Redox Electrolyte Solution Research Articles

Universal electrolyte for DSSC οperation under both simulated solar and indoor fluorescent lighting

A universal I−/I3− redox electrolyte solution for well performing DSSCs under both indoor and outdoor lighting conditions, is presented and characterized. Electrolyte attributes, including color and mass transport properties, were tailored and a good synergy between triiodide content, organic solvent mixture and additives in the electrolyte has been achieved, allowing DSSC adaptability under different lighting types and illumination intensities. The optical and redox properties were evaluated and the electrolyte was tested in DSSCs under different illumination conditions: simulated solar light (from 0.1 to 1 sun) and indoor light (from 100 to 1000 lux). Under indoor lighting, the electrolyte leads to power conversion efficiency (PCE) values close to the ones for the best performing iodide-based devices reported in the literature. Moreover, significantly superior performance, along with a constantly high fill factor were achieved under 100 mW/cm2 simulated solar light, paving the way for the development of low cost DSSCs with good ''pan-illumination'' performance, independently of the lighting conditions.

Double-network thermocells with extraordinary toughness and boosted power density for continuous heat harvesting

Double-network thermocells with extraordinary toughness and boosted power density for continuous heat harvesting

Intrinsic Electrocatalytic Activity of Gold Nanoparticles Measured by Single Entity Electrochemistry

AbstractUnderstanding the real nature of the intrinsic electrocatalytic performance of the counter electrode is crucial for realizing high conversion efficiency of solar energy in photoelectric devices. However, owing to the average effects, a thorough insight into such electrocatalytic process remains challenging in ensemble measurements. Here, we realized the real‐time detection of electrocatalytic reduction of a single Au nanoparticle (NP) in sulfide/polysulfide redox electrolyte solution using single entity collision electrochemistry. Owing to the high‐sensitivity electrochemical measurement, size‐dependent electrocatalytic behaviors of polysulfide reduction were observed for single AuNP collision. Our approach allowed the electrocatalytic properties of AuNPs for polysulfide electrolyte reduction at the single NPs level to be quantified, and the turnover number as a function of particle size were further estimated. Our result has further pointed out the importance of NP size control in the electrocatalysis study.

Modeling Standing Waves in a Thin Layer Sonoelectrochemical System

Recent experiments in thin layer (cell height 1 cm) sonoelectrochemistry demonstrate that sonication increases the rates of electrocatalysis. Heterogeneous electron transfer rates increase substantially upon ultrasound irradiation. A model is developed using Partial Differential Equations and the Galerkin Method (Finite Element Method) to understand catalytic effects observed when sound waves are applied in a thin layer electrochemical cell. The cell consists of a hollow Teflon cylinder with a transducer. A three electrode system is used with the electrodes spaced equally apart and parallel to the cylinder base. The cylinder is filled with a redox probe and electrolyte solution such that a meniscus forms at the top. The transducer propagates sound waves through the solution (i.e., sonication). A comparison of cyclic voltammograms before, during, and after sonication reveals enhancement effects from the sound waves. For redox probes with slow kinetics, sonication simultaneously increases currents and appears to shift the reaction to more reversible. Data indicate that a combination of the height and shape of the meniscus play a vital role in this enhancement effect. Numerical analysis is used in conjunction with partial differential equations to develop the model. It takes into account the shape and height of the meniscus as well as the pressure, density, elasticity, mass, and surface tension of the fluid near the meniscus. The model then reveals the standing waves of the system. Standing waves occur in a stationary medium when waves are traveling in opposite directions and cause interference with one another. In general, if this scenario occurs with waves of approximately the same amplitude there is no net change in energy, another indication that the height of the meniscus is relevant. To help quantify this phenomenon, four equations are used. Among them are two common equations used in fluid mechanics, a conservation of momentum equation (Compressible Euler Equation), a conservation of mass equation, a boundary motion equation, and an equation that describes the surface tension on the meniscus. The boundary motion equation includes a term that describes the position of the surface. The Galerkin Method is used to convert our continuous problem to a discrete one via the weak formulation with a proper basis. A reduction of the wave equation to an eigenvalue problem gives way to our standing waves. The wave interference taking place in the cell can be seen the provided picture. This model is important for prediction of behavior of the sound waves before and after they contact the top boundary and the role of the meniscus convexity. The model proves that the sound waves contact the meniscus and are then reflected back down into solution. Lastly, the model provides all possible standing waves in the cell. This will lead to advances in understanding the experimental system for applications in electrocatalysis. Figure 1

Achievement of over 1.4 V photovoltage in a dye-sensitized solar cell by the application of a silyl-anchor coumarin dye

A dye-sensitized solar cell (DSSC) fabricated by using a novel silyl-anchor coumarin dye with alkyl-chain substitutes, a Br3−/Br− redox electrolyte solution containing water, and a Mg2+-doped anatase-TiO2 electrode with twofold surface modification by MgO and Al2O3 exhibited an open-circuit photovoltage over 1.4 V, demonstrating the possibility of DSSCs as practical photovoltaic devices.

Modeling Standing Waves in a Thin Layer Sonoelectrochemical System

Recent experiments in thin layer sonoelectrochemistry demonstrate that sonication increases the rates of electrocatalysis. Heterogeneous electron transfer rates increase substantially upon ultrasound irradiation. A model is developed using Partial Differential Equations and the Finite Element Method to understand catalytic effects observed when sound waves are applied in a thin layer electrochemical cell. The cell consists of a hollow Teflon cylinder with a transducer as the base, seen in Figure 1. A three electrode system is used with the electrodes spaced equally apart and parallel to the cylinder base. The cylinder is filled with a redox probe and electrolyte solution such that a meniscus forms at the top. The transducer propagates sound waves through the solution (i.e., sonication). A comparison of cyclic voltammograms before, during, and after sonication reveals enhancement effects from the sound waves. For redox probes with slow kinetics, sonication simultaneously increases currents and appears to shift the reaction to more reversible. Data indicate that a combination of the height and shape of the meniscus play a vital role in this enhancement effect. Partial differential equations is used in conjunction with numerical analysis to develop and solve the model, respectively. The model takes into account the shape and height of the meniscus as well as the pressure, density, elasticity, mass, and surface tension of the fluid near the meniscus. Then the Finite Element Method reveals the standing waves of the system. Standing waves occur in a stationary medium when waves are traveling in opposite directions and cause interference with one another. In general, if this scenario occurs with waves of approximately the same amplitude there is no net change in energy, another indication that the height of the meniscus is relevant. To help quantify this phenomenon, four equations are used, seen in Figure 2. Following this basis functions in the finite element space are used to convert our continuous problem to a discrete one via the weak formulation. A reduction of the wave equation to an eigenvalue problem gives way to our standing waves in the form of an eigenfunction expansion. The wave interference taking place in the cell can be seen in Figure 1. This model is important for prediction of behavior of the sound waves before and after they contact the top boundary and the role of the meniscus convexity. This will lead to advances in understanding the experimental system for applications in electrocatalysis. Figure 1

Modeling Standing Waves in a Thin Layer Sonoelectrochemical System

Recent experiments in thin layer (cell height 1 cm) sonoelectrochemistry demonstrate that sonication increases the rates of electrocatalysis. Heterogeneous electron transfer rates increase substantially upon ultrasound irradiation1. A model is developed using Partial Differential Equations and the Galerkin Method (Finite Element Method) to understand catalytic effects observed when sound waves are applied in a thin layer electrochemical cell. The cell consists of a hollow Teflon cylinder with a transducer as the base seen in Figure 1. A three electrode system is used with the electrodes spaced equally apart and parallel to the cylinder base. The cylinder is filled with a redox probe and electrolyte solution such that a meniscus forms at the top. The transducer propagates sound waves through the solution (i.e., sonication). A comparison of cyclic voltammograms before, during, and after sonication reveals enhancement effects from the sound waves. For redox probes with slow kinetics, sonication simultaneously increases currents and appears to shift the reaction to more reversible. Data indicate that a combination of the height and shape of the meniscus play a vital role in this enhancement effect. Numerical analysis is used in conjunction with partial differential equations to develop the model. It takes into account the shape and height of the meniscus as well as the pressure, density, elasticity, mass, and surface tension of the fluid near the meniscus. The model then reveals the standing waves of the system. Standing waves occur in a stationary medium when waves are traveling in opposite directions and cause interference with one another. In general, if this scenario occurs with waves of approximately the same amplitude there is no net change in energy, another indication that the height of the meniscus is relevant. To help quantify this phenomenon, four equations are used. Among them are two common equations used in fluid mechanics, the Compressible Euler Equation (Equation 1), and a conservation of mass equation (Equation 2): u here represents velocity, ρ is the density, and P is the pressure. The other two equations are a boundary motion equation (Equation 3) and an equation that describes the surface tension on the meniscus (Equation 4). Ψ is a separate equation that describes the position of the surface, β is a surface tension coefficient, and H is the mean curvature of the meniscus. The Galerkin Method is used to convert our continuous problem to a discrete one via the weak formulation with a proper basis. A reduction of the wave equation to an eigenvalue problem gives way to our standing waves. The wave interference taking place in the cell can be seen below in Figure 1. This model is important for prediction of behavior of the sound waves before and after they contact the top boundary and the role of the meniscus convexity. This will lead to advances in understanding the experimental system for applications in electrocatalysis. Figure 1

Self-limiting electropolymerization of ultrathin, pinhole-free poly(phenylene oxide) films on carbon nanosheets

Poly(phenylene oxide) (PPO) is an electronically insulating, solid polymer film which can be prepared via electropolymerization of phenol on electronically conductive surfaces. The self-limiting nature of this reaction allows the formation of pinhole-free films with nanometer thickness on high aspect ratio micro- and nano-structured substrates. In this work, we investigated the electrodeposition of PPO on carbon nanosheets (CNS). The highly corrugated CNS morphology makes a perfect model substrate to study the conformal electrodeposition of PPO films for advanced energy storage devices based on three-dimensional large-area surfaces. Uniform PPO films of about 8nm were successfully coated over CNS layers with area enhancements of over 200×. The films uniformly covered the CNS petals and were electronically insulating as confirmed from cyclic voltammetry tests in redox electrolyte solutions. These thin conformal PPO films are of interest for blockage of electronic leakage in supercapacitor applications or even as ion conducting buffer layers in lithium ion batteries.

Optically transparent FTO-free cathode for dye-sensitized solar cells.

The woven fabric containing electrochemically platinized tungsten wire is an affordable flexible cathode for liquid-junction dye-sensitized solar cells with the I3(-)/I(-) redox mediator and electrolyte solution consisting of ionic liquids and propionitrile. The fabric-based electrode outperforms the thermally platinized FTO in serial ohmic resistance and charge-transfer resistance for triiodide reduction, and it offers comparable or better optical transparency in the visible and particularly in the near-IR spectral region. The electrode exhibits good stability during electrochemical loading and storage at open circuit. The dye-sensitized solar cells with a C101-sensitized titania photoanode and either Pt-W/PEN or Pt-FTO cathodes show a comparable performance.

Sonoelectrochemistry: Modeling Acoustic Energy in a Thin Layer System

Recent experiments in thin layer sonoelectrochemistry demonstrate that sonication increases the rates of electrocatalysis. Heterogeneous electron transfer rates increase substantially upon ultrasound irradiation1. A model is developed to understand catalytic effects observed when sound waves are applied in a thin layer electrochemical cell.The cell consists of a hollow Teflon cylinder with a transducer as the base seen in Figure 1. A three electrode system is used with the electrodes spaced equally apart and parallel to the cylinder base. The cylinder is filled with a redox probe and electrolyte solution such that a meniscus forms at the top. The transducer propagates sound waves through the solution (i.e., sonication). A comparison of cyclic voltammograms before, during, and after sonication reveals enhancement effects from the sound waves. For redox probes with slow kinetics, sonication simultaneously increases currents and appears to shift the reaction to more reversible. Data indicate that a combination of the height and shape of the meniscus play a vital role in this enhancement effect.Numerical analysis is used in conjunction with partial differential equations to develop the model. It takes into account the shape and height of the meniscus as well as the pressure, density, elasticity, mass, and surface tension of the fluid near the meniscus. The model then reveals the standing waves of the system. Standing waves occur in a stationary medium when waves are traveling in opposite directions and cause interference with one another. In general, if this scenario occurs with waves of approximately the same amplitude there is no net change in energy, another indication that the height of the meniscus is relevant. This model is important for prediction of behavior of the sound waves before and after they contact the top boundary and the role of the meniscus convexity. This will lead to advances in understanding the experimental system for applications in electrocatalysis. Duda, C. G. Thin Layer Sonoelectrochemistry. Ph.D. thesis, University of Iowa. 2012. 2. Leddy, J.; Duda, C.G.; and Leddy, W.J. provisional patent application, 2013.

Influence of dye chemistry and electrolyte solution on interfacial processes at nanostructured ZnO in dye-sensitized solar cells

Abstract A dye-sensitized solar cell is a complex system where an optimal interplay between three components, the semiconductor, dye and electrolyte, should be accomplished for good performance and stability. In this work, two dyes with different interaction with nanostructured ZnO are tested for application in ZnO-based dye-sensitized solar cells. The first dye is a perylene derivative, which adsorbs to the ZnO surface via an anhydride group. The short circuit photocurrent of the assembled solar cell increases with immersion time in the dye solution until it saturates and remains constant, indicating that the ZnO–dye interface is stable in the dye solution. However, it was found that the electrode bleaches at longer times in the redox electrolyte solution under illumination, both for organic solvent and ionic liquid-based systems, indicating a relatively weak bonding of the dye to the ZnO surface. The second dye is an indoline derivative, which has two carboxylic acid groups that can bond to the ZnO surface. In this case, the short circuit photocurrent is found to decrease strongly after a certain immersion time, indicating that the ZnO surface is attacked by the dye. In this case, however, for cells prepared with short immersion times, there is less bleaching after a prolonged time in the electrolyte solution indicating a stronger ZnO–dye bond. For both cases, ionic liquid electrolytes are observed to both exacerbate the bleaching process and to accelerate the recombination kinetics as compared to organic electrolyte solutions, thus illustrating the interaction of the ionic liquid with the semiconductor surface. The results highlight the importance of balanced dye interaction with the semiconductor surface, and the influence that the electrolyte solution can have.

Schottky junction/ohmic contact behavior of a nanoporous TiO2thin film photoanode in contact with redox electrolyte solutions

The nature and photoelectrochemical reactivity of nanoporous semiconductor electrodes have attracted a great deal of attention. Nanostructured materials have promising capabilities applicable for the construction of various photonic and electronic devices. In this paper, a mesoporous TiO2 thin film photoanode was soaked in an aqueous methanol solution using an O2-reducing Pt-based cathode in contact with atmospheric air on the back side. It was shown from distinct photocurrents in the cyclic voltammogram (CV) that the nanosurface of the mesoporous n-TiO2 film forms a Schottky junction with water containing a strong electron donor such as methanol. Formation of a Schottky junction (liquid junction) was also proved by Mott–Schottky plots at the mesoporous TiO2 thin film photoanode, and the thickness of the space charge layer was estimated to be very thin, i.e., only 3.1 nm at −0.1 V vs Ag/AgCl. On the other hand, the presence of [Fe(CN)6]4− and the absence of methanol brought about ohmic contact behavior on the TiO2 film and exhibited reversible redox waves in the dark due to the [Fe(CN)6]4−/3− couple. Further studies showed that multiple Schottky junctions/ohmic contact behavior inducing simultaneously both photocurrent and overlapped reversible redox waves was found in the CV of a nanoporous TiO2 photoanode soaked in an aqueous redox electrolyte solution containing methanol and [Fe(CN)6]4−. That is, the TiO2 nanosurface responds to [Fe(CN)6]4− to give ohmic redox waves overlapped simultaneously with photocurrents due to the Schottky junction. Additionally, a second step photocurrent generation was observed in the presence of both MeOH and [Fe(CN)6]4− around the redox potential of the iron complex. It was suggested that the iron complex forms a second Schottky junction for which the flat band potential (Efb) lies near the redox potential of the iron complex.

Nitrogen-Containing Heterocycles’ Interaction with Ru Dye in Dye-Sensitized Solar Cells

Interactions of N-containing heterocycles such as pyrazole, imidazole, pyridine, pyrimidine, pyrazine, and 4-t-butylpyridine (TBP) with the neutral and oxidized Ru(II)-polypyridyl dye (N719) have been studied using density functional theory (DFT). All of the heterocycles formed two hydrogen bonds with N719; one via the N atom with a lone pair of heterocycles and the H atom of the carboxyl group for the dye ligand, and the other via the H atom adjacent to the N atom forming the other hydrogen bond and the O atom with a lone pair of carboxyl groups. Additionally, the heterocycles were examined as an additive in an I−/I3− redox electrolyte solution of a dye-sensitized TiO2 solar cell. As the positive atomic charge on the S atom of the NCS ligand in the oxidized state decreased via interaction with the heterocycles, it became difficult for the oxidized dye to interact with I−, which prevented the dye from regenerating and led to a decrease in the short-circuit photocurrent density (Jsc) of the dye-sensitized solar cell. The DFT results also indicated that the heterocycles moved the energy of both HOMO and LUMO levels up, as well as raised the Fermi level of TiO2. This produced lower electron injection efficiency from photoexcited dye into the TiO2 conduction band and thereby decreased the Jsc value.

TiO2 Band Shift by Nitrogen-Containing Heterocycles in Dye-Sensitized Solar Cells: a Periodic Density Functional Theory Study

A density functional theory (DFT) method (periodic DMol3) with full geometry optimization was used to study the adsorption of nitrogen-containing heterocycles such as pyrazole, imidazole, 1,2,4-triazole, pyridine, pyrimidine, pyrazine, and 4-t-butylpyridine (TBP) on TiO2 anatase (101), (100), and (001) surfaces. All structures displayed a negative shift in the TiO2 Fermi level upon adsorption of N-containing heterocycles. Additionally, the heterocycles were examined as an additive in an I-/I3- redox electrolyte solution of dye-sensitized TiO2 solar cell. The DFT results indicated that the negative shift of TiO2 Fermi level was due to the adsorbate dipole moment component normal to the TiO2 surface plane, and corresponded to the enhanced open-circuit photovoltage (Voc) and the reduced short-circuit photocurrent density (Jsc) in a dye-sensitized solar cell.

Modeling RedOx-based magnetohydrodynamics in three-dimensional microfluidic channels

RedOx-based magnetohydrodynamic (MHD) flows in three-dimensional microfluidic channels are investigated theoretically with a coupled mathematical model consisting of the Nernst-Planck equations for the concentrations of ionic species, the local electroneutrality condition for the electric potential, and the Navier-Stokes equations for the flow field. A potential difference is externally applied across two planar electrodes positioned along the opposing walls of a microchannel that is filled with a dilute RedOx electrolyte solution, and a Faradaic current transmitted through the solution results. The entire device is positioned under a magnetic field which can be provided by either a permanent magnet or an electromagnet. The interaction between the current density and the magnetic field induces Lorentz forces, which can be used to pump and/or stir fluids for microfluidic applications. The induced currents and flow rates in three-dimensional (3D) planar channels obtained from the full 3D model are compared with the experimental data obtained from the literature and those obtained from our previous two-dimensional mathematical model. A closed form approximation for the average velocity (flow rate) in 3D planar microchannels is derived and validated by comparing its predictions with the results obtained from the full 3D model and the experimental data obtained from the literature. The closed form approximation can be used to optimize the dimensions of the channel and to determine the magnitudes and polarities of the prescribed currents in MHD networks so as to achieve the desired flow patterns and flow rates.

All-plastic dye-sensitized solar cell using a polysaccharide film containing excess redox electrolyte solution

All-plastic dye-sensitized solar cells (DSSCs) using TiO2 paste (SP-210), indium tin oxide-coated polyethylene naphthalate (PEN/ITO) film and a Pt-coated PEN/ITO film counter electrode, were fabricated with 1.0 wt% κ-carrageenan solid electrolyte containing the redox electrolytes (I-/I3-) solution. The cell performance of the all-plastic cell using 1.0 wt% κ-carrageenan solid electrolyte was investigated in comparison with liquid type one, showing that the values of Voc (open circuit voltage) and ff (fill factor) were the same, but Jsc (short circuit current) and η (conversion efficiency) were slightly larger for the liquid type electrolyte cell than the solid type one. A higher conversion efficiency was exhibited by UV-irradiation before heat treatment of the PEN/ITO/TiO2(SP-210) film than that after heat treatment. The effect of heat treatment conditions was investigated according to a modified hydrothermal treatment of the PEN/ITO/TiO2(SP-210) film, showing that the ff values become high (>0.7) by hydrothermal treatment. The electrochemical impedance spectroscopy (EIS) has shown the important comparison of each charge transport processes between the all-plastic and the conventional cells.

Electrochemical reaction with RedOx electrolyte in toroidal conduits in the presence of natural convection

Transport processes in an upright, concentric, annular, electrochemical reactor filled with RedOx electrolyte solution are studied experimentally and theoretically. The electrodes form the two vertical surfaces of the reactor. The theoretical calculations consist of the solution of the Navier–Stokes and the Nernst–Planck equations accounting for species’ diffusion, migration, convection, and electrochemical reactions on the electrodes’ surfaces as a function of the difference in electrodes’ potentials and the average concentration of the electrolyte. Since the convection is driven by density gradients, the momentum and mass transport equations are coupled. The current transmitted through the electrolyte is significantly enhanced by natural convection. The current is measured as a function of the difference in the electrodes’ potentials. To obtain the reaction rate constants, an inverse problem is solved and the reaction rate constants are determined by minimizing the discrepancy between theoretical predictions and experimental observations. As an example, we study the reversible electrochemical reaction Fe +++ + e − = Fe ++ on platinum electrodes.

Electrochemical properties of undoped hydrogen terminated CVD diamond

The electronic properties of hydrogen terminated intrinsic single crystalline CVD diamond in redox–electrolytic solutions are characterized by cyclic voltammetry experiments and pH-sensitive measurements of ion-sensitive field effect transistor (ISFET) structures. The data show insulator-metal transitions of diamond if immersed into redox–electrolyte solutions with chemical potentials (unoccupied states) below the valence-band maximum. Cyclic voltammetry current peaks are electron exchange rate limited due to the small density of holes in a few Ångström thick conducting layer at the surface of diamond. H-terminated diamond is sensitive to pH of electrolytes, following the Nernst prediction with about − 58 mV/pH. Due to Coulomb repulsion of H +-ions from the H-terminated diamond surface, the enlarged tunneling distance works like a gate insulator and is indicated as reason for the confinement of holes in ISFET structures.

Redox-couple interactions with undoped single crystalline CVD diamond

Intrinsic CVD diamond is well known as insulator, however if the surface is hydrogen terminated, a surface conduction can be detected if exposed to air and covered by an adsorbate layer. In this paper we show that hydrogen terminated intrinsic single crystalline CVD diamond undergoes an insulator metal transition if immersed into redox electrolyte solutions with chemical potentials below the valence-band maximum. We have applied cyclic voltammetry experiments using different redox couples to characterize this phenomenon. The experiments reveal a large chemical window (> 4.6 V) of undoped diamond. We detect the same formal potentials as reported in the literature for H-terminated boron doped polycrystalline diamond. The peaks show comparable symmetries but are shifted towards higher and lower potentials, and are significantly broadened. We attribute this to the rate limited electron transfer at the interface. The results are discussed based on the transfer doping model.

Insulator−Metal Transition of Intrinsic Diamond

Insulator-metal transitions are translations characterized by a sudden change in electron transport properties from localized to itinerant behavior of electrons. In most cases, it is detected in strongly disordered semiconductors as a function of temperature with statistically distributed donors and/or acceptors. Here, we show that reversible insulator-metal transitions take place in H-terminated intrinsic (undoped) single crystalline diamond, if immersed into redox-electrolyte solutions at room temperature. The change is generated by valence-band electrons of diamond, which tunnel into the redox-electrolyte, where they occupy empty electronic sites. We applied cyclic voltammetry experiments to characterize the insulator-metal transition in combination with different redox couples. The experiments show that transfer doping accounts for the experimentally detected phenomena, and that intrinsic diamond is a promising candidate for chemical sensing in a variety of applications.