Impedance Of Porous Electrodes Research Articles

Impedance of porous electrodes in the presence of electroactive species and solution resistance

In a recent paper, we have presented behavior of the porous electrodes in the presence of electroactive species and the absence of the dc and ac potential gradients. This paper presents a model without dc potential gradient but including ac potential drop due to the solution resistance in pores. In this case, a high-frequency straight line at 45° characteristic of porous electrodes appears at high frequencies on the complex plane plots followed by two semicircles. This high-frequency line increases with the increase of the electrode porosity, i.e. Thiele modulus, and solution resistivity. Fitting these impedances to the classical de Levie’s model does not reproduce well the parameters of the second semicircle although the overall fit is good for moderate porosities.

Impedance of porous electrodes in the presence of electroactive species

Behavior of model porous electrodes in the presence of electroactive species and absence of the potential gradient is presented. The presence of dc current forces the concentration gradient in pores. In typical electrochemical applications, the concentration gradient is much more important than the potential gradient. In the absence of potential gradient analytical solutions for the concentration, current, and impedance are obtained. Two cases are considered: a) with only one electroactive species in the bulk of the solution and b) with the redox couple in the bulk of the solution. In these cases, two semicircles form on the complex plane plots. The influence of the porosity, concentration, and potential on current, faradaic and total impedances, and resistances of both semicircles are presented and discussed.

Symmetric-cell characterization of the redox flow battery system: Application to the detection of degradations

A combined modelling-experiments approach is presented with the aim of determining in-situ key characteristics of a RFB. The porous electrode impedance was calculated and measured without and with a faradaic reaction, by means of a symmetric cell. The technique was applied to the specific [Fe(CN)6]3-/[Fe(CN)6]4- couple whose interest for RFB application is substantial. The interpretation of Electrochemical Impedance Spectroscopy (EIS) data enabled the determination of the key membrane and electrode resistances and the investigation of their evolutions during the battery operation. The effects of the cation composition in the supporting electrolyte, the carbon material and the cycling were analysed. The results gave insights on degradation mechanisms that can hinder RFB performance, and delineated the phenomena occurring during electrolyte circulation or during battery operation.

Comment on “Current-distribution effects on the impedance of porous electrodes and electrodes covered with films” by M. Tjelta and S. Sunde [J. Electroanal. Chem. 737 (2015) 65–77

In our recently published paper [M. Tjelta and S. Sunde, J. Electroanal. Chem. 737 (2015) 65–77] we inferred the valid frequency range for our impedance model through comparison of data for different truncations of the series solution. To avoid undue focus on the data beyond the stated range of validity in one of our figures and recurring misinterpretation of it, we reproduce it in this comment with data beyond the upper frequency limit of the validity range removed.

Current-distribution effects on the impedance of porous electrodes and electrodes covered with films

An analytical formula for the impedance of a disk electrode for general local interfacial impedances has been derived. The formula indicates that the current and potential distributions will affect the impedance data when the measurement frequency is close to or higher than certain critical value. For capacitive or Warburg-like local interfacial impedances, both relevant for porous and intercalation electrode films, the criterion is the same as that previously obtained by Orazem, Tribollet and co-workers through numerical simulation (Huang et al., 2007). Numerical simulation performed here for square and rectangular electrodes show that the criterion will be similar to that for the disk but with the shorter edge length of the electrode playing the role of the disk radius. The local impedances calculated for the square and rectangular electrodes are qualitatively similar to those of the disk. However, for square or rectangular electrodes the current-distribution effects lead to a different appearance in the various plots of the global impedance (impedance-plane plots, Bode plots, etc.) than those for the disk. This is suggested to be due to the “edge-like” and “center-like” local admittances receiving different weight when summed up to the global impedance of the electrode.

Kinetics of hydrogen evolution from acidic solutions on pressed micro graphite electrodes modified with carbon nanotubes. II. Impedance studies

The kinetics of hydrogen evolution on pressed micro graphite electrodes modified with multi-walled carbon nanotubes (MWCNTs) is studied in chloride environments with a constant ionic strength equal to unity with the use of impedance spectroscopy. The impedance of electrodes containing from 0.1 to 8 wt % MWCNTs in solutions of a general composition x M HCl + (1 − x) M NaCl at a currentless potential and at E = −0.6 V (N.H.S.) is interpreted as impedance of porous electrodes. The data obtained indicate the absence of diffusion limitations of the processes that proceed at a currentless potential and of the hydrogen evolution reaction. When the content of MWCNTs in a micro graphite electrode increases to 8 wt % at E = −0.6 V (N.H.S.), the increase in the contribution of the double layer capacitance and the adsorption capacitance to the Faraday impedance is the most pronounced, which means that MWCNTs promote a specific interaction of the electrolyte ions and atomic hydrogen with the surface of the cathodically polarized electrode.

On the impedance of porous electrodes – double-layer charging and charge transfer on an inhomogeneous inside electrode surface

A model of the impedance of porous electrodes was derived. The model generalized the de Levie model in so far that an inhomogeneous inside electrode surface is assumed. Then the charge transfer resistance and the double-layer capacity were assumed to be distributed. The width of the distribution is characterized by a distribution parameter. The inhomogeneities led to characteristic deviations of the Nyquist impedance plots compared to those for homogeneous inside electrode surfaces: the slope angle of the straight line for higher frequencies was less than π/4, the slope angle of π/2 of the straight line obtained for double-layer charging only and lower frequencies decreased and the semicircle behaviour obtained for double-layer charging and a Faradaic reaction in parallel were replaced by a depressed semicircle. The model allows the determination of the following parameters: the total effective double-layer capacity, the total effective charge transfer resistance, the distribution parameter and the total resistance of the electrolyte inside the pores. The impedance of porous zinc electrodes in weakly alkaline electrolyte can be theoretically well described with the new model. A scanning force microscopy 2-D-image of the morphology of the zinc electrode was performed and discussed.

The structure, morphology and electrochemical impedance study of the hydrogen evolution reaction on the modified nickel electrodes

Composite layers of modified amorphous nickel were prepared by simultaneous electrodeposition of Ni and TiO 2 on a Cu substrate from a solution containing TiO 2 (anatase) particles suspended by stirring. Scanning electron microscopy, X-ray diffractometry, Auger spectroscopy and absorption spectroscopy, were used for physical and chemical characterization of the layers. Obtained deposits exhibit an amorphous structure of the Ni-P matrix in which the crystalline component, TiO 2, is embedded. Additionally, the presence of non-stoichiometric oxide, Ti 2O 3, formed on a boundary of the TiO 2 grain and nickel matrix in consequence of the reduction conditions during the electrodeposition, was revealed by auger electron spectroscopy (AES). The hydrogen evolution reaction (HER) was investigated on the Ni-P+TiO 2 and compared with Ni-P electrode in 5 M KOH at 25°C using steady-state polarization and electrochemical impedance spectroscopy (EIS). In order to explain the electrochemical behaviour of the electrode materials, electrical equivalent circuits containing: (i) the constant-phase element (CPE), (ii) the porous electrode impedance, and (iii) two-CPE elements were compared and verified. The ac impedance behaviour of the electrodes may be well described using the two-CPE or porous electrode model in case of the Ni-P+TiO 2, and a simple CPE model for the Ni-P. The results obtained from the EIS and steady-state measurements allowed for the determination of the mechanism and kinetics of the HER. It has been found that an increase in electrochemical activity of the Ni-P+TiO 2 electrode is due to both the increase in the real surface area and the presence of titanium oxides TiO 2 and Ti 2O 3, as compared with the Ni-P electrode.