Activated Carbons as Nanoporous Electron-Ion-Exchangers

The electrochemical, ion-exchange, sorption, hydrophilic-hydrophobic properties of several types of electrodes based on activated carbon (AC) and its porous structure are studied. By the method of standard contact porosimetry it is found that AC exhibits both hydrophilic and hydrophobic porosity and has the high specific surface area (600–2600 m2/g) which explains its use in supercapacitors and for capacitive deionization (CDI) of water. A new phenomenon of superhydrophilicity caused by swelling of surface groups (SG) in water is observed. The measurements of the AC surface conductivity show that even in pure water AC exhibits considerable ionic conductivity, which makes possible its use in the production of pure water by CDI. AC is shown to be an electron-ion-exchanger which is its unique feature. It is found that in addition to the electric double layer capacitance, the pseudocapacitance of redox-reactions of surface groups makes a substantial contribution into its total capacitance. The deep cathodic charging to negative potentials vs. reversible hydrogen electrode (RHE) in concentrated H2SO4 allows reaching the specific preudocapacitance of AC of 1100 F/g mainly due to the high pseudocapacitance of the reaction of hydrogen intercalation into carbon. This corresponds to the formation of a new compound C6H. In pure water, the specific capacitance of AC reaches 66 F/g and does not increase with the increase in KCl concentration, which suggests that this capacitance is associated with surface groups.