Recyclable HF-free Ti3C2Tx 3D-printed supercapacitors: their second life in sodium-ion batteries

2D MXenes represent a useful class of materials in various applications and the main constraint for their bulk production is the requirement of hazardous hydrogen fluoride (HF) as an etching agent. Molten salt synthesis is one of the emerging HF-free techniques to produce MXenes, where a mixture of etching salts is heated till their melting point to etch the MAX phase. Here, we etched Ti3AlC2 MAX using the molten salt synthesis method to obtain 2D Ti3C2Tx, by lowering the typical high reaction temperature (similar to 700 degrees C) to 400 degrees C using oxalic acid as an organic additive, which contributes in reducing the overall melting point of the etching salt mixture. Then, the electrochemical properties of Ti3C2Tx were demonstrated by designing recyclable 3D printed supercapacitors using modified polylactic acid (PLA)/conductive graphene 3D electrodes. A real life application of recyclable 3D printed supercapacitors was demonstrated by powering a digital thermometer. Further, the used supercapacitors were recycled to collect the conductive carbon and constructed a sodium-ion battery using it as a conducting additive of the Ti3C2Tx anode and powered up a glucometer. A zero-waste device with the 'concept 3R' (recycle, recover and reuse) reduces the carbon footprint by keeping the materials out of landfills. Concerning environmental safety and e-waste management, this work establishes a green synthesis of Ti3C2Tx and demonstrates the use of recyclable materials in 3D printed devices for energy storage devices.