Nickel Hexacyanoferrate Electrodes for Continuous Cation Intercalation Desalination of Brackish Water – USA – Illinois
Using porous electrodes containing redox-active nickel hexacyanoferrate (NiHCF) nanoparticles, we construct and test a device for electrochemical water desalination in a two flow-channel device where the electrodes are in direct contact with an anion-exchange membrane. Upon reduction of NiHCF, cations intercalate into it and the water in its vicinity is desalinated; at the same time water in the opposing electrode becomes more saline upon oxidation of NiHCF in that electrode. In a cyclic process of charge and discharge, fresh water is continuously produced, alternating between the two channels in sync with the direction of applied current. Compared to capacitive deionization using porous carbon electrodes, a higher salt adsorption capacity per cycle is achieved, much lower cell voltages are needed, and the energy costs of desalination can be significantly reduced. Electrochemical water desalination with porous electrodes can make use of two fundamentally different mechanisms for salt storage. The first mechanism is capacitive electrosorption, where ions are held in electrical double layers (EDLs) formed in the micropores of porous electrodes comprised
of ideally polarizable material (e.g., carbon) . In the second mechanism, which has recently begun research exploration [2–6], intercalation electrodes are used where ions are stored within the sites of a solid-state host compound. The first mechanism, capacitive electrosorption, is used in Capacitive Deionization (CDI), a process in which ions are held near the carbon surface in the diffuse part of the EDL. CDI electrodes are made of carbon (carbon nanotubes, graphene, activated carbon powder, etc.) which can be processed into porous, ion- and electron-conducting, thin electrode films, suspensions, or fluidized beds . CDI based on capacitive EDL charging is a promising method, but to reach a certain salt adsorption capacity (SAC; a typical number being of the order of SAC=5-15 mg/g, referring to mass of NaCl removed, per total mass of carbon in a two-electrode cell, measured at a standard cell voltage of Vcell=1.2 V), the energy input is not insignificant [8–10], while the current efficiency (quantifying the fraction of current input that results in salt adsorption) of CDI cells can be well below unity, implying that in the charging process not only counterions adsorb but also coions desorb from the electrode . In CDI with membranes, or using improved chargingschemes, can be close to unity . Like capacitive carbons, intercalation host compounds (IHCs) can be incorporated into porous electrode films and can adsorb charge, but the ion storage mechanism of IHCs is fundamentally
different from EDL charging. In intercalation electrodes, ions are stored in the crystallographic sites of the IHC as a result of its redox activity. Water desalination using IHCs, which is currently much less developed and utilized than CDI, has the advantage that to reach a certain SAC a much lower voltage and energy is needed than using capacitive electrosorption, because the change in electrode potential with electrode charge can be much lower. Also, IHCs have the potential to selectively remove one ion (e.g., Na+) out of a multi-ion mixture with other ions of the same valence and charge.