(419g) Compositions, Structures, and Properties of Mesoporous-Fe,N-Containing Carbon Materials for Oxygen Reduction Electrocatalysts

Compositions,
structures, and properties of mesoporous-Fe,N-containing carbon materials for
oxygen reduction electrocatalysts

Shona Becwar, Zachariah
Berkson, Niels Zussblatt, Donghun Kim, Bradley
Chmelka

University
of California, Santa Barbara, Santa Barbara, California, USA

Mesoporous
Fe,N-carbon materials prepared by high-temperature condensation and pyrolysis
of small molecule precursors exhibit high and accessible surface areas, high
electrical conductivities, and high electrocatalytic activities that are
comparable to much more expensive activated-carbon-supported precious-metal
(Pt) fuel cell electrocatalysts. The synthesis conditions, precursor compositions,
and choice of templating material (e.g., salt-mixture or mesoporous-SBA-15
silica) influence the types, quantities, and distributions of Fe and N
heteroatom environments, which strongly affect macroscopic oxygen reduction
activities. It has been challenging to measure and correlate local Fe or N
heteroatom environments and their distributions in graphitic carbon materials
with macroscopic electrocatalytic properties. This has been due, in part, to
their paramagnetic and/or conductive properties, complicated distributions of
disordered and ordered regions, and heterogeneous material compositions. Nevertheless,
advanced 1D & 2D solid-state ¹H, ¹³C, ¹⁵N,
and ¹³C{¹⁵N}, variable temperature, spin-lattice
relaxation, and fast MAS NMR techniques, in combination with ⁵⁷Fe
Mossbauer and Raman spectroscopy, X-ray diffraction, and electron microscopy,
elucidate the local environments and mutual proximities of Fe and N heteroatoms
in mesoporous-Fe,N-carbon electrocatalysts.

While inclusion of N heteroatoms is
known to improve the oxygen reduction activities of carbon-based
electrocatalysts,¹ these results
establish that only specific N environments, identified and quantified by solid-state
NMR, correlate with increased oxygen reduction activity. Materials synthesized
with ¹³C,¹⁵N-enriched precursors yield increased NMR
signal sensitivity that enables powerful solid-state low-temperature (95K) 2D ¹³C{¹⁵N}
dipolar-mediated NMR correlation analyses with improved spectral resolution.
These analyses enable the identification and quantification of the
non-stoichiometric electrically- and catalytically-active N-heteroatom sites
(Fig. 1). Furthermore, relaxation-resolved ¹⁵N NMR spectra and ¹⁵N
spin-lattice (T₁) relaxation time measurementsresolve
¹⁵N signals from ¹⁵N species proximate to paramagnetic Fe
heteroatoms and provide approximate ¹⁵N-Fe distances.Insights
from solid-state NMR analyses provide detailed new insights into the types,
atomic environments, and distributions of ¹⁵N heteroatoms in
mesoporous Fe,N-carbon materials, which, until now, have been infeasible to
distinguish by scattering or other spectroscopic techniques. Correlating these
insights with increasing ORR activity provides heteroatom specific goals for
the design of improved electrocatalysts for applications in energy storage and
conversion.

References

(1)     Gong, K.; Du,
F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube Arrays with
High Electrocatalytic Activity for Oxygen Reduction. Science 2009,
323 (5915), 760–764.