sustainable H2 production in a photoelectrochemical cell having stacked TiO2 NTs photoanodes. Without any external bias, the integrated system, which comprises an MFC and a three-sheet TiO2 NTs photoanode-based PEC, generated a photocurrent of 0.44 mA/cm2 with an H2 production rate of 0.45 ± 0.03-m3 H2/m3 day under 1 sun illumination (100 mW/cm2), which is ∼ 1.5–1.9-fold higher than other tested systems. This study demonstrates the synergetic effect between MFCs and PECs, in which electrons recovered from wastewater biodegradation in MFCs significantly increase H2 generation in PECs without the need for an external power source.

Efficient charge migration in TiO2@PB nanorod arrays with core–shell structure for photoelectrochemical water splitting

TiO2@PB heterojunction nanorod arrays as photoanodes for photoelectrochemical cells were prepared by a simple hydrothermal and electrodeposition method, and show excellent photoelectrochemical performance under irradiation due to efficient charge migration.

Pulse electrodeposited zinc sulfide as an eco-friendly buffer layer for the cadmium-free thin-film solar cells

Zinc sulfide (ZnS) is an emerging alternate n-type buffer layer for the cadmium-free thin-film solar cells. The present research adopts a unique mixture of glycerol and tartaric acid for the emergence of phase-pure ZnS during pulse electrodeposition . Besides, proper optimization of pulse parameters aid in producing crystalline ZnS films with no post-deposition heat treatment. The as-deposited films exhibit cubic ZnS as confirmed from X-Ray Diffraction (XRD) and micro-Raman analyses. The optical study reveals an average transmittance of 71% in the wavelength spread of 300–900 nm with a band gap of 3.8 eV. The n-type semiconducting behavior of ZnS is affirmed from the Mott-Schottky analysis and the flat band potential is inferred to be −1.0 V vs SCE . The crystalline ZnS films produced from the economic pulse electrodeposition method devoid of heat treatment step, with their demonstration as photoanode in photoelectrochemical cells are suitable for the relevant applications. • A room temperature electrodeposition process for ZnS with better control over the morphology and composition. • Phase-pure ZnS is obtained using pulse electrodeposition with no post heat-treatment. • A novel method using a blend of organic reagents, tartaric acid and glycerol in the electrolyte.

Hurdles and recent developments for CdS and chalcogenide‐based electrode in “Solar electro catalytic” hydrogen generation: A review

AbstractThere are tremendous efforts to realize the idea of employing photoelectrochemical (PEC) technology for generating chemical energy from solar energy. The most important feature of this technology is to coordinate the energetics of an electrochemical reaction with the solar spectrum. So, the photoelectrochemistry of a semiconducting material plays a vital role in this arena. Exploring an efficient and stable semiconductor material for such purpose is an essential prospect. There are several sulfide selenide‐based semiconductors viz. II‐VI (Zn/Cd = S/Se) semiconductors those exhibit high potential in PEC application. Nonetheless, they suffer by certain corrosion issues, which are important to be reviewed and are discussed here. This review highlights the significant role of cadmium chalcogenides among various materials explored in this technology. CdS is the most studied system among all those with perfect band gap and band‐edge position exhibiting material, desired for photoanode in PEC cell. Next, recent progresses achieving enhancement in the performance of CdS photoanodes by doping, modifying the surface, morphology reconstruction, and engraving heterostructure is discussed in detail. The role of the nanostructured techniques, improved photo electrochemistry with redox couples, and advanced interfacial and band gap engineering is explored to overcome the lack of chemical stability of CdS. The review concludes with the bright scope of CdS as an efficient PEC material in the form of combined semiconductor systems with an optimum band gap, bandedge, and their complimentary properties rather than a single component material.

Electrodeposition of BiVO4 with needle-like flower architecture for high performance photoelectrochemical splitting of water

Photoelectrochemical (PEC) water splitting is a green and sustainable approach capable of driving mass hydrogen production in the future. To realize this vision, development of a well-performing photoelectrode is highly demanded. In this comprehensive study, electrodeposition technique was applied for fabricating BiVO4 films by regulating the deposition time from 1 min until 9 min. Interestingly, the morphology, crystallinity, chemical structure, and optical properties of BiVO4 films depend strongly on the deposition time. It is found that BiVO4 layer deposited for 7 min with a cross-section thickness of around 321.1–326.5 nm showed the optimum performance, whereby the photocurrent reached up to ~0.32 mA/cm−2 at 1.23 V vs. RHE. The deposited BiVO4 represents tiny and long petals, similar to “needle” nanostructures, which is embedded closely into compact agglomerates. Such morphology enables the BiVO4 films to perform efficiently as photoanode in PEC cells. Besides, high crystallinity is detected from the sharp peaks of XRD and Raman analysis, as well as good light absorption capability that are the main contributors to the enhancement of PEC performance. In addition to the facile fabrication offered by electrodeposition method, the non-toxic attributes and the impressive PEC performance of the optimum BiVO4 layer could serve as an interesting option for other applications such as gas sensors, solar cells, degradation of pollutants and photocatalytic water splitting.

New insight into the effect of interface supercapacitance on the performance of titanium dioxide/carbon nanowire array for photoelectrochemical water oxidation

The electrode/electrlyte interface is of great signifance to photoelectrochemical (PEC) water oxidation as the reaction mainly occur here. Herein, we focus on the effect of supercapactance of the electrode/electrlyte interface on the performance of PEC. It is discovered that the supercapacitor on the interface is crucial because it links the charge transport and solution ion adsorption on its two sides. In this study, we demonstrate an approach to promote the performance of TiO2 nanowire array (TiO2 NWs) photoanode in photoelectrochemical cells (PECs) by increasing its supercapacitance. A 2−5 nm carbon layer was coated and the interface supercapacitance increases by about 150 times. This enhances the separation rate of electron-hole pairs by collecting more holes. Meanwhile, it also promotes the water oxidation rate by adsorbing more OH− on its surface. As a result, the photocurrent density of C-TiO2 NWs was about 8 times higher than that of its carbon-free counterpart. This approach of increasing the supercapacitance of photoanodes would be attractive for enhancement of the efficiency of PECs and this work demonstrate the importance of supercapacitance of the interface for PECs.

Mediated Growth of Carbon Nitride Films via Spray‐Coated Seeding Layers for Photoelectrochemical Applications

AbstractPolymeric carbon nitride (CN) has emerged as a promising semiconductor in photoelectrochemical devices, thanks to its unique electronic and catalytic properties, low price, stability in various chemical environments, and benign nature. Decent progress in the deposition and growth of CN layers on substrates has been achieved using several deposition and growth methods; however, the properties of the layer, including the quality of its contact with the substrate and its structural properties, are still largely dependent on the surface properties of said substrate. Here, a new approach is introduced in which a spray‐coated seeding layer composed of CN monomers directs the growth of a homogenous, thick CN layer on a substrate by calcination at high temperature in the presence of melamine vapor. Uniform CN layers with strong adhesion to the substrate are obtained. The influence of the seeding layer and the vapor composition on the photoelectrochemical, optical, and structural properties is studied in detail. The best‐performing electrode, based on urea as the seeding layer, demonstrates good activity as a photoanode in photoelectrochemical cells, reaching up to 300 µA cm−2 in the presence of a hole scavenger.

Effect of SILAR-anchored ZnFe2O4 on the BiVO4 nanostructure: An attempt towards enhancing photoelectrochemical water splitting

Development of high performance photoanodes for application in solar-driven photoelectrochemical cells is considered a grand challenge. Various structures and materials have been studied to overcome the current performance limit of the photoanode in photoelectrochemical cells; however, the enhancement in both stability and photocurrent has not been realized to date. In our study, the successive ionic layer adsorption and reaction method is used to coat ZnFe2O4 nanoparticles on the BiVO4 photoanodes. Various characterizations about structural, morphological, and optical characterizations confirm the presence of anchored ZnFe2O4 nanoparticles over BiVO4. Showing remarkable stability, the photocurrent density of optimized BiVO4/ZnFe2O4 is significantly increased compared with that of the bare BiVO4 nanostructured thin film. Mott-Schottky and electrochemical impedance spectroscopy analyses demonstrate that the appropriate number of the successive ionic layer adsorption and reaction cycles leads to efficient charge transfer. Furthermore, the correlation among structural, morphological and optical properties is discussed here.

Two‐Dimentional Nanostructured Metal Oxide/Sulfide–Based Photoanode for Photoelectrochemical Water Splitting

The utilization of solar energy plays a vital role in relieving energy crisis and associated environmental problems. Photoelectrochemical (PEC) water splitting, which can directly transform solar energy into clean hydrogen energy, is one of the most promising strategies to utilize solar energy. 2D nanostructured metal oxides/sulfides have been widely applied as photoanodes in PEC cells due to their unique configuration and features, which can efficiently improve light absorption, enlarge electrode/electrolyte contacting interfaces, and promote carrier transfer and injection. This article begins with the introduction of performance parameters, carrier dynamics measurement, and mechanism exploration techniques for photoanodes, which is followed by an overview of research progress in using 2D nanostructure metal oxides/sulfides to fabricate efficient photoanodes for PEC water splitting. Subsequently, the recent representative strategies to optimize the PEC performance of 2D metal oxides/sulfides photoanodes, including structure engineering, defect engineering, element doping, heterojunction construction, and surface modification, are summarized. Finally, the exciting advances and future research directions for designing highly efficient 2D photoanodes are provided.

Efficient and stable photoelectrochemical hydrogen generation using optimized colloidal heterostructured quantum dots

Colloidal semiconductor quantum dots (QDs) are considered as promising building blocks to fabricate efficient and stable optoelectronic devices, thanks to their size/shape/composition-dependent electronic and optical properties based on the quantum confinement effect. However, inadequate light absorption and undesirable charge recombination events still limit the efficiency and long-term stability of solar energy to fuel conversion in QD-based photoelectrochemical (PEC) cells. In this work, we engineer the optoelectronic properties and band alignment in colloidal heterostructured CdS/CdSe core/shell QDs by tuning the shell thickness. Starting with a 3.0 nm CdS QD core (in diameter), we investigate the changes in structural and optical properties as a function of CdSe shell thickness (0.6–1.9 nm). We show that the optimization of the shell thickness can significantly broaden the light absorption range towards longer wavelengths and enhance the rate of photoelectron separation and transport in photoanodes made of QDs sensitized TiO2 mesoporous film. Complemented by theoretical modeling, we find that the band alignment in this heterostructured system can be engineered from quasi type-II (electrons are localized over the entire QDs but holes are mostly delocalized in shell) to inverted type-I (both electrons and holes are largely delocalized in shell) by changing the shell thickness. As a proof-of-concept, employing QDs with optimal shell thickness of 1.6 nm together with carbon nanotube doped TiO2 as photoanode in PEC cells, we obtained a very high photocurrent density of ~ 16.0 mA/cm2 (at 0.9 V vs. the reversible hydrogen electrode, RHE), which is the highest value ever reported for PEC cells based on CdS/CdSe QDs. We also observed an excellent long-term stability (maintaining 83% of its initial value after four hours) under one sun illumination (AM 1.5G, 100 mW/cm2). These results indicate that optimizing the design and band engineering of heterostructured core/shell QDs is a facile and efficient approach to enhance the performance of QDs-based PEC cells and other optoelectronic devices.