Gas adsorption effects on structural and electrical properties of activated carbon fibers

Abstract

Activated carbon fibers (ACF) are microporous carbon consisting of a three-dimensional network of micrographites having the size of ∼3 nm. We investigate structural and electronic properties of ACFs in relation to the adsorption of helium, nitrogen, and oxygen gases, by means of adsorption isotherm, ESR (electron spin resonance), magnetic susceptibility, and electrical conductivity measurements. The linewidth of ESR associated with dangling bond spins on micrographites decreases with gas uptake at low pressures below 0.1 kPa regardless of gas species. The similar behavior of oxygen to that of nonmagnetic helium and nitrogen demonstrates that adsorbed oxygen molecules having chemisorption feature are stabilized in the singlet ground state in the low-pressure range. Taking into account that the linewidth is governed by dipole–dipole interaction between dangling bond spins, the reduction in the ESR linewidth proves the swelling of micropores induced by gas uptake. The conductivity decreases with gas uptake in the same pressure range regardless of gas species. This behavior is also explained by the modification of microstructure of ACFs. At higher gas pressures 0.1–10 kPa where adsorption is characterized as physisorption for all gaseous species, nitrogen, and helium do not change the ESR linewidth, whereas the linewidth increases with introduced oxygen pressure, due to the dipolar field of paramagnetic oxygen molecules. The mean distance between a dangling bond spin and an oxygen molecule is estimated at ∼0.8 nm.