Cells with porous three-dimensional electrodes

Two contrasting examples of porous three-dimensional electrodes are shown in Fig, 2.35. The Swiss roJI cell (Fig. 2.35(a)) consists of an electrode-separator or spacer-electrode sandwich spirally wound, usually with axial flow through the mesh electrodes. The mass transport is promoted by textured electrodes and/or plastic mesh turbulence promoters which also serve as membrane-electrode spacers. The interelectrode gap is small (0.2-2 mm), providing a low cell voltage... 

Cells with three-dimensional electrodes providing enlarged specific electrode area and improved mass transport due to the specific fluid dynamics inside the three-dimensional structure are, for example, the porous flow-through cell [68], the RETEC (RETEC is a trademark of ELTECH Systems Inc., Cardon, Ohio) cell [15], the packed-bed cell [69-71],... 

In addition, Shen et al. [75] prepared a novel fliree-dimensional electrode using polypyrrole (Ppy) and polystyrene spheres (PS) covered by a platinum catalyst instead of the conventional gas diffusion electrode, in order to reduce the sealing effect in liquid fuel cells. This new type of porous structured electrode allows liquid alcohol to penetrate the catalyst layer quite easily. The approach results in an increased active surface area for electrochemical reactions. The electrochemical active areas of platinum in Pt/Ppy/PS electrodes and E-TEK Pt/C electrodes, calculated by cyclic voltammograms [76, 77], are 4.5 and 23.6 cm g respectively, indicating a larger EAS for the three-dimensional electrode. Preliminary studies show an improved performance for methanol oxidation on a three-dimensional electrode as compared with a conventionally prepared electrode with the same platinum loading. 

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 alkaline fuel cell (AFC) with its liqnid alkaline electrolyte KOH uses gas diffusion electrodes with a hydrophobic porous part, which is not flooded by the alkaline electrolyte, and a hydrophilic part containing electrolyte and thus leading to a three-dimensional three-phase boundary layer. As the electrode potentials in alkaline electrolyte are shifted towards more negative values, corrosion is less problematic. Raney Nickel and silver are the state-of-the-art catalysts. The practical use... 

For this three-dimensional porous gas diffusion, electrodes are used in fuel cells to provide a three-dimensional reaction zone and the diffusion of the reactant species to the electro-active sites by radial diffusion. The pore sizes and particles used are on the order of nanometers, resulting in the effective diffusion layer thickness being several orders of magnitude smaller. Limiting current densities on the order of 1-10 A/cm can be reached with such designs. 

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