Compositions, Structures, and Properties of Mesoporous-Fe,N-Containing Carbon Materials for Oxygen- and Sulfur-Reduction Electrocatalysts

Abstract

Mesoporous N- and Fe,N-carbon materials, prepared by polymerization of small molecule precursors and subsequent pyrolysis, exhibit high and accessible surface areas, high electrical conductivities, and high electrocatalytic activities that are comparable to more expensive activated-carbon-supported precious-metal (Pt) fuel cell electrocatalysts. Their favorable properties are due to a combination of factors, including high nitrogen contents that increase the number of active moieties, selective formation of certain types of N moieties at surface sites, relatively large (3 nm) mesopores to promote facile diffusion, and sufficient electron conductivity to surface N environments where the reactions occur. The resulting materials exhibit electrocatalytic properties that are comparable to a standard catalyst containing 20 wt% Pt supported on activated carbon. This is evident in Figure 1a, which shows current-voltage plots associated with the reduction of oxygen for 20 wt% Pt-C and for various N- and Fe,N-mesoporous carbon materials that were prepared identically except for the use of different mesostructured porogen templates. These results show that the oxygen reduction activities of the mesoporous N-carbons are significantly higher when a ZnCl2/NaCl salt mixture is used to produce mesoporosity, compared to mesoporous silica. In addition, the incorporation of a small amount (0.5 wt%) of Fe in the mesoporous N-carbon greatly improves oxygen reduction reactivity, which is shown to be superior to the commercial 20 wt% Pt-C. Similarly favorable activities and trends are observed for these materials as electrocatalysts for sulfur reduction.While the inclusion of N heteroatoms has previously been demonstrated to improve the reduction activities of carbon-based electrocatalysts,1 the atomic-level origins of such properties have remained elusive. This has been due to the multicomponent and non-stoichiometric compositions of these porous graphitic solids, their complicated distributions of disordered and ordered regions, and their paramagnetic and/or conductive properties. Nevertheless, advanced solid-state NMR techniques, in combination with Raman spectroscopy, X-ray diffraction, electron microscopy, and DFT modeling, yield detailed new insights on the atomic environments and interactions of N heteroatoms in mesoporous N-carbon electrocatalysts.2 Most importantly, two-dimensional (2D) 13C-15N correlation NMR spectra resolve distinct signals that enable the types and distributions of different N heteroatom environments to be established and correlated with electrocatalytic properties. For example, as shown in Figure 1b, the distributions of correlated of 15N and 13C signals in the 2D contour-plot spectrum of the salt-templated mesoporous N-carbon material manifest at least four distinct types of N-heteroatom environments: pyrrolic (purple), graphitic (blue), edge/isolated pyridinic (yellow), and pyrazinic/pyridinic (red) moieties. Green-shaded regions indicate correlated signals that arise from atomically proximate (<1 nm) moieties. These assignments are corroborated by DFT calculations to determine the chemical shieldings of different N-carbon moieties, which are shown as points overlaying the experimental 2D spectrum. Similar 2D spectra acquired for silica-templated mesoporous N-carbons yield different quantities and distributions of these N-carbon moieties, which account for the significantly different (lower) macroscopic oxygen reduction activities (Figure 1a). Furthermore, 1D 15N{1H} CP-MAS NMR spectra enable surface N species to be selectively distinguished from interior moieties (e.g., graphitic N) by the formers’ interactions with adsorbed water. In combination, these powerful multi-nuclear NMR analyses unambiguously establish that certain types of N-carbon moieties are more important to electrocatalytic performance than others.Incorporation of non-precious transition metals, such as Fe, significantly increases electrocatalytic activity.3,4 This is similarly the case for mesoporous N-carbon materials investigated here that were synthesized under otherwise identical conditions, except for the addition of a small (0.5 wt%) Fe (Figure 1a). The important role(s) of Fe, however, are challenging to establish, due to its generally dilute loading and its paramagnetic (and possibly ferromagnetic) properties. Nevertheless, 57Fe Mössbauer spectra, solid-state 15N NMR spectra, and 15N spin-lattice (T 1) relaxation time analysesresolve 15N signals from 15N species that are proximate to paramagnetic Fe heteroatoms, from which 15N-Fe distances are estimated. These analyses provide detailed new insights into the types, atomic environments, and distributions of 15N heteroatoms in mesoporous N- and Fe,N-carbon materials, which, until now, have been infeasible to distinguish by scattering or other spectroscopic techniques. Understanding the roles of these moieties in the catalytic reduction of oxygen or sulfur is expected to yield new design criteria for syntheses of high performance non-precious-metal electrocatalysts for diverse fuel cell and battery applications. References (1) Gong, K.; et al. Science 2009, 323, 760–764.(2) Becwar, S. M.; et al. submitted.(3) Donghun, K.; et al. ACS Appl. Mater. Interfaces 2018, 10, 25337−25349.(3) Al-Zoubi, T.; et al. J. Am. Chem. Soc. 2020, 142, 5477–5481. 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