Standalone Photo–Rechargeable Capacitor Integrating DSSCs with a Bi//NiCo2S4 Asymmetric Supercapacitor for High–Efficiency Indoor and Outdoor Energy Storage

The integration of photovoltaics with electrochemical energy storage (EES) into monolithic photo–rechargeable systems represents a promising approach for sustainable, off–grid power sources, particularly in the context of the Internet of Things (IoT). Herein, we report a high–performance photo–capacitor (PC) constructed by coupling two series–connected dye–sensitized solar cells (DSSCs) with an asymmetric Bi//NiCo2S4 supercapacitor in a three–electrode configuration. Nickel cobalt sulfide (NiCo2S4) serves as a dual–function common electrode, providing both excellent electrocatalytic activity toward the I3-/I- redox reaction and high supercapacitive performance, while bismuth (Bi) is employed as a storage electrode, offering favorable energy alignment and wide operational potential. The optimized PC achieves a photovoltage exceeding 1.2 V under one–sun illumination and 0.9 V under 1000 lux LED illumination, with an areal capacitance of 170.4 mF cm-2 and an overall energy–storage efficiency of 7.05% under low–intensity solar irradiation (30 mW cm-2). The device demonstrates excellent rate capability, long–term cycling stability, and superior performance under dim–light conditions, confirming its potential as a standalone power supply for both indoor and outdoor IoT applications. This work highlights a strategic combination of band alignment, electrode design, and three–terminal architecture to realize efficient, standalone solar energy storage.