Photoelectrodes In Photoelectrochemical Cells Research Articles

Development of Bio-Photoelectrochemical Hybrids for Solar Energy Conversion

Recent advancements in solar energy conversion have identified bio-photoelectrochemical hybrids as one of the most promising sustainable techniques. This study integrates BiVO4 and TiO2 semiconductors with photoautotrophic microorganisms (e.g., Algae) to enhance solar energy conversion. Thin films of these materials were prepared and characterized, revealing a relatively smooth surface for BiVO4 while structures for TiO2 thin films deposited as nanorods. Optical properties showed band gaps of ≤ 3.1 and 2.4 eV for TiO2 and BiVO4, respectively. The point of zero charge of materials was investigated, indicating that naturally occurring biofilm formation might not be favorable for as-prepared materials. To overcome this challenge, we aimed to use polyelectrolytes to enhance attachment of cells on the surface of semiconductor and in this regards we determined the biocompatibility of using this approach. Photoelectrochemical (PEC) measurements were conducted to evaluate solar energy conversion efficiency. This study offers insights into optimizing biosystem attachment, biofilm stability, and PEC performance of coupled semiconductors with photoautotrophic microorganisms as one photoelectrode in PEC cell, advancing sustainable solar energy conversion technologies.

(Invited) Crosscutting Materials and Molecular Catalysts in Hybrid Photoelectrodes for Liquid Solar Fuel Production

The Solar Hub entitled the C enter for Hybrid Approaches in Solar Energy to Liquid Fuels, or CHASE, is advancing the quest for liquid fuels from water, nitrogen, and/or carbon dioxide feedstocks with sunlight as the only energy source. To realize this quest, CHASE utilizes hybrid photoelectrodes based on molecular catalysts integrated with visible light absorbing semiconductors with controlled microenvironments to achieve liquid solar fuel production. These crosscutting materials serve as hybrid photoelectrodes in photoelectrochemical cells. This presentation will focus on recent advances on photocatalysis at hybrid Si interfaces with particular attention to the individual electron transfer and proton coupled electron transfer step(s) relevant to water oxidation and carbon dioxide reduction catalysis. The impact of CHASE findings on solar energy conversion and the quest for liquid solar fuels will be discussed.

Photostability improvement by the suppression of photocorrosion with a NiOx capping layer on CuO photoelectrodes

An increase in the photocurrent density and photostability of CuO photoelectrodes is important for the efficient photoactivity of the CuO photoelectrodes in photoelectrochemical cells. Herein, a NiOx capping layer was deposited on a CuO photoelectrode to act not only as a protective layer but also as a co-catalyst to reduce the photocorrosion that causes the low photostability of CuO photoelectrodes. It was found that CuO photoelectrode photocorrosion was reduced by controlling the NiOx deposition time and the NiOx precursor concentration. As the NiOx capping layer deposition time or NiOx precursor concentration increased, the NiOx capping layer completely covered the surface of the CuO photoelectrode and improved the crystallinity of the NiOx/CuO heterojunction photoelectrode. The photoelectrochemical properties of the NiOx/CuO heterojunction photoelectrode were improved with the help of the NiOx capping layer. As a result, an improved photocurrent density and photostability were obtained with a NiOx capping layer that was deposited for 10 min and with a NiOx precursor concentration of 25 mM. The photostability was particularly improved to 71.9%, which is approximately 3 times higher than the 22.7% of the single CuO photoelectrode.

Surface-Modified Titanium Dioxide Nanofibers with Gold Nanoparticles for Enhanced Photoelectrochemical Water Splitting

High-stability, high-efficiency, and low-cost solar photoelectrochemical (PEC) water splitting has great potential for hydrogen-energy applications. Here, we report on gold/titanium dioxide (Au/TiO2) nanofiber structures grown directly on a conductive indium tin oxide substrate, and used as photoelectrodes in PEC cells for hydrogen generation. The titanium dioxide nanofibers (TiO2 NFs) are synthesized using electrospinning, and are surface-modified by the deposition of gold nanoparticles (Au NPs) using a simple photoreduction method. The structure and morphology of the materials were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The surface plasmon resonance (SPR) of the Au NPs was investigated by ultraviolet-visible (UV-Vis) diffuse reflectance spectroscopy. The PEC properties of the as-prepared photoelectrodes were measured. The obtained photoconversion efficiency of 0.52% under simulated-sunlight illumination by a 150 W xenon lamp of the Au/TiO2 NFs structure with 15 min UV irradiation for Au NP deposition was the highest value from comparable structures. Working photoelectrode stability was tested, and the mechanism of the enhanced PEC performance is discussed.

Use of ionic liquid TEA-PS.BF4 as media synthesis of ZnO based on coprecipitation method

ZnO was synthesized by coprecipitation method and the 3-triethylammonium-propanesulfonic acid tetrafluoroborate (TEA-PS∙BF4) ionic liquid (IL) (0–10 wt%) was used during the synthesis. The Rietveld refinements of materials obtained showed that the TEA-PS∙BF4 influences the microstructure, i.e., microstrain increase and crystallite size decrease when compared to the ZnO without TEA-PS∙BF4. Moreover, the addition of IL in the synthesis made the crystals more isotropic. The scanning microscopy studies have shown that the films of ZnO are porous and have agglomerated particles of different sizes (between 16 and 180 nm). The data obtained - for the ZnO films as photoelectrodes in photoelectrochemical cells - from the j-V curves showed that the cells achieved photocurrent efficiencies of up to 1.03%, fill factor of 0.63 and a current density of 3.03 mA cm−2. The relationship between the percentage of IL and morphological/microstructure characteristics of ZnO films were also considered as a possible origin of the increase of up to 78% in photoconversion efficiency for the photoelectrode with 1 wt% of TEA-PS.BF4 compared to the cell without the addition of TEA-PS∙BF4.

New Hybrid Photoelectrodes Composed of TiO2 and Nanostructured Conjugated Porous Polymers for Enhanced Photocurrents in Solar Energy Conversion Cells

Photoelectrochemical water splitting is one of the most interesting alternatives to produce hydrogen in a clean way by solar energy conversion.(1) Despite the huge potential and great advances, new materials need to be developed in order to take this technology to a commercial level. At present, different materials as oxides, oxisulfides and metal chalcogenides are being investigated as photoelectrodes in photoelectrochemical cells. However, achieving high STH (Solar To Hydrogen) using a single material as a photoelectrode is a very tricky objective. Therefore, hybrid materials are getting a lot of attention lately. (2) In this work, we present a hybrid material formed by the heterojunction of a novel synthesized organic conductive polymer and TiO2 nanocrystals. The nanostructured conjugated porous polymer is based on dithiothiophene moiety (Nano-CMPDTT) and was synthesized by Sonogashira cross coupling reaction from precursors in mini-emulsion conditions. In order to elucidate the electronic structure and the ability of this material to be used as a photocatalyst, HOMO and LUMO positions were determined by cyclic voltammetry. The energy diagram shows an ideal position of the energy bands in order to use the synthesized polymer as an electron injector to TiO2 in photocatalytic reactions. In order to test the photoelectrochemical behavior of this hybrid material, TiO2 NCs suspensions and organic polymer has been deposited by spin coating in ITO glasses. The formed films have been characterized by X-ray diffraction, SEM, EDX and AFM. Photoelectrochemical measurements have been performed in a three electrode cell configuration, using the hybrid material as the working electrode. The hybrid material presents an enhancement in photovoltages and photocurrents values. Electrochemical Impedance Spectroscopy (EIS) was performed to confirm the improved charge transfer observed when illuminating the hybrid material in comparison to the TiO2 nanocrystals alone. By simulating the electron charge transfer by using an equivalent circuit, a decrease in the resistance associated with this phenomenon was found. This confirms that the presence of the polymer in the hybrid material improves the absorption of light, charge transfer and reduces electron-hole recombination, making this hybrid a good candidate to be used as a photoelectrode for the hydrogen evolution reaction.

Semiconducting materials for photoelectrochemical energy conversion

To achieve a sustainable society with an energy mix primarily based on solar energy, we need methods of storing energy from sunlight as chemical fuels. Photoelectrochemical (PEC) devices offer the promise of solar fuel production through artificial photosynthesis. Although the idea of a carbon-neutral energy economy powered by such ‘artificial leaves’ is intriguing, viable PEC energy conversion on a global scale requires the development of devices that are highly efficient, stable and simple in design. In this Review, recently developed semiconductor materials for the direct conversion of light into fuels are scrutinized with respect to their atomic constitution, electronic structure and potential for practical performance as photoelectrodes in PEC cells. The processes of light absorption, charge separation and transport, and suitable energetics for energy conversion in PEC devices are emphasized. Both the advantageous and unfavourable aspects of multinary oxides, oxynitrides, chalcogenides, classic semiconductors and carbon-based semiconductors are critically considered on the basis of their experimentally demonstrated performance and predicted properties. Photoelectrochemical (PEC) devices offer the promise of efficient artificial photosynthesis. In this Review, recently developed light-harvesting materials for PEC application are scrutinized with respect to their atomic constitution, electronic structure and potential for practical performance in PEC cells.

Vertically aligned TiO2/(CdS, CdTe, CdSTe) core/shell nanowire array for photoelectrochemical hydrogen generation

Type-II TiO2/CdS, TiO2/CdTe and TiO2/CdSTe heterostructured core/shell nanowire arrays (NWAs) on FTO substrates are synthesized via physical vapor deposition of CdS, CdTe and the alloyed CdSTe layer onto the hydrothermally pre-grown TiO2 NWAs. Their morphologies, microstructures, and optical properties are characterized in detail. As photoanodes for photoelectrochemical (PEC) hydrogen generation, the ternary CdSTe alloy sensitized TiO2 NWAs exhibits a photocurrent density of 4.52 mA cm−2 under 1 sun illumination at 0.4261 V vs. RHE, much higher than that of the TiO2/CdS (2.97 mA cm−2) and TiO2/CdTe (0.46 mA cm−2) electrodes. This highest photocurrent density level of the alloy TiO2/CdSTe electrode is attributed to the broadened light absorption range and enhanced charge separation efficiency according to the optical and electrochemical impedance spectra investigation. Our result implies a promising application of CdSTe sensitized TiO2 photoelectrode in PEC cell and other photoelectronic devices.

Preparation of chemical bath synthesized ternary Ag–Sn–S thin films as the photoelectrodes in photoelectrochemical cell

In this study, ternary Ag–Sn–S thin films are grown on glass and indium–tin-oxide coated glass substrates using chemical bath deposition. The structural, optical, electrical, and photoelectrochemical properties of the Ag–Sn–S thin films on substrates are investigated as a function of the [Ag]/[Ag + Sn] molar ratio in the precursor solution. X-ray diffraction patterns show that the samples change from Ag2S/Ag8SnS6, Ag8SnS6, Ag8SnS6/Ag4Sn3S8, and Ag8SnS6/Ag2S/SnS mixing phases with a decrease in the [Ag]/[Ag + Sn] molar ratio in the precursor solution. With a decrease in the [Ag]/[Ag + Sn] molar ratio in the precursor solution, the (0 2 2) diffraction peak of samples shifts to lower angles. A pure orthorhombic Ag8SnS6 phase is obtained in the samples prepared with [Ag]/[Ag + Sn] molar ratios in the precursor solution of 0.5–0.6. The energy band gaps of the samples obtained from transmittance and reflectance spectra are in the range of 1.17–1.48 eV. The carrier concentrations and mobilities of the samples are in the ranges of 2.5 × 1012–3 × 1014 cm−3 and 14.02–23.05 cm2/V s, respectively. The maximum photoelectrochemical performance of the samples in aqueous solution containing SO32− and S2− ions is 1.13 mA/cm2 at an external potential of 0 V vs. an Ag/AgCl reference electrode under 100 mW/cm2 light illumination from a Xe lamp source.

Synthesis and characterization of nanocrystalline MoBi2Te5 thin films for photoelectrode applications

Molybdenum bismuth telluride thin films have been prepared on clean glass substrate using arrested precipitation technique which is based on self-organized growth process. As deposited MoBi2Te5 thin films were dried in constant temperature oven at 110°C and further characterized for their optical, structural, morphological, compositional, and electrical analysis. Optical absorption spectra recorded in the wavelength range 300–800 nm showed band gap (E g) 1.44 eV. X-ray diffraction pattern and scanning electron microscopic images showed that MoBi2Te5 thin films are granular, nanocrystalline having rhombohedral structure. The compositional analysis showed close agreements in theoretical and experimental atomic percentages of Mo4+, Bi3+, and Te2− suggest that chemical formula MoBi2Te5 assigned to as deposited molybdenum bismuth telluride new material is confirmed. The electrical conductivity and thermoelectric power measurement showed that the films are semiconducting with n-type conduction. The fill factor and conversion efficiency was characterized by photoelectrochemical (PEC) technique. In this article, we report the optostructural, morphological, compositional, and electrical characteristics of nanocrystalline MoBi2Te5 thin films to check its suitability as photoelectrode in PEC cell.

Analysis of the operation of thin nanowire photoelectrodes for solar energy conversion

The solar energy conversion properties of thin Si and GaP nanowire photoelectrodes in photoelectrochemical cells have been examined through sets of finite-element simulations. A discussion describing the motivation behind nanostructured, high aspect ratio semiconductor photoelectrode designs and a brief survey of current experimental results reported for nanostructured semiconductor photoelectrodes in photoelectrochemical cells are presented first. An analysis is then shown that outlines the primary recombination pathways governing the steady-state current-potential behaviors of thin, cylindrical nanowire photoelectrodes, with explicit expressions detailing the differences between planar and cylindrical photoelectrodes arising from the solution of carrier fluxes in planar and cylindrical geometries. Results from finite-element simulations used to model the key features of thin nanowire photoelectrodes under low-level injection conditions are shown that illustrate which recombination pathway(s) is operative under various experimental conditions. Specifically, the respective effects of non-uniform doping, tapering along the length, variation in charge carrier mobilities and lifetimes, changes in nanowire radius, and changes in the density of surface defects on the observable photocurrent-potential responses are reported. These cumulative results serve as guides for future experimental work aimed at improving the attainable solar energy conversion efficiencies of doped semiconductor nanowire photoelectrodes. Lastly, separate simulations that model lightly doped nanowire photoelectrodes under high-level injection conditions are discussed. These results suggest discrete, ohmic-selective contacts may afford a way to circumvent the stringent doping requirements discussed herein for thin nanowire photoelectrodes.

Engineering Impurity Distributions in Photoelectrodes for Solar Water Oxidation

A strategy is introduced to decouple the optical absorption and electronic transport processes required for operation of oxide photoelectrodes in photoelectrochemical cells. Results indicate optical absorption at visible wavelengths and the related water photo-oxidation efficiencies can be enhanced by spatially segregating the functional impurity concentrations that facilitate their associated physical processes.

Photoassisted electrolysis of water for hydrogen generation with TiO2 aggregate film

In this paper, the nanocrystallite aggregates of TiO2 were synthesized and characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Brunauer-Emmett-Teller (BET). The aggregates are of submicron size, formed by nano-sized crystallites and able to offer both a large specific surface area and desirable size comparable to the wavelength of visible light. Therefore, the TiO2 aggregates were also studied as photoelectrode in photoelectrochemical cell for hydrogen generation. The results show that the hydrogen generation rates are 0.47 ml/h*cm2 and 0.27 ml/h*cm2 during the first test with and without illumination, respectively. The current density also presented continually increasing during the light-on period. This was attributed to the photogenerated current, which benefited from the TiO2 aggregates and may significantly enhance the electrolysis rate of water.

Semiconductor and ceramic nanoparticle films deposited by chemical bath deposition

Chemical bath deposition (CBD) has been used to deposit films of metal sulfides, selenides and oxides, together with some miscellaneous compounds, beginning nearly 140 years ago. While it is a well-known technique in a few specific areas (notably photoconductive lead salt detectors, photoelectrodes and more recently, thin film solar cells), it is by and large an under-appreciated technique. The more recent interest in all things 'nano' has provided a boost for CBD: since it is a low temperature, solution (almost always aqueous) technique, crystal size is often very small. This is evidenced by the existence of size quantization commonly found in CBD semiconductor films. The intention of this review is to provide readers, many of whom may not even be aware of the CBD technique, with an overview of how the technique has been used to fabricate nanocrystalline semiconductor (this terminology also includes oxides often classified as ceramics) films and some properties of these films. The review begins, after a short introduction, with a general description of the CBD method, designed to give the reader a basic knowledge of the technique. The rest of the review then focuses on nanocrystalline (or, in the few cases of amorphous deposits, nanoparticle) films. The various factors which determine crystal size are first discussed. This is followed by some of the many examples of size quantization observed in the films. Since CBD films are usually porous, surface effects can be very important, and various surface-dependent properties (light emission and surface states) as well as surface modification, are treated: (although some properties, like emission, can be strongly dependent on both surface and 'bulk'). Because of the fact that many CBD films have been made specifically for use as photoelectrodes in photoelectrochemical cells, there is next a chapter on this topic with a few examples of such photoelectrodes. Film structure and morphology follows with examples of patterning, porosity and crystal shape. The review concludes with some of the author's opinions as to what the near future holds for CBD development in general.

Light-induced aggregation of nanocrystalline CdS as photoelectrodes in photoelectrochemical cells (PECs)

The aggregation of size-quantizd nanocrystalline CdS particles (Q-CdS) in the film which serves as photoelectrodes in PECs were investigated by the photoelectrochemical method. The results show the aggregation of particle in PECs obviously occurs in the electrolyte( S 2− Sn 2− ). The photocurrent action spectra and absorption spectra of photoelectrodes display that the onset of the response is gradually shifted to red wavelengths with the increase of the illuminating time. The observations are understood in terms of the formation of larger aggregates as bulk CdS. The mechanism can be explained by the photoanodic dissolution and reform of CdS.

Photoelectrochemical Behavior of the GaAs/AlxGa1-x As Superlattice Electrode/Electrolyte Interface

Single and multiple quantum wells of lattice-matched superlattices material GaAs/AlxGa1-x As have been studied as photoelectrodes in photoelectrochemical cells containing nonaqueous electrolyte. Structured photocurrent spectra in the potential range of -1.8 to 1.0 V (vs standard calomel electrode) were obtained. The quantum yields for both superlattice electrodes were estimated and compared.

Nanocrystalline Photoelectrochemical Cells: A New Concept In Photovoltaic Cells

Semiconductor films with small crystal size normally exhibit prohibitively large recombination losses in photovoltaic cells. We show that porous nanocrystalline films of and can be used as photoelectrodes in photoelectrochemical cells with relatively low recombination losses. Spectral response measurements show how the recombination losses depend on film thickness. These photoelectrochemical cells operate due to charge separation at the semiconductor‐electrolyte interface rather than by a built‐in space charge layer as normally occurs in photovoltaic or photoelectrochemical cells. The rapid removal of one charge by the electrolyte explains the low recombination loss. The mechanism of these photoelectrochemical cells regarding the direction of photocurrent flow and the value of photovoltage is discussed.

Photoelectrochemistry of strained-layer and lattice-matched superlattice electrodes: effects due to buffer layers

Strained-layer and lattice-matched superlattice electrodes have been studied and compared as photoelectrodes in photoelectrochemical cells, and the effects of the presence of buffer layers in the strained-layer systems have been established. Photocurrent spectroscopy and photomodulated reflectance spectroscopy of superlattice electrodes, etched superlattice electrodes, and buffer layer structures reveal that highly strained superlattices (1.8% mismatch) have poorly defined quantization effects in their quantum wells; strained-layer superlattices with less mismatch (0.9%) have better defined quantization, but this is not reflected in their photocurrent spectra. On the other hand, lattice-matched superlattice electrodes exhibit extremely well-defined quantization effects that are clearly exhibited in multiple photocurrent peaks that match theoretical predictions; photomodulated reflectance spectra exhibit 17 transitions that represent all the possible allowed transitions in the quantum wells, including all light and heavy hole transitions, as well as unconfined transitions above the well barriers. The present work indicates that previous results reported for the photoelectrochemistry of highly strained superlattices probably reflected a photoresponse that was influenced more by the buffer layers than by the superlattice layers. The occurrence of hot electron transfer from photoexcited superlattice electrodes remains to be demonstrated unequivocally.

TiO 2-based photoelectrodes in photoelectrochemical cells: Performance and mechanism of O 2 evolution

Titanium laminae oxidized in air and CO 2 atmosphere have been investigated in the photo-oxidation of water. Factors influencing the photoactivity are the presence of the sub-oxide interface, the overall layer thickness and the non-stoichiometry of the rutile layer. The mechanism of O 2 evolution and the presence of some intermediates in the oxidation of water over titanium oxide photoanodes has been related to structural properties, derived from XRD and XPS electrochemical properties (polarization currents, cyclic voltammetry, FBp, zpc) and to the performance in PEC cell of five specimens of lowest and highest photoelectrochemical efficiency.

Layered transition metal thiophosphates (MPX 3) as photoelectrodes in photoelectrochemical cells

Layered crystals of the transition metal thiophosphates were synthesized and characterized for use as photoelectrodes in photoelectrochemical cells. Crystals incorporating tin and manganese show n-type response while those with iron and nickel show p-type response. These materials have a measured indirect bandgap of about 2.1 eV. They show ability to photoelectrolyze water in acid solutions with onset potentials which change in a Nernstian way as the pH of the solution changes. The onset potential is near zero volts versus a saturated calomel electrode at pH 2. At n-type crystals, oxygen could be evolved upon irradiation at underpotentials of 850 mV and at p-type crystals, hydrogen could be evolved at underpotentials of 400 mV, indicating a net gain in energy conversion. All crystals were unstable in basic solution. Liquid junction photovoltaic cells in iodide-triiodide acid solution using these layered materials were also constructed and found to have low efficiencies.