Electrochemical capacitance and ionic transport in the mesoporous shell of a hierarchical porous core–shell carbon structure

Abstract

A three-dimensional (3D) hierarchical porous carbon structure was prepared with possible variations in porosity at three levels of length scales. The carbon structure was template-synthesized from a core–shell silica sphere assembly. The as-synthesized carbon featured a semi-ordered porous structure with hollow macro-cores (330 nm) surrounded by a mesoporous shell containing uniform pores of 3.9 nm and distinct interstitial space between the core–shell domains. The mesoporous shell thickness was stepwise increased from 0, 25, 50 to 100 nm while keeping an identical core size to create a family of hierarchical porous structures for a systematic investigation of electrochemical capacitance and ionic transport. The shell thickness affected the overall porosity and relative porosities of the shell, core, and interstitial regions. A thicker mesoporous shell possessed a higher surface area which led to a proportional increase in electrochemical capacitance which can be fully realised at low scan rates. For the carbon structure with the maximum shell thickness of 100 nm, electrochemical capacitance per unit area and power density declined at high scan rates and high currents when ionic transport through long mesopores became limiting. The power density of the better as-synthesized porous carbon was up to 11.7 kW kg⁻¹ when the corresponding energy density was 5.9 W h kg⁻¹.