LAUSR

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 (Ti3C2Tx) 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 (H3PO4 and H2SO4) lead to larger absorption peak shifts and a higher change of transmittance than the neutral electrolyte (MgSO4) (Δλ is 100 nm vs 35 nm and ΔT770 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 Ti3C2Tx 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.