Architectural design of hierarchically ordered porous carbons for high-rate electrochemical capacitors

Abstract

The rate capability of carbon-based electrochemical capacitors (ECs) is an important issue for applications and the three-dimensional (3D) hierarchically ordered porous carbon (HOPC) can reduce the diffusion length to improve the rate performance by the unique architectural design. In this study, a modified dual-templating strategy for architectural design of 3D HOPC as a promising electrode material for high-rate EC applications was developed by using polystyrene and Pluronic F127 as macro- and meso-porous templates, respectively. Tetraethyl orthosilicate was also added as the silica precursor, and then the sol–gel-derived SiO₂ was removed by sodium hydroxide to form the second mesopore in HOPC (HOPC-s) for enhancing the accessible surface areas and pore structures. The HOPC-s formed by the templating strategy is composed of highly ordered macropores, macroporous windows, bimodal mesopores, and micropores, resulting in high specific surface area (1112 m² g⁻¹), high total pore volume (1.18 cm³ g⁻¹) and an easily accessible environment for fast electrolyte ion transport. The specific capacitance of the HOPC-s electrodes can reach 316 F g⁻¹ at 25 mV s⁻¹ and maintain excellent capacitive retention at a high scan rate of 1000 mV s⁻¹ when compared with that of ordered mesoporous carbon (OMC-s). Electrochemical impedance spectroscopy fitting shows that the pore electrolyte resistance of HOPC-s is 3 times lower than that of OMC-s, which is attributed to the hierarchical macroporous structures and short mesoporous channels. In the symmetric capacitor test, the HOPC-s also shows excellent power capability, and the energy density of 4–10 W h kg⁻¹ can be maintained over the power density range of 1–14 kW kg⁻¹. In addition, the capacitance of HOPC-s in the polycarbonate-containing ionic liquid shows 80% retention at a scan rate of 500 mV s⁻¹, indicating that the unique hierarchical structure can provide efficient ion-buffering capacity for high-rate performance. These exceptional electrochemical performances clearly demonstrate that 3D HOPC-s is a superior material to solve the poor ion transport limitation, which can open an avenue to fabricate high-rate ECs with high power and energy densities for energy storage.