Understanding Multi-Stage Charge Storage on Nanoporous Carbons in Zn-Ion Hybrid Capacitors

Zn-ion hybrid capacitors (ZIHCs) are promising high-power energy storage devices. However, the underlying charge storage mechanisms, especially the influence of proton storage, remain poorly understood. Herein, the model porous carbons are synthesized having similar specific surface areas (SSAs) and surface chemistry but different pore sizes. They highlight the role of supermicropores and small mesopores (0.86-4 nm) enabling a high capacity of 198 mAh g^-1 (capacitance of 446 F g^-1), while larger mesopores (4-13 nm) significantly enhance cycling stability, exceeding 0.6 million cycles. Electrochemical studies, including EQCM analysis, reveal a 4-stage charge-storage process under cathodic polarization, comprising adsorption and desolvation of hydrated Zn²⁺ ions, followed by water reduction, catalyzed by Zn²⁺, and formation of H_ad. The rising pH leads to the formation of insoluble zinc hydroxysulfate hydrates (ZHS). Depending on the pore architecture, the precipitation of ZHS has different effects on the overall stability of cycling. The study overall: (i) presents a simplified method for pore control in carbon synthesis; (ii) discuss the effect of pore size on charge storage and cycling stability in respect of ZHS formation; (iii) sheds light on the charge storage mechanism indicating the important contribution of cation effect known from electrocatalysis on faradaic charge storage mechanism.