High‐Energy Aqueous Ammonium‐Ion Hybrid Supercapacitors

Abstract The development of novel electrochemical energy storage devices is a grand challenge. Here, an aqueous ammonium‐ion hybrid supercapacitor (A‐HSC), consisting of a layered δ‐MnO 2 based cathode, an activated carbon cloth anode, and an aqueous (NH 4 ) 2 SO 4 electrolyte is developed. The aqueous A‐HSC demonstrates an ultrahigh areal capacitance of 1550 mF cm −2 with a wide voltage window of 2.0 V. An amenable peak areal energy density (861.2 μWh cm −2 ) and a decent capacitance retention (72.2% after 5000 cycles) are also achieved, surpassing traditional metal‐ion hybrid supercapacitors. Ex situ characterizations reveal that NH 4 + intercalation/deintercalation in the layered δ‐MnO 2 is accompanied by hydrogen bond formation/breaking. This work proposes a new paradigm for electrochemical energy storage.