Simple-Pore and Thin-Film Models of Porous Gas Diffusion Electrodes

Abstract

Two physical models for the mode of operation of porous gas-diffusion electrodes for fuel cells are discussed. For nonwetted electrodes, the simple-pore model is proposed, in which reactant gas dissolves in electrolyte and diffuses to the submerged electrode surface near the three-phase boundary of solid-liquid-gas. For normal gas solubilities and diffusion coefficients, this model will lead to low mass transport-limited currents unless the pore diameter is less than about 1 micron. For wetted double-layer structure electrodes, the thinfilm model appears reasonable on the basis of experimental observation. An approximate mathematical treatment of this model leads to useful simple solutions, which can form the basis for design of experiments to test the model. SF: of the major throretical problems associated ivith the 0 use of porous gas-.diffusion electrodes in fuel cells is an understanding of the mode of operation of the electrode. (l'he other major problems are those of electrocatalysis and electrokinetics.) The mode of operation must include the mechdnism by \L.hich gas (fuel or oxidant) gets to a site on the electrode surface whew it can react electrochemically. An understanding of this mechanism iyill aid in defining the optimum geometrical structure of the electrode for a particular reaction, A physical model of ho\v a gas-diffusion electrode works should be capable of (explaining in general terms how the limiting-current of the system depends on electrode structure and degree of electrolyte penetration, on reactant concentration, and on electrolyte resistance. Also, the model must be consistent with the shapes of the current-voltage curves observed experimentally (although a complete explanation of this facet must generally include knowledge of the electrokinetics of the system). Practical gas-diffusion systems are so complex in structure that it is not likely that any simple model will give perfect predictions of behavior. At present it seems reasonable to invcstigate only whether a model is capable of predicting the general variations pi.oduced by changes in conditions.The simpler problem of tivo-phase porous diffusion electrodes has received some study (2, 3, 72, 76. 78-20)> and reccnt work ('1. 5, 77) has shown that for systems ivith simple electrokinetics, it is posa$ible to describe the mode of operation of the electrode with considerable precision. Therefore, grneral concepts used .in these studies can be considered as verified and may be used \vith confidence for the more complex cases presented here. Physical Models,At room temperature and 1 atm.. the solubility of gases such as hydrogen and oxygen in aqueous solvents is about 10-6 mole per cc. Diffusion coefficients for the dissolved gases are about 10-5 sq. cm. per second. Experience of many Lvorkers has shown that porous gas-diffusion electrodes completely flooded with electrolyte are not capable of supporting much current, and it is generally accepted that a three-phase boundary of solid-gas-liquid is necessary to obtain current densities in ...

Thin-film flooded agglomerate model for silver-based oxygen depolarized cathodes

“…These electrodes have many similarities to ODCs used for the chlor-alkali electrolysis. The first developed models were based on a simple pore and thin-film approach proposed by Austin and Will [8,9] and extended by Srinivasan [10,11]. At the same time, dual scale porosity models were used by Markin and Bushtein [12,13].…”

Behavior of Hydrophilic Porous Oxygen Electrodes with Silver Electrocatalyst

“…Comparison with the thin-i~lm theory.--The thinfilm theory by Austin et al (11) takes into account the diffusion of oxygen through a liquid film and the film resistivity. This theory predicts polarization curves of the form i = k 9 ~l 1/2, if the activation polarization is small.…”

Liquid–Solid Boundaries Dominate Activity of CO2 Reduction on Gas-Diffusion Electrodes

“…The ultimate question that differentiates the scientific conclusions of many works of CO2R on GDEs is whether CO2 reduction occurs at a triple-phase boundary (TPB) or a double-phase boundary (DPB) (see Figure 1). 10,[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] The meaning of "TPB" is also not unanimous and is oftentimes ambiguous, sometimes with an atomistic context and other times a diffusive mass-transport context. The boundary thickness might be determined in the TPB by the fractions of a nanometer needed to transition from gas to liquid or solid, while in the DPB by the 10s to 1000s of nanometers CO2 might diffuse through liquid from the gas phase before it chemically or electrochemically reacts.…”