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.     

Optimizing Weights of Platinum in the Active Layer of the Cathode of a Hydrogen–Oxygen Fuel Cell with a Solid Polymer Electrolyte

The active layer of the cathode of a hydrogen–oxygen fuel cell with a solid polymer electrolyte is computer simulated. The active mass of the electrode consists of substrate grains (agglomerates of carbon particles with Pt particles embedded into them) and grains of a solid polymer electrolyte (Nafion). The substrate grains presumably contain hydrophobic pores, which facilitate the oxygen penetration into the active mass. A calculation of characteristics of such an electrode focuses on the optimization of platinum weights. The principal parameters of the system are concentration and size of grains of substrate and Nafion, Pt concentration in substrate grains, average diameter of hydrophobic pores in substrate grains, and the electrode polarization. The optimum, at a given electrode polarization, electrochemical activity of the active layer, its thickness, and the platinum weight are calculated. A link between these quantities and principal parameters of the active layer is revealed.     

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.     

Cathode of a fuel cell with a solid polymer electrolyte: Calculating overall currents in the presence of a gas-diffusion layer

Mathematical apparatus, which makes it possible to perform calculations of the current-voltage characteristics of cathodes of fuel cells with a solid polymer electrolyte in conditions where there are present extraneous diffusion restrictions is proposed. In so doing, the partial pressure of oxygen and the absolute pressure of gas in the gas chamber may assume any values. First of all presented are the results of calculations of the current-voltage characteristics intrinsic to active layers of the air and oxygen cathodes, which are performed under the assumption that the extraneous diffusion restrictions are absent altogether. Thereafter, in the same conditions (at the same parameters that characterize the active layer of a cathode), obtained are results of a calculation of the current-voltage characteristics inherent in the air and oxygen cathodes in the presence of extraneous diffusion restrictions. Afterward there is performed an analysis of the way a gas-diffusion layer restricts the process of generation of current in a cathode and of what measures should be taken in order for the extraneous diffusion restrictions to become less significant.     

Calculation of optimum thickness of active layer of oxygen and air cathodes of fuel cell with Nafion and platinum

It is shown that, for the electrodes of fuel cells with solid polymer electrolyte, the dependence of overall current on the active layer thickness contains an extremum. There is an optimum thickness of active layer, at which the overall current reaches its maximum possible value. The nature of this dependence is explained. The character of the distribution of electrochemical process intensity over the depth of active layer of cathode with solid polymer electrolyte is analyzed. The optimum thicknesses of active layers of oxygen and air cathodes of fuel cells with Nafion and platinum and the corresponding overall currents and contents of catalyst in the active layer are calculated. In the calculations, the temperature of fuel cell, the pressure in the cathode gas chamber, and the cathodic potential were varied. The optimization of active layer thickness of cathode with solid polymer electrolyte can reduce the platinum consumption, i.e. its amount per 1 kW of power produced in a membrane-electrode assembly.     

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.     

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.     

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.     

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.     

The effect of crosslinked networks with poly(ethylene glycol) on sulfonated polyimide for polymer electrolyte membrane fuel cell

To investigate the effect of crosslinking by a hydrophilic group on a sulfonated polyimide electrolyte membrane, sulfonated polyimide end‐capped with maleic anhydride was synthesized using 1,4,5,8‐naphthalenetetracarboxylic dianhydride, 4,4′‐diaminobiphenyl, 2,2′‐disulfonic acid, 2‐bis [4‐(4‐aminophenoxy)phenyl] hexafluropropane and maleic anhydride. The sulfonated polyimides end‐capped with maleic anhydride were self‐crosslinked or crosslinked with poly(ethylene glycol) diacrylate. A series of the crosslinked sulfonated polyimides having various ratios of sulfonated polyimide and poly(ethylene glycol) diacrylate were prepared and compared with uncrosslinked and self‐crosslinked sulfonated polyimides. The synthesized sulfonated polyimide films were characterized for FTIR spectrum, thermal stability, ion exchange capacity, water uptake, hydrolytic stability, morphological structure, and proton conductivity. The formation of sulfonated polyimide was confirmed in FTIR spectrum. Thermal stability was good for all the sulfonated polyimides that exhibited a three‐step degradation pattern. Ion exchange capacity was the same for both the uncrosslinked and the self‐crosslinked sulfonated polyimides (1.30 mEq/g). When the crosslinked sulfonated polyimides with poly(ethylene glycol) were compared, the ion exchange capacity was decreased as 1.27 > 1.25 > 1.23 mEq/g and water uptake was increased as 23.8 < 24.0 < 24.3% with the increase in poly(ethylene glycol) diacrylate content. All the crosslinked sulfonated polyimides with poly(ethylene glycol) diacrylate were stable for over 200 h at 80 °C in deionized water. Morphological structure and mean intermolecular distance were obtained by WAXD. Proton conductivities were measured at 30, 50, 70, and 90 °C. The proton conductivity of the crosslinked sulfonated polyimides with poly(ethylene glycol) diacrylate increased with the increase in poly(ethylene glycol) diacrylate content despite the fact that the ion exchange capacity was decreased. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1455–1464, 2005     

Effect of anodic current on carbon dioxide reforming of methane on Pt electrode in a cell with solid oxide electrolyte

The carbon dioxide reforming of methane in a cell with a solid oxygen-conducting 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.