Empirical in operando analysis of the charge carrier dynamics in hematite photoanodes by PEIS, IMPS and IMVS

Slow hole diffusion limits the efficiency of p-type dye-sensitized solar cells based on the P1 dye

NiO electrodes are widely applied in p-type dye-sensitized solar cells (DSSCs) and photoelectrochemical cells, but due to excessive charge recombination, the efficiencies of these devices are still too low for commercial applications. To understand which factors induce charge recombination, we studied electrodes with a varying number of NiO layers in benchmark P1 p-DSSCs. We obtained the most efficient DSSCs with four layers of NiO (0.134%), and further insights into this optimum were obtained via dye loading studies and in operando photoelectrochemical immittance spectroscopy. These results revealed that more NiO layers led to an increasing light harvesting efficiency (η LH), but a decreasing hole collection efficiency (η CC), giving rise to the maximum efficiency at four NiO layers. The decreasing η CC with more NiO layers is caused by longer hole collection times, which ultimately limits the overall efficiency. Notably, the recombination rates were independent of the number of NiO layers, and similar to those observed in the more efficient n-type DSSC analogues, but hole collection was an order of magnitude slower. Therefore, with more NiO layers, the beneficial increase in η LH can no longer counteract the decrease in η CC due to slow hole collection, resulting in the overall efficiency of the solar cells to maximize at four NiO layers.

High-throughput parallel testing of ten photoelectrochemical cells for water splitting: case study on the effects of temperature in hematite photoanodes

High-throughput testing of photoelectrochemical cells and materials under well-defined operating conditions can accelerate the discovery of new semiconducting materials, the characterization of the phenomena occurring at the semiconductor-electrolyte interface, or the understanding of the coupled multi-physics transport phenomena of a complete working cell. However, there have been few high-throughput systems capable of dealing with complete cells and applying variations in real-life operating conditions, like temperature or irradiance. Understanding the effects of the variations of these real-life operating conditions on the performance of photoelectrode materials requires reliable and reproducible measurements. In this work, we report on a setup that simultaneously tests ten individual, identical photoelectrochemical cells whilst controlling temperature. The effects of temperature from 26 to 65 °C were studied in tin-doped hematite photoanodes for water splitting - as a reference case - through cyclic voltammetry and electrochemical impedance spectroscopy. The increase of surface-state-mediated charge recombination with temperature mainly penalized the energy conversion efficiency due to the reduction of the photovoltage produced. For parallel measurements in the ten individual cells, standard deviations from 20 to 60 mV for the onset potentials and less than 0.2 mA cm-2 for saturation current densities quantified the reproducibility of the results.

Photoelectrocatalytic Hydrogen Generation: Current Advances in Materials and Operando Characterization

Photoelectrochemical (PEC) hydrogen generation is a promising technology for green hydrogen production yet faces difficulties in achieving stability and efficiency. The scientific community is pushing toward the development of new electrode materials and a better understanding of the underlying reactions and degradation mechanisms. Advances in photocatalytic materials are being pursued through the development of heterojunctions, tailored crystal nanostructures, doping, and modification of solid-solid and solid-electrolyte interfaces. Operando and in situ techniques are utilized to deconvolute the charge transfer mechanisms and degradation pathways. In this review, both materials development and Operando characterization are covered for advancing PEC technologies. The recent advances made in the PEC materials are first reviewed including the applied improvement strategies for transition metal oxides, nitrites, chalcogenides, Si, and group III-V semiconductor materials. The efficiency, stability, scalability, and electrical conductivity of the aforementioned materials along with the improvement strategies are compared. Next, the Operando characterization methods and cite selected studies applied for PEC electrodes are described. Operando studies are very successful in elucidating the reaction mechanisms, degradation pathways, and charge transfer phenomena in PEC electrodes. Finally, the standing challenges and the potential opportunities are discussed by providing recommendations for designing more efficient and electrochemically stable PEC electrodes.

Precise analyses of photoelectrochemical reactions on particulate Zn0.25Cd0.75Se photoanodes in nonaqueous electrolytes using Ru bipyridyl complexes as a probe

Recombination of photoexcited carriers at interface states is generally believed to strongly govern the photoelectrochemical (PEC) performance of semiconductors in electrolytes. Sacrificial reagents (e.g., methanol or Na2SO3) are often used to assess the ideal PEC performance of photoanodes in cases of minimised interfacial recombination kinetics as well as accelerated surface reaction kinetics. However, varying the sacrificial reagents in the electrolyte means simultaneously changing the equilibrium potential and the number of electrons required to perform the sacrificial reaction, and thus the thermodynamic and kinetic aspects of the PEC reactions cannot be distinguished. In the present study, we propose an alternative methodology to experimentally evaluate the energy levels of interfacial recombination centres that can reduce PEC performance. We prepare nonaqueous electrolytes containing three different Ru complexes with different bipyridyl ligands; redox reactions of Ru complexes represent one-electron processes with similar charge transfer rates and diffusion coefficients. Therefore, the Ru complexes can serve as a probe to isolate and evaluate only the thermodynamic aspects of PEC reactions. Recombination centres at the interface between a nonaqueous electrolyte and a Zn0.25Cd0.75Se particulate photoanode are elucidated using this method as a model case. The energy level at which photocorrosion proceeds is also determined.

Unassisted Photoelectrochemical H2O2 Production with In Situ Glycerol Valorization Using α-Fe2O3

Photoelectrochemical (PEC) H2O2 production via two-electron O2 reduction is promising for H2O2 production without emitting CO2. For PEC H2O2 production, α-Fe2O3 is an ideal semiconductor owing to its earth abundance, superior stability in water, and an appropriate band gap for efficient solar light utilization. Moreover, its conduction band is suitable for O2 reduction to produce H2O2. However, a significant overpotential for water oxidation is required due to the poor surface properties of α-Fe2O3. Thus, unassisted solar H2O2 production is not yet possible. Herein, we demonstrate unassisted PEC H2O2 production using α-Fe2O3 for the first time by applying glycerol oxidation, which requires less bias compared with water oxidation. We obtain maximum Faradaic efficiencies of 96.89 ± 0.6% and 100% for glycerol oxidation and H2O2 production, respectively, with high stability for 25 h. Our results indicate that unassisted and stable PEC H2O2 production is feasible with in situ glycerol valorization using the α-Fe2O3 photoanode.

Fabrication of ZnO Scaffolded CdS Nanostructured Photoanodes with Enhanced Photoelectrochemical Water Splitting Activity under Visible Light

CdS, characterized by its comparatively narrow energy band gap (∼2.4 eV), is an appropriate material for prospective use as a photoanode in photoelectrochemical water splitting. Regrettably, it encounters several obstacles for practical and large-scale applications, including issues such as bulk carrier recombination and diminished conductivity. Here, we have tried to address these challenges by fabricating a novel photoelectrode (ZnO/CdS) composed of one-dimensional ZnO nanorods (NRs) decorated with two-dimensional CdS nanosheets (NSs). A facile two-step chemical method comprising electrodeposition along with chemical bath deposition is employed to synthesize the ZnO NRs, CdS NSs, and ZnO/CdS nanostructures. The prepared nanostructures have been investigated by UV-visible absorption spectroscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy. The fabricated ZnO/CdS nanostructures have shown enhanced photoelectrochemical properties due to the improvement of the semiconductor junction surface area and thereby enhanced visible light absorption. The incorporation of CdS NSs has been further found to promote the rate of the charge separation and transfer process. Subsequently, the fabricated ZnO/CdS photoelectrodes achieved a photocurrent conversion efficiency 3 times higher than that of a planar ZnO NR photoanode and showed excellent performance under visible light irradiation. The highest applied bias photon-to-current conversion efficiency (% ABPE) of about ∼0.63% has been obtained for the sample with thicker CdS NSs on ZnO NRs with a photocurrent density of ∼1.87 mA/cm2 under AM 1.5 G illumination. The newly synthesized nanostructures further demonstrate that the full photovoltaic capacity of nanomaterials is yet to be exhausted.

De-Pinning Fermi Level and Accelerating Surface Kinetics with an ALD Finish Boost the Fill Factor of BiVO4 Photoanodes to 44

With the rapid development of performance and long-term stability, bismuth vanadate (BiVO4 ) has emerged as the preferred photoanode in photoelectrochemical tandem devices. Although state-of-the-art BiVO4 photoanodes realize a saturated photocurrent density approaching the theoretical maximum, the fill factor (FF) is still inferior, pulling down the half-cell applied bias photon-to-current efficiency (HC-ABPE). Among the major fundamental limitations are the Fermi level pinning and sluggish surface kinetics at the low applied potentials. This work demonstrates that the plasma-assisted atomic layer deposition technique is capable of addressing these issues by seamlessly installing an angstrom-scale FeNi-layer between BiVO4 and electrolyte. Not only this ultrathin FeNi layer serves as an efficient OER cocatalyst, more importantly, it also effectively passivates the surface states of BiVO4 , de-pins the surface Fermi level, and enlarges the built-in voltage, allowing the photoanode to make optimal use of the photogenerated holes for achieving high FF up to 44% and HC-ABPE to 2.2%. This study offers a new approach for enhancing the FF of photoanodes and provides guidelines for designing efficient unassisted solar fuel devices.

Distribution of Relaxation Times Based on Lasso Regression: A Tool for High-Resolution Analysis of IMPS Data in Photoelectrochemical Systems

Intensity-modulated photocurrent spectroscopy (IMPS) has been largely employed in semiconductor characterization for solar energy conversion devices to probe the operando behavior with widely available facilities. However, the implementation of IMPS data analysis to complex structures, whether based on the physical rate constant model (RCM) or the assumption-free distribution of relaxation times (DRT), is generally limited to a semi-quantitative description of the charge carrier kinetics of the system. In this study, a new algorithm for the analysis of IMPS data is developed, providing unprecedented time resolution to the investigation of μs to s charge carrier dynamics in semiconductor-based systems used in photoelectrochemistry and photovoltaics. The algorithm, based on the previously developed DRT analysis, is herein modified with a Lasso regression method and available to the reader free of charge. A validation of this new algorithm is performed on a α-Fe2O3 photoanode for photoelectrochemical water splitting, identified as a standard platform in the field, highlighting multiple potential-dependent charge transfer paths, otherwise hidden in the conventional IMPS data analysis.

Selective Valorization of Glycerol to Formic Acid on a BiVO4 Photoanode through NiFe Phenolic Networks

The integration of the glycerol oxidation reaction (GOR) with the hydrogen evolution reaction in photoelectrochemical (PEC) cells is a desirable alternative to PEC water splitting since a large quantity of glycerol is easily accessible as the byproduct from the biodiesel industry. However, the PEC valorization of glycerol to the value-added products suffers from low Faradaic efficiency and selectivity, especially in acidic conditions, which is beneficial for hydrogen production. Herein, by loading bismuth vanadate (BVO) with a robust catalyst composed of phenolic ligands (tannic acid) coordinated with Ni and Fe ions (TANF), we demonstrate a modified BVO/TANF photoanode for the GOR with a remarkable Faradaic efficiency of over 94% to value-added molecules in a 0.1 M Na₂SO₄/H₂SO₄ (pH = 2) electrolyte. The BVO/TANF photoanode achieved a high photocurrent of 5.26 mA·cm^-2 at 1.23 V versus reversible hydrogen electrode under 100 mW/cm² white light irradiation for formic acid production with 85% selectivity, equivalent to 573 mmol/(m²·h). Transient photocurrent and transient photovoltage techniques and electrochemical impedance spectroscopy along with intensity-modulated photocurrent spectroscopy indicated that the TANF catalyst could accelerate hole transfer kinetics and suppress charge recombination. Comprehensive mechanistic investigations reveal that the GOR is initiated by the photogenerated holes of BVO, while the high selectivity to formic acid is attributed to the selective adsorption of primary hydroxyl groups in glycerol on TANF. This study provides a promising avenue for highly efficient and selective formic acid generation from biomass in acid media via PEC cells.

How titanium and iron are integrated into hematite to enhance the photoelectrochemical water oxidation: a review

Hematite has been considered as a promising photoanode candidate for photoelectrochemical (PEC) water oxidation and has attracted numerous interests in the past decades. However, intrinsic drawbacks drastically lower its photocatalytic activity. Ti-based modifications including Ti-doping, Fe₂O₃/Fe₂TiO₅ heterostructures, TiO₂ passivation layers, and Ti-containing underlayers have shown great potential in enhancing the PEC conversion efficiency of hematite. Moreover, the combination of Ti-based modifications with various strategies towards more efficient hematite photoanodes has been widely investigated. Nevertheless, a corresponding comprehensive overview, especially with the most recent working mechanisms, is still lacking, limiting further improvement. In this respect, by summarizing the recent progress in Ti-modified hematite photoanodes, this review aims to demonstrate how the integration of titanium and iron atoms into hematite influences the PEC properties by tuning the carrier behaviours. It will provide more cues for the rational design of high-performance hematite photoanodes towards future practical applications.

Photoelectrocatalytic Properties of a Ti-Modified Nanocrystalline Hematite Film Photoanode

Photoelectrocatalytic oxidation of methanol, ethylene glycol, glycerol, and 5,6,7,8-tetrahydro-2-naphthol on thin-film nanocrystalline hematite electrodes fabricated by electrochemical deposition and promoted with spin-coated titanium has been studied. It is shown that the modification of hematite transforms it into material exhibiting high activity in the photoelectrochemical process of substrate oxidation upon illumination with light in the visible region of the spectrum. The highest activity is observed in the reaction of photoelectrocatalytic oxidation of glycerol. Results of intensity-modulated photocurrent spectroscopy (IMPS) suggest that the effect is due to an increased rate of charge transfer in the process of photoelectro-oxidation and efficient suppression of the recombination of generated electron-hole pairs. Therefore, thin-film photoanodes based on modified hematite are promising for practical application in the photooxidation of glycerol, a by-product of biofuel production, as well as in the photoelectrochemical degradation of other organic pollutants, including those formed during the production of pharmaceuticals.

Intensity modulated photocurrent spectroscopy to investigate hidden kinetics at hybrid perovskite–electrolyte interface

The numerous assorted accounts of the fundamental questions of ion migration in hybrid perovskites are making the picture further intricate. The review of photo-induced ion migration using small perturbation frequency domain techniques other than impedance spectroscopy is more crucial now. Herein, we probe into this by investigating perovskite–electrolyte (Pe–E) and polymer-aqueous electrolyte (Po–aqE) interface using intensity modulated photocurrent spectroscopy (IMPS) in addition to photoelectrochemical impedance spectroscopy (PEIS). We reported that the electronic-ionic interaction in hybrid perovskites including the low-frequency ion/charge transfer and recombination kinetics at the interface leads to the spiral feature in IMPS Nyquist plot of perovskite-based devices. This spiral trajectory for the perovskite-electrolyte interface depicts three distinct ion kinetics going on at the different time scales which can be more easily unveiled by IMPS rather than PEIS. Hence, IMPS is a promising alternative to PEIS. We used Peter’s method of interpretation of IMPS plot in photoelectrochemistry to estimate charge transfer efficiency \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$({Q}_{ste})$$\end{document}(Qste) from the Rate Constant Model. The \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${Q}_{ste}$$\end{document}Qste at low-frequency for Pe–E interface exceeds unity due to ion migration induced modified potential across the perovskite active layer. Hence, ion migration and mixed electronic-ionic conductivity of hybrid perovskites are responsible for the extraordinary properties of this material.

Polymer Photoelectrodes for Solar Fuel Production: Progress and Challenges

Converting solar energy to fuels has attracted substantial interest over the past decades because it has the potential to sustainably meet the increasing global energy demand. However, achieving this potential requires significant technological advances. Polymer photoelectrodes are composed of earth-abundant elements, e.g. carbon, nitrogen, oxygen, hydrogen, which promise to be more economically sustainable than their inorganic counterparts. Furthermore, the electronic structure of polymer photoelectrodes can be more easily tuned to fit the solar spectrum than inorganic counterparts, promising a feasible practical application. As a fast-moving area, in particular, over the past ten years, we have witnessed an explosion of reports on polymer materials, including photoelectrodes, cocatalysts, device architectures, and fundamental understanding experimentally and theoretically, all of which have been detailed in this review. Furthermore, the prospects of this field are discussed to highlight the future development of polymer photoelectrodes.

Photoelectrochemical Performance Improving Mechanism: Hybridization Appearing at the Energy Band of BiVO4 Photoanode by Doped Quantum Layers Modification

Surface passivation of the photoelectrode by wide bandgap semiconductor quantum layer is an important strategy to improve work stability and surface state inhibition. However, an inevitable energy barrier is generated during the quantum tunneling process of the photocarriers. To overcome this shortage, a tandem photo-generated hole transfer route is fabricated on BiVO₄ photoanode by doped dual-quantum layers modification, Ni-ZnO (5 nm) and Rh-SrTiO₃ (≈10 nm). Modulated photoelectrochemical (PEC), Scanning Kelvin Probe (SKP), and DFT calculation method results indicate that a tandem hole ohmic contact route is formed in the photoanode to reduce the quantum tunneling energy barrier, meanwhile, the photon absorption capacity of BiVO₄ is improved after doped quantum layers modification. Both a phenomenal attribute to the energy band hybridization between Ni, Rh 3d orbits in quantum layers with BiVO₄ photoanode. Then, the modified BiVO₄ photoanode achieves the recoded photocurrent density of 6.47 and 5.18 mA cm^-2 (Na₂ SO₃ electrolyte, V_RHE  = 1.23 V) under simulated sun light (100 mW cm^-2 AM 1.5 G) by xenon lamp illumination without and with UV composition cutting down to ≈5%, respectively. Generally, this work will highlight a potential application in the fields of PEC water splitting and photovoltaic conversion for various semiconductor nanomaterials.

Reaction Kinetics of Photoelectrochemical CO2 Reduction on a CuBi2O4-Based Photocathode

The CO₂ reduction reaction (CO₂RR) is an essential step in natural photosynthesis and artificial photosynthesis to provide carbohydrate foods and hydrocarbon energy in the carbon-neutral cycle. However, the current solar conversion efficiencies and/or product selectivity of the CO₂RR are very sluggish due to its complicated multiple-step charge transfer reactions. Here, we systematically investigate the charge transfer reaction rate during CO₂ reduction on CuBi₂O₄ photocathodes, where the surface is modified with 3-aminopropyltriethoxysilane (APTES). We discover that the surface amine group increases the charge separation rate, significantly enhancing the surface charge transfer reaction rate. However, the surface acidity has less influence on the first-order reaction, indicating that a rate-determining step (RDS) exists in the early stage of the photoelectrochemical cell (PEC) processes. Moreover, the intensity-modulated photocurrent spectroscopy (IMPS) confirms that both surface charge transfer and the recombination rate on APTES-coated CuBi₂O₄ are larger than bare CuBi₂O₄ while possessing comparable charge transfer efficiencies. Overall, the surface charge transfer reactions under the PEC condition require designing more effective nanostructured photoelectrodes and powerful characterization methods to intrinsically increase the charge separation and transfer rate while reducing the recombination rate.

Photo-enhanced ionic conductivity across grain boundaries in polycrystalline ceramics

Grain boundary conductivity limitations are ubiquitous in material science. We show that illumination with above-bandgap light can decrease the grain boundary resistance in solid ionic conductors. Specifically, we demonstrate the increase of the grain boundary conductance of a 3 mol% Gd-doped ceria thin film by a factor of approximately 3.5 at 250 °C and the reduction of its activation energy from 1.12 to 0.68 eV under illumination, while light-induced heating and electronic conductivity could be excluded as potential sources for the observed opto-ionic effect. The presented model predicts that photo-generated electrons decrease the potential barrier heights associated with space charge zones depleted in charge carriers between adjacent grains. The discovered opto-ionic effect could pave the way for the development of new electrochemical storage and conversion technologies operating at lower temperatures and/or higher efficiencies and could be further used for fast and contactless control or diagnosis of ionic conduction in polycrystalline solids.

Operational Spectroelectrochemical Investigation on the Interfacial Charge Dynamics of Copper Bismuth Oxide Based Photocathode

Copper bismuth oxide (CBO) is an emerging photocathode in photoelectrochemical (PEC) water splitting but exhibits limited performance due to the severe recombination of photogenerated charges at the semiconductor-liquid junction (SCLJ). For the first time, a set of operational spectroelectrochemical experiments including electrochemical impedance spectroscopy (EIS), transient photocurrent spectroscopy (TPS), and intensity-modulated photocurrent/voltage spectroscopy (IMVS, IMPS) are designed to investigate the charge dynamics at the SCLJ. It is indicated that there are dense surface states above the valence band of CBO, inducing the "Fermi level pinning" (FLP) effect at the SCLJ. The kinetic parameters speculated by IMVS and IMPS indicate the charge transfer efficiency of below 10% with even a bias of ∼0.7 V applied. TPS confirms the sluggish dynamics because of the charging behavior of the surface states. It is expected that this work would provide new connotations of charge dynamics at the SCLJ for the further optimization of CBO-based PEC systems.

Solar-to-Pharmaceutical Raw Material Production: Photoelectrochemical Naphthoquinone Formation Using Stabilized BiVO4 Photoanodes in Acid Media

In the quest for efficient use of solar energy to produce high-value-added chemicals, we first achieved the photoelectrochemical (PEC) diketonization of naphthalene, using a BiVO₄/WO₃ photoanode, to obtain naphthoquinone, an important pharmaceutical raw material with excellent efficiency by solar energy conversion. In the electrochemical (EC) reaction using F-doped SnO₂ (FTO) substrates and a 0.5 M H₂SO₄ H₂O-acetone (60 vol %) mixed solution containing 5 mM naphthalene, we produced a small amount of naphthoquinone evolution in the dark. However, naphthoquinone (η_NQ)'s Faradic efficiency and its evolution rate at 1.7 V_Ag/AgCl were only 28.5% and 0.48 μmol·cm^-2·h^-1, respectively. The PEC reaction using a WO₃ photoanode had very low efficiency for naphthalene diketonization, with low η_NQ and evolution rate values at 1.1 V_Ag/AgCl of 0.3% and 0.039 μmol·cm^-2·h^-1, respectively. In contrast, the BiVO₄/WO₃ photoanode strongly enhanced the PEC reaction, and the η_NQ and evolution rates at 1.1 V_Ag/AgCl were boosted up to 37.5% and 4.7 μmol·cm^-2·h^-1, respectively. The evolution rate of the PEC reaction in the BiVO₄/WO₃ photoanode was 10 times higher than that of the EC reaction with the FTO substrate regardless of the very low bias voltage. This result suggests that the BiVO₄-based photoanode was very efficient for the selective oxidation of naphthalene even in acid media because of the acetone-mixed electrolyte's anti-photocorrosion effect and the multilayering of WO₃ and BiVO₄. At a naphthalene concentration of 20 mM, the naphthoquinone evolution rate reached its maximum value of 7.1 μmol·cm^-2·h^-1. Although η_NQ tended to decrease with the increase in the electric charge, it reached 100% at a low bias voltage of 0.7 V_Ag/AgCl. An intensity-modulated photocurrent spectroscopy analysis indicated the rate constant of charge transfer at the photoanode surface to the naphthalene molecules was strongly enhanced at a low bias voltage of 0.7-1.1 V_Ag/AgCl, resulting in the high η_NQ value. The acid-resistant BiVO₄/WO₃ photoanode functioned in acetone-mixed electrolytes enabled the realization of a new PEC oxidation reaction driven by solar energy to produce high-value-added pharmaceutical raw materials.

Improved Photocatalytic Water Splitting Activity of Highly Porous WO3 Photoanodes by Electrochemical H+ Intercalation

WO3 photoanodes are widely used in photoelectrochemical catalysis, but typically the as-synthesized material is annealed before application. It is therefore desirable to explore less energy-intensive treatments. In this study, WO3 films of up to 3.9 μm thickness were obtained by galvanostatic anodization of tungsten foil in a neutral-pH Na2SO4 and NaF electrolyte, also containing a NaH2PO2 additive (to suppress O2 accumulation on the pore walls). Additionally, the WO3 photoanodes were modified by applying a cathodic reduction (H+ intercalation) and anodic activation treatment in-situ. XPS spectra revealed that intercalation modifies WO3 films; the amount of W5+-O and O-vacancy bonds was increased. Furthermore, subsequent activation leads to a decrease of the W5+ signal, but the amount of O-vacancy bonds remains elevated. The as-prepared and reduced (intercalated & activated) films were tested as OER photoanodes in acidic 0.1 M Na2SO4 media, under illumination with a 365 nm wavelength LED. It was observed that thinner films generated larger photocurrents. The peculiarities detected by XPS for reduced films correlate well with their improved photocatalytic activity. Photo-electrochemical impedance and intensity modulated photocurrent spectroscopies were combined with steady-state measurements in order to elucidate the effects of H+ intercalation on photoelectrochemical performance. The reduction results in films with enhanced photoexcited charge carrier generation/separation, improved conductivity, and possibly even suppressed bulk recombination. Thus, the intercalation & activation adopted in this study can be reliably used to improve the overall activity of as-synthesized WO3 photoanodes, and particularly of those that are initially poorly photoactive.

Plasmonic Photoelectrochemistry: In View of Hot Carriers

Utilizing plasmon-generated hot carriers to drive chemical reactions has emerged as a popular topic in solar photocatalysis. However, a complete description of the underlying mechanism of hot-carrier transfer in photochemical processes remains elusive, particularly for those involving hot holes. Photoelectrochemistry enables to localize hot holes on photoanodes and hot electrons on photocathodes and thus offers an approach to separately explore the hole-transfer dynamics and electron-transfer dynamics. This review summarizes a comprehensive understanding of both hot-hole and hot-electron transfers from photoelectrochemical studies on plasmonic electrodes. Additionally, working principles and applications of spectroelectrochemistry are discussed for plasmonic materials. It is concluded that photoelectrochemistry provides a powerful toolbox to gain mechanistic insights into plasmonic photocatalysis.

Combining Modulated Techniques for the Analysis of Photosensitive Devices

Small-perturbation techniques such as impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS) are useful tools to characterize and model photovoltaic and photoelectrochemical devices. While the analysis of the impedance spectra is generally carried out using an equivalent circuit, the intensity-modulated spectroscopies are often analyzed through the measured characteristic response times. This makes the correlation between the two methods of analysis generally unclear. In this work, by taking into consideration the absorptance and separation efficiency, a unified theoretical framework and a procedure to combine the spectral analysis of the three techniques are proposed. Such a joint analysis of IS, IMPS, and IMVS spectra greatly reduces the sample space of possible equivalent circuits to model the device and allows obtaining parameters with high reliability. This theoretical approach is applied in the characterization of a silicon photodiode to demonstrate the validity of this methodology, which shows great potential to improve the quality of analysis of spectra obtained from frequency domain small-perturbation methods.

Evolution of Surface Oxidation on Ta3N5 as Probed by a Photoelectrochemical Method

In this work, we present an in situ method to probe the evolution of photoelectrochemically driven surface oxidation on photoanodes during active operation in aqueous solutions. A standard solution of K₄Fe(CN)₆-KPi was utilized to benchmark the photocurrent and assess progressive surface oxidation on Ta₃N₅ in various oxidizing solutions. In this manner, a proportional increase in the surface oxygen concentration was detected with respect to oxidation time and further correlated with a continuous decline in the photocurrent. To discern how surface oxidation alters the photocurrent, we experimentally and theoretically explored its impact on the surface carrier recombination and the interfacial hole transfer rates. Our results indicate that the sluggish photocurrent demonstrated by oxidized Ta₃N₅ arises because of changes in both rates. In particular, the results suggest that the N-O replacement present on the Ta₃N₅ surface primarily increases the carrier recombination rate near the surface and to a lesser degree reduces the interfacial hole transfer rate. More generally, this methodology is expected to further our understanding of surface oxidation atop other nonoxide semiconductor photoelectrodes and its impact on their operation.

Lithium ion battery degradation: what you need to know

The expansion of lithium-ion batteries from consumer electronics to larger-scale transport and energy storage applications has made understanding the many mechanisms responsible for battery degradation increasingly important. The literature in this complex topic has grown considerably; this perspective aims to distil current knowledge into a succinct form, as a reference and a guide to understanding battery degradation. Unlike other reviews, this work emphasises the coupling between the different mechanisms and the different physical and chemical approaches used to trigger, identify and monitor various mechanisms, as well as the various computational models that attempt to simulate these interactions. Degradation is separated into three levels: the actual mechanisms themselves, the observable consequences at cell level called modes and the operational effects such as capacity or power fade. Five principal and thirteen secondary mechanisms were found that are generally considered to be the cause of degradation during normal operation, which all give rise to five observable modes. A flowchart illustrates the different feedback loops that couple the various forms of degradation, whilst a table is presented to highlight the experimental conditions that are most likely to trigger specific degradation mechanisms. Together, they provide a powerful guide to designing experiments or models for investigating battery degradation.

Understanding the varying mechanisms between the conformal interlayer and overlayer in the silicon/hematite dual-absorber photoanode for solar water splitting

Dual-absorber photoelectrodes have been proved to have great potential in the photoelectrochemical (PEC) water splitting application due to their broadband absorption and suitable energy-band position, while the surface/interface issues are still not clearly resolved and understood. Here, during the preparation of a silicon/hematite dual-absorber photoanode achieved via synthesizing a Sn-doped hematite film on the silicon nanowire (SiNW) substrate, we separately introduced the conformal overlayer and interlayer of an Al₂O₃ thin film by atomic layer deposition. With the thickness-optimized interlayer (overlayer) of the Al₂O₃ thin film, the photocurrent density at 1.23V_RHE can be enhanced from 0.85 mA cm^-2 to 1.51 mA cm^-2 (1.25 mA cm^-2), and the on-set potential has a cathodic shift of ∼0.32 V. Although both the overlayer and interlayer modification can substantially improve the PEC performance, the underlying mechanisms are obviously different. The overlayer can only reduce the carrier recombination on the top surface and in the bulk of the hematite film; in contrast, the interlayer not only passivates the SiNW surface and bottom surface of the hematite film, but also the top surface of the photoanode due to Al³⁺ thermal diffusion from the bottom to the top surface of the hematite film and the resultant Al₂O₃ formation. This work deepens our understanding for the roles of the surface and interface engineering in the achievement of high-performance PEC systems based on dual or more absorbers.

Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes

This review provides a comprehensive overview on characterisation techniques for light-driven redox-catalysts highlighting spectroscopic, microscopic, electrochemical and spectroelectrochemical approaches.

Heteroatom Doping Strategy for Establishing Hematite Homojunction as Efficient Photocatalyst for Accelerating Water Splitting

Hematite (α-Fe₂ O₃ ) is one of the promising photocatalysts for water oxidation, owing to its stable, abundant and visible-light responsive features. Enhancing electrical conductivity and accelerating oxidation evolution kinetics are expected to improve photocatalytic ability of hematite toward water oxidation. In this work, strategies of doping heteroatoms and developing pn homojunction are adopted to enhance the photocatalytic ability of hematite electrodes. The Ti and Mg dopants are separately incorporated in two layers of hematite electrodes via two-step hydrothermal reaction and one-step annealing process. The effect of regrowth time for synthesizing Mg-doped hematite on the photoelectrochemical performance of Mg-doped and Ti-doped hematite (Mg-Fe₂ O₃ /Ti-Fe₂ O₃ ) electrode is studied. The size of rod-like structure and gaps in-between play important roles on the photocatalytic ability of Mg-Fe₂ O₃ /Ti-Fe₂ O₃ . The optimized Mg-Fe₂ O₃ /Ti-Fe₂ O₃ electrode is prepared by using merely 10 min for synthesizing the Mg-doped hematite top layer, which shows the highest photocurrent density of 2.83 mA/cm² at 1.60 V_RHE along with the highest carrier density of 5.89×10¹⁶  cm^-3 and the smallest charge-transfer resistance. This largely improved photoelectrochemical performance is attributed to the more donor generation with heteroatom-doping and more efficient charge cascade with homojunction establishment. Other p-type metals are encouraged to dope in hematite as the second layer to couple with the n-type Ti-doped hematite for developing efficient pn homojunction and improve the photocatalytic ability of hematite in the near future.

Unique Layer-Doping-Induced Regulation of Charge Behavior in Metal-Free Carbon Nitride Photoanodes for Enhanced Performance

Photoinduced charge carrier behavior is critical in determining photoelectrocatalytic activity. In this study, a unique layer-doped metal-free polymeric carbon nitride (C₃ N₄ ) photoanode is fabricated by using one-pot thermal vapor deposition. With this method, a photoanode consisting of a phosphorus-doped top layer, boron-doped middle layer, and pristine C₃ N₄ bottom layer, was formed as a result of the difference in thermal polymerization kinetics associated with the boron-containing H₃ BO₃ -melamine complex and the phosphorus-containing H₃ PO₄ -dicyandiamide complex. This layer-doping fabrication strategy effectively contributes to the formation of dual junctions that optimizing charge carrier behavior. The ternary-layer C₃ N₄ photoanode exhibits significantly enhanced photoelectrochemical water oxidation activity compared to pristine C₃ N₄ , with a record photocurrent density of 150±10 μA cm^-2 at 1.23 V vs. RHE. This layer-doping strategy provides an effective means for design and fabrication of photoelectrodes for solar water oxidation.

Photoelectrocatalytic activity of In(iii)-modified TiO2 photoanodes in the visible spectrum region

Nanocrystalline film photoanodes of titanium dioxide modified with In(iii) ions in the concentration range from 1.0 to 10 mass% (0.23 to 2.52 at%) are manufactured using the sol–gel method.

Experimental and numerical analysis to identify the performance limiting mechanisms in solid-state lithium cells under pulse operating conditions

Solid-state lithium batteries could reduce the safety concern due to thermal runaway while improving the gravimetric and volumetric energy density beyond the existing practical limits of lithium-ion batteries. The successful commercialisation of solid-state lithium batteries depends on understanding and addressing the bottlenecks limiting the cell performance under realistic operational conditions such as dynamic current profiles of different pulse amplitudes. This study focuses on experimental analysis and continuum modelling of cell behaviour under pulse operating conditions, with most model parameters estimated from experimental measurements. By using a combined impedance and distribution of relaxation times analysis, we show that charge transfer at both interfaces occurs between the microseconds and milliseconds timescale. We also demonstrate that a simplified set of governing equations, rather than the conventional Poisson-Nernst-Planck equations, are sufficient to reproduce the experimentally observed behaviour during pulse discharge, pulse charging and dynamic pulse. Our simulation results suggest that solid diffusion in bulk LiCoO2 is the performance limiting mechanism under pulse operating conditions, with increasing voltage loss for lower states of charge. If bulk electrode forms the positive electrode, improvement in the ionic conductivity of the solid electrolyte beyond 10-4 S cm-1 yields marginal overall performance gains due to this solid diffusion limitation. Instead of further increasing the electrode thickness or improving the ionic conductivity on their own, we propose a holistic model-based approach to cell design, in order to achieve optimum performance for known operating conditions.

Sb2S3 Thickness-Related Photocurrent and Optoelectronic Processes in TiO2/Sb2S3/P3HT Planar Hybrid Solar Cells

In this work, a comprehensive understanding of the relationship of photon absorption, internal electrical field, transport path, and relative kinetics on Sb₂S₃ photovoltaic performance has been investigated. The n-i-p planar structure for TiO₂/Sb₂S₃/P3HT heterojunction hybrid solar cells was conducted, and the photon-to-electron processes including illumination depth, internal electric field, drift velocity and kinetic energy of charges, photo-generated electrons and hole concentration-related surface potential in Sb₂S₃, charge transport time, and interfacial charge recombination lifetime were studied to reveal the key factors that governed the device photocurrent. Dark J-V curves, Kelvin probe force microscope, and intensity-modulated photocurrent/photovoltage dynamics indicate that internal electric field is the main factors that affect the photocurrent when the Sb₂S₃ thickness is less than the hole diffusion length. However, when the Sb₂S₃ thickness is larger than the hole diffusion length, the inferior area in Sb₂S₃ for holes that cannot be diffused to P3HT would become a dominant factor affecting the photocurrent. The inferior area in Sb₂S₃ layer for hole collection could also affect the V_oc of the device. The reduced collection of holes in P3HT, when the Sb₂S₃ thickness is larger than the hole diffusion length, would increase the difference between the quasi-Fermi levels of electrons and holes for a lower V_oc.

Carrier recombination and transport dynamics in superstrate solar cells analyzed by modeling the intensity modulated photoresponses

The dynamics of carrier recombination and transport of two CuInS2 superstrate solar cells was studied by intensity modulated photovoltage and photocurrent spectroscopy (IMVS and IMPS respectively). For the analysis of the resulting data two different approaches were implemented. In the first approach, the typically used analysis in Dye Sensitized Solar Cells (DSSC) was adapted to obtain the characteristic times of the processes involved. The second approach was based on the fittings of both the IMVS and IMPS data to the solution of the continuity equation. These fittings allow the calculation of different dynamic parameters of the cells. Moreover, consistency between the obtained parameters was observed, in good agreement with the typical analysis for DSSC. The resulting dynamics was associated with the presence and distribution of defect states among the samples. Moreover, from the performed analysis, a relation between the results and the post-treatment applied to the solar cells could be established. The difference in the dynamics of the cells is mainly observed in the difference between the electron lifetimes of both solar cells.

Generalized Distribution of Relaxation Times Analysis for the Characterization of Impedance Spectra

Impedance spectroscopy is a universal nondestructive tool for the analysis of the polarization behavior of electrochemical systems in frequency domain. As an extension and enhancement of the standard impedance spectroscopy, the distribution of relaxation times (DRT) analysis was established, where the spectra are transferred from frequency into time domain. The DRT helps to analyze complex impedance spectra by identifying the number of polarization processes involved without prior assumptions and by separating and quantifying their single polarization contributions. The DRT analysis, as introduced in literature, claims to be a model-free approach for the characterization of resistive-capacitive systems. However, a data preprocessing step based on impedance models is often required to exclude non-resistive-capacitive components off the measured impedance spectra. The generalized distribution of relaxation times (GDRT) analysis presented in this work is dedicated to complex superposed impedance spectra that include ohmic, inductive, capacitive, resistive-capacitive, and resistive-inductive effects. The simplified work flow without preprocessing steps leads to a reliable and reproducible DRT analysis that fulfills the assumption of being model-free. The GDRT is applicable for the analysis of electrochemical, electrical, and even for non-electrical systems. Results are shown for a lithium-ion battery, a vanadium redox flow battery, and for a double-layer capacitor.

Nanoplatelet cryoaerogels with potential application in photoelectrochemical sensing

Semiconductor nanoparticle based porous 3D assemblies are interesting materials for various applications in the fields of photovoltaics, catalysis, or optical sensing. For use as photoelectrodes in photoelectrochemical sensors they need to be characterised by a high porosity, a good photostability, and a high charge carrier mobility. Our work reports on the preparation of cryoaerogel photoelectrodes based on CdSe nanoplatelets and their photoelectrochemical characterisation by means of linear sweep voltammetry (LSV) and intensity modulated photocurrent spectroscopy (IMPS). The obtained open-pored cryoaerogel films were observed to produce much higher photocurrents than comparable drop-cast films. By means of IMPS, the performance differences could be linked to the occurrence of charge carrier movement, which could solely be proven for the cryoaerogels. In a proof-of-principle experiment, the potential of the prepared photoelectrodes for application in photoelectrochemical sensing was moreover demonstrated.

Multilayered Hematite Nanowires with Thin-Film Silicon Photovoltaics in an All-Earth-Abundant Hybrid Tandem Device for Solar Water Splitting

The concept of hybrid tandem device structures that combine metal oxides with thin-film semiconducting photoabsorbers holds great promise for large-scale, robust, and cost-effective bias-free photoelectrochemical water splitting (PEC-WS). This work highlights important steps toward the efficient coupling of high-performance hematite photoanodes with multijunction thin-film silicon photocathodes providing high bias-free photocurrent density. The hybrid PEC-WS device is optimized by testing three types of multijunction silicon photocathodes with the hematite photoanode: amorphous silicon (a-Si:H) tandem: a-Si:H/a-Si:H and triple junction with microcrystalline silicon (μc-Si:H): a-Si:H/a-Si:H/μc-Si:H and a-Si:H/μc-Si:H/μc-Si:H. The results provide evidence that the multijunction structures offer high flexibility for hybrid tandem devices with regard to tunable photovoltages and spectral matching. Furthermore, both photoanode and photocathode are tested under various electrolyte and light concentration conditions, respectively, with respect to their photoelectrochemical performance and stability. A 27 % enhancement in the solar-to-hydrogen conversion efficiency is observed upon concentrating light from 100 to 300 mW cm^-2 . Ultimately, bias-free water splitting is demonstrated, with a photocurrent density of 4.6 mA cm^-2 (under concentrated illumination) paired with excellent operation stability for more than 24 h of the all-earth-abundant and low-cost hematite/silicon tandem PEC-WS device.

Rational Design of Photoelectrodes with Rapid Charge Transport for Photoelectrochemical Applications

Photoelectrode materials are the heart of photoelectrochemical (PEC) cells, which hold great promise to address global energy and environmental issues by converting solar energy into electricity or chemical fuels. In recent decades, significant research efforts have been devoted to the design and construction of photoelectrodes for the efficient generation and utilization of charge carriers to boost PEC performance. Herein, insights from a literature study on the relationship between the architecture and charge dynamics of photoelectrodes are presented. After briefly introducing the fundamental theories of charge dynamics in nanostructured photoelectrodes, the development of photoelectrode design in 1D polycrystalline nanotube arrays, 1D single-crystalline nanowire arrays, and hierarchical and mesoporous nanowire arrays is reviewed with a focus on the interplay between architecture and charge transport properties. For each design, commonly used synthetic approaches and the corresponding charge transport properties are discussed. Subsequently, the applications of these photoelectrodes in PEC systems are summarized. In conclusion, future challenges in the rational design of photoelectrode architecture are presented. The basic relationships between the architectures and charge dynamics of photoelectrode materials discussed here are expected to provide pertinent guidance and a reference for future advanced material design targeting improved light energy conversion systems.

Radiative and Non-Radiative Recombination Pathways in Mixed-Phase TiO2 Nanotubes for PEC Water-Splitting

Anatase and rutile mixed-phase TiO2 with an ideal ratio has been proven to significantly enhance photoelectrochemical (PEC) activity in water-splitting applications due to suppressing the electron–hole recombination. However, the mechanism of this improvement has not been satisfactory described yet. The PEC water oxidation (oxygen evolution) at the interface of TiO2 photoanode and electrolyte solution is determined by the fraction of the photogenerated holes that reach the solution and it is defined as the hole transfer efficiency. The surface and bulk recombination processes in semiconductor photoanodes majorly influence the hole transfer efficiency. In this work, we study the hole transfer process involved in mixed-phase TiO2 nanotube arrays/solution junction using intensity-modulated photocurrent and photovoltage spectroscopy (IMPS and IMVS); then, we correlate the obtained hole transfer rate constants to (photo)electrochemical impedance spectroscopy (PEIS) measurements. The results suggest that the enhanced performance of the TiO2 mixed-phase is due to the improved hole transfer rate across the TiO2/liquid interface as well as to the decrease in the surface trap recombination of the holes.

Regulating the Silicon/Hematite Microwire Photoanode by the Conformal Al2O3 Intermediate Layer for Water Splitting

Dual-absorber photoelectrodes have been proved to possess greater potential than the single-absorber systems in the applications of photoelectrochemical (PEC) cells (e.g., solar-driven water splitting); however, the mismatching of the energy bands and substantial carrier recombinations at the two absorber interfaces are normally subsistent. Here, we introduce an intermediate layer of conformal Al₂O₃ into the silicon/hematite (Si/α-Fe₂O₃) microwire photoanode for enriching the understanding of the interaction among the interlayer, inner absorber, and outer absorber. Our results show that the Si/Al₂O₃/α-Fe₂O₃ microwire photoanode with the thickness-optimized Al₂O₃ can lead to a substantial increase in the photocurrent from 0.83 to 2.08 mA/cm² at 1.23 V_RHE (under 1 sun irradiation) and an obvious decrease in the onset potential relative to the counterpart without Al₂O₃. By analyzing the PEC responses under various monochromatic lights, PEC impedance spectroscopy, and intensity-modulated photocurrent spectroscopy, we ascribe the improvements to the fact that the suitable-thickness Al₂O₃ can passivate the Si microwire surfaces and the bottom surfaces of the α-Fe₂O₃ film and give rise to Al doping into the post-synthesized α-Fe₂O₃. These essential causes promote the carrier separation in α-Fe₂O₃, diminish the photoanode surface recombination rate, and then increase the surface charge-transfer efficiency.

The Relation of Phase-Transition Effects and Thermal Stability of Planar Perovskite Solar Cells

A power conversion efficiency of over 20% has been achieved in CH3NH3PbI3-based perovskite solar cells (PSC), however, low thermal stability associated with the presence of a phase transition between tetragonal and cubic structures near room temperature is a major issue that must be overcome for future practical applications. Here, the influence of the phase transition on the thermal stability of PSCs is investigated in detail by comparing four kinds of perovskite films with different compositions of halogen atoms and organic components. Thermally stimulated current measurements reveal that a large number of carrier traps are generated in solar cells with the perovskite CH3NH3PbI3 as a light absorber after operation at 85 °C, which is higher than the phase-transition temperature. Electrochemical impedance spectroscopy measurements further exclude effects of a possible morphology change on the formation of carrier traps. These carrier traps are detrimental to the thermal stability. The thermogravimetric analysis does not show a decomposition for any of the materials in the temperature range relevant for operation. The perovskite alloys do not have this phase transition, resulting in effectively suppressed formation of carrier traps. PSCs with improved thermal stability under the standard thermal cycling test are demonstrated.

Detecting and identifying reversible changes in perovskite solar cells by electrochemical impedance spectroscopy

Reversible changes in perovskite solar cells (PSC) are detected and analysed by electrochemical impedance spectroscopy (EIS).

Metal Oxide/(oxy)hydroxide Overlayers as Hole Collectors and Oxygen-Evolution Catalysts on Water-Splitting Photoanodes

Solar water splitting provides a mechanism to convert and store solar energy in the form of stable chemical bonds. Water-splitting systems often include semiconductor photoanodes, such as n-Fe₂O₃ and n-BiVO₄, which use photogenerated holes to oxidize water. These photoanodes often exhibit improved performance when coated with metal-oxide/(oxy)hydroxide overlayers that are catalytic for the water-oxidation reaction. The mechanism for this improvement, however, remains a controversial topic. This is, in part, due to a lack of experimental techniques that are able to directly track the flow of photogenerated holes in such multicomponent systems. In this Perspective, we illustrate how this issue can be addressed by using a second working electrode to make direct current/voltage measurements on the catalytic overlayer during operation in a photoelectrochemical cell. We discuss examples where the second working electrode is a thin metallic film deposited on the catalyst layer, as well as where it is the tip of a conducting atomic-force-microscopy probe. In applying these techniques to multiple semiconductors (Fe₂O₃, BiVO₄, Si) paired with various metal-(oxy)hydroxide overlayers (e.g., Ni(Fe)O _xH _y and CoO _xH _y), we found in all cases investigated that the overlayers collect photogenerated holes from the semiconductor, charging to potentials sufficient to drive water oxidation. The overlayers studied thus form charge-separating heterojunctions with the semiconductor as well as serve as water-oxidation catalysts.