A stable aqueous ammonium ion hybrid supercapacitor based on pre-intercalated MnOx electrodes and ammonium sulphate electrolyte

Ammonium ions (NH4+) as charge carriers show potential both in aqueous ammonium ion capacitors and rechargeable batteries. Herein, we report the energy storage process of an aqueous ammonium ion capacitor (AAIC) consisting of an electrodeposited MnO2 and activated carbon as electrodes. The AAIC utilizes NH4(SO4)2 as electrolytes and the electrochemical studies revealed the phase transformation of MnO2 electrodes during the charge-discharge process. The transformed MnOx electrode delivered a maximum capacitance of ∼175 F g−1 at 1 A g−1 and delivered ∼89% of capacitance retention after 1000 continuous charge-discharge cycles. Ex-situ studies proved that NH4(SO4)2 intercalation/deintercalation resulted in the phase changes of MnO2 during the reversible charge-discharge process. The fabricated AAIC delivered an energy density of 15.93 Wh kg−1 with 90% cycling stability after 5000 charge-discharge cycles. Hence, these findings provide a more fundamental understanding of the charge storage mechanism of AAIC using NH4(SO4)2 electrolyte.