In situ measurement of conductivity and active surface area of porous electrodes by the current step method—I. Theory

If the electrical conductivity of a plate-shaped porous electrode immersed in an electrolyte is measured by the four-point d.c. method using a current step, the potential drop, Δφ₁, is a function of the time, t. The course of this function can be derived theoretically by solving two simultaneous partial differential equations describing the potential distribution in both the solid and liquid phases. It is shown that the Δφ₁ − t dependence permits not only to determine the conductivity of the electrode but also to estimate its true surface area and the conductivity of the pore electrolyte.     

Effect of flow rate and constant operating current on the behaviour of a recirculating electrochemical reactor system

A theoretical analysis is presented for the current-time, voltage-time and power-time relationships and for the energy requirement and process time for the cathodic extraction of copper in a porous flow-through electrode operated as a batch recirculation system under galvanostatic conditions. The anode reaction considered is O₂ evolution and the secondary cathode reaction is H₂ evolution. The copper extraction is under diffusion control, plug flow is assumed in the reactor and perfect mixing in the reservoir. Comparison is made with the case of operation at the limiting current throughout the process duration.     

Flow-through electrodes composed of parallel screens

The electrolysis of 2.0 × 10^?3 M K₃Fe(CN)₆ in 1 M KCl in water at room temperature (22°C) has been investigated on flow-through porous electrodes composed of parallel platinum screens. The effect of changing the number of screens on the limiting current and the limiting degree of conversion of substrate has been studied. The results obtained confirm the previously derived model of limiting current on porous electrodes under conditions of solution flow. The experimental results have also been presented in form of mass transfer correlations for Sherwood number and j_D-factor. The potential drop in the electrode composed of six screens has been measured and compared with a theoretical equation. The effect of error in the determination of flow rate on the accuracy of presentation of experimental data is discussed.     

Characterization of oxidized reticulated vitreous carbon electrode for oxygen reduction reaction in acid solutions

The oxidation of reticulated vitreous carbon (RVC) and its impact on the oxygen reduction reaction (ORR) in H₂SO₄ solutions has been studied. The results are compared with that of a planar glassy carbon (GC) electrode. The oxidation process was characterized by using different electrode configurations, GC (planar) and RVC electrodes both with flooded (batch process) and flow-through assembly. Cyclic voltammetry, potentiodynamic and rotating ring-disk electrode voltammetry were used for the characterization of the ORR. Anodically oxidized GC and flooded RVC are similar in that the ORR on both electrodes gave a more defined limiting current plateau. For the flow-through porous electrode, the oxidation process caused a distribution of the oxidation extent within the bed thickness, as evident from the SEM images, and only about half of the porous electrode was utilized in the oxidation process. X-ray photoelectron spectroscopy (XPS) measurements confirmed the above distribution and a gradient of the oxygen-to-carbon ratio was obtained within the porous bed. Oxidation of RVC led to an enhancement of its electrocatalytic properties towards ORR. H₂O₂ production was tested at the oxidized RVC from flowing acid solutions. The oxidation of RVC resulted in higher current efficiencies and higher outlet concentrations of the H₂O₂ acid solutions.     

The behaviour of a fixed bed porous flow-through electrode during the production of p-aminophenol

The behaviour of a fixed bed porous flow-through monel electrode has been investigated for the reduction of nitrobenzene to p-aminophenol in an aqueous acidic electrolyte and under essentially activation controlled conditions. Chemical yields have been determined experimentally as well as calculated from polarisation curves for a plane electrode. Potential and current distributions have been interpreted by means of a simple one-dimensional model.     

Kinetics of oxygen reduction at composite electrodes with controlled three-phase boundaries by patterning YSZ column

The oxygen reduction mechanism was investigated at the porous LSM-patterned YSZ composite electrode by employing the ac-impedance spectroscopy and the potentiostatic current transient (PCT) technique. For this purpose, the dense YSZ pellet was patterned by a laser beam, and was then coated with the LSM slurry. The length of three-phase boundaries (TPBs) per unit area l_TPB was effectively controlled by varying the width of the YSZ column. From analyses of the ac-impedance spectra and the cathodic PCTs obtained from the electrodes based upon the modified transmission line model (TLM), it was first experimentally confirmed that the effective migration length l_m decreased with increasing l_TPB under the mixed migration and charge-transfer control. Secondly, as the value of l_TPB increases, the charge-transfer resistance R_ct is decreased to a more extent but the ion migration resistance R_i is reduced to a less extent. Finally, from a comparison of the cathodic PCTs measured on the porous LSM-YSZ composite electrode to those measured on the porous LSM-patterned YSZ composite electrode, the oxygen reduction kinetics at that porous composite electrode was discussed in terms of the steady-state current density i_st and the time to reach the steady-state current density t_st.     

Study of reversible electrode processes with unsymmetrical cyclic reciprocal derivative chronopotentiometry

In this paper, the theoretical study of cyclic reciprocal derivative chronopotentiometry (CRDCP) for totally reversible electrode processes with symmetrical and unsymmetrical programmed currents is presented. The main viewpoints are: (1) for symmetrical programmed current, the amplitudes of the successive currents are the same, i.e. I(t) = (−1)ⁱ⁺¹I₀, whereas the transition times of each current step, τ_i, have different values. The properties of the dt/dE-E curves corresponding to each current cycle also differ a lot. (2) For unsymmetrical programmed current proposed in this work for the first time, the applied current successively reversed at each transition time steps to different amplitude. The use of this unique programmed current is advantageous versus symmetrical programmed current since the transition times obtained are equal to each other anticipatively, and the dt/dE-E curves also coincide with each other for the successive cycles. In this case, the results obtained in CRDCP are almost similar to those of cyclic voltammetry (CV). The characteristic parameters obtained in the dt/dE-E curves in both cases are quantitatively related to the species concentrations adjacent to the electrode surface and afford simple diagnosis criteria to characterize the reversibility of electrode processes. Properties of reversible electrode processes have also been further studied through the use of more than one current cycle.     

Double-layer capacity measurements as a method to characterize porous fuel cell electrodes

Growing interest in porous Teflon-bonded carbon and catalyst carbon electrodes has resulted in increasing research efforts to improve performance and lifetime of these electrodes. The impedance method is demonstrated as a useful instrument to investigate these semihydrophobic electrodes. The double-layer capacity, derived by rather simple calculations from impedance spectra, is correlated with important parameters such as Teflon content, fabrication pressure and drying temperature of the electrode as well as to the deterioration processes which are still limiting the lifetime of the electrode. Information needed for improving these electrodes can be deduced from C_D measurements. Further interpretation of the impedance spectra and calculation of kinetic data are possible with the aid of appropriate equivalent circuits.     

Limiting current on porous graphite electrodes under flow conditions

The flow electrolysis of a solution of 2.0×10^–3 M K₃Fe(CN)₆ in 2 M KCl in water on porous graphite electrodes is described. The electrodes have been composed of crushed graphite of three grain size ranges: 0.7÷1.0, 0.4÷0.7, and 0.25÷0.4 mm. Electrodes of three different heights have been investigated, and the solution flow rate ranged between 0.009 and 0.25 ml s^–1. The dependence of the logarithm of the limiting current and of the limiting degree of conversion on the logarithm of the flow rates is shown for the electrodes. The obtained experimental results are interpreted by means of the described earlier semiempirical model of the limiting current on flow-through porous electrodes.     

流通型碳纤维电极的电沉积行为

研究了电极构型对流通型碳纤维电极从ＡｕＣｌ－４稀溶液中电沉积金行为的影响．在顺流操作方式下，整个电极能够在极限电流条件下工作，利用电沉积分布曲线计算了传质系数与流速的关系式．逆流方式可以使电沉积分布更为均匀，从而提高整个电极的利用效率．利用数学模型对各种实验曲线进行了模拟，计算结果与实验结果相符．     

Passivation of porous carbon cathodes in lithium-thionyl chloride cells

The passivation of porous carbon cathodes in 1.8 M LiAlCl₄–SOCl₂ solution has been studied. The electrodes were galvanostatically discharged for a range of current densities and electrode thicknesses. The results show that the passivation of the cathode is controlled by the rate of diffusion of the LiCl product away from the electrode. This was shown by the ability of passivated glassy carbon to recover capacity for SOCl₂ reduction when allowed to stand in the electrolyte solution.The results show that there is a limiting cathode thickness beyond which additional carbon loading fails to increase the passivation time.     

Transient behaviour of porous electrodes with high exchange current densities

A theoretical analysis of the non-steady-state reaction distribution in a porous electrode with a high exchange current density is made by application of a simplified macrohomogeneous model for porous electrodes. The dimensionless transfer current for short times is given as an expansion in time, and two terms in a moderate time solution are also presented. At moderate times the current is split into time-dependent and time-independent parts, but this distinction is not apparent in the short time solution. For the limiting case of reversible kinetics, the analysis specifies the fraction of the superficial current density that will be distributed through the electrode. Treatments for the PbO₂ and LiAl electrodes are presented as examples.     

Investigation of the processes on iron and nickel electrodes at high corrosion current densities in solutions of high chloride content

Iron and nickel anodic dissolution has been examined by galvanostatic pulse measurements in solutions of high chloride concentrations with current densities in the range 1–100 A/cm². The transition time τ for the formation of salt layers was determined at different compositions of the electrolyte and current densities i. The relation of Sand viz i √τ = $\text{1}\text{2}$Z_MF √$\text{π}\text{D}$ (c_s — c) could be applied to the results. The increase of the electrode potential by some volts at t ? τ is explained by a poreless salt layer directly on the metal surface. This layer is non-electron-conducting and prevents the chlorine or oxygen evolution though the electrode potential is very positive. The measured additional ohmic drop U_Ω,p at t ≥ τ is explained by a porous precipitated salt layer formed on the poreless film. at times t > τ potential oscillations for both metals and electropolishing of nickel was observed.     

Mathematical Modeling of Operation of a Reactor with Flow-Through Three-Dimensional Electrodes at the Limiting Diffusion Current under Non-Steady-State Conditions

The results of mathematical modeling of metal electrodeposition in reactors with three-dimensional flow-through electrodes in a limiting diffusion current mode taking into account the nonsteady-state state of electrode and electrolysis were presented. An algorithm was developed for calculating the overall density of the current that passes through the electrode, which supports the electrode work in the limiting diffusion current mode throughout the metal electrodeposition process. A numerical study was performed to investigate the changes in the main characteristics of the process and electrode with time for the model electrochemical system.     

The rate of adsorption and adsorption displacement in the system hydroquinone-sulfite-iodide—II. Chronoamperometric measurements at a rotating gold disc electrode

The rates of adsorption of iodide and of adsorption displacement of sulfite by iodide were measured at a rotating gold disc electrode in solutions containing hydroquinone. The reaction was initiated by a potential step from a cathodic state of total desorption to an anodic state of adsorption and parallel hydroquinone oxidation. The anodic current of hydroquinone was measured as a function of time. In solutions of pH = 8.1 the adsorption of iodide was diffusion controlled. The current decreased linearly with time and iodide concentration; it suddenly reached its final value as soon as about $\text{1}\text{3}$ of the electrode surface was covered. In sulfite containing solution at pH = 10.5 with small additions of iodide, first the current decreased rapidly corresponding to the diffusion of sulfite which covered the electrode surface. In the measure as iodide appeared at the surface, the current increased up to a limiting value which could be used to measure the adsorption equilibrium of iodide in presence of sulfite. The rate of sulfite displacement is determined by the diffusion rate of iodide. However, the rate of increase of current as well as the maximum current of hydroquinone is proportional to c²_I?. It is assumed that hydroquinone is discharged the presence of adsorbed iodide via a bridge mechanism, the bridge consisting of about two iodide molecules.     

The kinetics of silver bromide film formation on the silver anode

The anodic potentiodynamic formation of an AgBr film on an Ag rotating disc electrode was studied in aqueous bromide solutions. As the electron transfer step (1) is intrinsically fast,

and as the film remains porous throughout its growth, the rate of film growth is limited by physical parameters such as ionic diffusion and migration in the solution.The anodic E/I curves for AgBr film formation were calculated quantitatively by computer on the basis of the following model of film growth. Film formation occurs initially by the nucleation of islands of film to a critical thickness, and then these islands spread laterally until only small pores remain between them. As these pores become small, the resistance of the solution within them becomes rate limiting (at the anodic current peak). After the peak, the concentration of bromide ions at the pore base falls to zero and current is then controlled by the diffusion of bromide ions into the lengthening pores of the film.This lateral spreading mechanism and the subsequent retention of the porous film morphology has been substantiated by comprehensive Scanning Electron Microscope investigations.     

Electrochemical absorption of hydrogen in transition metal oxides

The current response of an electrode which absorbs reactant on a potential step was computed by digital simulation and by Boltzmann substitution under assumption of diffusion coefficient being a linear function of concentration. The results were expressed as an correction factor α in Sand equation i = ?αF ΔC(D/πt)^{$\text{1}\text{2}$}. Similar results were obtained for admittance of such electrodes. Results were tested on absorption of hydrogen in sodium tungsten bronze Na_0.72WO₃. The potential step method is suitable for evaluation of concentration-potential-diffusion coefficient profiles.     

The zinc-chlorine battery: half-cell overpotential measurements

The voltaic performance of the zinc-chlorine battery was investigated by measuring the individual potentials of the Zn and Cl₂ electrodes versus a reference electrode. The overpotential at the Zn electrode is very small for both the charging and the discharging processes, and the use of a flow-through porous Cl₂ electrode is advantageous. Energy efficiency in the region of 65% has been achieved for a complete cycle. Tafel polarization data for both the zinc and the Cl₂/graphite electrodes was obtained using rotating zinc hemisphere and graphite disc electrodes. Exchange current densities and Tafel slopes are reported.     

Sonoelectrochemistry of molecular and colloidal redox systems at carbon nanofiber-ceramic composite electrodes

An electrically conducting composite electrode consisting of a ceramic paper substrate, which has been densely coated with carbon nanofibers of ca. 100 nm diameter, is employed in the presence of 24 kHz (8 W cm⁻²) power ultrasound emitted from a glass horn system. The carbon composite electrode remains stable during prolonged use in the presence of ultrasound. Sonovoltammetric limiting currents for the reduction of Ru(NH₃)₆³⁺, the oxidation of hydroquinone, and the reduction of colloidal hydrous iron oxide are reported. A comparison of sonovoltammograms obtained at a porous carbon nanofiber-ceramic composite electrode with those obtained at a conventional glassy carbon electrode shows that (i) the average mass transport limited current density at carbon nanofiber-ceramic composite electrodes is increased by approximately one order of magnitude, and (ii) due to the porous topography of the electrode surface, molecules remain resident within the diffusion layer for a longer period of time. Colloidal hydrous iron oxide is not reduced at conventional glassy carbon electrodes but can be reduced at the porous carbon nanofiber-ceramic composite electrode, presumably due to a more effective particle-carbon nanofiber electrode surface interaction.