Current-potential Characteristics Research Articles

Reversible Redox Probes to Determine Band-Edge Locations and Dopant Concentrations of Nano-TiO2 Thin-Films: Settling the Mott-Schottky Conundrum.

Knowing the exact location of the semiconductor band-edges is key for mechanistic insights into their use for water and CO2 photo/electrocatalysis. In this regard, a reliable strategy for nano-semiconductors did not exist yet. We demonstrate the use of reversible redox probes on nano-semiconductor electrodes to determine their band-edge locations in aqueous solutions. Rectifying current-potential (i-U) characteristics with the high work function Fe(CN)63-/Fe(CN)64- redox couple yielded the exact flatband potential at various pH whereas the reversible i-U characteristics with the low work function Ru(NH3)63+/Ru(NH3)62+ redox couple provided the conduction band-edge location and dopant concentration for a 30nm thin-film n-TiO2. The methodology can be extended to other nano-semiconductors and serves as an alternative to and goes beyond the capabilities of the Mott-Schottky procedure for bulk semiconductor electrodes.

Simple Approach for Reconciling Cyclic Voltammetry with Hydrogen Adsorption Energy for Hydrogen Evolution Exchange Current.

Cyclic voltammetry (CV) is a standard technique to analyze the current-potential characteristics of the hydrogen evolution reaction (HER). Herein, we develop a computational quantum-scaled CV model for the HER building on the Butler-Volmer relation for a one-step, one-charge transfer process. Owing to a universal and absolute rate constant verified by fitting to experimental cyclic voltammograms of elemental metals, we show that the model quantifies the exchange current─the main analytical descriptor for HER activity─solely using the hydrogen adsorption free energy obtained from density functional theory calculations. Furthermore, the model resolves controversies over analytical studies for HER kinetics.

Reconciling the Volcano Trend with the Butler-Volmer Model for the Hydrogen Evolution Reaction.

The volcano trend has been widely utilized to forecast new optimum catalysts in computational chemistry while the Butler-Volmer relationship is the norm to explain current-potential characteristics from cyclic voltammetry in analytical chemistry. Herein, we develop an electrochemical model for hydrogen evolution reaction exchange currents that reconciles device-level chemistry, atomic-level volcano trend, and the Butler-Volmer relation. We show that the model is a function of the easy-to-compute hydrogen adsorption energy invariably obtained from first-principles atomic simulations. In addition, the model reproduces with high fidelity the experimental exchange currents for elemental metal catalysts over 15 orders of magnitude and is consistent with the recently proposed analytical model based on a data-driven approach. Our findings based on fundamental electrochemistry principles are general and can be applied to other reactions including CO2 reduction, metal oxidation, and lithium (de)intercalation reactions.

Origin of the Electrical Barrier in Electrolessly Deposited Platinum Nanoparticles on p-Si Surfaces

Pt deposited by either sputtering or electron-beam (e-beam) evaporation on p-Si forms an ohmic contact, with zero photovoltage and very little photogenerated charge-carrier collection. However, electro- or electroless deposition of Pt onto p-Si produces a rectifying junction that generates a photovoltage of ∼300 mV under simulated 1 sun illumination. To explain these differences, we have characterized junctions formed by electroless or e-beam deposition of Pt onto H-terminated or oxide-coated p-Si substrates using impedance spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electrochemical current density vs potential (J–E) characteristics. When Pt was deposited electrolessly, XPS and TEM measurements revealed a thin interfacial SiOx layer of 1–6 nm thickness under the Pt overlayer. Moreover, open-circuit potential measurements under illumination on electrolessly deposited Pt on a p-Si electrode showed that the junction was a function of the Nernstian potential of the contacting electrolyte solution. Creating an analogous junction by e-beam deposition of Pt required oxidation of the Si surface prior to Pt deposition, followed by etching in HF to remove oxide on the exposed Si surface. The resulting structure has both an interfacial SiOx layer under the Pt and a H-terminated Si surface on the bare areas. Additionally, under a H2 atmosphere, Pt can adsorb hydrogen that can diffuse to the SiOx/Pt interface and produce a dipole layer. This information allowed formulation of a model for the charge transfer across p-Si/SiOx/Pt interfaces. When in contact with a solution having a kinetically facile redox couple, the current is carried across the Si/electrolyte interface, and the electrode has the properties of a semiconductor/liquid junction. In contrast, when in contact with a solution with a large kinetic barrier to interfacial charge transfer, such as the hydrogen evolution reaction, the current instead passes predominantly through the SiOx layer to the Pt and then reacts with protons in the solution. In this situation, the junction to the semiconductor is buried and occurs at the Si/SiOx/Pt interface. The Si/SiOx/Pt contact displays an increase in barrier height due to the hydrogen-induced dipoles. Consequently, the barrier height for an electrode made by electroless deposition of Pt onto Si is determined by the pathway that the electrons traverse to reach the solution.

Enhancement of Characteristics of Nitrogen-Doped Graphene Composite Materials Prepared by Ball Milling of Graphite with Melamine: Effect of Milling Speed and Material Ratios

Nitrogen-doped graphene has been prepared using the ball milling method that is known to be eco-friendly, scalable and economic. The parameters studied in the synthesis were the mass ratio of starting materials (graphite and melamine) and speed of the ball milling. To determine its structure and properties, the nitrogen-doped graphene was characterized using Field Emission Scanning Electron Microscopy with Energy Dispersive X-Ray (FESEM-EDX), RAMAN Spectroscopy and X-Ray Diffraction (XRD). Based on FESEM-EDX analysis, the doped composite exhibited nitrogen content of ~3.5%. The nitrogen-doped graphene was examined as a replacement for platinum catalysts in fuel cells. Different composite catalysts were evaluated using a Rotating Disk Electrode (RDE) to test the Oxidation Reduction Reaction (ORR) performance. Based on ORR performance comparison, the composite with highest performance was then used to fabricate a Membrane Electrode Assembly (MEA). Testing on MEA performance was conducted on a Fuel Cell Station, where Open Circuit Voltage (VOC) of 0.14 V was obtained. The results indicate that the ball milling method may produce an efficient nitrogen-doped graphene MEA electrode from graphite and melamine only. Compared with a platinum counterpart, the new composite material electrode showed soundly high current-potential characteristics and fuel conversion efficiency.

Efficient inverted perovskite solar cells with CuSeCN as the hole transport material

We demonstrate herein a mixed solvent containing organic amines and water for the solution processing of CuSeCN and apply it as the hole-transport material for stable and efficient inverted perovskite solar cells for the first time. The obtained CuSeCN film is characterized with X-ray diffraction spectrum, X-ray photoelectron spectroscopy, scanning electron microscope, and current-potential characteristics. High-quality perovskite film can be formed on the CuSeCN film obtained from the spin-coating of CuSeCN solution in a mixture solvent of H2O, ethanolamine (ETA), ethylenediamine (EDA), diethanolamine (DTA) with volume ratio of 2:6:1:1. The CuSeCN-based perovskite solar cell yields power conversion efficiency of 15.61% at forward scan and 15.97% at reverse scan with negligible hysteresis and good stability. As compared to the typically used thioether-based solvents for the solution processing of CuSCN, the mixture solvent proposed in this work is less volatile and much cheaper, which is critical for practical applications of perovskite solar cells.

Orbital boost characteristics of spacecraft by electrodynamic tethers with consideration of electric-magnetic-dynamic energy coupling

This paper investigates spacecraft orbital boost maneuver by a fully insulated electrodynamic tether using a coupled multiphysics dynamic model that considers electric-magnetic-dynamic energy coupling. The current-potential characteristics of the electrodynamic tether is described by a simplified analytical model, where revised Parker-Murphy model is employed to evaluate electric current generation at plasma contactors together with libration dynamics of tether. The 13th-order Earth's magnetic field model is used to account influence of inhomogeneity of Earth's magnetic field on orbital parameters of electrodynamic tether system, especially in inclined orbits. Parametric analysis has been conducted for orbital boost maneuver from 400 km to 1,200 km at four different orbital inclinations. The results show that (i) the orbit of electrodynamic tether system will change from circular to elliptical orbits, especially in inclined orbits, (ii) the voltage of onboard power supply at the anode affect electric current generation, (iii) the coupled multiphysics model is necessary to characterize the interaction of electrodynamic tether with the surrounding space environment and the controllability of electric current in electrodynamic tether, (iv) the motion of orbital boost maneuver by electrodynamic tether is stable, which is different from the deorbit by electrodynamic tether.

Investigation of the effect of alkyl substitution on DSSC efficiency of Ru(II) complexes bearing tridentate benzimidazole ligands

In this study, tridentate 2,6-bis(5,6-dimethyl-1H-benzimidazol-2-yl)pyridine ligand (L1) and eight derivatives (L2–9) were prepared by substitution with alkyl chains and Ru(II) complexes of these ligands (C1–9) were synthesized. The synthesized ligands and their Ru(II) complexes were characterized by FTIR, UV–vis, 1H-NMR, and ESI/MS spectroscopic methods. The absorption and the emission spectra of the complexes were examined, HOMO–LUMO energy levels of the complexes were determined by cyclic voltammetry and their use as photosensitizer in dye-sensitized solar cells (DSSCs) was investigated. The effects of alkyl chain substitution on absorption in the visible region and on the current–potential characteristics of the complexes are discussed. The effects of this modification on the important parameters for DSSCs such as absorption, current density were examined and power conversion efficiency values of the Ru(II) complexes are reported.

Electrostatic Investigation of 4-Dicyanomethylene-2,6-dimethyl-4H-pyran (DDP) Dye with Amide Derivatives in Water.

Cyclic voltammetry (CV) studies of 4-dicyanomethylene-2,6-4H-pyran (DDP) dye with alkyl-substituted amides were carried out in an aqueous solution. Formamide and substituted amide interaction with DDP dye were characterized by fluorescence spectral techniques in an aqueous solution, but the electrochemical nature and the interaction at the interface region between dye–amide remains largely unexplored. The introduction of formamide to DDP dye exhibits an increase in the peak current accompanied with potential values gradually shifting more toward a less positive region. A large variation in the current–potential characteristics is observed in alkyl-substituted amides. The cyclic voltammograms of alkyl amides are found to be entirely different from each other. The role of alkyl substitution in the amide molecular framework influences the reduction potential of the dye in an aqueous medium. The mode of interaction of the dye with alkyl-substituted amides is predominantly due to the electrostatic behavior, even though hydrogen-bonding interactions coexist throughout the aqueous phase. The binding constant parameter (K), free-energy changes (ΔG), and the variation in the potential behavior of the dye in the presence of formamide and alkyl amides authenticate that the nature of interaction operates by both hydrogen-bonding mode and electrostatic interactions. Electrochemical techniques when coupled with fluorescence methods provide an efficient method of determining the interaction at the bulk and the interface regions of a water-soluble dye with nonfluorophoric solutes.

Synthesis and DSSC Applications of Ru(II) Complexes Bearing Benzimidazole Type Ligands

In this study, the synthesis of 7 Ru(II) polypyridine complexes (C1–7) and their characterization by FT-IR, UV–Vis, 1H-NMR and ESI–MS spectroscopic methods have been reported. The use of the complexes as photosensitizers in dye-sensitized solar cells (DSSCs) has been investigated. The photovoltaic characteristics of the fabricated DSSC devices were examined under 1.5 G solar simulation and TiO2 was used as a semiconductor. Absorption and emission spectra of the complexes were examined, cyclic voltammograms of the Ru complexes were performed and HOMO–LUMO energy levels were calculated. Thus, electrochemical properties of the complexes and the energy diagrams, which shows the charge transfer characteristics of the complexes were studied. The alterations in the current–potential characteristics of the Ru sensitizers by modifications in ligands have been investigated. The effects of the substitutions on ligands are evaluated and power conversion efficiency value up to half of the reference sensitizer was obtained by C6 complex.

Potential-current characteristics in SiC/ZrB2 composite ceramics

ZrB2/SiC composite ceramics were fabricated to improve the electrical conductive properties of SiC matrix. The debinding and sintering temperatures were determined by computation of Gibbs free energy. As a result, all the samples have the relative density above 99%, and have excellent mechanical and electrical properties. The effects of ZrB2 content on the microstructure, mechanical and electrical properties were systematically studied. With increasing ZrB2 content, as-prepared composites show great improvement in their mechanical properties. Importantly, the introduction of ZrB2 weakened varistor nonlinear characteristic of composite and reduced its resistivity. The reason is the evolution of grain boundary in conductive paths. The sharp decrease of resistivity indicates the formation of percolation paths. The percolation threshold at 1 mA cm−2 obtained via percolation model is 10.7963 vol% (19.7098 wt%) ZrB2. This value is much less than conventional composites, because the percolation path originates from grain boundary breakdown other than continuous conductor chains.

Mechanism and Kinetics of HER and OER on NiFe LDH Films in an Alkaline Electrolyte

The mechanism and kinetics of hydrogen evolution (HER) and oxygen evolution (OER) reactions on nickel iron layered double hydroxide (NiFe LDH) in a basic electrolyte are investigated. The deposited film reported an overpotential of 247 and 245 mV at 10 mA/cm2 toward the HER and OER, respectively. A least squares procedure was performed to fit a theoretical current density model with experimental linear sweep voltammetry (LSV) results, and the chemical reaction rate constants for the OER and HER steps were identified. Electrochemical impedance spectroscopy (EIS) measurements were taken at different potentials, and the resulting kinetic model demonstrates a good agreement between theoretically calculated faradaic resistance and experimental EIS results. The HER results indicated the Heyrovsky step as rate controlling, with a dependence of reaction mechanism on potential. At low potential, the mechanism begins with a Volmer step, followed by parallel Tafel and Heyrovsky steps. At higher potential, the mechanism becomes consecutive combination of the Volmer and Heyrovsky steps. The OER data point to the formation of the adsorbed peroxide as rate controlling. The HER and OER kinetic data were combined into a model capable of predicting the electrolysis cell current-potential characteristics, which can be used for process design and optimization.

Effect of substrate on the structural, optical and electrical properties of SnS thin films grown by thermal evaporation method

Tin sulfide thin films were deposited on different substrates by thermal evaporation technique. The effect of substrate on the growth of SnS thin films has been studied thoroughly. X-Ray Diffractrometry pattern shows that the deposited films are showing (111) orientation having primitive type orthorhombic structure and those deposited on glass/ZnO:Al has better crystallinity and less dislocation density than the others. Small cracks are visible on the surface and that may be due to the higher dislocation density or strain exerted on the surface. Different types of substrate also shifts the absorption edge of the films deposited on different substrates. Electrical characteristics obtained from Hall Effect measurements show the films deposited on glass/ZnO:Al has better mobility and less resistivity than the films deposited on other substrates. Electric potential-current characteristics shows ohmic and rectifying behavior as the substrate is changed.

Investigation of Direct Water Photoelectrolysis Process Using III-N Structures

GaN, GaN/AlGaN and GaN/InGaN-based structures were used to study water photoelectrolysis in KOH-based electrolyte, measurement of current-potential characteristics, investigation of electrode corrosion and for hydrogen generation. The corrosion process of p-n AlGaN/GaN structure starts in the p-layers, spreads via vertical channels associated with threading defects, and continues laterally along the n-layers, where large local hollows and voids were observed. The H2 production rate of 0.3-0.6 ml/cm2×h was measured for n-GaN structure.

Tailoring Photoelectrochemical Performance and Stability of Cu(In,Ga)Se2 Photocathode via TiO2-Coupled Buffer Layers

We report on the photoelectrochemical (PEC) performance and stability of Cu(In,Ga)Se2 (CIGS)-based photocathodes for photocatalytic hydrogen evolution from water. Various functional overlayers, such as CdS, TiO2, ZnxSnyOz, and a combination of the aforementioned, were applied on the CIGS to improve the performance and stability. We identified that the insertion of TiO2 overlayer on p-CIGS/n-buffer layers significantly improves the PEC performance. A multilayered photocathode consisting of CIGS/CdS/TiO2/Pt exhibited the best current-potential characteristics among the tested photocathodes, which demonstrates a power-saved efficiency of 2.63%. However, repeated linear sweep voltammetry resulted in degradation of performance. In this regard, we focused on the PEC durability issues through in-depth chemical characterization that revealed the degradation was attributed to atomic redistribution of elements constituting the photocathode, namely, in-diffusion of Pt catalysts, out-diffusion of elements from the CIGS, and removal of the metal-oxide layers; the best-performing CIGS/CdS/TiO2/Pt photocathode retained its initial performance until the TiO2 overlayer was removed. It was also found that the durability of CIGS photocathodes with a TiO2-coated metal-oxide buffer layer such as ZnxSnyOz was better than those with a TiO2-coated CdS, and the degradation mechanism was different, suggesting that the stability of a CIGS-based photocathode can be improved by careful design of the structure.

Organic photovoltaic annealing process analysis using impedance spectroscopy

Organic solar cells based on P3HT:PCBM without annealing process, with annealing process before and after cathode deposition have been investigated in dark and under light by using high magnitude and phase resolution impedance spectrometer. The impedance data in dark can be interpreted as the properties and positions of layers and interfaces of the sample to nanoscale range. Therefore, the fabrication process and device nanoscale layer and interface quality can be monitored. Under light conditions, the impedance data obtained were fitted into equivalent circuit and electronic parameters such as diffusion time of electrons, τd and effective carrier lifetime, τn were calculated. Correlation of the nanoscale layers and interface parameters to the dynamic carrier performance parameters were established. Applying the high magnitude and phase resolution impedance method to understand the properties of organic thin-film solar cells in both static and dynamic status will give useful insights not only on the device performance improvement in terms of process, layer morphology and interfaces quality; but also deeper understanding of how the device kinetic characteristics at the interface states induced correlate with the improvement of current-potential characteristics under device operation conditions.

Electric-field response based experimental investigation of unsaturated soil slope seepage

Rainfall is one of the important factors causing the failure of slope, such as the occurrence of transverse cracks and localized slumps. The process of rainfall seepage was studied with an indoor soil slope model based on the Network Parallel Electrical Method. The responses of geoelectric-field parameters were analyzed to infer the evolution process of rainfall seepage path. The variations of geoelectric-field parameters also contribute to our understanding of the behavior of groundwater seepage. The results show that the seepage velocity and seepage position of groundwater can be obtained according to the exciting current and primary field potential response characteristics of seepage field. The primary field potential, exciting current, spontaneous potential and apparent resistivity are sensitive to the water flow. When the position of the seepage surface reaches a certain electrode, the spontaneous potential, primary field potential and exciting current rapidly increase, while the apparent resistivity decreases gradually. The result of apparent resistivity can reflect the variation of the water content in the 3D structural soil slope and the position of infiltration surface. The results of study can provide the theoretical basis for studying the behavior of moisture flow in soil slope under rainfall condition.

Influence of Dopant Type and Orientation of Silicon Anodes on Performance, Efficiency and Corrosion of Silicon–Air Cells with EMIm(HF)2.3F Electrolyte

Intermediate term discharge experiments were performed for Si–air full cells using As-, Sb- and B-doped Si-wafer anodes, with 〈100〉 and 〈111〉 orientations for each type. Discharge characteristics were analyzed in the range of 0.05 to 0.5 mA/cm2 during 20 h runs, corrosion rates were determined via the mass-change method and surface morphologies after discharge were observed by laser scanning microscopy and atomic force microscopy. Corresponding to these experiments, potentiodynamic polarization curves were recorded and analyzed with respect to current-potential characteristics and corrosion rates. Both, discharge and potentiodynamic experiments, confirmed that the most pronounced influence of potentials – and thus on performance – results from the dopant type. Most important, the corrosion rates calculated from the potentiodynamic experiments severely underestimate the fraction of anode material consumed in reactions that do not contribute to the conversion of anode mass to electrical energy. With respect to materials selection, the estimates of performance from intermediate term discharge and polarization experiments lead to the same conclusions, favoring 〈100〉 and 〈111〉 As-doped Si-wafer anodes. However, the losses in the 〈111〉 As-doped Si-anodes are by 20% lower, so considering the mass conversion efficiency this type of anode is most suitable for application in non-aqueous Si–air batteries.

Power Generation by Reverse Electrodialysis in a Microfluidic Device with a Nafion Ion-Selective Membrane.

An energy conversion microchip consisting of two circular microchambers and a Nafion-filled microchannel is fabricated using standard micro-electro-mechanical systems (MEMS) techniques. When the chambers are filled with KCl solutions with different concentrations, the Nafion microchannel acts as a cation-selective membrane and results in the generation of electrical power through a reverse electrodialysis (RED) process. The current-potential characteristics of the Nafion membrane are investigated for devices with various microchannel lengths and electrolyte concentration ratios. It is shown that for a given voltage, the current and generated power increase with a reducing channel length due to a lower resistance. In addition, a maximum power density of 755 mW/m2 is obtained given an electrolyte concentration ratio of 2000:1 (unit is mM). The optimal device efficiency is found to be 36% given a channel length of 1 mm and a concentration ratio of 1000:1 (mM). Finally, no enhancement of the short circuit current is observed at higher concentration ratios.

(Invited) Hybrid Organic/Inorganic Bismuth-Based Materials for Solution-Processed Thin-Film Photovoltaics

The lead–halide-based hybrid organic–inorganic perovskite material, methylammonium lead iodide (MAPbI3), has recently garnered increased attention among researchers worldwide as a promising photovoltaic material. As of 2015, perovskite photovoltaic devices have achieved laboratory efficiencies in excess of 20%. Many challenges regarding the stability and toxicity of the MAPbI3 material, however, remain at the forefront of current research. Due to the toxicity of lead and a purely fundamental interest in dicationic organic groups in place of monocationic MA+, less toxic dicationic bismuth–halide materials are the object of my research. The bismuth–iodide networks of the materials under investigation were hypothesized to form chains that extend in one dimension, whereas in MAPbI3 the lead–iodide network forms a lattice in three dimensions. If a small dicationic organic group is incorporated into the structure, however, it was hypothesized that the bismuth–iodide chains are in close enough proximity to each other to provide for long-range charge transport in three dimensions, similar to that observed in MAPbI3. A library of dicationic bismuth–halide materials has been investigated to determine optimal dicationic groups based on film quality, optical band gap, and current density versus potential (J–E) characteristics. XRD data of dicationic bismuth–halide thin films have suggested larger average crystal grain size than MAPbI3 thin films and SEM images of dicationic bismuth–halide thin films have indicated greater surface coverage than MAPbI3 thin films. UV–Vis absorption spectra of dicationic bismuth–halide thin films have revealed onsets of light absorption in the range of 565 nm to 620 nm, which correspond to optical band gaps between 2.2 and 2.0 eV. J–E measurements have indicated that dicationic bismuth–halide photovoltaic devices exhibit a photovoltage of almost 400 mV (Fig. 1). Thermal stability tests suggest that dicationic bismuth–halide thin films are significantly more robust than MAPbI3 thin films. Large single crystals have been obtained for a dicationic bismuth–halide material, and single-crystal XRD data have indicated that the material formed binuclear clusters of Bi2I10 4-, with two dicationic organic groups and two H2O molecules associated with each binuclear cluster. To my knowledge, this represents the first report of a crystal structure of this material. Efficient photovoltaic devices featuring a dicationic bismuth–halide material may offer a more environmentally friendly, stable alternative to MAPbI3 photovoltaic devices that currently dominate emerging photovoltaic technology research. Figure 1