Doped and Decorated Graphene Foam Electrocatalysts

Abstract

Graphene is an ideal material for electrochemical applications due to high electrical conductivity, large surface area, and stability over a wide electrochemical potential window. However graphene is expensive; it is difficult to control porosity (and thereby mass diffusion); and it is difficult to dope effectively. Simple carbon foams with large micron-scale pores, very thin walls, and very large surface area (e.g. 2500 m2/g) can be synthesized at low cost by thermal decomposition of sodium ethoxide.[1,2] Crucially, by decomposing nitrogen-containing alkoxides, nitrogen-doped graphene powders are made.[3] These do not contain transition metal contamination, as is the case with carbon nanotubes and carbon black. Therefore they can be used to probe the fundamental oxygen reduction reaction (ORR) activity of nitrogen-doped carbon in acid – a relatively under-represented research topic, plagued by contamination issues.[4,5] We record surprisingly high mass activity and onset potential for the ORR, with high electron transfer number (3.6). The negligible metal content was confirmed by ICP-AES. This work shows that 4-electron ORR is possible even in the absence of transition metals, most likely at (or adjacent to) tertiary nitrogen sites.[6] The activity in alkaline medium is also of interest, due to the development of new alkaline ion exchange membranes and the faster ORR kinetics. The nitrogen-doped graphene foams presented here display negligible degradation in cyclic voltammograms over 60,000 load potential cycles, demonstrating the applicability of such metal-free non-precious electrocatalysts in application that require long lifetimes.[7] [1] S. M. Lyth, H. Shao, J. Liu, K. Sasaki, Int. J. Hydrogen Energy, 2014, 39, 376[2] J. Liu, D. Takeshi, K. Sasaki, S. M. Lyth, J. Electrochem. Soc. 2014, 161, F838[3] S. M. Lyth, Y. Nabae, N. M. Islam, et al., eJournal of Surface Science and Technology, 2012, 10, 29-32.[4] S. M. Lyth, Y. Nabae, N. M. Islam, S. Kuroki, M. Kakimoto, S. Miyata, J. Nanosci. Nanotechnol. 2012, 12, 4887[5] S. M. Lyth, Y. Nabae, N. M. Islam, S. Kuroki, M. Kakimoto, S. Miyata, J. Electrochem. Soc. 2011, 158, B194[6] J. Liu, D. Takeshi, D. Orejon, K. Sasaki, S. M. Lyth, J. Electrochem. Soc. 2014, 161, F544[7] J. Liu, K. Sasaki, S. M. Lyth, ECS Trans. 2014, 64, 1161 Figure 1