Fuel cell cathode with Nafion and platinum: Calculating its overall performance with consideration of gas-and vapor-exchange processes in the gas-diffusion layer

The overall performance of cathodes with solid polymer electrolytes is calculated for three different cases in which gas-and vapor-exchange processes in the gas-diffusion layer are considered. First of all, specific results are presented for calculating the overall performance of oxygen and air cathodes under the assumption that external diffusion restrictions are completely absent. Then with these same conditions (i.e., all the parameters characterizing the active layer of the cathode remain the same), results of the overall performance are given for the idealized case in which gas exchange in the gas-diffusion layer causes the appearance of limiting currents, but in this case vapor exchange is completely ignored. Finally, an analysis is carried out to determine how vapor-exchange processes in the gas-diffusion layer alter overall cathode performance and what measures should be taken to reduce the significance of external diffusion restrictions.     

Active layer of the cathode of a fuel cell with a solid polymer electrolyte: The effect of the Nafion concentration on the overall characteristics

A computer-aided simulation of the structure of the active layer of the cathode of a fuel cell with a solid polymer electrolyte (Nafion) is performed under the assumption about equidimensionalness of dimensions of grains of the substrate (with platinum crystallites in them) and grains (agglomerates of molecules) of Nafion. It is analyzed how the Nafion concentration affects principal parameters, which include the specific surface area, in the vicinity of which electrochemical process goes on; the effective ionic electroconductivity, and the effective diffusion coefficient of a gas. It is demonstrated how one can determine the Nafion concentration at which the overall current takes on a maximum value. Dependences of the optimum value of the overall current and the thickness of the active layer and the weight of platinum, which correspond to it, on the Nafion concentration are calculated. It is demonstrated that there in principle cannot exist one individual optimum concentration of Nafion, which is suitable for all techniques used for the preparation of the active layer. The mutual relationship between values of the effective diffusion coefficient of a gas and the effective ionic electroconductivity of Nafion determines the value of the optimum of the Nafion concentration.     

Mass transfer in porous systems, flooded in part with water, of gas diffusion and catalytic layers of the cathode of a fuel cell with a solid polymer electrolyte

Mass transfer in porous gas diffusion and catalytic layers of the cathode of a hydrogen-air fuel cell with a solid polymer electrolyte is considered. The transport processes are considered with allowance made for the partial flooding of porous systems of these layers with water, which forms during the fuel cell operation. The consideration also allows for the influence of the diluent gas present when air oxygen is used as the oxidant. The fraction of water-flooded pores is calculated within percolation theory as a function of structural parameters of the porous system. Conditions leading to the beginning of the gas diffusion layer flooding are presented.     

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.     

Hydrophobized oxygen cathode of a fuel cell with a liquid electrolyte: Calculating overall currents and thicknesses

The mechanism governing operation of hydrophobized cathodes is discussed. A model is proposed for the active-layer structure. The model consists of equidimensional hydrophobic (agglomerates of polytetrafluoroethylene particles) and hydrophilic (agglomerates of carbon black particles with the catalyst on them) grains. The percolation characteristics of the model are calculated: the presence of a gas cluster and an ionic cluster is established, the specific area of contact between these clusters is determined, the magnitude of ionic conductivity is assayed, and so forth. The “model of cylindrical gas pores” is selected for calculating the overall current. Formulas for the bulk current density are determined. The overall characteristics of a cathode with a platinum catalyst on a carbonaceous carrier (on carbon black) in 7 M KOH at a temperature of 60°C are calculated with allowance made for the fact that the Tafel plots for the process of reduction of oxygen on platinum have two segments with different slopes.     

Model for the multicomponent gas diffusion in a fuel cell porous electrode

A simple analytically solvable model for the diffusion of a multicomponent vapor-air mixture is proposed. The model, which accurately accounts for the oxygen and water balance on the gas diffusion cathode of a fuel cell, is used for computing polarization characteristics of a fuel cell with a polymer electrolyte. An analytical solution for the cathodic overvoltage in the extreme cases of high and low current densities is derived. The results are compared with the available theoretical and experimental data. It is shown that the solutions of the proposed model coincide with the solutions provided by the Bernardi-Verbrugge model, which is far more involved.     

Hydrogen-oxygen (air) fuel cell with nafion and platinum: Calculating overall characteristics,comparing the performance of a cathode with polymeric electrolyte with a hydrophobized cathode with liquid electrolyte

Results of calculating the major overall characteristics of both an individual cathode and the whole hydrogen-oxygen (air) fuel cell with Nafion and platinum are shown. The effect of varying the parameters of both the active layer and the polymeric-electrolyte membrane on the overall characteristics of such a fuel cell is analyzed. The mechanisms of operation of active layers of hydrophobized cathodes and cathodes containing Nafion are compared. These two electrode types demonstrate a qualitative difference in the current generation mechanisms. As a result, the current in cathodes with Nafion increases more actively with the increase in over-potential (in proportion with exp [η₀/2], where η₀ is the cathodic overpotential) as compared with the case of hydrophobized cathodes (here the current ～ exp[η₀/4]). This explains the fact that a fuel cell with Nafion demonstrates so high power characteristics as compared with a fuel cell with hydrophobized electrodes and liquid electrolyte.     

Synthesis and test of palladium-based nanostructured anodic electrocatalysts for hydrogen fuel cells with solid polymer electrolyte

Palladium-based nanostructured electrocatalysts on the Vulcan XC-72 carbon support for fuel cells with solid polymer electrolyte are synthesized and studied. In particular, electrochemical studies of the synthesized catalysts are carried out and membrane-electrode assemblies are assembled on their basis and tested. The test results indicate that platinum can be replaced with palladium in the hydrogen electrode of the fuel cells.     

Calculating a Characteristic Bulk Current Density in the Cathode of a Hydrogen-Oxygen Fuel Cell with a Solid Polymer Electrolyte

The magnitude of currents of electrodes in hydrogen-oxygen fuel cells of all types is shown to be fully determined by values of the effective coefficient of gas diffusion, the effective coefficient of ionic conduction, and the characteristic bulk current density. The characteristic bulk current density is estimated in two versions for cathodes with Nafion: the catalyst is distributed in the bulk of substrate grains or at their external surface. The currents commensurate with those observed in experiments are given only by the second version. Means of computer-aided simulation are used to imitate the formation of fractal films composed of the catalyst particles on the surface substrate grains. The simulation means made it possible to link the magnitude of the specific surface area of platinum particles with its weight content in substrate grains. Electrochemical characteristics of the cathode with Nafion-the potential dependence of the optimum magnitude of the overall current and the thicknesses of the active layer and the weight of platinum in it, as well as the magnitudes of the optimum current generated by a unit weight of platinum—are calculated. A notion of “ norm” is introduced for the characteristic bulk current density of the cathode. 1 × 10^?3 A cm^?3 is the electrochemical-process intensity, which the technology of preparation of active layers of cathodes can provide at this stage in the development of fuel cells with a solid polymer electrolyte.     

Theory of porous electrodes: Calculation of overall cathode characteristics for the case where the polarization curve has segments with different slopes

It is demonstrated how one should carry on with calculations of overall currents and other parameters that characterize active layers of porous electrodes in the case where polarization curves for the catalyst display two or more segments with different slopes and exchange currents. A calculation of overall currents presumes that the active layer of an electrode has an optimum thickness, over which the current reaches a maximum. The entire range of values of the cathode potential is considered, specifically, the high potentials (from a steady-state potential up to the point where there is observed an inflection in the polarization curve), the intermediate potentials (near the front surface and near the rear surface of the active layer there are realized segments of the polarization curve with different slopes), and the low potentials (throughout the entire thickness of the active layer there is observed a second segment of the polarization curve). To give an example, calculations of overall characteristics of a cathode with Nafion and platinum are performed.     

Active layer of the oxygen cathode in a fuel cell with Nafion and platinum: The role played by the diffusion and ohmic restrictions and the selection of the working thickness of the active layer

The basic parameters that characterize the operation of the active layer of a cathode with Nafion are the effective coefficient of the diffusion of oxygen, the effective ionic conductance, and the thickness of the active layer. One of the deficiencies intrinsic to the fuel cells containing Nafion is their extreme sensitivity to the heat and moisture exchange. Nafion demands an optimum degree of humidification. Upon thoroughly draining the active layer of a cathode with Nafion, its effective ionic conductance substantially lowers, and large diffusion restrictions arise following the flooding of pores in the active layer. The goal of this work is to perform a comparison of values of some dimensional characteristics pertaining to the flooded and thoroughly drained active layers of a cathode with similar indicators of an active layer in its optimum (normal) state. It is demonstrated how one should perform the selection of the working thickness of an active layer that would provide for the efficiency of its functioning.     

Effect of Bi0.75Y0.25O1.5 electrolyte additive in collector layer to properties of bilayer composite cathodes of solid oxide fuel cells based on La(Sr)MnO3 and La(Sr)Fe(Co)O3 compounds

The sintering features, electroconductivity, and electrochemical characteristics of bilayer electrodes with functional composite layers based on La(Sr)MnO₃ (LSM) and La(Sr)Fe(Co)O₃ with LSM collector layer and Bi(Y)O_1.5 (YDB) electrolyte additive in contact with Ce (Sm)O₂(SDC), La(Sr)Ga(Mg)O₃, and Zr(Sc)O₂ electrolytes were studied. YDB additive to the electrode collector layer was shown to produce a positive effect to the properties of the studied electrode systems. The maximum electrochemical activity and electroconductivity was observed for the electrodes with 5 wt % of YDB electrolyte additive in the collector layer. Thus, electroconductivity of electrodes is almost doubled and 100 mV cathode overvoltage current density is increased by 30% at the temperatures of 800 to 900°C and up to 10-fold at 650 to 700°C. The collector layer sintering temperature of bilayer electrodes can be reduced from 1150 to 1000°C without loss of electrochemical activity. The service life tests (about 1200 h) of composite electrodes with LSM2-SDC functional layer and 90% LSM2 + 10% YDB collector layer in contact with SDC electrolyte showed the time dependences of polarization resistance tending to saturation and described with damped exponent. Original Russian Text ? N.M. Bogdanovich, D.I. Bronin, G.K. Vdovin, I.Yu. Yaroslavtsev, B.L. Kuzin, 2009, published in Elektrokhimiya, 2009, Vol. 45, No. 4, pp. 486–494.