Atomic Environments in N‑Containing Graphitic Carbon Probed by First-Principles Calculations and Solid-State Nuclear Magnetic Resonance

To
understand the nature and structure of N-doping centers in carbon
materials, we combine two-dimensional (2D) solid-state NMR experiments
and chemical shift calculations for 15N, 13C,
and 1H nuclei from density functional theory (DFT). Comparisons
of predicted chemical shifts with experimental 2D 13C–15N spectra show good agreement and the calculations explain
the spectral broadening seen in the experiments. The major differences
between the chemical shifts of graphitic/pyridinic/pyrrolic N-moieties
are understood by comparing the electronegativities of the various
environments. Moreover, the signal broadening is explained using four
different factors: (1) the standalone N/C geometry, (2) the effect
of a second N atom nearby, (3) the first or second neighbor C atom
difference, and (4) the influence of residual water, which is important
to understand the electrocatalytic environment. An intuitive correlation
between the charge of the probed atom and the chemical shift is validated:
the smaller the charge, i.e., higher electron density, the more shielded
the nucleus is, and hence the smaller the associated chemical shift.
These results can improve the understanding of the nature of heteroatom
sites in nitrogen–carbon materials and contribute to the rational
design of these materials with desired electronic properties and improved
electrochemical performance.