*3.3. Compression Test and Microstructures of Hydrogels*

Figure 3a shows the stress-strain curves of these hydrogels under compression. The measured stresses at the 90% strain level were 52.3 ± 0.3, 86.1 ± 3.9, 108.4 ± 4.3, and 152.3 ± 5.1 kPa for TOCNF-CNT/PVA, TOCNF-CNT@PANI/PVA-1, TOCNF-CNT@PANI/PVA-3, and TOCNF-CNT@PANI/PVA-2, respectively. Thus, the stress of TOCNF-CNT@PANI/PVA-1 hydrogel with PANI at the 90% strain level was almost 1.6-fold than that of TOCNF-CNT/PVA. PANI nanoparticles combined with TOCNF-CNT nanofiber to form TOCNF-CNT@PANI composite fibers with a "core-shell" structure. The composite fibers based on good dispersibility and interfacial adhesion inside the hydrogel effectively transferred the load, thereby improving the mechanical strength of PVA hydrogel [39]. With the increase of PANI content, the stress of TOCNF-CNT@PANI/PVA increased first and then decreased. The stress of TOCNF-CNT@PANI/PVA-3 was 108.4 ± 4.3 kPa, which was lower than that of TOCNF-CNT@PANI/PVA-2 with 152.3 ± 5.1 kPa. This phenomenon could be attributed to the TOCNF-CNT biological template being insufficient to carry and disperse these excess PANI. Aggregated PANI prevented effective cross-linking between PVA and borax and disrupted the integrity of the network in the hydrogel. Under external force, the stress concentration caused by agglomeration would weaken the mechanical strength [40].

The TOCNF-CNT@PANI/PVA-2 possessed the highest mechanical strength in all the hydrogels. Its σ value (152.3±5.1 kPa) at ε= 90% and *E*<sup>e</sup> value (61.0±0.8 kPa) in the σ-ε curve were 2.9-fold and 4.2-fold more than those (σ = 52.3 ± 0.3 kPa, *E*<sup>e</sup> = 14.4 ± 0.3 kPa) of TOCNF-CNT/PVA hydrogel. The specific compressive stress (σs) value of TOCNF-CNT@PANI/PVA-2 was 128 kPa cm3 g<sup>−</sup>1, which was 2.8-fold larger than that of TOCNF-CNT/PVA with 45.9 kPa cm3 g<sup>−</sup>1. In Figure 3b, TOCNF-CNT@PANI/PVA-2 had the largest energy absorption (*E*a) value. In the *E*a*-*ε curves of hydrogels, the *E*<sup>a</sup> with ε = 90% was selected to compare the mechanical properties of hydrogels. In particular, the *E*<sup>a</sup> value of TOCNF-CNT@PANI/PVA-2 at <sup>ε</sup> <sup>=</sup> 90% was 3.2 <sup>±</sup> 0.5 kJ m<sup>−</sup>3, which was approximately 4 times larger than TOCNF-CNT/PVA with 0.8 <sup>±</sup> 0.4 kJ m<sup>−</sup>3. All the values of strength and physical properties are collected in Table 1.

**Figure 3.** (**a**) Stress-stain curves under compression; (**b**) energy absorption-strain curves of hydrogels; (**c**) SEM image of TOCNF-CNT@PANI/PVA-2 composite hydrogel; (**d**) idealized 3D cross-linking network of TOCNF-CNT@PANI/PVA-2 composite hydrogel.


**Table 1.** Physical-mechanical characteristics of various hydrogels.

Note: (1) σ means stress; (2) ε means strain; (3) ρ means density; (4) σ<sup>s</sup> means specific stress; (5) *E*<sup>a</sup> means energy absorption; (6) *E*<sup>e</sup> means compressive elastic modulus; (7) *Wc* means water content.

The improvement of mechanical properties was due to the effective enhancement of TOCNF-CNT@PANI composite fibers with a "core-shell" structure. In addition, the CNTs were entangled with each other to form a lot of contact junctions in the interstitial space between the PVA molecular chain. These contact junctions built the continuous conductive network in the hydrogel matrix. Figure 3c shows the microstructure of TOCNF-CNT@PANI/PVA-2 composite hydrogel, and Figure 3d presents the schematic diagram of the 3D network structure. The composite hydrogel possessed an interconnected porous structure, and each pore had a diameter of 200–500 nm. The wall of the pore was formed by a hydrogel matrix with a thickness of 10–30 nm. The entangled TOCNF-CNT@PANI composite fibers penetrated through the hole wall and built a hierarchical network. The specific framework could effectively promote the transport of electrons, improving the electrical conductivity of the hydrogel. Among them, TOCNFs were important to promote the formation of hierarchical microstructure, which profited from their excellent natural characteristics of hydrophilicity, high aspect-ratio, mechanical strength, and flexibility. TOCNFs combined with CNTs through hydrogen bonding and chain entanglement and served as nanocarriers to disperse CNTs in aqueous media [13]. The well-dispersed TOCNF-CNT nanohybrids were coated by PANI to form TOCNF-CNT@PANI composite fibers with "core-shell" structure. The composite fibers could further improve the mechanical strength and electrical properties of hydrogel [41]. The CNTs in composite fiber could effectively transfer the force from the PVA molecule chains. Moreover, an efficient and stable CNTs electric network

could improve the electrical conductivity of TOCNF-CNT@PANI/PVA composite hydrogel. The results showed that the hierarchical network microstructure inside the composite hydrogel increased the interface between the electrolyte and the electroactive material, demonstrating a broad application prospect in flexible electrodes.
