Limiting Current of Oxygen Reduction on Gas-Diffusion Electrodes for Phosphoric Acid Fuel Cells

Various models have been devoted to the operation mechanism of porous diffusion electrodes. They are, however, suffering from the lack of accuracy concerning the acid-film thickness on which they are based. In the present paper the limiting current density has been measured for oxygen reduction on polytetrafluorine-ethyl bonded gas-diffusion electrodes in phosphoric acid with and without fluorinated additives. This provides an alternative to estimate the film thickness by combining it with the acid-adsorption measurements and the porosity analysis of the catalyst layer. It was noticed that the limiting current density can be accomplished either by gas-phase diffusion or liquid-phase diffusion, and it is the latter that can be used in the film-thickness estimation. It is also important to mention that at such a limiting condition, both the thin-film model and the filmed agglomerate model reach the same expression for the limiting current density. The acid-film thickness estimated this way was found to be of 0.i ~m order of magnitude for the two types of electrodes used in phosphoric acid with and without fluorinated additives at 15O~ The cathode of phosphoric acid fuel cells (PAFC) consists of a porous carbon substrate and a catalyst layer which is composed of a carbon-supported platinum catalyst bonded with polytetrafluoroethylene (PTFE). The presence of PTFE in the catalyst layer gives hydrophobicity so that the catalyst layer, during operation, is only partially in contact with acid. The performance of the electrode is dependent on a number of factors such as pore-size distribution, electrolyte composition, active surface area, and electrochemical kinetics. The working mechanism of such an electrode as well as those used in molten carbonate fuel cells (MCFC) has been modeled by means of the thin-film model ~,2 and more appropriately with the agglomerate and filmed agglomerate models? -7 All of these approaches suffer from the drawback that the film thickness is not taken into account in the model in a satisfying way. By fitting the theoretical expression for the initial slope of the polarization curves to the experimentally determined values on the basis of a simple thin-fi lm model, Wilemski 2 found an unlikely value, ca. 0.5 ~tm, for the electrolyte film thickness for a molten-carbonate fuel-cell (MCFC) cathode. On the other hand, by use of a theory of the l iquid-fi lm stability, the value of the molten carbonate film thickness on a NiO substrate was estimated to be ca. 0.003 ~m, i.e., two orders of magnitude smaller. 8 Springer and Raistrick, 9 in their ac impedance analysis, suggested that only by assuming the presence of an external electrolyte film covering the agglomerate was it possible to explain a separate low-frequency mass-transfer impedance loop, while the agglomerate itself could not lead to such a loop, therefore Yuh and Selman 6 used the size of the low-frequency impedance loop for estimating the electrolyte film thickness. The thickness was estimated to be ca. 0.1 ~m by assuming that the low-frequency loop was * Electrochemical Society Active Member. caused by a Nernst-type diffusion limitation in the electrolyte film. On the other hand, Yuh and Selman 5 used the limiting current supposedly measured at low oxygen concentration (less than 3% oxygen) l~ to estimate the film thickness. By assuming that the limiting current was due to the oxygen diffusion through the film and was independent of gas composition, they found the film thickness to be ca. O. 1 ~m. As they pointed out later, 6 the limiting current cannot always be observed on a MCFC cathode even at low partial pressure of oxygen; hence the observed limiting currents were suspected to be due to the depletion of the gaseous oxygen at high current levels. In the present work, well-defined limiting currents have been measured on a PAFC cathode, and effort has been made to estimate the acid-film thickness by combining the limiting current, the electrode microstructure, and acid adsorption measurements for two kinds of electrodes and for 100% phosphoric acid electrolyte with and without fluorinated compounds. The success of fluorinated compounds as possible electrolyte additives for PAFC has been demonstrated in our previous papers, 11,12 where promising compounds are specified such as potassium perfluorohexanesulfonate, C6FI3SO3K (hereafter referred to as C~ salt), potassium nonafluorobutanesulfonate (C4FgSO3K), and perfluorotributylamine [(C4Fg)3N]. Limiting Current Density for Gas-Diffusion Electrode Models A gas-diffusion electrode contains pores with a variety of pore sizes. From the conventional porosimetry, a set of data for cumulative pore volume (V) and specific pore-wall surface (A) as a function of pore radius (r) can be obtained. It is known that the total pore volume (V 0 and total specific surface area (At) can be obtained by integration over a range of pores of increasing radius, assuming that the pores are cylindrical Downloaded 28 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp J. Electrochem. Soc., Vol. 141, No. 11, November 1994 9 y~ = ;; dV(r) [1]