How to Perform Reliable IR Compensation in Electrocatalysis?

Abstract

iR compensation is one of the most used techniques in electrochemical analysis, which is designed to correct the voltage loss between the working electrode and the reference electrode in the electrolyte (Rsolution).[1] It is conducted using two methods frequently, namely the after-the-scan and the on-the-fly methods. A factor is often engaged when performing the iR compensation, which is an empirical value (e.g., 80%~100%) chosen by researchers. After the compensation, the experimental data (mainly electrocatalytic indicators like overpotential and Tafel slope) can be greatly “improved”.[2] However, the abuse of iR compensation, especially in electrocatalysis studies, can generate misleading and exaggerated results.In this study, we present a detailed analysis of the applicability of the iR compensation technique in current electrocatalysis studies by revisiting the technical assumptions.[3] A few considerations are discussed. Firstly, it is pointed out that the resistance obtained from the electrochemical impedance spectroscopy is not always the resistance that needs to be compensated. The main reason is the involvement of the catalyst material resistance (Rcatalyst) and the contact resistance between the substrate and the catalyst material (Rcontact) (Figure 1). Depending on the types of activity (intrinsic activity, sample activity, electrode activity, and industrial activity) of interest, one should choose different resistance values. For example, only Rsolution should be compensated when studying the electrode activity, but both Rcontact and Rsolution should be compensated for sample activity analysis. Secondly, the resistance may be potential/time-dependent, meaning the value should be measured at the reaction condition, other than the often-used open circuit potential. Thirdly, the distribution of resistance on the catalyst surface sites is uneven, suggesting that one single value cannot be used for sites at different surface traits. Fourthly, most researchers use linear sweep voltammetry (LSV) where the current is always contributed by both the Faradic current and non-Faradic current. However, only the Faradic current should be compensated because non-Faradic current does not flow between the working and reference electrodes.Regarding these considerations and experimental methods, we further suggested a standard workflow to perform iR compensation in electrocatalysis studies.[3] Figure 1. Voltage loss across a three-electrode system with a modified working electrode and the corresponding equivalent circuit diagram. (1) surface of the substrate; (2) surface of the catalyst contacting the substrate; (3) surface of the catalyst contacting the electrolyte solution; (4) outer boundary of the diffuse layer; (5) junction of the reference electrode. Reference: Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons, Inc., 2001 Anantharaj, S.; Noda, S. J. Mater. Chem. A 2022, 10, 17, 9348– 9354 Zheng, W. ACS Energy Lett. 2023, 8, 4, 1952–1958 Figure 1