Investigating the NH<sub>4</sub><sup>+</sup> Preintercalation\nand Surface Coordination Effects on MnO<sub>2</sub> for Ammonium-Ion\nSupercapacitors

Ion\npreintercalation is an effective method for fine-tuning the\nelectrochemical characteristics of electrode materials, thereby enhancing\nthe performance of aqueous ammonium-ion hybrid supercapacitors (A-HSCs).\nHowever, much of the current research on ion preintercalation lacks\ncontrollability, and the underlying mechanisms remain unclear. In\nthis study, we employ a two-step electrochemical activation approach,\ninvolving galvanostatic charge–discharge and cyclic voltammetry,\nto modulate the preintercalation of NH<sub>4</sub><sup>+</sup> in\nMnO<sub>2</sub>. An in-depth analysis of the electrochemical activation\nmechanism is presented. This two-step electrochemical activation approach\nendows the final MnO<sub>2</sub>/AC electrode with a high capacitance\nof 917.4 F g<sup>–1</sup>, approximately 2.4 times higher than\nthat of original MnO<sub>2</sub>. Furthermore, the MnO<sub>2</sub>/AC electrode retains approximately 93.4% of its capacitance after\n10 000 cycles at a current density of 25 mA cm<sup>–2</sup>. Additionally, aqueous A-HSC, comprising MnO<sub>2</sub>/AC and\nP-MoO<sub>3</sub>, achieves a maximum energy density of 87.6 Wh kg<sup>–1</sup>. This study offers novel insights into the controllable\nion preintercalation approach via electrochemical activation.