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Fiber-shaped supercapacitors (FSCs) are promising energy storage devices for portable and wearable electronics due to their miniaturized size, flexibility, and knittability. Despite the significant progress in this area, it is still a challenge to develop large capacitance and high energy density FSCs for practical applications. In this work, a hybrid fiber composed of reduced graphene oxide and polyaniline nanoparticles (r-PANI-GOF) is synthesized via in situ synthesis of polyaniline nanoparticles both on the surface and inside of graphene fibers. The areal specific capacitance of a single r-PANI-GOF electrode is as large as 1755 mF cm−2 in the three-electrode system. The r-PANI-GOF hybrid fibers were also used as electrodes for making an all-solid-state FSCs. This whole device has a specific areal capacitance of up to 481 mF cm−2 and a high areal energy density of 42.76 μWh cm−2. The hybrid fiber electrodes with a high capacitance, and excellent flexibility may become new candidates for the development of fiber-shaped high-performance energy storage devices.

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As EHD printing relies on the electrical force between the nozzle tip and the collecting surface to eject the polymer melts or solutions, the electrical field strength gradually decreases as the printing layer increases. [7,8] In addition, due to the inherent low conductivity nature of polymeric fibers, extensive residual charges are entrapped and accumulate inside the printed structures, which generate electrical repellent forces to the successively-printed fibers and disrupt the precision stacking process at a relatively great printing layer. [9][10][11] These factors significantly limit the height of EHD-printed scaffolds smaller than 3 mm. [12,13] It still remains a substantial challenge to fabricate microfibrous scaffolds with relatively large height and wellordered fiber organizations. [14] Currently, several pioneering explorations have been attempted to address the issues of unstable electrical field strength and the fiber repellent force for the EHD printing of 3D polymeric structures. [15][16][17] Luo et al. reported an innovative solutionbased EHD printing strategy by utilizing fibrous paper as the collector substrate, which can transfer the electrical charges from the printed fibers to the grounded conductive plate via the ions migration inside the residual solvent of the newly-deposited fibers. [18] Such charge transport mechanism dynamically changes the newly-deposited fibers into locally grounded electrical poles to attract the precision stacking of successive fibers. This phenomenon simultaneously alleviates the decrease of electrical strength and the repulsion of printed fibers, which facilitates the EHD printing of a hollow cylinder with a height of 1.3 cm. However, this strategy is not suitable for melt-based EHD printing since the polymer melts, with higher viscosity and extremely lower conductivity compared to polymer solutions, cannot freely transfer residual charges from the printed fibers to the grounded substrates. Recently, Wunner et al. reported a melt-based EHD printing strategy to fabricate large-volume scaffolds with highly-ordered architectures by dynamically changing the nozzle-to-collector distance and applied voltages to maintain constant electrical field strength during the whole printing process. [19] The maximum height of the printed structures reached 7.1 mm, which