Visualization of local ionic concentration and diffusion constants using a tailored electrochemical strain microscopy method

A tailored electrochemical strain microscopy technique is presented and used to analyze the ionic mobility and diffusion coefficients in composite Si/C anodes. The resulting surface displacement after a voltage pulse is proportional to the ionic concentration change and is measured by the deflection of an atomic force microscopy tip. The results show a higher ionic mobility at the steps of silicon composite anode microcrystals compared to the crystal centers. Diffusion coefficients are extracted from the time dependence of the surface displacement. Mappings with nanoscale resolution of local diffusion coefficients are displayed. The results demonstrate higher diffusion coefficients at the steps.Bundesministerium für Bildung und Forschung (Federal Ministry of Education and Research

Composite Materials with Combined Electronic and Ionic Properties

In this work, we develop a new type of composite material that combines both electrocatalytic and ionic properties, by doping a silver metal catalyst with an anion-conducting ionomer at the molecular level. We show that ionomer entrapment into the silver metallic structure is possible, imparting unique properties to the catalytic character of the metallic silver. The novel composite material is tested as the cathode electrode of fuel cells, showing significant improvement in cell performance as compared with the undoped counterpart. This new type of material may then replace the current design of electrodes in advanced fuel cells or other electrochemical devices. The possibility to merge different properties into one composite material by molecular entrapment in metals can open the way to new materials, leading to unexplored fields and applications

Enveloping of Catalyst Powder by Ionomer for Dry Spray Coating in Polymer Electrolyte Membrane Fuel Cells

This study presents innovative concepts for improving performance of membrane electrode assemblies (MEAs) prepared by the dry-spraying method introduced by the German Aerospace center (DLR). Dry-spraying is a time and cost effective method that involves solvent-free spraying of catalyst powder on polymer electrolyte membrane. The issue which is resolved in this work is the large ionomer particle size in the conventional method. With mechanical grinding, particle size of the ionomer less than 100 nm were not been achieved. However, here the reactive interface of dry-sprayed MEA is optimized by improving ionic conductivity. Our approach is to modify a carbon support by partially enveloping with Nafion® ionomer followed by incorporating Pt black with it. Additionally, commercial catalyst powder was also modified by two-step preparation process with Nafion® dispersion. In this research, both of these modified powders are sprayed over membrane; hot-pressed; characterized, and have shown improved ionic network and distribution, which corresponds to their higher performances. The improvement in the performance does not correlate with electrode surface area but with the ionomer resistance of the catalytic layer. Therefore, with this study we demonstrate a pathway and methodology to further improve performance by optimizing ionomer structure and networks in the catalytic layer

High-resolution analysis of ionomer loss in catalytic layers after operation

The function of catalytic layers in fuel cells and electrolyzers depends on the properties of the ionically conductive phase, which are most commonly perfluorinated ionomers based on Nafion and Aquivion. An analysis by atomic force microscopy reveals that the ultrathin ionomer films around Pt/C agglomerates have a thickness distribution ranging from 3.5 nm to 20 nm. Their conductivity and gas permeation properties determine the fuel cell performance to a large extend. For electrodes in Aquivion-based membrane-electrode-assemblies operation-induced structure changes were investigated by means of material- and conductivity-sensitive atomic force microscopy, infrared spectroscopy and electron-dispersive X-ray analysis. The observed thinning of the ultrathin ionomer films was mainly caused by polymer degradation deduced from reduced swelling after long-time operation and a significant loss of ionomer with operation time detected by infrared spectroscopy. From the linear thickness increase of the ultrathin films with rising humidity, a mainly layered structure of the ionomer was deduced. An influence of thickness of such ultrathin ionomer films on fuel cell lifetime was found by analysis of differently prepared membrane-electrode-assemblies, where a linear increase of irreversible degradation rate with ionomer film thickness in the electrodes of unused membrane-electrode-assemblies was found.Seventh Framework Programme (European Union Seventh Framework Programme

Photoelektrische Messungen mit der Rastersondenmikroskopie zur Mikroanalyse reiner und modifizierter Halbleiteroberflächen im Elektrolyten

A novel method for measuring the local photocurrent together with the topography on semiconductor samples with a resolution less than 1 nm is described . The setup is based on a scanning tunneling microscope and is especially able to measure the short circuit photocurrent in order to distinguish it from pure photoconductivity. With measurements in ambient environment on a tungsten diselenide model semiconductor it is shown that the System consisting of the tunneling tip, the tunnel gap and the semiconductor behaves as a nanosized MIS (metal-insulator-semiconductor) solar cell. The spatially resolved photocurrent is a quite sensitive tool for analyzing the electronic variations on a semiconductor surface. Variations in flat band potential as well as changes in the recombination rate can be detected and linked to structures of the surface, below the surface, or on metal particle modified surfaces . It is possible to visualize the space charge zones along steps as well as around catalyst particles on the surface . Measuring the photocurrent makes it possible to directly observe effective barriers of copper particles and of size effects influencing the width of the space charge layer at the surface

High-resolution imaging of ion conductivity of Nafion membranes with electrochemical atomic force microscopy

Conductive electrochemical AFM images demonstrating the complex nature and structure of Nafion surface conductivity are presented. Nanoscale regions with high currents determining the overall total membrane current can be distinguished frommajority domains with lower currents and non-conductive areas. The different conductive domains form ordered structures and showa specific dynamic behaviour. These observations were compared to the structural and electrical models in the literature. None of the models is able to explain all aspects of the current images. The existence of inverted micelles seems to be quite probable since the formation of agglomerates like chains and larger ordered clusters is clearly visible. This aspect is best described by the model of Schmidt-Rohr and Chen. In addition, the highly dynamic behaviour and distribution of conductive channels of Nafion leading to the formation of new current pathways also indicates the formation of different meso-phases with a high local fluctuation rate. The other discussed models also predict structural features which are in agreement with our observations like the formation of super-structures and agglomeration of fibers. The structural characterisation reflects the situation at or near the membrane surface and might differ from the bulk structure since the surface energy may have a large influence on the formation of structures during the membrane solidification process. The quite large dynamics of conductivity changes of Nafion reflected in the formation of newcurrent pathways even at roomtemperature leads to the assumption that the internal structure of Nafion is subject to significant changes due to humidity and temperature variations. The local variation of individual structures may reflect the variation of concentration of hydrophilic and hydrophobic groups during membrane solidification. The minimization of surface free energy during a self-assembling process is essential for the formation of different phases and subsequent structures like chains, etc. as well as higher order clustering

Irreversible Damage of Polymer Membranes During Attenuated Total Reflection Infrared Analysis

Analyzing polymer membranes by attenuated total reflection infrared spectroscopy (ATR-IR) can lead to irreversible damage to the material and induces systematic errors in the data. Attenuated total reflection infrared spectroscopy is a common tool for analyzing the surface of polymer membranes. In order to provide sufficient contact between the membrane and the internal reflection element (i.e., the ATR crystal), pressure is applied via a metal stamp. This procedure, however, can lead to mechanical damage. In this work, we study this damage using the example of a polyethersulfone (PES) membrane for water filtration and we show how the damage can be avoided. Attenuated total reflection infrared spectroscopy, laser-scanning microscopy (LSM), and atomic force microscopy (AFM) are employed to understand the mechanically-induced phenomena at the molecular and macroscopic scales. The data reveal that the mechanical impact does not only result in a compressed membrane structure with smaller pores, but it also leads to deformations at the molecular level. Moreover, in light of the mechanical damage, a detailed analysis of the PES IR spectrum indicates that several previous vibrational assignments of peaks may be incorrect and that many published results may be biased and should be revisited

Photoelektrische Messungen mit der Rastersondenmikroskopie zur Mikroanalyse reiner und modifizierter Halbleiteroberflaechen im Elektrolyten Projektabschlussbericht

A novel method for measuring the local photocurrent together with the topography on semiconductor samples with a resolution less than 1 nm is described. The setup is based on a scanning tunneling microscope and is especially able to measure the short circuit photocurrent in order to distinguish it from pure photoconductivity. With measurements in ambient environment on a tungsten diselenide model semiconductor it is shown that the system consisting of the tunneling tip, the tunnel gap and the semiconductor behaves as a nanosized MIS (metal-insulator-semiconductor) solar cell. The spatially resolved photocurrent is a quite sensitive tool for analyzing the electronic variations on a semiconductor surface. Variations in flat band potential as well as changes in the recombination rate can be detected and linked to structures of the surface, below the surface, or on metal particle modified surfaces. It is possible to visualize the space charge zones along steps as well as around catalyst particles on the surface. Measuring the photocurrent makes it possible to directly observe effective barriers of copper particles and of size effects influencing the width of the space charge layer at the surface. (orig.)16 refs.SIGLEAvailable from TIB Hannover: RA 831(3718) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekStiftung Volkswagenwerk, Hannover (Germany)DEGerman