Electrochemical Aging and Characterization of Graphite-Polymer Based Composite Bipolar Plates for Vanadium Redox Flow Batteries

Three bipolar plates (BPP) comprised of a composite of polypropylene or polyvinylidene fluoride polymer and varying average graphite particle size were studied for application in a vanadium redox flow battery (VRFB). The BPPs were electrochemically aged via 3000 cyclic voltammetry curves in 1.8 M VOSO4 + 2.0 M H2SO4 electrolyte. After every 500th cycle the aging progression was determined by performing cyclic voltammetry on the bipolar plates in 0.1 M H2SO4 solution where the double layer capacitance, the quinone/hydroquinone and the vanadium species redox activity were quantitatively evaluated. Prior to the aging, the composite plates were extensively characterized using various physical methods. The performed studies reveal that the wettability, surface roughness and accessible porosity of the bipolar plates significantly influence their electrochemical stability. Cycling tests in vanadium redox flow single cells at a constant current density of 60 mA cm-2 revealed a close correlation of the cell efficiencies to the electrochemical stability of the bipolar plates. Thus, the proposed electrochemical characterization method can be an effective foresight to predict the applicability of a bipolar plate in a vanadium redox flow battery

Overview: State-of-the Art Commercial Membranes for Anion Exchange Membrane Water Electrolysis

One promising way to store and distribute large amounts of renewable energy is water electrolysis, coupled with transport of hydrogen in the gas grid and storage in tanks and caverns. The intermittent availability of renewal energy makes it difficult to integrate it with established alkaline water electrolysis technology. Proton exchange membrane (PEM) water electrolysis is promising, but limited by the necessity to use expensive platinum and iridium catalysts. The expected solution is anion exchange membrane (AEM) water electrolysis, which combines the use of cheap and abundant catalyst materials with the advantages of PEM water electrolysis, namely a low foot print, large operational capacity, and fast response to changing operating conditions. The key component for AEM water electrolysis is a cheap, stable, gas tight and highly hydroxide conductive polymeric AEM. Here we present target values and technical requirements for AEMs, discuss the chemical structures involved and the related degradation pathways, and give an overview over the most prominent and promising commercial AEMs (Fumatech Fumasep® FAA3, Tokuyama A201, Ionomr Aemion™, Dioxide materials Sustainion®, and membranes commercialized by Orion Polymer), and review their properties and performances of water electrolyzers using these membranes

Layered composite membranes based on porous PVDF coated with a thin, dense PBI layer for vanadium redox flow batteries

A commercial porous polyvinylidene fluoride membrane (pore size 0.65 μm, nominally 125 μm thick) is spray coated with 1.2–4 μm thick layers of polybenzimidazole. The area resistance of the porous support is 36.4 mΩ cm2 in 2 M sulfuric acid, in comparison to 540 mΩ cm2 for a 27 μm thick acid doped polybenzimidazole membrane, and 124 mΩ cm2 for PVDF-P20 (4 μm thick blocking layer). Addition of vanadium ions to the supporting electrolyte increases the resistance, but less than for Nafion. The expected reason is a change in the osmotic pressure when the ionic strength of the electrolyte is increased, reducing the water contents in the membrane. The orientation of the composite membranes has a strong impact. Lower permeability values are found when the blocking layer is oriented towards the vanadium-lean side in ex-situ measurements. Cells with the blocking layer on the positive side have significantly lower capacity fade, also much lower than cells using Nafion 212. The coulombic efficiency of cells with PVDF-PBI membranes (98.4%) is higher than that of cells using Nafion 212 (93.6%), whereas the voltage efficiency is just slightly lower, resulting in energy efficiencies of 85.1 and 83.3%, respectively, at 80 mA/cm2

Numerical sensitivity studies on the variability of climate-relevant processes in the Barents Sea

The Barents Sea is a key region in the North Atlantic/Arctic Ocean climate system because of the intense ocean-atmosphere heat exchange and the formation of sea ice. The latter process is connected with salt input, so-called ‘‘brine release,’’ whereby water masses of Atlantic origin can be transformed into dense shelf bottom waters. To investigate the sensitivity of simulated, climate-relevant processes to different but wellestablished and realistic initial and boundary data, a high-resolution coupled ice-ocean model is applied to the Barents Sea. The model is based on the Hamburg Shelf Ocean Model and runs on a 7 x 7 km grid, based on the International Bathymetric Chart of the Arctic Ocean topography. The model is initialized with different temperature and salinity data from the Arctic Climate System Study BarKode data set and is forced with National Centers for Environmental Prediction atmospheric data. Eight sensitivity experiments with initial and boundary conditions in different combinations are performed over a period of 6 years (1979–1984). Results are analyzed with special emphasis on the ocean-atmosphere heat exchange, the ice extent, and the brine release. The experimental variability is compared to the interannual climatic variability in order to assess the role of different forcing terms for regional climate modeling. Our results show that the experimental variability can be partly of the same order than the interannual variability, which suggests that data uncertainties could easily bias the results of climate variability studies. Modification of the Barents Sea inflow had the strongest effect on model results. The ocean-atmosphere heat flux proved to be the most sensitive parameter to oceanic and atmospheric anomalies, whereas the ice extent and the corresponding salt input is more invariant to different boundary conditions

Erfolge für die Brennstoffzelle – Standards setzen und nutzen

Brennstoffzellen-Stacks und-Komponenten aus Deutschland werden inzwischen weltweit vertrieben. Was deutschen Herstellern bei den Systemen bisher nur wenig gelungen ist, ist für die Komponenten längst Standard. Einige Komponenten wie z.B. Gas-Diffusionslagen und Luftfilter liefern deutsche Zulieferer auch in die Serienfahrzeuge. Bei den Stack-Herstellern sind die Produkte mit deutschem Kern für den Endkunden nach der Systemintegration kaum erkenntlich. Dabei ist neben der allgemeinen CE-Kennzeichnung des Herstellers die Produkthaftung der wichtigere Bestandteil für die Unternehmerverantwortung. Die Sicherheit von Brennstoffzellensystemen wird für verschiedene Eigenschaftsprofile in der IEC 62282 Normenreihe für Brennstoffzellen beschrieben. Im Beitrag wird eine Übersicht dieser Normenreihe vorgestellt

Study of the Ionomer Distribution in Catalyst Layers by Atomic Force Microscopy

Within our contribution we utilize and give an overview on the AFM technique used to image fuel cell and electrolyser catalyst layers

Visualization of the Ionomer Distribution in Catalyst Layers by Atomic Force Microscopy

The structural composition of the catalyst layer in fuel cells and electrolyzer has a crucial impact on the performance and stability during operation. Made of the catalyst material itself and an ionomeric binder combined to a porous assembling, the catalyst layer is the place where the electrochemical reaction takes place. This requires the conductance of electrons and ions as well as mass transport of educts and products to and from the reactive catalytic centers, respectively. The ionomer enables the conductance of cations (usually H+ in proton exchange membrane technologies) or anions (usually OH- in anion exchange membrane technologies) where a homogenous distribution of the ionomer around the catalytic centers is expected to result in an effective process. In contrast, either too thin or too thick coverages might cause high ionic conductance or mass transport resistances [1, 2]. Thus, structural investigation of the ionomer distribution complements the evaluation between preparation of the catalyst layer and electrochemical characterization and thus allows performance optimization of fuel cells and electrolyzers. In general, visualization of the ionomer in the catalyst layer is technically challenging, but recently the feasibility of materials-sensitive atomic force microscopy (AFM) was demonstrated to analyze the ionomer distribution in fuel cell and electrolyzer [3,4]. AFM analyzes the catalyst layer under ambient conditions and do not require vacuum conditions or high radiation that might harm the ionomer, but also close to fuel cell and electrolyzer conditions can be resembled. The different nanomechanical properties of the ionomer and the catalyst result in material contrast in adhesion force images enabling the visualization of the ionomer distribution (c.f. Figure). In this contribution, we give an overview how the AFM technique can be used for different applications in fuel cell and electrolyzer research. Thus, the average ionomer layer thickness is analyzed depending on the ionomer loading in the catalyst layer and measurement conditions like elevated temperature and defined relative humidity are varied. Later allows the insights to occurring ionomer swelling which might have an impact on the performance and stability during operation