(292h) Oxygen and Sulfur Reduction Activities of High N-Content Nanoporous Fe,N-Carbon Electrocatalysts Correlate with Atomic-Scale Compositions and Structures

Nanoporous iron- and nitrogen-functionalized (Fe,N) carbon materials exhibit high oxygen and sulfur reduction activities that are comparable to or surpass those of activated-carbon-supported-Pt or doped graphene electrocatalysts used in hydrogen fuel cells or lithium-sulfur batteries. Favorable properties include high nitrogen contents (>10 mol%), high fractions of N moieties at surface sites, 3-nm pores to facilitate diffusion, and electron conductivity to surface Fe and N environments where reactions occur. Such properties have been challenging to combine, understand, and control, due in part to the materials’ non-stoichiometric compositions, high electrical conductivities, heterogeneous surfaces, and complicated structural order and disorder, all of which have important influences on their transport, adsorption, and reaction behaviors. Nevertheless, such challenges can be overcome by characterizing nanoporous Fe,N-carbon materials over multiple length scales by using X-ray scattering, high-resolution transmission electron microscopy, Raman spectroscopy, density functional theory, macroscopic electrochemical reaction analyses, and especially solid-state nuclear magnetic resonance (NMR) spectroscopy. In combination, these complementary techniques enable the local bonding environments and interactions of key nitrogen and iron moieties to be identified and correlated with the oxygen and sulfur reduction properties of different mesoporous Fe,N-carbon electrocatalysts. Such reactions respectively occur at the cathodes of H₂ proton-exchange-membrane fuel cells or Li-sulfur batteries and are often the rate-limiting processes in these devices. Interestingly, the types, quantities, and distributions of N-heteroatom environments, especially surface pyridinic moieties, are shown to strongly influence the reduction activities and to depend strongly on the properties of the nanopore-generating template used. The analyses quantitatively correlate the atomic-scale compositions and structures of nanoporous Fe,N-carbon materials with their macroscopic O₂ and sulfur reduction properties, yielding design criteria for their use as non-precious-metal electrocatalysts in diverse electrochemical applications.