Determination of the Electroactive Surface Area of Supported Ir-Based Oxygen Evolution Catalysts by Impedance Spectroscopy: Observed Anomalies with Respect to Catalyst Loading

Abstract

Recently, several successive papers have proposed a method for determining the actual electrochemically active surface area (ECSA). The method is based on measurements of the impedance response within a suitable potential window and the use of a simple equivalent circuit for the extraction of the so-called adsorption capacity, which is related to the phenomenon of specific adsorption of intermediates in the electrochemical reaction of interest, in particular the oxygen evolution reaction (OER). In this paper, the applicability of the proposed method to the characterization of Ir-based nanostructured electrocatalysts is investigated in more detail. As a model catalyst, we use a commercial sample of Ir nanoparticles dispersed over a high surface area carbon support (Vulcan XC-72). We test the methodology-predicted linear scaling of adsorption capacity with increasing amount (and hence surface area) of catalyst. Systematic experiments are performed with different loadings of the same catalyst. The analysis shows significant deviations from the scaling predicted by the model. To further verify this unexpected result, we extend the analysis to the other model parameters, namely the so-called charge transfer and adsorption resistances and the empirically added double layer capacitance. Interestingly, the analysis shows scaling anomalies only for the first two parameters, which, like the adsorption capacitance, are related to catalytically active sites, but not for the double layer capacitance, which measures the total conductive surface of the studied material. Based on the results, we propose possible reasons for the anomalies and confirm the main hypotheses by impedance simulations using the full physical model - instead of the simplified equivalent circuit.