Activity Of Nitrogen-doped Carbon Research Articles

Local electronic structure and activity of nitrogen-doped carbon

Local electronic structure and activity of nitrogen-doped carbon

Local electronic structure and activity of nitrogen-doped carbon

Local electronic structure and activity of nitrogen-doped carbon

Gram-Scale Synthesis of Nitrogen-Doped Carbon Foams As Template-Free, Metal-Free, Non-Precious Electrocatalysts for Oxygen Reduction and CO2 Conversion

Carbon foams are ideal materials for electrochemical applications due to their large surface area, high electrical conductivity, and stability over a wide electrochemical potential window. However, many of these materials are expensive; use complex multi-step synthesis protocols; and require the use of nanostructured template materials which must be removed with strong acid or alkaline. It is often difficult to control the porosity (and thereby optimize mass diffusion); and it is difficult to dope effectively with heteroatoms. We synthesize carbon foams at low cost and gram-scale, with large micron-scale pores, very thin walls, and very large surface area (e.g. 2500 m2/g), by thermal decomposition of sodium ethoxide.[1,2] Crucially, by synthesizing and subsequently decomposing nitrogen-containing metal alkoxides, nitrogen-doped carbon foams can be made.[3] The nitrogen content can be varied over a wide range (e.g. <0.5 to 15 at%) by simply changing e.g. the precursor ratios. In addition, the ratio of e.g. pyridinic to tertiary nitrogen bonding can be tailored by changing the pyrolysis temperature. These materials do not contain transition metal contamination (confirmed by ICP-AES), unlike carbon nanotubes or 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 topic, plagued by contamination issues.[4,5] We observe surprisingly high mass activity and onset potential for the ORR, with high electron transfer number. This work shows that 4-electron ORR is possible even in the absence of transition metals, most likely associated with tertiary nitrogen sites.[6] In alkaline medium, these catalysts are comparable to Pt/C, and undergo negligible degradation even over 60,000 load potential cycles.[7] Finally, these materials are investigated for their electrochemical CO2 conversion activity to carbon monoxide, formic acid, methane, and ethane.

Cooperative Catalysis for Selective Alcohol Oxidation with Molecular Oxygen.

The activation of dioxygen for selective oxidation of organic molecules is a major catalytic challenge. Inspired by the activity of nitrogen-doped carbons in electrocatalytic oxygen reduction, we combined such a carbon with metal-oxide catalysts to yield cooperative catalysts. These simple materials boost the catalytic oxidation of several alcohols, using molecular oxygen at atmospheric pressure and low temperature (80 °C). Cobalt and copper oxide demonstrate the highest activities. The high activity and selectivity of these catalysts arises from the cooperative action of their components, as proven by various control experiments and spectroscopic techniques. We propose that the reaction should not be viewed as occurring at an 'active site', but rather at an 'active doughnut'-the volume surrounding the base of a carbon-supported metal-oxide particle.

Doped and Decorated Graphene Foam Electrocatalysts

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