The slow reaction of the oxygen reduction reaction (ORR) at the cathode of proton electrolyte membrane fuel cells is a major barrier towards commercialisation. To date, carbon supported platinum catalyst has been the preferred for the ORR. Nevertheless, there is limitation for widespread usage of this material due to catalyst instability as a result of corrosion of carbon and platinum at high potentials. This brought about the motivation in understanding and developing of non-precious metal catalysts for ORR.
This project focuses on utilizing various types of nitrogen-functionalised carbon material as non-precious metal catalyst support for the ORR. Differences in structure, surface composition, surface area and pore volume of these carbonaceous materials are expected to affect the dispersion of metal particles which may in turn induces high nitrogen doping. Commercial activated carbon (AC), high surface area graphite (HSAG), carbon nanotubes (CNT), graphene (GRA) and synthesized ordered mesoporous carbon (CMK-3) are reported in this study as potential support for ORR catalysts. The work also investigated the role of iron in incorporating nitrogen from two different nitrogen precursors (ammonia gas and a nitrogen-containing ligand, 2, 4, 6-Tris (2-pyridyl)-1, 3, 5-triazine (TPTZ)). Catalysts preparation used wet impregnation method followed by a heat treatment in gaseous ammonia or nitrogen that consequently produced active sites for ORR. The electrocatalytic activity of the catalysts was investigated in a conventional-three electrodes cell that filled up with 0.5 Molar sulphuric acid.
Iron and nitrogen functionalised CNT showed higher ORR activity than the AC and HSAG catalysts. It was observed the heat treatment in hydrogen and subsequently, in ammonia gas successfully incorporated a certain level of nitrogen content in the CNT, resulting in its significant activity for ORR.
By using TPTZ as nitrogen precursor, more nitrogen was incorporated into the CNT, enhancing the electrocatalytic activity. It was found that the content and types of nitrogen surface groups were mainly affected by the heat treatment temperatures. High ORR activity was obtained from the highest temperature due to the formation of high quaternary-N content. By varying the iron loading on the CNT, the nitrogen content increased with increasing iron loading but the ORR activity was influenced by the amount of pyridinic-N and quaternary-N content.
Pristine graphene showed an ORR activity due to oxygen surface functionalities. The activity was further enhanced by incorporating iron into the graphene. In the case of graphene, the heat treatment in ammonia resulted in zero nitrogen content. Iron and nitrogen functionalised graphene catalyst prepared using TPTZ showed a remarkable ORR activity due to high nitrogen content from the TPTZ decomposition and the activity was even higher than of the CNT catalysts. In the case of CMK-3, nitrogen-doped CMK-3 prepared in ammonia produced higher activity as compared to nitrogen-doped CMK-3 using TPTZ. Of all types of carbon supports used in this project, catalysts supported on graphene showed the highest electrocatalytic activity in acid media, indicating graphene was a good support for the reaction. Impurity free, high defects density, high surface area and exceptional electrical conductivity of a graphene material over other carbonaceous supports provides easy access of oxygen molecule to the active sites and therefore, enhanced the electrocatalytic activity of the ORR.