High‐Energy Aqueous Ammonium‐Ion Hybrid Supercapacitors

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‐MnO2 based cathode, an activated carbon cloth anode, and an aqueous (NH4)2SO4 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 NH4+ intercalation/deintercalation in the layered δ‐MnO2 is accompanied by hydrogen bond formation/breaking. This work proposes a new paradigm for electrochemical energy storage. Aqueous ammonium‐ion hybrid supercapacitors (A‐HSCs) based on a layered δ‐MnO2 cathode, an activated carbon cloth anode, and an aqueous (NH4)2SO4 electrolyte are successfully developed. The A‐HSCs deliver an ultrahigh capacitance, amenable area energy density, and decent durability. This study proposes a new paradigm for electrochemical energy storage.