Porous, electrodes diffusion currents

Stable, conductive electrodes would also be a problem. Preliminary experiments, were carried out in a cell, using simulated flue gas nearly identical to that shown in Fig. 24. In these tests, the membranes were hot-pressed from mixed powders of electrolyte (ternary eutectic of [Na, Li, K]2 S04) with LiA102 as matrix. The electrodes were constructed of cold-pressed Li20-9Cr203, partially sintered to give a highly-porous gas-diffusion structure. The tests were encouraging up to 50% of the S02 was removed from the simulated flue gas with the application of current. Simultaneously, a stream of concentrated S03 and Oz was evolved at the anode. 

Not included in this survey are limiting currents at porous electrodes, because they usually are not controlled exclusively by convective diffusion [for exceptions, see (HI lb)], limiting currents due to limited gas solubility at an electrode (N8b), or limiting currents recorded by electrochemiluminescence (H6c, C12b). 

The next important stage in the development of porous gas-diffusion electrode is an investigation of influence of thickness of PANI layer (or more easy controlled parameters like PANI mass or electrochemical capacity) on the local currents of O2 electroreduction (table 3). 

Whereas current-producing reactions occur at the electrode surface, they also occur at considerable depth below the surface in porous electrodes. Porous electrodes offer enhanced performance through increased surface area for the electrode reacdon and through increased mass-transfer rates from shorter diffusion path lengths. The key parameters in determining the reaction distribution include the ratio of the volume conductivity of the electrolyte to the volume conductivity of the electrode matrix, the exchange current, the diffusion characteristics of reactants and products, and the total current flow. The porosity, pore size, and tortuosity of the electrode all play a role. 

For this case of both diffusion and local mass transfer in the porous electrode, we define the dimensionless total current density as... 

The main reason a porous gas electrode is so active,7 therefore, is that it allows particularly large maximum diffusion currents by diffusion through (fairly) thin meniscus layers. But this thesis brings a corresponding antithesis because it implies that farther up the pore where there is no meniscus but bulk solution, the gaseous... 

A diffusion layer grows into the bulk of the electrolyte phase. The practical solution of this transport problem is use of porous electrodes [17,18]. A quantitative treatment of the problem of current density distribution is given on the basis of electrotechnique [17,61] and of electrochemical kinetics [62,63]. A simple but very useftd model, assuming that the transport length L is about the pore diameter dp, was given by Vetter [64]. This model is schematically represented in Fig. 10(a) for the case of an electrode of the second kind. 

A special kind of porous electrodes is gas diffusion electrodes which mostly have been used in fuel cells. A gas diffusion anode for hydrogen [68-73] may consist of three layers a metal current collector, a hydrophobic gas-porous layer, and a catalyst layer. The hydrophobic layer is often based on polytetrafluorethylene, which is made electric con-... 

Besides the activation overpotential, mass transport losses is an important contributor to the overall overpotential loss, especially at high current density. By use of such high-surface-area electrocatalysts, activation overpotential is minimized. But since a three-dimensional reaction zone is essential for the consumption of the fuel-cell gaseous reactants, it is necessary to incorporate the supported electrocatalysts in the porous gas diffusion electrodes, with optimized structures, for aqueous electrolyte fuel-cell applications. The supported electrocatalysts and the structure and composition of the active layer play a significant role in minimizing the mass transport and ohmic limitations, particularly in respect to the former when air is the cathodic reactant. In general, mass transport limitations are predominant in the active layer of the electrode, while ohmic limitations are mainly due to resistance to ionic transport in the electrolyte. For the purposes of this chapter, the focus will be on the role of the supported electrocatalysts in inhibiting both mass transport and ohmic limitations within the porous gas diffusion electrodes, in acid electrolyte fuel cells. These may be summarized as follows ... 

The potentiostatic method is less ambiguous than the galvanostatic one. Its application, however, requires more sophisticated instrumentation. The rise time of the potentiostat should be fast enough to ensure rapid step change of the potential. Errors may arise from slow rise times as well as from current integration. With porous electrodes, all sites may not be under the same potential diffusion of reactant into or out of the pores may be slow compared with the potential change, which can lead to incorrect estimates of surface coverage and utilization. 

The principal problem as far as electrocatalysis is concerned is the relation of the current density using a porous electrode to that using a planar electrode, i.e., the rate unaffected by mass transport and diffusion. 

A theoretical analysis of the current distribution and overpotential-current density relations for two models of porous gas diffusion electrodes—the simple pore and thin film models—has been carried out (108,109). The results of the analysis for the simple pore model will be summarized here. The reactant gas diffuses through the pore to the gas-electrolyte interface at 2 = 0, where it dissolves in the electrolyte and the dissolved gas diffuses through the electrolyte to the various electrocatalytic sites along the pore at which the reaction occurs (Fig. 25). It is assumed that the first and second steps of diffusion of reactant gas through the electrolyte-free part of the pore (z < 0) and of dissolution of gas at the gas-electrolyte interface are fast. 

---