Quantifying Chemical Capacitance and Diffusion Coefficients in Hydrogenated La _0.6 Sr _0.4 FeO _{3– <i>δ</i>} <i>via</i> <i>Operando</i> Spectro-Electrochemical Characterizations

Hydrogenation in redox-active oxides is widely used to modulate electronic structure and, in turn, optical, magnetic, and transport properties in electrochromic, energy-storage, and ionotronic devices. Quantitatively resolving hydrogenation is therefore critical for rational device design. However, a general <i>operando</i> framework that directly quantifies potential-dependent proton concentration while decoupling ionic and electronic transport and linking both to defect thermodynamics remains limited. Herein, using La_0.6Sr_0.4FeO_3-<i>δ</i> thin films as a model system, we develop an integrated methodology that combines <i>operando</i> spectro-electrochemistry, electrochemical impedance spectroscopy, and defect-chemistry analysis to interrogate hydrogenation across applied potentials. The <i>operando</i> spectro-electrochemical signal, calibrated by coulometric titration, maps absorbance changes to proton concentration, enabling direct determination of its potential dependence. Impedance spectroscopy separates ionic and electronic contributions, yielding potential-dependent chemical capacitance and ionic conductivity. Defect-chemistry analysis further supports the reliability of the observed potential-dependent trends. We believe this approach is readily transferable to other redox-active oxides.