Modulating surface electron density of hydrophilic/high-conductive MXene/Ni(OH)₂/NF heterostructures for efficient asymmetric supercapacitors

To address the growing demand for efficient and stable energy storage in the context of green energy, in this study, an acid etching method was employed to in-situ grow Ni(OH)₂ nanosheet arrays on a nickel foam (NF) substrate. Subsequently, negatively charged Ti₃C₂T_x MXene nanosheets were electrophoretically deposited onto the Ni(OH)₂/NF surface to result in a MXene/Ni(OH)₂/NF composite electrode material. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations revealed the homogeneous dispersion distribution of nanosheets on the material surface. The composite material exhibited high electrical conductivity and good hydrophilicity by rapid water infiltration within 150 ms. Furthermore, the Motte-Schottky effect establishes a distinctive pathway for electron transfer, which leads to the creation of electron vacancies or ‘holes’ on the MXene surface. This phenomenon significantly boosts MXene's electrochemical activity and enhances its affinity for OH⁻ ions in the electrolyte, which consequently improves the performance of electrochemical capacitors. Electrochemical tests verified that the prepared MXene/Ni(OH)₂/NF electrode displayed an areal specific capacitance of 0.223 mAh cm⁻² at a current density of 1 mA cm⁻². The electrode displayed excellent rate performance with a capacity retention rate of 71 % when the current density was increased by five times. In addition, the assembled asymmetric capacitor with MXene/Ni(OH)₂/NF electrode demonstrated an excellent cycling stability with only a 16.38 % capacity loss after 5000 cycles. It also exhibited the remarkable specific capacitance and long-term stability by achieving a high energy density of 43.68 Wh kg⁻¹ and a power density of 424.96 W kg⁻¹. These results highlight the electrode's exceptional performance in terms of energy storage and power delivery.