Exploring the excess free charge distribution under catalytic conditions using a quartz crystal microbalance methodology

Understanding the distribution of surface charge at electrified interfaces is pivotal for advancing energy conversion systems, such as electrolyzers and fuel cells, and energy storage devices, including electric double−layer capacitors, pseudocapacitors and batteries. However, directly measuring surface charge on solid materials remains challenging. This study introduces the electrochemical quartz crystal microbalance (EQCM) as a novel approach to discern and quantify excess free charge on electrode surfaces. The EQCM leverages the oscillation frequency of a quartz crystal as a sensitive indicator of mass changes on a gold electrode. By utilizing the crystal’s frequency response to the electrostatic attraction of excess free charges, the method identifies potential regions where positive and negative excess charges exist. Our findings reveal a potential region where the EQCM detects no frequency change, flanked by regions showing a marked increase in -∆f. Moreover, we demonstrate that the EQCM methodology enables the identification and quantification of excess charge even during the catalytic oxygen reduction reaction (ORR). By comparing the EQCM’s frequency response with capacitance measurements obtained via cyclic voltammetry, we estimate the amounts of excess free charge and Faradaic charge across the entire pH scale. Additionally, we construct excess free charge distribution diagrams for Ar- and O2−saturated solutions by analyzing the correlation between excess free charge and frequency response. These diagrams visually depict charge fluctuations at specific pH levels and applied potentials, both before and during catalysis.