Eliminating Flooding-related Issues in Electrochemical CO₂-to-CO Converters: Two Lines of Defense

By using silver (Ag) in nanostructured (nanowire, nanosphere, etc.) or thin-layer form as a catalyst for electrochemical CO2 reduction, very high CO-forming selectivity of almost 100% can be achieved. Supported by gas diffusion layers (GDLs),  the reactant CO2 in the gas phase can approach and potentially access active Ag sites, which allows current densities in the range of a few hundred mA cm-2 to be reached. Yet, the stability of gas diffusion electrode (GDE) based electrochemical CO2-to-CO converters is far from perfect, and the activity of GDE cathodes, especially when operated at high current densities, often significantly decays during electrolyses after no more than a few hours. The primary reason of stability losses in GDE-based CO2-to-CO electrolysers is flooding: that is, the excess wetting of the GDE that prevents CO2 from reaching Ag catalytic sites. In the past years, the authors of this paper at Empa and at the University of Bern, cooperating with other partners of the National Competence Center for Research (NCCR) on Catalysis, took different approaches to overcome flooding. While opinions differ with regard to where the first line of defense in protecting GDEs from flooding should lie, a comparison of the recent results of the two groups gives unique insight into the nature of processes occurring in GDE cathodes used for CO2 electrolysis.

Real-Time Monitoring of the In Situ Microfluidic Synthesis of Ag Nanoparticles on Solid Substrate for Reliable SERS Detection

A sharpened control over the parameters affecting the synthesis of plasmonic nanostructures is often crucial for their application in biosensing, which, if based on surface-enhanced Raman spectroscopy (SERS), requires well-defined optical properties of the substrate. In this work, a method for the microfluidic synthesis of Ag nanoparticles (NPs) on porous silicon (pSi) was developed, focusing on achieving a fine control over the morphological characteristics and spatial distribution of the produced nanostructures to be used as SERS substrates. To this end, a pSi membrane was integrated in a microfluidic chamber in which the silver precursor solution was injected, allowing for the real-time monitoring of the reaction by UV-Vis spectroscopy. The synthesis parameters, such as the concentration of the silver precursor, the temperature, and the flow rate, were varied in order to study their effects on the final silver NPs' morphology. Variations in the flow rate affected the size distribution of the NPs, whereas both the temperature and the concentration of the silver precursor strongly influenced the rate of the reaction and the particle size. Consistently with the described trends, SERS tests using 4-MBA as a probe showed how the flow rate variation affected the SERS enhancement uniformity, and how the production of larger NPs, as a result of an increase in temperature or of the concentration of the Ag precursor, led to an increased SERS efficiency.

Stereometric and fractal analysis of granulated silver films used in thin-film hybrid structures

Silver nanostructures are of interest to be used in hybrid thin-film structures with various materials. In this work, we analyse 3D AFM images of granulated silver film nanostructures prepared by thermal evaporation. The advanced AFM data study aims to understand how film thickness and postannealing affect the nanostructure morphology changes. For the first time, the evaluation of surface statistical parameters and fractal geometry were used to characterise the nanostructure morphology of Ag island films. The samples with gravimetric thickness of 2, 4 and 10 nm were analysed before and after annealing at 200°C for 10 min. The statistical processing revealed the essential variation of parameters with Ag film thickness increment and as a postannealing result. The nonmonotonic variation of surface roughness, skewness and fractal dimensions were found. It is caused by the features of the film growth process with the thickness increment and thermally activated diffusion of Ag nanoparticles during annealing. LAY DESCRIPTION: Silver nanostructures are of great scientific interest due to their unique properties and wide applicability. In this work, we analyse microscopic 3D images of silver nanostructures deposited on quartz in vacuum chamber. Such thin-film nanostructures consisted of silver nanoparticles attached to the surface of quartz substrate. We aimed at studying how effective film thickness and postannealing affect the nanostructure shapes and forms. Effective thickness is the equivalent thickness of a smooth film. For this purposes we used special parameters called statistical parameters and fractal geometry to investigate nanostructures. The samples with effective thickness of 2, 4 and 10 nm were analysed before and after annealing at 200°C for 10 min. It has been shown, that effective Ag film thickness and postannealing results in variation of nanostructures forms. We have shown that the variation of statistical parameters were nonmonotonic. It is caused by the features of the film growth process with the thickness increment and thermally activated diffusion of Ag nanoparticles during annealing.

Various Silver Nanostructures on Sapphire Using Plasmon Self-Assembly and Dewetting of Thin Films

Silver (Ag) nanostructures demonstrate outstanding optical, electrical, magnetic, and catalytic properties and are utilized in photonic, energy, sensors, and biomedical devices. The target application and the performance can be inherently tuned by control of configuration, shape, and size of Ag nanostructures. In this work, we demonstrate the systematical fabrication of various configurations of Ag nanostructures on sapphire (0001) by controlling the Ag deposition thickness at different annealing environments in a plasma ion coater. In particular, the evolution of Ag particles (between 2 and 20 nm), irregular nanoclusters (between 30 and 60 nm), and nanocluster networks (between 80 and 200 nm) are found be depended on the thickness of Ag thin film. The results were systematically analyzed and explained based on the solid-state dewetting, surface diffusion, Volmer-Weber growth model, coalescence, and surface energy minimization mechanism. The growth behavior of Ag nanostructures is remarkably differentiated at higher annealing temperature (750 °C) due to the sublimation and temperature-dependent characteristic of dewetting process. In addition, Raman and reflectance spectra analyses reveal that optical properties of Ag nanostructures depend on their morphology.