*3.7. Electrochemical Properties of Composite Hydrogels*

In order to evaluate the effect of the incorporated PANI on the electrochemical behavior of the composite hydrogel electrode, a CV test was performed, as shown in Figure 7a. In the CV test, the potential range was <sup>−</sup>0.2 to 0.8 V at a scan rate of 40 mV s<sup>−</sup>1, in 6 M KOH electrolyte with platinum sheet counter electrode and Hg/HgO reference electrode. In Figure 7a, the CV curve of TOCNF-CNT/PVA exhibited regular rectangular and symmetric shapes, which reflected the typical characteristics of the electric double layer charge (EDLC) storage. Moreover, the CV curves of composite hydrogel containing PANI possessed a larger current density and different shape. The increase in current density indicated greater capacitance, which was due to the pseudo-capacitance effect of PANI. The deformation of the CV curve was attributed to the diffusion and migration of limited ions in the polymer block and the ohmic resistance due to the thick polymer layer [47]. However, the voltammograms of PANI-based hydrogel possessed clear faradaic oxidation and reduction peaks. Three pairs characteristic peaks arose at 0, 0.4, and 0.6 V; the peaks arose at 0 and 0.6 V were related to the redox behavior of PANI through the leucoemeraldine and pernigraniline states; the peaks at 0.4 V were assigned to the electron transition from the protonation/deprotonation of PANI [14,19]. Among these voltammograms, TOCNF-CNT@PANI/PVA-2 possessed the largest loop area, corresponding to the highest specific capacitance. Furthermore, the G-CD behaviors of these composite hydrogel electrodes were measured at 0.4 A g−<sup>1</sup> current density from <sup>−</sup>0.2 to 0.8 V with 6 M KOH electrolyte (Figure 7b). The G-CD curves of the TOCNF-CNT/PVA hydrogel-based electrode exhibited a symmetrical triangle, indicating that it was an electric double-layer capacitor with reversible capacitance characteristics. For all the samples, the G-CD profiles were nearly triangular, demonstrating their excellent capacitive performances. Based on Equation (3), the *C*s was calculated from the G-CD curves data. The *C*s values of TOCNF-CNT/PVA, TOCNF-CNT@PANI/PVA-1, TOCNF-CNT@PANI/PVA-2, and TOCNF-CNT@PANI/PVA-3 were 84.9, 127.3, 226.8, and 184.4 F g−<sup>1</sup> at 0.4 A g−<sup>1</sup> current density, respectively. It was observed that composite hydrogel containing PANI possessed a higher specific capacitance than TOCNF-CNT/PVA hydrogel. The 3D network structure of TOCNF-CNT could load PANI and enable greater contact with electrolytes, thereby forming more active sites inside the hydrogel. Moreover, when CNTs were used as the filler for PANI to build a "core-shell" structure composite, the porous structure could further improve the capacitance performance. The high specific capacitance originated from two different charge storage methods: (1) the EDLC storage in CNTs nano-core and (2) the oxidation and reduction chemistry (pseudo-capacitance) of the PANI nano-shell [48].

The specific capacitances of composite hydrogels remained approximately 80%. For all samples, the relationships between the specific capacitance and the current density are shown in Figure 7c. At the same current density, these composite hydrogels containing PANI possessed higher specific capacitances than TOCNF-CNT/PVA hydrogels, indicating that PANI significantly increased the specific capacitance of the composite hydrogel. The sp2-hybridized carbon atoms of CNTs formed π-π stacking interactions with the quinoid ring of the PANI without destroying the graphitized plane of CNTs [49,50]. Among these hydrogels, the TOCNF-CNT@PANI/PVA-2 possessed the largest specific capacitance. It could be attributed to the appropriate ratio of PANI to CNTs, which allowed the PANI to better combine with the CNT networks. The developed pore structure and large specific surface area were beneficial to the charge accumulation and enhanced the specific capacitance.

Nyquist plots from EIS of the composite hydrogel electrodes are shown in Figure 7d. The Nyquist plots of hydrogel electrodes showed a typical semicircle in the high-frequency region. The intercept of semicircle represented the equivalent series resistance (ESR), and the diameter of semicircle represented the charge-transfer resistance (*R*ct) of the interface. Correspondingly, the diameter of a semicircle of TOCNF-CNT@PANI/PVA-2 hydrogel was the smallest in all samples, indicating that TOCNF-CNT@PANI/PVA-2 hydrogel possessed the lowest resistance. It was because the cross-linked 3D network structure in composite hydrogels provided an ideal charge-transfer path. Nyquist plots showed a straight line at the low-frequency region, and nearly vertical shape reflected the ideal capacitance characteristics [51,52].

**Figure 7.** (**a**) CV (cyclic voltammetry) curves at 40 mV s−<sup>1</sup> scan rate; (**b**) G-CD (galvanostatic charge-discharge) curves at 0.4 A g−<sup>1</sup> current density; (**c**) specific capacitance of the hydrogel electrodes at different current densities; (**d**) Nyquist plot of hydrogel electrode and enlarged illustration of high-frequency region.
