Online Electrochemical Mass Spectrometry Combined with the Rotating Disk Electrode Method for Direct Observations of Potential-Dependent Molecular Behaviors in the Electrode Surface Vicinity

We newly developed a rotating disk electrode-online electrochemical mass spectrometry (RDE-OLEMS) to investigate potential-dependent molecular behaviors in electrode surface vicinity under mass transport-controlled conditions of reacting molecules. The potential-dependent molecular behaviors were investigated by using a quadrupole mass spectrometer (Q-mass) where the molecules are collected through a gas-sampling tip located in near the electrode surface. For the oxygen reduction reaction (ORR) on the polycrystalline Pt electrode, the potential-dependent Q-mass ion signal intensities of O 2 (m/z = 32) that are ascribable to the dissolved oxygen molecules increased linearly with the disk electrode rotation rates without substantial interference from the collection tip, clearly showing that the dissolved O 2 for ORR can be monitored by the RDE-OLEMS. For electrochemical carbon dioxide reduction (ECR) on the polycrystalline Au electrode, the potential-dependent Q-mass ion signal intensities of CO (m/z = 28) generated by the ECR increased with increasing disk rotation rates from 0 (without disk rotation) to 300 rpm in the potential region from −0.4 to −1.4 V vs. the reversible hydrogen electrode. The results demonstrate that the RDE-OLEMS enables us to evaluate the potential-dependent behaviors of reactant and product molecules present near the electrode surface under the mass transport-controlled condition.