Pseudocapacitive deionization with nanolamellar metal carbides (MXene CDI)

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Capacitive deionization (CDI) is an emerging technology, promising high efficiency and low energy consumption for the removal of ions from aqueous media with low molar concentration (below 100 mM). By this virtue, CDI is particularly attractive for deionization of brackish water or industrial water streams. The process of CDI is similar to the operation of double-layer capacitors (supercapacitors): ions are electrosorbed onto the surface of highly porous electrodes and effectively removed from the feed water stream. Regeneration of the electrodes is then accomplished by discharging or voltage reversal. Until now, the majority of works in the field of CDI has been limited to the synthesis, analysis, and application of nanoporous carbon considering the high surface area and relatively high chemical stability. With the focus on carbon, most works have explored purely electrostatic deionization by ion immobilization via electrochemical double layers. This is why mostly carbon as a capacitive material has been applied for CDI so far.The proposed project will, for the first time, explore a purely pseudocapacitive material for capacitive deionization, namely MXene. Pseudocapacitance is distinguished by a capacitor-like electric response (linear charge-vs.-voltage), while charge storage is accomplished by a Faradaic process (ion intercalation). MXenes are a novel class of two-dimensional metal carbides, carbonitrides, and nitrides, first described in 2011. They are derived by chemical treatment of MAX phases, which are ternary metal carbides, carbonitrides, and nitrides - a large family of nanolamellar materials with over 60 members. Chemical etching by HF selectively removes atoms from the A-layer in MAX, leaving only M and X atoms left in the structure. Without the interlayer A-atoms, the three-dimensional MAX structure is transferred to two-dimensional nanolamellar MXene.The requested funding for one PhD student position will combine the two core topics of material science and aqueous electrochemical application. The PhD student will optimize the synthesis of V2C MXene from the MAX phase V2AlC and chemically modify its surface functionalities (oxidation / removal of surface functional groups etc.) by thermal methods. The impact of structure and surface chemistry on the electrochemical behavior of MXene (with / without modification) in aqueous media with NaCl as the salt will be explored. Besides CDI salt removal capacity and desalination rate, the longevity and structural changes during electrochemical cycling (including in situ methods) will be studied and a qualitative statement on the relations between MXene properties and CDI performance will be derived.