Electrical double layer formation in nanoporous carbon materials

Abstract

Environmental separation processes such as removal of heavy metals from aqueous solutions, electrosorption in groundwater remediation, and capacitive desalination, as well as energy storage in supercapacitors, are based on the electrical double layer (EDL) formation within nanoporous carbon materials. This research is focused on the nano-scale phenomena of EDL formation inside the confined space of nanopores. The electrosorption behavior of nanoporous carbon materials was characterized by measuring the double-layer capacitance using cyclic voltammetry. The presence of micropores results in the occurrence of EDL overlapping, corresponding to a considerable loss of the double-layer capacitance. Hence, pore size distribution plays an important role in determining the double-layer capacitance. EDL formation has significant influence on ion transport and sorption inside nanopores. The data obtained by simple diffusion and electrochemically-aided diffusion experiments demonstrated the size-exclusion effects on pore accessibility by ions. A larger ion-exclusion volume prevents larger ions from penetrating inside the pores. Batch equilibrium electrosorption experiments using nanoporous carbon materials showed that selective electrosorption, imposed by the difference in the size of hydrated ions, occurs in a competitive environment. Molecular modeling based on Monte Carlo methods was developed to simulate the EDL formation in a slit-type nanopore. Simulation results indicated that the competition in asymmetries of ion charge and size not only determines the screening of surface charge but also affects the electrolyte distribution within charged pores. In a mixture of electrolytes, the charge/size competitive effects can dominate pore accessibility. Multivalent counterions with large size have the energetic advantage of screening surface charge. On the other hand, small monovalent counterions present a ¡§size affinity¡¨ to access the pores. Therefore, electrosorption selectivity of counterions with different properties is a result of a counterbalance between minimization of potential energy and size-exclusion effects. Manipulation of electrosorption selectivity to separate ions could in principle be achieved via tuning the EDL formation inside the pores. The findings of the thesis have several significant implications for the development of advanced techniques for selective separation of ions in environmental systems and energy storage.