Gaseous diffusion electrode

T. Kaz and N. Wagner [2002] Method for Production of Multi-Layer Electrode or Electrode Assembly and Gaseous Diffusion Electrode, US Patent, US 2002150812 Al. 

When the electrode is completely immersed in the electrolyte solution, only a two-phase interface (i.e., liquid-solid) is present in the electrode structure. In form it may be either a consolidated powdered active carbon or a confined but unconsolidated bed of carbon particles. These are u.sed for flow-through porous electrodes in many electrochemical systems. The other mode of operation is the gas-diffusion electrode, in which the electrode pores contain both the electrolyte solution and a gaseous phase. Numerous publications [29-31] have reported on a theoretical analysis of flow-through porous electrodes and gas-diffusion electrodes, which takes into account the physicochemical characteristics of carbon electrode materials. There does not seem to be a uniform explanation for the effects of structural and chemical heterogeneity in carbons. 

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 ... 

With gaseous reactants, a high flux of reactant to the site of electron transfer is achieved using a gas diffusion electrode (GDE) originally, such electrodes were developed extensively for fuel cells but are now frequently incorporated into other technologies. 

In the active parts of a gas diffusion electrode, the pore electrolyte is contained in the small pores between the catalyst particles and the carbon support particles. The larger pores are then filled with gas. Gas has to diffuse through a thin film of electrolyte and in the small pores that contain the pore electrolyte. This introduces an additional mass transfer resistance that can be described with an agglomerate model. Such a model is described with a diffusion-reaction equation for the gaseous species dissolved in the pore electrolyte with the charge transfer reactions as source or sink. The solution to this reaction diffusion model is used to calculate new reaction terms that replace the reaction terms in the balance of charge, material, mass, and energy. These models can often be solved analytically for steady-state if the reactions are of... 

Successful operation of metal/air batteries depends on an effective air electrode. As a result of the interest in gaseous fuel cells and metal/air batteries over the past 30 years, a significant effort has been aimed at improved high-rate, thin air electrodes, including the development of better catalysts, longer-lived physical stmctures, and lower-cost fabrication methods for such gas diffusion electrodes. 

Oscillatory behavior observed as periodic potential transients at constant current or periodic current transients at constant potential is found frequently when more than two parallel electrode reactions are coupled. Usually, an upper and a lower current-potential curve limit the oscillation region. These two curves represent stable states [139] according to the theory of stability of electrode states [140]. Oscillatory phenomena occurring during the oxidation of certain fuels on solid electrodes are discussed in this section. The discussion is not extended to porous electrodes because the theory of the diffusion electrode has not been developed to the point to allow an adequate description of the complex coupling of parallel electrode reactions and mass transport processes in the liquid and gaseous phase. 

A patent apphcation describing pyrolysis of poly(phenylene ether) (PPE) leading to the formation of highly electrically conductive and permeable gaseous carbons, which could be used as base materials for catalysts and for the preparation of the fuel cell gas diffusion electrodes, was submitted in 2003 by Cabasso et al. [ 111 ]. This was achieved by oxidization of PPE in an oxygen-containing atmosphere, at a temperature close to its Tg, followed by its carbonization in an inert atmosphere at high temperatures [111]. 

The species diffusivity, varies in different subregions of a PEFC depending on the specific physical phase of component k. In flow channels and porous electrodes, species k exists in the gaseous phase and thus the diffusion coefficient corresponds with that in gas, whereas species k is dissolved in the membrane phase within the catalyst layers and the membrane and thus assumes the value corresponding to dissolved species, usually a few orders of magnitude lower than that in gas. The diffusive transport in gas can be described by molecular diffusion and Knudsen diffusion. The latter mechanism occurs when the pore size becomes comparable to the mean free path of gas, so that molecule-to-wall collision takes place instead of molecule-to-molecule collision in ordinary diffusion. The Knudsen diffusion coefficient can be computed according to the kinetic theory of gases as follows... 

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