Studies of Hydrocarbon Fuel Cell Anodes by the Multipulse Potentiodynamic Method

Abstract

The behavior of methane through butane fuels parallels that previously observed with methane, ethane, and propane with a perchloric acid electro-lyte although quantitative differences are observed. A temperature range from 60 ~ to 120 ~ was covered. With both electrolytes all of the hydrocarbons form one class of surface species which is oxidized at relatively low potentials. Hydrocarbons above methane form a second more refractory class of surface species in both cases. As the molecular weight of the fuel increases, greater quantities of the refractory species accumulate on the electrode. These species play a role in determining the maximum current that can be carried by the electrode. They seem connected with the current and voltage oscillations frequently encountered with hydrocarbon fuel cells. The behavior of ethane and related hydrocarbons on smooth platinum electrodes in the presence of a perchloric acid electrolyte has been previously investigated with the multipulse potentiodynamic (MPP) method in this laboratory (1,2). More recently the method has been extended to porous fuel cell anodes of more complex structure with which the behavior of methane and ethane was examined in detail, and a few preliminary experiments were performed with propane (3, 4). For this purpose a miniature version of a previously described (5) conducting porous -Teflon fuel cell electrode (containing platinum black as catalyst) was employed. The elec-trolyte was ,again perchloric acid. The work with porous-Teflon electrodes has now been extended to phosphoric acid systems in the temperature range from 60 ~ to 120~ Saturated linear hydrocarbons from methane through butane have been included in the new studies which are the subject of the present paper. As in the previous work, particular attention was given to the behavior of the species that form on the surface of the electrode. In addition data were again obtained to relate this behavior to the overall polarization curves for the fuel electrodes. Experimental The equipment .and general procedures utilized in these investigations have been described previously (3). In brief, %he 0.2 cm diameter Teflon-bonded, platinum black electrode was mounted in a three compartment , Teflon cell. Platinized platinum flags served as the hydrogen reference and counter electrodes. The former communicated with the working anode through a Luggin capillary. The cell was operated in an air thermostat enabling control of the temperature to within 0.1~ The 75% phosphoric acid electrolyte solution used for this work was prepared from reagent grade phos-phoric acid and quartz distilled water. Electrolytic grade hydrogen was used in the reference electrode Present address: Silicone Products Department, General Elec-trio Company, Waterford, New York, chamber, .and Phillips research grade hydrocarbons were used as the fuels. Tank argon, deoxygenated by passage over heated copper turnings, was used as the "fuel" for obtaining solvent blanks. Tank argon was also used for degassing the solution. The electronic instrumentation and circuit have been described previously (6). The potential-time sequence applied to the anode for adsorption studies at constant potential is shown in Fig. 1. The significance of the steps is covered below. (A) Pretreatment step (15 sec) to remove oxidiz-able impurities and to produce a layer of "adsorbed oxygen" which serves to block fuel adsorption. The solution is vigorously stirred and purged with argon to remove molecular oxygen and oxidation products formed. (B) Potential step, during which the oxygen layer formed in (A) is maintained, and the solution is purged for an additional minute. The solution is then allowed to become quiescent for 1 rain. (C) Reduction step (15 sec) during which the ad-sorbed oxygen layer is reduced. At this low potential (0.06v) the adsorption of hydrocarbons is blocked. This step was included so that the reduction of the surface and the adsorption of the hydrocarbon would not occur simultaneously. However, omission of this step (A}