Oxygen-deficient Fe3O4-x/Fe hybrid nanoparticles anchored on defect-rich carbon nanotubes for high-performance supercapacitors and alkaline hydrogen evolution reaction

Developing cost-effective multifunctional electrode materials for both energy storage and hydrogen production remains a key challenge for sustainable energy technologies. In this work, Fe 3 O 4-x /Fe hybrid nanoparticles were anchored on defect-rich carbon nanotubes via a hydrothermal method (denoted as Fe10-CNT). As a supercapacitor electrode, Fe10-CNT delivers a high specific capacitance of ∼474 F g −1 and an energy density of ∼16 Wh kg −1 at 1 A g −1 , while retaining ∼72.5% of its specific capacitance (∼343 F g −1 ) and energy density (∼12 Wh kg −1 ) at a high current density of 10 A g −1 , demonstrating excellent rate capability in a three-electrode system. Moreover, Fe10-CNT exhibits good stability with ∼93% capacitance retention after 5000 charge-discharge cycles. For the alkaline hydrogen evolution reaction, Fe10-CNT requires an overpotential of 170 mV at 10 mA cm −2 and a Tafel slope of 165 mV dec −1 , compared with 290 mV and 346 mV dec −1 for activated CNT (A-CNT), respectively. Notably, 2000 electrochemical redox cycles induce self-activation of the catalyst, with the overpotential decreasing to 77 mV and the Tafel slope decreasing to 40 mV dec −1 . Comprehensive microscopy and spectroscopy analyses reveal that the as-prepared Fe10-CNT possesses an Fe 3 O 4-x /Fe hybrid structure with reduced oxygen coordination around the Fe sites, while post-activation X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) analyses demonstrate a dynamic electronic and local structural reconstruction toward a metallic-Fe-enriched surface, accompanied by further reduction in oxygen coordination around the Fe sites and increased electron density during electrochemical operation. We envision that these findings will provide a solid foundation for the rational design of multifunctional electrode materials through controlled electronic and local structural engineering, enabling next-generation sustainable energy storage and hydrogen production technologies.