Greener and scalable MXene fabrication enabled by supercritical CO2: a mini-review

Abstract

Supercritical CO₂ (scCO₂) has a gas-like diffusivity and a liquid-like density, which speeds up transport and allows for tunable termination chemistry during MXene synthesis. This mini review compares scCO₂-assisted routes to HF and LiF–HCl systems in terms of time to conversion, monolayer yield, oxidation stability, and readiness for scale-up. Optimized scCO₂ protocols show a conversion time of about 0.5 to 3 hours (compared to 12 to 72 hours), a mono/few-layer yield of more than 60%, and better air stability, with kilogram-scale batches presented. We evaluate the effects of pressure, temperature, water activity, and co-solvent polarity, and we demonstrate how RESS blow-down and in situ spectroscopy can speed up the process. Application snapshots (supercapacitors, Li/Na-ion batteries, HER, EMI) demonstrate consistent improvements due to termination control and diminished restacking. There are still route-specific limits, comprising narrow p–T windows, batch-to-batch termination scatter, and compression/heat duties that set techno-economics. We suggest a reporting checklist that includes PMI, E-factor, specific energy, CO₂ intensity, and water use. We also describe hybrid scCO₂-electrochemical/microwave pathways and closed-loop engineering that are needed to connect the promise of lab-scale research with real-world use. When looked at with clear, metrics-based criteria, scCO₂ is a safer, greener, and more scalable way to make MXene.