Semiconductor-electrolyte Interface Research Articles

Enhanced water splitting on (010) facet-exposed BiVO4 photoanode with improved carrier injection efficiency

Transition metal oxide semiconductors, noted for their stability and suitable bandgap, are promising photoanodes for water splitting. Surface engineering is critical to tackle issues like low carrier mobility and charge recombination, stemming from atomic arrangement and Fermi level differences. While exposing dominant crystal facets boosts photocatalytic capability, it can hinder carrier injection into the electrolyte. In this study, BiVO4 films with various facet exposures were synthesized and characterized using scanning electron microscopy and x-ray diffraction to confirm their morphology and crystalline structure. Mott–Schottky analysis was employed to investigate changes in the band structure near the semiconductor–electrolyte interface, revealing that high (010)-BiVO4 facet exposure enhances carrier separation but reduces injection efficiency. The results from photoconductive atomic force microscopy tests demonstrated that enhanced band bending at the semiconductor interface improves hole transfer. Coating the (010)-BiVO4 photoanode with MoS2 and an amorphous ZrO2 interlayer yielded a photocurrent density of 0.6 mA cm−2 at 1.2 V (vs RHE) under AM 1.5 G illumination, tripling the pristine photoanode's performance and nearly tripling water splitting efficiency. Mechanism revealing the improved photoelectrochemical performance is attributed to a greater band bending on the BiVO4 surface, enhancing hole injection dynamics. This work provides a feasible strategy for a deeper understanding of the intrinsic mechanisms of facet engineering and improving the activity of photoanodes.

Development of Semi-Empirical and Machine Learning Models for Photoelectrochemical Cells

We introduce a theoretical model for the photocurrent-voltage (I-V) characteristics designed to elucidate the interfacial phenomena in photoelectrochemical cells (PECs). This model investigates the sources of voltage losses and the distribution of photocurrent across the semiconductor–electrolyte interface (SEI). It calculates the whole exchange current parameter to derive cell polarization data at the SEI and visualizes the potential drop across n-type cells. The I-V model’s simulation outcomes are utilized to distinguish between the impacts of bulk recombination and space charge region (SCR) recombination within semiconductor cells. Furthermore, we develop an advanced deep neural network model to analyze the electron–hole transfer dynamics using the I-V characteristic curve. The model’s robustness is evaluated and validated with real-time experimental data, demonstrating a high degree of concordance with observed results.

Ion sensors based on organic semiconductors acting as quasi-reference electrodes

Thin-film devices that transduce the chemical activity of ions into electronic signals are essential components in various applications, including healthcare diagnostics and environmental monitoring. Combinations of organic semiconductors (OSCs) and ion-selective materials have been explored for developing solution-processable ion sensors. However, the necessity of reference electrodes (REs) and operational stability in ion-permeable OSCs have posed questions regarding whether reliable measurements with thin-film components are attainable with OSCs. Herein, we report electric double-layer transistors (EDLTs) with OSCs in single-crystal forms for ion sensing. Our EDLTs demonstrated high operational stability, with a one-to-one relationship between the source electrode potential and device resistance, and served as quasi-REs (qRE). When our EDLT is served as qRE, its drift was as small as 0.5 mV/h and comparable to that of commonly employed REs. In our system, the semiconductor-electrolyte interface is self-passivated by the alkyl chains of OSCs in single-crystal structures, with the two-dimensional transport layer appearing unaltered upon gating. EDLT arrays with ion-selective and nonselective liquid junctions enable ion concentration sensing without a conventional RE. These findings provide opportunities to develop thin-film devices based on OSCs for easy integration and reliable measurements.

Promoting effect of interfacial hole accumulation on photoelectrochemical water oxidation in BiVO4 and Mo-doped BiVO4

Hole transfer at the semiconductor-electrolyte interface is a key elementary process in (photo)electrochemical (PEC) water oxidation. However, up to now, a detailed understanding of the hole transfer and the influence of surface hole density on PEC water oxidation kinetics is lacking. In this work, we propose a model for the first time in which the surface accumulated hole density in BiVO4 and Mo-doped BiVO4 samples during water oxidation can be acquired via employing illumination-dependent Mott-Schottky measurements. Based on this model, some results are demonstrated as below: (1) Although the surface hole density increases when increasing light intensity and applied potential, the hole transfer rate remains linearly proportional to surface hole density on a log-log scale. (2) Both water oxidation on BiVO4 and Mo-doped BiVO4 follow first-order reaction kinetics at low surface hole densities, which is in good agreement with literature. (3) We find that water oxidation active sites in both BiVO4 and Mo-doped BiVO4 are very likely to be Bi5+, which are produced by photoexcited or/and electro-induced surface holes, rather than VOx species or Mo6+ due to their insufficient redox potential for water oxidation. (4) Introduction of Mo doping brings about higher OER activity of BiVO4, as it suppresses the recombination rate of surface holes and increases formation of Bi5+. This surface hole model offers a general approach for the quantification of surface hole density in the field of semiconductor photoelectrocatalysis.

Effect of Electrolyte Composition over CO2 Reduction

The rapid utilization of fossil fuels escorted by an excess of CO2 emissions has led to global energy and environmental crisis. Therefore, the necessity for a clean, renewable and sustainable source of energy is a growing concern for the present and future society. In this regard, economically viable CO2 reduction would be a critical turnover in research, as it has the potential to fulfil a substantial need for clean energy. However, even though many efforts have been done in this field, there is still room for improvements concerning efficiency, material stability, and catalytic enhancement in regard of kinetics and selectivity. Herein, we provide the experimental proof for enhancement of the CO2 reduction efficiency and selectivity from the SEI (semiconductor-electrolyte interface) side through the use of carbonates, borates, sulphates and alkali cations as the electrolyte as well as an overview of the latest developments on Cu2O based PEC CO2 reduction for solar fuel production. Cu2O is a low-cost semiconductor and one of the most promising candidates for PEC CO2RR. However, its stability and performance is still unsatisfactory, thus the CO2 reduction products vary from one investigated system to another, such as: CH3OH, CO, HCOOH, CH3COOH, and CH3CH2OH. Moreover, the instability of Cu2O causes it, to be rarely used for the routine CO2 reduction reaction. In this paper, we use a very facile electrodeposition method, which offers a high level of reproducibility and the possibility of using a new electrode in each experiment, in order to focus our efforts on following the phenomena occurring in the double layer during the photocatalytic run. In this way, we were able to correlate the final CO2RR performance with a reorganization of the cations and anions near the photocatalyst surface. It is shown in the literature that factors such as: carbonate concentration, local pH, the presence of alkali metal cations, the geometry of the anionic group, CO2 solubility, conductivity, as well as pH changes along with the number of H+ in the electrolyte, play a significant role in regulating the partial CO2RR current. In this paper, we would like to shed new light on the influence of the electrolyte composition, cation-anion interaction and local reaction environment around the catalyst on the performance of CO2RR. We found out that the specific interaction between the alkali cation and the anionic group of particular geometry contributes to the formation of a kind of a “rigid layer” within the double diffusion, close to the photocatalyst surface layer, which accounts more for an apparent CO2RR current than the decrease in the finite Warburg element, which is a central key parameter for the diffusion coefficient values. The effectiveness of the strength of the cation-anion interaction in the formation of the “rigid layer” around the photocatalyst surface is found to increase the PEC CO2RR performance. Elucidating this mechanism provides useful information for creating further experimental design and the new pathways for addressing highly efficient PEC CO2RR systems. The authors have noticed a significant knowledge gap in the existing literature, as no prior publications have delved into the concurrent and combined impact of both cations and anions on PEC CO2RR. In this pioneering publication, we present the inaugural investigation of this this intricate phenomenon.

Electrochemical impedance spectroscopy analysis of dye-sensitized solar cells composed of electrospun composite photoanodes: A comparative study of natural and synthetic sensitizers

In this study, a transmission line model is applied to the electrochemical impedance spectroscopy (EIS) data of the fabricated dye-sensitized solar cells (DSSCs) to evaluate the charge transfer mechanism through the cells. Natural dye from black plum (Syzygium cumini) fruit was used as a cell sensitizer (SC-DSSC) and compared its photovoltaic and electron transport capabilities to those of a cell using a synthetic sensitizer (N719-DSSC). TiO2/ZnO electrospun composite nanofibers were used as the semiconductor layer of the photoanode to enhance electron transfer. The EIS analysis revealed the role of electron resistances through shant, interfaces, and electrolyte solution by measuring the electron transfer kinetic parameters of each element. Based on the results, the SC-DSSC and N719-DSSC are appropriate photovoltaic cells because their ratios of effective electron diffusion length to photoanode thickness are 12.5 and 2.8, respectively. The EIS analysis showed that the electrospun composite nanofiber coated on the photoanode reduces the semiconductor layer's electrical resistance to the cell's total resistance. The extracted natural dye also boosted electron lifetime to 3.68 ms and diffusion coefficient to 54.3×10−6 m2/s while minimizing back-electron recombination at the semiconductor-electrolyte interface. Moreover, the semiconductor-electrolyte interface resistance is over 85% of the overall resistance for both DSSCs and controls electron transport through the cells, which is due to the dye-semiconductor binding intensity. Based on the photovoltaic data, the SC-DSSC cell efficiency was lower than N719-DSSC which is attributed to its higher electron transfer rate-controlling element. Thus, enhancing dye-semiconductor interactions will decrease the rate-controlling impedance and enhance cell performance.

Effect of molybdenum doping Zn-Co spinel oxide microspheres for efficient electrocatalytic hydrogen production in alkaline media and study supported by DFT

The rational design of efficient transition metal-based electrocatalysts for the hydrogen evolution reaction (HER) is critical for the water-splitting process. Industrial water-alkali electrolysis requires large current densities at low overpotentials, which are always limited by the intrinsic activity. Here, ZnCo2−xMoxO4 (x ≤ 0.10) spinel oxide microsphere electrocatalysts were synthesized for the process in alkaline media. Mo doping in the ZnCo2O4 crystal structure enhanced the active sites, which were responsible for the enhancement in the HER process. The ZnCo2−xMoxO4 (x = 0.06) electrocatalyst exhibited exceptional HER activity, as evidenced by an overpotential of 195 mV, Tafel slope of 81.4 mVs−1 and high stability for 40 h with 1000 cycles linear sweep voltammetry. The surface and electrochemical characterization revealed that 6% Mo-doping exhibits improved the HER activity due to a significantly higher electrochemical surface area and accelerated charge transfer kinetics at the semiconductor electrolyte interface. Density functional theory (DFT) for exploring and explaining the effect of the Mo dopants on the HER activity elucidated that hydrogen and water molecules are adsorbed on the surface of the Mo-doped ZnCo2O4 slab structures. The results show that Mo dopants enhance the chemical activity, due to water adsorption, and HER activity for enhanced electrocatalytic processes. This work provides a new insight into low-metal-cost materials for efficient and durable HER electrocatalysts and successfully showcases the catalyst model for a detailed mechanistic insight into the HER process.

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.

An Optical-Electronic-Catalytic Coupled Multi-Physical Simulation for Sunlight-Driven CO2 Reduction Device Based on Light Absorbers Patterned with Island Electrocatalysts

Photoelectrochemical (PEC) reduction of CO2 to value-added chemicals is a promising approach to convert solar energy but it is often suffered from sluggish kinetics and photocorrosion at the semiconductor-electrolyte interface.1, 2 In general, there are two requirements for a good performing solar-driven CO2 reduction: i) effective catalysts for a fast surface redox reaction and ii) an effective semiconductor/catalyst interface for rapid charge transfer across interfaces.3 The deposit of a continuous electrocatalyst layer on the photoabsorber surface can reduce kinetic overpotentials for CO evolution reaction (COER) and can enhance the photoelectrode stability. However, the optical obscuration significantly attenuates the magnitude of the incident illumination. Reducing the coverage (f c) of the catalyst on the photoabsorber surface can reduce the optical obscuration but worsen the reaction kinetics. Moreover, the pinch-off effect occurs as the size of the catalyst is small enough to provide a facile path for carriers to transport across interfaces. The PEC CO2 reduction with the utilization of the pinch-off effects could potentially be advantageous to balance the kinetics and the optical absorption. Understanding the trade-offs between the reduction in photocurrent density due to optical obscuration and the decrease in kinetic overpotentials due to improved active area is important in optimizing PEC CO2 reduction devices.To elucidate the effects of the optical performance, interfacial carrier transport, and electrochemical activity for the PEC CO2 reduction, three possible interfacial configurations in PEC were considered in this study: i) Bare p-Si photocathode case. The bare p-Si photocathode immersed in aqueous electrolyte and CO2R occurs at the semiconductor-electrolyte (SC/E) interface; ii) Layered catalyst case. The continuous layer catalyst is deposited on the surface of p-Si photocathode and forms a semiconductor-metal (SC/M) interface; iii) Patterned catalyst case. Producing a patterned catalyst with a low geometric coverage fraction on the surface of p-Si photocathode. The Schottky junctions with a low barrier height formed by the SC/M are surrounded by the Schottky junctions with a high barrier height formed by the SC/E. We developed a multiphysics model based on the finite element method, including electromagnetic wave propagation, charge transport in bulk semiconductor, species transport in the electrolyte, and charge-transfer kinetics of surface redox reactions.The optical performance and electrochemical performance of CO2R reduction were compared in the three cases. Under the bare p-Si case, the saturation photocurrent densities (J sa) and the onset potential (V on) were -13.40 mA/cm2 and -0.80 V vs. RHE at -1 mA/cm2, respectively. Under the layered catalyst case, the J sa and the V on were -0.84 mA/cm2 and -0.50 V vs. RHE at -0.10 mA/cm2, respectively. Under the patterned catalysts case (f c is 0.1 in this case), the J sa is -12.30 mA/cm2 and V on is -0.69 V vs. RHE at -1 mA/cm2. The magnitude of J sa is influenced by the extent of light obscuration and the magnitude of V on is influenced by the photoelectrode activity and the pinch-off effects. The V on for the patterned case is higher than that of the bare p-Si case due to the improved electrode activity combined with the pinch-off effects. In the bare p-Si case and the patterned catalyst case, the CO2 mass transfer limits the CO partial current density (J CO) to further increase due to high J sa while CO2 mass transfer is not the limitation for COER at high overpotentials for the layered catalyst case due to significant light obscuration. The pinch-off effect also increased the applied voltage corresponding to the maximum J CO for the patterned case (-1.21 V vs. RHE) compared to that for the bare p-Si case (-1.35 V vs. RHE) due to the deceased transport resistance of electrons.The influence of different interfacial configurations, solar concentration ratio, different coverage of catalysts, and different sizes of catalysts for optical loss, pinch-off effect, CO2 mass transfer, current distribution at interfaces, and electrochemical performance are discussed in detail for the optimal design photoelectrode. This framework allows us to provide design guidelines for PEC CO2 reduction devices.

Integration of Charge-Carrier Dynamics and Electrochemical Modeling in a Unified Microkinetic Model for the Oxygen Evolution Reaction in Water Splitting

Water-splitting in a photo-electrochemical cell is a promising method for the sustainable production of hydrogen. However, these cells are not yet able to achieve the efficiencies required for commercial use. The reaction that generates oxygen at the photo-anode, the oxygen evolution reaction (OER), is commonly regarded as the performance limiting reaction. Modeling of the OER can provide several opportunities [1,2]: (1) to identify and verify the prominent reaction mechanism by comparison to experimental electrochemical data, (2) to determine variables that are difficult to obtain from experiments, such as the surface coverage of the intermediate OER reaction species at the semiconductor-electrolyte interface, and (3) to rapidly explore new semiconductor materials for increased oxygen evolution.Previously in our group, a time-dependent non-linear microkinetic model of the OER has been developed. [3,4] This model consists of two parts: (1) A microkinetic model of the reaction kinetics at the semiconductor-electrolyte interface. The model equations describe the time-evolution of the current density and the intermediate species under applied potential. (2) A simplified description of the charge-carrier transport dynamics at the surface of an iron oxide (hematite) semiconductor. This model is able to simulate the steady-state behavior, such as current-voltage curves. However, the dynamic behavior and the simulation of electrochemical impedance spectra are not possible with the current model.Therefore, in this work a more detailed description of the charge-carrier dynamics is developed and implemented. While in the earlier model [3, 4] the two charge-carriers, electrons and holes, were simulated only at the surface of the semiconductor, in this work, also processes in the bulk of the semiconductor are included, through addition of a space-dependency of the charge-carriers in the semiconductor. The advantage is that we can fully include drift-diffusion effects, charge transfer at the interface, electron-hole pair generation, and the recombination processes at the interface and in the bulk of the semiconductor. The resulting partial differential equations are connected to the state-space system of the reaction kinetics through semi-discretization in the spatial dimension.In addition, we will present a global sensitivity analysis to characterize the influence of the model input parameters, such as free energies and reaction rates, on the output, i.e. current density. [5] This analysis, using Sobol's method, presents the influence of each parameter on the uncertainty of the model output, the current density, at different potentials. [6] As a result, this analysis can indicate exactly which parameters to research to decrease the output uncertainty most effectively.This work presents for the first time an integration of the spatial charge-carrier dynamics and the electrochemical model of the OER into one combined model. As such, we can investigate the limiting processes in both components simultaneously, while the sensitivity analysis allows us to probe these limiting processes under a wide range of conditions, such as varying illumination levels and temperatures. The combined model, in conjunction with global sensitivity analysis, will be used to accurately determine the surface coverages of the OER, which can be validated by comparison with experimental data, e.g., from ATR-FTIR measurements. [7] Additionally, by implementing different materials, in addition to iron oxide, we aim to identify the most promising catalyst for OER.[1] X.Q. Zhang, A. Bieberle-Hütter. ChemSusChem 2016 9, 1223-1242.[2] B. Samanta, Á. Morales-García, F. Illas, N. Goga, J.A. Anta, S. Calero, A. Bieberle-Hütter, F. Libisch, A.B. Muñoz-García, M. Pavone, M.C. Toroker. Chem. Soc. Rev. 2022 51, 3794-3818.[3] K. George, M. van Berkel, X. Zhang, R. Sinha, A. Bieberle-Hütter. J. Phys. Chem. C 2019 123 (15), 9981–9992.[4] K. George, T. Khachatrjan, M. van Berkel, V. Sinha, A. Bieberle-Hütter. ACS Catalysis 2020 10 (24), 14649–14660.[5] I.M. Sobol. Math. Model. Comput. Exp. 1993 1 (4), 407-414[6] B.F.H. van den Boorn, M. van Berkel, A. Bieberle-Hütter. Adv. Theory Simul. 2022, 2200615.[7] A. Bieberle-Hütter, A. Bronneberg, K. George, M.C.M. van de Sanden. J. Phys. D: Appl. Phys. 2020 54, 133001.Figure 1: Framework of the microkinetic model with two components: (1) the electrochemical model with the OER reaction mechanism, and (2) the charge-carrier dynamics with drift-diffusion, charge transfer and recombination processes. The results of the global sensitivity analysis of the electrochemical model are represented by the sensitivity indices (SOER), which express the influence of each of the input parameters on the output, i.e., the current density, as a percentage. Figure 1

Electrochemical reduction induced OVD in BiOBr nanosheets for sensitive photoelectrochemical detection of doxycycline

The regulation of semiconductor-electrolyte interface (SEI) is a promising way to optimize the photoelectrochemical performance of the semiconductor. Herein BiOBr nanosheets were electrochemically reduced to form oxygen vacancy defects (OVD) on the surface, thus increasing the carrier concentration and narrowing the space charge region at SEI. The best photoelectric response was obtained when the reduction potential was −0.2 V vs. Ag/AgCl. The OVD modified BiOBr structure was confirmed by different techniques. The electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) plots evidenced the carrier concentration increase and the internal electric field enhancement. The time resolved photoluminescence (TRPL) measurements reflected the life time of photogenerated carriers was prolonged obviously. The optimized sample was chose for photoelectrochemical (PEC) detection of doxycycline (DOC). The fabricated PEC sensor could sensitively distinguish DOC solutions with concentrations varying from 0.1 μM to 100 μM. The photocurrent exhibited a linear relationship against the logarithm of concentration. The sensitivity was 8.21nA per logarithm and the limit of detection (LOD) was 0.026 μM at 3σ/S. The PEC sensor also displayed good anti-interference capacity and excellent stability. Our work shed light on the control of PEC performance of BiOX (X = Cl, Br, I) through introducing electrochemically induced OVD.

Interplay between Collective and Localized Effects of Point Defects on Photoelectrochemical Performance of TiO2 Photoanodes for Oxygen Evolution

AbstractAmong the various photoanode materials investigated for photoelectrochemical water splitting cells, TiO2 stands out due to its abundance, stability, and favorable valence band edge for water oxidation. In this study, the importance of introducing and combining oxygen and titanium vacancy point defects in anatase TiO2 photoanodes to improve their performance is unveiled, achieving a photocurrent density of 0.73 (±0.015) mA cm−2 at +1.23 VRHE under 100 mW cm−2 of simulated sunlight or 26.4 mA cm−2 at +1.23 VRHE under 100 mW cm−2 of 365 nm light. The characterization by X‐ray photoelectron spectroscopy, surface photovoltage, and electron paramagnetic resonance demonstrates that these oxygen and titanium vacancies can have both collective and localized positive effects on the material, leading to a narrowing of the bandgap, an increase in donor density, and an increase in hydroxyl groups on the surface of TiO2. These result in enhanced light absorption, conductivity, and photovoltage, as well as a more negative flat‐band potential and increase in hole flux to the semiconductor–electrolyte interface. These findings provide valuable insights into the role of point defects in modulating the properties of TiO2 and have important implications for the development of high‐performance TiO2‐based devices.

Photodegradation of CuBi2 O4 Films Evidenced by Fast Formation of Metallic Bi using Operando Surface-sensitive X-ray Scattering.

CuBi2 O4 has recently emerged as a promising photocathode for photo-electrochemical (PEC) water splitting. However, its fast degradation under operation currently poses a limit to its application. Here, we report a novel method to study operando the semiconductor-electrolyte interface during PEC operation by surface-sensitive high-energy X-ray scattering. We find that a fast decrease in the generated photocurrents correlates directly with the formation of a metallic Bi phase. We further show that the slower formation of metallic Cu, as well as the dissolution of the electrode in contact with the electrolyte, further affect the CuBi2 O4 activity and morphology. Our study provides a comprehensive picture of the degradation mechanisms affecting CuBi2 O4 electrodes under operation and poses the methodological basis to investigate the photocorrosion processes affecting a wide range of PEC materials.

Bifunctional photothermal effect to promote band bending and water oxidation kinetics for improving photoelectrochemical water splitting

The photothermal effect is proved to favor the photoelectrochemical (PEC) water oxidation, but the impact of the photothermal effect on the band bending in the heterojunction has seldom been revealed. In this work, we constructed a Z schematic heterojunction by heteroatoms doping, the carbon quantum dots (CQDs) was anchored to trigger the local heat under the NIR irradiation. With the photothermal effect, the photocurrent density of (Ti, Zn)–Fe2O3@Ti–Fe2O3 reached 1.02 mA/cm2 (1.23 VRHE), 2.30 mA/cm2 (1.60 VRHE), which yielded an increased by 1.3 folds, 1.7 folds, respectively. Detailed studies showed that the photothermal effect would enlarge the contact potential difference in the heterojunction and band bending at the semiconductor-electrolyte interface (SEI). Furthermore, the oxygen evolution reaction (OER) kinetics was also accelerated, which was evidenced by the Tafel slope decreasing from 88 mV/decade to 78 mV/decade. Despite the photovoltage being reduced, the advantages of the photothermal effect still out weighted the disadvantages.

Modified Poly(Vinyl Alcohol) Based Polymer Electrolyte for Dye Sensitized Solar Cells (DSSCs)

AbstractDue to increasing energy demand day by day, it must be necessary to have an alternate source of energy beside natural resources. Sun is the ultimate source of solar energy. So, from this solar energy, various types of solar cells are fabricated easily which are available in markets but these are somewhat costly. So, there is requirement of a low‐cost alternative to costly crystalline solar cells, which is fulfilled by dye sensitized solar cell (DSSC). Due to many potential properties like good ionic conductivity, thin film forming ability and many more, polymer electrolyte plays a vital role in improving the efficiency of DSSC which cannot be obtained by using volatile liquid electrolytes which are used earlier. The reliance of DSSCs on liquid electrolytes imposes a limit and restriction on the manufacture of DSSC modules. Liquid electrolyte faces many problems like corrosion, leakage, and evaporation. When the photoanode comes into touch with the volatilized redox‐electrolytes solution, the charge distribution at the semiconductor–electrolyte interface changes, resulting in photo corrosion on the photoelectrode. Furthermore, electron recombination at semiconductor–liquid electrolytes interfaces is discovered to cause performance losses. The concept of ionic conductivity observed in polymer materials when complexed with salt is a significant step forward in the development of DSSC devices.In this review, the fabrication of DSSC using polymer electrolyte, specifically PVA based solid polymer electrolyte is discussed. Here solid polymer electrolytes are of special interest due to thin film forming ability and overall good performance. Due to inherent properties like cost‐effectiveness, better tensile strength, and high hydrophilicity, PVA is chosen as a novel applicant for host polymer. The recent advancement of DSSC using modified PVA polymer electrolyte by incorporation of nanosized fillers, plasticizers, polymer blend, by the formation of conducting polymer electrolyte are discussed in detail. The improvement in ionic conductivity and enhanced efficiency of DSSC by using above mentioned factors are also discussed.

Collective excitations of germinating pollen grains at critical points

In plants, the germinating pollen grain (pollen tube) is a single, elongated cell that serves as a conduit through which gametes pass. Pollen tubes display a fast growth rate, which under certain conditions, changes periodically and is accompanied by ion exchange with the growth environment. Therefore, pollen tubes exposed to various abiotic conditions may adversely affect or improve their reproductive performance and fertility. We examined a collection of live pollen tubes of tobacco (Nicotiana tabacum L.) and hyacinth (Hyacinthus orientalis L.) using a non-invasive semiconductor–electrolyte interface technique in the vicinity of the germination temperature or optimum growth temperature of a pollen grains/tubes. The time series measurements and numerical calculations, performed using information theory methods, represent signatures of collective dynamics in living cells at critical—molecularly encoded—germination and growth temperatures. This method (and soil pH data) can facilitate assisted plant migrations from one ecosystem to another as the Earth faces climate change.

Dynamic semiconductor-electrolyte interface for sustainable solar water splitting over 600 hours under neutral conditions.

Photoelectrochemical (PEC) water splitting that functions in pH-neutral electrolyte attracts increasing attention to energy demand sustainability. Here, we propose a strategy to in situ form a NiB layer by tuning the composition of the neutral electrolyte with the additions of nickel and borate species, which improves the PEC performance of the BiVO4 photoanode. The NiB/BiVO4 exhibits a photocurrent density of 6.0 mA cm-2 at 1.23 VRHE with an onset potential of 0.2 VRHE under 1 sun illumination. The photoanode displays a photostability of over 600hours in a neutral electrolyte. The additive of Ni2+ in the electrolyte, which efficiently inhibits the dissolution of NiB, can accelerate the photogenerated charge transfer and enhance the water oxidation kinetics. The borate species with B─O bonds act as a promoter of catalyst activity by accelerating proton-coupled electron transfer. The synergy effect of both species suppresses the surface charge recombination and inhibits the photocorrosion of BiVO4.

WO3/Ag2S type-II hierarchical heterojunction for improved charge carrier separation and photoelectrochemical water splitting performance

In the present work, WO3/Ag2S heterojunction was fabricated to achieve an improved photoelectrochemical (PEC) water splitting performance. To prepare the working electrodes, a two-step method was adopted which includes, a thin film of WO3 deposited using DC sputtering and well-separated Ag2S nanorods fabricated by glancing angle deposition. The PEC response was studied for bare WO3, Ag2S, and WO3/Ag2S heterojunction. The as-prepared WO3/Ag2S heterojunction samples revealed higher absorption as well as a higher photocurrent density of 2.40 mA/cm2 (at 1 V Ag/AgCl) as compared to bare WO3 thin film (0.34 mA/cm2). The enhancement in the photocurrent density of WO3/Ag2S electrodes could be ascribed to the formation of the type-II heterojunction between WO3 and Ag2S which effectively separates and transfers the charge carriers at the interface. In addition, increased trapping of light due to vertically tilted Ag2S nanorods structures results in effective absorption of light. Furthermore, electrochemical impedance spectra measurements showed that WO3/Ag2S samples have lower charge transfer resistance at the semiconductor electrolyte interface with high flat band potential. This work provides a deeper insight into the role of the interface formed between WO3 and Ag2S for the photoelectrochemical water splitting response.

A Newly Developed Inverted Rotating Disk Electrode (IRDE) Setup for the Investigation of Photoelectrochemical Reactions

A new setup for an inverted rotating disk electrode (IRDE) was developed for the investigation of photoassisted or photoelectrochemical processes involving mass transfer reactions, as in electrodeposition or etching processes. In this setup, rotation of the disk electrode is achieved by magnetic coupling between a driver magnet (i.e. a magnetic stirrer) and the follower magnet included in the sample holder. For photoelectrochemical experiments, the cell allows a homogeneous illumination and removal of gaseous reaction products, while still providing leak tightness. In this way, controlled flow conditions are provided with the possibility to simultaneously illuminate the sample surface from a distance as small as 20 mm. At the same time gases formed during the reaction can easily ascend through the electrolyte.The light source is placed on top of the cell and illumination is done through the electrolyte. A low liquid level (≥ 15 mm) allows a small distance between the sample and the light source and therefore a high light intensity on the sample surface. Illumination is done using a high-power LED. Due to the high power, thermal effects need to be taken into consideration and especially an effective cooling of the LED is essential.As a wide band gap semiconductor, silicon carbide (SiC) has gained large interest due to its high chemical and physical stability. Since photoassisted anodic etching of n-type 4H silicon carbide in potassium hydroxide solutions [1] requires both UV illumination and the removal of gaseous reaction products, this reaction is investigated in the newly developed IRDE setup. Possible applications for such a process are defect decoration [2], trench etching, and dicing.Photoassisted anodic etching of n-type SiC occurs in three steps: (1) hole generation in the material through illumination with light with an energy higher than the bandgap of the semiconductor (2) oxidation of silicon and carbon through the holes driven to the semiconductor-electrolyte interface by an external potential and (3) dissolution of the oxidation products in the electrolyte. In accordance with the three steps of the reaction, different processes can be limiting: For low light intensities (either through a large distance between the sample and the LED or through low driving currents through the LED) the number of photon-induced holes is limiting. At low electrolyte concentrations, the transport of hydroxyl ions and the dissolution of the oxidation products determine the etch rate. For high light intensities and high electrolyte concentrations, the external potential and thereby the oxidation reaction are limiting for the overall current flow. These different reaction regimes can be distinguished in cyclic voltammetry measurements in the newly developed IRDE cell.[1] D. H. van Dorp und J. J. Kelly, „Photoelectrochemistry of 4H–SiC in KOH solutions,“ Journal of Electroanalytical Chemistry, Bd. 599, Nr. 2, pp. 260-266, 2007.[2] J. L. Weyher, S. Lazar, J. Borysiuk und J. Pernot, „Defect-selective etching of SiC,“ physica status solidi, Bd. 202, Nr. 4, pp. 578-583, 2005. Figure 1

(Invited) Photoluminescence Monitored Digital Photocorrosion of GaAs/AlGaAs Nanoheterostructures

Conventional etching techniques do not have the ability to achieve precision of monolayer etching of crystalline solid materials. In recent years, atomic layer etching (ALE) has emerged as a method addressing this challenge based on the application of self-limiting sequential reactions. Nevertheless, monitoring of the ALE process, important for real time information about the location of the etching front, is not trivial and could be an expensive task. We have investigated the innovative process of digital photocorrosion (DIP) of GaAs/AlGaAs nanoheterostructures, which is based on photoexcitation driven decomposition of these materials.1 The process is carried out in a flow cell filled with etchants designed for selective removal of the photocorrosion products. For instance, water dilutes relatively easily As and most of the Ga oxides, but dilution of Ga2O3 or Al oxides and hydroxides requires dedicated etchants. Under optimized conditions, DIP allows etching of (001) GaAs/AlGaAs nanoheterostructures with average rates approaching 1 Å/cycle. The process could be monitored in situ by measuring, e.g., open circuit potential2 or photoluminescence3 effect. The sensitivity of these effects to the surface states of a semiconductor allows for convenient marking of the location of a GaAs/AlGaAs interface when crossed by the etching front.In this presentation, I will discuss mechanisms of DIP of GaAs/AlGaAs nanoheterostructures and the conditions for formation of stoichiometric surfaces. Examples of the DIP process applied for formation of an unusual density alkanethiol self-assembled monolayers and operation of optical devices for detecting electrically charged molecules immobilized in the vicinity of a semiconductor-electrolyte interface will also be discussed. Finally, I will also address the potential of this economically attractive invention for the research of other semiconductor materials.__________________1. S. Aithal, N. Liu and J. J. Dubowski, Journal of Physics D: Applied Physics 50 (3), 035106 (2017).2. S. Aithal and J. J. Dubowski, Appl Phys Lett 112 (15), 153102 (2018).3. M. R. Aziziyan, H. Sharma and J. J. Dubowski, Acs Appl Mater Inter 11 (19), 17968-17978 (2019).