Flux-Assisted Double Perovskite Ba₂CuTaO₆ Electrodes with Ultralow Charge-Transfer Resistance for High-Rate Supercapacitors

Abstract

A scalable potassium chloride flux–assisted solid-state strategy is demonstrated for the synthesis of phase-pure Ba2CuTaO6 double perovskite with an ordered rock-salt arrangement of CuO6 and TaO6 octahedra at 950 °C. Structural and compositional analyses by X-ray diffraction, HRTEM/SAED, and STEM–EDS mapping confirm improved crystallinity, homogeneous Ba:Cu:Ta stoichiometry (2:1:1), and reduced structural disorder compared with conventional solid-state synthesis. When evaluated as a pseudocapacitive electrode in 6 M KOH, the flux-derived Ba2CuTaO6 delivers a specific capacitance of 343 F g-1 at 1 A g-1, together with an ultralow charge-transfer resistance of 8.7 Ω and cycling retention of ~93.2%. The electrochemical performance surpasses that of CuTaO4 and BaTaO4 reference oxides by approximately four-fold, highlighting the beneficial role of double-perovskite electronic synergy. The enhanced charge-storage behavior is attributed to reversible Cu2+/Cu+ redox activity, hydroxyl-enriched surface chemistry, and dominant surface-controlled pseudocapacitive processes. Two-electrode electrochemical performance reveals that Ba2CuTaO4 delivers energy density of ~55 Wh kg-1 at current density of 1 A g-1, which is about ~7× and 18× higher than CuTaO4 and BaTaO4 respectively. The combination of rapid ion transport, reduced grain-boundary resistance, and robust structural stability positions the flux-engineered Ba2CuTaO6 as a promising and viable double-perovskite electrode for high-power electrochemical energy-storage applications.