Structural Evolution of BaMoO₄ Upon Zn Doping and Its Influence on Electrochemical Behavior in Hybrid Supercapacitors

Abstract

Barium molybdate (BaMoO₄) based materials are emerging as stable and environmentally benign electrodes; however, their electrochemical roles and tunability remain insufficiently explored. In this work, pristine and Zn-modified BaMoO₄ materials were synthesized via a solid-state chemical route and systematically investigated as complementary electrodes for hybrid supercapacitors. Structural characterization confirms that pristine BaMoO₄ crystallizes in a well-defined scheelite-type tetragonal phase with tetrahedral morphology, whereas Zn doping induces noticeable structural disorder, reduced crystallinity, and the formation of Zn-rich Mo-O regions. Electrochemical studies in 2 M Na₂SO₄ reveal that pristine BaMoO₄ exhibits predominantly electric double-layer capacitive behavior, and limited accessible redox sites with excellent cycling stability, making it suitable as a positive electrode. In contrast, Zn-modified BaMoO₄ operates effectively in the negative potential region, delivering an enhanced pseudocapacitive response (303 F g⁻¹), attributed to reversible Zn²⁺-associated redox processes. By pairing these two functionally distinct electrodes, a BaMoO₄/Zn-BaMoO₄ hybrid supercapacitor was assembled, achieving an extended aqueous operating voltage of 1.8 V and a specific capacitance of 76 F g⁻¹ at 0.33 A g⁻¹, with 87% capacitance retention after 2000 cycles. Rather than maximizing absolute capacitance and energy density values, this study demonstrates a controlled structural modification strategy using a Zn dopant, in which targeted lattice and morphological changes enable polarity-selective charge storage within an oxide family, offering a stable and sustainable platform for aqueous hybrid energy storage systems.