Apparatus for spatially resolved impedance spectroscopy on DMFC and - PowerPoint PPT Presentation

Apparatus for spatially resolved impedance spectroscopy on DMFC and PEM Fuel Cells with special regard to fuel concentration oscillations in gas channels T. Sanders a , T. Baumhöfer a , D. U. Sauer a , A. Schröder b , K. Wippermann b , J. Mergel b , D. Stolten b a Electrochemical Energy Conversion and Storage Systems Prof. Dr. rer. nat. Dirk Uwe Sauer Institute for Power Electronics and Electrical Drives (ISEA) RWTH Aachen University Univ.-Prof. Dr. ir. R. W. De Doncker b Institute of Energy Research, IEF-3: Fuel Cells Forschungszentrum Jülich GmbH

Motivation � “Flipping” effect observed in measurements � Caused by measurement method � Not a consequence of MEA state Source: Oscillations in Gas Channels II. I. A. Schneider et al. Journal of The Electrochemical Society 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 2 T. Baumhöfer

Outline � The cause for the measurement effect � Influence of various factors � Model based � An advanced measurement setup � “EIScell” measurement hardware 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 3 T. Baumhöfer

Commonly used measurement setup � The full cell is excited by one voltage source V AC Z = n � Spatially resolved current measurement I n I 1 V AC Oxygen flow I 2 I 3 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 4 T. Baumhöfer

Influence of concentration oscillations Oxygen flow 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 5 T. Baumhöfer

Influences � Absolute oxygen concentration Spectra: 10 Segments, linear channel dU = RT 1 2 2 2 dI z F q C 1 1 � Frequency � Limited Diffusion � Excitation amplitude � No influence 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 6 T. Baumhöfer

Advanced measurement setup � Single excitation, sequential measurement V � Problem: in-plane resistance n Z = n I + I − + I � Smaller concentration oscillation amplitude n n n + 1 1 � Influence only from neighboring segments V 1 I 1 V 2 I 2 V 3 I 3 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 7 T. Baumhöfer

Comparison Cell excitation Single segment excitation 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 8 T. Baumhöfer

Comparison (Zoom) Cell excitation Single segment excitation 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 9 T. Baumhöfer

EIScell � 54 independent channels � ± 2.5 V, ± 2.5 A per channel � AC response from all I,V � simultaneously � DC measurements � I-V curves � Automated � Integration of test stand control 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 10 T. Baumhöfer

Contacting 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 11 T. Baumhöfer

Shunt Resistors 25 mA 25 mA � Voltage drop across shunt resistors leads to an equalization of measured currents 25 mA 25 mA � For the given test cell resistances below 10m � would be needed � Not suitable for measurement of 25 mA 25 mA small currents V meas = mA ⋅ m Ω = µ V 1 10 10 In-plane Shunt Segments Resistances Resistors 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 12 T. Baumhöfer

Shunt Resistors 32 mA 25 mA � Voltage drop across shunt resistors leads to an equalization V diff of measured currents 36 mA 50 mA � For the given test cell resistances below 10m � would be needed V diff � Not suitable for measurement of 32 mA 25 mA small currents V meas = mA ⋅ m Ω = µ V 1 10 10 In-plane Shunt Segments Resistances Resistors 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 13 T. Baumhöfer

Compensated Shunt Resistor � Measurement signal range can be optimized (variable shunt resistance) V meas = 1 mA ⋅ 1 Ω = 1 mV � No influence by wire resistance (Kelvin connection) � Parallel measurement up to high frequencies 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 14 T. Baumhöfer

First Measurement Results Oxygen flow 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 15 T. Baumhöfer

Summary � “Flipping” in spectra is no result from the MEA state, but a measurement influence caused by concentration oscillations. � By using another measurement setup the effect can be reduced. � Magnitude of improvement dependent on relation between in-plane and segment impedance � A compensated shunt resistor allows for precise spatially resolved current measurement without affecting the current distribution. 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 16 T. Baumhöfer

Thank you for your attention bh@isea.rwth-aachen.de 6/23/2009 Institute for Power Electronics and Electrical Drives Slide 17 T. Baumhöfer

Apparatus for spatially resolved impedance spectroscopy on DMFC and PEM Fuel Cells with special regard to fuel concentration oscillations in gas channels T. Sanders a , T. Baumhöfer a , D. U. Sauer a , A. Schröder b , K. Wippermann b , J. Mergel b , D. Stolten b a Electrochemical Energy Conversion and Storage Systems Prof. Dr. rer. nat. Dirk Uwe Sauer Institute for Power Electronics and Electrical Drives (ISEA) RWTH Aachen University Univ.-Prof. Dr. ir. R. W. De Doncker b Institute of Energy Research, IEF-3: Fuel Cells Forschungszentrum Jülich GmbH