A highly efficient transition metal nitride-based electrocatalyst for oxygen reduction reaction: TiN on a CNT-graphene hybrid support

Transition metal nitrides of group 4-6 (Mo2N, W2N, NbN, Ta3N5, and TiN) were synthesized by the ureaglass route and screened for oxygen reduction reaction (ORR) electrodes in PEMFCs. In terms of electrochemical stability and activity, TiN was selected as the most promising candidate as a catalyst for ORR. To further enhance the activity for ORR, TiN was modified with nanostructured carbon supports including CNTs, graphene (GR), and CNT-GR hybrid. The obtained nanocarbon-supported TiN catalysts exhibited small particle sizes of TiN (<7 nm) and a good TiN-support interaction with reduced aggregation and no free-standing TiN particles away from the supports compared to bare TiN. In particular, TiN supported on the CNT-GR hybrid (TiN/CNT-GR) showed greatly enhanced ORR activity than bare TiN and other supported TiN catalysts. It exhibited a high onset potential (0.83 V) and the highest current density among the reported nitride-based electrocatalysts. The enhancement was ascribed to a synergistic effect between TiN nanoparticles (NPs) and CNT-GR hybird support, roles of which were to provide active sites for ORR and a facile electron pathway to NPs, respectively. Besides, TiN/CNT-GR exhibited large mesopores that could allow easy access of the electrolyte due to the formation of a 3-D CNT-GR structure assembled between 2-D graphene and 1-D CNTs. Further, it showed an excellent methanol tolerance compared to the commercial Pt/C catalyst. Thus, our TiN/CNT-GR could be a promising ORR electrocatalyst for PEMFCs and DMFCs.