## 3 Results and Discussion
### Electrochemical Characterization of the Packed Bed  
Chronoamperometry and cyclic voltammetry experiments were performed to study the electrochemical effects which occur inside the particulate fixed bed electrode, between the material surface and the mobile phase environment, when a potential is applied. Unlike conventional systems, which often operate in static conditions, the major goal here was to determine the operating quality by analyzing the response of the MWCNT packed bed directly in the flow cell. Accordingly, a constant potential was applied in the chronoamperometry measurements to first determine the potential-dependent current profile and its stabilization over time. The changes in the current profile  
Figure 2: Static binding capacities of (**a**) two carbon acids isomers at pH = 7 and (**b**) four amino acids at pI (neutral charged). The adsorbed amount is related to the dry mass of CNT-K. The experiment was performed at RT, 24 h, 500 rpm, with 2 or 3 technical replicates, triple determination at 216 or 280 nm. led to a greater understanding of the electrochemical double layer (EDL) rearrangement, and later, the aging effects of the MWCNT matrix, which occur due to structural changes of the matrix or of the surface groups' chemistry. As the through flow of the mobile phase is expected to play a role in the EDL, it was kept constant at 1 mL/min during all experiments. Regarding the chronoamperometry experiments (Figure 3), immediate changes in the current strength are recognizable when a constant potential is applied to the particulate electrode over a defined process time. Simultaneously, the enforced changes in the electrode surface charge provoke a rearrangement of the ions in solution. The time needed to load the capacitor and to reach a stable current corresponds to the EDL rearrangement rate, whereas the mean current in this region after the exponential rearrangement represents the resistance in a non-ideal capacitor setup. As the profiles of the electric current converge to a constant value and do not achieve zero, which would be typical for an ideal capacitor, capacitive and faradaic current must exist in the PCC system. By comparing the constitutions of several packed CNT-K columns chromographically with tracer experiments and electrochemically with chronoamperometry, we determined that the higher the current strength, the greater the CNT mass in the system. This correlates with the fact that more particle-particle contacts are available, as the MWCNT matrix is more homogeneous. The analysis of the response of various columns chronoamperometrically enables us not only to identify the constitution of the column, but also record the aging of the MWCNT matrix. This information on the life cycle of the particulate packed bed is directly connected to the quality of the particulate electrode and is extremely relevant for the consistency of results in a chromatographic, electrically driven process. Figure 3: Chronoamperometry experiments in DI-water (1 mL/min) to determine the constitution of the packed bed in a column A: (**a**) electrode response at the beginning, (**b**) electrode response after aging, (**c**) current/potential diagram showing mean current after electrochemical double layer (EDL) rearrangement. The chromatographic interpretation of changes in the column constitution can be found in Figure S9 (tracer experiment). The chronoamperograms in Figure 3 demonstrated that the influence of the low volume flow rate significantly lengthens the EDL rearrangement time in comparison with static experiments, where it only takes a few seconds [61]. Figure 3a presents a chronoamperogram of a packed bed shortly after the packing process. The EDL rearrangement time is approximately 60 s for +800 mV and 120 s for \(-\)800 mV, and the current profiles maintain high mean currents after the exponential current decrease/increase.