An Ultra‐Low‐Temperature Alternating Current Filter

Abstract Traditional alternating current filter based on aluminum electrolytic capacitors (AECs) suffer from abrupt drop of filtering capability at ultra‐low temperatures (≤−30 °C), which greatly hinders the reliable working of electronics at extremely cold conditions. Herein, an ultra‐low‐temperature alternating current (AC) filter for the first time enabled by high‐frequency supercapacitor based on covalently bonded hollow carbon onion‐graphene hybrid structure is reported. It is found that the covalent bonding junctions enable high electronic conductivity and efficient ion adsorption/desorption behavior in the hybrid structure. Moreover, the hybrid structure owns positive curvature and shallows pores for fast ion diffusion kinetics. Consequently, the supercapacitor exhibits a record short resistor‐capacitor time constant (τ_RC) of 0.098 ms at 120 Hz at room temperature. Combining with low‐melting‐point electrolyte, the supercapacitor possesses excellent filtering capability and can output stable direct current signal with low fluctuation coefficients in a temperature range of −50 to 0 °C. More interestingly, the filter presents high negative phase angle, low dissipation factor, short τ_RC, and high capacitance retention below −30 °C, whereas AEC cannot work properly owing to its phase angle<45°. This work realizes the fabrication of an ultra‐low‐temperature AC filter, which presents a critical step forward for promoting the development of ultra‐low‐temperature electronics.