933 research outputs found

    Development of Bifunctional Electrodes for Closed-loop Fuel Cell Applications

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    Unitized regenerative fuel cells (URFC) in combination with photovoltaic modules are attractive for space missions because they enable extended operation times and low weight. During the planetary day, electrical energy is stored which can be converted into electricity by the fuel cell during the night. All air-independent applications such as spacecraft or space stations would profit signify¬cantly from such energy conversion devices. A unitized regenerative fuel cell is a combined energy con¬version and storage system based on H2 and O2 which combines the advantages of fuel cells and secondary batteries. Substantial advantages of the specific energy density can be expected from the use of a URFC (400-1000 Wh/kg) in comparison to secondary batteries (220-250 Wh/kg for future advanced Li-polymer batteries). An important topic is the function of so-called bifunc¬tional oxygen electrodes which generally require the combination of favourable properties for both operating modes. In particular, different catalysts for oxygen reduction and for oxygen evolution are needed. This contribution investigates various electrode designs with Pt and IrO2 catalysts. For that purpose, the DLR dry spraying technique for the manufacturing of electrodes is applied for by mixing the two different catalysts together (Pt and IrO2) or applying the catalyst on different areas of electrode or even realising different layers with both catalysts. The different options are explained in Fig. 1. Of interest was to compare the simple mixing of the catalysts (option 1), the layered electrode with two compositions (option 2) and the segmented approach with dedicated areas with just one catalyst (option 3)

    Protective Coatings for Low-Cost Bipolar Plates and Current Collectors of Proton Exchange Membrane Electrolyzers for Large Scale Energy Storage from Renewables

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    Hydrogen produced by proton exchange membrane (PEM) electrolysis technology is a promising solution for energy storage, integration of renewables, and power grid stabilization for a cross-sectoral green energy chain. The most expensive components of the PEM electrolyzer stack are the bipolar plates (BPPs) and porous transport layers (PTLs), depending on the design. The high cost is due to the fact that the employed materials need to withstand corrosion at 2 V in acidic environment. Currently, only titanium is the material of choice for the anode side. We use vacuum plasma spraying (VPS) technology to apply highly stable coatings of titanium and niobium to protect stainless steel BPPs from the oxidative conditions on the anode side. The latter is able to decrease the interfacial contact resistance and improves the long-term stability of the electrolyzer. Furthermore, porous transport layers (PTL) can be realized by VPS as well. These coatings can be produced on existing titanium current collectors acting as macro porous layers (MPL). Lastly, free standing multifunctional structures with optimized tortuosity, capillary pressure and gradient porosity are used as current collectors. The coatings and porous structures developed by VPS enable the reduction of the required material and costs without performance losses

    Application of In-Situ Diagnostic Methods for the Study of SOFC Operational Behaviour

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    In order to optimise the operational behaviour of fuel cells and minimise cell degradation it is very helpful to use in-situ and ex-situ analytical methods. The application of advanced diagnostic methods by monitoring cell characteristics under real operating conditions provides valuable information to be used for the development of degradation mitigation strategies. The application of these methods in SOFC development is significantly limited mainly due to the experimental problems that are associated with the high operating temperature. However, an increasing effort in developing and implementing non-traditional analytical methods such as spatially resolved measurements and imaging techniques for SOFC development has been made in recent years. The presentation gives an overview of in-situ diagnostic methods that are applied at DLR Stuttgart for the study of SOFCs. It includes spatially resolved measurements with an experimental segmented cell configuration where different techniques such as IV characteristics, impedance spectroscopy, gas chromatography and temperature measurement are involved. The investigation by means of segmented cells aims at the determination of local effects and the identification of critical operating conditions during technically relevant SOFC operation. Recently, a new test setup with transparent optical access has been built up allowing for microscopic observation of processes within the cell as well as for application of in-situ laser Raman spectroscopy to determine highly resolved concentrations of gas species along a flow channel. Examples of analytical studies by applying these diagnostic methods are presented and potentials and limitations of the different techniques are discussed

    Reversible and Irreversible Degradation Phenomena in PEMFCs

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    The presentation is focused on reversible and irreversible degradation phenomena in polymer electrolyte membrane fuel cells (PEMFCs). Analytical methods for the determination of component degradation will be presented and a new systematic approach for the analysis of reversible and irreversible degradation rates in an operating fuel cell will be introduced. A detailed description of voltage loss rates and particularly of the discrimination between reversible and irreversible voltage losses will be given. A major motivation of the presented work is the lack of common description procedures and determination approaches of voltage losses in durability tests of fuel cell. This issue results in severe difficulties in the comparison of results obtained by different testing facilities or within different research projects especially if only one value for a degradation rate is reported. In order to systematically analyze voltage losses we have performed single cell durability measurements of several hundreds of hours in 25 cm2 lab-scale cells. Specific test protocols containing regular refresh procedures were used for this purpose (see Figure 1). This enables distinguishing between reversible and irreversible voltage losses. To test the refresh procedures and analyze their effect on cell performance, parameters such as the duration of e.g. a soak time step have been varied. Between these refresh steps the cells were typically operated for 50 to 150 h. Conventional 5-layer MEAs with PFSA membranes, carbon supported Pt-catalysts and hydrophobized carbon fiber substrates with micro porous layers as GDLs were used for this study. For in-situ diagnostics of the operated cells polarization curves, impedance spectra, and CVs were recorded in order to determine the impact of the refresh procedures on the cells. Ex-situ methods were used to determine the causes for the reversible and irreversible voltage losses. Using different methods, detailed information about the physical composition of the individual fuel cell components can be obtained in order to optimize them and increase cell durability. Depending on the examined component and the analytical objective infrared absorption spectroscopy (FTIR), Raman, and X-ray photoemission spectroscopy (XPS) can be used to analyze the degradation effects and the sources for reversible and irreversible voltage loss during fuel cell operation. An overview of the different methods and their application will be given. It will be shown, that a combination of complementary methods is necessary to gather a comprehensive view of the occurring processes and mechanisms. As an example, depth profiling techniques combined with XPS can be used to determine the composition changes inside the fuel cell electrodes

    Application of Electrochemical Impedance Spectroscopy on Different Battery Circuits

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    For the operation of a battery pack, the cell state estimation plays a central role. For that, enough information about the current charge condition (SoC, state of charge) and the health status (SoH, state of health) of the individual cells or cell strings must be available. One way to draw out conclusions about the state of charge and health provides the electrochemical impedance spectroscopy (EIS) [1]. The test cells are thereby stimulated with an alternating current signal, and the resulting voltage signal is detected. These results in cell impedances, which are addicted to the signal frequencies and the respective cell states. This poster shows an experimental platform which uses the EIS to detect asymmetries in SoC and/or SoH on circuited cells. For that, the behavior of the amplitudes and frequencies of the signals should be analyzed, because for the calculation of the precise impedance, these factors are crucial. Thereby the required alternating current and voltage signals are acquired and analyzed separately for each single cell. As cell type lithium iron-phosphate round cells of the size 18650 are used. The investigations are made on a series circuit (Fig.1) made up of three cells and on a parallel circuit made up of two strings, each having two cells in series. It shows that both a series and a parallel connection within the working range the experimental platform impedances of individual cells can be determined. For these cases, differences in state of charge and state of health can be highlighted and assigned to the respective cells

    Effect of the Inlet Gas Humidification on PEMFC Behavior and Current Density Distribution

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    Water management represents one of the main challenges in the design and operation of PEMFCs. The influence of inlet gas humidification on cell performance is analyzed using in-situ diagnostic tools, such as cyclic voltammetry and segmented cell current density measurements, supported by post-mortem ex-situ investigations. Particular attention is paid to the effect of low humidity conditions in both cathode and anode, under which the cell is observed to suffer severe voltage decline. A simple onedimensional water balance model is proposed to contribute to the understanding of the various operation regimes observed in PEMFCs under medium-to-low humidification conditions

    Durability Testing of Polymer Electrolyte Fuel Cells Under Stationary and Automotive Conditions

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    Our presentation focuses on durability testing and degradation of fuel cells. A major motivation of our work is the lack of common description procedures and determination approaches of voltage losses in durability tests of fuel cell for both stationary and automotive applications; this issue leads to severe difficulties in the comparison of results obtained by different institutions or within different projects, especially if only a single value for the degradation rate is reported. In this context, special attention is devoted to the discrimination between so called reversible and irreversible voltage losses. The first are permanent and determine the maximum lifetime of a fuel cell. The latter strongly depend on the chosen operation conditions and can be recovered by specific procedures. In order so systematically address voltage losses we have performed single cell durability measurements of several hundreds of hours in 25 cm2 lab-scale cells using different test protocols containing regular refresh procedures (soak time) allowing to distinguish between reversible and irreversible losses. Furthermore, operation strategies to minimize reversible degradation without using the time consuming refresh procedures are provided. To test the refresh procedures and analyze their effect on cell performance, parameters such as duration of the soak time steps have been varied. Between these refresh steps the cells were typically operated for 50 to 150 h. As samples conventional 5-layer membrane electrode assemblies were used with PFSA membranes, Pt-based catalysts and hydrophobized carbon fiber substrates with micro porous layers as GDLs. For in-situ diagnosis of the operated cells polarization curves, electrochemical impedance spectra, and cyclic voltammograms were recorded in order to determine the impact of the operation conditions and the refresh procedures on degradation. The interpretation of the degradation of the measured membrane electrode assemblies is supported by post-mortem analysis using physical characterization techniques. Additionally, we provide possible approaches to quantitatively determine irreversible voltage decay rates. For instance, voltage values before or after voltage recovery steps can be used to calculate the irreversible loss rate. The advantages and drawbacks of different approaches are discussed. One clear conclusion is that short time tests in the range of 100 hour are not conclusive since this time is too short to make a reliable discrimination between reversible and irreversible losses; also, the decay rate of reversible loss observed after each refresh step increases substantially upon long time operation independent on the type of the refresh procedure. In summary, in our presentation strategies for determination of fuel cell voltages loss rates are compared, evaluated and assessed according to their suitability to distinguish between reversible and irreversible degradation rates; a description of voltage loss rates is proposed. Moreover, operation strategies to minimize reversible degradation are provided. The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant No. 621216 (SecondAct) and No. 303452 (Impact)

    Проект модернизации оборотного водоснабжения ТЭЦ ООО "Юргинский машзавод"

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    ТЭЦ являются одним из основных загрязнителей атмосферы твёрдыми частицами золы, окислами серы азота, другими веществами, оказывая вредное воздействие на здоровье людей, а также углекислым газом, способствующим возникновению «парникового эффекта». Поэтому предлагается сократить вредные выбросы путем оптимизации водно-химического режима ТЭЦ. Thermoelectric plant is one of the major polluters of the atmosphere solid particles of ash, nitrogen oxides, sulfur, and other substances, exerting harmful effects on human health, as well as carbon dioxide, contributing to the emergence of the "greenhouse effect." It is therefore proposed to reduce emissions through the optimization of water chemistry thermoelectric plant

    Mass transport limitations in concentrated aqueous electrolyte solutions : theoretical and experimental study of the hydrogen-bromine flow battery electrolyte

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    Modelling and simulation is a powerful tool to support the development of novel flow cells such as electrolysers and flow batteries. Electrolytes employed in such cells often consist of aqueous solutions of highly concentrated solutes at elevated temperatures. Such conditions pose numerous challenges in conventional model parametrisation because of non-ideal behaviour of the electrolytes. The aim of this work is to study mass transport of electroactive species in highly-concentrated media. We selected the hydrogen-bromine flow battery posolyte, HBr (aq) and Br2, as an exemplary flow battery electrolyte and we leveraged chronoamperometric techniques involving ultramicroelectrodes to study diffusion and migration of bromide and bromine at high concentration and temperature. We successfully simulated the current densities of HBr/Br2 redox reactions in solutions up to 8 mol L–1 using advanced mass transport theory which agreed well with the results obtained with ultramicroelectrodes. While uncharged species transport (Br2) can be credibly modelled using conventional theories such as Fick’s law, charged species (Br–) require special treatment as the diffusion coefficient vary with concentration up to 50 % with respect to the limiting value at infinite dilution. The transport of charged species without added supporting electrolyte occurs via both migration and diffusion and the contribution of migration current may be up to 50 % of the total current. At HBr concentration  0.6 mol L–1 migration appears to be suppressed due to the “self-screening” effect of the electrolyte. Proper experimental electrolyte characterisation under operating conditions similar to the actual flow cell applications is indispensable to establish predictive models and digital twins of electrochemical devices. Straightforward transfer of concepts known in electro-analytical chemistry to flow cells modelling may lead to erroneous simulations or model overfitting
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