MXenes are attractive for use in a variety of industrial applications; however, scaleup of the material via different etching techniques is not well explored. The synthesis process or route influences the structure, properties, quality, and terminal groups of MXenes, altering properties and performance. Here, we compare the acid-etching and molten salt-etching approaches to MXene synthesis, with an emphasis on structure, properties, and scalability, both at the clay (etched, multilayered) stage and delaminated nanosheets (few to single-layered). We compared electrical conductivity, electromagnetic interference (EMI) shielding, supercapacitor performance, yield, and process scalability: Notably, the conductivity was superior for acid-etched nanosheets at 2200 S/cm to salt-etched nanosheets at 0.66 S/cm. For EMI shielding effectiveness and capacitance, acid-etched nanosheets performed the highest. The yield of the acid-etched nanosheets was about 50% and salt-etched nanosheets was 15%. When the salt-etching process was scaled up to 25g of MAX, the properties remained like the 1g batch, with a lower yield of ~2%. The acid-etched process was scaled-up from 1g to 200g MAX phase, with similar properties and yield. Although the acid-etching process appears superior for most metrics, the molten-salt etching method is still in its infancy and has advantages regarding process safety.
We also demonstrate that molten salt etching can be accomplished electrothermally, radically reducing material synthesis time. We utilized Joule heating with molten salt etching precursors to etch MAX phase into multilayered Ti₃C₂T_Z MXenes. This new out-of-oven Ti₃C₂T_Z synthesis decreased etching time by 98% from molten salt etching and reduced energy usage from 1.56 x 10⁷ J/g to 1.01 x 10⁶ J/g per batch, paving the way for commercialization of MXene etching.

Yufan Zhang

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