Combining Weakly Hydrogen‐Bonded Electrolyte with Vertical Ion Channels Enables Low‐Temperature AC‐Line Filtering Electrochemical Capacitors

ABSTRACT Electrochemical capacitors (ECs) are expected to replace traditional aluminum electrolytic capacitors (AECs) in AC‐line filtering due to their advantages in miniaturization and circuit integration, yet their frequency response is severely limited by slow ion transport at low temperature. Herein, we demonstrated that the electrochemical interface integrating weakly hydrogen‐bonded electrolyte environment within vertical ion channels enables ECs to deliver millisecond‐level ultrafast response even at −50°C for AC‐line filtering. With aids of atomic calculation and spectroscopic characterizations, Mg(ClO 4 ) 2 aqueous electrolyte is shown to efficiently weaken hydrogen bonding and possess the lowest anion kinetic barrier (E a ), resulting in its superior low‐temperature performance over other aqueous electrolytes. By integrating this weakly hydrogen‐bonded electrolyte within vertical graphene electrode, the resulting EC unit delivers an ultra‐fast response with a relaxation time constant as low as 5.7 ms at −50°C, along with a high areal energy density of 1.49 mFV 2 /cm 2 at 120 Hz and stable 60‐Hz filtering capability at −50°C. This work bridges the gap between ultra‐fast response and low‐temperature adoptability in ECs and expands their applications in practical low‐temperature AC‐line filtering applications.