Computer modeling of positive electrode operation in lithium-ion battery: Model of equal-sized grains,percolation calculations

A computer simulation of the negative electrode (anode) operation in a lithium-ion battery is performed. A complete research program is carried out in accordance with the recommendations of the theory of porous electrodes: the “model of equal-sized grains of two types” was studied, percolation properties of the anode active layer were researched, values of effective coefficients were calculated for charge transfer and mass transport, a complete system of equations describing operation of the anode is presented. Two specific cases of galvanostatic mode of anode discharge are considered in detail: an “ideal” anode and anode with nanosize particles. Working anode parameters are calculated: optimum bulk concentration of graphite in the active layer, active layer thickness, time of complete anode discharge, its specific electric capacitance and final potential on the active/layer interelectrode space interface. Advisability of working with anodes with nanosize grains and electrolyte with enhanced specific conductivity is shown.     

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.     

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.     

Active layer of fuel cell electrode with polymer electrolyte: Modeling of support grains, calculation of overall cathode characteristics

The active layer of the cathode of a fuel cell with polymer electrolyte (Nafion) is considered. The optimum carbon support structure is constructed using computer simulation: its carbon “skeleton” possesses the maximum outer surface area and provides electronic conductivity of the grains, support cubes, along the three coordinate axes. Nafion is absent in the support grain, so that the grain is capable of participating only in the transport of oxygen molecules, it possesses no proton conductivity. An estimate of all parameters of an optimum support grain is provided; in particular, the value of the effective Knudsen diffusion coefficient of oxygen is established. After this, effective proton conductivity and effective Knudsen diffusion coefficient are calculated already on the whole active layer scale, according to the model of equally sized cube grains of three types. In conclusion, the overall current in the active layer of a cathode with a polymer electrolyte was calculated for the percolation cluster consisting only of Nafion grains and the Knudsen diffusion of oxygen created only by a combined gas percolation cluster consisting of void grains and all support grains. The overall current value for t = 80°C and pressure of p* = 101 kPa proved to be low, hundreds of mA/cm². The current value can apparently be increased to several A/cm² if the support grains are developed that would simultaneously possess both proton conductivity and ability to sustain oxygen diffusion.     

Cathodic active layer in a polymer electrolyte fuel cell: Model of combined grains,calculation of overall characteristics

A new type of the cathodic active layer structure for a polymer electrolyte fuel cell is proposed. This structure is based on combined grains and gas pores. Combined grains represent nonporous agglomerates of carbon black particles (catalyst carrier) and Nafion molecules. This type of cathodes has the following advantages: (1) in combined grains, complete utilization of the catalyst occurs, (2) limitations on the oxygen delivery into the active layer are almost totally lifted, and (3) the danger that pores will be flooded with evolved moisture is actually released. The overall characteristics of cathodes with combined grains are calculated. The advantages of such oxygen or air cathodes are demonstrated, namely, not only their enhanced power density but also the lower index of platinum consumption, i.e., the platinum amount per kW of electric energy produced in the membrane-electrode block, as compared with conventional cathodes.     

Effect of operation parameters on the sputtering and emission processes in radiofrequency glow discharge. A comparison with the direct-current mode

In this paper, a comparison of the direct current (DC) and radiofrequency (RF) operating modes in glow discharge optical emission spectrometry (GD-OES) is carried out using the same discharge chamber, based on the Marcus design, powering alternatively with DC or RF energy. The effect of discharge pressure, the DC bias voltages and the delivered power divided by the DC-bias voltage on the sputtering rates, emission intensities and emission yields achieved for conducting materials was investigated in order to characterize both discharge types. Our results show that the effect of plasma variables on sputtering rates and emission yields using a DC-GD based on the chamber described by Marcus, can be considered to follow trends similar to those of the well-known DC-Grimm source. However, if the effect of plasma variables are compared for a DC-GD and a RF-GD, both generated by the same source as designed by Marcus, the behaviour of the DC and RF operations of the source proved to have some differences. Thus, at a fixed delivered power, the sputtering rate in the DC-GD decreases noticeably with pressure while the reverse effect is observed in the RF-GD. Moreover, under selected operating conditions, using the tin emission line (Sn I 380.102 nm), lower sputtering rates and higher emission yields were observed for the RF-GD than for the DC-GD source. Extension of known theoretical expressions and concepts from analytical DC-GD to RF-GD-OES work appears rather involved and is not yet possible.     

Influence of mobile phase,source parameters and source type on electrospray ionization efficiency in negative ion mode

Electrospray ionization (ESI) efficiency is known to be affected by the properties of the analytes, source design and source parameters. In this study, the ionization efficiency of 17 acidic compounds at various conditions in ESI negative ion mode was evaluated. Namely, the influence of organic solvent content in the mobile phase, ionization source parameters, ionization source geometry and functionality (conventional ESI, ESI with thermal focussing and with additional internal nebulizer gas) was studied. It was observed that the ionization efficiency in thermal focussing ESI is only marginally affected by the organic solvent composition, while for conventional ESI and ESI with internal nebulizer gas, the ionization efficiency increases significantly with increasing organic modifier content. For all ionization sources and mobile phase compositions, the ionization efficiency values between different setups showed good correlation. Copyright © 2016 John Wiley & Sons, Ltd.     

SnO2 negative electrode for lithium ion cell: in situ Mössbauer investigation of chemical changes upon discharge

In situ ¹¹⁹Sn Mössbauer study of an SnO₂ electrode was performed during discharge of a lithium ion cell. The first step is lithium intercalation into the SnO₂ host structure. This lithium intercalation results in reinforcement of the SnO₂ lattice instead of direct decomposition of the oxide upon reduction. This first step is followed by the reduction of tin dioxide into unusual tin species (possibly “exotic” forms of Sn(II) or Sn(0)). The last step of the discharge consists in Li-Sn alloy formation. However, non-reduced SnO₂ is present nearly up to the end of the discharge despite a very low discharge regime. It seems highly probable that this fact is related both to slow Li diffusion and disconnection of SnO₂ particles due to Li₂O formation. The working electrode appears to be rather far from equilibrium during continuous discharge, which means that ideal succession of well-defined stages cannot describe the real phenomena involved in the operating battery.     

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.     

Confidence limits,error bars and method comparison in molecular modeling. Part 1: The calculation of confidence intervals

Computational chemistry is a largely empirical field that makes predictions with substantial uncertainty. And yet the use of standard statistical methods to quantify this uncertainty is often absent from published reports. This article covers the basics of confidence interval estimation for molecular modeling using classical statistics. Alternate approaches such as non-parametric statistics and bootstrapping are discussed.     

Computer simulation of active layers in the electric double layer supercapacitor: Optimization of active layer charging modes and structure,calculation of overall characteristics

The structure and functioning modes of active layers in an electric double layer capacitor (EDLC) with an aqueous electrolyte are simulated by means of a computer. A model of active layers prepared from activated carbon materials is proposed, percolation estimates are performed and effective ionic conductivities are calculated. The polarization of active layers includes a sequence of two charging processes: first, galvanostatic and then potentiostatic. The proposed program of calculations involves mutual matching and optimization of seven parameters characterizing the active layer and conditions of charging processes. According to calculations, galvanostatic polarization of wide pores in the EDLC biporous active layer up to the limiting potential followed by potentiostatic polarization of fine pores allows the capacity C _sp = 246 F/g and the energy W _sp = 107 kJ/kg to be obtained in fractions of second.     

Sensitivity of doses and associated uncertainties in bioassay estimates to ICRP Publication 66 Respiratory Tract Dosimetric Model with respect to the parameters: Ventilation rates, time budget, total and regional deposition, oral versus nasal breathing and clearance rates of compartments

An analysis of the reliability of the ICRP's dose coefficients for intake of radionuclides, applied on the Human Respiratory Tract Dosimetry Model proposed in ICRP Publication No. 66 was carried out, with respect to the following variables: ventilation rates, time budget, total deposition, regional deposition for the four respiratory tract compartments (alveolar-interstitium, bronchioles, bronchi and extrathoracic), oral versus nasal breathing patterns, and variation in clearance rates of compartments. The analysis was done by calculating reliability factors defined as the square root of the ratio of larger to smaller dose coefficient calculated at the extreme values for the model parameter being tested, in intervals of values of the effective dose. Calculations for each of the variables were carried out for an adult, using these 12 radionuclides: ³H, ⁶⁰Co, ⁹⁰Sr, ⁹⁵Zr, ¹⁰⁶Ru, ¹²⁵Sb, ¹³¹I, ¹³⁷Cs, ²¹⁰Pb, ²²⁶Ra, ²³⁸U and ²³⁹Pu. For AMAD = 0.1 mm the analysis associated with the total deposition in the compartments indicated a Reliability Category II. For AMADs = 1.0 and 10 mm the analysis associated with the deposition in the extrathoracic compartments indicated a Reliability Category II. For AMAD = 10 mm the deposition in the compartments of the tracheobronchial region also showed a Reliability Category II. Most results for all other parameters for the studied AMADs were found to be in Category I. The corresponding impacts on the uncertainties in the predicted bioassay results for these twelve radionuclides were also determined. This analysis is especially helpful when doses are estimated through bioassay measurements employing the ICRP Publication 66respiratory tract model.