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

Abstract

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.