Abstract: Electrochemical reactivity is driven by the composition and structure of electrode surfaces. Monitoring surface chemistry in operando is thus crucial to understanding the behavior of electrodes yet is often inaccessible either due to resource limitations or to technical challenges in replicating realistic reaction environments. The invention presents a color impedance spectroscopy (CIS)-based technique to access operando surface measurements for mixed ionic-electronic conductors (MIECs). The CIS technique tunes the depth of charge carriers' movement within an electrode material by modulating the frequency of an applied AC electrochemical signal and monitors these changes spectroscopically. The results enable surface sensitivity in conventional bulk spectroscopies and provide new opportunities to characterize the operational behavior of MIEC electrodes.