New Insights Into Energy Storage in Aqueous Zn/Porous Carbon Hybrid Capacitors: The Critical Role of High‐Voltage Charging

ABSTRACT

Aqueous zinc ion hybrid capacitors (ZIHCs) typically pair a battery‐type Zn anode with a capacitor‐type porous carbon cathode. Their capacity is conventionally thought to be limited by the intrinsic electric double‐layer (EDL) capacitance and pseudocapacitance arising from the porous structure and surface chemistry of the cathode. This longstanding view has constrained the achievable energy density of ZIHCs. In this work, we challenge this paradigm by demonstrating that charging at high voltage substantially enhances the capacity of commercial cathodes in a 2 M ZnSO ₄ electrolyte. Leveraging this insight, we develop two simple strategies to significantly increase the energy density of ZIHCs. An ex situ approach nearly doubles the capacity of a commercial cathode, yielding an order of magnitude increase in the specific energy E _total based on the total active mass of anode and cathode when integrated into a Zn‐limited configuration. Alternatively, an in situ strategy, which directly charges the device to a high voltage, achieves comparable enhancement while simplifying fabrication. Both strategies deliver exceptional cycling stability (over 60,000 cycles at 5.0 A g ⁻¹ ) and are successfully validated in pouch cell prototypes. Overall, this work provides a deeper mechanistic understanding and offers simple, low‐cost routes to high‐energy‐density aqueous ZIHCs.