Influence of mass transport on the performance of carbon gas-diffusion air electrodes in alkaline solution

The effect of mass transport on the electrochemical behaviour of carbon gas-diffusion air electrodes in alkaline solution was investigated on the basis of ΔE(i) curves. These curves are obtained by subtraction of potential values for an electrode operating with airE _air(i) from potential values for the same electrode operating with pure oxygenE _oxygen(i) at the same current densityi. Three different regions on these curves connected with different modes of mass transport are recognized. A model of the gas-diffusion air electrode which takes into account the diffusion of the gas, diffusion of the dissolved gas, electrochemical reaction and IR drop is used to explain the experimental results.     

Mechanism of hydrogen oxidation on a platinum-loaded gas diffusion electrode

The mechanism of hydrogen oxidation on a platinum-loaded gas-diffusion electrode has been investigated. Experimental potential–current curves, especially in the low overpotential range, have been measured for H2–N2 mixtures with a small content of hydrogen and for pure H2. Theoretical relations have also been presented. Comparing the experimental and theoretical relations, it is concluded that the hydrogen oxidation occurs according to the Volmer–Tafel mechanism. The reactivity of the electrode has a large effect on the kinetic parameters for hydrogen oxidation. The limiting current is determined by diffusion of hydrogen for a very reactive gas diffusion electrode and by the Tafel reaction for a gas diffusion electrode with a low reactivity. The transfer coefficient for the Volmer reaction V is 0.5 and i0,V/i0,T 0.1 for a very reactive gas diffusion electrode. V increases and i0,V/i0,T ratio decreases with decreasing reactivity of the gas diffusion electrode.     

Anomalous behaviour of hydrogen extraction from hydride-forming metals and alloys under impermeable boundary conditions

Hydrogen injection into and extraction from hydride-forming electrodes has been routinely investigated under the assumption that the electrode is homogeneous in structure and hydrogen transport through such electrodes is purely controlled by hydrogen diffusion. However, various kinds of abnormal behaviours in hydrogen transport, which could hardly be explained in terms of the diffusion-control model, have been quite frequently reported by many researchers. This review provides a comprehensive survey of the anomalous behaviours of hydrogen transport observed in such hydride-forming electrodes as Pd and metal-hydrides, with particular emphasis on hydrogen extraction under the impermeable boundary conditions during the potentiostatic current transient measurement involving the potential stepping. After a brief discourse on the conventional diffusion-control model, the topics related to the boundary conditions at the electrode surface during hydrogen extraction are extensively reviewed. In particular, it is shown that the diffusion-controlled constraint should be no longer valid at the electrode surface for hydrogen extraction in case hydrogen diffusion is influenced by either the interfacial charge transfer reaction or the hydrogen transfer reaction between adsorbed state on the electrode surface and absorbed state at the electrode sub-surface. Subsequently, the atypical behaviours of current transient due to hydrogen diffusion in the presence of traps and in the coexistence of two hydride phases are treated in detail. Each of the hydrogen extraction models suggested is discussed with the aid of the anodic current transients numerically calculated based upon the theoretical electrode potential curve, and then it is exemplified by hydrogen extraction from Pd and metal-hydrides in aqueous solutions.     

Evaluation of Ag-based gas-diffusion electrode for two-compartment cell used in novel chlor-alkali membrane process

The oxygen-depolarized cathode has become attractive by reducing energy consumption and also CO₂ emissions. The initial gas-diffusion electrode was for a three-compartment cell in which the alkaline electrolyte was separated from the oxygen feeder. During the electrolysis, the electrolyte penetrated into the hydrophobic gas-diffusion layer. Thus the pores in the diffusion layer were flooded with the electrolyte and the performance decreased.We proposed a new gas-diffusion electrode (GDE) having a large pore radius for a two-compartment cell in which the GDE was pressed on the membrane, and the electrolyte was transported through the GDE. The previous electrode was used for a three-compartment cell, which contained the anolyte, catholyte and oxygen-feeder compartments. First, a model analysis was made to simulate the cathodic overpotential curves of the GDE's for the two-compartment and three-compartment cells. Experiments were made using a half cell for the cathodic polarization curve and a zero-gap cell for testing the increased durability. The two-compartment cell with Ag–CC (carbon cloth) had the highest performance among the GDE's. However, the decrease in performance during a longer period, such as 600 days, was considered to be due to catalyst aggregation and not flooding the pores based on the cathodic potential curve and the SEM images.     

Electrochemical degradation of 2,4-dichlorophenol on a palladium modified gas-diffusion electrode

Pd/C catalyst was prepared by hydrogen reduction method and used for the Pd/C gas-diffusion electrode. It was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) techniques. The electrochemical degradation of 2,4-dichlorophenol was investigated in a diaphragm electrolysis system, feeding firstly with hydrogen gas then with air, using the Pd/C gas-diffusion electrode and the carbon/polytetrafluoroethylene (C/PTFE) gas-diffusion electrode as a cathode, respectively. The results indicated that the self-made Pd/C gas-diffusion cathode can not only reductively dechlorinate 2,4-dichlorophenols by feeding hydrogen gas, but also accelerate the two-electron reduction of O₂ to hydrogen peroxide (H₂O₂) by feeding air. Therefore, both the removal efficiency and the dechlorination degree of 2,4-dichlorophenol reached about 100% after 80 min, and the average removal efficiency of 2,4-dichlorophenol in terms of total organic carbon (TOC) exceeded 76% after 160 min by using Pd/C gas-diffusion cathode, which were better than that of the C/PTFE gas-diffusion cathode. The analysis of high-performance liquid chromatography (HPLC) identified that 4-chlorophenol, 2-chlorophenol, and phenol were the dechlorination products, and hydroquinone, benzoquinone, maleic, fumaric, crylic, malonic, oxalic, acetic and formic acids were the main oxidation intermediates. A reaction scheme involving all these intermediates was proposed.     

Transient measurements at the wall-jet ring disc electrode

Theory is presented for the wall-jet ring disc electrode (WJRDE) which predicts the current transient at the ring resulting from a potential step at a WJRDE disc for the case where the ring is potentiostatted so as to reverse the reaction which occurs at the disc. The shape of such transients, in which double-layer charging effects are minimal, is revealed to be very sensitive to differences in the diffusion coefficient of the electroactive species which reacts at the disc and that of the corresponding product. Accordingly such measurements are recommended for the study of complex electrode processes in order to separate mass transport parameters (diffusion coefficients) from homogeneous kinetic phenomena.     

Reduction of metal ions in dilute solutions using a GBC-reactor Part II: Theoretical model for the hydrogen oxidation in a gas diffusion electrode at relatively low current densities

The GBC-reactor is based on the combination of a gas diffusion anode and a porous cathode. A theoretical model for gas diffusion electrode, valid at relatively low current densities, is derived. This is based on the pseudohomogeneous film model including an approximation of the Volmer–Tafel mechanism for the hydrogen oxidation kinetics. Results show a severe mass transfer limitation of the hydrogen oxidation reaction inside the active layer of the gas diffusion electrode, even at low current densities. Empirical formulae are given to estimate whether leakage of dissolved hydrogen gas into the bulk electrolyte occurs at specific process conditions. A simplified version of the model, the reactive plane approximation, is presented.     

Mass transport in a hydrogen gas diffusion electrode

Experimental data are presented concerning the diffusion-limited current density for hydrogen oxidation in a gas diffusion electrode (GDE) under various conditions. These current densities were obtained using mixtures of hydrogen and inert gases. To elucidate the dependence of the overall mass transport coefficient on the gas phase diffusion coefficient and the liquid phase diffusion coefficient of the hydrogen, a simplified model was derived to describe the transport of hydrogen in a GDE based on literature models. The GDE consists of a hydrophobic and a hydrophilic layer, namely a porous backing and a reaction layer. The model involves gas diffusion through the porous backing of the GDE combined with gas diffusion, gas dissolution and reaction in the reaction layer of the electrode. It was found that the transport rate of hydrogen under the experimental circumstances is determined by hydrogen gas diffusion in the pores of the porous backing, as well as in the macropores of the reaction layer. Diffusion of dissolved hydrogen in the micropores of the reaction layer, through the liquid, is shown to be of little significance.     

Rotating nanoparticle array electrode for the kinetic study of reactions under mixed control

A method for the determination of the intrinsic electrocatalytic activity of metal nanoparticles for reactions under mixed diffusion-activation control is proposed. The working electrode consists of a nanoparticle array supported on a rotating disc of an inert conducting substrate. This electrode configuration reduces significantly the contribution of the reactant diffusion to the experimental current-potential curves and consequently, the kinetic parameters of the reaction can be evaluated more accurately. A model was developed for the diffusion process in the proposed configuration, which includes a parameter related to the degree of dispersion of the nanoparticles on the electrode surface (active area factor). It allows one to evaluate changes in the current-potential plot under conditions of constant particle diameter, which is essential for the appropriate analysis of the electrocatalytic activity of nanoparticles. On this basis the dependences of the current and current density for the hydrogen oxidation reaction on overpotential, rotation rate, particle size and active area factor were derived. The validity of these expressions was verified through the analysis of experimental results.     

Effect of mass transfer on the current distribution in monopolar and bipolar electrochemical reactors with a gas-evolving electrode

The current distribution in an electrochemical reactor with vertical parallel-plate electrodes was experimentally determined. The research was performed with monopolar and bipolar electrodes. The reactor has a gas-evolving electrode and at the counter electrode an electrochemical reaction with combined diffusion and charge-transfer kinetic control, takes place. Therefore the kinetics at the counter electrode are influenced by the bubble-induced convection and by the forced convection of the electrolyte. These reactors are found in many electrochemical processes, for example, electrowinning of metals and electrosynthesis. The test reactions were hydrogen evolution at the cathode and the anodic oxidation of sulphite to sulphate from basic solutions. The current distribution shows a minimum at a distance of approximately six times the equivalent diameter of the reactor from the inlet region. This minimum is a consequence of the interaction between forced convection and the bubble-induced convection, which shifts the mass transfer coefficient of the anodic reaction along the reactor. The effects of the total current, the volumetric electrolyte flow rate and the metal phase resistance on the current distribution are also analysed. The experimental current distribution data are compared with theoretical expectations and good agreement is found.     

A novel approach for computing tertiary current distributions based on simplifying assumptions

A novel yet efficient method for the computation of simplified tertiary current density and surface concentration distributions in electrochemical processes is presented. The method is rooted in the important physiochemical property that the activation potential is constant and uniform for given electrode material during the electrolysis. The technique is attractive because it involves a single iterative procedure against the conventional doubly iterative procedure. The initial assumption of current distribution along the electrode is also not necessary, as it involves only an assumption of a suitable power series to solve steady state laminar convective diffusion. Accordingly the method is relevant only for electrodes of constant activation polarization, but this holds good for situations where the electrode configurations are such that the primary current density distribution is almost uniform and for situations where the Wagner number is high. To illustrate the utility of the technique the procedure is applied to some realistic problems encountered in electrochemical engineering such as the current distribution either in plane-parallel plate electrode with electrolyte flowing between them or a moving electrode with the electrolyte stationary.     

Partially immersed gas-diffusion electrodes

A new type of partially immersed carbon gas-diffusion electrode with a sandwich-like structure consisting of two active layers each side of the gas-supplying layer is described. The current collector grid is embedded into the electrically conductive gas-supplying layer. A theoretical model of the partially immersed electrodes is proposed and experimentally confirmed. The model can be used for the theoretical prediction of the steady state behaviour of partially immersed electrodes from experimental data obtained with non-immersed gas-diffusion electrodes. An experimental zinc-air battery of 1900 A h capacity with the new type of partially immersed carbon air gas-diffusion electrodes has shown a specific energy density of 250 W h kg^–1 and 420 W h 1^–1 at a current density of 0·5 mA cm^–2.     

The cycling behaviour of zinc electrodes in a nickel oxide-zinc accumulator

The behaviour of porous zinc electrodes was investigated by monitoring the potential distribution along the surface of the electrode during charging and discharging in a nickel oxide-zinc battery. Impedance measurements of the zinc electrode were taken as a function of state-of-charge and of the cycle number. The composition of the electrode was varied with HgO and PbO additives. Shape change was observed for all electrodes. The observed potential distribution has led to the tentative explanation that the shape change phenomenon is caused by diffusion of zincate along the surface.     

Numerical simulation of the current, potential and concentration distributions along the cathode of a rotating cylinder Hull cell

Numerical simulations of the non-uniform current, potential and concentration distributions along the cathode of a rotating cylinder Hull (RCH) cell (RotaHull^® cell) are performed using finite element methods. Copper electrodeposition from an acid sulfate electrolyte is used as a test system. Primary, secondary and tertiary current distributions are examined. The importance of controllable and uniformly accessible hydrodynamics along the length of the RCH cathode is demonstrated. Charge transfer kinetics are described by a Tafel approximation while mass transport is considered using a Nernstian diffusion layer expression. The effects of applied current density and electrode rotation speeds on the distribution of potential and current along the RCH cathode are investigated. An expression of the primary current distribution and a dimensionless mass transport correlation facilitate comparisons with the simulations.     

A study of current distribution in a DEM cell during bromate formation

The anodic oxidation of potassium bromide to potassium bromate is performed in an undivided cell with hydrogen evolution the major reaction at the counter electrode. The cell used is a dished electrode membrane (DEM) cell. Current density distribution, measured using a segmented electrode, shows a variation in the two principle dimensions; along the length of the electrode and over the width of the electrode. Current densities are highest at the electrolyte flow inlet and also exhibit a localized maximum along the electrode length. The variation in current density is due to the influence of electrolytic gas evolution on the effective electrolyte conductivity and mass transport and also due to the change in shape of the dished electrode, which influences mass transport, electrical potential field and flow at the cell inlet and exit.     

Electrochemical reduction of carbon dioxide and the effect of the enzyme carbonic anhydrase 11 on iron corrosion

The corrosion of pure iron in carbon dioxide saturated solutions at low temperatures has been investigated in this study. Both dc polarisation and ac impedance techniques were used in conjunction with a rotating disc electrode to investigate the reaction sequence leading to hydrogen evolution at 30°C. The results indicate that protons from the low pH solution, the bicarbonate ion and the undissociated carbonic acid molecule contribute to the reaction sequence leading to eventual hydrogen evolution. The corrosion mechanism involves both activation and diffusion processes. A rotating disc electrode in combination with ac techniques and the enzyme carbonic anhydrate II, EC 4.2.1.1, were used to characterise the diffusion effects. A chemical–electrochemical cathodic reaction mechanism is proposed.     

Effet du pH en milieu basique sur la cinetique d''oxydation de l''hydrogene a une electrode de platine

The polarization curves of hydrogen oxidation in several alkalines media are determined by the rotating disc method. The electrode are made of smooth platinum or platinum covered by a thin layer of black platinum. For the total range of pH we show that the limiting process is always the diffusion of dissolved hydrogen to the electrode.     

采用常规条形流场的质子交换膜燃料电池阴极数值模拟(Ⅱ)电池性能影响因素的分析

利用已建立的数学模型考察了阴极扩散特性参数、阴极流场几何参数、电极电阻、催化剂活性、流道上O₂浓度变化对H₂-Air PEMFCs性能的影响.计算表明,增大氧有效扩散系数、减小扩散层厚度可增大电池的工作电流密度;提高氧还原反应交换电流密度、减小电极电阻、优化流道尺寸可改善电池性能;沿反应气体流动方向逐渐增加MEA的催化剂负载量可提高电流密度分布的均匀性.     

Tungsten carbide-based electrochemical sensors for hydrogen determination in gas mixtures

An amperometric study of gas-diffusion electrodes (GDE) catalysed by two types of tungsten carbide, WC₍₁₎ and WC₍₂₎, which differ considerably in their specific surface area (0.5 and 6 m² g^–1), was carried out. The H₂–air gas mixture (H₂ 1–4%) measurements show that for this range of hydrogen concentration the hydrogen limiting diffusion current (i _d(H2)) may be attained so that a curve of limiting current density against hydrogen concentration can be obtained. The response and stability of the electrode performance were compared to those of platinum catalysed GDEs. The most promising for use in amperometric hydrogen sensing is the WC₍₁₎ catalyst of small specific surface area. Electrodes catalysed with this catalyst show inferior response time in comparison to electrodes catalysed with the other two catalysts (WC₍₂₎ and Pt) but their overall stability is much better.