Optimized synthesis of composition-engineered BiVO₄–polypyrrole hybrid electrodes for high-energy solid-state supercapacitors

Developing electrode materials with high electrochemical activity, rapid charge transport, and long-term stability remains a key challenge for advanced supercapacitor technologies. In this work, a series of BiVO₄-polypyrrole (BV-PPy) composite electrodes was synthesized by gradually increasing the BiVO₄ concentration within a conductive polypyrrole matrix via an in situ oxidative polymerization approach. The aim was to identify the optimized BiVO₄ content that maximizes electrochemical activity for supercapacitor applications. Structural and spectroscopic analyses confirm the successful incorporation of BiVO₄ within the conductive PPy matrix, forming a strongly coupled hybrid structure that promotes efficient electron transport and abundant electroactive sites. Among the prepared composites, the optimized B₃V-PPy electrode exhibits superior electrochemical performance, delivering a high specific capacitance of 573 F g⁻¹ at 1.5 A g⁻¹ with good rate capability. Electrochemical impedance spectroscopy reveals reduced internal resistance and enhanced charge-transfer kinetics due to the synergistic interaction between BiVO₄ nanoparticles and the PPy network. Kinetic analysis indicates a hybrid charge-storage mechanism, dominated by surface-controlled capacitive reactions with additional diffusion-controlled faradaic contributions. A symmetric solid-state supercapacitor assembled using the optimized B₃V-PPy electrode and a PVA/Na₂SO₄ gel electrolyte operates stably within a 1.4 V potential window, delivering an energy density of 69.83 Wh kg⁻¹ at a power density of 1050 W kg⁻¹. The device also demonstrates good durability with 71% capacitance retention after 15,000 cycles. The enhanced electrochemical performance is attributed to the optimized balance between the conductive PPy network and the redox-active BiVO₄ phase, which facilitates rapid electron transport, efficient ion diffusion, and improved utilization of electroactive sites. These results demonstrate that tuning the BiVO₄ concentration in the polypyrrole matrix is a promising strategy for designing high-performance hybrid electrodes for advanced solid-state supercapacitor applications.