All-MXene (2D titanium carbide) solid-state microsupercapacitors for on-chip energy storage

On-chip energy storage is a rapidly evolving research topic, opening doors for the integration of batteries and supercapacitors at the microscale on rigid and flexible platforms. Recently, a new class of two-dimensional (2D) transition metal carbides and nitrides (so-called MXenes) has shown great promise in electrochemical energy storage applications. Here, we report the fabrication of all-MXene (Ti₃C₂T_x) solid-state interdigital microsupercapacitors by employing a solution spray-coating method, followed by a photoresist-free direct laser cutting method. Our prototype devices consisted of two layers of Ti₃C₂T_x with two different flake sizes. The bottom layer was stacked large-size MXene flakes (lateral dimensions of 3-6 μm) serving mainly as current collectors. The top layer was made of small-size MXene flakes (∼1 μm) with a large number of defects and edges as the electroactive layer responsible for energy storage. Compared to Ti₃C₂T_x micro-supercapacitors with platinum current collectors, the all-MXene devices exhibited a much lower contact resistance, higher capacitances and better rate-capabilities. Areal and volumetric capacitances of ∼27 mF cm^-2 and ∼357 F cm^-3, respectively, at a scan rate of 20 mV s^-1 were achieved. The devices also demonstrated excellent cyclic stability, with 100% capacitance retention after 10000 cycles at a scan rate of 50 mV s^-1. This study opens up a plethora of possible designs for high-performance on-chip devices employing different chemistries, flake sizes and morphologies of MXenes and their heterostructures.