Impedance of a rotating disc electrode with a reversible reaction

This paper presents a model of electrode impedance for the case of a fast reversible reaction. The various contributions of the impedance were analysed with particular emphasis on the charge transfer resistance: this resistance was shown to be also dependent on mass transfer phenomena. For the case of significant mass transfer control,the diameter of the high-frequency loop increases with the absolute value of the overpotential. The various physicochemical parameters involved in the expression for impedance were determined through previous measurements. The impedance model was validated by experimental measurements carried out with the hexacyanoferrate (II)–(III) couple ona Pt RDE.     

A thermodynamic estimation of the minimum concentration attainable in a flow-through porous electrode reactor

The minimum concentration attainable in a porous flow-through reactor is estimated by applying the thermodynamics of electrochemical cells with a knowledge of the maximum reactor operating potential. This predicted concentration is the equilibrium wall concentration at the back of the reactor and is qualitatively compared to the experimentally measured minimum average bulk values observed by various authors for the deposition of copper, silver, lead, and mercury, and for the oxidation of ferrous ions. It is suggested that a knowledge of the current versus reactor operating potential will elucidate the lower limits observed for any metal system. In particular, the case of antimony removal is discussed.     

Calculation of pH near the surface of an electrode covered with a porous film

The pH adjacent to the surface of a film-covered electrode is generally a difficult quantity to measure. Consequently, we have developed a model to calculate the concentration distribution of a reacting species in a porous film on an electrode surface. Reactions at both the electrode surface and at the film surface within the pore structure are considered. Effects of pore geometry, diffusion rate and reaction rate on concentration distribution are computed from a one-dimensional model. Use of the model in predicting hydroxide ion concentration near the surface of a passivating zinc electrode is demonstrated.     

Theoretical analysis of transient solution phase concentration field in a porous composite electrode with time-dependent flux boundary condition

Journal of Applied Electrochemistry - Energy conversion and storage in a Li-ion cell involves multiple closely coupled transport processes, such as species diffusion through solid and solution...     

Arsenic species interactions with a porous carbon electrode as determined with an electrochemical quartz crystal microbalance

The interactions of arsenic species with platinum and porous carbon electrodes were investigated with an electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry in alkaline solutions. It is shown that the redox reactions in arsenic-containing solutions, due to arsenic reduction/deposition, oxidation/desorption, and electrocatalyzed oxidation by Pt can be readily distinguished with the EQCM. This approach was used to show that the arsenic redox reactions on the carbon electrode are mechanistically similar to that on the bare Pt electrode. This could not be concluded with just classical cyclic voltammetry alone due to the obfuscation of the faradaic features by the large capacitative effects of the carbon double layer.For the porous carbon electrode, a continual mass loss was always observed during potential cycling, with or without arsenic in the solution. This was attributed to electrogasification of the carbon. The apparent mass loss per cycle was observed to decrease with increasing arsenic concentration due to a net mass increase in adsorbed arsenic per cycle that increased with arsenic concentration, offsetting the carbon mass loss. Additional carbon adsorption sites involved in arsenic species interactions are created during electrogasification, thereby augmenting the net uptake of arsenic per cycle.It is demonstrated that EQCM, and in particular the information given by the behavior of the time derivative of the mass vs. potential, or massogram, is very useful for distinguishing arsenic species interactions with carbon electrodes. It may also prove to be effective for investigating redox/adsorption/desorption behavior of other species in solution with carbon materials as well.     

Electrochemical degradation of organic dyes with a porous gold electrode

Electrochemical degradation of methylene blue (MB) and rhodamine B (RhB) was studied at porous gold (PAu) electrode in presence of KCl as supporting electrolytes. PAu electrode with three-dimensional porous structure was prepared by paper-based method and used as working electrode. Based on the cyclic voltammetric test, the redox potential of of MB and RhB contained in KCl solution was found and the electrochemical degradation was conducted at fixed applied voltage. The direct oxidation of organic dyes involved using PAu and Pt electrodes, while the indirect electrolysis was mediated by active chlorine electro-generated from KCl solution. The results showed that PAu electrode with porous structure has high electrochemical activity with fast kinetics for the destruction of RhB (0.0448 min^-1) and low energy consumption (0.315 kWh/m³), compared to smooth Au foil and reported others. In addition, the durability test for 10 serial degradation showed that PAu electrode has a good reproducibility and high adaptability for practical application.     

Impedance measurements during the cycling of a zinc electrode

Impedance spectra have been obtained during the cycling of a zinc electrode both in a Leclanché-cell-type electrolyte and in an alkaline zincate solution. Changes in impedance plots indicate an increase in the electrode area and a variation of the electrode kinetics with cycling. The kinetics of zinc deposition appear to be very sensitive to the contamination of electrolytes by dissolution products. The results confirm a correlation between the presence of an additive such as NBu₄ Br and the increase of the cycle life of the zinc electrode.     

Enhanced mass transfer to a rotating cylinder electrode with axial flow

A study of the rotating concentric cylindrical electrode has been made, in which the enhanced mass transfer rate by turbulence promotors to a smooth cylinder has been measured. When a special polypropene cloth was applied in the annulus an increase in the Sherwood number was detected, up to six times the value for a smooth cylinder at low Taylor numbers.Nomenclature A electrode area, dl (m²) - C ₀ bulk concentration (mol m^–3) - D diffusion coefficient (m² s^–1) - e annular gap,R-r (m) - F Faraday's constant, 96487 (As mol^–1) - I _l limiting current (A) - k _l mass transfer coefficient,I _l /nFC ₀ A (m s^–1) - l electrode height (m) - n number of electrons - r, R radius of inner and outer cylinder (m) - u axial liquid velocity (m s^–1) - angular velocity (rad s^–1) - kinematic viscosity (m² s^–1) - liquid density (kg m^–3) Dimensionless numbers Re _a axial Reynolds number 2eu/ - Re rotational Reynolds number 2r ²/ - Sc Schmidt number /D - Sh ^* rotational Sherwood number 2rk _l /D - Sh combined flow Sherwood number 2ek _l /D - St Stanton numberSh/Re /Sc - Ta Taylor number=re/(e/r)^1/2 - a, b, c power indices     

Impedance model of electrolyte-insulator-semiconductor structure with porous silicon semiconductor

We present a generic impedance model for the porous silicon|electrolyte structure that is valid for a range of interfacial layers and bias in these structures. The model is validated using three widely different porous structures: short irregular silicon columns and pores, long cylindrical silicon columns and pores; and branched interconnected silicon microchannels and voids in a mesh structure. The model incorporates appropriate RC or constant phase elements for the different parts of the porous structure, namely, the top of the silicon columns (channels)|electrolyte, the column (channel) walls|electrolyte in the pores/channels, and the electrolyte|semiconductor interface at the base of the pores/channels. This physical model underscores the effects of column/channel depletion and accumulation, either due to applied bias or change of surface charge, to the impedance spectra of the device. The model helps to explain why the porosity needs to be optimized for specific applications and helps as a measurement tool for optimization.     

新型多孔Ti/BDD薄膜电极电化学氧化降解低浓度茜素红

运用热丝化学气相沉积（HFCVD）的方法制备了以多孔钛为基体的掺杂硼金刚石（porous Ti/BDD）薄膜电极,并测试了它的主要物理性质,SEM表明金刚石相生长良好并且能均匀地分布在基体表面和孔内,Raman光谱表明电极的金刚石相纯而且质量很高。采用循环伏安法研究了酸性条件下茜素红在多孔Ti/BDD电极上的电氧化行为。通过改变阳极电流密度、支持电解质Na2SO4的浓度来研究茜素红在多孔Ti/BDD电极上的电化学氧化降解的效果影响。结果表明：电流密度40 mA/cm2、支持电解质浓度0.5 mol/L为较理想的工艺参数,总电流效率达到30.2%。在相同条件下,发现多孔Ti/BDD薄膜电极氧化降解茜素红与平板Ti/BDD薄膜电极相比具有更高的电流效率。紫外可见光光谱证实了多孔Ti/BDD电极能够有效地电氧化降解茜素红。     

Doped carbon nanotube array with a gradient of nitrogen concentration

Carbon nanotube (CNT) arrays doped with nitrogen were synthesized using chemical vapor deposition by pyrolysis of xylene/pyridine and ferrocene with a modified precursor feeding technique. The system produced a gradient of nitrogen concentration that modified the CNTs from a hollow cylinder to a bamboo-shaped structure which contains a series of compartments whose lengths are gradually reduced as the nitrogen concentration increases.     

Local current densities in a two rotating disc electrode system with an axial electrolyte inlet measured by an autoradiographic method

The distribution of local current densities in a rotating electrolyser with axial electrolyte inlet in a laminar flow regime was studied by the autoradiographic method. With all systems studied, the local current density decreased monotonically from a maximum value at the inner to a minimum at the outer boundary of the electrode. Experimental results are compared with the numerical solution of the convective diffusion equation by the finite element method.Nomenclature a, b constants - c concentration - c ₀ concentration in the bulk phase - d optical density - D diffusion coefficient - F Faraday constant, 96 487 C mol^–1 - h interelectrode distance - j local current density - J _{n, dif} density of diffusion flux in outer normal direction - n number of electrons transferred in the elementary step - diffusion flux - Q volume rate of flow - r radial coordinate - r ₀ inner electrode radius - r ₁ outer electrode radius - r _v radius of inlet orifice - r _d outer disc radius - t time - v _r radial velocity component of liquid - v _z normal velocity component of liquid - z normal coordinate - thickness of the layer in which the equation of convective diffusion is solved - v kinematic viscocity - angular velocity     

Impedance of electrode reactions under dc polarization

The impedance of electrode reactions under polarization with direct current (i_dc) is theoretically analysed for the reaction where a first- or second-order chemical reaction on the electrode surface follows the charge-transfer process: A → νB + n e⁻, B → C. Discussion is developed upon the dependence of impedance on ac frequency and dc cd or electrode potential, for the cases of either of the above steps being rate-determining.

It is shown that the plot of reactance (1/ωC_s) υs resistance (R_s), which are the components of impedance in the series equivalent circuit, gives a semi-circle on the complex plane, irrespective of the rate-determining step in the reaction. The value of RT/nF, the rate constant of the surface chemical reaction, and also the double-layer capacitance, can be determined by analysing the semi-circle. The dependence of angular frequency corresponding to the maximum mf 1/ωC_s on i_dc can be taken as a criterion for the determination of the rate-determining step.     

Impedance of the sintered nickel positive electrode

Impedance measurements have been made on a mini electrode disposed between two large (identical) counter electrodes. A general mechanism is obtained for the electrode behaviour from the experimental data. The electrode reaction is two step and complex. Where comparison is possible agreement is obtained with earlier work.     

Resorcinol-formaldehyde based porous carbon as an electrode material for supercapacitors

Porous carbon materials were prepared using resorcinol and formaldehyde catalyzed by KOH in a sol-gel process followed by carbonization, during which the KOH serves as an activating agent and generates pores mainly located in the micropore range. With an increase of mass ratio of KOH to resorcinol from 1 to 4, both the specific surface area and the pore volume of the carbons increased, from 522 to 2760 m²/g and 0.304 to 1.347 cm³/g, respectively, but the average pore diameter decreased from 4.4 to 2.5 nm. Samples were investigated as electrode materials in supercapacitors and the relevant electrochemical behavior was characterized by cyclic voltammetry, electrochemical impedance spectroscopy and constant current charge-discharge experiments using 30% KOH aqueous solution as electrolyte. The highest specific capacitance of up to 294 F/g was obtained at a current density 1 mA/cm² for the sample with mass ratio of KOH to resorcinol of 2. Only a slight decrease in capacitance for the same sample, from 294 to 242 F/g, was observed when the current density increased from 1 to 30 mA/cm². The specific capacitance only decayed 3% at a current density 30 mA/cm² after 1000 cycles, which indicates that the sample possesses excellent power property and cycle durability.     

Fracture behavior of cylindrical porous alumina with pore gradient

In this study, laminated cylindrical porous alumina materials with a pore gradient along the radial direction are fabricated by combining a lower sintering temperature with the presence of an organic-former (PMMA particles). It has been proved that pore morphology can be controlled by the lamination method in the centrifugal molding technique. The fracture behavior of the porous laminated alumina with a pore gradient is investigated by using the ring compression testing. An analytical formula to account for the pore gradient is introduced to calculate fracture strength based on the curved beam theory. There are two types of fracture behavior, which are not related to the lamination process and vary with the amount of pore-former. In the case of the same porosity, a reduction in fracture strength is observed with increase in the number of laminated layers, and the minimum strength is found in continuous graded cylinders.     

Complex impedance spectra of porous electrode with fractal structure

Transmission line model (TLM) is often used for the analysis of porous electrode since the current distribution in the pore can be calculated. In the present paper, the TLM is applied to the impedance analysis of complicated porous electrode with fractal structure. Activated carbon is used as electrode material for electric double layer capacitor and its pore has branch structure. Considering the fractal structure of the activated carbon, we proposed new equivalent circuit with TLM for three sizes of pores: macropore, mesopore and micropore. In this model, we defined the ratio ξ of electric double layer parts in whole inner wall of the pore because the inner wall is separated to electrolyte/electrode interface and branch base parts. Calculated electrochemical impedance of blocking electrode shows the lumped constant region in low frequency range and the distributed constant region in low frequency range. General TLM yields the straight line with slope of 45° in the distributed constant region on the Nyquist plots. In the present analysis, it was found that the slope was 22.5° at ξ = 0 in the case of double-pores (macropore and mesopore) and 11.25° at ξ = 0 in the case of triple-pores (macropore, mesopore and micropore).     

Dissolved oxygen concentration in an undivided rotating cylinder electrode reactor

The steady state mass balance for dissolved oxygen in an undivided rotating cylinder electrode reactor was established. Cases involving the absence or presence of diffusion controlled oxygen reduction reaction were treated. The results show that in the absence of the oxygen reduction reaction and under low electrolyte velocities employed industrially, the rotating cylinder electrode reactor, exhibiting a continuous stirred tank reactor behaviour, yields lower dissolved gas concentrations than a plug flow reactor. Therefore, the rotating cylinder electrode reactor leads to lower redissolution rates of the detached metallic particles by oxygen corrosion. The effect of the dissolved gas concentration in the inlet electrolyte also demonstrates the benefits of a prior degassing step in the flow sheet. The simpler equation derived in the absence of oxygen reduction may be used in its presence for inlet dissolved oxygen concentrations below saturation and industrially relevant electrolyte velocities ranging from approx. 0 to 0.3 m s⁻¹ if metal deposition current efficiency is higher than 87% or, conversely, if the dissolved metal concentration to dissolved oxygen concentration ratio is greater than 27. An even simpler equation based on the added hypothesis of total desorption of the gas produced can also be used with electrolyte velocities below 0.025 m s⁻¹. Finally, the derived equations allow optimization of the rotating cylinder electrode reactor.