We evaluated the electrochemical CO2 reduction reaction (ECR) on low-index Au single crystal surfaces (Au(111), (100), and (110); Au­(hkl)) and discussed the surface-atomic-arrangement-dependence of Au on the ECR. Online-electrochemical mass spectrometry (OLEMS) results revealed that the onset potential of the quadrupole mass spectrometer (Q-mass) ion signal for the reduction product carbon monoxide (CO; m/z = 28) is ca. 0.3 V lower on the Au(110) surface than on the Au(111) and (100) surfaces. Furthermore, the Au(110) surface showed the highest selectivity for CO generation in the potential region of −0.4 V to −1.4 V vs reversible hydrogen electrode (RHE); the relative, OLEMS-corrected ECR partial current density for generated CO at −0.76 V was ca. 20-fold higher compared with the Au(100) and (111) surfaces. The Tafel slope of Au(110) at the onset potential region (around −0.4 V) was much smaller than that of the Au(111) and (100) surfaces, suggesting that Au(110) shows the fastest ECR kinetics among the low-index Au surfaces. The results obtained in this study reveal that the ECR efficiency as well as the selectivity for CO generation on Au electrode surfaces can be dominated by the surface atomic arrangements and that relative Faradaic selectivity evaluation by OLEMS is helpful for discussion of the ECR process.

We investigated electrochemical CO2 reduction (ECR) on 0.1 monolayer‐thick‐Co and Sn‐deposited Au(110) surfaces (Co/Au(110), and Sn/Au(110)). Scanning tunneling microscopic images showed quasi‐one‐dimensional Co and Sn islands with different aspect ratios growing along the trenches of the missing‐row direction of the (1×2) reconstructed Au(110) surface. The selectivity and partial current density of the CO and H2 evolutions correlated with those of the deposited metals. CO evolution selectivity of the former Co/Au(110) increased compared with that of the Au(110), while that of the Sn/Au(110) significantly decreased. Co/Au(110) showed 1.4‐fold higher CO evolution activity than that of the clean Au(110) at −1.35 V vs. reversible hydrogen electrode. In contrast, the H2 evolution of the latter surface was significantly enhanced at a potential lower than −0.1 V. The results showed that site separations of Au and alloying elements of Co and Sn at the topmost surface determine the ECR product selectivity of alloy electrodes.

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 O2 (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 O2 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.

Potential-dependent molecular behavior of dissolved O2 in the vicinity of a polycrystalline Pt electrode surface was studied in O2-saturated 0.1 M HClO4 solution using a newly developed online electrochemical mass spectrometry (OLEMS) with rotating disk electrode (RDE) method (RDE-OLEMS). A gas sampling tip was positioned at ~50 μm from the electrode surface; O2 concentration in the vicinity of the Pt electrode at open circuit potential (~0.8 V vs. RHE) depended on the disk rotation rates, and linear sweep voltammetry (LSV) of the Pt electrode showed diffusion-limiting currents in the RDE-OLEMS layout. Furthermore, simultaneously recorded RDE-OLEMS data for the potential-dependent O2 concentration were consistent with the LSV data recorded for a negative-going sweep. We have successfully applied the newly developed RDE-OLEMS method and it is effective for in-situ analysis of gaseous electrochemical reaction products and dissolved molecules in the vicinity of the electrode under electrochemical conditions.

We investigated the oxygen evolution reaction (OER) activity changes of cobalt oxide (CoOx) thin films on Ir(111) and Pt(111) substrates by repeated OER measurements in 0.1 M KOH. Atomic force microscopy and X‐ray photoelectron spectroscopy analysis of the as‐prepared CoOx/Ir(111) and CoOx/Pt(111) showed similar surface morphologies of the CoOx thin films and almost the same OER overpotentials, which were estimated to be around 430 mV. However, after three OER measurements, the overpotential of CoOx/Ir(111) decreased by 70 mV, whereas that of CoOx/Pt(111) increased slightly. Structural analysis showed that CoOx/Ir(111) revealed the island‐like nanostructures of CoOx dispersed on Ir(111) surface, accompanied by the generation of CoOOH. In contrast, for CoOx/Pt(111), the Pt(111) substrate remains covered by the CoOx thin film. The results suggest that the interaface at CoOx (CoOOH) nano‐islands and Ir(111) substrate are responsible for reducing the OER overpotential.