The need for developing energy storage devices with high energy and power densities has motivated tremendous effort in the research of supercapacitors. Major challenges still exist in improving the working voltage window and specific capacitance of the devices. Here, we report a systematic effort in achieving remarkable supercapacitor performance. Firstly, a flexible CNT film/double side carbon tape/CNT film@MnO2 (CDC@MnO2) positive electrode with a large voltage window of 0-1.2 V was fabricated via a facial one-step water bath process. The large interfacial area created by the CDC sandwich structure enabled a high MnO2 mass loading up to 6.6 mg cm(-2), resulting in a high area capacitance of 1.1 F cm(-2) at a current density of 1 mA cm(-2). Secondly, inspired by the superior electrochemical performance of the CDC@MnO2 positive electrode, a CDC@Fe2O3 film with area capacitance up to 339.1 mF cm(-2) was fabricated as the negative electrode. Thirdly, a flexible asymmetric supercapacitor was successfully assembled which possesses a large voltage window of 0-2.1 V, a high volume capacitance of 6.8 F cm(-3), and a high energy density of 4.1 mWh cm(-3) at the power density of 22.3 mW cm(-3).