Electrochromic Effect in Titanium Carbide MXene Thin Films Produced by Dip-Coating

MXenes, a large family of 2D transition metal carbides and nitrides, have shown potential in energy storage and optoelectronic applications. Here, the optoelectronic and pseudocapacitive properties of titanium carbide (Ti ₃ C ₂ T _x ) are combined to create a MXene electrochromic device, with a visible absorption peak shift from 770 to 670 nm and a 12% reversible change in transmittance with a switching rate of <1 s when cycled in an acidic electrolyte under applied potentials of less than 1 V. By probing the electrochromic effect in different electrolytes, it is shown that acidic electrolytes (H ₃ PO ₄ and H ₂ SO ₄ ) lead to larger absorption peak shifts and a higher change of transmittance than the neutral electrolyte (MgSO ₄ ) (Δλ is 100 nm vs 35 nm and ΔT _{770 nm} is ≈12% vs ≈3%, respectively), hinting at the surface redox mechanism involved. Further investigation of the mechanism by in situ X-ray diffraction and Raman spectroscopy reveals that the reversible shift of the absorption peak is attributed to protonation/deprotonation of oxide-like surface functionalities. As a proof of concept, it is shown that Ti ₃ C ₂ T _x MXene, dip-coated on a glass substrate, functions as both transparent conductive coating and active material in an electrochromic device, opening avenues for further research into optoelectronic and photonic applications of MXenes.