Role of different drying techniques on the morphology of carbon aerogel: A combinatorial analysis using gas adsorption and small-angle scattering

Abstract

The choice of drying process plays a crucial role on the morphology, vis-a-vis the physicochemical properties, of carbon aerogel (CA). In this article the effect of four common drying processes, namely, open-air drying (OAD), modified open-air drying with solvent exchange (MOAD), freeze-drying (FD) and critical-point drying (CPD), on the morphological characteristics of carbon aerogel has been studied using a combination of gas adsorption measurements and small angle (neutron and X-ray) scattering. The investigations reveal a wide diversity in the pore-size distribution in the samples. OAD and MOAD results CA having predominantly micropores with the lowest specific surface area and mesopore area as a consequence of collapse of pores during the drying process. On the other hand, the FD leads to blending of micro-to-meso-porous structures and CPD results a meso-to-macro-porous material. The scattering analysis reveals that the average size of primary particles as well as the agglomerates is the largest for CPD with high polydispersity, due to the fusion of the gel particles at the applied high pressure while drying, and the lowest fractal dimension signifying more open and less dense structure. The Porod analysis confirms the higher surface area for CPD as well. The relative low number density, bigger sizes and small packing fraction for CPD-CA and FD-CA complement the trend as observed in the gas adsorption. Finally, inline with the observed morphological diversity, the electrochemical measurement showed that the CPD-CA and FD-CA maintained desirable capacitive behavior with specific capacitance of 89 and 91 F/g, respectively .