(83c) Thermodynamic Modeling of Electric Double Layer in Capacitive Deionization Cell Electrodes with Condensation Theory
AIChE Annual Meeting
2018
2018 AIChE Annual Meeting
Transport and Energy Processes
Fuel Cells, Electrolyzers, and Electrochemical Devices
Monday, October 29, 2018 - 8:40am to 9:00am
In this work, we propose the electric double layer consist of a Stern layer and a diffuse layer. The counterions in the Stern layer are considered tightly attached (or condensed) to the surface of the electrode, while the diffuse layer contains free counterions and small amount of coions which are in Donnan equilibrium with the bulk electrolyte solution. The scenario is analogous to that of aqueous polyelectrolytes, where counterion condensation happens when the charge density of the polyelectrolyte is higher than a critical point above which the counterions condense to the surface of the polyion. As a limiting law, Manningâs counterion condensation theory has been shown to describe the condensation phenomenon and calculate the activity coefficients of mobile ions in polyelectrolyte solutions. [3] Assuming the Stern (condensed) layer is a plate capacitor and equality of the chemical potentials of solutes in the bulk solution and in the diffuse layer, the amount of condensed counterions, Stern and Donnan potential drops, salt adsorption capacity and electrode surface charge density can be calculated with the condensation model. The nonideality of mobile ions can also be captured with Manningâs counterion condensation theory. Experimental measurements of CDI performance for aqueous NaCl and CaCl2 solutions of various concentrations under different cell voltages are conducted. The salt adsorption data can be well explained with our model.
Reference
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- Biesheuvel, P. M., H. V. M. Hamelers, and M. E. Suss. "Theory of water desalination by porous electrodes with immobile chemical charge."Colloids and Interface Science Communications 9 (2015): 1-5.
- Manning, Gerald S. "Limiting laws and counterion condensation in polyelectrolyte solutions I. Colligative properties."The journal of chemical Physics 3 (1969): 924-933.