The Sedimentary Carbon-Sulfur-Iron Interplay – A Lesson From East Anglian Salt Marsh Sediments

We explore the dynamics of the subsurface sulfur, iron and carbon cycles in salt marsh sediments from East Anglia, United Kingdom. We report measurements of pore fluid and sediment geochemistry, coupled with results from laboratory sediment incubation experiments, and develop a conceptual model to describe the influence of bioturbation on subsurface redox cycling. In the studied sediments the subsurface environment falls into two broadly defined geochemical patterns – iron-rich sediments or sulfide-rich sediments. Within each sediment type nearly identical pore fluid and solid phase geochemistry (in terms of concentrations of iron, sulfate, sulfide, dissolved inorganic carbon (DIC), and the sulfur and oxygen isotope compositions of sulfate) are observed in sediments that are hundreds of kilometers apart. Strictly iron-rich and strictly sulfide-rich sediments, despite their substantive geochemical differences, are observed within spatial distances of less than five meters. We suggest that this bistable system results from a series of feedback reactions that determine ultimately whether sediments will be sulfide-rich or iron-rich. We suggest that an oxidative cycle in the iron-rich sediment, driven by bioirrigation, allows rapid oxidation of organic matter, and that this irrigation impacts the sediment below the immediate physical depth of bioturbation. This oxidative cycle yields iron-rich sediments with low total organic carbon, dominated by microbial iron reduction and no methane production. In the absence of bioirrigation, sediments in the salt marsh become sulfide-rich with high methane concentrations. Our results suggest that the impact of bioirrigation not only drives recycling of sedimentary material but plays a key role in sedimentary interactions among iron, sulfur and carbon

Die Normal-Wasserstoffelektrode als Bezugselektrode in der Direkt-Methanol-Brennstoffzelle

Direct methanol fuel cells (DMFCs) are able to directly convert the chemical energy of methanol into electrical energy. The engineering effort required for handling methanol is not great and therefore the design of DMFC systems is much simpler than that of other fuel cell systems. A disadvantage of DMFCs in comparison to hydrogen-driven fuel cells is the low electrochemical efficiency as a result of several physicochemical processes. These processes, important for the conversion of methanol, are coupled and have to be adjusted in the operation of the DMFC. The aim of the present work is the further development of electrochemical measuring methods for investigating the physicochemical processes in the DMFC during operation. Interest is focused on the reference electrode and impedance measurements. The main part of a DMFC is the membrane electrode assembly (MEA). Developments have so far concentrated on analysing the entire MEA consisting of two substrate layers, two microlayers, two catalyst layers and one membrane. Due to the uncertainty about the contribution of each layer to the measured signal, it is very difficult to physically interpret the measured information. Therefore, the first part of this work concentrates on a new procedure for the production of catalyst-coated membranes (CCMs). The CCMs are used in the test cells without substrates or microlayers and therefore the measured signal contains only the contributions of the catalyst layers and the membrane. For electrode-selective measurements it is necessary to split this cell signal into two single electrode signals. For this purpose a special test cell with a third electrode inside the cell, a so-called reference electrode, has been developed. The technique of using reference electrodes in fuel cells is well known. Different simulations have shown that this method involves several sources of errors. Because of these errors it is not clear whether the measured signals are useful or not, and no experiments systematically investigating these errors are known. For this reason, the influence of different errors on the application of reference electrodes and the effect on the measured voltage signal is investigated in the present work. It can be shown that impedance measurements are indispensable for validating the single electrode signals measured with the reference electrode. A precondition for the application of impedance spectroscopy is a mathematical model for the investigated system. An impedance model for the whole frequency range of the DMFC anode catalyst layer was not available and therefore had to be developed. With this model, the reference electrode measurements can be combined with the impedance measurements and this combination enables the measured signals to be validated. The combination of the methods presented here considerably reduces the error potential of reference electrode measurements. Single electrode measurements for the DMFC anode, measured with a hydrogen reference electrode, are demonstrated for small current densities. The problems with these kinds of measurements have not been completely solved, but with the methods presented it is possible to detect the influence of measuring errors and to test the reliability of the measured signals

Cost-optimized design point and operating strategy of polymer electrolyte membrane electrolyzers

Green hydrogen is a key solution for reducing CO2 emissions in various industrial applications, but high production costs continue to hinder its market penetration today. Better competitiveness is linked to lower investment costs and higher efficiency of the conversion technologies, among which polymer electrolyte membrane electrolysis seems to be attractive. Although new manufacturing techniques and materials can help achieve these goals, a less frequently investigated approach is the optimization of the design point and operating strategy of electrolyzers. This means in particular that the questions of how often a system should be operated and which cell voltage should be applied must be answered. As existing techno-economic models feature gaps, which means that these questions cannot be adequately answered, a modified model is introduced here. In this model, different technical parameters are implemented and correlated to each other in order to simulate the lowest possible levelized cost of hydrogen and extract the required designs and strategies from this. In each case investigated, the recommended cost-based cell voltage that should be applied to the system is surprisingly low compared to the assumptions made in previous publications. Depending on the case, the cell voltage is in a range between 1.6 V and 1.8 V, with an annual operation of 2000–8000 h. The wide range of results clearly indicate how individual the design and operation must be, but with efficiency gains of several percent, the effect of optimization will be indispensable in the future

Faculty Spotlight- Dr. Oliver Glanz

Published on Oct 31, 2018 Andrews University Teaching and Learning: Dr. Glanz our student would like to take a moment and say thanks for your wholistic care.https://digitalcommons.andrews.edu/auvideo/1434/thumbnail.jp

A completely slot die coated membrane electrode assembly

This work shows how to manufacture completely coated membrane electrode assemblies (CC-MEAs) for PEM water electrolysis by only using a slot die. Platinum, Nafion®, and IrO2 dispersions are successively coated to the respective dried layer. For comparison reasons, MEAs with the same Iridium loading of 2.1 mg cm−2 and Platinum loading of 0.4 mg cm−2, assembled with a commercial membrane of the same 20 μm thickness, were produced via decal method. Differences in polarization curves are attributed to the lower high frequency resistance of CC-MEAs determined by impedance spectroscopy. The easy-to-scale CC-MEA method presented here offers the advantages of direct membrane deposition (DMD) without the challenge of homogenously coating a porous transport layer (PTL). Therefore, it allows a free choice of different PTLs – regardless if in sintered form or as expanded metal. The comparability between the produced CC-MEAs and published DMD results is shown by means of cross-sectional and electrochemical measurements

Layer Formation from Polymer Carbon-Black Dispersions

It has been well-established that effects such as cracking are observable when wet layers are dried. In particular, the layer thickness, as well as the surface tension of the liquid, is responsible for this behavior. The layer formation of polymer electrolyte fuel cells and electrolyzer electrodes, however, has not yet been analyzed in relation to these issues, even though the effect of cracks on cell performance and durability has been frequently discussed. In this paper, water propanol polymer-containing carbon-black dispersions are analyzed in situ with regard to their composition during drying. We demonstrate that crack behavior can be steered by slight variations in the initial dispersion when the solvent mixture is near the dynamic azeotropic point. This minor adjustment may strongly affect the drying behavior, leading to either propanol or water-enriched liquid phases at the end of the drying process. If the evaporation of the solvent results in propanol enrichment, the critical layer thickness at which cracks occur will be increased by about 30% due to a decrease in the capillary pressure. Microscopic images indicate that the crack area ratio and width depend on the wet layer thickness and initial liquid phase composition. These results are of much value for future electrode fabrication, as cracks affect electrode properties

Impacts of Porous Transport Layer Compression on Hydrogen Permeation im PEM Water Electrolysis

Gas permeation through a membrane electrode assembly (MEA) is an important issue in the development of polymer electrolyte membrane (PEM) water electrolyzers, because it can cause explosions and efficiency losses. The influence of operating pressure, temperature and MEA modifications on the permeation was already investigated. However, most of the studies pay no attention to the compression of the porous transport layer (PTL) of the MEA when assembling it in a test cell to carry out the experiments.This paper deals with the impact of the PTL compression on hydrogen permeation and cell voltage. Polarization, impedance and permeation measurements are used to demonstrate that the compression significantly affects the MEA's properties. Measurements show either a linear or nonlinear correlation between current density and hydrogen permeation, depending on the compression.The results indicate that the compression of the PTL must be taken into account for developing MEAs and comparing different permeation measurements