Indirect laser photothermal writing of high areal capacitance micro-supercapacitors with expanded thick MXene electrodes

Recently, a power source unit designable in the form of an in-plane device architecture has gained tremendous attention due to its practical accessibility toward next-generation miniaturized electronic systems. Among various energy storage units, micro-supercapacitors employing MXene-based electrodes have been recognized as viable candidates owing to their characteristic advantages of long-term cycling stability, high frequency response, and fast discharge/charge rate. In this study, we suggest an indirect green laser irradiation technique using a double-layered plasmonic photothermal heater to overcome the most critical drawback of MXene-based electrodes, the formation of highly stacked assemblies. We have designed a photothermal activation method in which the optical energy irradiated from the green laser source is converted to thermal energy inside the photothermal heater and then transferred efficiently to the composite layer comprising MXene and graphene oxide sheets. The fast laser scanning process at a speed of 400 mm/s enables an instantaneous thermal evolution of stacked MXene/GO layers as thin as 40 μm into unstacked MXene/RGO electrodes as thick as 450 μm. Our study reveals that the degree of expansion is adjustable according to the laser power, which decisively determines electrochemical performance of MXene-based micro-supercapacitors, allowing an areal capacitance as high as 97.4 mF/cm². Based on the comprehensive interpretation on various electrochemical parameters, we clarify that a significant improvement in expanded MXene/RGO electrodes is attributable to the generation of relatively conductive 2D sheets with deep porous channels, which facilitates the energy and power density of 3.3 μWh/cm² and 0.4 mW/cm², respectively.