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 integration of batteries and supercapacitors at microscales 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 (Ti3C2Tx) solid-state interdigital microsupercapacitors by employing a solution spray-coating, followed by a photoresist-free direct laser cutting method. Our prototype devices consisted of two layers of Ti3C2Tx with two different flake sizes. The bottom layer was stacked large-size MXene flakes (typical 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 Ti3C2Tx micro-supercapacitors with platinum current collectors, the all-MXene devices exhibited much lower contact resistance, higher capacitances and better rate-capabilities. The areal and volumetric capacitances of ~27 mF cm-2 and ~337 F cm-3, respectively, at a scan rate of 20 mV s-1 were achieved. The devices also demonstrated their excellent cyclic stability, with 100% capacitance retention after 10,000 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.