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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.

Herein a modified floating electrode (MFE) approach for investigating the electrochemical phenomena at a gas/electrode/liquid reaction interface is introduced. Such investigation is in sharp contrast to conventional electrochemical techniques, which measure the properties of electrode/liquid interfaces. MFE is based on an apparatus that enables electrocatalytic conversion under enhanced mass transport of reactant gas. This is enabled by the floating regime of the working electrode that presents a low mass transport barrier for the gas. The present MFE is designed to take the advantage of transmission electron microscopy (TEM) grids with a deposited electrocatalyst of choice, to be used as working electrodes. The applicability of MFE is demonstrated on the example of oxygen reduction reaction (ORR), an essential segment in the sector of electrochemical energy conversion. The approach is validated on two state-of-the-art industrial benchmarks ORR electrocatalysts, a carbon-supported platinum (Pt/C) nanoparticulated electrocatalyst and an alloyed counterpart (Pt-Co/C). It is shown that MFE enables acquisition of the two most vital catalyst features in one measurement sequence. Firstly, it allows for rapid electrochemical performance measurements of potential ORR electrocatalysts under high oxygen transport, specifically high current densities. Secondly, it enables the local characterization of nanostructural events via identical location transmission electron microscopy (IL-TEM).