High-energy flexible supercapacitors based on free-standing metal oxide electrodes for ammonium ion storage

The booming of portable electronics stimulated the development of flexible and wearable energy storage solutions, where ammonium ions exhibit superiority over traditional metal ions. To further obtain the high-energy ammonium ion-based supercapacitor, it is essential to explore the electrode materials with high capacity, rate performance and cycling stability. Herein, we explored α-MnO2 nanowires with a tunnel structure and synthesized in-situ on flexible active carbon cloth (ACC), as the cathode material, exhibiting a substantial mass loading of 18 mg cm−2, and an areal capacitance of 3132.6 mF cm−2 (2 mA cm−2). Simultaneously, to match the MnO2@ACC cathode, in-situ growth of h-WO3 pre-intercalated with NH4+ on ACC to serve as the anode material, with a mass loading of 17 mg cm−2 and an areal capacitance of 2643.9 mF cm−2 (2 mA cm−2). Subsequently, quasi-solid-state ammonium ion flexible supercapacitor (A-FSCs) are fabricated by utilizing these two distinct electrode materials, thereby expanding the operating voltage range to optimize the electrochemical performance. This work provides an approach for achieving capacity matching between the cathode and anode materials in asymmetric A-FSCs. The assembled MnO2@ACC//WO3@ACC A-FSC device demonstrates exceptional areal capacitance (1128.89 mF cm−2 at 2 mA cm−2), long stability (the capacity retention rate of 98.20 % after 20,000 cycles), high energy density of 508.1 μW h cm−2 at the power density of 1.8 mW cm−2, and promising practical applications.