The Oxygen Reduction Reaction on Pt(322) and Pt(553) Electrodes Modified with Cotpp and Au

Abstract

1. Purpose Polymer electrolyte fuel cells (PEFCs) attract attention as a clean and highly efficient energy system. It is necessary to activate the oxygen reduction reaction (ORR) for the wide spread of fuel cells because of the high cost and the limitation of natural resources of Pt. One of the strategies for the enhancement of the ORR activity is modification of the Pt electrode surfaces. Kodama et al. reported that the ORR activity of Pt(322) = 5(111)-(100) is enhanced by Au-deposition at the terrace edge [1]. Feng et al. revealed that metal-porphyrin and its metal-organic frame (MOF) have high ORR activity [2]. Higher density of d-band vacancy also enhances the ORR activity [3]. Cobalt tetraphenyl porphyrin (CoTPP) has electron attracting property to increase the density of d-band vacancy; improvement of ORR activity is expected. In this paper we have studied the ORR activity on Pt high index planes modified with cobalt tetraphenyl porphyrin (CoTPP) and Au. 2. Experimental Pt(322) = 5(111)-(100) and Pt(553) = 5(111)-(111) electrodes, were prepared by Clavilier’s method. Both electrodes have 5 atomic rows of (111) terrace, but the step structure of Pt(322) is different. from that of Pt(533) (Pt(322):(100)-step, Pt(553):(111)-step). Au was deposited on Pt surface according to the method of Kodama et al. [1], CoTPP modification was done according to Itaya et al. [4]. Linear sweep voltammograms of the ORR were measured using rotating disk electrode (RDE) in 0.1 M HClO4 saturated with O2. Potential was scanned from 0.05 V (RHE) up to 1.0 V (RHE) at scanning rate 0.010 V s−1 and rotation rate 1600 rpm. The activity for the ORR is estimated using the specific activity j kat 0.90 V (RHE). 3. Results and discussion Voltammograms of bare and Au-modified Pt(322) and Pt(553) are shown in Fig. Redox peaks due to the adsorption/desorption of hydrogen at the terrace edge appear at 0.27 and 0.12 V (RHE) on Pt(322) and Pt(553), respectively. Other broad peaks are due to the hydrogen desorption/adsorption at terrace. The peaks due to the step disappear after Au deposition on Pt(322), whereas the peaks due to the terrace are intact. These results suggest that Au atoms were deposited selectively at terrace edge. On Pt(553), however, the peaks due to the step and terrace shrink after the Au deposition, suggesting that both step and terrace are modified by Au atoms. Selectivity of Au modification depends on the step structure of Pt substrate. The ORR activity of Pt(322) increases from 2.8 mA cm-2 to 3.1 mA cm-2 after the Au deposition, however that of Pt(553) decreases from 2.4 mA cm-2 to 0.9 mA cm-2. Au deposition on the terrace prevents the ORR on Pt(533). This fact indicates that Au deposition at only terrace edge is necessary for the enhancement of ORR activity. Au deposited Pt(322) was further modified by CoTPP. The ORR activity decreases from 3.1 mA cm-2 to 2.3 mA cm-2after the modification of CoTPP. Five atomic rows of (111) terrace is as large as the size of CoTPP; CoTPP may block the active sites for the ORR at the terrace. It is necessary to investigate the ORR on surfaces with wider terrace. 4. Acknowledgement This work was supported by New Energy Development Organization (NEDO). 5.