A conductive polymer coated MoO₃ anode enables an Al-ion capacitor with high performance

Electrochemical capacitors are becoming promising energy conversion/storage and power output devices. However, high cost and low energy density are two serious disadvantages. By integrating the advantages of Li-/Na-ion batteries and electrochemical capacitors, Li-/Na-ion capacitors have been explored recently. Al is very cheap and is the most abundant metal element on the earth. There are few reports on Al-ion capacitors due to the challenges in finding a suitable anode with large capacitance and good rate performance. Here, the feasibility of assembling an Al-ion capacitor with good electrochemical performance is demonstrated. The Al-ion capacitor is assembled by using a composite of MoO₃ nanotubes coated by a conductive polypyrrole (PPy@MoO₃) as an anode, which functions via a redox intercalation/deintercalation of Al³⁺ ions in aqueous solution. It delivers a capacitance of 693 F g^-1, about 3 times higher than that of electrode materials for sodium-ion capacitors in aqueous solution. Combined with an activated carbon (AC) cathode, the Al-ion capacitor presents an energy density of 30 W h kg^-1 and an excellent cycling life with 93% capacitance retention after 1800 cycles. This finding provides another energy storage device with low cost and promotes the application of capacitors.