Enhancement of the ORR Activity and Durability of Pt/C by the Modification with Melamine and N-Containing Aromatic Compounds

Abstract

The ORR activity and durability of Pt catalysts have to be enhanced for the wider spread of PEFCs. We pay special attention to possibility that surface modification of a Pt catalyst increases its ORR activity. On the Pt surface, Pt-OH species is stabilized too much by network structure of water molecules adsorbed there [1]. The overstabilization of Pt-OH impedes the ORR because this species is an intermediate of ORR. Then, we try to enhance the ORR activity by destabilizing Pt-OH. The perturbation of the water network would destabilize Pt-OH. We try to perturbate the water network by adding an external organic compound to enhance the ORR activity.We tested a wide variety of compounds as an enhancer of ORR of Pt. Although most of compounds tested exhibit no influence of inhibitory effects on the ORR, we found that certain kind of N-containing aromatic compounds -melamine (and tetraazaporphyrin)- enhance the ORR activity of Pt catalysts significantly [2-4]. With the addition of melamine, ORR activity at 0.9 V vs. RHE is increased by 1.6 times although the ECSA decreased (Fig. 1). The enhancement of the ORR activity is reasonably ascribed to the destabilization of Pt-OH, judging from the change of cyclic voltammograms of Pt/C after the addition of melamine.This melamine modification method is also applied for another highly active catalyst (Pt/Pd/C core-shell catalyst) to obtain a further activated catalyst. The mass activity of Pt/Pd/C core-shell catalyst increases by 2.4 times after melamine modification and reaches 3600 AgPt -1 at 0.9 V vs. RHE [5].Melamine also enhances the durability considerably. Under an accelerated durability test (ADT) (80 °C, 0.6 V (3s) - 1.0 V (3s), square wave, 10000 cycles), Pt/C undergoes severe degradation; the ECSA significantly decreases because of the increase in the size of Pt nanoparticles (Fig. 1). However, after the ADT in the presence of melamine, the decrease in ECSA is significantly suppressed [6]. This beneficial effect is due to the suppression of the increase in size of Pt nanoparticles. A melamine polymer (poly-melamine sulfonate)-modified Pt/C also exhibits higher durability than unmodified Pt/C [7]. The suppression of Pt-OH by melamine participates in the enhancement of the durability.In this talk, we introduce discovery of melamine as an ORR enhancer, effect of melamine on the ORR activity/durability of Pt/C, and mechanisms of functions of melamine.This paper is based on the results obtained from projects, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).