The Hierarchical Structure of Nanoporous Carbon Electrode Materials Elucidated by Water Sorption: A Comparison of Multiple Structural Models
The total amount of energy that an electrical double layer capacitor (EDLC) can store depends on the voltage and the accessible surface area for ion electrosorption. Nanoporous carbon materials with a high specific surface area, such as carbide derived carbon (CDC), make ideal electrodes for EDLC devices. CDC materials have fine-tuned pore sizes in the subnanometer range which are controlled by the initial carbide (TiC) and annealing conditions. Water can enter TiC-CDC pores and give filling fractions in excess of 0.6 g/g (H2O/C). Recent reports of water's diffusional dynamics dependence on CDC pore size indicate confinement effects similar to that observed in pores of 16 Å. Though other studies have gleaned insights into the porous structure of CDC, there is currently no direct structural information on the sorption of water at intermediate stages between completely empty and full. Small-angle neutron scattering (SANS) was used to find to what degree pores are filled with water (D2O) at intermediate stages of loading for four TiC-CDC powders, each with different pore size distributions. We present models to reproduce the SANS data and extract information on the structure of CDC, consisting of subnanometer pores as well as mesopores. The subnanometer pores exhibit a narrow size distribution whereas the mesopores are polydisperse. In addition, the structure of the room temperature ionic liquid (RTIL) [C4mim]+[Tf2N]- and water confined inside the micropores (< 2 nm) of carbide derived carbon (CDC) were investigated using small angle x-ray scattering (SAXS). RTILs, composed entirely of ions, have high thermal and electrochemical stability which make them interesting candidates as electrolytes in EDLCs. Water is usually removed from working devices due to the breakdown of H2O at low voltages, but the interaction between the two fluids in confinement is still not fully understood. SAXS/WAXS measurements were carried out on dry CDC, samples which were partially (50%), completely (100%), and over-filled with RTIL (150%), as well as RTIL-filled samples exposed to100% water-saturated air. This study provides information on the length scales of the CDC porosity, the extent to which RTIL enters micropores, and how confinement affects the charge-ordered structure of the RTIL.
Espitia, Jose Ali, "The Hierarchical Structure of Nanoporous Carbon Electrode Materials Elucidated by Water Sorption: A Comparison of Multiple Structural Models" (2018). ETD Collection for University of Texas, El Paso. AAI10823094.