Renate Hiesgen

Nanosized IrO(x)-Ir Catalyst with Relevant Activity for Anodes of Proton Exchange Membrane Electrolysis Produced by a Cost-Effective Procedure.

We have developed a highly active nanostructured iridium catalyst for anodes of proton exchange membrane (PEM) electrolysis. Clusters of nanosized crystallites are obtained by reducing surfactant-stabilized IrCl3 in water-free conditions. The catalyst shows a five-fold higher activity towards oxygen evolution reaction (OER) than commercial Ir-black. The improved kinetics of the catalyst are reflected in the high performance of the PEM electrolyzer (1 mg(Ir) cm(-2)), showing an unparalleled low overpotential and negligible degradation. Our results demonstrate that this enhancement cannot be only attributed to increased surface area, but rather to the ligand effect and low coordinate sites resulting in a high turnover frequency (TOF). The catalyst developed herein sets a benchmark and a strategy for the development of ultra-low loading catalyst layers for PEM electrolysis.

Microscopic Analysis of Current and Mechanical Properties of Nafion® Studied by Atomic Force Microscopy

The conductivity of fuel cell membranes as well as their mechanical properties at the nanometer scale were characterized using advanced tapping mode atomic force microscopy (AFM) techniques. AFM produces high-resolution images under continuous current flow of the conductive structure at the membrane surface and provides some insight into the bulk conducting network in Nafion membranes. The correlation of conductivity with other mechanical properties, such as adhesion force, deformation and stiffness, were simultaneously measured with the current and provided an indication of subsurface phase separations and phase distribution at the surface of the membrane. The distribution of conductive pores at the surface was identified by the formation of water droplets. A comparison of nanostructure models with high-resolution current images is discussed in detail.

Spatial distribution and dynamics of proton conductivity in fuel cell membranes: potential and limitations of electrochemical atomic force microscopy measurements

The proton conductivity of a Nafion 112 membrane is measured with a high spatial resolution using electrochemical atomic force microscopy. Image analysis reveals an inhomogeneous conductivity distribution which is attributed to the limited connectivity of hydrophilic domains. This information relates to the micro-morphology which is due to phase separation of the hydrophobic polymer backbone and the hydrophilic pendant groups. The direct images relate to a different length scale and are complementary to the x-ray diffraction investigations which provide only average information. Furthermore, the measured current values reveal an interesting correlation with the size of the conductive areas. A bimodal conductivity distribution suggests that there are different mechanisms which contribute to the proton current in Nafion. Additionally, time dependence in local conductivity is found and interpreted in terms of redistribution of water in the membrane. A statistical analysis of the current distribution is performed and compared with theoretical simulations. Evidence is found for the existence of a critical current density. On a timescale of seconds the response of the conductive network is probed by applying voltage steps to the atomic force microscope tip.

Low-Cost and Durable Bipolar Plates for Proton Exchange Membrane Electrolyzers

Cost reduction and high efficiency are the mayor challenges for sustainable H2 production via proton exchange membrane (PEM) electrolysis. Titanium-based components such as bipolar plates (BPP) have the largest contribution to the capital cost. This work proposes the use of stainless steel BPPs coated with Nb and Ti by magnetron sputtering physical vapor deposition (PVD) and vacuum plasma spraying (VPS), respectively. The physical properties of the coatings are thoroughly characterized by scanning electron, atomic force microscopies (SEM, AFM); and X-ray diffraction, photoelectron spectroscopies (XRD, XPS). The Ti coating (50 μm) protects the stainless steel substrate against corrosion, while a 50-fold thinner layer of Nb decreases the contact resistance by almost one order of magnitude. The Nb/Ti-coated stainless steel bipolar BPPs endure the harsh environment of the anode for more than 1000 h of operation under nominal conditions, showing a potential use in PEM electrolyzers for large-scale H2 production from renewables.

AFM as an analysis tool for high-capacity sulfur cathodes for Li-S batteries.

Summary In this work, material-sensitive atomic force microscopy (AFM) techniques were used to analyse the cathodes of lithium–sulfur batteries. A comparison of their nanoscale electrical, electrochemical, and morphological properties was performed with samples prepared by either suspension-spraying or doctor-blade coating with different binders. Morphological studies of the cathodes before and after the electrochemical tests were performed by using AFM and scanning electron microscopy (SEM). The cathodes that contained polyvinylidene fluoride (PVDF) and were prepared by spray-coating exhibited a superior stability of the morphology and the electric network associated with the capacity and cycling stability of these batteries. A reduction of the conductive area determined by conductive AFM was found to correlate to the battery capacity loss for all cathodes. X-ray diffraction (XRD) measurements of Li2S exposed to ambient air showed that insulating Li2S hydrolyses to insulating LiOH. This validates the significance of electrical ex-situ AFM analysis after cycling. Conductive tapping mode AFM indicated the existence of large carbon-coated sulfur particles. Based on the analytical findings, the first results of an optimized cathode showed a much improved discharge capacity of 800 mA·g(sulfur)−1 after 43 cycles.

Improving the activity and stability of Ir catalysts for PEM electrolyzer anodes by SnO2:Sb aerogel supports: does V addition play an active role in electrocatalysis?

Low Ir loading oxygen evolution reaction (OER) catalysts with superior activity and durability for proton exchange membrane (PEM) electrolyzers are an important topic in industry and academia. One possible strategy for addressing this challenge is the use of support materials that are stable under highly corrosive acidic environments at a high working potential (>1.4 V). Moreover, highly porous structure is another key criteria for OER catalyst support to achieve a high electrochemical surface area. Here, we report a novel Ir supported on a SnO2:Sb aerogel OER catalyst (Ir/SnO2:Sb-mod-V), which was prepared under ambient pressure by using vanadium additives. It shows an unrivaled activity and enhanced stability, on which vanadium does not play any active role but demonstrates the influences that changes the porosity of the aerogel support and affects the impurity content of the chlorine. By taking advantage of the high porosity of the aerogel substrate, Ir/SnO2:Sb-mod-V allows a decrease of more than 70 wt% for precious metal usage in the catalyst layer while keeping a similar OER activity compared to its unsupported counterpart.

Problems of roughness measurements using STM

Abstract Scanning tunneling microscopy (STM) seems to be the ideal tool for measurements of surface roughness on a nanometer scale. However, especially in the nanometer size range the extension and form of the tunneling tip comes into play. As an example for the problems arising in investigations of the surface topography a study of indium tin oxide (ITO) films is presented. Roughness measurements are performed using various methods. Several examples of pictures are discussed in which only multiple images of the tunneling tip are seen. A convolution of the substrate and of the STM tip topography occurs. Roughness parameters determined on the basis of such images reflect also the form of the tunneling tip. Reliable data for the surface can be obtained only with very fine tips. All etching techniques tested turned out to be not very reliable. However, in a few cases very sharp tips have been produced by electrochemical etching techniques, leading to the first images of an ITO surface with an ordered domain structure.

Nanoscale Investigation of Nafion Membranes after Artificial Degradation

In this contribution we report on the nanostructure and conductivity of freshly prepared as well as artificially degraded Nafion membranes investigated by contact atomic force microscopy (AFM),conductive AFM, and pulsed force-mode (PFM)-AFM. The different techniques can provide complementary information on structure and conductivity. Conductive AFM gives information about the internal structure of ionic clusters during current flow. High resolution current images of the membrane were used to directly compare the measured nanostructure of the single conductive channels with model predictions from the literature. The influence of H2O2 treatment as a method for artificial degradation is investigated. The analysis of adhesion forces demonstrates a significant change of thesurface properties with different membrane treatment. Topography and adhesion measurements clearly show materials changes with high resolution and correlate with changes in conductivity distributions.

Nanoscale Semiconductor Interface Characterization by Photo-STM

For many years there has been a continuing trend insemiconductor technology to develop smaller devices,which are needed, for example, to maximize the integrationdensity. With structures built from quantum wires andquantum dots reaching nanometer dimensions, there is ademand for characterization techniques that are able to re-trieve information about the electronic and structural prop-erties of such small areas. Since the invention of the scan-ning tunneling microscope in 1981

Analysis of aged Polymer Electrolyte Fuel Cell (PEFC) components by non traditional methods

The ageing of microporous layers (MPL) of fuel cell gas diffusion layers has been quantitavely analyzed using a special atomic force microscopy technique, namely the so-called HarmoniX technique. From the change of mean adhesion force under dry and wet conditions an increased loss of polytetrafluoroethylene (PTFE) at the cathode was found. With ionic current measurement in tapping and contact mode by AFM, activated Nafion was investigated before fuel cell operation with high resolution and individual ionic channels were imaged in one cluster. These measurements were compared to the current distribution of membranes after 1600 h of fuel cell operation under OCV. Distinct current levels were found which demonstrate the existence of an interpenetrating ionic network with different branches not directly connected at the surface. SEM/EDX investigations of the specially designed fuel cells indicate an important role of platinum in degradation of membranes.