Ph.D Thesis


Ph.D StudentAmikam Gidon
SubjectCapacitive-Faradaic Fuel Cells for Ions
Separation
DepartmentDepartment of Civil and Environmental Engineering
Supervisor ASSOCIATE PROF. Youri Gendel
Full Thesis textFull thesis text - English Version


Abstract

A novel method for separation of ions from water and wastewater was proposed in this study. The technique utilizes micro-scale porous conducting particles (e.g., activated carbon) loaded with bifunctional catalyst capable of hydrogen oxidation and oxygen reduction reactions. Within the proposed process of ions removal, the particles act as micro-scale adsorbing capacitive-Faradaic fuel cells (CFFCs) which require air and hydrogen gas for the adsorption of ions during the water treatment step and desorption of ions in the brine production.

The CFFC research program comprised three sections: (i) proof of concept for the separation of ions from water and wastewater using micro-scale CFFCs; (ii) CFFCs for ions separation: macro-scale configurations and polarization mechanism; (iii) separation and hydrogenation of nitrate and perchlorate ions by microscale CFFCs loaded with bimetallic catalyst.

In the first section the process was proved for the removal of perchlorate and copper ions from deionized water (and groundwater for ClO4-) using two types of CFFCs prepared from Lewatit AF5 carbon, and a powdered activated charcoal, loaded with platinum. During adsorption of ClO4- ions, the reduction of atmospheric oxygen on Pt (Faradaic electrode of the CFFCs) results in depletion of electrons from the carbonaceous part (i.e., the capacitive electrode) of the micro-scale fuel cell, which leads to adsorption of anions in the electric double layer. The hydrogen oxidation reaction that occurs during the regeneration of the ClO4- - loaded CFFCs results in accumulation of electrons in the capacitive electrode and in repulsion of perchlorate ions into the regenerant solution. The principle of the novel technique was also proved for separation of Cu2 cations using micro-scale CFFCs made of Lewatit AF5 loaded with Pt catalyst.

In the second section the polarization mechanism of capacitive-Faradaic fuel cells is reported. The mixed potentials of Faradaic (Pt and Ti/Pt-IrO2) and capacitive (activated carbon powder and fleece) electrodes were measured in aerated and hydrogenated NaCl solutions at varied pHs. Next, two configurations of divided and undivided macro-scale CFFCs were investigated: (i) system with two fixed electrodes made of Ti/Pt-IrO2 and activated carbon fleeces, and (ii) CFFC with one fixed Ti/Pt-IrO2 and one flowing carbon electrodes. Adsorption-desorption of pH and Cl-, SO42- anions agreed to the proposed capacitive-Faradaic mechanism, but the behavior of NH4 and Na cations contradicts it. This phenomenon is attributed to Faradaic processes and competitive adsorption-desorption of H and OH- ions that require further detailed investigations.

In the third section, the utilization of the CFFCs for separation and hydrogenation of nitrate and perchlorate ions is reported. The process utilizes micro-scale CFFC made of activated carbon particles loaded with a Pt-Cu or Pt-Re catalyst capable of oxygen reduction, hydrogen oxidation, and nitrate or perchlorate hydrogenation reactions, respectively. Oxygen reduction on Pt results in adsorption of anions from the treated water to the activated carbon. To repel anions into the regenerant solution the dissolved H2 is oxidized on Pt electrode of the CFFC. Simultaneously the nitrate or perchlorate ions are reduced by the H2 on the Pt-Cu or Pt-Re (respectively) catalyst of the CFFC.