All-MXene 3D Aerosol-Jet Printed Microsupercapacitors

Abstract

The Internet of Things (IoT) promises users a wider gamut of features and enhanced ease-of-use for devices in both professional and domestic settings. For such devices to have a future however, they will require compact energy storage, especially considering the widespread abandonment of cables in electronic devices for both power and data transfer. One possible answer to this requirement are micro-supercapacitors (MSCs), specifically those based on the MXene Ti3C2Tx, which has been well established as a material for supercapacitors owing to its high capacitance, cycling stability and conductivity.1 Specifically, the creation of Ti3C2Tx supercapacitors via printing techniques such that energy storage can be directly incorporated into the structure of electronics is very appealing and an area of much recent study.2,3 A plurality of these printing techniques have produced planar supercapacitors consisting of a dense MXene film which, while highly conductive, has a limited surface area for intercalation and ion transport.4 These planar devices also have intrinsically inferior space efficiency in real-world electronics. To address this issue, very recent studies have demonstrated 3D printing of nanomaterial-based supercapacitors, however these have been achieved using composites5 or post-processing steps such as freeze-drying.6 In this work, we show that aerosol-jet printing (AJP) of aqueous Ti3C2Tx inks can be used to directly fabricate high aspect ratio 3D structures up to 1 mm in height and with feature sizes as small as 20 µm. These can be realised without the aid of any supporting structures and in a wide range of geometries such as walls, pillars and pyramids. No post-processing is necessary, as only aqueous dispersions are used, without the need for additives, binders or additional drying steps. When such 3D features are integrated into symmetric interdigitated MSCs with gel electrolyte, the 3D structured electrodes exhibit 29% higher areal capacitance than comparable, mass-equivalent planar electrodes, up to 33 mF/cm2 at 10 mV/s.Figure 1 shows the comparison between 2D (top row) and 3D (bottom row) Ti3C2Tx MSCs of equivalent mass. Cyclic voltammograms show a 29% increase in areal capacitance for the 3D device, 33 mF/cm2 at 10 mV/s. The difference in geometry between the two devices is evident in the low and high magnification SEM images, of particular note are the uniform 3D Ti3C2Tx pillars with height and diameter of approximately 300 µm and 20 µm, respectively.