Electrode Reaction Rate Research Articles

Crevice Corrosion Behaviors Between CFRP and Stainless Steel 316L for Automotive Applications

Carbon fiber reinforced plastics (CFRP) are promising lightweight materials for vehicle applications. 316L is one of the most widely used types of austenite stainless steels and applied in lots of automotive applications. The existence of crevices will result in galvanic corrosion and crevice corrosion when CFRPs and 316L are directly connected. A crevice former for the galvanic system was therefore designed and applied to evaluate the crevice corrosion behaviors and study the mechanism of galvanic crevice corrosion through several electrochemical techniques in this research. The results showed that the crevice corrosion of galvanic systems grew from crevice mouth to the inside crevice and could be divided into four steps, metastable pitting corrosion at the crevice mouth, initiating step of crevice corrosion, propagating step and ending step of crevice corrosion. Because of the influences of the galvanic system, electrode reaction rates were speeded up and the passivation region was shortened at the initiating stage of crevice corrosion. Corrosion rate was observed to be higher in the galvanic system than that in normal crevice systems.

Electroreduction of the bromate anion on a microlectrode in excess acid: solution of the inverse kinetic problem

Acidic aqueous solutions of bromic acid salts (bromates) are promising electrolytes for redox flow batteries due to their record high power capacity and the rate of electrode reactions proceeding in the autocatalytic regime. The paper gives a comparison of the results of mathematical modeling and experimental measurements of steady state currents of bromate anion electroreduction in sulfuric acid medium on microelectrodes of various radii. An algorithm of solving the inverse problem suitable for determination of the key transport and kinetic process parameters was proposed and tested.

Influence of Gd Doping on the Structure and Electrocatalytic Performance of TiO2 Nanotube/SnO2−Sb Nano‐coated Electrode

AbstractGd‐doped TiO2−NT/SnO2−Sb (NT=nanotube) electrodes were prepared by using a solvothermal synthesis approach with a nano‐sized catalyst coating. Phenol degradation and total organic carbon (TOC) removal followed pseudo‐first‐order kinetics in the experimental range. A maximum rate was achieved by using a Gd doping ratio of 2 % (molar ratio based on Gd/Sn), which was 56.5 % and 68 % higher than that of the control (Gd/0 %) for phenol degradation and TOC removal. The results from the UV scan of the electrolyte showed that introducing an appropriate amount of Gd could promote the electrochemical incineration process, and thus effectively degrade the chemical intermediates during phenol oxidation. In addition, the Gd/2 %‐doped electrode had the longest accelerated life time of 25 h, which was 25 % higher than that of the control. A suitable Gd doping ratio could diminish the SnO2 crystal size and increase the specific surface area, speeding up the electrode's reaction rate, thus promoting the oxygen evolution potential. A regular and compact morphology with a smallest particle size of 9.5 nm was obtained on the Gd/2 %‐doped electrode, which prompted a smaller charge‐transfer resistance and higher electrical double‐layer capacitance than that of the control. The results from X‐ray photoelectron spectroscopy and electron paramagnetic resonance suggested that a maximal of surface active sites (i. e. oxygen vacancy) was formed on the Gd/2 %‐doped electrode, which provided abundant positive charge for adsorbing more oxygen species (37.5 %) than the control (21.5 %), and greatly enhanced the formation of ·OH to attack the targeted pollutant.

Study on Kinetics of Anode Process of Chelated Iron Fuel Cell for Sulfur Recovery Unit of FLNG

Floating liquefied natural gas (FLNG) refers to water-based liquefied natural gas (LNG) operations employing technologies designed to enable the development of offshore natural gas resources. For environmental protection and safety, a sulfur recovery unit must be designed on the FLNG to purify hydrogen sulfide (H2S) contained in natural gas. The chelated-iron sulfur recovery process was a promising technique for the removal of H2S for waste minimization and resource recovery. In this process, an iron (Ⅲ) complex was used as an oxidant for dissolved H2S in the desulfurization catalysis into elemental sulfur. The catalyst was reduced to iron（Ⅱ）complex, and then underwent separation for removal of elemental sulfur. Using oxygen in the air as the oxidant, iron（Ⅱ）complex was oxidized to iron (Ⅲ) complex, thus realizing the recycling of the chelated-iron solution. However, in the traditional direct air regeneration process, the direct contact between air and chelated-iron catalyst can lead to degradation and subsequent failure of the chelated agent. Thus, the technology has encountered the problem of chelate degradation which made it difficult to ensure the reliable and economical operation. Furthermore, due to limitation of the FLNG space, the liquid chelated iron sulfur recovery process must also meet the requirements of a small footprint and light weight. A chelated-iron fuel-cell process was designed to achieve rapid regeneration of chelated iron. Since the fuel cell separated the direct chemical reaction between the chelated iron catalyst and air, it can effectively avoid the failure of chelated agent. Therefore, the fuel-cell-assisted chelated-iron process became a preferred solution for sulfur recovery for FLNG because it achieved fast regeneration of chelated iron catalyst, while producing electric energy. The electrochemical reactions were expressed as Anodic reaction: Fe 2+ L→Fe 3+ L+e - (1) Cathodic reaction: O 2+2H2O+4e -→ 4OH - (2) During the sulfur recovery reaction cycle, the oxidation rate of sulfur ions was very fast, and the overall reaction rate was limited by the regeneration rate of chelated iron ions. Therefore, by increasing the regeneration rate of the iron complex, the amount of catalyst can be effectively reduced, thereby minimizing the volume and weight of the sulfur recovery device of FLNG. In order to successfully complete the catalyst regeneration fuel cell design, it is necessary to study the kinetic mechanism of the anodic electrode reaction of different anodic materials in chelated iron catalysts fuel cell, so as to select the appropriate electrode materials. In this paper, cyclic voltammetry, polarization curves, and EIS measurements were used to study quantitatively the kinetic parameters of electrode processes for different electrode materials, including platinum, platinum plating titanium, graphite, ruthenium-titanium, and Harris alloy. Results showed that: (1) As the anode of the chelated iron catalyst fuel cell, reversible oxidation of ferrous ions as shown in reaction (1) occurs on the platinum, platinum-plated titanium, and graphite electrodes. The value of the electrode reaction rate constant K was almost the same on each of these electrodes, and the electrochemical reaction rate was fast. The rate-determining step was the diffusion of Fe2+. (2) When using ruthenium-titanium and Harris alloys as anode materials of the chelated iron catalyst fuel cell, many side reactions occurred on the electrode, and the reaction rate was smaller. However, when the suitable oxidation potential was applied, the reaction rate was increased while the occurrence of side reactions can be reduced. (3) The average current density of the platinum-plated titanium electrode used as the chelated-iron catalyst fuel cell anode was the largest of the six tested materials. Because the effective surface area was much larger than the apparent area, more reactive sites were available to carry out the electrode reactions. Therefore, the platinum-plated titanium electrode was the preferred anode material for chelated-iron catalyst fuel cells.

Neutral Red and Ferroin as Reversible and Rapid Redox Materials for Redox Flow Batteries.

Neutral red and ferroin are used as redox indicators (RINs) in potentiometric titrations. The rapid response and reversibility that are prerequisites for RINs are also desirable properties for the active materials in redox flow batteries (RFBs). This study describes the electrochemical properties of ferroin and neutral red as a redox pair. The rapid reaction rates of the RINs allow a cell to run at a rate of 4 C with 89 % capacity retention after the 100th cycle. The diffusion coefficients, electrode reaction rates, and solubilities of the RINs were determined. The electron-transfer rate constants of ferroin and neutral red are 0.11 and 0.027 cm s-1 , respectively, which are greater than those of the components of all-vanadium and Zn/Br2 cells.

Determination of Effective Surface Area and Reaction Rate Constant By Cyclic Voltammetry Considering Ohmic Resistance and CPE Effects

One of the most commonly used electroanalytical techniques for determining the effective surface area and reaction rate constant in a redox system is linear sweep voltammetry due to its sensitive response to a reaction and easy implementation. In linear sweep voltammetry, a linear potential scan is applied to an immobile electrode submerged in an unstirred analyte. The responses to the applied potential is the total current which consists of the faradaic and non-faradaic currents. The faradaic current is the current generated by the reaction of interest, while the non-faradaic current is generated by an electrical double layer. As the faradaic processes received high attention in modeling [1-3], the non-faradaic current, which can be described by the constant phase element (CPE), received less attention. The conventional approach to evaluate the effective surface area and reaction rate constant is to assume that the non-faradaic current is linear and the ohmic drop is negligible. For a quasireversible reaction, by extrapolating the baseline, the peak current and potential are obtained and applied to the relation of Matsuda and Ayabe [4] and Nicholson’s method [5] to determine for the effective surface area and the reaction rate constant, respectively. However, this conventional approach could lead to an inaccurately estimated peak current and potential since the non-faradaic current is not always linear and the ohmic drop, which results in the peak potential shifting, occasionally cannot be negligible. In addition, the faradaic and non-faradaic currents are coupled through ohmic drop effects. Our research group has successfully developed numerical modeling of voltammetry, including the combined effects of ohmic resistance, CPE, mass transfer, and faradaic processes [6]. In this model, the CPE parameters, i.e. the pseudo-capacitance and the dispersion coefficient, are required as inputs. Generally, the CPE parameters were evaluated by electrochemical impedance spectroscopy (EIS). However, the EIS analysis is time-consuming and requires prior knowledge about the equivalent circuit of a system to obtain the CPE parameters. Furthermore, it measures only impedance of a system at one specific potential in frequency domain, while a voltammetry method is a potentiodynamic measurement, which operates under a wide potential window. Therefore, the semi-theoretical approach is introduced to evaluate the CPE parameters under voltammetry condition. This approach possesses several advantages for evaluating the CPE parameters, such as time-saving, simple, and straightforward. In this study, by considering the effects of ohmic resistance and CPE, an approach for determining the electrode effective surface area and reaction rate constant using linear sweep voltammetry was developed. By fitting the peak current and potential of the numerical simulation to those of the experimental data (see Fig. 1a), the electrode effective surface area and reaction rate constant can be determined. As the non-faradaic current becomes relatively more important at faster scan rates, it is difficult to extrapolate the baseline, as depicted in Fig. 1b. This difficulty could lead to underestimation of the effective surface area since the relation of Matsuda and Ayabe, needs the faradaic peak current as an input. Furthermore, Nicholson’s method, which evaluates the reaction rate constant directly from the peak separation, would underestimate the reaction rate constant if the ohmic drop effect was appreciable. Therefore, owing to the problem that the non-faradaic current and ohmic drop effect occasionally could not be avoided, using our approach that takes them into consideration would lead to more accurate prediction of such parameters. Acknowledgment The authors would also like to express their gratitude to the Ministry of Education, Culture, Sports, Science and Technology, Japan, for providing financial support under the scholarship program for foreign students.

On the Limitations of the Doyle-Fuller-Newman Model across Operating Temperatures in Predicting Lithium-Ion Battery Dynamics

Despite their promising potential, lithium-ion batteries have not witnessed wide adoption in large-scale applications such as electrified propulsion. This is attributed to: a) safety concerns and performance degradation due to aging, and b) lack of a comprehensive understanding of battery behavior. Lithium-ion transport processes are non-linear and span multiple length scales. As such, ion transport can be modeled on a multiplicity of scales. Macroscale battery models describing mass and charge transport are well suited as a reference model for developing model-based control and estimation strategies. The battery research community, for long, has considered the Doyle-Fuller-Newman (DFN) macroscale model [1, 2] as the benchmark to evaluate the predictive ability of reduced-order and simplified electrochemical models. However, macroscale models are approximate representations of micro-scale battery dynamics, and are vulnerable as predictive tools under specific operating conditions. The motivation to pursue this work is driven by the need to develop tools to guide battery researchers on the use of the most appropriate models for predicting battery dynamics. This work specifically identifies the operating temperature conditions that violate the underlying assumptions of the continuum equations of the DFN model, and leads to loss of model predictiveness. Starting with the Poisson-Nernst-Planck (PNP) micro-scale equations, a multiple-scale expansion is applied to rigorously derive macroscopic equations of mass and charge transport [3]. Physics-based conditions are identified under which classical macroscale models accurately describe lithium-ion micro-scale dynamics with an accuracy prescribed by the homogenization technique. These conditions are represented schematically in the form of phase diagrams. The temperature-dependent dynamics of lithium-ion battery electrodes are examined using the electrolyte phase diagram [4], and the results obtained indicate that standard macroscopic models fail to describe micro-scale processes in batteries that are operated above critical temperature conditions. The results predicted by analytical studies in previous work [4] are confirmed through numerical simulations in the present work. The equations of the full-homogenized macroscale (FHM) model developed in [3] are resolved using the finite element modeling software COMSOL Multiphysics®. Numerical simulations are performed using the FHM model and the DFN macroscale model developed in COMSOL by Plett [5]. The performance of both models is assessed against data from experiments conducted on 18650 cylindrical lithium-ion cells with nickel manganese cobalt oxide cathode at the Battery Aging and Characterization Laboratory at Clemson University. 2A (1 C-rate) constant current discharge experiments are conducted at 5°C, 23°C, 45°C, and 52°C. The measured battery voltage responses are used to assess the predictive ability of the FHM and DFN models. To prevent any bias, the same geometrical and stoichiometric parameters values are used in both models, and kept constant across different temperatures. For each data set, five parameters are identified for both the models as a function of temperature: electrode diffusion coefficients and reaction rate constants, and contact resistance. The results indicate that the DFN model, which predicts battery dynamics accurately at 5°C and 23°C, fails to replicate the same at 45°C and 52°C towards the end of cell discharge. Simulations results for the performance of both models at these temperatures are illustrated in Fig. 1. The FHM model accurately predicts battery response under all temperature conditions. The lack of accuracy of the DFN model at higher temperatures is due to the inability of the electrolyte equations to capture micro-scale dynamics, indicated by a phase diagram study [4]. The outcome of this work will enable the development of physics-based control strategies to prolong battery useful life for battery management system applications. References Doyle, T. F. Fuller, and J. Newman, “Modeling of galvanostatic charge and discharge of the lithium/polymer/insertion cell”, J. Electrochem. Soc., vol. 140, no. 6, pp. 1526-1533, (1993).Doyle and J. Newman, “The use of mathematical modeling in the design of lithium/polymer battery systems”, Electrochimica Acta, vol. 40, no. 13-14, pp. 2191-2196, (1995).Arunachalam, S. Onori, and I. Battiato, “On veracity of macroscopic Lithium-ion battery models”, J. Electrochem. Soc ., vol. 162, no.10, pp. A1940-A1951, (2015).Arunachalam, S. Onori, and I. Battiato, “Temperature-dependent multiscale-dynamics in Lithium-Ion battery electrochemical models”, in Proceedings of the 2015 American Control Conference, pp. 305-310, IEEE, (2015).L. Plett, “Battery Management Systems, Volume I: Battery Modeling”, Artech House, (2015). Figure 1

Platinum-3D Graphene Oxide Areogel Nanocomposite for Direct Electrochemistry and Electrocatalysis of Horseradish Peroxidase

Aiming to improve the electron transfer efficiency between redox protein/enzyme and electrode, Pt-3D graphene oxide aerogel (3DGA) nanocomposite was synthesized via a facile one-step hydrothermal method and used as a new biocompatible carrier for protein/enzyme. The morphology and structure of Pt-3DGA nanocomposite were characterized by SEM, TEM, XPS, XRD, Raman and BET. Then a new modified electrode was fabricated with Pt-3DGA-horseradish peroxidase (HRP) biocomposite suspension and Nafion solution coated on carbon ionic liquid electrode (CILE). The performances of Nafion/Pt-3DGA-HRP/CILE were further checked by various methods with the well-maintained 3D structure, good retention of bioactivity of HRP and the enhanced electrochemical ability. Direct electrochemistry of HRP was investigated with a pair of well-shaped redox peaks appeared with the peak-to-peak separation as 78 mV. Electrochemical parameters including electron transfer coefficient (α) and heterogeneous electrode reaction rate constant (ks) are calculated to be 0.52 and 3.35 s−1, respectively. Nafion/Pt-3DGA-HRP/CILE shows outstanding electrocatalytic activity towards trichloroacetic acid (TCA) reduction with a wide detection range of 0.5 ∼ 42.0 mmol/L. Nafion/Pt-3DGA-HRP/CILE can be successfully utilized for the analysis of TCA contents in real samples with good recovery.

Nanostructured Electrodes for Low-Temperature Operating SOFCs

Production of electricity by directly utilizing abundantly available fuels such as natural gas can meet the energy demands in various sectors. To make a robust, reliable and cost effective SOFC technology, the operating temperature of SOFCs has to be lowered down to 500°C. Albeit low ionic conductivity of electrolytes and hampered electrode kinetics at such low temperatures significantly affect the efficiency. Reducing the thickness of fast oxide-ion conducting electrolytes (e.g. Ce0.9Gd0.1O2- δ) and by using highly electrically conducing electrodes can lower the ohmic losses; however sluggish electrode reaction rates at low-temperatures remains a difficult problem causing huge non-ohmic losses. Nanostructuring of electrocatalyst on an electrically conducting electrode can significantly improve the reaction rate for hydrogen oxidation and oxygen reduction, thereby enhancing the efficiency of SOFCs. Additionally, operating SOFCs at a low temperature range would reduce particle growth and extend cell life-time compared to high-temperature SOFCs. In this study, the effect of nanostructured infiltrates on both catalytically active and inactive porous electrodes will be analyzed in detail. Improvement in electrode reaction rate will be quantified using electrochemical impedance spectroscopy (EIS). The main focus of the study will be on understanding how nanostructured electrodes could improve the efficiency and long-term stability of LT-SOFCs.

Concentration Impedance in Testing of Solid Oxide Cells Revisited

Concentration impedance in the context of solid oxide cells (SOC) has been discussed in the literature [1-6]. We divide the concentration impedance into diffusion impedance and conversion impedance. Diffusion impedance is defined as impedance due to a reactant concentration gradient caused by a slow (relative to the electrode reaction rate) diffusion through a porous boy of a stagnant gas layer. Conversion impedance is defined as impedance caused by a decrease in an electrode reactant concentration due the conversion of the reactant by electrode reaction. Mainly the diffusion impedance has been treated in spite of the fact that the conversion impedance often is of greater importance. Actually not all workers within the SOC field recognize the concept of conversion impedance. Other workers claim that the two kinds of concentration impedance cannot be distinguished. It is correct that the two phenomena will interact, and thus often it may be difficult to separate them, but we have tools that make it possible to distinguish between the two types of impedance. The diffusion impedance due to diffusion through a porous body, e.g. an electrode support layer, will not be sensitive to reactant gas flow rate, whereas the conversion impedance will be very sensitive to the reactant flow rate. Diffusion impedance results in the simple, clean case in a Warburg element, i.e. a 45 ° slope followed by a relative sharp decrease by decreasing low frequencies down to the x-axis in a Nyquist plot. In contrast to this, clean conversion impedance can always be modeled by a resistor in parallel with an ideal capacitor, which most often has a rather huge value in the order of 0.1 – 1 F cm-2. The paper and presentation will explain how the two types of concentration impedance may be extracted from impedance spectra in a manner that they are separated. Both simple examples and more difficult cases with very high fuel utilization at high current density, where the interaction between diffusion and conversion impedance is in particular strong, will be given. Special emphasis will be put on the case of plug flow condition, which is the relevant case for practical cells and stacks.

Effects of silver halide complexes on optical and electrochemical properties of silver deposition-based electrochromic device

Electrochromic-based optical modulation devices have attracted much interest because of potential application for electric papers and smart windows. We have reported Ag deposition-based optical modulation device which showed reversible changes of optical states between clear transparent and silver-mirror. In the device, formation of silver halide complexes greatly affected the optical states of silver electrodeposit. In this research, the effects of silver halide complexes were investigated by changing halide anion species in terms of the optical properties of the EC devices. The stability of the silver halide complex induced change in reduction voltage to form silver nanoparticles. Diffusion coefficients and electrode reaction rates also have effects on morphology of deposited Ag nanoparticles and resulting optical properties.

Highly defective mesoporous carbon – ionic liquid paste electrode as sensitive voltammetric sensor for determination of chlorogenic acid in herbal extracts

In this report, the preparation of a modified carbon paste electrode based on highly defective mesoporous carbon (DMC) and room temperature ionic liquid 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM.PF6) was described for sensitive voltammetric detection of chlorogenic acid (CGA) in herbal extracts. DMC with desired defective structure was synthesized via a facile method using nanosilica as a hard template, sucrose as a carbon source, and KNO3 as a defect causing agent. The characteristics of modified electrode (IL/DMC/PE) were investigated by cyclic voltammetry, electrochemical impedance spectroscopy and chronocoulometry. The electrochemical parameters of CGA on the surface of modified electrode were determined with the results of the charge transfer coefficient (α), the number of electron transfer (n), the electrode reaction rate constant (ks) and saturating absorption capacity as 0.5, 2, 3.1s−1 and 1.42×10−9molcm−2 respectively. The proposed sensor exhibited a remarkably enhanced effectiveness toward the electrochemical reaction of CGA in aqueous solution. The electrocatalytic behavior was further exploited as a sensitive detection scheme for the CGA determination by Square wave voltammetry (SWV). Under optimized conditions, the linear response range and detection limit were found to be 2×10−8–2.5×10−6molL−1 and 1×10−8molL−1 (S/N=3) respectively. The method was successfully applied for determination of CGA in extracts of Calendula officinalis and Echinacea purpurea.

Direct electrochemistry and electrocatalysis of lobetyolin via magnetic functionalized reduced graphene oxide film fabricated electrochemical sensor

A novel lobetyolin electrochemical sensor based on a magnetic functionalized reduced graphene oxide/Nafion nanohybrid film has been introduced in this work. The magnetic functionalized reduced graphene oxide was characterized by fourier transform infrared spectroscopy, atomic force microscope, X-ray diffraction, transmission electron microscopy and thermogravimetric analysis. The scanning electron microscopy characterized the morphology and microstructure of the prepared sensors, and the electrochemical effective surface areas of the prepared sensors were also calculated by chronocoulometry method. The electrochemical behavior of lobetyolin on the magnetic functionalized reduced graphene oxide/Nafion nanohybrid modified glassy carbon electrode was investigated by cyclic voltammetry and differential pulse voltammetry in a phosphate buffer solution of pH6.0. The electron-transfer coefficient (α), electron transfer number (n), and electrode reaction rate constant (Κs) were calculated as 0.78, 0.73, and 4.63s−1, respectively. Under the optimized conditions, the sensor based on magnetic functionalized reduced graphene oxide/Nafion showed a linear voltammetric response to the lobetyolin concentration at 1.0×10−7 to 1.0×10−4mol/L with detection limit (S/N=3)of 4.3×10−8mol/L. The proposed sensor also displayed acceptable reproducibility, long-term stability, and high selectivity, and performs well for analysis of lobetyolin in real samples. The voltammetric sensor was successfully applied to detect lobetyolin in Codonopsis pilosula with recovery values in the range of 96.12%–102.66%.

(Invited) Solvate Ionic Liquids for Advanced Lithium Batteries

Certain concentrated mixtures of salts and solvents are not simply "solutions" anymore, but they may be described as "solvate ionic liquids", in which the solvents strongly coordinate the cation and/or the anion of the salts to form stable “solvate ions”. The new family of ionic liquids can be obtained by simply mixing glyme (triglyme (G3) or tetraglyme (G4)) with lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) in a molar ratio of 1:1 [1-3]. The equimolar complex [Li(G3 or G4)1][TFSA] maintains a stable liquid state over a wide temperature range and can be regarded as a solvate ionic liquid consisting of a [Li(glyme)1]+ complex cation and a [TFSA]- anion, exhibiting high self-dissociativity (ionicity) at room temperature [1, 2]. The electrochemical oxidation of [Li(glyme)1][TFSA] takes place at the electrode potential of ~4.6 V vs. Li/Li+, while the oxidation of solutions containing excess glyme molecules ([Li(glyme)x][TFSA], x > 1) occurs at lower than 4 V [1]. This enhancement of oxidative stability is due to the donation of lone pairs of ether oxygen atoms to the Li+ cation, resulting in the highest occupied molecular orbital (HOMO) energy level lowering of a glyme molecule, which is confirmed by ab initio molecular orbital calculations. The solvation state of a Li+ cation and ion conduction mechanism in the [Li(glyme)x][TFSA] solutions strongly suggests that Li+ cation conduction in the equimolar complex takes place by the migration of [Li(glyme)1]+ cations, whereas the ligand exchange mechanism is overlapped when interfacial electrochemical reactions of [Li(glyme)1]+ cations occur. The ligand exchange conduction mode is typically seen in a lithium battery with a configuration of [Li anode | [Li(glyme)1][TFSA] | LiCoO2 cathode] when the discharge reaction of a LiCoO2 cathode, i.e., de- solvation of [Li(glyme)1]+ and insertion of the resultant Li+ into the cathode, occurs at the electrode-electrolyte interface. The battery can be operated for more than 200 charge-discharge cycles in the cell voltage range of 3.0–4.2 V, regardless of the use of ether-based electrolyte, because the ligand exchange rate is much faster than the electrode reaction rate [1].Another intriguing aspect of the solvate ionic liquids is unusual solubility. The theoretical capacity of the S cathode is 10 times higher than that of conventional cathode materials used in current Li–ion batteries. However, Li–S batteries suffer from the dissolution of lithium polysulfides, which are formed by the redox reaction at the S cathode. In the equimolar complexes [Li(G3 or G4)][TFSA] consisting of [Li(glyme)1]+ complex cation and [TFSA]- anion, both cations and anions are low coordinating ions with low Lewis acidity and basicity, respectively. The [Li(G3 or G4)][TFSA] molten complexes do not readily dissolve other ionic solutes due to the low coordinating nature of the cation and anion, which leads to the stable operation of the Li–S battery over more than 400 cycles with discharge capacities higher than 700 mAh g-S-1 and with coulombic efficiencies higher than 98% throughout the cycles [4]. Furthermore, the addition of a nonflammable fluorinated solvent, which does not break the solvate structure of the glyme–Li salt molten complexes, greatly enhances the power density of the Li–S battery [4].An important electrode reaction, which is enabled by the use of the solvate ionic liquids, is electrochemical Li+ intercalation into graphite electrodes [5]. Li+-intercalated graphite was successfully formed in an equimolar molten complex, [Li(G3)1][TFSA]. The desolvation of Li+ ions took place at the graphite/[Li(G3)1][TFSA] interface in the electrode potential range 0.3‒0 V vs Li. In contrast, the cointercalation of G3 and Li+ (intercalation of solvate [Li(G3)1]+ cation) into graphite occurred in [Li(G3) x ][TFSA] electrolytes containing excess G3 (x > 1). This cointercalation causes exfoliation of the graphite. The electrode potential for the formation of Li+-intercalated graphite (desolvation of solvate [Li(G3)1]+ cation) changes greatly, depending on the activities of free G3 in the electrolyte. In the solvate ionic liquid, the activity of the free solvent is very low, and consequently, the electrode potentials for the formation of Li+-intercalated graphite become higher than that for cointercalation, and the cointercalation of G3 is inhibited in [Li(G3)1][TFSA].

Specific Response of Quinone Redox at 2-Aminoethanethiol Modified Electrode

Self-assembled monolayers of alkanethiols on electrode surface have been the subjects of intensive studies in both fundamental interests of surface chemistry and their technological significance. The self-assembly is one of the most widely used techniques to construct ordered molecular structures and to control surface characteristics, due to its facile procedure. 2-aminoethanethiol has been used as an anchor to introduce functional molecules to electrode surfaces as it has a reactive amino group and its carbon chain is short. We have found that a 2-aminotehanethiol modified Au electrode shows a specific electrochemical response to p-hydroquinone in aqueous solution. That is, a peak separation (ΔE p) in the CV appeared to be very large at the first potential cycle, but it drastically decreased upon successive potential cycling. We have been studying condition where the specific response takes place and its mechanism. Figure 1A shows typical CVs of an aminoethanethiol modified Au disk electrode in a 0.1 M sodium perchlorate solution containing 5 mM hydroquinone at 100 mV s-1. The ΔE p for quinone redox was found to be 530 mV for the first cycle, indicating a slow electron transfer (Fig. 1A curve a). After application of +0.9 V vs. Ag/AgCl for 30 min at which hydroquinone is oxidized, the ΔE p largely decreased to 120 mV (Fig. 1A curve b) and the CV overlapped with that of a bare electrode. On the other hand, application if –0.5 V was applied, the CV remained virtually constant. Interestingly, when the electrode was left in the solution at open circuit or –0.5 V for 3 h, the ΔE p increased to the original value (530 mV). These results indicate that the changes in the ΔE p is reversible and not originating from peeling of the film, and the increase can be attributed to an interaction between aminoethanethiol and hydroquinone, whereas the decrease is due to oxidation of hydroquinone. A Au electrode modified with 2-dimethylanimoethanethiol showed similar behavior (Fig. 1B), indicating that Schiff base formation from amino group of 2-aminoethanethiol and p-quinone is not the dominant cause of the changes in the electrochemical response. At the current stage, we speculate two possible mechanisms for the changes in the electrochemical response. The first one is changes in structure of aminoethanethiol due to changes in an interaction with quinone, although it is known that short alkyl chains on the electrode surface do not block electron transfer. The second one is a chemical reaction between hydroquinone and aminoethanethiol, which produces derivatives with slow electrode reaction rate constant. We currently investigate those two possibilities with STM measurements for an aminoethanethiol modified electrode before and after the electrolysis and with NMR measurements for the reaction. Figure 1

Detailed kinetic study of the electrochemical Bunsen reaction in the sulfur–iodine cycle for hydrogen production

The electrochemical Bunsen reaction has been proposed as an innovative and promising method compared with the traditional Bunsen reaction for the sulfur–iodine cycle, and its kinetics were studied in this work. Polarization curves were measured using platinum electrode in the different concentrations of SO2–H2SO4–H2O and I2–HI–H2O solutions at a temperature range of 303–353K. Fundamental kinetic parameters, namely exchange current density, Tafel slope, asymmetry factor, were determined for anodic SO2 oxidation reaction and cathodic I2 reduction reaction according to the Tafel plots. The activation energy and pre-exponential factor for the electrode reactions were also analyzed using Arrhenius relation. Results indicated that an increase in the SO2 concentration in the anolyte and the I2 concentration in the catholyte reduced the activation energy, contributing to the easier anode and cathode reaction. Take high electrode reaction rate and easier electrode reactions into account, the initial SO2 concentration of 1.509mol/L and the I2/HI molar ratio of 0.5 could be optimal.

The mechanism of dissolution of minerals in acidic and alkaline solutions: Part VI a molecular viewpoint

The theory of dissolution proposed here and in this series of papers predicts that the order of reaction with respect to H+ ions in solutions should in the series 0.5, 1.0, 1.5…. Experimentally this is found to be the case. The mechanism of dissolution is based on the removal of material from the surface that results in the formation of cations and anions in parallel ‘partial’ reactions. These reactions result in the charging of the surface and the formation of a potential difference across the solid–solution interface. This potential difference in turn influences the rate of removal from the surface. The purpose of this paper is to discuss in further detail the particular mathematical form of the dependence on potential difference that has been chosen. Three plausible models are examined in order to understand the mechanistic steps of dissolution underlying the mathematical form of the potential dependence: (i) the Marcus model, (ii) ionic transfer model of Fawcett, and (iii) the ‘make-before-break’ model of Gileadi. This mathematical form for the potential dependence implies a somewhat surprising link between the dissolution of a metal and the dissolution of a mineral. This link is strengthened by demonstrating that both the rates of mineral dissolution and metal electrode reactions are correlated with the rate of water self-exchange. Since they are correlated with the same variable, they must be correlated with one another. It is proposed that mineral dissolution reactions and dissolution/deposition reactions at metal electrodes share similar reaction trajectories, and hence the similarity between functional dependencies of the rate of ion formation on potential difference across the Helmholtz layer.

Analysis of Material and Charge Transfer in the Fiber Catalyst Layer for Polymer Electrode Fuel Cells

Nanofiber electrocatalysts for polymer electrode fuel cells has been proposed in recent years. In this study, we analyzed material transfer rate and electrode reaction rate in the nanofiber catalyst layer using rotating disk electrode. Carbon alloy nanofiber was examined for a cathode catalyst. We focused on effects of the fiber length and catalyst loading on the catalyst layer activity. The transport situation was discontinuously changed at a higher loading amount of relatively long catalysts. The material transfer rate was increased at higher catalyst loading with longer nanofibers. This phenomena is explained by the difference of layer structure between the fiber catalyst and the particle-like catalyst. Relatively long (6.9 mm, for example) catalyst with catalyst loading at around 1 mg cm−2 is appropriate condition of catalyst layer because of its high material transfer rate inside the catalyst layer.

Understanding Direct Borohydride – Hydrogen Peroxide Fuel Cell Performance with a Calibrated Numerical Model

Hydrogen peroxide – direct borohydride fuel cells (H2O2-DBFCs) have high theoretical energy density and cell voltage [1-3], yet these desirable characteristics have not been demonstrated in a practical system. Parasitic chemical reactions competing with the desired electrochemical reactions are partly to blame [4]. Efforts to improve H2O2-DBFC performance have been hindered by its complexity; the reactions have many intermediate steps and pathways that are difficult to resolve experimentally [5, 6]. Furthermore, the relative rates of electrode reactions are influenced by transport in the reactant flow channels, where advection, diffusion, and migration occur [7]. A numerical model can make the complexity manageable and offer new insights by revealing relationships that are difficult to measure. To this end, a H2O2-DBFC model has been developed and calibrated to experiments with a single-cell H2O2-DBFC. The calibrated model was used to investigate the relationships between transport in the flow channels, parasitic side reactions, and DBFC performance.In the H2O2-DBFC shown in Fig. 1, a Nafion 117 membrane separates parallel flow channels. The channel walls are graphite electrodes coated with electro-deposited catalysts. Fuel (1-50 mM NaBH4 / 2 M NaOH) is oxidized at the Au anode and oxidizer (10-40 mM H2O2 / 1 M H2SO4) is reduced at the Pd:Ir cathode, while Na+ and H2O cross the membrane.The steady-state finite-difference model includes diffusion, migration, and advection in the flow channels, transport across the membrane, multiple charge transfer reactions at each electrode to yield mixed potentials, and chemical parasitic reactions at each electrode. It is formulated in MATLAB and solved by a modified Newton solver in SUNDIALS.Model features were chosen to reflect experimental observations. For example, experiments showed that low cathode potential caused by oxidizer-lean stoichiometry can lead to H+ reduction at the cathode, therefore this reaction was included in the model. The model was calibrated to experiments by fitting predicted polarization curves to measured curves. Electrode reaction rates were modeled with global expressions that summarized many intermediate steps, and the fitted parameters were the rate constants in those global expressions. Predicted vs. measured polarization curve comparisons showed good agreement (R2 ≥ 0.920) over the fuel cell operating space. The predicted and measured limiting current densities agreed, indicating transport was modeled accurately. Results from the calibrated model show compact concentration boundary layers in both channels, which limit reactant utilization and current density. Transport limits are most pronounced at the cathode because BH4 - has higher diffusivity than H2O2, BH4 - transport is aided by migration, and BH4 - can provide up to 8e- while H2O2 only consumes up to 2e-. Decreasing inlet concentrations, flow rates, and cell voltage result in lower losses to parasitic side reactions as these changes decrease reactant concentrations near the electrodes, which favors charge transfer reactions. Peak conversion efficiency coincides with peak power density because thermodynamic efficiency and parasitic reaction rates both decrease (relative to charge transfer reaction rates) with increasing current density. This work shows that advection, migration and diffusion must be included in H2O2-DBFC models, and that global reactions are sufficient to capture the essential features of complex H2O2-DBFC reactions.

Assessment of organic acid-rich bio-sap to generate electricity

The study has focused on electricity generation from organic acid-rich bio-substrate like star fruit (Averrhoa carambola). The sap of star fruit was selected as an electrolyte due to the presence of significant amounts of organic acids such as citric acid and ascorbic acid. To preserve the sap, 2% phenol by volume was used to reduce the growth of microorganisms, and the addition of phenol did not affect the initial pH. It was observed that due to an increase in the electrode surface area, reaction rate and current generation had been amplified. Internal resistance also decreased rapidly because of the large electrode surface area. Furthermore, internal resistance was the significant barrier in electricity generation, which was also successfully controlled by the baffle flow agitation system. Moreover, the baffle flow agitation system reduces the formation of dead zones and increases the total dissolved solids inside the electrochemical cell compartments during operation.