Impacts of different length scales on the electrochemical capacitance of a 3D hierarchical porous carbon structure

Abstract

The transport coupled electrochemical processes were investigated with a 3D hierarchical porous carbon structure prepared with possible variations of porositity at three length scales. The carbon structure was template-synthesized from a core-shell silica sphere assembly. The as-synthesized carbon featured an semi-ordered porous structure with hollow macro-cores a few hundred nanometers diameter surrounded by a mesoporous shell containing uniform pores of 3.9 nm. The spherical core-shell domains were assembled with distinct interstitial space between them. In one set of experiments, the mesoporous shell thickness was stepwise increased from 0, 25, 50 to 100 nm while keeping an identical core size of 330 nm to create a family of hierarcical porous structures for a systematic investigation of electrochemical capacitance and ionic transport. A thicker mesoporous shell possessed a higher surface area leading to a proportional increase in electrochemical capacitance which can be fully realised only at low scan rates. For the carbon structure with a 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 one as-synthesized porous carbon was as high as 11.7 kW/kg with a corersponding energy density of 5.9 Wh/kg. In another set of investigations, structures with the same overall particle size but varying core diameter and shell thickenss were synthesized and tested.