Mechanistic Insights Into Supercapacitive Swing Adsorption via Acid-Base Titrations

The mechanism of supercapacitive swing adsorption (SSA) has been investigated by pH measurements and acid-base titrations of water-extracted, charged activated carbon electrodes at voltage windows varying from 0-0.5 to 0-1.4 V. 15%CO₂/85%N₂ mixtures and pure N₂ were used as feed gases. Deionized water extracts from negatively charged electrodes showed a pH increase for both feed gases. Titration of extracts from negative electrodes exposed to CO₂ consumed more HCl titrant compared to those without CO₂. Conversely, extracts from the positive electrodes consumed more KOH titrant when only N₂ was present. The pH values and titrant amounts consumed increased with increasing voltage window for both electrodes. Larger amounts of acid consumed for negative electrode extracts in the experiments with CO₂ are explained by the adsorption of additional protons created from CO₂ hydrolysis. Smaller amounts of base are consumed for the positive electrode in the presence of CO₂ due to the reaction of bicarbonate with acid generated at the positive electrode. The results explain that CO₂ is adsorbed to the negative electrode and desorbed from the positive electrode upon charging. They are evidence for the ionic liquid-solid mechanism of SSA, which assumes that CO₂ sorption is driven by selective proton adsorption to the negative electrode.