1. State Key Laboratory of Clean Energy Utilization, College of Energy Engineering, Zhejiang University, Hangzhou 310027, China;2. Hangzhou Global Scientific and Technological Innovation Center, Zhejiang University, Hangzhou 311215, China
Abstract:
Supercapacitors that can withstand extremely low temperatures have become desirable in applications including portable electronic devices, hybrid electric vehicles, and renewable energy conversion systems. Graphene is considered as a promising electrode material for supercapacitors owing to its high specific surface area (up to 2675 m2·g-1) and electrical conductivity (approximately 2 × 102 S·m-1). However, the restacking of graphene sheets decreases the accessible surface area, reduces the ion diffusion rate and prolongs the ion transport pathways, thereby limiting the energy storage performance at low temperatures (typically < 100 F·g-1 at sub-zero temperatures). Herein, we fabricate a supercapacitor based on holey graphene and mixed-solvent organic electrolyte for ultra-low-temperature applications (e.g., -60 ℃). Reduced holey graphene oxide (rHGO) was synthesized as the electrode material via an oxidative-etching process with H2O2. Methyl formate was mixed with propylene carbonate to improve the electrolyte conductivity at temperatures ranging from -60 to 25 ℃. The as-fabricated supercapacitor showed a high room-temperature capacitance of 150.5 F·g-1 at 1 A·g-1, which was almost 1.5 times greater than that of the supercapacitor using untreated reduced graphene oxide (rGO; 101.4 F·g-1). The improved capacitance could be attributed to the increased accessible surface rendered by the abundant mesopores and macropores on the holey surface. As the temperature decreased to -60 ℃, the rHGO supercapacitor still delivered a high capacitance of 106.2 F·g-1 with a retention of 70.6%, which was superior to other state-of-the-art graphene-based supercapacitors. Electrochemical impedance spectra tests revealed that the ion diffusion resistance in rHGO was significantly smaller than that in rGO and less influenced by temperature with a lower activation energy. This was because the holey morphology can provide transport pathways for ions and reduce the ion diffusion length during charging/discharging, consequently diminishing the diffusion resistance at low temperatures. Specifically, at -60 ℃, the energy density of supercapacitor reached up to 26.9 Wh·kg-1 at 1 A·g-1 with a maximum power density of 18.7 kW·kg-1 at 20 A·g-1, surpassing the low-temperature performance of conventional carbon-based supercapacitors. Moreover, after 10000 cycles at -60 ℃ with a current density of 5 A·g-1, 89.1% of capacitance was retained, suggesting the stable and reliable power output of the current supercapacitor at extremely low temperatures.
