Current research on supercapacitors focuses on achieving high specific energy by expanding the voltage window and improving specific capacitance through advanced electrode design. This study presents a new type of pseudocapacitive integrated electrode developed by decorating α-Fe2O3 nanoparticles onto NH4V3O8 multiwalled nanotubes using a simple and efficient method. α-Fe2O3 stores energy through conversion reactions, while NH4V3O8 facilitates intercalation-based storage. The difference in work function between α-Fe2O3 nanoparticles and NH4V3O8 multiwalled nanotubes generates a built-in electric field at the heterointerface, as confirmed by density functional theory calculations. This built-in electric field enables simultaneous operation at both positive and negative potentials, thereby supporting sulfate ion conversion and sodium ion intercalation. These mechanisms are validated by in situ Raman and ex situ X-ray photoelectron spectroscopy analyses. Owing to the coexistence of multiple energy storage mechanisms and the presence of a built-in electric field, the assembled full cell delivers a high specific energy (79 Wh/kg), specific power (5996 W/kg), and a broad voltage window of 2.2 V. These findings emphasize the effectiveness of the integrated electrode design and represent a significant advancement toward realizing next-generation energy storage technologies for a wide array of applications, ranging from portable electronics to expansive renewable power infrastructures.