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applied sciences Article A Carbon Nanotube Packed Bed Electrode for Small Molecule Electrosorption: An Electrochemical and Chromatographic Approach for Process Description Tatjana Trunzer *, Timothy Stummvoll , Melanie Porzenheim, Paula Fraga-García and Sonja Berensmeier * Bioseparation Engineering Group, Department of Mechanical Engineering, Technical University of Munich, 85748 Garching, Germany; timo.stummvoll@tum.de (T.S.); melanie.porzenheim@tum.de (M.P.); p.fraga@tum.de (P.F.-G.) * Correspondence: t.trunzer@tum.de (T.T.); s.berensmeier@tum.de (S.B.); Tel.: +49-89-289-15750 (S.B.) Received: 9 January 2020; Accepted: 4 February 2020; Published: 7 February 2020 Featured Application: The potential-controlled sorption of charged molecules using multi-walled carbon nanotubes as electrodes is an innovative step to economizing chromatographic processes. However, any process design requires understanding the basic operators, such as material properties and effects triggered by a potential at the solid-liquid interface, to define the framework for future applications. Abstract: Triggering the interaction of nanomaterials with molecules by means of electrical potentials in aqueous media remains challenging, especially if 3D through-flow systems are used as electrodes, as in potential-controlled liquid chromatography (PCC). In this paper, multi-walled carbon nanotubes (MWCNTs) function as a particulate packed bed electrode in order to study the system’s response to various applied potentials and electrolyte compositions. The process principle was analyzed using chronoamperometry and cyclic voltammetry. Applying an electrical potential to the hydrophilic MWCNTs induces the presence of both capacitive and faradaic currents. This leads, over time, to a degradation of the electrode due to structural changes of the MWCNT matrix and an increase in redox reactions on the surface. The role of the electrochemical double layer (EDL) is highlighted as a main player in the process, directly influencing the adsorption capability of the electrode. The EDL rearrangement time and coverage radius depend on the composition of the mobile phase and on the potential applied. The capacity of the electrode for the target (maleic acid) increases at high positive potentials (+800 mV vs. Ag/AgCl), while the presence of electrolytes leads to a capacity decrease. Our research enhances the understanding of capacitive through-flow cells. Keywords: multi-walled carbon nanotubes; aqueous system; chronoamperometry; cyclic voltammetry; electrochemical double layer; potential-controlled chromatography 1. Introduction Chromatography, and ion exchange chromatography in particular, is a widely used separation and purification technique at the biotechnological industrial scale. However, the process consumes a large amount of salt for elution and requires specifically functionalized resins. An alternative which circumvents these disadvantages is potential-controlled chromatography (PCC) or electrochemically modulated liquid chromatography (EMLC), both of which have been known to scientists for several decades [1–8]. Here, the surface charge of a conductive resin is modified by an applied electrical potential. Consequently, a selective interaction between charged or polarized molecules and the Appl. Sci. 2020, 10, 1133; doi:10.3390/app10031133 www.mdpi.com/journal/applsciPDF Image | Carbon Nanotube Packed Bed Electrode
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