No abstract

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.

With the recent growth of interest in wearable electronics, gel electrolytes or solid electrolytes of energy conversion devices such as batteries and capacitors have been actively studied. All-solid-state supercapacitor has advantages such as high power density, outstanding mechanical stability, and long cycle life. However, the low energy density of supercapacitor makes it difficult to replace the battery. Therefore, a hybrid supercapacitor combining a battery and a supercapacitor has been significantly attracted. In this study, aluminum as a battery electrode and graphene as a capacitor electrode were hybridized in a cell and ethyl acrylate and ionic liquid were used to make a polymer gel electrolyte. Aluminum has high energy density due to three-electron reaction and can be considered as a promising battery type electrode of a hybrid supercapacitor. Conversely, graphene is appropriate for capacitor type electrodes owing to its large specific surface area (~2675 m2 g-1) and high theoretical capacity (550 F g-1 ). With the increase of charge/discharge cycling, the dissolution and deposition reactions of aluminum were repeated and the contact of the aluminum anode and gel electrolyte decreases, which can result in a decrease in capacitance and cycle stability. Two strategic approaches were applied to improve the cell performance and charging-discharging behaviors were investigated with the variations of aluminum surface morphology. Firstly, aluminum metal was electropolished to remove the surface oxide film and applied as an anode. Secondly, thin aluminum metal film was prepared with electroplating and tested as an anode. Electrochemical Impedance Spectroscopy (EIS) was used to characterize the gel electrolyte and to measure the contact resistance between electrolyte and the surface-treated aluminum. SEM and XRD were used to analyze the aluminum surface structure.