Pushing the Limits: Hydrothermally Synthesized Bi ₂ MnCoO ₆ Electrodes with Exceptional Cyclability for Asymmetric Supercapacitors

Perovskite oxides are revolutionizing energy storage with their robust stability and versatile redox chemistry. Here, a novel bismuth‐based double perovskite, Bi 2 MnCoO 6 (BMCO), is introduced as a next‐gen supercapacitor electrode, synthesized via a hydrothermal synthesis. The strategic placement of Bi 3+ at the A‐site imparts superior electronic conductivity and ion transport. Simultaneously, the Mn/Co B‐site duo unlocks a rich, reversible redox palette (Mn 2+/3+/4+ , Co 2+/3+ ), driving exceptional pseudocapacitance. In three‐electrode tests, BMCO delivers an impressive 627 Fg −1 specific capacitance at 0.5 Ag −1 and maintains near‐ideal reversibility. Pushing the material into a real device, an asymmetric supercapacitor (ASC) is assembled by pairing BMCO as the cathode with activated carbon anode in 6 M KOH and achieves a 23 Whkg −1 energy density and up to 14 kWkg −1 power density. Most strikingly, the BMCO//AC ASC endures 100,000 charge–discharge cycles at 5 Ag −1 with ≈100% Coulombic efficiency. A pronounced “self‐activation” in the first 10,000 cycles boosts capacitance to 150%, as electrolyte penetration unlocks hidden redox sites. Thereafter, the device stabilizes at ≈120% of its original capacitance, proving remarkable long‐term durability, setting a new benchmark for perovskite‐based ASCs. This work not only elevates perovskite materials into the forefront of high‐performance supercapacitors but also demonstrates how precision cation design and eco‐friendly hydrothermal processing can yield ultra‐stable energy storage solutions.