**1. Introduction**

Nowadays, with the rapid development of personal wearable electronic devices, miniaturized energy storage devices with mechanical flexibility and even self-healing functions have attracted more and more attention; among them, the supercapacitor is a research hotspot [1,2]. The supercapacitor is a promising new energy storage device, which combines the advantages of high power from double-electric layer capacitors and high energy from batteries [3]. However, traditional supercapacitors are susceptible to various unavoidable mechanical deformations and cannot meet the needs of flexible wearable electronic devices [4]. Therefore, flexibility and self-healing characteristics are requirements for wearable energy storage devices. In order to integrate flexibility, self-healability, and electrochemical performance into a supercapacitor device, searching suitable electrode and electrolyte material has become a key to the manufacture of intelligent devices [5].

Hydrogels are considered as ideal candidates because of their flexible three-dimensional (3D) networks, high deformability, and hydrophilic property [6]. Firstly, hydrogel polymer network with high-water content dissolves ions and provides high ion conductivity, which can be compared with the ionic conductivity of a liquid while maintaining the solid shape and size to avoid liquid leakage during various mechanical deformation. It is an ideal choice for flexible supercapacitor electrolyte and separator materials [7]. Secondly, electrically conductive hydrogel (ECH) is a new functional material that combines flexible 3D hydrogel network with conductive nano-fillers. Due to the inherent porous structure, excellent conductivity, and flexibility of the ECH, it is the perfect choice for flexible electrode materials. The continuous conductive network inside the ECH provides electron transfer pathways. The hierarchical pore structure can ensure sufficient contact between the active material and the electrolyte ions to improve the electrochemical performance [8]. Finally, a supercapacitor assembled from the hydrogel-based electrode and electrolyte, because of the inherent viscoelasticity and flexibility of the hydrogel, will possess an improved interface between the electrode and the electrolyte [9]. However, the related researches on self-healable and flexible hydrogel-based supercapacitors are limited so far [2].

A novel freestanding and moldable hydrogel with excellent self-healing performance was synthesized by incorporating cellulose nanofibers (CNFs) into polyvinyl alcohol-borax (PVA) hydrogel matrix. Conductive materials were incorporated into self-healable PVA hydrogels to form multifunctional ECHs, which had potential applications in flexible and self-healable supercapacitors. In our present study, carbon nanotubes (CNTs) and polyaniline (PANI) were the electrochemically active materials in electrodes. 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-cellulose nanofibers (TOCNFs) were the bio-dispersant that helped CNTs disperse in water. PANI was coated on the surface of TOCNF-CNT nano-hybrids through in-situ oxidative polymerization to form "core-shell" TOCNF-CNT@PANI composites. These composites built a hierarchically reinforced and conductive 3D network in the self-healable PVA hydrogel matrix. The hydrogel-based electrodes and electrolytes were used to assemble symmetrical solid-state all-in-one supercapacitors with flexible and fast self-healable performances.
