Evaluating surface charge density enhancements at different pH values in capacitive deionization electrodes

Tahel Malka, Faculty Of Mechanical Engineering, Technion - Israel Institute of Technology, Haifa, Israel

Capacitive deionization (CDI) is a method to desalinate water based on porous, high surface area, charged microporous carbon electrodes. By electrically charging these electrodes, they electrosorb ions dissolved in feedwater, storing ions in the electric double layers within the

micropores. The accumulation of the salt in term of charges is similar to the principles of supercapacitors, and thus CDI cells store energy like storage devices.

Improving the CDI performance can be done by chemical treatments (oxidation, amination, etc.), to increase the concentration of the electrode's charged surface groups, which allows for a higher salt adsorption than in untreated electrodes. It is known that in a CDI cycles the pH value inside the electrodes is dynamic, and affects the surface charge density. Evaluating the changes in the surface charge density of the electrodes along a wide range of pH values can be done by using chemical titration.

By measuring the different titrant volumes required to reach the same pH values between an electrode titration and a blank titration, we can calculate the micropore’s surface charge density in units of moles per micropore volume. Using this data, the specific electrode charge for CDI electrodes is determined for various pH values in the system and for different electrodes types: oxidized, aminated, or pristine. We also apply this data towards modeling selectivity features in a CDI system, and evaluate qualitatively the efficacy of electrode treatment processes.


Evaluating surface charge density enhancements at different pH values in capacitive deionization electrodes Tahel Malka, Faculty Of Mechanical Engineering, Technion - Israel Institute of Technology, Haifa, Israel Capacitive deionization (CDI) is a method to desalinate water based on porous, high surface area, charged microporous carbon electrodes. By electrically charging these electrodes, they electrosorb ions dissolved in feedwater, storing ions in the electric double layers within the micropores. The accumulation of the salt in term of charges is similar to the principles of supercapacitors, and thus CDI cells store energy like storage devices. Improving the CDI performance can be done by chemical treatments (oxidation, amination, etc.), to increase the concentration of the electrode's charged surface groups, which allows for a higher salt adsorption than in untreated electrodes. It is known that in a CDI cycles the pH value inside the electrodes is dynamic, and affects the surface charge density. Evaluating the changes in the surface charge density of the electrodes along a wide range of pH values can be done by using chemical titration. By measuring the different titrant volumes required to reach the same pH values between an electrode titration and a blank titration, we can calculate the micropore’s surface charge density in units of moles per micropore volume. Using this data, the specific electrode charge for CDI electrodes is determined for various pH values in the system and for different electrodes types: oxidized, aminated, or pristine. We also apply this data towards modeling selectivity features in a CDI system, and evaluate qualitatively the efficacy of electrode treatment processes.

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