## 3 Results and Discussion
### Electrochemical Characterization
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **X\({}_{\text{ef}}\) (M)** & **t\({}_{\text{Cl-}}\)** & **D\({}_{\text{Na+}}\)/D\({}_{\text{Cl-}}\)** & **PS\({}_{\text{Cl-}}\) (\%)** \\ \hline NPAS-Sf & +0.0122 & 0.742 & 0.348 & 33.0 \\ NPAS-Ph & +0.0013 & 0.627 & 0.595 & 3.1 \\ NPAS-And & +0.0015 & 0.652 & 0.534 & 9.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Effective fixed charge (X\({}_{\text{ef}}\)), counter-ion transport number (t\({}_{\text{Cl-}}\)), ionic diffusion coefficient ratio (D-/D\({}_{+}\)), and anionic permselectivity (PSCL-) for the studied samples. Figure 5: (**a**) Variation of \(\Delta\Phi\) values with concentration ratios for NPAS-Sf (\(\bullet\)), NPAS-Ph (\(\blacktriangledown\)), and NPAS-And (\(\circ\)). Dashed line: NaCl solution diffusion potentials; solid line: ideal anion-exchanger (t\({}_{-}\) = 1 and t\({}_{+}\) = 0). Scheme of ions distribution in charged nanoporous structures as a function of (**b**) charge and porosity for samples with similar pore size; (**c**) pore size and porosity for samples with similar surface charge. lower frictional effect through the asymmetric NPAS-And sample, probably associated to its partial opener structure. The influence of pore radii and porosity on interfacial effects and diffusive transport have already been analyzed in a previous paper [47], and it is schematically shown in the Supplementary Information in Figure S2b. The partial exclusion of co-ions from nanopores due to electrical interactions causes an increase of solution concentration at the solid-liquid interfaces (concentration polarization), which can be minimized by solutions stirring; consequently, a comparison of \(\Delta\Phi\) values obtained for the same external solution concentrations under stirring and no-stirring conditions might give information on interfacial effects. Figure 6 shows a comparison of \(\Delta\Phi\) values determined for the NPASs under both experimental conditions, where slight differences depending on sample structure can be observed. Reductions of around 17% and 10% in \(\Delta\Phi\) values at a high NaCl solution concentration (c\({}_{v}>0.03\) M) were obtained for NPAS-Sf and NPAS-And samples, respectively, as a result of concentration-polarization, but no differences seemed to exist at the solution/NPAS-Ph interfaces due to its larger pore size. Electrochemical impedance spectroscopy (EIS) is an alternating current (a.c.) technique commonly used for electrical characterization of homogeneous and heterogeneous systems, which allows for the estimation of sample/solution electrical resistance and capacitance by using equivalent circuit models as well as interfacial effects [42; 43]. Figure 7 shows the impedance plots, Nyquist plot (\(-\)Zimg vs. Zreal, Figure 7a), and Bode plots (Zreal vs. frequency, Figure 7b, and \(-\)Zimg vs. frequency, Figure 7c), where  
Figure 6: Effect of solution stirring on \(\Delta\Phi\) values. (**a**) NPAS-Sf: (\(\bullet\)) solutions stirred, (\(\circ\)) solutions not stirred; (**b**) NPAS-Ph: (\(\blacksquare\)) solutions stirred, (\(\square\)) solutions not stirred; (**c**) NPAS-And: (\(\blacklozenge\)) solutions stirred, (\(\circ\)) solutions not stirred. The inset shows possible concentration–polarization effect. slight differences for the bulk contribution of the system sample/0.002 M NaCl solution can be observed; however, more significant differences existed in the case of the NPAS-Sf/solution sample when the interface contribution (f < 4 kHz) was considered, this fact was attributed to the higher solution concentration-polarization presented by this sample, in agreement with concentration potential results previously discussed. For comparison reasons, in Figure 7b,c, values obtained for the NaCl solution alone (without any sample in the test-cell) are also indicated.