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 )₂ SO₄ 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.