Double Layer Capacity Research Articles

Measurement of protonic resistance of catalyst layers as a tool for degradation monitoring

The ionic resistance of a PEM fuel cell catalyst layer, as well as the double layer capacity and the high frequency resistance are estimated from the impedance spectra of H2/N2 operated cell, thanks to a volumetric electrode model. The impedance spectra are calculated in the case of a homogeneous distribution of resistivity and capacity, and considering both through plane and in plane heterogeneities.Then, results of the degradation of a MEA subjected to four different accelerated stress tests are presented. They consist of (i) a constant current operation, (ii) 1.2 V potential hold with nitrogen and hydrogen (iii) open circuit voltage (OCV) hold with air and hydrogen, and (iv) a start-up protocol where heterogeneous degradations are observed using a linear segmented fuel cell. This technique can provide useful information about the evolution of catalyst layer microstructure during AST.

Analysis of charging-induced structural damage in electrochemical systems.

Electrochemical charging of an electrochemical system results in Maxwell stress and a variation in the surface energy of the electrode. The charging-induced variation in the surface energy of the electrode and Maxwell stress introduce mechanical deformation and create stresses in the electrode. To relax the strain energy stored in the electrode, surface cracking and buckling can occur. Under the condition that the charging-induced change in the surface energy is the dominant factor controlling the in-plane deformation of a planar electrode, i.e. the bonding between the adsorbate and the substrate is weak, the variation in the surface energy of the electrode can be described by the Lippmann equation. Using the Lippmann equation, both the surface charge density on the electrode and the surface energy of the electrode are formulated as a function of the electrode potential. Analytical relations between the critical electrode potential and average damage size have been obtained for the charging-induced cracking and buckling in a planar, thin-film electrode. The results show that surface cracking will prevail over local buckling in accordance with experimental observations. Both critical electrode potentials are inversely proportional to the square root of the magnitude of the differential capacity of the electrical double layer at the electrode potential of zero charge.

Microwave-assisted preparation of carbon nanofiber-functionalized graphite felts as electrodes for polymer-based redox-flow batteries

Abstract A simple and fast microwave-assisted protocol to functionalize commercially available graphite felts (GFs) with carbon nanofibers (CNFs) for the application as electrode materials in redox-flow batteries (RFB) is demonstrated. As catalyst for the CNF synthesis nickel acetate is applied and ethanol serves as the carbon source. By the in-situ growth of CNFs, the active surface of the electrodes is increased by a factor of 50, which is determined by the electrochemical double layer capacities of the obtained materials. Furthermore, the morphology of the CNF-coating is investigated by scanning electron microscopy. Subsequently, the functionalized electrodes are applied in a polymer-based redox-flow battery (pRFB) using a TEMPO- and a viologen polymer as active materials. Due to the increased surface area as compared to an untreated graphite felt electrode, the current rating is improved by about 45% at 80 mA cm −2 and, furthermore, a decrease in overpotentials is observed. Thus, using this microwave-assisted synthesis approach, CNF-functionalized composite electrodes are prepared with a very simple protocol suitable for real life applications and an improvement of the overall performance of the polymer-based redox-flow battery is demonstrated.

Boron-doped nanoporous carbons as promising materials for supercapacitors and hydrogen storage

Boron-doped carbon nanostructures were obtained via template and bulk synthesis from sucrose in the presence of boric acid. Synthesized B-containing mesoporous materials are characterized by high spatial ordering, developed surface (up to 870 m2/g), and porosity (up to 1.2 cm3/g). The obtained B-doped microporous materials exhibit high interfacial capacitance (up to 0.36 F/m2), which is higher than the available literature data and enables the use of synthesized samples as materials of supercapacitors. The capacity of electric double layer up to 300 F/g using sulfuric acid as the electrolyte was accumulated on microporous B-doped carbons.

High capacity supercapacitor material based on reduced graphene oxide loading mesoporpus murdochite-type Ni6MnO8 nanospheres

Mesoporous materials often deliver a series of attractive properties for low-cost and environmentally-friendly energy storage/conversion technologies. Based on this, we prepared mesoporous murdochite-type Ni6MnO8 nanospheres (NMN) with a uniform morphology by using a template route. Then as-prepared NMN were loaded uniformly on reduced graphene oxide (RGO) sheets via electrostatic interaction to fabricate RGO-NMN composite (RGO-NMN), which combines the advantages of both RGO such as double-layer capacity and NMN such as high pseudocapacitity. By the aid of a nanosized mesoporous silica template, NMN and RGO-NMN display a large specific surface areas and concentrated pore diameter distribution. The large surface area and abundant pores can provide a promising route for easy access of electrolyte in energy storage electrodes and favor the availability of Faradic reaction in electroactive sites greatly. Indeed, following electrochemical analysis uncovered a remarkable value of specific capacitance (862.5Fg−1 and 498.3Fg−1 at 0.5Ag−1 for RGO-NMN and NMN) and a predominant long-term stability with slight decays less than 10% after 9000 cycling test in both NMN and RGO-NMN electrodes.

Impedance Spectroscopy of Direct Ethanol Fuel Cells with Anodes Containing Pt Group Metals

Alcohols might be a promising alternative to gaseous hydrogen in fuel cells technologies. In this study ethanol electrooxidation and oxygen electroreduction were studied by using classical electrochemical methods and electrochemical impedance spectroscopy (EIS). Studies have been carried out for both Pt and Pt-Ru catalysts for the cathode and anode separately in 0.5 M sulfuric acid solutions and for the assembled fuel cells with Nafion membranes. EIS investigations for anodes were carried out in the potentiodynamic conditions because otherwise a stationary state necessary for impedance measurements does not occur [1]. Impedance of the ethanol electrooxidation was recorded in voltammetric experiments with a sinusoidal perturbation of single frequency and small amplitude additionally applied at the electrode. After several voltammetric runs impedance spectra were built for selected electrode potentials. Thus obtained time and potential resolved impedance spectra were characterised by good reproducibility and their validity was confirmed in Kramers–Kronig transformations. The impedance spectra were qualitatively similar to those reported earlier and obtained by using Dynamic EIS (DEIS) experiments [1]. Complex mechanism of ethanol oxidation manifest itself as a negative resistance of the charge transfer, which is related to decrease in the rate of ethanol electrooxidation with increasing anodic potential. This effect is also accompanied by a significant drop in the capacity of the double layer and can be ascribed to the accumulation of carbon monoxide at the electrode surface. Impedance measurements were also performed for assembled fuel cells under different loads with current perturbation equal to 5% of the overall current density. Since the stationary state was assured by a constant flow of the reactants, in this configuration impedance was recorded in a broader frequency range and for much lower frequencies than in ex situ experiments. Equivalent circuit were proposed to model experimentally recoded spectra. The resistance of the charge transfer and the capacity of the double layer at the cathode were virtually independent from the oxygen partial pressure within studied range. This effect, observed particularly for current densities greater than 4 mA cm-2, suggest nearly constant potential of the cathode. Ethanol concentration largely affects impedance of the anode. For higher ethanol concentration a decrease in the double layer capacity accompanied by an increase in the absolute values of the negative resistance were observed. These effects reflect a strong poisoning of the electrode surface. Results points out that the current efficiency is strongly limited by the electrooxidation process at the anode. The influence of other experimental conditions such as temperature and fuel to oxygen stoichiometry on the impedance of direct ethanol fuel cells will be also discussed. [1] E.Desilets, A. Lasia, Electrochimica Acta 78 (2012) 286-293

On the origin of enhanced electrochemical behavior of oxidized activated carbon

The amount of ions stored within the electrochemical double layer is dominantly determined by the surface area of porous electrode. High surface area indicates high capacitance. However, this is not the case when the pore is extremely small, ∼less than 1nm. To observe this phenomenon, we have carried out an experiment by comparatively investigating the electrochemical performance of activated carbon (AC) and sulfuric acid treated AC (SAC). The results show that the electrochemical performance of SAC involving the specific capacitance, rate capability and cycling stability is significantly increased as compared to pristine AC. These are attributed to the improved porosity by differentiating the respective contributions of electrochemical double layer capacity and pseudo-capacity from SAC and AC, respectively.

Effect of oxidative treatment on the electrochemical properties of aligned multi-walled carbon nanotubes

Vertically aligned multi-walled carbon nanotubes (MWNTs) were grown on the surface of electroconductive silicon substrate by catalytic chemical vapor deposition (CCVD) of a mixture of toluene and ferrocene vapors at 800°С. The anisotropic structure of the array that is due to the mutual orientation of MWNTs makes such materials attractive for use as supercapacitor electrodes. The effect of iron nanoparticles encapsulated in MWNTs as a result of synthesis on the electrochemical capacity of the sample in a 1 М H2SO4 solution was studied. Iron was removed during the thermal treatment of the MWNT array in a 20% H2SO4 solution under the normal or hydrothermal conditions. The contribution of redox processes involving iron was shown to be comparable to the contribution of the double-layer capacity of MWNTs. The hydrothermal treatment allows easy separation of the array from the silicon substrate without any loss of electric coupling of MWNTs.

Correlating the Layer Properties of Ni-alumina Composite Coatings and the Mechanism of Codeposition

<p>Ni-Al<sub>2</sub>O<sub>3</sub> composite coatings electrodeposited from Watt’s electrolyte include Al<sub>2</sub>O<sub>3</sub> nano-and submicron particles. The effect of particle size and concentration of the particles in the electrolyte were investigated on the morphology and incorporation value of particles into the deposits. The influence of alumina particles on the electrodeposition behavior of Ni was also studied by means of electrochemical impedance spectroscopy. The results achieved from the impedance measurements and the correlation with the layer characterization could help to better understand the codeposition mechanism derived from the impact of different particle characteristics including size and concentration on the nature of the double-layer. The increase in the concentration of particles from 1 to 20 g/l resulted in an increase of the double-layer capacity and decrease of the charge transfer resistance, while the addition of submicron particles had a higher influence on the characteristics of the double-layer compared to the nano particles. Although the alumina particles with submicron size could stimulate the incorporation of particles faster than those with nano size, the strengthening performance of the layers not only depended on the incorporation value of the particles, but also on the microstructure of the deposits.</p>

The activity of nanocrystalline Fe-based alloys as electrode materials for the hydrogen evolution reaction

In view of alkaline water electrolysis, the activities for the hydrogen evolution reaction of nanocrystalline Fe-based electrode materials were investigated and compared with the activities of polycrystalline Fe and Ni. Electrochemical methods were used to elucidate the overpotential value, the charge transfer resistance and the double layer capacity. Structural properties of the electrode surface were determined with SEM, XRD and XPS analyses. Thus, a correlation between electrochemical and structural parameters was found. In this context, we report on a cyclic voltammetric activation procedure which causes a significant increase of the surface area of Fe-based electrodes leading to a boost in effective activity of the activated electrodes. It was found that the intrinsic activity of activated Fe-based electrodes is very high due to the formation of a nanocrystalline surface layer. In contrast, the activation procedure influences only the intrinsic activity of the Ni electrodes without the formation of a porous surface layer.

Numerical model for electro-poling

A theoretical model for describing the time evolution of concentration profiles in thermal electro-poling experiments is being presented. The model is based on a Monte Carlo formalism. It takes into account the field dependent mobility of cations as well as electrons. Model calculations are presented for a 46S4 bio-glass. Upon application of a DC field Na+ ions start to move toward the cathode giving rise to a depletion zone. The electric field increases in this zone. Once a specific threshold value is reached the electrons become mobile and move toward the anode leaving behind a basically neutral zone. Thus, the full double layer capacity has developed after a few seconds. The concentration depletion profiles continue to develop on a much longer time scale of tens of minutes. In the 46S4 glass Na+ ions as well as Ca++ ions develop a depletion profile. The numerical calculations account for all essential experimental observations available in the literature. The model is also capable of predicting under which conditions two different cations move on the same or on different time scales.

Influence of Fluoride Ion on the Performance of Pb-Ag Anode During Long-Term Galvanostatic Electrolysis

Anodic potential, morphology and phase composition of the anodic layer, corrosion morphology of the metallic substrate, and oxygen evolution behavior of Pb-Ag anode in H2SO4 solution without/with fluoride ion were investigated and compared. The results showed that the presence of fluoride ions contributed to a smoother anodic layer with lower PbO2 concentration, which resulted in lower double layer capacity and higher charge transfer resistance for the oxygen evolution reaction. Consequently, the Pb-Ag anode showed a higher anodic potential (about 35 mV) in the fluoride-containing electrolyte. In addition, the fluoride ions accelerated the detachment of loose flakes on the anodic layer. It was demonstrated that the anodic layer formed in the fluoride-containing H2SO4 solution was thinner. Furthermore, fluoride ions aggravated the corrosion of the metallic substrate at interdendritic boundary regions. Hence, the presence of fluoride ions is detrimental to oxygen evolution reactivity and increases the corrosion of the Pb-Ag anode, which may further increase the energy consumption and capital cost of zinc plants.

Cytosine derivatized bis(2,2'-bithienyl)methane molecularly imprinted polymer for selective recognition of 6-thioguanine, an antitumor drug.

A molecularly imprinted polymer (MIP) was designed and synthesized to serve as a functional material for selective recognition of 6-thioguanine (6TG), an antitumor drug. For that, the newly synthesized functional monomer, cytosine-bis(2,2'-bithienyl)-(4-carboxyphenyl)methane ester (Cyt-S4), revealed Watson-Crick type nucleobase pairing of 6TG. Formation of the Cyt-S4 and 6TG complex of the 2:1 stoichiometry was postulated based on the DFT calculations at the B3LYP/3-21G((⁎)) level and experimentally confirmed by fluorescence titration. The molecularly imprinted polymer (MIP) film was deposited by potentiodynamic electropolymerization on a Pt disk electrode as well as on an Au-coated glass slide and on an Au-quartz crystal resonator. The statistical model of formation of this film was successfully simulated by molecular dynamics. Completeness of the subsequent 6TG template extraction from MIP was confirmed by the UV-visible spectroscopy. An imprinting factor of 2.9 for the MIP film was determined by piezoelectric microgravimetry using ECQM. The double-layer capacity and alternating current measurements under flow-injection analysis (FIA) conditions were selected to transduce the 6TG recognition signal into the change of the double-layer capacity dependence on the 6TG concentration in solution for different supporting electrolyte concentrations. Detectability of the resulting chemosensor was 10 µM 6TG for the 0.5 M KF carrier solution in FIA. Selectivity of the chemosensor with respect to common interferences was high, e.g., it exceeded 130 to 2-amino-6-methylmercaptopurine, a 6TG metabolite.

Adsorption of anionic surfactant at the electrode-NaClO4 solution interface.

Adsorption of 1-decanesulfonic acid at the electrode–NaClO4 solution interface was determined by double-layer differential capacity measurements. At potentials less than −1,200 mV, the adsorption of the anionic surfactant on the electrode does not occur. Low concentrations of the anionic surfactant (below cmc) causes slight changes in the zero charge potential, E z, and the surface tension at this potential, γ z. The adsorption of the anionic surfactant was analyzed using the constants obtained from the following isotherms: Frumkin, corrected Flory–Huggins, and virial.Graphical

Impact of Inhomogeneous Catalyst Layer Properties on Impedance Spectra of Polymer Electrolyte Membrane Fuel Cells

A physically-based 1D impedance model of the cathode catalyst layer that accounts for inhomogeneous catalyst layer (CL) properties is developed. The impact of locally changing proton conductivity, volumetric double-layer capacity, ORR rate and effective porosity on the characteristics of impedance spectra is analyzed. It was found that a non-constant conductivity and capacity profile within the CL results in a specific fingerprint in the high frequency range of the spectra where at homogeneous conditions a linear 45° branch would be expected. Furthermore, a well pronounced splitting of a high and low frequency loop can be provoked by introducing non-uniformly distributed CL properties. The simulation results show that electrochemical impedance spectroscopy is a valuable tool to investigate the inhomogeneity of the catalyst layer originated by the electrode preparation itself or by degradation processes.

The study of capacitive deionization behavior of a carbon nanotube electrode from the perspective of charge efficiency.

In this work, the capacitive deionization (CDI) performance of a single-walled carbon nanotube (CNT) electrode has been studied from the point view of charge efficiency theory. It is revealed here that the charge efficiency of a CNT electrode is strongly dependent upon the cell voltage and solution concentration. Either the high cell voltage or the low ionic strength results in a high charge efficiency, implying that CDI is expected to be a promising technique for an aqueous solution with low ionic strength. Additionally, it is found that the high decay constant and high electrical double-layer capacity are beneficial to enhance electrosorption performance.

Electrooxidation of 2-chlorophenol and 2,4,6-chlorophenol on glassy carbon electrodes modified with graphite–zeolite mixtures

This work reports the behavior of zeolite–graphite electrodes for the oxidation of 2,4,6-trichlorophenol (2,4,6-TCP) and 2-chlorophenol (2-CP) in a pH 3, 0.5 M Na2SO4 electrolyte using cyclic voltammetry, chronoamperometry, electrochemical impedance and galvanostatic electrolysis. Three zeolites with different hydrophobic/hydrophilic balance were used, in order to determine the possible effect of this parameter. On glassy carbon (GC) modified with 80–20 % graphite–zeolite mixtures the voltammetric peak charge of 2-CP oxidation was higher than that on GC modified with graphite alone, clearly showing a catalytic effect of the zeolites. ZSM-5 was the more active zeolite, probably due to its hydrophobic character, which should favor its interaction with the also hydrophobic 2-CP. In agreement with this, the charge-transfer resistance of GC modified with a graphite-ZSM-5 mixture was about half that of GC modified with graphite alone, and its double-layer capacity about 50 % higher, indicating a modification of the interface which favours the reaction. On the contrary, for 2,4,6-TCP both the oxidation peak charge, and the charge-transfer resistance, for GC electrodes modified with graphite–zeolite mixtures was about the same as that of GC modified with graphite alone. This lack of catalytic action could be due to the fact that the oxidation of 2,4,6-TCP requires the release of at least one chlorine atom, this release being so difficult that it cannot be promoted by any of the zeolites studied. However, in prolonged galvanostatic electrolysis practically all the current went to the oxidation of 2,4,6-TCP, while in the case of 2-CP only a small fraction was oxidized. Probably with 2-CP at long times surface poisoning blocked the reaction, as reflected by the fact that the electrolysis potential rose to a value 0.2 V higher than that reached by TCP, and so high that most of the current went to oxygen evolution. The zeolites ZA and ZY decreased the mineralization of 2,4,6-TCP, since they promoted a one-electron oxidation reaction, so that its concentration at 80 min was about 80 % of that observed with graphite alone. The oxidation of 2-CP was not significantly affected by any zeolite as compared with that on graphite.

Isoconductivity method to study adhesion of yeast cells to gold electrode

Abstract In this paper, we used impedance spectroscopy and gold electrodes to detect the presence of yeast cells and monitor the attachment of these cells to the electrodes. We analyzed the effect of conductivity changes of the medium and the attachment on the electrode-electrolyte interface impedance. A three-electrode cell was designed to produce a uniform electric field distribution on the working electrode and to minimize the counter electrode impedance. Moreover, we used a small AC overpotential (10 mV) to keep the system within the linear impedance limits of the electrode-electrolyte interface. This study proposes a new method to differentiate the impedance changes due to the attachment of yeast cells from those due to conductivity changes of the medium. The experiments showed that when the difference between the cell suspension and base solution conductivities is within the experimental error, the impedance changes are only due to the attachment of yeast cells to the electrodes. The experiments also showed a strong dependence (decrease) of the parallel capacity of the electrode electrolyte interface with the yeast cell concentration of suspension. We suggest that this decrease is due to an asymmetrical redistribution of surface charges on both sides of cell, which can be modeled as a biologic capacity connected in series with the double layer capacity of the interface. Our results could help to explain the rate of biofilm formation through the determination of the rate of cell adhesion.

Preparation of Carbonaceous Fiber Sheets Derived from Bamboo and Application to Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells

Bamboo is one of sustainable natural resources with high CO2 immobilization capacity; on the other hand, its relatively high vitality induces several environmental serious problems recently. To solve the problems, new and some favorable applications of bamboo are widely being developed and investigated. To utilize the feature of bamboo is very important for the application development. The microstructure of bamboo is fibrous and is composed mainly of cellulose, giving us a cellulose fiber easily. Moreover, its fibrous morphology was even kept after carbonization at relatively high temperature. We then attempted to prepare the carbonaceous fiber sheet by using bamboo as a starting material and employ it as gas diffusion layer of proton exchange membrane fuel cells (PEMFCs) [1].In this presentation, the detail regarding to preparation procedure of bamboo fiber, its carbonization and typical character is demonstrated.Bamboo was cut down to 80 mm in length by a metallic chopper and the inner and outer skins were carefully descaled by specially developed cutting machine (Silver-loy Co. Ltd., Japan). Then, it was pressed repeatedly by specially developed roll press machine (Silver-loy Co. Ltd., Japan) to form several breaches. Thus obtained bamboo strips were employed for further study. The specimens were boiled in 0.1 mol dm-3 sodium hydroxide aqueous solution for 3 h by using commercially available electric pot (Panasonic, SR-P37). After that, the samples were washed repeatedly by water and treated by an electric blender at 37000 rpm for 1 min for defiberize. Thus obtained bamboo mono filaments were molded and treated at various temperatures under argon atmosphere to obtain carbonaceous fiber sheets.The size and morphology of the fibers in the sheets were investigated by an optical microscope (OM) and field emission scanning electron microscope (FE-SEM). X-ray diffraction (XRD) and Raman spectroscopy was used to evaluate the carbonization degree. The electrochemical behavior of carbonaceous fiber sheets was investigated in 0.1 mol dm-3 perchloric acid aqueous solution under nitrogen atmosphere by cyclic voltammetry. Furthermore, a gas diffusion electrode was fabricated by using carbonaceous fiber sheet as a gas diffusion layer and Pt/C catalyst. PEMFC operation test was carried out by using Nafion117 membrane as the electrolyte.Figure 1 shows FE-SEM image of carbonaceous bamboo fiber after heated at 3000ºC. The diameter of fiber was ca. 6 μm, which is similar to carbon fiber that used in the fuel cell.Figure 2 shows XRD patterns of carbonaceous bamboo fiber after heated at various temperatures. The peak corresponding to 002 diffraction of graphite was appeared and become to be sharpened with increasing in the heat treatment temperature.Both of the in-plane electrical conductivity, which is measured by a four-probe method, and that measured under consolidation condition is lower than that of the carbon paper composed by carbon fibers.Figure 3 shows the cyclic voltammograms of carbonaceous fiber sheets after the heat treatment at various temperatures. The double layer capacity was decreased as an increase in the heat treatment temperature, suggesting that carbonization developed at higher temperature whereas the surface area decreased. The results of PEMFC operation test will be mentioned in the presentation.Reference[1] T. Kinumoto et al., ECS Trans., accepted for publication at 2013.AcknowledgementThis study is supported by The Environment Research and Technology Development Fund (ERTDF) from Ministry of the Environment, JAPAN.

A Metal-Free Organic Aqueous Capacitor Using Quinonic Compounds Couple

Redox-capacitors are electrochemical energy storage devices using the electric double layer capacity and pseudo-capacity derived from redox reactions. Because of its higher energy density than conventional electric double layer capacitors, higher power density than batteries and long cycle life, redox-capacitor is a promising candidate for grid-scale stationary energy storage device.We demonstrated the aqueous redox-capacitors using inexpensive quinonic and hydroquinonic organic compounds as redox active materials, which facilitate charge/discharge by proton insertion/extraction in aqueous electrolyte. These quinonic/hydroquinonic compounds are supported in nanoporous carbon. This capacitor exhibited high energy density (＞20 Wh kg-1) and high power density (＞5 A g-1) without the degradation after 10000 charge/discharge cycles [Sci. Rep. 4, 3591 (2014).].In this study, we investigated the dependence of loading rate of quinonic compound to carbon materials for further enhancement of energy density. It was found that quinonic compound, which is highly loaded in carbon (>70 wt%), can be utilized by adding a slight amount of carbon with high electrical conductivity. Finally, the high energy density of approximately 30 Wh kg-1 was achieved in the metal-free organic aqueous capacitor using quinonic compounds couple.