Strategic Design of CeO ₂ –Mn ₃ O ₄ Nanostructures for Dual Functional Applications in Solid‐State Supercapacitors and Overall Water Splitting Catalysis

The scalable Co‐precipitation route is successfully adopted to fabricate CeO 2 –Mn 3 O 4 mixed‐phase nanostructures with systematically varied cerium contents (1%–5%) for multifunctional energy storage and electrocatalytic applications. Structural and morphological analyses verify the formation of a well‐integrated CeO 2 –Mn 3 O 4 mixed‐phase nanocomposite with homogeneous cerium dispersion, while X‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy confirm mixed Ce 3+ /Ce 4+ and Mn 3+ /Mn 4+ redox states accompanied by oxygen vacancy‐rich defect structures that promote enhanced electronic conductivity and charge transfer behavior. Electrochemical measurements reveal that the optimized Ce@Mn 3 O 4 3% electrode exhibits superior pseudocapacitive performance, delivering a high specific capacitance (Cs) of 597 F/g at 5 mV/s with excellent cycling durability, retaining 82.9% of its capacitance after 6000 cycles. When assembled into a solid‐state supercapacitor using a PVA‐KOH gel electrolyte, the device achieves a maximum Cs of 148 F/g and an energy density of 55.8 Wh/kg at a power density of 3588 W/kg, along with 87.4% capacitance retention over prolonged cycling. The optimized Ce@Mn 3 O 4 3% electrode demonstrates bifunctional electrocatalytic activity for alkaline water splitting, requiring a low overpotential of 397 mV at 10 mA/cm 2 during oxygen evolution and maintaining stable performance under continuous operation. These results highlight the synergistic role of the CeO 2 –Mn 3 O 4 mixed‐phase nanocomposite and defect‐engineered nanostructuring in enhancing both electrochemical and catalytic properties, positioning CeO 2 –Mn 3 O 4 as a promising candidate for an integrated energy storage system.