Computer simulation of active layers in double-layer supercapacitors: Galvanostatics,determination of effective coefficients,and calculation of overall characteristics

A computer simulation of the structure and modes of functioning of biporous active layers (activated carbon) in double-layer capacitors (DLCs) was performed. The charging of DLCs in a galvanostatic mode was studied. The main characteristics of DLCs (charging time, specific capacity, stored energy, and power) were calculated. DLCs with aqueous electrolyte of different types were studied: active layer with the “ideal” structure (type 1), active layer with a monoporous structure (2), and biporous active layer (3). A computer simulation of biporous active layers of DLCs involves the formulation of a model of the structure of the active layer, percolation evaluation, and calculation of the effective ion conductivities of both highly porous carbon grains and the whole active layer. When calculating the characteristics of the active layers of DLC, we analyzed the effect of the main parameters (charge current density and active layer thickness) on the charging process and overall characteristics. The central problem of calculation of a DLC with a real, nonmonoporos structure was formulated. In active layers generally having pores of three types (micro-, meso-, and macropores) in the galvanostatic mode of DLC charging, the wide pores are polarized first. In this case, the limiting acceptable potential is achieved, and galvanostatic charging should be stopped and changed to potentiostatic charging. As a result, a large number of micropores can remain unpolarized. Therefore, it is important to perform a theoretical search for means to carry out complete adsorption of ions in micropores and obtain high specific capacities of DLCs.     

Computer simulation of active layer of fuel cell electrode with polymer electrolyte: Complete combined carbon support grains, calculation of overall characteristics

Full computer simulation of the active layer of a fuel cell cathode with polymer electrolyte and complete combined carbon support grains is carried out. The active layer structure included two types of equal-size cubic grains (combined support grains and voids) together forming a cubic lattice. Also, the structure of combined grains was modeled; a carbon cluster was formed in them, with the oxygen reduction process occurring on its surface; the rest of the grain volume was filled by polymer electrolyte. The completeness of the grains consisted in the fact that they were characterized by 3D electron conductivity, ability to take part in the transport of protons in the active layer and the carbon cluster in the grains had the maximum possible surface area. Calculation of overall currents of oxygen cathodes with full combined carbon support grains, Nafion, and platinum yielded the following result. At t = 80°C, pressure p* = 101 kPa, cathode potential E ₀ = 0.8 V, and optimum active layer thickness Δ* = 20 μm, maximum overall current I _max = 0.38 A/cm², maximum power density W _max = 0.31 W/cm². At potential E ₀ = 0.7 V, Δ* = 9.8 μm, I _max = 1.13 A/cm², W _max = 0.79 W/cm². At potential E ₀ = 0.6 V, Δ* = 3.8 μm, I _max = 2.95 A/cm², W _max = 1.76 W/cm². At potential E ₀ = 0.5 V, Δ* = 1.4 μm, I _max = 7.71 A/cm², W _max = 3.86 W/cm². The overall current values are higher than those observed experimentally at the given cathode potentials. The discrepancy is explained by the fact that calculations of active cathode layers with a practically regular structure were carried out. All combined support grains in them are full and identical, while in fact the active layer structure is not characterized by the properties of fullness and equivalence. The second circumstance is that experimental active layers rarely have a strictly optimum thickness. Meanwhile deviation from this optimum results in losses in current. Transition to cathodes with combined grains has additional advantages. (1) In such grains, all platinum participates in current generation, the catalyst utilization degree reaches 100%. (2) Oxygen can enter the active layer not through small Knudsen pores, but through large (with the diameter of hundreds and more nm) gas pores, in which usual molecular gas diffusion occurs, so that diffusion limitations in the active layer become less significant. 3. In the active layer, the danger of gas pore flooding by evolving water decreases. Now, water vapor is much more easily removed from large gas pores directing then into the gas-diffusion layer pores.     

Anodic film formation on osmium electrodes: Part II. Galvanostatic and potentiostatic measurements

The formation and reduction of anodic films on Os electrodes in 2 M HCl and HClO₄ solutions were studied by anodic and cathodic charging curves. The galvanostatic oxidation of Os in HClO₄ shows the formation of OsO₂ as an intermediate step to OsO₄ that goes in the solution. The cathodic charging curves at Os electrodes previously oxidized at constant potential reveal the anodic film to be made up of a reversibly desorbed oxygen layer and an oxide phase reduced irreversibly. Both layers increase with time under potentiostatic conditions following a logarithmic equation until a constant value is reached. At all times, the content of OsO₂ in the anodic film at high potentials is larger than that of chemisorbed oxygen.In HCl solutions only the reversible reduction of an oxygen layer is observed. The growth of this film also complies with a direct logarithmic law before attaining a limiting coverage. The charge involved in the reduction increases linearly with the potential at a given time of formation. The results are discussed in terms of a Temkin-type isotherm and a place-exchange mechanism.     

Computer-aided simulation of the cathodic active layer in fuel cells with solid polymer electrolyte: the nature of overall current transient

Total computer-aided simulation of the structure and current-generation processes in the cathodic active layer of a fuel cell with solid polymer electrolyte is carried out. Not only the transport structure of the active layer but also the structure of support grains (agglomerates of carbon particles with platinum-covered surface) are modeled. The process of active layer functioning under potentiostatic conditions is studied. It is demonstrated for the first time how the moisture exchange in the pores of support grains affects the cathode overall characteristics. The time variations of the overall current, the average temperature of the active layer, and the total degree of water-flooding of support-grain pores within the active layer are calculated by numerical methods. It is shown that for the fuel cell voltage of 0.6 V and its working temperature of 80°C, the flooding process dominates over the process of drying of pores in support grains. In 10–15 s, all support-grain pores turn out to be entirely filled with water. Then they begin functioning not in the kinetic mode (in the moment of switching-on the current, the Knudsen diffusion of oxygen in the support grains is observed) but in the inner-diffusion mode. As a result, the overall cathodic current decreases from its initial value of 4.323 A/cm² to its final value of 0.526 A/cm² and the active layer temperature decreases from the initial value of 102°C to the final value of 82.5°C. The overall current transient is studied also experimentally, the qualitative coincidence of theoretical and experimental data is demonstrated.     

A cathode for solid polymer electrolyte fuel cells: Designing the optimal structure of the active layer

The complete computer simulation of the cathodic active layer with solid polymer electrolyte (Nafion) is carried out. The active layer structure can be described by 8 parameters. In designing the optimal structure, it is shown that to provide the high overall characteristics of the cathode and save the catalyst, 0.5 of the active layer volume should be set aside for the support grains (agglomerates of carbon particles covered with platinum and containing Nafion incorporations and microvoids). Protons and oxygen molecules must be supplied to the active layer by means of peculiar combined percolation clusters. The latter consist of a combination of support grains with either Nafion grains (to produce “protonic” clusters) or grains-voids (to afford “gas” clusters). The volume fractions of Nafion grains and grain-voids are assumed to be 0.25 and 0.25. The computer simulation of the support grain structure is also carried out. Their composition, i.e., the volume fractions of the carbon component (g _e), Nafion (g _ii), and microvoids (g _gg), is varied. The support grains play the key role in the active layer functioning. It is impossible to organize three full-value percolation clusters (electronic, protonic, and gas); hence, one has to have one or two combined clusters in the active layer. Thus the double load fells on the support grains. Their optimal structure should not only sustain the transport of protons and electrons in the active layer but also create the best conditions for the electrochemical process in each grain. The maximum current I _max (realized upon reaching the optimal active layer thicknesses Δ*) is calculated. The dependences of I _max and Δ* on the main parameters characterizing the support grains (g _e and g _ii) are analyzed. Here, two goals are sought: (1) to obtain the high currents, (2) to provide the low consumption of platinum per power unit. To solve the first problem, one has to work with high values of g _e. The second problem requires the opposite: the values of g _e must be minimal possible.     

Etude du comportement électrochimique du manganèse dans le tampon borate

The anodic behavior of the manganese electrode in borate buffer solutions have been studied by potentiostatic and galvanostatic electrochemical techniques. Three distinct steps determined on the charging curves are attributed to the formation of Mn (OH)₂, MnOOH and MnO₂. The manganese dioxide formed anodically in pH 11 borate solution may be reduced cathodically with high faradaic efficiency.     

Investigation of ferromagnetic layer dissolution characteristics during pulse electrodeposition of FeCoNiCu/Cu multilayers

Minimization of interface roughness of electrodeposited metallic multilayers with nanometer thick sublayers during pulse electrolysis largely depends on the choice of potential pairs and electrolyte. In this study, a systematic effort has been made to identify the different origins of less noble ferromagnetic (FM) layer dissolution during two pulse potentiostatic electrochemical deposition (ECD) of FeCoNiCu/Cu multilayers and its subsequent minimization. Detailed investigations by employing cyclic voltammetry and chronoamperometry, it was established that the FM layer dissolution not only depends on copper layer discharge potential (E _Cu) but also on FM layer discharge potential (E _FM) and Cu²⁺ concentrations. Both the cell capacitance effect and galvanic displacement reactions are accountable for corrosion of FM layer. It was found that potentials, E _FM = ?2.3 V vs. SCE and E _Cu = ?0.6 vs. SCE are the optimum pair for growth of FeCoNiCu/Cu multilayers with minimum dissolution of FM layer.     

XPS spectroscopic study of potentiostatic and galvanostatic oxidation of Pt electrodes in H2SO4 and HClO4

The surface oxides produced from potentiostatic and galvanostatic oxidation of Pt electrodes in HClO₄ and H₂SO₄ are examined using X-ray photoelectron spectroscopy. The oxide I species produced as the initial oxidation product by successively more anodic potentiostatic oxidation in 0.2 M HClO₄ is found to have a Pt²⁺ oxidation state, a binding energy characteristic of neither PtO, Pt(OH)₂ or PtO₂, and a limiting thickness of 8 Å. Galvanostatic oxidation in HClO₄ and H₂SO₄ is found to produce PtO₂·H₂O as an unlimiting growth oxide or a limiting growth oxide layer depending on the concentration of the acid electrolyte. The incorporation of the acid electrolyte anion in the surface layer is shown to have an effect on which type of oxide layer is produced. X-ray decomposition and chemical modification by Ar⁺ stripping are shown to produce chemical artifacts complicating any interpretation of a Pt oxide surface layer.     

Computer simulation of positive electrode operation in lithium-ion battery: Optimization of active mass composition

The work of the positive electrode (cathode) of a lithium-ion battery is simulated. The model of equally sized grains of three types: the intercalating agent grains with a volume fraction g, the electrolyte grains with a volume fraction g _i, and the carbon black grains with a volume fraction g _e is studied. The optimal composition of cathode active mass providing maximum specific capacity of cathode is determined. It is shown that a fraction of carbon black grains should be as small as possible: g _e = 0.35. The variation in the fraction of intercalating agent grains within the allowable limits (0 ?? g ?? 0.3) changes the main parameters of cathode active mass: a fraction of electrochemically active intercalating agent grains g* (g* < g); a specific surface area S, on which the electrochemical process proceeds; and the conductivity k* by lithium ions in the ionic percolation cluster, which forms in the cathode active mass. The parameters g* and S decrease and parameter k* steeply increases with decreasing g. Therefore, in the range of possible values of g, specific capacity of cathode reaches the maximum value at g = g _opt. The value of g _opt is determined under the galvanostatic mode of cathode discharge. The cathode working parameters: the active layer thickness, discharge time, specific capacity, and potential at the cathode active layer/interelectrode space interface at the instant of discharge completion are calculated in relation to a fraction of intercalating agent grains g.     

Fluorination effect of activated carbon electrodes on the electrochemical performance of electric double layer capacitors

The surface of phenol-based activated carbon (AC) was fluorinated at room temperature with different F₂:N₂ gas mixtures for use as an electrode material in an electric double-layer capacitor (EDLC). The effect of surface fluorination on EDLC electrochemical performance was investigated. The specific capacitance of the fluorinated AC-based EDLC was measured in a 1 M H₂SO₄ electrolyte, in which it was observed that the specific capacitances increased from 375 and 145 F g⁻¹ to 491 and 212 F g⁻¹ with the scan rates of 2 and 50 mV s⁻¹, respectively, in comparison to those of an unfluorinated AC-based EDLC when the fluorination process was optimized via 0.2 bar partial F₂ gas pressure. This enhancement in capacitance can be attributed to the synergistic effect of increased polarization on the AC surface, specific surface area, and micro and mesopore volumes, all of which were induced by the fluorination process. The observed increase in polarization was derived from a highly electronegative fluorine functional group that emerged due to the fluorination process. The increased surface area and pore volume of the AC was derived from the physical function of the fluorine functional group.     

Theoretical study of the electrokinetic and electrochemical behaviors of two-layer composite membranes

A theoretical study concerning the effect of structure (porosity, pore radius and layer thickness) and surface characteristics (zeta potential) of two-layer composite membranes on global streaming potential (SP_g), membrane potential (Em_g) and membrane conductivity (λ_g) is presented. To this end, each layer of the composite membrane (composed of a support layer and a filtering layer) was modeled as a bundle of identical capillary tubes with connections between pores of the two layers (the pores in the filtering layer being smaller than those of the support layer). The global parameters SP_g, Em_g and λ_g were calculated by using the theory of thermodynamics of irreversible processes and a space charge model. SP_g, Em_g and λ_g were expressed as a function of the individual parameters of each layer SP⁽ⁱ⁾, Em⁽ⁱ⁾ and λ⁽ⁱ⁾, respectively, the length fraction of the support layer, the porosity and pore radius ratios. It was shown that the electrokinetic (streaming potential and membrane conductivity) and electrochemical (membrane potential) behaviors of such composite membranes vary between that of single layers. For streaming potential, the results indicate that the contribution of the filtering layer to the global streaming potential is very little influenced by zeta potentials of both types of pores. It appears that the individual streaming potential of the filtering layer greatly dominates the global streaming potential. This is due to the fact that the streaming potential of the filtering layer is weighted by the pore radius ratio which is a predominant parameter in determining the global streaming potential. In contrast to the streaming potential, the contribution of the filtering layer to the global membrane potential (Em_g) or membrane conductivity (λ_g) depends more or less on the zeta potentials of both kinds of pores and the corresponding electrokinetic radii as well. As to the membrane potential, the contribution of the filtering layer to Em_g is all the more sensitive to the zeta potentials than the electrokinetic radii are small. The filtering layer greatly dominates the global membrane potential when its pores are narrow (with regard to the Debye length) and strongly charged. For the electrolyte conductivity inside pores, the smaller pores (inside the filtering layer) have an effect all the more dominant on the apparent membrane conductivity than their zeta potential is low and that of larger pores (inside the support layer) is high.     

Properties of arsenic in molten potassium tetrachlorogallate at 300°C: Formation of gallium arsenide

Redox properties of As(III) species have been studied in molten KGaCl₄. The plot of equilibrium potential of As vs acidity is interpreted in terms of reaction AsCl⁴_? + 3 e = As + 4 Cl^?. X-Ray analysis of electrodeposits shows that only As is formed under potentiostatic conditions. Conversely, gallium arsenide is observed under galvanostatic conditions, for which the electrode potential becomes very negative.     

Application of convolution procedures to chronopotentiometry

Convolution procedures are used to extract the faradaic information from chronopotentiometric data, in conditions where significant distortion by double layer charging occurs. The faradaic component of the imposed current is obtained, after measurement of the double layer capacitance, by differentiation of the initial chronopotentiogram. Convolution of this current with the function (πt)^?1/2 leads to a potential-convoluted current relationship freed from the effect of double layer charging. The kinetic characterization of the system using a combined analysis of this relationship and that relating the faradaic current to the electrode potential is discussed for the various types of reaction mechanism. The efficiency of the proposed procedure is tested on the galvanostatic reduction of fluorenone in DMF.     

Pitting of Al and Al–Si alloys in KSCN solutions and the effect of light

The pitting corrosion susceptibility of pure Al and three Al-Si alloys, namely (Al-6%Si), (Al-12%Si) and (Al-18%Si) has been studied in 0.04 M KSCN solution. Measurements were carried out under the effect of various experimental conditions using cyclic polarization, potentiostatic and galvanostatic techniques. In all cases, the potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of a thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion, at a certain critical potential, pitting potential (E_pit), as a result of breakdown of the passive film by SCN^? anions. Cyclic polarization measurements allowed the determination of the pitting corrosion parameters, namely the pitting potential and the repassivation potential (E_rp). Alloyed Si decreased the passive current (j_pass) and shifted both E_pit and E_rp towards more positive values. Thus alloyed Si suppressed pitting attack. The effect of illumination on passivity and the initiation of pitting corrosion on Al in KSCN solutions was also studied. It is observed that illumination of Al leads to an increase in its pitting corrosion resistance-apparent from j_pass, E_pit, and E_rp measurements in aggressive KSCN solutions.     

Kinetics of passivity of NiTi in an acidic solution and the spectroscopic characterization of passive films

Anodic polarization of nitinol in acetic acid under galvanostatic conditions produces oxide films composed mainly of TiO₂. An exponential current-field relation is valid during ionic conduction through the growing oxide, in which the field coefficient is related to the jump distance. Transport processes in anodic films have been discussed in terms of a cooperative mechanism developed for amorphous oxide films on valve metals, in which both metal and oxygen ions were involved in ionic conduction. For more crystalline oxide structure of passive films on nitinol, formed during a prolonged potentiostatic conditions, the charge transfer takes place only through the oxygen vacancies as mobile species via a high-field-assisted mechanism. Based on the results of the Mott–Schottky analysis, these films behave as n-type semiconductors indicating that oxygen vacancies formed during the film formation and growth act as electron donors. The barrier/protecting and electronic/semiconducting properties of the passive films as well as their chemical composition were studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy.     

Cathode of a hydrogen-oxygen fuel cell with a solid polymer electrolyte: The effect of the flooding of pores by water on the characteristics of the active layer

A computer model of the active layer of the cathode of a hydrogen-oxygen fuel cell with a solid polymer electrolyte is studied. The active mass of the electrode consists of equidimensional grains of the substrate (agglomerates of carbon particles with platinum particles embedded in them) and a solid polymer electrolyte (Nafion). The flooding by water can be experienced by both the pores in the substrate grains, which facilitate the oxygen penetration into the active layer of the electrode, and the voids between the grains. All possible versions of the flooding of these pores by water are considered. A calculation of the optimum, at a given polarization of the electrode, value of electrochemical activity, the thickness of the active layer, and the weight of platinum is performed. The major parameters of the system are the concentrations of grains of the substrate and solid polymer electrolyte, the size of these grains, the platinum concentration in the substrate grains, the average diameter of pores in the substrate grains, and the polarization of electrodes. The ultimate aim of the work is to estimate how the flooding of pores of the active layer of the cathode by water affects the magnitude of the optimum current, the effective thickness of the active layer, and the weight of platinum.Translated from Elektrokhimiya, Vol. 41, No. 1, 2005, pp. 35–47.Original Russian Text Copyright © 2005 by Chirkov, Rostokin.     

Understanding active chlorine species production using boron doped diamond films with lower and higher sp3/sp2 ratio

The present study was motivated by the reports that promote the use of boron doped diamond (BDD) anode for electrochemical disinfection. The discussion about the production of undesirable active chlorine species on diamond films is still open. For this reason, the influence of sp³/sp² ratio on the performance on the evolution of chlorine-related species was investigated by polarization and electrolytic techniques in order to establish whether their formation and consumption related to either chemical or electrochemical reactions. The results demonstrated that dissolved Cl₂, ClO₂⁻, ClO₂, ClO₃⁻ and ClO₄⁻ species can be electrochemically formed at both BDD electrodes. However, the concentration trends are different, indicating that the relation of sp³/sp² ratio has a key role in the electrochemical route to produce ClO₃⁻ and ClO₄⁻.     

On the Reactivity of Silver Electrodes in a Chlorine Radiofrequency Plasma — Plasma Oxidation vs. Thermal Oxidation

The plasma‐electrochemical growth of an ion‐conducting film by the oxidation of a metal in an electronegative plasma is investigated and compared with results from thermal oxidation. As model system we studied the oxidation kinetics of silver electrodes in a Cl₂ rf plasma. The electrochemical control of the reaction by external currents through the product layer using the plasma as a fluid electrode was achieved. Both potentiostatic and galvanostatic control of the reaction was applied. The morphology of the product layer and its temporal evolution was investigated using SEM. A formation of silver chloride surface patterns in the oxidation process takes place if a simple stability criterion is not fulfilled. Specific surface morphologies were found under different experimental conditions. The morphology of the product layer is influenced by the external electric current and the substrate temperature. The influence of the plasma phase on the thermodynamics and kinetics of the oxidation process is discussed. The role of excited plasma species, the electrical charging of the surface and radiation from the plasma are taken into account.