Aqueous AC‐Line Filtering Electrochemical Capacitors Featuring 2.5‐V Voltage Window and Kilohertz Response Using Boron‐Doped Diamond Nanoarrays

Abstract Electrochemical capacitors show great promise in alternate current (AC) line filtering for modern miniaturized electronics, yet their widespread adoption is significantly hindered by limited voltage windows and slow response. Herein, an aqueous electrochemical capacitor for wide‐voltage and ultrafast‐response filtering is developed using the boron‐doped diamond electrodes that feature both the ultra‐wide potential window of 2.93 V and vertical‐oriented nanoarray architecture. This capacitor exhibits a wide voltage window up to 2.5 V larger than those of reported aqueous filtering capacitors to date and a high‐frequency response capability with a characteristic frequency f ₀ = 3338 Hz, as well as a phase angle of −80.4° and a high specific energy density of 365.6 µFV ² cm ⁻² at 120 Hz. These superior properties enable effective ripple smoothing of rectified AC signals with complex waveforms, high frequency (60–1000 Hz), and large voltage amplitudes up to 5 V. This work presents an enlightening strategic design of filtering electrochemical capacitors featuring wide voltage windows and high‐frequency response, thereby providing an effective solution to address key challenges in the integration and miniaturization of next‐generation electronic devices.