Optimizing porous structure of carbon electrodes for temperature-independent capacitance at sub-zero temperatures

Fundamental understanding of ion transport in porous carbon materials with different pore size is of great importance for carbon electrode-based supercapacitors to realize the ability of rapid charge/discharge process and high energy density at broad operating temperatures. Herein, hierarchically porous carbon frameworks are synthesized as model electrodes by using a tunable one-pot strategy in the presence of salt template and pore-forming agent. Density functional theory calculation reveals that the size of micropores can significantly influence the solvation energy and desolvation time of electrolyte, thereby affecting the ion transport kinetics and the specific capacitance of porous carbon electrode, particularly at low temperatures. The accordingly optimized carbon electrode material with rational micropore size and interconnected hierarchical pores exhibits the specific capacitance of 200 and 245F·g−1 at −40 and 60 °C, respectively. The as-fabricated symmetric supercapacitor can deliver an exceptional energy density of 63.4 Wh·kg−1 under power density of 1.5 kW·kg−1 at −40 °C. In addition, the device has the ability for fast charge/discharge process at low temperature, as evidenced by the energy density of 34.3 Wh·kg−1 under an ultrahigh power density of 37.5 kW·kg−1 at − 40 °C.