(Invited) Separation and Hydrogenation of Ions By Micro-Scale Capacitive-Faradaic Adsorbing Fuel Cells

Abstract

Capacitive deionization processes are aimed at separation of ions from water and wastewater. Since late 1960-s, when the CDI concept was proposed, the technique was improved via incorporation of ion exchange membranes (MCDI), utilization of slurry electrodes (FCDI) and introduction of one or two battery electrode(s) into the process to enhance the salt adsorption capacity of the cell and to reduce the energy demand of the process. In all these processes the monopolar electrodes must be physically connected to the DC power supply via wires, and cathodes and anodes should be separated to prevent short-circuiting. These and other requirements of CDI processes result in serious drawbacks, limitations and sophisticated design of water treatment reactors.A novel CDI-based method for separation of ions from water and wastewater using micro-scale (10µm – 5 mm) fuel cells is proposed1. The fuel cells comprise activated carbon particles loaded with bi-functional catalyst for hydrogen oxidation and oxygen reduction reactions. Within the proposed process of ions removal, the particles act as the micro-scale capacitive-faradaic adsorbing fuel cells (MS-CF-AFC) which require oxygen and hydrogen gases for adsorption of ions during the water treatment step and desorption of ions in brine production.Within the O2-induced adsorption of anions, thereduction of oxygen on Pt (faradaic electrode of the MS-CF-AFC) 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 electrical double layer of carbon. The hydrogen oxidation that occurs during the regeneration of the anions-loaded MS-CF-AFCs results in accumulation of electrons in the capacitive electrode and in repulsion of perchlorate ions into the regenerant solution. This way the chemical energy of H2 and O2 is converted by the MS-CF-AFCs into the electrical energy which is utilized for separation of ions. For the separation of cations, the MS-CF-AFCs are powered by the H2 gas during the cationsadsorption step and by the O2 gas in brine production.The O2-induced adsorption of anions (and desorption of cations) results in pH increase in the treated water. Consequently, within the MS-CF-AFC treatment the separated anions are “exchanges” with OH- ions. The rate of the process is faster at lower pH values in the treated water. Higher anions adsorption capacity is achieved at higher O2 pressure. The H2-induced desorption of anions (and adsorption of cations) is accompanied by decrease in the pH. Thus, the cations removal process can be regarded as an exchange of separated cations with the H+ ions. The H2-induced step of the MS-CF-AFC processes is faster at higher pHs, and higher cations removal capacity is obtained at higher H2 pressure. To enhance the salt adsorption capacity of MS-CF-AFCs for anions and cations the activated carbon in the cells can be decorated with appropriate surface groups, e.g., carboxylic and amine groups (respectively).The process was proved for the removal of perchlorate, copper and nitrate ions using pure solutions of NaClO4, KNO3 and CuCl2 salts in deionized water and in groundwater, using batch and packed-bed reactors for two types of platinum loaded (5wt%) MS-CF-AFCs prepared from Lewatit AF5 microporous carbon and a powdered activated charcoal.The main disadvantage of the process described above is its inability to “deactivate” toxic NO3 - and ClO4 - ions as the process only concentrates the pollutant (similarly to the well-known ion-exchange). It was shown that Pt-Cu MS-CF-AFCs are capable to adsorb nitrate ions from water and to reduce them into the N2 gas during the H2-induced step of anions desorption from the MS-CF-AFCs.The proposed MS-CF-AFCs can be performed in batch, continuous stirred (CSTR), fixed-bed and other types of reactors normally applied in adsorptive water treatment processes.Potential advantages of the MS-CF-AFCs process are: (1) the process can be utilized for separation of all types of anions and cations (appropriate carbon modification might be required); (2) the process can be performed in any type of adsorption reactors; (3) desalinating hybrid capacitive-faradaic micro-scale fuel cells do not require any wiring as opposite to CDI, previously proposed desalinating fuel cells and battery electrode desalination processes; (4) the regeneration of the MS-CF-AFCs does not require any concentrated solutions of acids, bases or salts; and (5) the hydrogen required for the process can be produced on-site using hydrogen generators and air can be utilized as the oxygen source.[1] Gendel, Y., Amikam, G., Nativ, P., 2020. Separation of Anions and Cations from Water and Wastewater Using Micro-Scale Capacitive-Faradaic or Pseudocapacitive-Faradaic Adsorbing Fuel Cells. US Provisional Patent Application 62/983,689. Figure 1