Flexible supercapacitors are demanded for energy storage of wearable electronics. In this paper, a simple strategy for preparing flexible carbon fibers (CFs) with good energy storage capacity using a mixed acid treatment process is reported. When the volume ratio of concentrated sulfuric acid to concentrated nitric acid is 3:1, the carbon fiber electrodes have the best electrochemical performance with a high capacitance of 27.83 F g⁻¹ at 15 mA g⁻¹ and extremely high capacitance retention of 79.9% after 500 cycles at 100 mA g⁻¹. Furthermore, their energy density can reach 3.86 Wh kg⁻¹ with a power density of 7.5 W kg⁻¹. Such an excellent electrochemical performance of carbon fiber electrodes is attributed to their surface rich oxygen-containing functional groups, rough surface, and a certain number of graphene quantum dots (GQDs). Importantly, the all-solid-state flexible supercapacitor performs excellent bending stability performance with a capacitance retention of almost 100% after 500 times of bending at 180°, showing good prospects and applications in the field of flexible energy storage devices.

Electrochemical water splitting for hydrogen production is a key technological route toward the large-scale generation of green hydrogen. However, the anodic oxygen evolution reaction (OER) suffers from sluggish kinetics and high overpotential, necessitating the development of non-noble metal catalysts that simultaneously possess low cost, high activity, and excellent stability. In this work, a nitrogen-doped graphene quantum dots@nickel–iron layered double hydroxide (N-GQDs@NiFe-LDH) composite catalyst was in situ constructed via a facile hydrothermal strategy. Benefiting from the electronic modulation and structural confinement effects of N-GQDs, the intrinsic catalytic activity and structural stability of the catalyst were simultaneously enhanced. The as-prepared catalyst requires an overpotential of only 320 mV to deliver a current density of 500 mA cm⁻² and maintains 98% of its initial activity after 100 h of chronoamperometric stability testing, demonstrating promising potential for practical applications. Multiscale characterizations revealed that N-GQDs formed strong electronic interactions with Ni/Fe active sites at the interface, significantly enhanced interfacial electron transport, and accelerated the OER kinetics. This study demonstrates that the N-GQDs@NiFe-LDH catalytic system constructed via an interfacial heterostructure engineering strategy provides a new insight for the rational design and development of efficient non-noble-metal OER electrocatalysts.