High-Performance Carbon-Nanotube-Based Supercapacitors at a Wide Temperature Range: Geometrical Effect on Diffusion of Electrolytic Ions

Supercapacitors (SCs) characterized by excellent charge and discharge rates, high power density, and stable cycling performance, exhibit crucial applications in various fields, while it faces significant performance degradation at low temperature. Herein, we systematically investigate the electrochemical performances of carbon nanotube (CNT)-based SCs over a temperature range of -18 to 60 °C and establish quantitative relationships between CNT geometry and temperature-dependent electrochemical kinetics in symmetric SCs. The results and analysis supported by electrochemical impedance spectroscopy (EIS) with Warburg diffusion analysis, Arrhenius modeling of ion diffusion kinetics, and multiterm self-discharge modeling reveal exceptional low-temperature resilience where CNT-based SCs retain >85% peak specific capacitance (75.76 F/g at 0.5 A/g) with 87% rate retention at 20 A/g (-18 °C), attributable to minimized diffusion barriers in CNTs with shorter length and wider channel that reduce Arrhenius activation energy by 33% (CNT-8-L: <i>Q</i> = 15.40 kJ/mol vs CNT-3-L: 23.07 kJ/mol). In addition, a symmetric CNT-based SC is successfully employed to power a digital thermometer. The self-discharge compensation effect enables optimized energy deliver of the CNT-based SC for over 40 min to the digital thermometer at -18 °C (approximately 4 times longer than at 60 °C) through suppression of current leakage as well as ion diffusion, although the specific capacitance is lower. The experimental findings and analyses contribute to the design and optimization of CNT geometries for low-temperature applications and deepen our understanding of the underlying energy storage mechanisms.