Surface Functionalization and Capacitance Enhancement of Activated Carbon through Mechanical Activation

Abstract

Lithium-ion capacitors (LICs) have the potential to combine the high energy density of lithium ion batteries and the high power density of supercapacitors into one device. In this study, we have investigated surface functionalization of activated carbon (AC) powder through mechanical activation for cathodes of LICs with non-aqueous electrolytes. Traditionally, surface functionalization is carried out via chemical activation or plasma treatment. Here, we show, for the first time, that surface functionalization can be achieved with mechanical activation using high-energy ball milling of AC powder in air at room temperature. It is found that mechanical activation is an effective way to create functional groups on the AC surface, thereby imparting pseudocapacitance and increasing the specific capacitance of the AC powder. The surface functionalization has led to an increase in the specific capacitance from 15.8 F/g for the as-received AC (AR-AC) to 33.5 F/g for the AC with 6-h high-energy ball milling (6h-AC). Note that although high-energy ball milling has resulted in significant reduction in the specific surface area of the AC powder, the surface functionalization induced by mechanical activation has drastically increased the areal specific capacitance from 2.3 μF/cm2 for the AR-AC to 68 μF/cm2 for the 6h-AC. As a result, the specific capacitance of AC has been increased through high-energy ball milling. The areal specific capacitance for the 6h-AC is 3 times the theorectical areal specific capacitance of the electrical double layer for graphene (21 μF/cm2), indicating the existence of the pseudocapacitance due to surface redox reactions in the 6h-AC powder. The presence of the pseudocapacitance due to surface redox reactions is also confirmed by cyclic voltammery (CV). The high specific capacitance achieved through mechanical activation is fairly stable over charge/discharge cycles, which is slightly better than the cyclic stability displayed by the AR-AC. X-ray photoelectron spectroscopy (XPS) analysis reveals that the pseudocapacitance displayed by the 6h-AC is mainly due to C=O functional groups introduced during mechanical activation. This work has opened up a new route to increase the specific capacitance of low cost and widely used AC powder for LICs.