Rational Design of Dual-Heterojunction ZnO Nanorod/MnO ₂ Nanoneedle/Graphitic Carbon Nitride Nanosheet Composites for Photorechargeable Supercapacitors

Photorechargeable supercapacitors are emerging as next-generation self-powered systems that integrate light harvesting and electrochemical energy storage in a single electrode configuration. Despite their promise, challenges such as inefficient utilization of photoexcited charge carriers and sluggish charge transfer dynamics have limited their performance. Herein, a photorechargeable supercapacitor is engineered with in situ hydrothermal growth of ZnO nanorods as the photoactive layer and MnO 2 /g-C 3 N 4 as the energy storage layer, assembled through a simple drop-casting technique. The hierarchical electrode configuration establishes dual type II and S-scheme heterojunction pathways, where g-C 3 N 4 /ZnO promotes directional electron transfer to FTO, while MnO 2 /g-C 3 N 4 preserves charge carriers, enhancing the redox activity. As a result, the fabricated device exhibited a remarkable 764.8% increase in areal capacitance under light illumination, with 140.2% capacitance retention after 80 cycles under light and 74.8% after 21,000 cycles in the dark at 80 μA/cm 2 . The device also delivers a maximum areal energy density of 0.131 μWh/cm 2 under illumination, nearly 10 times higher than in dark conditions. For practical application, the four cells are connected in series and photocharged under real sunlight, which delivered a photovoltage of 1.63 V. This study demonstrates that the synergistic interfacial design and light-induced charge carrier dynamics significantly boost the photorechargeable supercapacitor performance, pushing the technology toward practical application and a sustainable alternative to conventional charging-dependent energy storage systems.