Hybrid supercapacitors based on X-site Ba(II) ions substituted by Sr(II) in Langbeinite-type phosphates

Abstract

The compounds KBa1-xSrxCr2(PO4)3 (with x = 0.00; 0.25; 0.50; 0.75; 1.00) were synthesized by a solid-state reaction, and they were thoroughly characterized by different spectroscopic and microscopic techniques. Their structures were indexed in a cubic system with a P213 space group forming a 3D framework built on CrO6 octahedra and PO4 tetrahedra sharing vertices leading to identical Cr2P3O18 (U) units. The interconnection between the tetrahedral and octahedral groups leads to the formation of two large closed cavities (K, MII)(1) and (K, MII)(2), statistically occupied by K+ and M2+ (M = Ba, Sr) atoms. Electron paramagnetic resonance spectroscopy confirmed the presence of paramagnetic Cr3+ ions, showing the effects of substituting the Ba2+ ions with smaller Sr2+ ions on the dipolar coupling between the Cr3+ centers. The obtained materials and active carbon were used as electrode materials in hybrid SC devices. At the same time, their electrochemical properties were assessed by potentiostatic electrochemical impedance spectroscopy, cyclic voltammetry, and galvanostatic charge–discharge measurements, showing promising results with a maximal specific capacity (3.86 F/g), energy density (343 mWh/kg), and power density (30.9 kW/kg) in the case of KBa0.5Sr0.5Cr2(PO4)3, proving them as good candidates for positive and/or negative electrode materials for energy storage applications.