Enhancement of ORR Activity and Durability of Pt Single Crystal Electrodes Modified with Alkylamines

Abstract

1. Purpose Fuel cell is high-efficiency and clean energy system. However, high overpotential of oxygen reduction reaction (ORR) and large Pt loading in electrocatalyst are grave issues for the general use. Miyake et al. reported that platinum nanoparticles modified with octylamine (OA) and alkylamine with pyrene group (PA) give higher activity and durability at the molar ratio OA/PA=7/3 compared with bare Pt nanoparticles [1]. However, the platinum nanoparticles are not shape controlled or well-defined. We have investigated surface structures which enhance the ORR activity and durability using Pt single crystal electrodes modified with OA/PA. The surfaces examined are Pt(100), Pt(110), Pt(111), Pt(331)=3(111)-(111) that has the highest ORR activity [2] and Pt(775)=7(111)-(111). In the notation of high index planes such as n(hkl)-(mno), the value of n, Miller indices (hkl) and (mno) show the number of terrace atomic rows, the structures of terrace and step, respectively. 2. Experimental Linear sweep voltammograms of the ORR were measured with rotating disk electrode (RDE). Potential was scanned from 0.05 V (RHE) to 1.0 V (RHE) at scanning rate 0.010 V s-1 and rotation rate 2000 rpm. The activity for the ORR was estimated using the specific activity at 0.90 V (RHE) j k according to the Koutecky-Levich equation. Durability test was conducted using rectangular wave from 0.60 V step to 1.0 V. All the potentials were referred to RHE. 3. Result and Discussion The ORR activities of Pt(111) modified with OA/PA = 9/1, 8/2, 7/3 (molar ratio) were twice or three times higher than that of bare Pt(111). The ORR activity increases with the increase of the ratio of PA. The activity of Pt(775) = 7(111)-(111) modified with OA/PA=9/1 was also higher than that of bare Pt(775). Pt(100) and Pt(110) modified with OA/PA=9/1 gave lower ORR activity than bare electrodes. These results show that PA on wide (111) terrace enhances the ORR activity. The ORR activity of Pt(111) was enhanced after the durability test because active (110) type defects are formed on the surface. The ORR activity of Pt(111) modified with OA/PA was 1.8 mA/cm2 after 2000 cycles, higher than that of bare Pt(111) 1.0 mA/cm2. Adsorbed amines on wide (111) terrace preserved the high activity after the durability test. The activity after 2000 cycles decreased with the increase of the ratio of PA. This result differs from the previous study [1]. The ORR activity of Pt(775) modified with OA/PA was as high as that of bare Pt(775) after the durability test. Durability of Pt(775) was not improved by OA/PA modification. 4. Acknowledgement This work was supported by New Energy Development Organization. 5.