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0692.tif Journal of The Electrochemical Society The Effect of Phosphoric Acid Concentration on Electrocatalysis To cite this article: Jeffrey T. Glass et al 1989 J. Electrochem. Soc. 136 656 View the article online for updates and enhancements. This content was downloaded from IP address 71.205.160.15 on 12/04/2022 at 05:36 https://doi.org/10.1149/1.2096705 https://googleads.g.doubleclick.net/pcs/click?xai=AKAOjsvq0K51R-6GFlF2LqdPe5fjcPSWTMw52HFNhhk6MnEJgjvlBpuB2VaA2Rr-0yQcVqn30HMNa-kdy6HD2cTw2GTaf5UFC3QEsS2nNI5HnFBqaqPgc-fosuL7DB_vaotubVgdoyvzgP1sBrtXhLG12AYvS6t9DgnHwwy9txht4zGu1w34Frs9tjk7uwT2fzyp9sD6HYeY6wFEBHLTCQQDjEKvzoqpU4A9xStMJ7GoAUmkag94U6FKNhIjPnGx0M-fBx2bkuVxxXwRdyICQo-YUzG4YkTN5fX_JdQ&sig=Cg0ArKJSzJo2q341ewLA&fbs_aeid=[gw_fbsaeid]&adurl=https://ecs.confex.com/ecs/242/cfp.cgi%2520 656 J. Electrochem. Soc., Vol. 136, No. 3, March 1989 �9 The Electrochemical Society, Inc. Dr. M. Fujii assisted in meeting the publication costs of this article. REFERENCES 1. T. Watanabe, A. Fujishima, and K. Honda, in "Energy Resources Through Photochemistry and Catalysis," M. Gratzel, Editor, p. 359, Academic Press, Inc., New York (1983). 2. H. Gerischer~ Electroanal. Chem., 58, 263 (1975). 3. A. Fujishima and K. Honda, Nature, 238, 37 (1972). 4. R. L. Anderson, Solid-State Electron., 5, 341 (1962). 5. W. A. Harrison, J. Vac. Sci. Technol., 14, 1016 (1977). 6. G. Hodes, J. Manassen, and D. Cahen, Nature, 261, 406 (1976). 7. J. Manassen, G. Hodes, and D. Cahen, This Journal, 124, 532 (1977). 8. M. A. Russak, J. Reichman, H. Witzke, S. K. Deb, and S. N. Chen, ibid., 127,725 (1980). 9. G. Hodes, D.oCahen, J. Manassen, and M. David, ibid., 127, 2252 (1980). 10. J. Reichman and M. A. Russak, ibid., 128, 2025 (1981). 11. M.A. Russak and J. Reichman, ibid., 129, 542 (1982). 12. M. Skyllas-Kazacos, J. Electroanal. Chem., 148, 233 (1983). 13. A. B. Ellis, S. W. Kaiser, J. M. Bolts, and M. S. Wrighton, J. Am. Chem. Soc., 99, 2839 (1977). 14. G. Hodes, J. Manassen, and D. Cahen, ibid., 192, 5962 (1980). 15. M. A. Russak and C. Creter, This Journal, 131, 556 (1984). 16. CdS, ASTM 6-0314; CdSe, ASTM 8-459; CdTe, ASTM 15-770. 17. M. P. Lisitsa, V. N. Malinko, and S. F. Terekhova, So- viet Phys. Semicond., 3, 491 (1969). 18. S. Kambe, M. Fujii, T. Kawai, S. Kawai, and F. Naka- hara, Chem. Phys. Lett., 1@9, 105 (1984). 19. M. Fujii, T. Kawai, and S. Kawai, J. Chem. Soc., Chem. Commun., 53 (1985). 20. B. Ermolovich, V. V. Gorbunov, and I. D. Konosenko, Soviet Phys. Semicond., 11, 1061 (1977). 21. L. J. Nicastro and E. L. Offenbacher, RCA Rev., 34, 442 (1973). 22. R. K. Swank, Phys. Rev., 153, 844 (1969). The Effect of Phosphoric Acid Concentration on Electrocatalysis Jeffrey T. Glass, *'! George I.. Cahen, Jr.,* and Glenn E. Stoner* Applied Electrochemistry Laboratory, Department of Materials Science, University of Virginia, Charlottesville, Virginia 22901 ABSTRACT Phosphoric acid (H3PO4) is the electrolyte used in the most advanced H2/air fuel cell developed for power generation and for on-site integrated energy systems. Oxygen reduction kinetics in this electrolyte are relatively poor and thus limit the efficiency of such fuel cells. Therefore, in order to better understand oxygen interactions with plat inum in these cells, oxygen adsorption and the kinetics of oxygen reduction reaction (ORR) were studied utilizing a Pt rotating disk electrode in several concentrations of highly purified H3PO4. Although the kinetics of the ORR remained first order in all concentra- tions, it was found that its Tafel slope increased from -110 mV/dec in low H3PO4 concentrations to -134 mV/dec in 85 w/o H3PO4. Anodic adsorption isotherms indicated that the higher H3PO4 concentrations also hindered adsorption of oxygen from solution. These effects were attributed to the blockage of electroactive sites by the adsorption of H~PO4 molecules. Fuel cells have been utilized since the early 1960's in high cost, specialized energy conversion applications such as on the Gemini and Apollo space flights (1, 2). Only more recently, however, have they begun to approach practical- ity for terrestrial applications such as power station load leveling and electric vehicle power. Although several types of hydrogen-oxygen fuel cells are being studied by various investigators, phosphoric acid cells have several advantages (3-5). First, phosphoric acid is a very nonvol- atile (i.e., low vapor pressure) and noncorrosive acid mak- ing it fairly safe for consumer applications. It is very con- ductive at temperatures above 150~ thereby minimizing ohmic losses in the cell. It may be utilized at very high con- centrations (essentially 100%), thereby allowing high tem- perature operation without changes in concentration due to water loss. It is also relatively tolerant to impurities such as CO and CO2 found in reformed hydrogen at tempera- tures above 150~ On the other hand, the oxygen reduc- tion kinetics in this electrolyte are fairly poor. In general, however, phosphoric acid is believed to be one of the most promising electrolytes. Therefore, the properties of phos- phoric acid (H~PO4) are very important. In view of this, the present study investigates the electrochemical properties of this electrolyte with respect to oxygen interactions (i.e., the oxygen reduction reaction in 02 saturated solution and anodic coverage in N2 saturated solution) with plat inum (Pt). These oxygen interactions are the most important electrochemical phenomena in H3PO4 since the oxygen re- duction reaction (ORR) limits the efficiency of HJO2, phos- phoric acid fuel cells. Additionally, Pt electrode material was utilized because Pt is the most widely used and best characterized material in these systems. *Electrochemical Society Active Member. 1Present address: Kobe Development Corporation, Research Triangle Park, North Carolina 27709. It is worthwhile to examine previous reports of the ORR. An excellent review has been comprised by Tarasevich, Sadkowski, and Yeager (6) which covers the ORR as well as other oxygen processes at electrode surfaces. The mech- anistic complexity of the ORR is discussed in detail. De- spite this complexity certain conclusions can be made. For instance, the ORR on Pt generally proceeds through a di- rect four-electron pathway, yielding no appreciable solu- tion-phase peroxide as long as impurities are minimal (6-8). Yeager et al. have categorized materials depending on whether or not detectable peroxide generation predom- inates (8). "Class I" materials (graphite, gold, oxides, etc.) generally involve considerable desorption of peroxide in- termediates during oxygen reduction. For "class II" mate- rials (Pt, palladium, silver, etc.), especially Pt, if peroxide is formed at all during oxygen reduction, its decom- position is quite rapid, making it a less important species in the reaction scheme. Furthermore, on Pt it is believed that the first step in the ORR "is probably adsorption of 02 on the surface" (6). This has led to the proposal of three general adsorption geometries and subsequent reaction sequences. Regarding the ORR in H3PO4, previous investigations have determined that the Tafel slope on Pt electrodes is approximately 120 mV/dec (9-13). The only previous study involving H3PO4 concentration effects on the ORR utiliz- ing a Pt electrode was reported in Ref. 13. ORR rate con- stants and cyclic voltammograms were investigated in the various concentrations. The potential of zero charge on a mercury electrode in these different concentrations was also investigated using streaming mercury electrode tech- nique. It was concluded that most of the oxygen reduces to water directly through the four-electron transfer reaction in all concentrations studied. However, reaction rates were found to decrease in high concentrations and the onset of J. Electrochem. Soc., Vol. 136, No. 3, March 1989 �9 The Electrochemical Society, Inc. 657 oxide formation on Pt was also hindered. Tafel slopes of approximately 120 mV/dec were measured and found to be "practically independent" of acid concentration. The present research is similar to this previous study but concentrates on Tafel slope measurements in the various acid concentrations. Also, theoretical and experimental limiting currents are compared, reaction orders are evalu- ated and anodic coverages in N2-saturated solutions are given. Experimental A standard Pt rotating disk electrode, 0.76 cm in diame- ter with a Teflon sheath, was utilized in this study. The specimen was polished on 320, 400, 600, and 2400 grit wet SiC paper, followed by a 1 and 0.05 ~m A1203 aqueous slurry polish on napped cloth to a mirror finish. It was ul- trasonically cleaned for several minutes after both the 600 and 2400 grit SiC polishes. A plexiglass ring was used to help stabilize the specimen during polishing so that exces- sive "rounding" did not occur. After the final polish, the sample was rinsed in ethanol, rubbed lightly with cotton to remove the A1203 film and ultrasonically cleaned in dis- tilled water. This ultrasonic cleaning was repeated, fol- lowed by a soak in doubly distilled, organically filtered water (hereafter referred to simply as "high purity water") for at least lh immediately prior to electrochemical test- ing. After submersion in the electrolyte, the sample was electrochemically pretreated at 0.04V (RHE) for 5 min, 1.2V for 10 min, and 0.04V for 30 min to reduce