Startup Mattershift says it has achieved a breakthrough in making carbon nanotube (CNT) membranes at large scale. The startup is developing the technology’s ability to combine and separate individual molecules to make gasoline, diesel, and jet fuel using CO2 removed from the air. In an open-access paper in Science Advances, researchers from Mattershift and colleagues in the labs of Dr. Benny Freeman at The University of Texas at Austin and Dr. Jeffrey McCutcheon at the University of Connecticut confirmed that Mattershift’s large-scale CNT membranes match the characteristics and performance of small prototype CNT membranes previously reported on in the scientific literature.
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.
In Capacitive Deionization (CDI), charged ionic species are removed from aqueous solutions. The ions are adsorbed onto the surface of a pair of electrically charged electrodes, usually composed of highly porous carbon materials, upon applying an electrical voltage difference. Upon charging the electrodes with a voltage difference of typically 1-1.4 V, the salt ions present in the feed migrate to the electrode of opposite charge, cations to the cathode and anions to the anode, and form electrical double layers (EDLs) along the pore surface. Thus, the water flowing through the CDI cell is partially desalinated. In a discharging step, where either the applied voltage is shorted or the polarity reversed, the salt ions are released in a brine stream. The system architecture can be in flow-by or flow-through mode with the feed either streaming past the electrodes in parallel direction or streaming vertically through the electrode. New developments propose floating electrodes suspended in the aqueous solution that enable continuous operation. Various porous carbon materials have been suggested as electrode material, such as carbon aerogels, activated carbon and carbon nanotubes. An important factor is the sorption performance. The technology has been previously applied to brackish and seawater desalination, wastewater remediation and water softening, but has proven to be highly effective for solutions with low molar concentration such as brackish water. CDI does not require high pressure or temperature, as in membrane or thermal desalination, making the technology more energy efficient in comparison. It also has a higher accuracy in removing only particular salts and ions that enables the recovery of valuable compounds such as lithium among others. However, problems may arise in the regeneration phase as during reversed-polarity, repelled ions might be attracted to the oppositely charged electrode and by electrical shorting the only driving force is diffusion, which is slow and inefficient. The special applicability to brackish water offers a great potential for development, as the demand for desalination of brackish water is increasing due to salt intrusions into the groundwater in many regions worldwide. Nonetheless, many basic settings have not been uniquely defined until now and have to be further optimized.
the researchers developed a self-heating carbon nanotube-based membrane that only heats the brine at the membrane surface. The new system reduced the heat needed in the process and increased the yield of recovered water to close to 100 percent.
In addition to the significantly improved desalination performance, the team also investigated how the application of alternating currents to the membrane heating element could prevent degradation of the carbon nanotubes in the saline environment. Specifically, a threshold frequency was identified where electrochemical oxidation of the nanotubes was prevented, allowing the nanotube films to be operated for significant lengths of time with no reduction in performance. The insights provided by this work will allow carbon nanotube-based heating elements to be used in other applications where electrochemical stability of the nanotubes is a concern.