Multifunctional Copper-Doped Fe _<b>3</b> O _<b>4</b> /rGO Hybrid: Toward Energy Storage, Water Splitting, and Hydrovoltaic Applications

In this study, magnetite (Fe3O4)-based nanostructures were engineered in four different forms: pristine Fe3O4 (F), Cu-substituted Fe3O4 (FC), Fe3O4 incorporated with reduced graphene oxide (FR), and a ternary composite of Cu–Fe3O4 with rGO (FCR). The materials were obtained via a simple coprecipitation route and assessed for their suitability in supercapacitors, electrocatalytic water splitting, and hydrovoltaic energy harvesting. Extensive analyses, including structural, morphological, surface area (BET), chemical (XPS), and thermal studies, confirmed the successful formation and stability of the composites. Electrochemical testing of the symmetric FCR device revealed a specific capacitance of 193 F g–1 with an energy density of 26.79 W h kg–1 at 1 A g–1. The device preserved 89.06% of its initial capacitance and exhibited 91.22% Coulombic efficiency after 8000 charge–discharge cycles, confirming its robust durability. In water-splitting studies, the FCR electrode showed excellent bifunctional activity toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), achieving low overpotentials of 94 mV and 228 mV, respectively. A complete water-splitting system based on FCR required only 1.672 V to drive 10 mA cm–2 in an alkaline medium. Furthermore, hydrovoltaic evaluation demonstrated that FCR could generate an open-circuit voltage of 0.61 V and a short-circuit current of 0.6 μA. Collectively, these findings establish Cu–Fe3O4/rGO as a versatile multifunctional material with significant potential for integrated energy storage and conversion technologies.