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 urea-glass 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.