Hydrogen Bonding at MXene/Electrolyte Interface Enables Stable Ammonium Ion Energy Storage

Abstract

Aqueous ammonium ion (NH₄⁺) hybrid supercapacitors (AAHSCs) have attracted much attention due to their environmental friendliness and excellent electrochemical performance, but the mechanism of NH₄⁺ energy storage of AAHSCs has been unknown. In this work, Ti₃C₂T_x films were prepared by vacuum filtration and used as cathode materials for AAHSC. The charge storage mechanism of Ti₃C₂T_x films in different electrolyte solutions was systematically analyzed by means of ex-situ techniques and theoretical calculations. These analyses revealed an H–bonding (N–H…O) interaction between NH₄⁺ and oxygen-containing functional groups, elucidating the reason for the excellent electrochemical performance of AAHSCs. DFT and MD calculations further verified the existence of H–bonds, which act as a channel for charge transfer and facilitate the transfer of electrons from NH₄⁺ to O–atoms. The Ti₃C₂T_x film//AC-AAHSC assembled with 1 M (NH₄)₂SO₄ as the electrolyte solution has a high specific capacitance of 84.3 F/g at a current density of 1 A/g and excellent cycling stability of 96.2% (10,000 cycles). It has a high energy density of 108 Wh/kg at a power density of 2,160 W/kg. This work lays the theoretical foundation for the construction of high-performance MXene-based AAHSCs devices.