Direct STM Elucidation of the Effects of Atomic-Level Structure on Pt(111) Electrodes for Dissolved CO Oxidation

We sought to establish a new standard for direct comparison of electrocatalytic activity with surface structure using in situ scanning tunneling microscopy (STM) by examining the electrooxidation of CO in a CO-saturated solution on Pt(111) electrodes with steps, with combined electrochemical measurements, in situ STM, and density functional theory (DFT). On pristine Pt(111) surfaces with initially disordered (111) steps, CO oxidation commences at least 0.5 V lower than that for the main oxidation peak at ca. 0.8–1.0 V vs the reversible hydrogen electrode in aqueous perchloric acid solution. As the potential was cycled between 0.07 and 0.95 V, the CO oxidation activity gradually decreased until only the main oxidation peak remained. In situ STM showed that the steps became perfectly straight. A plausible reason for the preference for (111) steps in the presence of CO is suggested by DFT calculations. In contrast, on a pristine Pt(111) surface with rather straight (100) steps, the low-potential CO oxidation activity was less than that for the pristine, uncycled (111) steps. As the potential was cycled, the activity also decreased greatly. Interestingly, after cycling, in situ STM showed that (111) microsteps were introduced at the (100) steps. Thus, potential cycling in the presence of dissolved CO highly favors formation of (111) steps. The CO oxidation activity in the low-potential region decreased in the following order: disordered (111) steps > straight (100) steps > (100) steps with local (111) microsteps ≈ straight (111) steps

Imaging Individual Proton-Conducting Spots on Sulfonated Multiblock-Copolymer Membrane under Controlled Hydrogen Atmosphere by Current-Sensing Atomic Force Microscopy

The proton-conductive spots on the membrane surface of sulfonated poly­(arylene ether) multiblock copolymer were successfully imaged by current-sensing atomic force microscopy under hydrogen atmosphere at various temperatures and humidities. These spots should be connected to the proton-conductive paths inside the membrane. The average diameter of the spots was approximately 12 nm, consistent with the size of hydrophilic domains observed by transmission electron microscopy. The size of the proton-conducting spots was almost unchanged regardless of the temperature and humidity, whereas the number of the spots increased at higher humidity; the total area of the proton-conducting spots increased accordingly on the membrane surface. This increase in the conducting area at high humidity should be related to the bulk ionic conductivity measured by impedance spectroscopy

Effect of Surface Ion Conductivity of Anion Exchange Membranes on Fuel Cell Performance

Anion conductivity at the surfaces of two anion-exchange membranes (AEMs), quaternized ammonium poly­(arylene ether) multiblock copolymer (QPE-<i>bl</i>-3) and quaternized ammonium poly­(arylene perfluoro-alkylene) copolymer (QPAF-1), synthesized by our group was investigated using current-sensing atomic force microscopy under purified air at various relative humidities. The anion-conducting spots were distributed inhomogeneously on the surface of QPE-<i>bl</i>-3, and the total areas of the anion-conducting spots and the current at each spot increased with humidity. The anion-conductive areas on QPAF-1 were found on the entire surface even at a low humidity. Distribution of the anion-conducting spots on the membrane was found to directly affect the performance of an AEM fuel cell

Atomically Flat Pt Skin and Striking Enrichment of Co in Underlying Alloy at Pt₃Co(111) Single Crystal with Unprecedented Activity for the Oxygen Reduction Reaction

By the use of in situ scanning tunneling microscopy and surface X-ray scattering techniques, we have clarified the surface structure and the layer-by-layer compositions of a Pt skin/Pt₃Co­(111) single-crystal electrode, which exhibited extremely high activity for the oxygen reduction reaction. The topmost layer was found to be an atomically flat Pt skin with (1 × 1) structure. Cobalt was enriched in the second layer up to 98 atom %, whereas the Co content in the third and fourth layers was slightly smaller than that in the bulk. By X-ray photoelectron spectroscopy, the Co in the subsurface layers was found to be positively charged, which is consistent with an electronic modification of the Pt skin. The extremely high activity at the Pt skin/Pt₃Co­(111) single crystal is correlated with this specific surface structure