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 ac curacy concerning the acid-film thickness on which they are based. In the present paper the limiting current density has been measured for o xygen reduction on polytetrafluorine-ethyl bonded gas-diffusion electo rdes in phosphoric acid with and without fluorinated additives. This p rovides 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 diffusio n, and it is the latter that can be used in the film-thickness estimat ion. 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 thickn ess estimated this way was found to be of 0.1 mum order of magnitude f or the two types of electrodes used in phosphoric acid with and withou t fluorinated additives at 150-degrees-C.