## 3 Results and Discussion
### Sensitivity of the Electrochemical Process to the Environment  
Along with chronoamperometry, in which the effect of the ion concentration on the current strength is determined, cyclic voltammetry (CV) is used to enhance the understanding of the electrochemical effects occurring in the particle based, through-flow electrode (Figure 5). To get more information about the potential-dependent adsorption of ions from the mobile phase, pH and conductivity are measured simultaneously to obtain the current values. The typical CV for a CNT-K column is presented in Figure 5a using DI-water as the mobile phase (1 mV/s). A reproducible current profile exists when a positive potential is applied, which represents reversible surface modification that includes the rearrangement of the EDL and the surface reactions. If negative potential gradients are applied, changes in the current profile are recognizable and an exponential current increase is detected. These changes are particularly pronounced between \(-500\) and \(-800\) mV and become less significant with each cycle, because the initial surface properties change more markedly at the beginning due to compensation from redox reactions, which take place on the electrode surface. This type of particle aging, discussed in literature [53; 65], is consistent with our previous results (see Sections 3.3 and 3.5) and helps to understand and confirm them. Regarding the shape of the CV, the flat and almost  
Figure 4: Current/potential diagram to visualize the mean current after EDL rearrangement in a system with DI-water or 10 mM phosphate buffer as mobile phase (pH = 7.2, 1 mL/min). The MWCNT matrix was analyzed at an earlier stage (packed bed at initial condition) (a,c) and after long usage (after aging) (b,d). The experiments were performed with column B. The related chronoamperometry measurements and tracer experiments are illustrated in Figures S11 and S12. Information about the electric charge is given in Figures S13 and S14. rectangular profile corresponds to a capacitor under flow [51; 53; 61; 65; 66; 67] with current strength up to 200 \(\mu\)A [49; 51; 66]. Alongside the fluid dynamical effects, leakage current can shift the CV shape [68]. The CV in Figure 5b was run in another column with 0.1 mV/s to guarantee a complete picture of the voltage gradient influence on the current profile and pH value regarding a DI-water and buffer system. In the buffer system, the electrical current is higher and the CV shape is more pronounced, and the cathodic current shape is quite similar to anodic, suggesting a capacitor behavior and a reversible system [51]. The aging of the column due to redox reactions at the negative potential region is negligible in comparison to the CV with DI-water. It is very interesting to follow pH and conductivity changes in the flow cell following potential application and to compare both systems, water and buffer. These effects increase understanding of what happens on the electrochemical double layer. The effect of the CV experiment in Figure 5b (increase from 0 mV to +800 mV, followed by a decrease to -800 mV and back) on the pH-value and the conductivity of the mobile phase are presented in Figure 5c,d. Even if the electrical current of DI water system is lower under applied potential due to the smaller number of ions in comparison to the buffer system, the pH value oscillates substantially upon applying a potential gradient. If the potential changes, the ion concentration in the bulk is disturbed, immediately influencing the pH value due to the adsorption or desorption of oxonium and hydroxyl ions inversely to the applied potential value (pH decrease at positive potential and pH increase at negative potential). At the same time, the low amount of salt ions in DI-water is responsible for the change in the mobile phase conductivity. Figure 5: Cyclic voltammogram to compare two columns and two different mobile phases (1 mL/min).