## 3 Results
### Monolith Structure and Conductivity  
For the further characterization of CNT monoliths, PVA was applied as a structural reinforcement as previously described by [PERSON] et al. [17]. Especially for planar monoliths, utilized for electrosorption and described in Section 3.4, we found that small concentrations of PVA further increased the homogeneity and stability. In the following, we characterize monoliths comprised of untreated and oxidized CNTs to determine their suitability as electrodes in an electrosorption process. Our synthesized cylindrical monoliths were free-standing and characterized by a remarkably high structural stability. For example, three monoliths with a density of approximately 0.3 g cm\({}^{-3}\) supported a counterweight of 3000 g (see Figure S5), which roughly translates to over 5000 times their weight. Moreover, the tensile strength for planar monoliths was determined to 4.28 \(\pm\) 0.71 MPa, further emphasizing the high structural stability of our CNT monoliths and their suitability as technical electrodes. The surface area of the untreated nanotubes was determined to 181 \(\pm\) 2 m\({}^{2}\) g\({}^{-1}\). In this context, [PERSON] et al. reported a predominance of structurally closed tube ends for the used CNTs, possibly due to the synthesis process [31; 57]. Given that, the oxidation of the CNTs did not influence the tube structure as a specific surface area (SSA) of 175 \(\pm\) 2 m\({}^{2}\) g\({}^{-1}\) was measured for the oxidized sample (see Figure 6). Remarkably, the SSA could be preserved during monolith synthesis with untreated and oxidized nanotubes, as the SSA decreased only by 2%, respectively. This effect can be attributed to our additive-free gelation approach and the scarce utilization of PVA as a binder. Within the development of our synthesis protocol, we changed the incubation conditions of the wet gel in 1 wt-% PVA from several days as described by [PERSON] et al. [17] to a quick dispersion and centrifugation step with subsequent discard of the supernatant. Thus, we expect less PVA accumulation on the surface of the nanotubes, and therefore preservation of the CNTs' electrical and structural properties. Indeed, we were able to synthesize CNT monoliths with a reduced SSA loss compared to similar synthesis approaches [11; 24]. For instance, [PERSON] et al. [11] reported an SSA loss of 26% upon monolith synthesis. Hence, with our synthesis protocol, a vast number of possible binding sites for the later application as an electrode in an electrosorption process should be provided, which will be examined and discussed in Section 3.4. Additionally, we observed the preservation of the CNTs' exciting properties through conductivity measurements illustrated in Figure 6b. The electrical conductivity was found to be strongly dependent on the pretreatment of the nanotubes. For untreated monoliths, a conductivity of 74 \(\pm\) 9 S m\({}^{-1}\) was measured in a dried state. Utilizing monoliths comprised of oxidized nanotubes, the conductivity is almost doubled to 141 \(\pm\) 15 S m\({}^{-1}\). This indicates an improved particle network formation during the monolith synthesis of the mildly  
Figure 6: Specific surface area (**a**) and electrical conductivity (**b**) for untreated and oxidized monoliths. oxidized nanotubes. After incubation in DI-water, both materials experience a considerable reduction of conductivity. Here, DI-water is stored in the porous structure of the monoliths and inhibits the current flow. Monoliths made of untreated CNTs display a loss of conductivity of 51% to 36 \(\pm\) 9 S m\({}^{-1}\), while the oxidized monoliths possess a marginally higher conductivity of 42 \(\pm\) 13 S m\({}^{-1}\). Nonetheless, a high conductivity can be observed for the synthesized monoliths, in a dry and wet state, which is essential for the later electrosorption in aqueous systems. Compared to [PERSON] et al. [11] and [PERSON] et al. [17], on whom our synthesis protocol builds, we were able to increase the conductivity of MWCNT monoliths by at least one order of magnitude.