Scientific Highlights

June, 2013

Dosing Differential Electrochemical Mass Spectrometry (D-DEMS) for Li-O2 Batteries

The high-energy rechargeable Li-O2 battery has been subject to intensive research worldwide during the past years. The Li-O2 cell mainly comprises a negative (e.g. Li metal) and positive (e.g. porous carbon) electrode separated by an electronically insulating, but Li+ conducting electrolyte layer. In order to study the cell chemistry, a differential electrochemical mass spectrometry setup based on a set of valves, a pressure sensor and a quadrupole mass spectrometer has been developed. On galvanostatic discharge, oxygen dissolves in the non-aqueous electrolyte, reduces at the porous carbon surface to form mainly Li2O2, as determined from the linear decrease in the oxygen pressure corresponding to a ratio of 2e- per O2 consumed. On charge, the discharge product is oxidized, the lithium ions return to the negative electrode and oxygen gas evolves. Although the oxygen evolution rate initially reaches 2e-/O2, it rapidly drops as the cell over-potential increases. In addition, the evolution of CO2 at 4.3 V vs Li+/Li clearly demonstrates the existence of parasitic side reactions. The D-DEMS, as successfully developed at PSI, is a key tool for analyzing the O2 gas usage, without which conclusions on the cell rechargeability can hardly be drawn.


Presentation slides
Publication: ECL Annual Report 2012

Further publications: Lithium batteries - Phase Boundaries

April, 2013

Radiation grafted membranes developed at PSI outlast state-of-the art commercial membranes in the fuel cell

Components for the polymer electrolyte fuel cell (PEFC) are required to show high performance and durability under application relevant conditions. Furthermore, for commercial viability the materials and processes for component fabrication need to be of los cost. The polymer electrolyte membrane developed at PSI on the basis of the radiation grafting technique has the potential of being produced in cost-effective manner. In recent years, we have collaborated with the Belenos Clean Power to further develop the membrane to commercial competitiveness. We managed, by careful analysis and optimization of the synthesis conditions, to close the performance gap to commercial membranes (Nafion). The durability of the PSI membranes was evaluated in a dynamic (accelerated) test to simulate an automotive cycle. Our membrane showed a durability much superior to that of the unreinforced commercial Nafion 212 membrane. It even outlasted the reinforced and chemically stabilized Nafion XL-100 membrane, a state-of-the-art material for challenging automotive applications. Beyond fuel cells, the radiation grafting technology can be adapted to design polymer electrolyte materials for other electrochemical applications of current and future interest, such as water electrolyzers, redox flow batteries and next-generation lithium batteries.

Presentation slides
Publication:  

Further publications: LEC Publications

January, 2013

Partially reduced graphene oxide paper: a thin film electrode for electrochemical capacitors

One way to utilize graphene and its’ outstanding specific surface area of 2630 m2g-1 for supercapacitor electrodes is by preparing a so called free-standing graphene paper. Such a flexible, conductive graphene-paper electrode was prepared by a flow-directed filtration of graphene oxide dispersion followed by a gentle thermal reduction treatment of the filtrate. The prepared partially reduced graphene oxide paper (GOPpr) showed a dense packing of graphene sheets with a distinct interlayer distance of 4.35 Å. Its electrode quality for energy storage applications was tested by an electrochemical characterization in the aprotic electrolyte tetraethylamonium-tetrafluoroborat in acetonitrile (1M TEABF4 / AN) which is commonly used for supercapacitors. During the first positive or negative charging sweep GOPpr featurs an electrochemical activation. As a result the GOPpr becomes accessible for ion insertion and release. The achieved specific capacitance in galvanostatic discharge measurements reached up to 199 Fg-1 at 0.1 Ag-1 for a positively activated and polarized electrode. Even at high specific currents of 10 Ag-1 this value did not drop below 145 Fg-1. An even higher specific capacitance was determined from cyclic voltammetry where the positively activated electrode yielded up to 270 Fg-1 during the discharge sweep. It’s flexible nature, the simple processability and the outstanding rate handling capability for the anodically activated positively polarized electrode containing neither a conductive agent nor any polymeric binder is among the best reported in literature for organic electrolytes. Therefore GOPpr is a very promising candidate for a positive electrode material for energy storage devices relying on flexible materials and ion insertion / double layer formation.
Publication: Moritz M. Hantel, Tommy Kaspar, Reinhard Nesper, Alexander Wokaun and Rüdiger Kötz, Partially Reduced Graphene Oxide Paper: A Thin Film Electrode for Electrochemical Capacitors, J. Electrochem. Soc. 2013, Volume 160, Issue 4, Pages A747-A750. DOI

Further publications: Electrocatalysis and Interface Group

January, 2012

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

The optimization of thermochemical and electrochemical conversion systems is of high importance 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.

Citation: M. Kenzelmann, S. Gerber, N. Egetenmeyer, J.L. Gavilano, Th. Strässle, A. D. Bianchi, E. Ressouche, R. Movshovich, E.D. Bauer, J. L. Sarrao, and J.D. Thompson, Physical Review Letters 104, 127001 (2010)

Publication: [1] N. I. Prasianakis, T. Rosen, J. Kang, J. Eller, J. Mantzaras, F. N. Büchi, Simulation of 3D porous media flows with application to polymer electrolyte fuel cells, Comm. in Comp. Phys. (in press) (2012). [2] T. Rosén, J. Eller, J. Kang, N. I. Prasianakis, J. Mantzaras, F. N. Büchi, Saturation dependent effective transport properties of PEFC gas diffusion layers, (submitted) (2012)

Further publications: Fuel Cell Systems and Diagnostics Group

September, 2011

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.

Presentation slides
Publications: [1] R. Flueckiger, F. Marone, M. Stampanoni, A. Wokaun, F.N. Buechi, Electrochim. Acta, 56, 2254 (2011) [2] J. Eller, T. Rosen, F. Marone, M. Stampanoni, A. Wokaun, and F.N. Buechi J. Electrochem. Soc., 158, B963 (2011)

Further publications: Fuel Cell Systems and Diagnostics Group)


localcurrent.jpg

January, 2010

Local current measurement in PEFC

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.

Presentation slides
Publications: I.A. Schneider, G.G. Scherer, Handbook of Fuel Cells – Fundamentals, Technology and Applications. Edited by Wolf Vielstich, Hubert A. Gasteige, Harumi Yokokawa.Volumes 5&6, Part 4, Chapter 45, 2009: Advances in Electrocatalysis, Materials, Diagnostics and Durability. ISBN: 978-0-470-72311-1

Further publications: Fuel Cell Systems and Diagnostics Group