Influence of Morphology on Transport Properties and Interfacial Resistance in Nanoporous Carbons
L Liu and SK Bhatia, JOURNAL OF PHYSICAL CHEMISTRY C, 123, 21050-21058 (2019).
The drive for enhancing process efficiency by decreasing the system length scale to reduce transport resistance in nanomaterials brings into prominence the interfacial resistance, the relative importance of which scales inversely with system size; this critically limits the extent of efficiency enhancement possible. We investigate here the effects of morphology on the interfacial resistance for methane transport in carbons, by comparing the transport properties of a finite-sized ordered carbon nanotube, disordered activated carbon fiber (ACF-15), and silicon carbide derived carbon (SiC- DC), using molecular dynamics simulations. We find that while the ordered carbon nanotubes (CNTs), having a smooth energy landscape, provides the largest transport coefficient, the relative interfacial resistance in this material is also high and exceeds that in the disordered carbons, which have intracrystalline diffusivities that are more than 2 orders of magnitude smaller. This behavior is traced to the existence of large entry and exit zones, in which the fluid motion remains correlated with that at the ends. The sizes of these zones are influenced by the small interfacial momentum accommodation coefficient (or Maxwell reflection coefficient) characteristic of carbons and the level of disorder in the material. However, the interfacial resistivity in the CNT is smaller than that in the disordered carbons, due to the dramatically reduced contribution from bending of fluid streamlines at the interfaces. Nevertheless, the interfacial resistance of the CNT is larger than that of the disordered carbons, ACF-15 and SiC-DC, due to the reduced cross-sectional area of the CNT. It is found that the resulting overall corrected diffusivities in the ACF-15 and SiC-DC having a thickness around 50 nm are comparable to that in the corresponding CNT, indicating that at nanoscale thicknesses morphology is not a significant consideration from a transport viewpoint, and CNTs offer little advantage compared to disordered carbons.
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