## 3 Results and Discussion
### Electrochemical Characterization of the Packed Bed
The response of one package looks different after repeated use and several processing cycles: aging leads to a longer EDL rearrangement time and a reduction of the mean current (Figure 3b). This change is interpreted as a degradation of the MWCNT matrix, which is also detectable with tracer experiments (for the chromatographic characterization see Figure S9). A possible reason for the modification of the MWCNT matrix is the mutual repulsion of the tubes due to charging the surface through the repeated application of an electrical potential [62]. This aging effect reduces the number of particle-particle contact points and changes the fluid pathway through the column. In addition to structural changes, the increase in the column resistance might be due to irreversible redox reactions on the surface [63], as confirmed by XPS results (see Section 3.3). By plotting the mean current after EDL rearrangement over the applied potential, the relationship between current/potential and packed bed constitution is represented (see Figure 3c), showing higher mean current for freshly packed, more homogeneous MWCNT electrodes. In addition to the information on packing structure and aging effects, the slightly hyperbolic trend of the current, determined at higher potentials (positive and negative), indicates the presence of faradaic current, which points to redox reactions. This phenomenon appears at the outer measurement limit, between \(-\)500 and \(-\)800 mV, at which CNT material can oxidize. The linear regime, visible at lower applied potentials, represents dominating physicosptive effects and capacitive current. The x-axis intersection is around +100 mV and corresponds to the point zero current of the total working electrode. The further the applied potential is from this point, the higher the net current is and the more likely it is that faradaic current occurs. The electrical charge of the particle electrode is potential-dependent and increases with higher applied potential. Thus, more than \(-\)500 mC can be obtained at the beginning of the column life (if \(-\)800 mV are applied), while the electric charge decreases with column aging (Figure S10). Another characteristic factor calculated from the mean current of the chronoamperometry measurements is the specific resistance of the packed bed (see Equation S2), which is approximately \(1.44\times 10^{6}\)\(\Omega\) mm\({}^{2}\)/m in the best case at \(\pm\)800 mV (lowest resistance) (Figure 3a). This high resistance corresponds more to the resistance of tap water than to the expected MWCNT resistance (electrical conductivity, 1300 mS/cm [54] or even higher [64]). The high CNT-K column resistance is not surprising, as 93% of the MWCNT matrix contains DI-water, which has a poor conductivity (\(<\)0.02 mS/cm). Getting more than 0.5 g of particles into the system to increase the particle-particle contact points and to reduce the amount of water and the packed bed resistance is still a challenge because the CNT-K matrix swells in water and becomes tightly packed with high backpressure, thereby reducing the homogeneity of the packed bed and the flow profile quality.