Developing Ion Electroadsorption Stereoselectivity, by Pore Size Adjustment with Chemical Vapor Deposition onto Active Carbon Fiber Electrodes. Case of Ca²⁺/Na⁺ Separation in Water Capacitive Desalination

Abstract

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.

Carbon Coated Alumina Nanofiber Membranes for Selective Ion Transport

“…In the range of applied voltages from −0.5 to +0.8 V, no waves corresponding to redox reactions was observed. This behavior is typical for carbon materials . Note that the preliminary blowing of inert gas through the electrolyte can extend the electrochemical stability range to −0.8 … +1.0 V due to removal of adsorbed oxygen.…”

Carbons and Electrolytes for Advanced Supercapacitors

“…Planar solvated ions, like NO 3 − , are adsorbed in slit‐shaped pores, while spherical solvated ions, such as Cl − , are excluded. A similar study was carried out with Na + , Ca 2+ , and Mg 2+ which showed that electrosorption of hydrated monovalent sodium ions in micropores is preferred while the hydrated bivalent cations are excluded …”

“…New aspects have to be considered when decreasing the pore size of the carbon materials to the range of the ion sizes present in the system. Using carbons with an average pore size approaching the size of hydrated ions, Aurbach et al demonstrated it was possible to selectively electrosorb ions based on size . They defined this as ion sieving.…”

Time-dependent ion selectivity in capacitive charging of porous electrodes

“…In commercial desalination technologies, such as reverse osmosis and multistage flash distillation, freshwater is produced at relatively high energy cost and often requires re-mineralization for human consumption. For brackish water or wastewater it can be advantageous to use a different type of technology, namely techniques where ions are removed from the feed water under the influence of electrical field effects, such as in electrodialysis [8,9], capacitive deionization [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28], membrane capacitive deionization [29][30][31][32][33][34], desalination using microchannels [35], batteries [36], microbial desalination cells [37] and wires [38]. Such techniques have the potential to be energy-efficient as they focus on the removal of the (often relatively few) ions in the water to obtain freshwater in this way.…”