Reduction of metal ions in dilute solutions using a GBC-reactor: Part I: Experimental study on the laboratory scale reduction of ferric ions

To reduce metal ions in dilute solutions a new type of electrochemical reactor has been developed: the GBC-reactor. This reactor consists of a gas diffusion electrode coupled with a packed bed electrode. The working principle of the reactor is based upon two main reactions: the catalytic oxidation of hydrogen gas in the gas diffusion electrode and the simultaneous reduction of metal ions on the packed bed electrode. This process occurs spontaneously without the need for an external power supply when the Gibbs free energy of the total reaction is negative. To study the behaviour of the reactor the reduction of ferric ions was used as a model system. The experimental results, obtained from varying a number of key process parameters, could be described using a very simple macroscopic rate equation. It is concluded that the reduction of ferric ions in a GBC-reactor is controlled by both mass transfer and electrochemical kinetics. To carry out scale-up and optimization studies a reactor model incorporating the potential distribution in the packed bed electrode is, however, necessary.     

Reduction of metal ions in dilute solutions using a GBC-reactor: Part I: Experimental study on the laboratory scale reduction of ferric ions

To reduce metal ions in dilute solutions a new type of electrochemical reactor has been developed: the GBC-reactor. This reactor consists of a gas diffusion electrode coupled with a packed bed electrode. The working principle of the reactor is based upon two main reactions: the catalytic oxidation of hydrogen gas in the gas diffusion electrode and the simultaneous reduction of metal ions on the packed bed electrode. This process occurs spontaneously without the need for an external power supply when the Gibbs free energy of the total reaction is negative. To study the behaviour of the reactor the reduction of ferric ions was used as a model system. The experimental results, obtained from varying a number of key process parameters, could be described using a very simple macroscopic rate equation. It is concluded that the reduction of ferric ions in a GBC-reactor is controlled by both mass transfer and electrochemical kinetics. To carry out scale-up and optimization studies a reactor model incorporating the potential distribution in the packed bed electrode is, however, necessary.     

Modelling of three-dimensional bipolar electrodes with irreversible reactions

The behaviour of an electrochemical reactor with three-dimensional bipolar electrodes for irreversible reactions is analysed. Copper deposition at the cathodic side and oxygen evolution at the anodic one were adopted as test reactions at the bipolar electrode, from an electrolyte solution with a copper concentration lower than 1000 mg dm⁻³, pH 2 and 1 M Na₂SO₄ as supporting electrolyte. A mathematical model considering the leakage current is proposed, which can represent the tendency observed in the experimental data related to cathodic thickness and potential at both ends of the bipolar electrode. High values of leakage current were determined, which restricts the faradaic processes to small thicknesses at both ends of the bipolar electrode. Likewise, the performance of the bipolar electrochemical reactor for the treatment of effluents is experimentally and theoretically examined. In this case, the conversion for copper removal was 90.1% after 480 min of operation with one bipolar electrode and 94.8% after 300 min of operation with two bipolar electrodes at a total current of 3 A.     

填充床电极反应器中典型有机电合成反应系统的选择性分析

典型的有机电合成反应,常伴有相互竞争的电极副反应和均相反应,增加电极极化以提高反应速率会使选择性明显降低。填充床电极具有大的内表面,能在相对低的极化下,达到较高的表观电流密度,有利于缓解反应速率和选择性之间的突出矛盾。本文对填充床电极微分反应器（PBEDR）中典型有机电合成反应过程进行了理论分析,重点在于床层内超电势横向分布对选择性的影响。建立了描述超电势分布的普遍化数学模型,归纳出表征电极极化和副反应影响的量纲1数μ和ω,并用ADM（Adomian’s decomposition method）对该非线性微分方程模型求解;所获得的逼近解代数表达式,可方便地计算不同参数下超电势分布对平均选择性的影响,而毋需反复求解模型微分方程。最后给出了PBEDR硝基苯电还原制对氨基苯酚选择性分析实例,优化了反应器特征尺寸（填充床电极厚度）。结果证明：理论计算与实验数据令人满意地一致。     

In-situ infrared spectroscopic studies of electrochemical energy conversion and storage

With their ability to convert chemical energy of fuels directly into electrical power or reversibly store electrical energy, systems such as fuel cells and lithium ion batteries are of great importance in managing energy use. In these electrochemical energy conversion and storage (EECS) systems, controlled electrochemical redox reactions generate or store the electrical energy, ideally under conditions that avoid or kinetically suppress side reactions. A comprehensive understanding of electrode reactions is critical for the exploration and optimization of electrode materials and is therefore the key issue for developing advanced EECS systems. Based on its fingerprint and surface selection rules, electrochemical in-situ FTIR spectroscopy (in-situ FTIRS) can provide real-time information about the chemical nature of adsorbates and solution species as well as intermediate/product species involved in the electrochemical reactions. These unique features make this technique well-suited for insitu studies of EECS. In this Account, we review the characterization of electrode materials and the investigation of interfacial reaction processes involved in EECS systems by using state-of-the-art in-situ FTIR reflection technologies, primarily with an external configuration. We introduce the application of in-situ FTIRS to EECS systems and describe relevant technologies including in-situ microscope FTIRS, in-situ time-resolved FTIRS, and the combinatorial FTIRS approach. We focus first on the in-situ steady-state and time-resolved FTIRS studies on the electrooxidation of small organic molecules. Next, we review the characterization of electrocatalysts through the IR properties of nanomaterials, such as abnormal IR effects (AIREs) and surface enhanced infrared absorption (SEIRA). Finally, we introduce the application of in-situ FTIRS to demonstrate the decomposition of electrolyte and (de)lithiation processes involved in lithium ion batteries. The body of work summarized here has substantially advanced the knowledge of electrode processes and represents the forefront in studies of EECS at the molecular level.     

Simultaneous reactions at disk and porous electrodes

Advances in electrochemical engineering are reviewed, and the methodology of the analysis of electrochemical systems is outlined. Examples illustrative of current research concern simultaneous reactions for flow-through porous electrodes and the more fundamental system of a rotating disk electrode. Here the undesirable side reaction is the formation of dissolved hydrogen, and the main reaction is the deposition of copper from sulfuric acid solutions. Distributions of reaction rate, concentration, and potential describe the detailed system behavior. The side reaction is responsible for the poorly defined limiting-current plateau on the disk electrode and provides a limit for the maximum flow rate at which good recovery can be achieved with the porous electrode.     

An experimental study of a single layer packed bed cathode in an electrochemical flow reactor

Experimental measurements are reported for a packed bed electrode consisting of a single planar layer of uniform copper plated spheres located between platinum anodes and restrained by two plane porous PVC diaphragms. Two mass transfer controlled reactions, namely the reductions ofm-nitrobenzene sulphonic acid and copper sulphate, were investigated and the electrochemical mass transfer data in the range 23 <Re < 520 correlated by the equationShSc ^?1/3 = 0-83Re ^0.56, the Reynolds and Sherwood numbers being defined in terms of a particle diameter. Variations of electrode potential throughout the bed were found to be small enough to ensure reaction selectivity in the system.     

Effect of contact resistance between particles on the current distribution in a packed bed electrode

A microscopic, modelistic approach was carried out to elucidate the electrochemical reduction of a nuclear waste solution in a packed bed electrode. The interfacial surface reactions within the packed bed were taken into account and the particle–particle contact resistance through oxide films was found to be big enough to effect the potential distribution throughout the bed. On the basis of equations developed here, the contribution of the resistance of the oxide film to the potential and current distribution throughout the bed was compared with the macroscopic homogeneous approach.     

Application of volumetric electrodes to the recuperation of metals in industrial effluents—II. Design of an axial field flow through porous electrodes

The potential drop occurring in the bed of the flow-through porous electrode (fpe) changes the local metal-solution potential difference (lpd). Thus it may destroy the considered electrochemical reaction specificity or limit the electrolytic cell efficiency.So, it's necessary to compute the fpe geometry (bed height, particles diameter) which satisfies the lpd for a chosen pair flow-yield.This study, realized in the case of a fixed bed built with very conductive particles, proposes a generalized equations system which permits the fpe design. It's possible to do the same for an undistinguished electrochemical reaction by introducing a characteristic number of this system. From these equations a design diagram is established.A discussion about the axial field electrode limits as performing reactor follows.     

Mathematical Modeling and Experimental Studies of the Joint Electrodeposition of Gold and Silver from Sulfuric Acid Thiourea Solutions on Flow-Through 3D Electrode Taking into Account Its Nonstationary State

Mathematical models of electrochemical processes in reactors with flow-through 3D electrodes for the electrodeposition of metals from a polycomponent electrolyte solution have been presented. The parameters of the process and electrode, such as the flow rate of the electrolyte, porosity, specific surface, and the specific conductivity of the electrode and solution, at each electrode point during electrolysis have been taken into account. The mathematical models and methods for determining the electrochemical and hydrodynamic parameters of multicomponent electrochemical systems have been described. The data have been used to calculate industrial electrochemical processes in flow-through 3D electrodes. The adequacy of these methods has been shown. The experimental and calculated data on the electrodeposition of gold and silver from sulfuric acid thiourea solutions to carbon-graphite fiber cathodes have been analyzed. The dependences of electrodeposition parameters on the electrolysis conditions, the initial properties of the electrode-solution system, and the concentration ratio of gold and silver in the solution taking into account the joint reduction of hydrogen ions and molecular oxygen have been examined. The current and resulting regularities of the processes have been revealed. The adequacy of the mathematical models for industrial joint electrodeposition of gold and silver from sulfuric acid thiourea solutions on flow-through 3D electrodes taking into account the side electrode reactions has been shown. The appearance of anode zones on a cathode polarized carbon fiber flow electrode during the deposition of metals from multicomponent systems has been considered.     

The reaction engineering of electrochemical cells—I. Axial packed bed electrodes

It is shown that the graphical method of solving the coupled equations for an endothermic reaction in a plug-flow, non-isothermal reactor can be extended to electrochemical reactions in three-dimensional electrodes. The method of solution is described and as a first example, it has been applied to a simple reversible reaction in an axial packed bed electrode. The sensitivity to various parameters has been examined. The method is ideally suited to computer-aided design procedures.     

填充床鼓泡电化学反应器行为特性研究──Ⅰ.丙烯阳极直接氧化的数学模拟

本文建立了填充床鼓泡电化学反应器的一维数模．该模型既考虑了反应动力学，也考虑了体系的基本传递特性．模拟结果可以描述床层的径向电势、电流和浓度分布．     

POLARIZATION BEHAVIOR OF PACKED-BED ELECTRODES: A COMPARATIVE STUDY OF EXPERIMENTAL AND MODELING RESULTS

The current distribution and overall polarization behavior of electrodeposition at a flow-through (packed bed or fluidized bed) electrode are modeled by means of a one-dimensional model involving a primary reacting species and a simultaneous side reaction. The model equations are solved by orthogonal collocation; the time and storage requirements compare favorably to those of finite-difference methods. Experimental data obtained using a packed-bed electrode are compared with the model predictions and various methods of fitting the data are compared.     

POLARIZATION BEHAVIOR OF PACKED-BED ELECTRODES: A COMPARATIVE STUDY OF EXPERIMENTAL AND MODELING RESULTS

The current distribution and overall polarization behavior of electrodeposition at a flow-through (packed bed or fluidized bed) electrode are modeled by means of a one-dimensional model involving a primary reacting species and a simultaneous side reaction. The model equations are solved by orthogonal collocation; the time and storage requirements compare favorably to those of finite-difference methods. Experimental data obtained using a packed-bed electrode are compared with the model predictions and various methods of fitting the data are compared.     

Synergistic production of graphene microsheets by simultaneous anodic and cathodic electro-exfoliation of graphitic electrodes in aprotic ionic liquids

Electrochemically mediated exfoliation of graphite is a promising green and high throughput approach for production of graphene sheets (GNs). Previous research focused mostly on either anode or cathode exfoliation due to restrictions imposed by the investigated intercalating ions and insufficient consideration given to the design of the electrochemical cell. Consequently, in single graphite electrode studies, at the non-graphitic counter-electrode (e.g. Pt), unwanted electrode reactions such as gas evolution and electrolyte decomposition take place, leading to significant energy and chemical losses. Here, we report the simultaneous anodic and cathodic GN production in two types of electrochemical cells (undivided and divided) using aprotic electrolytes containing ionic liquids (ILs). We demonstrate a synergistic exfoliation effect when the iso-molded graphite anode and cathode are subjected to a constant cell potential, generating up to 3 times higher exfoliation yields compared to single-electrode studies on each side (∼6-fold improvement in total). Thorough characterization of the products collected from both electrode compartments confirmed the production of ultrathin GNs (<5 layers). The cathodic exfoliates were almost exclusively composed of GNs; whereas among the anodic products, in addition to the majority GNs, we detected traces of other morphologies such as nanoparticles, nanotubes, and larger rolled sheets.     

Macrokinetics of processes in hydrogen-oxygen fuel cells and electrolysers

A theory of the macrokinetics of electrode reactions in gas-liquid electrodes has been developed for the case of electrochemical reactions occurring at the electrodes of an hydrogen-oxygen fuel cell or of a water electrolyser. This theory takes account of the convective flows arising at electrodes of this type due to electrochemical reactions, which exert strong influences on transport phenomena. The concepts developed have been used for calculations of the large-scale macrokinetics in hydrogen-oxygen fuel cells and electrolysers with capillary membranes, which include mass transfer processes in the electrodes, the capillary membrane and in the boundary gas layers adjoining the electrode, as well as the flooding and drying of porous electrodes due to changes in the working conditions. Account is also taken of the self-regulation of the supply and removal of water vapour.     

Enhanced Na-storage properties of O3-type NaNi0.5Mn0.5O2 cathodes by doping and coating dual-modification strategy

Due to their high theoretical specific capacities and good structural compatibility, O3-layered oxides are considered as high-performance cathode candidates for sodium-ion batteries. However, serious side reactions and structural degradation during long-term cycling have greatly hindered their practical applications. Herein, O3-type NaNi_0.5Mn_0.5O₂ (NNMO) was prepared by partially substituting Ni²⁺ with Cu²⁺ and inducing CuO surface coating to enhance the structural stability without sacrificing the high capacity. In Particular, Cu²⁺ incorporated at a molar ratio of 0.10 greatly improved the electrochemical performance of pristine NNMO. The NaMn_0.5Ni_0.4Cu_0.1O₂ electrode exhibited a capacity retention of 64.5% after 300 cycles at 0.5 C. Microscopic and spectral studies revealed that the CuO coating layer suppressed detrimental side reactions upon cycling. Moreover, the bulk doping of Cu²⁺ ultimately enhanced the structural stability by mitigating irreversible phase transitions. Hence, the integration of bulk Cu²⁺ doping and CuO surface coating contributed to the increased structural stability and electrochemical performance of NNMO. The doping and coating dual-modification strategy established in this work effectively suppressed the side reactions and structural degradation of NNMO, and this work provides new insight into the effective design of electrode microstructures for use in high-performance batteries.     

Fluctuation analysis in electrochemical engineering processes with two phase flows

Fluctuation analysis in electrochemical systems appears to be a suitable method for obtaining information on the system dynamic behaviour, especially when the voltage or current fluctuations are due to elementary events on the electrode at a semi-macroscopic scale, for example growth and detachment of bubbles on a gas-evolving electrode or contacts between charged particles in a fluidized or circulating bed reactor, or pits in localized corrosion. Therefore this situation is largely encountered in electrochemical engineering processes with two phase flows. By analysing the current (voltage) fluctuations at constant potential (current) and/or the electrolyte resistance fluctuations, this technique provides quantitative parameters which are often inaccessible by traditional deterministic techniques, steady state or not, which deal with time-averaged signals. The technique also leads to a better understanding of the elementary processes on the electrode. Two examples are given: the first concerning a gas-evolving electrode and the second a circulating bed electrode. This paper was presented at the International Workshop on Electrodiffusion Diagnosis of Flows held in Dourdan, France, May 1993.     

The role of diffusion of ionic species and of interdiffusion in the solid state in electrolytic deposition processes from molten salts

Technical applications of electrocoating from molten salts involve the study of electrochemical reactions which are under the control of diffusion of electroactive species. Chemical reactions and ionic transport in the melt have previously been examined. Now the role of interdiffusion in the solid state is considered in the electrolytic deposition processes for coating metal and alloy substrates. The rate of diffusion into the substrate is the rate-controlling process. The study of the kinetics of incorporation by pulse electrochemical techniques is described.It is shown that the classical treatment used to describe the diffusion-controlled phenomena has to be modified to take account of the volume change of the electrode due to the metal incorporation. A mathematical analysis is presented to include the perturbation resulting from the bojndary motion which occurs during electrolysis at constant potential or at constant current (galvanodiffusion). Examples of application of this treatment in recent studies of metalliding reactions are given and discussed in relation to the experimental case of aluminiding iron.This paper was presented at a workshop on the electrodeposition of refractory metals, held at Imperial College, London, in July 1985.     

Hydrodynamic modelling of the liquid-solid behaviour of the circulating particulate bed electrode

Experimental and modelling investigations of the hydrodynamics of the internal flow structure in the circulating particulate bed electrode (CPBE) are reported. The CPBE, a hybrid between an expanded packed bed and an entrained/fluidized bed, is particularly well suited for many electrochemical applications such as metal recovery and pollution treatments for metal containing effluents. This study deals with the fundamental hydrodynamics and particle dynamics of the CPBE. A mathematical model of the CPBE has been developed which successfully describes the motion of the particles and the fluid in the bed. It is shown that many of the flow characteristics of the circulating bed can be predicted using fundamental data. The validity of the proposed model was demonstrated by comparing predictions to experimental observations of several bed characteristics under various operating conditions.