(365i) Compositions and Structures of High-N-Content Mesoporous Carbon Oxygen Reduction Electrocatalysts

Nitrogen- and transition-metal-containing carbon materials have been the subject of intense recent research interest, due to their desirable properties, including high oxidation stability, high conductivity, and high electrocatalytic activity, which enable their application as replacements for platinum-based oxygen reduction reaction (ORR) catalysts in fuel cells. Typically, these properties have been shown to be related to the N contents in the materials. However, N contents greater than ~4 wt% usually results in dramatically reduced electrical conductivity, and correspondingly reduced suitability for use as electrode materials. Nevertheless, a novel set of precursors have been identified which yield materials with order of magnitude greater N contents while retaining electrical conductivities up to 2.0 S/cm. When these materials are synthesized with high mesoporosities, surface areas in excess of 800 m²/g, and the inclusion of iron-containing salts, they have been found to exhibit high ORR activities. Understanding the molecular compositions and structures of these functionalized mesoporous carbon materials aids the optimization of synthesis conditions, which yields improved performances of the electrochemical devices. Solid-state ¹⁵N nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy measurements demonstrate that the structures and compositions of these materials can be strongly influenced by choice of templating material, transition-metals included, and synthesis conditions, which result in physiochemical material differences that can be used to enhance electrocatalytic activities. Of particular interest are synthesis choices which can selectively alter surface chemistry without changing pore structure, and thus provide insight into the identity of the oxygen reduction active sites. Comparing and correlating the molecular compositions with the electrocatalytic activities crucially establishes that our materials with tunable compositions and cost-effective precursors exhibit ORR activities comparable to current Pt-based catalysts.