Assessing optimal pore-to-ion size relations in the design of porous poly(vinylidene chloride) carbons for EDL capacitors

Abstract

In order to maximize the capacitance of electrical double layer (EDL) capacitors per unit electrode volume, higher surface areas are required. This leads to an increasingly greater subdivision of the carbon electrode, namely to pore systems of smaller pore size. When the pore size approaches the ion size, the EDL charging kinetics lows down because of multiple interactions of the ions with the surrounding pore walls. On the other hand, the ion electroadsorption capacity increases just because of this enhanced interaction. Therefore, there is a conflict between improving discharge kinetics and improving capacity. Knowing the effective ion size relative to the pore size can be helpful in optimizing the pore system design in electrodes for EDL capacitors. A thorough technique based on the adsorption of molecular probes in the gas phase and the electroadsorption of different ions was developed to assess pore dimensions. In this report, the technique is applied to the unique case of polyvinylidene chloride (PVDC) based carbon electrodes in an attempt to elucidate its extraordinary high EDL capacitance, in terms of the relation between effective ion size and pore size.