Aqueous zinc-ion hybrid capacitors (ZIHCs) combine the high power density of supercapacitors with the high energy density of batteries, offering intrinsic safety, low cost, and environmental sustainability. However, their specific capacity is severely limited by the kinetic and capacity mismatch between capacitive carbon cathodes and zinc anodes. In a study published in ENG. Chem. Eng., researchers at Xi’an Jiaotong University report a hierarchical porous carbon cathode derived from low-cost coal tar pitch that overcomes these limitations through a unique dual-ion relay storage mechanism.
The team synthesized CMK-x carbon nanosheets using magnesium oxide as a structure-directing agent and potassium hydroxide as an activator. Among the samples (pyrolyzed at 600–900 °C), CMK-700 exhibited the optimal structure: a high specific surface area of 2223.9 m²·g⁻¹, abundant oxygen-containing functional groups (10.15 at %), and a maximized ultramicroporous volume of 0.4836 cm³·g⁻¹. The material’s hierarchical pore network, comprising micropores, mesopores, and macropores, facilitates rapid ion transport and provides abundant active sites.
The charge-storage mechanism was elucidated through combined ex-situ characterization and molecular dynamics simulations. Larger solvated zinc ions [Zn(H₂O)₆]²⁺ (diameter ~0.86 nm) are stored in larger micropores (>1 nm) and mesopores via electric double-layer capacitance and chemical adsorption onto oxygen-containing functional groups. However, these ions cannot access ultramicropores (<1 nm). In contrast, smaller hydrated protons H₃O⁺ (diameter ~0.564 nm) can penetrate these ultramicropores, undergoing reversible chemical hydrogen adsorption/desorption. This proton relay mechanism unlocks the capacitive potential of otherwise inaccessible ultramicropores, contributing substantial pseudocapacitance.
MD simulations of 0.7 nm and 1.0 nm slit-pores confirmed that [Zn(H₂O)₆]²⁺ ions are hindered from entering ultramicropores due to their large size and high desolvation energy barrier, while H₃O⁺ ions readily diffuse into these confined spaces. This selective ion transport behavior validates the proposed “protons relaying zinc ions” storage mechanism.
Electrochemical testing showed that CMK-700 delivered a high specific capacity of 368.1 mAh·g⁻¹ at 0.5 A·g⁻¹, significantly outperforming CMK-600 (329.6), CMK-800 (321.2), and CMK-900 (255.0 mAh·g⁻¹). The GCD curves revealed a plateau-like region (0.3–0 V) contributing 171.3 mAh·g⁻¹, attributed to proton storage in ultramicropores. Rate capability remained excellent, with 133.7 mAh·g⁻¹ retained at 20 A·g⁻¹. Cycling stability was outstanding, with 86.39 % capacity retention after 21,000 cycles at 10 A·g⁻¹.
Ex-situ SEM revealed flake-like precipitates (identified as Zn(OH)₂ and Zn₄ClO₄(OH)₇) forming during discharge below 0.3 V and dissolving upon charging, confirming the coupling between proton chemical adsorption/desorption and reversible alkaline zinc salt precipitation/dissolution.
This work demonstrates that tailoring pore size distribution to match specific ion dimensions, combined with integrating proton storage, provides a powerful strategy for enhancing the energy density of aqueous zinc-ion hybrid capacitors.

DOI
10.1007/s11705-026-2681-3