(190g) Correlating Atomic-Scale Compositions and Structures of Mesoporous N-Containing Carbon Electrocatalysts with Oxygen and Sulfur Reduction Properties

Mesoporous N- and Fe,N-carbons exhibit high and stable activities for oxygen and sulfur reduction that are comparable to or surpass those of standard Pt-activated-carbon electrocatalysts. Favorable properties include high nitrogen contents (>10 atom%), high fractions of N moieties at surface sites, 3-nm mesopores to promote diffusion, and electron conductivity to surface N environments where the reduction reactions occur. The types, quantities, and distributions of N-heteroatom environments, especially those at surface sites, are shown to strongly influence macroscopic reduction activities. N- and Fe,N-mesoporous carbons synthesized using difference mesopore templates (e.g., salt versus silica) are explored to understand the atomic level differences that correlate with increased reduction activity, and how they can be optimized. Compared to 20 wt% Pt/activated carbon, the salt-templated mesoporous Fe,N exhibits the highest reduction activity.

While inclusion of N-heteroatoms improves carbon-based electrocatalyst reduction activity, the atomic-level origins of such properties have remained elusive. Two-dimensional ¹³C-¹⁵N NMR spectra resolve signals from four distinct types of N-heteroatom environments: pyrrolic, graphitic, edge/isolated pyridinic, and pyrazinic/pyridinic moieties, the quantities of which vary by porogen and account for different reduction activities. Chemical shift assignments are corroborated by DFT. Importantly, ¹⁵N-¹H NMR spectra enable surface N species to be selectively distinguished from interior moieties via interactions with adsorbed water. These analyses establish that certain types of N-carbon moieties are more important to electrocatalytic performance than others. The incorporation of non-precious transition metals (e.g., Fe) significantly increases electrocatalytic activity, which have been challenging to explain. Nevertheless, ⁵⁷Fe Mössbauer spectroscopy and solid-state ¹⁵N NMR and spin-lattice relaxation-time analysesresolve signals from ¹⁵N species that are proximate to paramagnetic Fe-heteroatoms, from which ¹⁵N-Fe distances are estimated. Understanding the roles of Fe and N-carbon moieties in electrocatalytic reduction yields new design criteria for syntheses of high performance non-precious-metal electrocatalysts with diverse fuel cell and battery applications.