Bidirectional activation technology towards foam-like carbon nanosheets and its coupling with oxygen-deficient α‐MnO2 for ammonium-ion hybrid supercapacitors

Background Aqueous non-metallic ammonium ions (NH 4 + ) have newly been developed as a promising charge carrier for electrochemical energy storage owing to their high safety, abundance, and tiny hydrated ionic size. Producing reliable electrode materials with excellent electrochemical performance, however, remains a significant issue. Methods we proposed a bidirectional chemical activation strategy, which effective coupling “in to out” and “out to in” etching manner, to synthesize foam-like porous carbon nanosheets (FCNSs) with the surface area much larger than that samples obtained from the unidirectional mode. Meanwhile, oxygen-deficient α‐MnO 2 nanorods can be rationally obtained through mild redox reaction, both of which display excellent capacitive performance in dilute (NH 4 ) 2 SO 4 electrolyte (0.5 M). Significant findings the as-synthesized FCNSs and oxygen-deficient α‐MnO 2 nanorods can deliver a specific capacitance of 257.5 F g −1 and 525.1 F g −1 , respectively, demonstrating their huge potential to assemble ammonium-ion hybrid supercapacitor (A-HSC). Benefitting from the feasible electrochemical performance for both anode and cathode, the resulting A-HSC displays a specific capacitance of 180.8 F g −1 (calculated based on both anode and cathode) within 0–1.6 V, and a low self-discharge rate, which outperforms the state-of-the-art A-HSC devices. Importantly, the energy storage mechanism has been elucidated from several ex-situ characterizations.