Nitrogen-Incorporated Amorphous Carbon Catalysts with Higher Reactivity toward Oxygen Reduction Reaction

Abstract

Introduction One of the challenges to be addressed in facilitating widespread dissemination of polymer electrolyte fuel cell (PEFC) is to reduce the amount of Pt catalysts used in PEFC. The rate of oxygen reduction reaction (ORR) (O2 + 2H+ + 4e- → 2H2O) is relatively lower at Pt surface and quite a large amount of Pt catalysts is required to increase O2 reduction rate, which is a serious problem for cathode electrodes. For instance, 100g of Pt is needed for a fuel cell vehicle. Pt usage of this sort is impractical in terms of resources as well as economics. In order to ensure widespread commercialization of PEFC, the development of non-Pt catalysts is essential. The other challenge is to improve the stability of catalysts. The activity of Pt catalysts is known to be decreased by surface poisoning during long-term use. The objective of this study is to realize non-Pt catalyst with ORR activity and higher stability by using amorphous carbon (a-C) incorporating nitrogen atoms. It has been reported that carbon alloy with quinolizinium structure at graphene edges exhibited the higher ORR activity [1]. Our research group has reported that a-C including amorphous phase and nano-sized sp 2 cluster could be a conductive material by incorporating nitrogen atoms. It works as an ideal polarizable electrode with higher overpotential toward water discharge and higher stability to electrochemically-induced corrosion [2]. In the study, a-C based catalysts with higher ORR activity and higher stability was tried to be realized by introducing quinolizinium structure at the sp 2cluster surface in a-C. Experimental a-C catalysts with quinolizinium structure were synthesized by plasma-enhanced CVD method. Vaporized acetonitrile + pyridine (6:4 at a molar ratio) mixture was used as a source material. a-C catalysts were synthesized under various conditions of RF power and substrate temperature. Chemical compositions and structures of the resulting a-C were examined by XPS and Raman spectroscopy. The reactivity of ORR was examined by linear sweep voltammograms using rotating disk electrode (RDE). Results and Discussion The size of sp 2 clusters of the resulting a-C catalysts was estimated from Raman spectra. The ratios of sp 2/sp 3-hybridized carbons and the density of N atoms in quinolizinium structure were estimated from XPS results. Hydrodynamic voltammograms using RDE (Figure 1) were carried out to estimate the number of electrons for O2 reduction in O2 saturated 1M KOH solution. The peak correspond to N atoms in quinolizinium structure (graphitec N) was included in N1s peak in XPS spectra of all a-C catalysts and observed at approximately 402 eV. These results indicate that functional groups active for O2 reduction are introduced in graphite regions in a-C. With increasing RF power of CVD synthesis, the ratio of sp 2/sp 3 from XPS exhibited a tendency to increase, the size of sp2 cluster to decrease, and the density of N atoms in quinolizinium structure to increase. It indicates that the active site for O2 reduction can be controlled by deposition condition. The number of electrons for O2 reduction was enhanced by these structural changes and reached 2.82 at a maximum (Figure 1). The value over 2 suggests that a-C catalysts with the activity of 4-electron ORR were successfully realized. It can be expected that the ORR activity is further enhanced by optimizing sp 2/sp 3, the size of sp 2cluster, and the density of N atoms in quinolizinium structure.