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Designing and modifying nanoporous metal foils to make them suitable for supercapacitor and catalysis is significant but challenging. In this work, Cu_xO nanoflakes have been successfully in situ grown on nanoporous Cu foil via a facile electrooxidation method. A Ga-assisted surface Ga-Cu alloying–dealloying is adopted to realize the formation of a nanoporous Cu layer on the flexible Cu foil. The following electrooxidation, at a constant potential, modifies the nanoporous Cu layer with Cu_xO nanoflakes. The optimum Cu_xO/Cu electrode (O-Cu-2h) delivers the maximum areal capacitance of 0.745 F cm⁻² (410.27 F g⁻¹) at 0.2 mA cm⁻² and maintains 94.71% of the capacitance after 12,000 cycles. The supercapacitor consisted of the O-Cu-2h as the positive electrode and activated carbon as the negative electrode has an energy density of 24.20 Wh kg⁻¹ and power density of 0.65 kW kg⁻¹. The potential of using the electrode as oxygen evolution reaction catalysts is also investigated. The overpotential of O-Cu-2h at 10 mA cm⁻² is 394 mV; however, the long-term stability still needs further improvement.