Atomic Undercoordination in Ag Islands on Ru(0001) Grown via Size-Selected Cluster Deposition: An Experimental and Theoretical High-Resolution Core-Level Photoemission Study

The possibility of depositing precisely mass-selected Ag clusters (Ag1, Ag3, and Ag7) on Ru(0001) was instrumental in determining the importance of the in-plane coordination number (CN) and allowed us to establish a linear dependence of the Ag 3d5/2 core-level shift on CN. The fast cluster surface diffusion at room temperature, caused by the low interaction between silver and ruthenium, leads to the formation of islands with a low degree of ordering, as evidenced by the high density of low-coordinated atomic configurations, in particular CN = 4 and 5. On the contrary, islands formed upon Ag7 deposition show a higher density of atoms with CN = 6, thus indicating the formation of islands with a close-packed atomic arrangement. This combined experimental and theoretical approach, when applied to clusters of different elements, offers the perspective to reveal nonequivalent local configurations in two-dimensional (2D) materials grown using different building blocks, with potential implications in understanding electronic and reactivity properties at the atomic level

PTFE Content in Catalyst Layers and Microporous Layers Effect on Performance and Water Distribution in Polymer Electrolyte Membrane Fuel Cells

This work describes the effects of catalyst layers CLs consisting of hydrophobic PTFE on the performance and water management of PEM fuel cells. Catalyst inks with various PTFE contents were coated on Nafion membranes and characterized using contact angle measurements, SEX EDX, and mercury porosimetry. Fuel cell tests and electrochemical impedance spectroscopy EIS were conducted under varying operating conditions for the prepared materials. At dry conditions, CLs with 5 wt. PTFE were advantageous for cell performance due to improved membrane hydration, whereas under humid conditions and high air flow rates CLs with 10 wt. PTFE improved the performance in high current density region. Higher PTFE contents bigger equal 20 wt. increased the mass transport resistance due to reduced porosity of the CLs structure. Operando neutron radiography was utilized to study the effects of hydrophobicity gradients within CLs and cathode microporous layer MPLC on liquid water distribution. More hydrophobic CLs increased the water content in adjacent layers and improved performance, especially at dry conditions. MPLC with higher PTFE contents increased the overall liquid water within the CLs and GDLs and escalated the water transfer to the anode side. Furthermore, the role of back diffusion transport mechanism on water distribution was identified for the investigated cell

Breakdown of the correlation between oxidation states and core electron binding energies at the sub-nanoscale

X-ray photoelectron spectroscopy is a powerful analytical tool to fingerprint the atomic oxidation state. However, the well-established trend that is found for bulk, ultra-thin films and surface oxides, which is usually characterized by an incremental binding energy shift upon increasing oxidation state, may not apply at the sub-nanoscale where reduced dimensionality and quantum size effects play a relevant role. Here we investigate through a combined experimental and theoretical approach the oxidation of size-selected Ag7 and Ag11 clusters supported on graphene, revealing an anomalous Ag 3d5/2 core level shift trend upon increasing O coverage. We show that the negative core level shift trend typical of Ag reverts back in the case of the highest Ag(III) oxidation state, an effect that is due to the peculiar electronic structure of Ag nano-oxides. Our results highlight the great care needed to extend at the zero-dimensional scale the knowledge acquired for the spectral interpretation of 3D and 2D materials