Transition Metal and Nitrogen-Doped Carbide-Derived Carbon/Carbon Nanotube Composites As Cathode Catalysts for Anion-Exchange Membrane Fuel Cells

Abstract

Energy is an essential part of our lives and causes a significant impact on the environment. As the energy carrier, hydrogen could be a feasible option as it is readily producible and cost-efficient.1 In the transport sector, the proton exchange membrane fuel cells, which use hydrogen as the fuel, are already used, but have issues regarding the usage of high amount of precious metals. However, replacing acidic media with alkaline gives better possibility to use non-precious metal catalysts, especially for the cathodic oxygen reduction reaction (ORR). This suggests that anion exchange membrane fuel cells (AEMFCs) have great promise as future energy devices.2 As non-precious metal catalysts, different transition metal and carbon-based materials have shown great promise.3 Herein, a nanocarbon composite based on carbide-derived carbon/carbon nanotube (CDC/CNT) is used as a support to prepare M−N−C type catalysts. The CDC/CNT composite offers a novel structure containing both micro- and mesopores that is beneficial for the AEMFC application. The composite material is doped with nitrogen and transition metals using pyrolysis at 800 °C in inert atmosphere. As precursors, metal acetates (iron(II) and cobalt(II) acetate) and 1,10-phenantroline are employed. Three M-N-C materials are prepared, namely Fe-N-CDC/CNT, Co-N-CDC/CNT, and bimetallic CoFe-N-CDC/CNT.4 Different physico-chemical characterization methods (SEM-EDX, XPS, XRD, MP-AES, and N2 physisorption) are used to study the catalyst materials, which proved the success of doping (metal content ca. 1 wt%) as well as the feasible micro- and mesoporous structure with defects present. The RRDE method was used for electrochemical testing to study the ORR pathway and activity. In alkaline medium all three catalyst materials exhibit good electrocatalytic activity for the ORR with the peroxide yield depending on the metal additive in order Co > CoFe > Fe. The materials are applied as cathodes in the AEMFC together with ETFE membrane. The MEA with the CoFe-N-CDC/CNT cathode shows the best performance in H2/O2 AEMFC and a very good performance in H2/air AEMFC by obtaining respective peak power densities of 1.12 and 0.80 W cm–2 (Fig. 1). Additionally, the CoFe-N-CDC/CNT exhibited good stability in both AEMFCs. This shows that the M-N-CDC/CNT materials are promising cathode catalysts for the AEMFC application.4 References O. Abe, A. P. I. Popoola, E. Ajenifuja, and O. M. Popoola, Int. J. Hydrogen Energy, 44, 15072-15086 (2019).S. Gottesfeld, D. R. Dekel, M. Page, C. Bae, Y. S. Yan, P. Zelenay, and Y. S. Kim, J. Power Sources, 375, 170-184 (2018).A. Sarapuu, E. Kibena-Põldsepp, M. Borghei, and K. Tammeveski, J. Mater. Chem. A, 6, 776-804 (2018).J. Lilloja, E. Kibena-Põldsepp, A. Sarapuu, J. C. Douglin, M. Käärik, J. Kozlova, P. Paiste, A. Kikas, J. Aruväli, J. Leis, V. Sammelselg, D. R. Dekel, and K. Tammeveski, ACS Catal., 11, 1920-1931 (2021), Figure 1