Ektrochimica Acta, Vol. 38. No. 7. pp. 913-918. 1993 Printed in Great Britain. @x3-4686/93 $6.00 + 0.00 Q 1993. Pergamott Press Ltd. INFLUENCE OF PHYSICOCHEMICAL PROPERTIES ON THE PERFORMANCE OF Pt/C POROUS ELECTRODES FOR OXYGEN REDUCTION IN PHOSPHORIC ACID N. GIORDANO,*E. PASSALACQUA, * P. L. ANToWccI,t L. PINO,* M. VIVALDI,* A. PA-~-II* and K. KINOSHITA~ *CNR Institute for Transformation and Storage of Energy, via Salita S. Lucia sopra Contesse 39,98126 S. Lucia, Messina, Italy tuniversity of Reggio Calabria, Faculty of Engineering, via E. Cuzzocrea 48,891OO Reggio Calabria, Italy fEnergy and Environment Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720, U.S.A. (Received 16 March 1992; zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML in revisedform 16 November 1992) Abstract-Oxygen reduction on Pt supported on carbon black in PTFE-bonded porous electrodes in 98% H,PO, at 170°C was investigated. The influence of the Pt surface area and the volume of H,PO, in the electrode structure (per cent acid occupation, PAO) on the performance of porous electrodes (ie current-potential behavior, potential at a constant current density of 200mAcm-2, oxygen gain) were evaluated. The results clearly show that the Pt surface area has a major influence on the Tafel slope, electrode polarization, electrode potential decay at constant current density, and oxygen gain. On the other hand, these parameters do not show a clear trend with PAO. Keywords: phosphoric acid fuel cells, oxygen reduction, oxygen gain, Tafel slope, activity decay INTRODUCTION The performance and operating life of Pt supported on carbon (Pt/C) in porous electrodes for oxygen reduction in phosphoric acid are very sensitive to the properties of the electrocatalyst and carbon support, as well as the structure of the electrode. Various factors such as the method of Pt/C electrocatalyst preparation, Pt particle size, activation process, wetting of electrode structure, PTFE content, surface properties of carbon, etc., may all be important in contributing to ihe steady-state performance and life of air electrodes in phosphoric acid fuel cells (pafc). These factors are interrelated in their influence on electrode performance; consequently, it is not easy to single out a contributing factor. However, it is pos- sible to detect a trend in the electrode performance as several of these factors are varied. The results of an extensive effort in this Labor- atory to investigate the physicochemical parameters that influence the performance and life of porous electrodes for oxygen reduction in pufcs have been reported[l-41. The performance of oxygen cathodes in phosphoric acid was influenced by electrode fabri- cation parameters[2), such as PTFE content and sintering temperature. These parameters affect the wettability of the porous electrode, and consequently its performance in phoshoric acid. Furthermore, the relative amount of acid which is absorbed in the electrode structure, called per cent acid occupation (PAO), and a major influence on the oxygen gain and Tafel slopeC23. Other studies, notably by Maoka[S, 61 and Mori and coworkersC7, 81 have also reported on the influence of electrode- fabrication parameters and PA0 on the performance of Pt/C porous electrode for oxygen reduction in phosphoric acid. Previous studies from this Laboratory have dealt with the effects of PA0 temperature of electro- catalyst activation, PTFE content and sintering temperature on electrode performance. The specific activity of supported Pt electrocatalysts for oxygen reduction at 900mV has been extensively studied[9- 191, but the influence of Pt surface area and other electrode properties on the short-term performance at higher current densities, say about 2OOmAcm-‘, has not been addressed in much detail. The main focus of this paper is on analysing the oxygen reduction performance at constant current density in short-term tests of about 48 h with Pt/C porous elec- trodes with different Pt surface areas. EXPERIMENTAL Electrocataly st/electrode preparation The Pt/C electrocatalysts were prepared using a carbon black (Ketjenblack, BET surface area, 950m2g-‘) for the support. Two different pro- cedures were used to prepare the supported electro- catalysts with 10 or 20 wt% Pt to obtain electrocatalyst that have various Pt surface area. Procedure 1. This procedure was developed by Jalan and Bushnell[20]. An aqueous solution of H,PtCI, (IOgl-‘) is heated to 60°C and then 4Oml of 30% H,O, is added to stabilize the colloidal Pt suspension that forms when 200ml of Na,S,O, solu- tion is added. A suspension of the Pt particles is added to homogeneous slurry of carbon black 913