Hierarchically 3D Fibrous Electrode for High‐Performance Flexible AC‐Line Filtering in Fluctuating Energy Harvesters

In light of the rapid development of intelligence and miniaturization in electronics, the growing demand for sustainable energy sources gives rise to a plethora of environmental/mechanical energy harvesters. However, the fluctuating nature of these generated energies frequently presents a challenge to their immediate usability. Although electrolytic capacitors can smooth fluctuating energy, lacking miniaturization and flexibility constrain their potential applications. Conversely, electrochemical capacitors (ECs), particularly fiber‐shaped electrochemical capacitors (FSECs), can offer superior flexibility. Nevertheless, the inherent trade‐off between ion transport and charge storage in fibrous electrodes poses a significant obstacle to their filtering capability. Here, a hierarchically 3D fibrous electrode that effectively balances ion transport and charge storage through its unhindered primary framework and intertwined secondary frameworks is presented. The resulting FSEC exhibits an exceptional specific areal capacitance of 1.37 mF cm−2 with a phase angle of −82° at 120 Hz, surpassing that of fiber‐shaped filter capacitors and most non‐fibrous filter ECs previously reported. Additionally, the FSEC displays excellent flexibility and high‐frequency response, rendering it well‐suited for filtering arbitrary ripple voltage and compatible with environmental/mechanical energy harvesters. These results demonstrate a promising approach for designing fibrous high‐frequency response electrodes and a foundation for portable environmental energy harvesting devices. A hierarchically 3D flexible fibrous electrode composed of an unimpeded primary framework of 3D electrochemically reduced graphene oxide and an intertwined secondary framework of poly(3,4‐ethylenedioxythiophene) nanofibrils is fabricated, which can regulate ion transport and charge storage. It exhibits superior performance in converting fluctuating environmental/mechanical energy into direct current signals.