Varying Electrolyte Concentration Research Articles

Multi-Scale Computational Fluid Dynamics Simulation of Alkaline Water Electrolyzers: A Numerical Investigation

The utilization of water electrolyzers for hydrogen production represents one of the promising technologies in the global transition away from fossil fuels towards sustainable energy sources. Among the various types of electrolyzers, alkaline electrolyzers in the zero-gap configuration with different separator types, like diaphragms and anion conducting membranes, have garnered significant attention, among others, due to their potential to utilize non-precious metal catalysts such as iron and nickel for the electrodes. This characteristic holds promise for reducing costs and increasing accessibility to hydrogen production technologies, including the establishment of largescale stationary systems and hydrogen production powered by renewable energy.In order to accelerate the development process and to understand the important mechanisms in more detail within these cell types, numerical Computational Fluid Dynamics (CFD) simulations are conducted and combined with experimental cell testing at the Juelich Research Center. The studies focus on investigating the influence of variations in the porous transport anode-electrode, particularly nickel fiber fleece and nickel foam, along with their characteristic properties on the performance of the anion exchange membrane water electrolysis cells. This includes investigating porosity gradients, optional additional catalyst layers and their loadings, and the effects of changing types and thicknesses of porous transport electrodes. These studies are conducted at varying electrolyte concentrations and operating conditions.Using the open-source platform OpenFOAM® and the developed libraries of OpenFuelCell21, CFD simulations are performed at both the micro and meso scales to examine the aforementioned questions (see Fig. 1).The model incorporates all key transport phenomena, such as two-phase fluid flow, described via an Eulerian-Eulerian approach, heat and mass transfer, electrochemical reactions, species transfer, and charge transfer across the various functional layers of the cell. It allows for the analysis of electrochemical reactions by employing the Butler-Volmer equation to describe electrokinetics across various types of electrochemically active porous electrodes.For the multi-physical simulation of the detailed flow within the porous electrodes at micro scale, microtomography images are used as geometrical input. In addition, the microscopic data are subsequently utilized for characterizing the porous electrodes in the macro-homogeneous simulations performed at the meso-scale.Literature: Zhang, S. Hess, H. Marschall, U. Reimer, S. Beale, W. Lehnert, openFuelCell2: A new computational tool for fuel cells electrolyzers, and other electrochemical devices and processes, Comput. Phys. Commun. 298 (2024) 109092. Figures:Figure 1: Numerical simulation of the flow through different micro-porous electrodes (left) which results are used for the macro-homogeneous simulations of alkaline electrolysis cell in 2-D and 3-D (right). Figure 1

Role of Acoustic and Volumetric Properties in View to Estimate the Intermolecular Interactions in the Solutions of Electrolytes and l‐Isoleucine

Abstractl‐Isoleucine is subjected to density and sound velocity measurements in aqueous solutions of potassium (KCl) and sodium chlorides (NaCl) as well as water, from 288.15 to 298.15 K. Through the use of these experimental data, a variety of volumetric and acoustical parameters are calculated, including values for molar volume (Vm), available volume (Va), adiabatic compressibility (β), nonlinear parameters (B/A), Wada constant (W), Rao's constant (R), and van der Wall's constant (a). The findings shed light on many electrostatic and hydrophobic interactions within the binary system involving electrolytes, water, and amino acids. The interactions between the electrolytes and l‐isoleucine are thoroughly examined under varying electrolyte concentrations and temperatures, revealing many interactions. Positive transfer quantities demonstrate the ion‐hydrophilic and solute–solvent interaction in the aqueous medium, particularly between l‐isoleucine and NaCl and KCl. The influence of electrolyte type and temperature on these interactions is discernible. The culmination of the study's outcomes is elaborated through analyzing solute–solvent and solute–solute interactions. In particular, the interaction hierarchy is identified as l‐isoleucine + water + KCl > l‐isoleucine + water + NaCl > l‐isoleucine + swater.

Fluid Physics Impacting Vanadium and Other Redox Flow Batteries

The Vanadium redox flow battery (VRFB) has been intensively examined since the 1970s, with researchers looking at its electrochemical time varying electrolyte concentration time variation equations (both tank and cells, for negative and positive half cells), its thermal time variation equations, and fluid flow equations. Chemical behavior of the electrolyte ions has also been intensively examined. Our focus in this treatment is a completely new approach to understanding the physics, chemistry, and electronics of the VRFB. Here, we develop complete theoretical equations by an analytical treatment affecting the fluid flow in the VRFB as well as all other redox flow batteries, providing background derivations applicable for all of the fundamental concepts required to properly understand flow batteries. With these concepts presented, calculations are done to determine actual values for fluid velocity, strain rate, angular fluid velocity, angular momentum, rotational kinetic energy, and gravity effects on fluid velocity in a redox flow battery.

Influence of Electrolyte Concentration on the Electrochemical Behavior of Copper Hexacyanoferrate as an Active Material for Zinc-Ion Batteries

This study investigates the impact of electrolyte concentration on the electrochemical behavior of copper hexacyanoferrate (CuHCF), a promising active material for aqueous zinc-ion battery electrodes. Cyclic voltammetry, charge-discharge measurements, and X-ray diffraction analysis were employed to assess the electrochemical reactions and structural integrity of the CuHCF electrode under varying electrolyte concentrations. The results revealed a significant influence of electrolyte concentration on the electrochemical performance of the CuHCF electrode. Specifically, the charge-discharge capacity exhibited an initial increase as the electrolyte concentration increased from 1.0 to 2.0 mol dm‒3, followed by a subsequent decrease. This decrease in capacity was attributed to the occurrence of an electrode/electrolyte interfacial reaction in the low-potential region of 0.0–0.3 V, coupled with structural changes in the CuHCF active material. Notably, these findings underscore the strong correlation between the electrochemical performance of the CuHCF electrode and the hydration structure of zinc ions, as well as the pH of the electrolyte solution. Thus, optimizing the electrolyte composition holds significant potential for enhancing the performance of aqueous zinc-ion batteries employing CuHCF electrodes.

Determination of the Ion Concentrations in VRFB by Non-Invasive Optical Techniques Due to Chemical Reactions, Complexes, and Side Reactions

The Vanadium redox flow battery has been intensively examined since the 1970s, with researchers looking at its electrochemical time varying electrolyte concentration time variation (both tank and cells, for negative and positive half cells), its thermal time variation, and fluid flow behavior. Chemistry behavior of the electrolyte ions have also been intensively examined too. Our focus in this paper is to examine the disturbance and effect of other chemical reactions and constituents to those typically found in the positive electrode electrolyte, while handling the less problematic negative electrode. Appropriate formulas are developed which allow assessment of the size of the effect, and their evaluation indicates something on the order of up to 14%. Therefore, for unambiguous non-invasive optical measurements, knowledge of the disturbance effect, and taking it into account will allow proper concentration determination. This treatment includes a comprehensive examination of identified side reactions which occur due to crossover ions between the electrodes.

Physics, electrochemistry, chemistry, and electronics of the vanadium redox flow battery by analyzing all the governing equations.

The vanadium redox flow battery has been intensively examined since the 1970s, with researchers looking at its electrochemical time varying electrolyte concentration time variation equations (both tank and cells, for negative and positive half cells), its thermal time variation equations, and fluid flow equations. Chemistry behavior of the electrolyte ions have also been intensively examined too. In this perspective, all of the phenomena have been examined, unified and presented together with their physical chemistry shown in the appropriate equations. This is done by providing the field equations for the battery, which are electronic, electrochemical, chemical, physics of fluid dynamics, and thermal physics of heat transport, in character. They are interpreted in new analytical equations providing fundamental scientific insights, as well as allowing engineering and manufacturing assessments. Graphs of pressure trend in electrodes, power and temperature tables for electrode and current collector loss mechanisms are provided. Current density, concentration, electrostatic, and overpotential functional dependences in the electrodes are given.

Analytical current-voltage formulas in electrodes and concentration differences for VRFB

The Vanadium redox flow battery has been intensively examined since the 1970s, with researchers looking at its electrochemical time varying electrolyte concentration time variation equations (both tank and cells, for negative and positive half cells), its thermal time variation equations, and fluid flow equations. Chemistry behavior of the electrolyte ions have also been intensively examined too. Our focus in this treatment is an approach to understanding the basic physics, chemistry, and electronics of the VRFB with a view towards practical engineering and manufacturing assessments and designs. Here we develop as rigorously as possible analytical formulas to specify the behavior of the surface and bulk like concentrations in the electrode regions of the battery, which are structurally made of porous materials. It is not necessary that the negative and positive electrodes be made of the same material, thicknesses, nor even with the same porosities.

Application of TiS2 as an Active Material for Aqueous Calcium-Ion Batteries: Electrochemical Calcium Intercalation into TiS2 from Aqueous Solutions

This study explores the potential of titanium disulfide (TiS2) as an active material for aqueous calcium-ion batteries (CIBs). We investigate the electrochemical redox reactions of calcium ions within TiS2 and assess its suitability for use in aqueous CIBs. Additionally, we examine the impact of varying electrolyte concentrations, ranging from 1.0 to 8.0 mol dm−3, on TiS2 electrode reactions. Our findings reveal that TiS2 exhibits distinct charge–discharge behaviors in various aqueous calcium-ion electrolytes. Notably, at higher electrolyte concentrations, TiS2 effectively suppresses the hydrogen generation reaction caused by water decomposition. In situ X-ray diffraction analysis confirms the intercalation of Ca2+ ions between the TiS2 layers during charging, which is a groundbreaking discovery, signifying TiS2’s applicability in aqueous CIBs. X-ray photoelectron spectroscopy analysis further supports the formation of a solid electrolyte interphase (SEI) on the TiS2 electrode surface, contributing to the suppression of electrolyte decomposition reactions. Furthermore, we investigate the influence of anions in the electrolyte on charge–discharge behavior. Our findings suggest that the choice of anion coordinated with Ca2+ ions affects the SEI formation and cycling performance. Understanding the role of anions in SEI formation is crucial for optimizing aqueous CIBs. In conclusion, this research underscores TiS2’s potential as an active material for aqueous calcium-ion batteries and emphasizes the importance of the electrolyte composition in influencing SEI formation and battery performance, contributing to sustainable and efficient energy storage technologies.

Nernst Equations and Concentration Chemical Reaction Overpotentials for VRFB Operation

The vanadium redox flow battery (VRFB) has been intensively examined since the 1970s, with researchers looking at its electrochemical time varying electrolyte concentration time variation equations (both tank and cells, for negative and positive half cells), its thermal time variation equations, and fluid flow equations. Chemistry behavior of the electrolyte ions have also been intensively examined too. Our focus in this treatment is a new approach to understanding the physics, chemistry, and electronics of the VRFB by looking at the equilibrium and non-equilibrium aspects of the battery. Detailed examination of the behavior inside the porous electrodes of the electrolyte concentration is done by distinguishing between the concentrations near the pore surface wall and volume elements away from the solid surface-electrolyte boundary. Relationship to the battery charging or discharging current and current density is performed allowing macroscopic overpotentials to be determined for each electrode. These are concentration overpotentials and their exact relationship to the battery potentials is developed carefully so that a rigorous basis is established. It is found that there are two overpotentials making up the negative electrode overpotential, due to the and ions. Similarly, there are two overpotentials making up the positive electrode overpotential, due to the and ions.

Nanoparticle Taylor Dispersion Near Charged Surfaces with an Open Boundary.

The dispersive spreading of microscopic particles in shear flows is influenced both by advection and thermal motion. At the nanoscale, interactions between such particles and their confining boundaries become unavoidable. We address the roles of electrostatic repulsion and absorption on the spatial distribution and dispersion of charged nanoparticles in near-surface shear flows, observed under evanescent illumination. The electrostatic repulsion between particles and the lower charged surface is tuned by varying electrolyte concentrations. Particles leaving the field of vision can be neglected from further analysis, such that the experimental ensemble is equivalent to that of Taylor dispersion with absorption. These two ingredients modify the particle distribution, deviating strongly from the Gibbs-Boltzmann form at the nanoscale studied here. The overall effect is to restrain the accessible space available to particles, which leads to a striking, tenfold reduction in the spreading dynamics as compared to the noninteracting case.

Mechanistic Underpinnings of Morphology Transition in Electrodeposition under the Application of Pulsatile Potential.

We quantitatively investigate the role of voltage fluctuation in terms of different waveforms on the electrodeposition dynamics and morphology for varying electrolyte concentrations. Dependent on the electrolyte concentration, a wide range of morphologies ranging from highly branched dendrites to comparatively closed packed electrodeposits has been captured. We mechanistically map the deposition dynamics by image analysis and demonstrate the highly porous dendritic dynamics to be independent of external perturbation. Additionally, comparatively closed packed morphological features show significant sensitivity toward the frequency and nature of the waveforms. The results provide fundamental insights into the correlation between the time scales of voltage fluctuation and growth dynamics. We comprehensively analyze the effect of the waveform nature on the average deposition height and show sinusoidal fluctuation to be preferred over square and pulse for metal batteries for lower deposition heights.

Surface modification and electrochemical performance of KMnO4-ultra-surface area activated carbon (USAC) composites as cathode material

The goal of this study was to analyse the impact of a combination of a sub-active material (KMnO4) with surface modification to the activated carbon surface for enhanced cathode performance. Activated carbon with ultra-specific surface area (USAC, TCK-PBM, 2560.4 m2/g) was used for the preparation of our composite with potassium permanganate (KMnO4). The physical characteristics of this As-treated USAC sample were analysed through XRD, SEM and EDX analysis. The XRD analysis was used to examine structural properties; surface analysis undertaken by SEM; EDX analysis was used to measure the metal content of the sample as well as the content after treatment with KMnO4. It was shown USAC treated with KMnO4 had more than a 10.5 μg acidic group by the Boehm titration method. The electrochemical properties of KMnO4-USAC were investigated for varying electrolyte concentration (0.1, 0.5, 1.2 and 6 M) at ambient temperature (25 °C) and the electrode showed great electrochemical performance after ten cycles.

Phase Inversions Observed in Thermoresponsive Pickering Emulsions Stabilized by Surface Functionalized Colloidal Silica.

In this study, the emulsification performance of functionalized colloidal silica is explored with the aim to achieve phase inversion of particle-stabilized (Pickering) emulsion systems. An increased understanding of inversion conditions can facilitate surfactant-free emulsion fabrication and expand its use in industrial applications. Phase inversion was achieved by adjusting the temperature but without changing the composition of the emulsion formulation. Silica nanoparticles modified with hydrophobic propyl groups and hydrophilic methyl poly(ethylene)glycol (mPEG) groups are used as emulsifiers, enabling control of the wettability of the particles and exploration of phase inversion phenomena, the latter due to the thermoresponsiveness of the attached PEG chains. The phase inversion conditions as well as the reversibility of the emulsion systems were examined at varying electrolyte concentrations and pH values of the suspensions. Transitional phase inversions, from oil-in-water and water-in-oil and back, were observed in functionalized silica particle-stabilized butanol emulsions at distinct temperatures. The phase inversion temperature was affected by electrolyte concentration and pH conditions due to salting-out effects, PEG-silica interactions, and the effects of the particle surface charge. Investigations of phase inversion conditions, temperature, and hysteresis effects in Pickering emulsions can improve the theoretical understanding of these phenomena and facilitate the implementation of low-energy emulsion preparation.

Potential-Dependent Volcano Plot for Oxygen Reduction: Mathematical Origin and Implications for Catalyst Design.

Climbing up the volcano peak stands as a challenging problem for oxygen reduction. Repeated efforts have been made to fine-tune the binding energy of oxygen reaction intermediates within a narrow region of 0.2 eV by adjusting the catalyst electronic structure. Herein, we address ourselves to two different, oft-neglected but nontrivial questions: (a) Does a superior oxygen reduction reaction catalyst in rotating disk electrode experiments still work well in practical fuel cells (usually at a different potential)? (b) For a given catalyst, can we place it on the volcano peak by adjusting the electrode potential (ϕM), which can be easily varied within 0.5 V in experiments, and the potential at the reaction plane in solution (ϕOHP), which is modulated by double-layer electrostatic effects? To answer these two questions, we articulate the mathematical origin of the volcano plot and reveal its dependence on ϕM and ϕOHP by combining a microkinetic model for the oxygen reduction reaction and a mean-field model for the double layer. Furthermore, we explore possible approaches of adjusting ϕOHP, for instance, by varying electrolyte concentration and particularly by tuning the electrostatic properties of the support material in a supported catalyst system. The investigation of how electrostatic properties of the support material affect the volcano plot of a supported catalyst opens an additional channel of catalyst-support interactions.

Effect of electrolyte concentration on morphological and photoluminescence properties of free standing porous anodic alumina membranes formed in oxalic acid

The free-standing porous anodic alumina membranes (AAM) were prepared in oxalic acid by varying electrolyte concentration from 0.1 to 0.9 M with a constant potential of 40 V at 8 °C. The influence of electrolyte concentration on the morphological and optical features of AAM has been studied. Regularity ratio (RR) of AAM was measured using WSxM software, highest RR obtained for AAM formed in 0.3 M. From X-ray diffraction (XRD) analysis, it was found that prepared AAM were amorphous in nature. UV–Vis absorbance spectra reveal that absorbance bands slightly moves towards the higher wavelength side with increasing electrolyte concentration. Photoluminescence (PL) of AAM has been studied and related with RR of AAM. PL measurements of AAM exhibit strong PL peak in blue region, having a slight change with increasing electrolyte concentration. PL bands were originating from two kinds of oxygen-deficient defects (F and F+), their distributions were discussed on the basis of PL spectrum.

High-Rate Electrochemical Reduction of CO2 to C2-3 Products Under Neutral, Aqueous Condition, Tracing the Evolution of Phase and Morphology of Cu2o Nanocubes Towards Rational Catalyst Design

The need of a sustainable society to advance towards a circular economy, in which there is a balance between the emission and capture of anthropogenic gases like carbon dioxide (CO2), is of utmost scientific and technological importance to ensure the increase or rather preservation of the current prosperity for future generations. The growing global population and increase in worldwide prosperity is tightly connected to a rise in power consumption. Traditionally, this energy demand is supplied by the combustion of fossil fuels, resulting in growing emission of greenhouse gases. In this, CO2 is a key issue which climatic influences and general mitigation is rigorously disused not only in scientific field, but in politics as well. Here, the electrochemical CO2 reduction reaction (CO2RR) is posing as one potential technology, to address questions of power storage for renewable energies and sustainable usage of natural resources. The CO2RR shows a diverse spectrum of products ranging from liquid fuels, as ethanol and propanol, to gaseous building blocks for the chemical industry, as ethylene and CO. The selectivity of this reaction is highly dependent on the nature of the catalyst and is always competing with the hydrogen evolution reaction (HER), due to the aqueous conditions. Recently, very selective and active catalysts have been developed for the production of CO and formic acid. Both compounds are comparably easy to produce, as the reduction only involves the transfer of 2 electrons. Unfortunately, the production of valuable hydrocarbons is much more difficult and suffers from losses in selectivity due to the highly complex reaction mechanism. So far, only copper showed acceptable production rates for hydrocarbons, owing to the favorable binding energies of intermediates. Recent studies have been focused on fundamental parameters to control the selectivity of this complex reaction on copper for C2+ species, ranging from catalyst design to electrolyte selection. On the catalyst side, many effects as the presence of (100) facets1, grain boundaries2 and oxides3 have been suggested to be beneficial, whereas influences of buffer capacity and local electric field brought additional advantages by choice of the electrolyte. While promising results were shown, many studies only focused on low-current density, which are far from an industrial application. Here, at least 200 mA cm-2 are needed for a functional electrolyzer. This poses as a discrepancy between research and application and raises the question if the results from fundamental studies can be transferred to full size electrolyzers.4 For this, we are presenting a cubic Cu2O catalyst, which we investigate in a comprehensive study, moving from initial tests in an H-Cell towards high currents on a Gas Diffusion Electrode (GDE) in a Flow-Cell setup. To deconvolute the parameters dictating CO2RR selectivity, we used X-Ray-Diffraction (XRD), quasi in-situ X-Ray Photon Spectroscopy (XPS) and operando X-ray Absorption Spectroscopy (XAS) to trace phase changes during reaction, in addition to morphological investigations by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). This complementary analysis depicts a highly dynamic system, in which the reduction of oxidized Cu progresses from the surface towards deeper layers, resulting in a purely metallic, defect-rich material. We further focus on performance tests of our Cu2O catalyst at high current density of up to 700 mA cm-2 in a flow-electrolyzer. By varying system parameters as mass loading and nafion content we observe strong changes in selectivity and activity during CO2RR, which we correlate to accessibility of the active copper sites and issues of mass transport. We further discuss the role of surface pH at high current density by varying electrolyte concentration and therefore buffer capacity. Our study suggests distinct differences between CO2RR in electrochemical cells for fundamental studies at low currents compared to tests at high currents in a flow-electrolyzer. Furthermore, we show how recent results from literature translate to performance at high current density and comment on the importance of electrode preparation. Y. Hori, I. Takahashi, O. Koga and N. Hoshi, Journal of Molecular Catalysis A: Chemical, 2003, 199, 39-47.A. Verdaguer-Casadevall, C. W. Li, T. P. Johansson, S. B. Scott, J. T. McKeown, M. Kumar, I. E. L. Stephens, M. W. Kanan and I. Chorkendorff, Journal of the American Chemical Society, 2015, 137, 9808-9811.H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser and B. R. Cuenya, Nature Communications, 2016, 7, 12123.T. Burdyny and W. A. Smith, Energy & Environmental Science, 2019, DOI: 10.1039/c8ee03134g.

Investigation of the free volume characteristics of PEO based solid state polymer electrolyte by means of positron annihilation spectroscopy

Ion conduction in poly (ethylene oxide), PEO, based electrolytes are explained considering the ion movement coupled with polymer segmental motions or free volume. Herein, we describe the application of positron annihilation spectroscopy to investigate the free volume structure of PEO-LiTFSI solid state electrolyte with varying electrolyte concentration. Addition of LiTFSI indicates plasticization effect through the increase in free volume size of polymer matrix. Depth dependent Doppler broadening measurements have shown a decrease in positron/positronium diffusion length and S-parameter value on addition of electrolyte. The observed decrease in S-parameter and positronium intensity could not be ascribed to positronium reaction (inhibition) with dissociated ions or salt aggregates as the variation does not follow the established inhibition expression. The SW correlation curves indicate that the chemical environment at positron/positronium annihilation sites in PEO-LiTFSI is similar to pristine PEO. It again confirms that reduction in positronium intensity and S-parameter cannot be attributed to positronium inhibition. The present study shows that resultant positron/positronium annihilation parameters can be directly attributed to the change in molecular packing or free volume structure of the solid state electrolytes.

Cu isotope fractionation during reduction processes in aqueous systems: evidences from electrochemical deposition

Redox processes are ubiquitous in Earth science, and redox transitions often lead to large fractionations of the stable isotopes of many transition metals such as copper. To get insights into the mechanisms of isotope fractionations induced by electrochemical processes, we examine the behavior of copper isotopes during the reduction reaction Cu2+ + 2e− = Cu0. All experiments have been conducted by applying a controlled current between the working electrode and the auxiliary electrode, i.e., the galvanostatic electrodeposition technique, in aqueous CuSO4 solutions. Controlling parameters were tested by varying electrolyte concentration (0.01–1 mol kg−1), stirring speed (0–500 rpm), current (0.1–0.5 A), time (35–600 s), and temperature (5–80 °C). In all cases, the plated Cu metal is enriched in the light isotope (63Cu) with respect to the solution. At room temperature, the Cu isotopic fractionation between the electroplated Cu and electrolyte is found to increase with electrolyte concentration and stirring speed, and to decrease with current and run duration. These trends can be interpreted by three competing processes: copper transport in the solution, kinetics of electrochemical reduction of copper ions and surface diffusion at the electrode, i.e., transport becomes important at low copper concentration, low stirring speed, high currents and large amount of copper precipitation. Copper isotope fractionation has a maximum near 35 °C, decreasing both towards higher and lower temperatures. In the temperature range of 35–80 °C, the dependence of temperature on isotope fractionation can be described by $$ \Delta^{65} {\text{Cu}}_{{{\text{Cu}}\left( 0 \right) - {\text{Cu}}({\text{II}}){\text{aq}}}} = - \left( {0.27 \pm 0.04} \right) \times 10^{6} T^{ - 2} + \left( {0.16 \pm 0.34} \right), $$ where ∆65CuCu(0)–Cu(II)aq (‰) represents the copper isotopic composition differences between the product (electroplated copper) and the reactant (electrolyte solution, CuSO4(aq)), and T is the temperature in K. At low temperature (down to 5 °C), a noticeable deviation from this trend suggests a change in the controlling mechanism, i.e., transport in the solution becomes important. Our findings are best explained by a two-step reduction process including reduction from Cu(II) to Cu(I) and a subsequent reduction of Cu(I) to Cu(0). The good agreement of our high-temperature data with the results from Ehrlich et al. (2004), who used a different experimental approach to precipitate Cu(I) mineral from CuSO4 solution, implies that transformation of Cu(II) to Cu(I) dominates the isotope fractionation observed during electrochemical reduction of Cu(II) to Cu(0). These findings support that copper isotopes can be used as effective tracers of redox processes. They may have implications to processes in hydrothermal systems and the formation of ore deposits, e.g., volcanic-hosted massive sulfides, as well as to processes in near surface aquatic environment and related supergene processes.

Experimental Validation of the Transmission Line Model Via Impedance Spectroscopy of an Ordered Array on Porous Carbon Electrode

In this paper, the interface of a porous substrate with an arranged and well-defined geometry is investigated for the impedance response with different electrolytes by using the well known electrochemical technique of Electrochemical Impedance Spectroscopy (EIS). Based on De Levie's Transmission Line Model (TLM) for porous electrodes a modification has been done in the model by incorporating the CPE behavior to reflect the non-ideal behaviour for real electrodes at low frequencies. The geometry used to validate the proposed model was fabricated from a smooth Glassy carbon electrode (GCE) surface to correlate the EIS response to that obtained by the model. The desired array was obtained by using laser micromachining tool on a glassy carbon substrate with a goal to study the influence of different electrolytes, varying electrolyte concentrations, the morphological features of pore- depth and radii of pore as well as different pore shapes. The outcomes of the experiments carried out on the in-house setup are compared with the MATLAB simulated results of the impedance. The transition from resistive to capacitive behavior is found to be shifted to lower frequency side and TLM behavior at high frequencies is more apparent as the concentration is increased. For the same concentration, different electrolytes are found to behave differently due to varying conductivity and ion sizes. In all the experiments carried out the impedance response showed CPE (constant phase element) behavior at the low frequency end while for high frequencies the nature/appearance of TLM behavior is found to be dependent on the pore depth to radius ratio. In addition the concept of time constant is also dealt with which exhibits differences for different geometries of the pores on glassy carbon. The results suggest that the GC electrode engaged here could provide a tractable model for studying electrochemical impedance behavior of porous electrodes without using any surface area analysis or pore volume analysis technique. Keywords: Transmission Line Model, Glassy carbon, EIS, MATLAB

Experimental studies of interfacial phenomena on innovative carbon nanomaterials in aqueous electrolyte solutions

Interfacial phenomena on the surface of carbon nanomaterials (CNM) in aqueous solutions in the presence of surfactants were studied experimentally and theoretically. The hydrodynamic radius and electrokinetic potential of particles of the dispersed phase and the concentration of functional groups on the CNM surface were found. The points of zero charge of solutions of the dispersed phase were measured, and so were their shifts with varying electrolyte concentrations, which make it possible to determine the pH values at which the CNM particles are electrically neutral.