Supersonically sprayed Sr-Fe₃O₄/Fe₃C textile electrodes as flexible supercapacitors

Flexible supercapacitors require electrode materials that simultaneously provide high conductivities, abundant electrochemically active sites, and rapid ion transport pathways. In this study, we developed hybrid Sr-Fe₃O₄/Fe₃C/reduced graphene oxide (rGO) electrodes supported on conductive carbonized cotton fabric via a combination of solvothermal synthesis and supersonic cold-spray deposition, followed by controlled annealing. The cold-spray process facilitated the uniform, binder-free deposition of rGO-enwrapped Sr-Fe precursor particles onto temperature-sensitive cotton textiles without structural damage. Subsequent carbonization induced a phase evolution toward a mixed Fe₃O₄/Fe₃C morphology that was strongly coupled with the graphitized carbon matrix. Systematic annealing between 750 and 825 °C revealed that the microstructure, crystallinity, and phase distribution were temperature-dependent. The optimal SF775 electrode was uniformly coated with Sr-Fe₃O₄/Fe₃C and rGO on a carbonized cotton fibrous network. The SF775 electrode delivered a maximum areal capacitance of 1050 mF cm^-2 at 5 mA cm^-2, along with an energy density of 145.8 µWh cm^-2 at a power density of 10 mW cm^-2. Furthermore, the device exhibited excellent mechanical flexibility and durability, retaining its performance after 1000 bending cycles. These results indicate that high-performance textile-based Sr-Fe₃O₄/Fe₃C electrodes produced via cold-spray deposition and thermal phase evolution are promising candidates for wearable supercapacitors.