DOI: 10.1002/celc.201402295 Experimental Aspects in Benchmarking of the Electrocatalytic Activity Viktor C ˇ olic ´, [a, b] Jakub Tymoczko, [a] Artjom Maljusch, [a] Alberto Ganassin, [a] Wolfgang Schuhmann, [a] and Aliaksandr S. Bandarenka* [a, b] 1. Introduction The development of efficient electrocatalysts is a progressively growing field of electrochemistry and material science. [1–4] The importance of this field is increasing due to the demand for new-generation energy conversion and storage devices, such as fuel cells and electrolyzers. [5–7] A number of promising catalytic materials for the key reac- tions taking place in these electrochemical devices, namely the oxygen reduction reaction (ORR), the oxygen evolution reac- tion (OER), the hydrogen oxidation reaction (HOR) and the hy- drogen evolution reaction (HER), have been identified. Howev- er, due to the scarcity of most catalytic materials, for the wider application of these key reactions, further optimization of elec- trocatalysts and the development of novel electrocatalysts using more abundant materials are essential. [8–10] Identification and optimization of electrocatalysts require very careful and accurate evaluation of their activity. [11] This is particularly important to compare the activity of newly devel- oped materials with the state-of-the-art electrocatalysts. How- ever, with a growing number of reports, it is often difficult to perform such a comparison as unfortunately, there is a lack of widely accepted protocols for precise catalytic activity meas- urements. The situation can become even more complicated due to the fact that modern equipment commonly used in electrochemistry, although being very powerful and robust, is still not ideal, and the measurement results can be affected by certain hardware-related issues. In this work, we consider three experimental aspects impor- tant for the benchmarking of electrocatalytic activity. They are unfortunately largely ignored in modern experimental electro- catalysis. First, we demonstrate that careful determination of the uncompensated resistance in order to correct the activity data for the iR drop using electrochemical impedance spec- troscopy (EIS) can be rather demanding. Further, using electro- deposited cobalt oxide films, we demonstrate how the uncom- pensated resistance changes with the electrode potential if a non-conducting gas phase is formed in the system, hence complicating the activity assessment. Finally, it is also shown that the correct choice for surface-limited reactions for the de- termination of the real surface area of catalytic electrodes plays a key role in ensuring more meaningful activity assess- ment based on electrochemical data collected under different conditions. 2. Results and Discussion 2.1. Determination of Uncompensated Resistance using Electrochemical Impedance Spectroscopy The uncompensated ohmic resistance emerging in an electro- chemical system is typically a sum of resistances on the path of the current to the working electrode. The origin of this ohmic drop can be various. However, in general, it can be di- With the high interest in improving the performance of elec- trocatalysts for technologically significant reactions, great ef- forts are directed at the assessment of the activities of various catalytic materials. For this purpose, it is important to compare the catalytic activities measured using different methods and under different conditions. To achieve this, it is of utmost im- portance to avoid certain methodological and instrumental issues that can severely affect the obtained experimental re- sults. Using well-defined systems, we demonstrate the impor- tance of experimental conditions in the assessment and bench- marking of the activity of catalytic processes for various reac- tions. Particularly, we demonstrate that the correction of the uncompensated ohmic resistance using impedance spectrosco- py measurements requires particular attention and additional procedures which are normally ignored. Additionally, we dem- onstrate how the uncompensated resistance changes with the potential if a non-conducting gas phase is accumulated in the system, hence influencing the activity measurement. It is fur- ther shown that a correct choice for surface-limited reactions for the determination of the real surface area of catalytic elec- trodes plays a key role in ensuring more meaningful activity assessment. [a] V. C ˇ olic ´, J. Tymoczko, Dr. A. Maljusch, A. Ganassin, Prof. W. Schuhmann, Prof. A. S. Bandarenka Analytical Chemistry: Center for Electrochemical Sciences (CES) Ruhr-Universität Bochum, Universitätsstr. 150 44780 Bochum (Germany) [b] V. C ˇ olic ´, Prof. A. S. Bandarenka Energy Conversion and Storage (ECS), Fachbereich Physik Technische Universität München, James-Franck-Straße 1 85748 Garching (Germany) E-mail : bandarenka@ph.tum.de # 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemElectroChem 0000, 00,1–8 &1& These are not the final page numbers! ÞÞ CHEMELECTROCHEM ARTICLES