Oxygen Reduction on Gas-Diffusion Electrodes for Phosphoric Acid Fuel Cells by a Potential Decay Method

The reduction of gaseous oxygen on carbon-supported plat inum electrodes has been studied at 150~ with polarization and potential decay measurements. The electrolyte was either 100 weight percent phosphoric acid or that acid with a fluorinated additive, potassium perfluorohexanesulfonate (C6FI3SO3K). The pseudo-Tafel curves of the overpotential vs. log (iiL/(iL -i)) show a two-slope behavior, probably due to different adsorption mechanisms. The potential relaxations as functions of log (t + ~) and log ( d ~ / d t ) have been plotted. The variations of these slopes and the dependence of the double-layer capacitance on the overpotential depended on the electrode manufacture and the kind of electrolyte {whether containing the fluorinated additive or not). Introduction Much experimental work has been done to determine the Tafel slope and exchange current for the electrochemical reduction of oxygen in acidic electrolytes. Two Tafel regions were found by Sepa and co-workers I on a rotating smooth platinum electrode in sulfuric acid; in the low polarization region the Tafel slope was 2.303 RT/F,, and in the high polarization region the Tafel slope was 2(2.303 RT)/F. The transition of the slopes was interpreted in terms of the first charge-transfer step being rate determining in both potential regions but under Temkin adsorption on an oxygen-covered surface in the low polarization region, and Langmuirian adsorption on an oxygen-uncovered surface in the high polarization region. In phosphoric acid electrolyte, a similar behavior was observed on smooth platinum electrodes. ~-5 However, gas-diffusion electrodes with high surface-area platinum (supported on carbon) are presently used in phosphoric acid fuel cells (PAFC). There is always the question of the applicability of kinetic data obtained on low-surface-area electrodes (wires, beads, foils, etc.) to high-surface-area dispersed electrocatalysts. It seems reasonable, therefore, that the kinetic parameters should be obtained on the highly dispersed electroeatalysts directly by using gas-diffusion electrodes. Compared with smooth platinum electrodes, complications are encountered with the carbonsupported platinum gas-diffusion electrodes, such as problems with supporting the electrocatalyst, preparative techniques, preconditioning, experimental conditions, etc. The electrochemical measurement involves the polarization losses caused mainly by the finite conductivity of the electrolyte, electrode matrix, and diffusion of reactants and products. Controversies therefore exist concerning measurements of kinetics of oxygen reduction on gas-diffusion electrodes. Tafel slopes ranging from 65 to 180 mV/decade have been reported. 6"12. Most investigators 6-9 believed that the working mechanism of gas-diffusion electrodes, among other things, was a main factor affecting the magnitude of the Tafel slopes. When controlled by the kinetic process, the electrode exhibits a Tafel slope smaller than 120 mV/decade (most likely ca. 90 mV/decade in concentrated phosphoric acid), whereas when flooded and operating under diffusion control, the electrode exhibits a behavior with a doubled Tafel slope, i.e., with a Tafel slope of ca. 180 mV/decade. The dependence of the oxygen reduction kinetics on the properties of platinum particles (size and structures) has been a controversial subject. In spite of the fact that earlier Zeliger, 1~ Bert et al., 11 and later Kunz and Gruver 7 and Vogel and Barfs ~2 have shown that the oxygen reduction was independent of the plat inum particle size, Blurton et al. ~3 and Bergoli ~ found a decrease in the specific platinum activity on oxygen reduction with diminishing particle size in 20 weight percent (w/o) H2SQ at 70~ and in * Electrochemical Society Active Member. 99 w/o H3PO4 at 177~ respectively. As shown by the work of Peuckert et al., 14 the reason for the structure sensitivity may be associated with the adsorption of surface oxide species. The effects of using alternative electrolyte and electrolyte additives have been explored extensively, such as fluoroalkane suffonic acids [e.g., trifluoromethane sulfonic acid (TFMSA), 15-17 and perfluoroalkane disulfonic acids18], perfluoro sulfonimide (e.g., PFSI 19), and perfluorinated salts. 2~ Beneficial effects of the use of perfluorinated surfactants as additives to the phosphoric acid in PAFCs have been approved, a~ A lower extent of anion adsorption of the perfluorinated additives, among others, has been suggested to be a primary factor responsible for the higher rate of oxygen reduction. A method of analysis of the potential decay on open circuit, following interruption of a polarization current, has been developed and applied to the study of adsorption behaviorsY -24 This potential decay technique is used here to examine the adsorption behavior in oxygen reduction on gas-diffusion electrodes in phosphoric acid containing perfluorinated additives. Potential Decay Method For an electrode reaction the current density i varies as a function of the overpotential, here b is the Tafel slope, d~l/d in i, io the exchange current density. Under polarization conditions such that the back reaction can be neglected, the course of potential decay, ~l(t), after the interruption of current, is given by the differential equation dn C ~ = io exp (~) [2] where C is the total capacitance of the electrode interface. For cases involving electroactive absorbed intermediates at significant or appreciable coverage (0 > 0.05), C = Cdl + Cp~. Here Cdl is the double-layer capacitance, and Cp, is the pseudocapacitance characterizing the potential dependence of the fractional coverage, 0, of the electroactive intermediates