Disentangling Catalytic Activity and Film Conductivity Reveals Temporally Decoupled Catalyst Evolution and...

Catalytic activity and electrical conductivity are critical to the performance of electrocatalysts and electrochemical devices, yet their combined assessment under operating conditions remains challenging. Here, we introduce operando Fourier-transform electrochemical source-drain impedance spectroscopy (FT-ESDIS), which simultaneously measures catalyst-film conductivity and charge-transfer resistance (Rct) with high temporal resolution at high current densities. During Fe incorporation into nickel (oxy)hydroxide (NiOxHy), FT-ESDIS revealed that catalytic activation and charge transport properties evolve on distinct time scales, demonstrating that activity alone cannot capture catalyst-layer evolution during operation. At higher overpotentials, nickel-containing (oxy)hydroxides (Ni(Fe)OxHy) showed an abrupt apparent conductivity increase upon oxidation, with conductivity remaining nearly unchanged thereafter, whereas iron (oxy)hydroxide (FeOxHy) exhibited a gradual increase in apparent conductivity. Previous reports attributed this overpotential-dependent increase to the formation of high-valent Fe species. However, we show that the increase arises predominantly from an electrolyte-mediated current artifact, in which a potential perturbation at one electrode changes the local solution potential at the neighboring electrode surface and modulates its Faradaic current, producing an apparent conductivity response without direct through-film transport. We further establish a quantitative relationship between this artifact and Rct, enabling its estimation and subtraction from the apparent conductivity. The corrected FeOxHy conductivity exhibits substantially weaker potential dependence, suggesting that charge transport involving high-valent Fe species, if present, contributes much less than inferred from the uncorrected response. By resolving kinetic and transport properties under operating conditions, FT-ESDIS provides a more comprehensive basis for interpreting electrocatalyst behavior and establishes transport and reactivity as complementary descriptors for electrochemical energy-conversion systems.