Ultrahigh Energy Density and Long-Life Cyclic Stability of Surface-Treated Aluminum-Ion Supercapacitors

In this study, aluminum–graphene supercapacitors (denoted as aluminum-ion supercapacitors; ASCs), consisting of a battery-type aluminum anode, a capacitor-type graphene cathode, and ionic liquid 1-ethyl-3-methylimidazolium chloride (EMImCl) and aluminum chloride (AlCl3) electrolyte, were prepared. This study primarily aimed to investigate the enhanced electrochemical performance of ASCs arising from changes in the surface oxide layer and morphology via electrochemical surface treatments, including electropolishing and electrodeposition of aluminum anodes. The ASC devices based on an electrodeposited anode at a current density of 3 A g–1 exhibited a high specific capacity of 211 F g–1 compared to that of the electropolished anode (∼186 F g–1); these were 20 and 5.7%, respectively, higher than that of the pristine aluminum anode. In particular, the electrodeposited ASC delivered an energy density of 151 W h kg–1 at a power density of 3,390 W kg–1. Furthermore, a maximum power density of 11,104 W kg–1 was achieved at an energy density of 124.3 W h kg–1. These values are among the best as compared to those of previously reported aluminum-based supercapacitors, suggesting the potential feasibility of these ASCs with outstanding energy and power densities for next-generation energy storage devices.