Certain microporous carbons have pores of molecular dimensions, the size of which can be adjusted over a wide range to fit the dimensions of a large variety of molecules. With knowing the dimensions of these molecules, size-calibrated porous carbons can be produced. In this work, we took advantage of the electronic conductivity of carbon in using it as an electrode in electrolytic solutions. By changing the electrode potential, we could induce electroadsorption and electrodesorption of ions of different dimensions into pore-calibrated carbons, thus enabling us to estimate effective ion sizes. A few fundamental questions such as whether ions accommodated in the electrodes' pores are in a solvated state, which is important for novel electrochemical energy storage devices and for microbiological systems, could be addressed. Analysis of the results indicates that all the cations that are employed are electroadsorbed in the electrode pores in hydrated states. Compared with the monovalent cations, the bivalent cations exhibit dimensions that make them almost twice as big. Monovalent anions are basically adsorbed in a nonhydrated state. The doubly charged sulfate anion is adsorbed in the hydrated state. The nitrate, a multiatom planar anion, has a smaller effective dimension than the monoatomic halogen anions, in accordance with the observation that the pores in many activated carbons are slit-shaped. The analysis of the results enabled us to establish a scale of ionic effective dimensions in aqueous media, as presented below (MTBE stands for methyl tert-butyl ether):  cations, [3.62 Å, N2] < Cs+ < K+ < Na+ < Li+ < [4.21 Å, CF4] < [5.05 Å, SF6] < [5.8 Å, MTBE] < Ba2+, Ca2+, and Mg2+; anions, [3.62 Å, N2] < NO3 - < Cl- < F- < Br- < [4.21 Å, CF4] < ClO4 - < [5.05 Å, SF6] < [5.8 Å, MTBE] < SO2-. We discovered that as long as the pore size is considerably greater than the ion size, the electric double-layer capacity at moderate concentrations is independent of both the size and charge of the ions in solutions.

By using carbon chemical vapor deposition (CVD) onto active porous carbon fibers, we were able to reduce the pore size of the active carbon to obtain selective carbon molecular sieves (CMS) of variable properties. The pore size of these carbons was characterized by the adsorption of molecules of different dimensions (molecular probe adsorption) from the gas phase. By manipulating the several experimental parameters of the CVD process, the pore size of the CMS could be finely tuned to fall between sizes of alkaline earth cations (Ca2+; Mg2+, 6−7 Å in diameter), and the much smaller size of hydrated monovalent cations (4 Å in diameter). This was proven by voltammetric measurements of the pore-tailored carbon electrodes in the corresponding salt solutions, which also provided a measure of the differential capacity of the electrical double layer (EDL) vs the potential (e.g., at potentials negative to the potential of the zero charge, the EDL capacity was very small for CaCl2 and MgCl2 solutions, while it was high for NaCl solutions). The CVD process applied herein did not significantly reduce the specific surface area and the pore volume of the carbons. This implies that the carbon deposits are only superficial and that the pore blocking effect takes place only at the pore mouth. Hence, the adsorption processes related to these carbons involve surface barrier mechanisms. The importance of the selectivity thus obtained relates to a possible use of these carbon electrodes in electrochemical water desalination processes. By using these carbons as electrodes in capacitive water deionization processes, it is possible to selectively remove Na+ cations, leaving Ca2+ and Mg2+ ions (which are important nutrients) in the treated water.

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In an attempt to form a proton-selective environment in porous carbons that is similar to that of protonconducting polymers, we used highly oxidized molecular sieving carbons that are covered by oxygen-containing surface groups. Naturally, it would be assumed that the free electron pairs of the surface oxygen atoms are those that provide specific adsorption sites for protons. However, we discovered that selective adsorption of protons takes place within the pores of the molecular sieving carbon electrode, even in the absence of surface oxides.

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