Elucidating Mn²⁺/Mn³⁺ and Ni⁰/Ni²⁺ RedoxSynergy in Hair-Derived Carbon-SupportedAg/Ni–MnO_<i>x</i> Supercapacitor

Despite their critical importance, developing sustainable high-performance supercapacitor (SC) electrodes with long-term stability poses significant challenges. Herein, we report a novel ternary composite electrode in which Ag/Ni-doped manganese oxide (Ag/NiO_<i>x</i>@Mn_<i>y</i>O_<i>z</i>) is supported on human hair-derived activated carbon (HHC). This composite is synthesized via a one-pot hydrothermal process followed by thermal annealing at 800 °C, a strategy that creates a conductive Ag/Ni bimetallic network and abundant oxygen vacancies in the NiO_<i>x</i> and Mn_<i>y</i>O_<i>z</i> phases. During operation, operando X-ray absorption spectroscopy (XAS) confirms reversible dual-ion redox transitions (Mn²⁺/Mn³⁺ and Ni⁰/Ni²⁺) in the cathode, highlighting the material’s enhanced redox activity. As a result, HHC-supported Ag/NiO_<i>x</i>@Mn_<i>y</i>O_<i>z</i> exhibits an exceptional specific capacitance (Cs) of 1770 F g^–1 at 5 mV s^–1 in three-electrode tests. When assembled into an asymmetric hybrid supercapacitor (AHSC), the device delivers a high energy density of 37.53 Wh kg^–1 and a power density of 2251.8 W kg^–1 at 3 A g^–1 while retaining ∼82% of its initial capacitance after 5000 charge–discharge cycles. These results confirm the effectiveness of our sustainable HHC-supported Ag/NiO_<i>x</i>@Mn_<i>y</i>O_<i>z</i> framework in addressing the enduring trade-off between energy density, power density, and cycling stability in next-generation SCs.