*3.1. Synthesis Process and Mechanism of TOCNF-CNT@PANI*/*PVA Composite Hydrogels*

The preparation process of hierarchical 3D network TOCNF-CNT@PANI/PVA composite hydrogel is illustrated in Figure 1. Firstly, TOCNF suspensions were prepared through a TEMPO oxidation treatment in an aqueous system following an ultrasonication treatment. TEMPO oxidizes the hydroxymethyl group at the glycose C6 position in the cellulose chain to a more active carboxyl group [11]. Secondly, the homogeneous TOCNF-CNT nanohybrid dispersions were obtained by mixing CNT powders with TOCNF dispersions through ultrasonication. Not only the fluctuation of counter ions on the surface of TOCNF fibers induced the dipoles of the carbon lattice in CNTs, but also the carboxyl groups of TOCNFs produced electrostatic repulsion, which ensured the stabilization of CNTs in water [10]. Thirdly, TOCNF-CNT with excellent dispersibility and high specific surface area were used as nanocarrier of PANI. TOCNF-CNT@PANI nanohybrids were synthesized by in-situ chemical polymerization with APS as oxidant and TOCNF-CNT as a biological template in acid medium. PANI formed a wholly uniform coating layer around TOCNF-CNT nanohybrid bundles (Figure 1d), which would bring enough pseudo-capacitance [12]. Finally, PVA and borax were introduced into the TOCNF-CNT@PANI nanohybrid dispersions to achieve TOCNF-CNT@PANI/PVA composite hydrogel through cross-linking reaction. Borax would decompose into B(OH)4 <sup>−</sup> ions in water, and B(OH)4 − ions would create a reversible connection between the CNF-CNT@PANI composite fibers and the PVA molecular chains, forming a dynamic 3D network structure in the hydrogel [6]. The hierarchical 3D network inside hydrogel is illustrated in Figure 1e; the CNTs provided the fast electron transport path, and the nano-coating layer of PANI-ensured electrons could only pass through a very short distance to the CNT networks with high conductivity, which improved the electrochemistry of TOCNF-CNT@PANI nanohybrid. In addition, the borate ions could combine with the adjacent hydroxyl groups to form dynamic cross-linking between PVA chains and TOCNF-CNT@PANI nanohybrids. The dynamic PVA-borate cross-linking network provided hydrogel with the moldable and self-healing performance. The TOCNF-CNT networks provided an additional platform to improve strength, toughness, and conductivity. The chain entanglement and hydrogen bonding between TOCNF-CNT@PANI nanohybrids and PVA formed a hierarchical 3D network in TOCNF-CNT@PANI/PVA hydrogels.

**Figure 1.** Schematic of the fabrication process of TOCNF-CNT@PANI/PVA composite hydrogel. (**a**) TOCNFs aqueous dispersion, (**b**) CNTs aqueous dispersion, (**c**) TOCNF-CNT nanohybrids aqueous dispersion, (**d**) TOCNF-CNT@PANI nanohybrids aqueous dispersion, (**e**) TOCNF-CNT@PANI/PVA composite hydrogel.
