Scientific Highlights ENE

Flow modeling in gas diffusion layers of PEFCs at the micro- and mesoscale.

The optimization of thermochemical and electrochemical conversion systems is of high mportance for a sustainable energy future society. Of particular interest regarding the performance of polymer electrolyte fuel cells (PEFCs) is the study of the gas flow in the gas diffusion layers (GDL). More specifically, permeability and diffusivity measurements in a model PEFC under normal operating conditions are highly desirable. As laboratory-measurements of these quantities under such conditions are very demanding, an alternative is the use of computer-based simulations. For this, two key elements are needed: a) an advanced numerical tool capable of modeling key microscale processes, and b) in-situ X-ray tomographic microscopy (XTM) scans of the GDL material. Physical modeling of 3D gas flows is accomplished through novel mesoscale computational algorithms based on the lattice Boltzmann method (LBM).

The provided figure illustrates computed flow streamlines through the GDL porous structure (carbon fiber paper Toray TGPH 060, domain size: 444x222x160 microns). The GDL microstructures, wherein the produced liquid water can be distinguished from the solid material, are obtained at the TOMCAT beamline of the Swiss Light Source (SLS). The results show that permeability and relative effective diffusivities of dry and partially liquid saturated GDL samples follow a relation proportional to (1-s)x, where (s) is the saturation level and the exponent x is approximately 3.

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X-Ray Tomography of Water in Operating Fuel Cell

Polymer electrolyte fuel cells (PEFC) convert the chemical energy of hydrogen with a high efficiency (40-70 %) directly into electricity. The product of the overall reaction is water, produced at the cathode of the cell. The interaction of liquid water with the porous structures of the cell is one of the mechanisms in the PEFC that are commonly believed to be key for further optimization with regard to performance, durability and cost. Synchrotron based X-ray tomographic microscopy (XTM) allows for the simultaneous in situ visualization of the water and carbonaceous structures in the gas diffusion layer (GDL) on the pore scale level [1, 2]. In-situ XTM scans of operating PEFC are performed within a few seconds per scan and pixel sizes of 2 - 3 µm. Experiments are made at the TOMCAT beamline of the Swiss Light Source (SLS).

The figure shows XTM surface renderings of the cathode channel with flow field plate, GDL, liquid water and catalyst layer.

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Shedding light on a dark state: The energetically lowest high-spin state of C₂

The Swan band emission between 400 and 700 nm is a prominent feature in all carbon-containing flames. The intense d 3∏g - a 3∏u electronic transition is widely used to detect the molecule in combustion and astronomy studies to characterize and test chemical mechanisms.

The quantitative interpretation of spectra requires precise molecular constants for the computation of the complex molecular spectra of C₂.

In this work we report on the deperturbation of the d 3∏g , v=6 state by double-resonant four-wave mixing. The high sensitivity and dynamic range of the method allows observation of 'extra lines'. These weak spectral features originate from nearby-lying, optically dark states that gain transition strength through the perturbation process. The study unveils the presence of the energetically lowest high-spin state of C₂ in the vicinity of the d 3∏g , v=6 state. This quintet state (5∏g) unravels major issues of the so-called high-pressure bands of C₂. The anomalous nonthermal emission initially observed in 1910 and later in numerous experimental environments is rationalized by taking into account 'gateway' states, i.e. rotational levels of the d 3∏g , v=6 state that exhibit significant quintet character through which all population flows from one electronic state to the other.

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A two-step solar thermochemical CO₂-splitting cycle using Zn/ZnO redox reactions

A two-step thermochemical cycle for reducing CO₂ into CO has been experimentally demonstrated using concentrated solar radiation as the energy source of high-temperature process heat. The first, endothermic, solar step is the thermal dissociation of zinc oxide into zinc. The second, exothermic, non-solar step is the reaction of zinc with CO₂, yielding CO and the initial zinc oxide, which is recycled to the first step. A Second-Law thermodynamic analysis for the net reaction CO₂=CO+0.5O₂ indicates the potential of reaching solar-to-chemical energy conversion efficiencies of up to 39%. CO can be used as combustion fuel for power generation or further processed to synthetic liquid fuels for transportation.

The adjacent scheme depicts the solar chemical reactor configuration for the production of Zn by ZnO-reduction at 2000 K. It features a cavity-receiver lined with AUTHOR_WWW/ENE.ZnO particles that are directly exposed to high-flux solar irradiation, providing a very efficient heat transfer mechanism to the reaction site.

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Hydrocarbon-fueled catalytic microcombustors

The design of portable power generation units using small-scale thermal engines requires a deep understanding of combustion and heat transfer processes at the microscale. Sophisticated CFD models accounting for flow-field description, combined heterogeneous and homogeneous chemical reactions and heat transfer mechanisms, have been used to delineate the stable combustion regimes of methane- and propane-fueled catalytic honeycomb microcombustors. Stability diagrams of hydrocarbon-fueled catalytic microreactors, in terms of maximum allowable heat losses to the environment versus channel inlet velocity, have identified favorable reactor wall materials for steady-state microreactor operation. Metallic materials (such as AUTHOR_WWW/ENE.FeCr alloy) display wider combustion stability envelopes compared to ceramic materials (e.g. cordierite), owing to the higher upstream heat transfer through the microreactor walls, which in turn enhances the preheating of the incoming fuel/air mixture. Comparison between different hydrocarbon fuels, namely methane and propane, revealed a significant impact of fuel transport properties on microreactor stability, particularly on the high-velocity (blow-out) branch of the stability envelope. While methane is less reactive than propane, both catalytically and in the gas phase, it compensates more efficiently for heat losses towards the environment than propane, owing to its higher diffusive transport towards the catalytic surface.

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Identification of the SCR active sites in Fe-ZSM-5

The identification of the SCR active sites in Fe-ZSM-5 is of utmost importance for the understanding and optimization of the catalyst performance. No method (e.g. UV/VIS, IR, EPR, EXAFS, XPS, XRD) can definitively distinguish between isolated iron species and iron oxide clusters of different nuclearity in the same sample. A statistical approach was used to solve this problem. From the correlation of the measured SCR activity with the calculated concentration of different species the temperature dependent activities of isolated, dimeric and oligomeric iron species and iron particles could be determined.

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Local current measurement in PEFCs

Major barriers for a successful commercialization of Polymer Electrolyte Fuel Cells (PEFCs) are insufficient lifetime and high cost of platinum catalyst. A comprehensive understanding of aging and transport phenomena on all relevant length scales is a key to improve durability and to reduce precious metal loading. Flow fields as used in PEFCs for the distribution of the reactant gases over the electrode area cause inhomogeneities. The importance of down the channel inhomogeneities has been realized. Inhomogeneities in the perpendicular to the flow channel direction, however, have not received adequate attention to date, possibly due to the lack of direct experimental evidence. A novel approach allows for the first time the direct measurement of the local cell current in channel and land areas of PEFCs with sub-millimeter resolution. The high potential of our method is demonstrated here in the evaluation of in-plane current transients during start-up of a PEFC and in transient flooding experiments in combination with neutron radiography for liquid water detection. The method provides key information that is badly needed for the understanding of transport and degradation phenomena and for the assessment of mitigation strategies.

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Evolution of Organic Aerosols in the Atmosphere

Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models.

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Vibrational Spectra of Adsorbates from DFT

The hydrolysis of isocyanic acid was studied experimentally and theoretically and a reaction mechanism on different catalysts was established. The decreasing NOx emission limits for diesel vehicles impel the further development of the existing NOx deactivation technologies, particularly the selective catalytic reduction (SCR) of nitrogen oxides with urea. In the urea-SCR process, urea is injected into the hot exhaust gas, where it thermally decomposes into isocyanic acid (HNCO) and ammonia. HNCO quickly hydrolyses on the surface of SCR catalysts and even faster on the surface of specialized hydrolysis catalysts. The theoretical Density Functional Theory (DFT) method with cluster model was used. The reaction path was studied by adsorption of water and isocyanic acid, followed by optimization of the adsorbates structure. Additionally, vibrational analyses of the adsorbates were made. In the experimental part activity tests and surface sensitive techniques (XRD, XPS and DRIFTS) were applied. Measured DRIFT spectra could be reproduced by theoretical calculations. Based on the computational screening of different catalysts we can confirm the experimental results, which show that TiO2 is the best catalyst for isocyanic acid hydrolysis. Presented methodology of DFT modeling and "virtual" catalyst screening can be used for successful combination with different experimental methods available at PSI and for variety of questions and complex catalytic systems.

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