Extended Data Fig. 3: Interfacial capacitance of Fe–0.45 wt% C measured by electrochemical impedance spectroscopy as a function of temperature and anodic polarization.

a, Interfacial capacitance at the rest potential strongly increases with temperature which is consistent with pseudo-capacitance caused by adsorbed species on the surface of the electrodes. Double-layer capacitance should hardly change over a small interval of 100 °C. On the other hand, specific adsorption, which causes pseudo-capacitance, requires activation energy and explains the strong effect of temperature. b, Differential capacitance curves of Fe–0.45 wt% C at different temperatures show capacitance values diminish to low values on anodic polarization. As pseudo-capacitance caused by adsorbed intermediates depends on their surface coverage, the results are consistent with the proposed mechanism involving surface coverage of an intermediate approaching zero.