Estimating charge-transport properties of fuel-cell and electrolyzer catalyst layers via electrochemical impedance spectroscopy

Abstract

Electrochemical impedance spectroscopy is the most common experimental technique for measuring the charge-transport properties of catalyst layers. Such a measurement relies on fitting the impedance spectrum with analytical impedance expressions. To date, a study that examines the suitability of the available analytical models for such analysis does not exist. In this work, a numerical one-dimensional catalyst-layer model is used to assess the appropriateness of the analytical models for estimating catalyst-layer charge-transport properties. An ohmic-heating-based approach to computing ohmic resistance is used to examine the relationship between conductivity, resistance, and impedance of various catalyst layers representative of those used in proton-exchange-membrane fuel cells and water electrolyzers. The ohmic-heating analysis indicates that the most commonly used impedance expressions (Eikerling and Kornyshev, 1999; Makharia et al., 2005) result in a threefold overestimation of protonic resistance and may not be applicable to the proton-conductivity estimation for electrolyzer catalyst layers that exhibit low electronic conductivity. A more recent analytical model (Kulikovsky, 2017) is shown to produce protonic- and electronic-transport properties that agree with numerical simulations. The results of this work are used to provide recommendations for the selection of analytical impedance expressions for a given operating regime (H2/O2 or H2/N2) and an observed shape of the measured spectrum in order to achieve an accurate charge-transport characterization of a catalyst layer. A novel graphical approach to analyzing H2/N2 spectra is also proposed. Although the considered analytical models are not applicable to heterogeneous catalyst layers and no general algebraic conductivity-resistance relationship exists in that case, it is shown that the H2/N2 impedance measurement of heterogeneous catalyst layers provides the total ohmic resistance. A two-dimensional fuel-cell model is used to show that the anode catalyst layer may cause a distortion of the impedance spectrum at frequencies above 5 Hz that obstructs the charge-transport characterization with analytical models.