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2005

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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.

Section: Calculating a Characteristic Bulk Current Density In The Catmentioning

confidence: 99%

Section: The Catalyst Is Distributed In the Bulkmentioning

confidence: 99%

Section: (19)mentioning

confidence: 99%

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

2005

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“…We can remove diffusion limitations on the oxygen supply by, for example, abandoning the "stochastic" version of the porous structure of the active layer studied in this investigation (version where the substrate grains and the electrolyte grains are distributed in the active layer randomly). We can also consider structures with "regular gas pores" [19] or even investigate an active layer with a completely regular structure [20,21].…”

Section: Formulation Of the Problemmentioning

confidence: 99%

“…The polarization at the rear side of the active layer η ∆k , which corresponds to the maximum current I kmax , may be obtained by combining expressions (17), (19), and (20). The final result is…”

Section: Formulation Of the Problemmentioning

confidence: 99%

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

2005

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The major difficulties in a hydrogen-oxygen fuel cell with a solid polymer electrolyte [1][2][3][4][5] emerge when it is attempted to ensure optimum operation of the cathode, i.e. the oxygen electrode. We will explore the same model of the active layer of a porous electrode as in [6]. Specifically, we will assume that the active layer may comprise a mixture of four constituents. The first, the substrate grains, consists of agglomerates of carbon particles, which provide for electron conduction. The second constituent is microparticles of platinum (catalyst) embedded into these agglomerates. The third constituent is particles of a hydrophobizing agent (polytetrafluoroethylene), which play the role of a binding agent and which can facilitate creation of hydrophobic pores in the active layer. At present the active mass of the cathode is prepared both with use of polytetrafluoroethylene and without it. And finally the fourth constituent is the grains of a solid polymer electrolyte (Nafion), which ensure ionic conduction.We will embark now on the description of the adopted computer model. For the porous electrode we will select a cube of conditional dimensions N s × N s × N s = . It is desirable to have a model of a macroscopic size. What this means is that the following condition must be fulfilled: N s should amount to a few tens of elements of structure; in what follows we will always assume that N s = 100. Let us explore a cubic lattice of points. Every point is a cube randomly filled either by a solid electrolyte or by a substrate with platinum particles embedded into it, or this is an empty space. Further on, to simplify analysis, we will assume as we did in [6] that concentrations of points filled with electrolyte and substrate are identical and denote their bulk concentration as g . This is a major parameter of the system under consideration. It is obvious that 0 ≤ g ≤ 0.5 (no poresvoids are present in the electrode at g = 0.5). The length of the edge of one cube (all cubes are identical geometrically) will be denoted with L . One more parameter of structure is the average degree of filling of the substrate grains by platinum, denoted by g Pt (0 ≤ g Pt ≤ 1) . The platinum concentration in a randomly selected substrate grain can assume any value; however, when averaged over all the substrate grains, it needs must be equal to g Pt .In [6] we discussed at length how the electron and ion clusters emerge in the model under investigation at N s 3 Abstract -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 t...

Computer Simulation of the Structure of Porous Electrodes with an Immobilized Enzyme and Nanosized Support Particles

2005

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The central problem in porous electrodes with an immobilized enzyme and nanosized particles of the support is the creation of an active layer, which is a composite comprising the support particles and enzyme molecules. For a composite to function efficiently, a connected macroscopic-size structure comprising support particles must be formed in the composite i.e. there must emerge a percolation cluster [1], which is labeled here an electron cluster [2]. This connected macroscopic-size structure connects the front and the rear surfaces of the porous electrode. The task of this structure is to conduct current and supply electrons to the enzyme molecules. It is obvious that only with the emergence of an electron cluster upon reaching a certain minimum concentration (percolation threshold) of the support particles in the composite the electrochemical activity of a porous electrode with an immobilized enzyme becomes other than zero. And now all the enzyme molecules that enter the composition of the composite happen to be divided into two classes. One class comprises molecules that are intimately linked to an electron cluster, having been supplied with electrons, these molecules are capable of realizing bioelectrocatalysis. We had labeled such enzyme molecules "active" [2]. The other class consists of the rest of the enzyme molecules; these are not intimately linked to an electron cluster, not supplied with electrons, and exhibit no electrochemical activity.In porous electrodes of fuel cells the setting of a function of electrocatalysis (bioelectrocatalysis-special case) apart from a function of transport of electrons required for performing an electrochemical process occurred not right away. In the first generation of hydrogen-oxygen fuel cells [3] (in what follows we will bear in mind only hydrogen-oxygen systems) a porous metallic matrix (it was prepared from skeleton catalysts) performed two functions simultaneously: it was both an electrocatalyst and a carrier of electrons. In so doing, any locus of such a matrix could have taken part in electrocatalysis. The porous electrodes for the second generation of fuel cells were prepared by mixing and subsequent caking of agglomerates of particles of a catalyst (usually it was platinum black) and agglomerates of particles of a hydrophobizing agent (polytetrafluoroethylene). The mechanism of the action of hydrophobized porous electrodes was dismantled for example in [4]. In contradistinction to porous electrodes functioning in fuel cells of the first generation, now the entire amount of platinum black could not have been provided for with electrons, only its connected portion that formed an electron cluster could. However, just as before, functions of electrocatalyst and conducAbstract -The efficiency of the operation of a porous electrode with an immobilized enzyme is defined, in particular, by a lucky structure of its active layer, which can contain nanosized particles of the support. The composites of such a kind are prepared with the aid of methods of colloidal chemistr...