New Insights into the Structure of Nanoporous Carbons from NMR, Raman, and Pair Distribution Function Analysis

Abstract

The structural characterization of nanoporous carbons is a challenging task as they generally lack long-range order and can exhibit diverse local structures. Such characterization represents an important step toward understanding and improving the properties and functionality of porous carbons, yet few experimental techniques have been developed for this purpose. Here we demonstrate the application of nuclear magnetic resonance (NMR) spectroscopy and pair distribution function (PDF) analysis as new tools to probe the local structures of porous carbons, alongside more conventional Raman spectroscopy. Together, the PDFs and the Raman spectra allow the local chemical bonding to be probed, with the bonding becoming more ordered for carbide-derived carbons (CDCs) synthesized at higher temperatures. The ring currents induced in the NMR experiment (and thus the observed NMR chemical shifts for adsorbed species) are strongly dependent on the size of the aromatic carbon domains. We exploit this property and use computer simulations to show that the carbon domain size increases with the temperature used in the carbon synthesis. The techniques developed here are applicable to a wide range of porous carbons and offer new insights into the structures of CDCs (conventional and vacuum-annealed) and coconut shell-derived activated carbons. ■ INTRODUCTIONNanoporous carbons are an important class of materials used in a range of applications including capacitive energy storage, gas storage, water treatment, and catalysis. 1−3 In each case, the nanoporosity and high specific surface areas (typically >1500 m 2 g −1 ), achieved by activating carbonaceous precursors, are exploited to store molecules or ions. In principle, carbon structures can be engineered for a given application, though characterization of the highly disordered structures poses a significant challenge. The challenges in determining local-and long-range structure make it extremely difficult to establish structure−function correlations beyond those simply derived from surface area and pore-size distributions.The structures of carbon materials 4,5 have been actively researched since the pioneering X-ray diffraction studies of Franklin. 6,7 She distinguished between graphitizing and nongraphitizing carbons, the former transforming into graphite upon heating to high temperature, and the latter showing no such transformation at temperatures as high as 3000°C. 7 For nanoporous carbons, analysis of the broad Bragg peaks is generally of limited use due to the long-range disordered structures of these materials. However, inclusion of the diffuse scattering in the analysis allows the extraction of a pair distribution function (PDF), which is a weighted histogram of atom-to-atom distances showing the likelihood of finding an atom pair separated by a certain distance. 8 PDF studies show that porous carbons often exhibit a high degree of local ordering, with a propensity for hexagonal carbon rings in which the carbon atoms are sp 2 hybridized. 9−11 Correlations in the PDFs typical...

An Atomistic Carbide-Derived Carbon Model Generated Using ReaxFF-Based Quenched Molecular Dynamics

“…To maintain consistency, all simulated PDFs used the same parameters. The peaks of these PDFs correspond well to the distances between carbons in an idealized graphene-like structure and follows previous interpretation in which no strong correlation can be observed between different sheets and/or pore walls [96,97]. The first three peaks at 1.43(2), 2.45(2) and 2.83(2) Å correspond to neighbors in a hexagonal ring and peaks at 3.75(5), 4.28(5) and 4.97(8) Å correspond to the location of carbon atoms in neighboring rings.…”

Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy

“…In PFG NMR experiments magnetic field gradient pulses are used to encode and decode the positions of the nuclear spins, with ionic diffusion probed over a given These marked reductions could arise from a number of factors: (i) the local reduction in in-pore diffusion due to collisions with the rigid pore walls, 30,31 (ii) tortuosity arising from the disordered arrangement of pores [32][33][34] in the carbon particles such that ions diffuse in an indirect way, and (iii) the structure and composition of the electrolyte in the pores may differ from neat electrolyte, our previous studies having shown that ions are partially desolvated in the pores. 6 The difference in diffusion coefficients between anions and cations is amplified upon confinement in the pores (Figure 2a), highlighting the important role of ion size, with the larger cations presumably taking more indirect pathways through the pore network (as the smallest pores are inaccessible).…”

Solid-state NMR studies of supercapacitors

“…The one exception to the trend is TiC-CDC-600 with an average pore size of 0.6 nm, for which a number of studies have shown that adsorbed electrolyte species exhibit smaller than expected ring current shifts. 57 , 26 , 58 However, 1 H MAS NMR experiments on the unloaded carbon indicate the presence of a significant amount of hydrogen in the material. This is indicative of a more fragmented structure in which the sp 2 -hybridisation is reduced by the extensive termination of bonds by hydrogen.…”

“…The validity and utility of the lattice simulation-DFT approach has subsequently been demonstrated for the structural study of microporous carbons with different pore and domain sizes. 58 Through comparison of the experimental and simulated ring current shifts for adsorbed BF 4 anions, approximate domain sizes were determined in absolute terms for a set of TiC-CDCs subjected to different high-temperature heat treatments. For TiC-CDC-600, the average domain size determined in this way is smaller than the size of a coronene molecule (7.5 Å diameter), whereas for TiC-CDC-1000, the average domain size is intermediate between coronene and circumcoronene (12.4 Å diameter).…”