Nitrogen and sulfur co-doped MXene ink without additive for high-performance inkjet-printing micro-supercapacitors

Although the evolution of inkjet printing in the extensible and configurable manufacturing of micro-supercapacitors (MSCs) is rapid, the performance of eco-friendly inks based on free additives need be improved. Meanwhile, MXenes, which are transition metal carbides and nitrides in two dimensions (2D), have received extensive attention due to their metallic conductivity, abundant functional groups, and extraordinary performance in applications such as energy storage. However, the pristine MXene (P-MXene) has various polar terminal groups and vacancy defects, resulting in poor environmental stability. Herein, the printable ink fabricated by doping nitrogen and sulfur on Ti₃C₂T_x MXene (N,S-MXene) is developed to improve the ambient stability and electrochemical properties; this ink is used to direct inkjet printing of the planar MSCs with an electrode spacing of 300 μm. These MSCs provide excellent performance, desirable customization, and seamless connection, delivering an extraordinary volumetric capacitance of 710 F cm⁻³, a remarkable energy density of 8.9 mWh cm⁻³ at a power density of 411 mW cm⁻³, and a long-term cycling stability up to 94.6 %, much better than previously reported MSCs fabricated by inkjet printing. These excellent properties are attributed to doping nitrogen and sulfur atoms that improve the redox reactivity, H⁺ adsorption, reaction kinetics, and antioxidation throughout the microelectrodes, depending on the results of DFT calculation and electrochemical tests. Therefore, this strategy makes the practical applications of MXenes more competitive, creating more possibilities for printed micro-electronics.