MAE 214 FUEL CELL FUNDAMENTALS & TECHNOLOGY Fuel Cell Analyses Methods NFCRC DR. JACK BROUWER MAE 214 Lecture #11 Spring, 2005 FC ANALYSES TECHNIQUES Potential Sweep Methods Linear Sweep Voltammetry (I-V) Cyclic Voltammetry Product Analysis Mass Spectrometry (MS), Fourier Transform InfraRed (FTIR), Gas Chromatography (GC), Vibrational Spectroscopy thin-film and gas/electrode analysis Electrochemical AC Impedance Spectroscopy Four Probe DC method for area specific resistance (ASR) Microstructural Characterization In-Situ, Ex-Situ Scanning Electron Microscope (SEM) Transmission Electron Microscope (TEM) Material Phase/Structure Analyses In-situ, Ex-situ X-Ray Powder Diffraction (XRD) Combinatorial Methods for Catalysts
Fuel Cell Processes Time scales of physical, chemical, electrochemical processes From: Ivers-Tiffée, Weber, and Schichlein, Univ. Karlsruhe Linear Sweep Voltammetry (LSV) Excitation: potential ramp/sweep at constant rate scan rate, ν=de/dt 5 mv/s - 10 V/s Response: voltammogram,, I vs. E Current, A E appl, V E 2 E o x E 1 E app, V Excitation Time, s Response E p E 1 E 2 x E o
Linear Sweep Voltammetry Applications Examine plots of I p vs. n 1/2 Negative deviations Slow kinetics Positive deviations Adsorption limited CC used to quantify I p, A I p, A n 1/2, (V/s) 1/2 slow kinetics adsorption limited n 1/2, (V/s) 1/2 Linear Sweep Voltammetry Applications: I-V V for fuel cell performance Study of kinetics Study of adsorption Determination of: Number of electrons, n; active area, A; and possibly diffusion coefficient, D o ; and product bulk concentration, C * o Characterization of new materials Advantages/ Disadvantages: In principle, high analytical sensitivity Reality: Not a good quantitative technique difficult to discriminate against double-layer charging current
Cyclic Voltammetry Cyclic Voltammetry (CV) Excitation: E 1 to E 2 and back to E 1 Response: I vs. E E app, V E 2 Excitation Diffusion limited response E 1 Time, s Difficult for high-temp. fuel cells due to fast adsorption/desorption desorption kinetics Current, A E a E 1 E 2 Response E appl, V E c Cyclic Voltammetry From: W. Vielstich, Univ. de São Paulo
Cyclic Voltammetry Diffusion limited voltammogram: From: W. Vielstich, Univ. de São Paulo Cyclic Voltammetry Polycrystalline platinum in 1M KOH (100 mv/s sweep rate) From: W. Vielstich, Univ. de São Paulo
Mass Spectrometry (MS) Product Analysis On-line MS Experimental Setup From: Torresi and Wasmus, Univ. de São Paulo Mass Spectrometry (MS) Measure product evolution vs. time (potential, ) Measure reactant consumption vs. time (potential, ) Cyclic Voltammetry concurrent with MS analyses CO 2 mass signal shown Product Analysis Often referred to as MSCV From: Torresi and Wasmus, Univ. de São Paulo
Product Analysis Can use any kind of gas/liquid analyses or mass measurement technique concurrent with test of electrochemical performance: For example: Fourier Transform InfraRed (FTIR) analyses (gas concentrations) Micro-balance (mass measurement) Electrochemical quartz micro-balance (EQMB) Gas Chromotography (gas concentrations) Liquid Chromotography (liquid concentrations) Vibrational Spectroscopy In-Situ FTIR Gas-solid interaction Sample characterization In-Situ Raman Spectro- microscopy Carbon/sulfur deposition Raman Spectroscope Microscope Lens Scattering Light Laser Beam Pressing Flange Quartz Window Teflon Spacer Thermocouple Cooling Jacket Sealing Glass Pt Wire Heating Element SOFC Single Alumina Tube Cell Pressing Flange Cooling Water Cooling Water Fuel Gas Fuel Gas O-ring Pt Wire Air Air Transmission FTIR Emission FTIR * Diffuse Reflectance FTIR Reflection Absorption FTIR * q * 80 o <q <89 o
Vibrational Spectroscopy CV potential scan (a), IR spectra (b), and CO and CO 2 band intensities Pt(100) electrode in 0.1M ethanol + 0.1M HClO 4 solution From: Rodes, Pérez, and Aldaz, Univ. de Alicante Vibrational Spectroscopy Spectra indicative of interactions between gases and electrode surface Shifts in location and magnitude of peaks in spectra due to: type of interaction between surface atoms and gas surface properties 2110 2136 gas properties location 2020 2045 adsorption configuration interaction site properties applied electric field gas concentrations gas species temperature Waterfall plot from Rodes, Pérez, and Aldaz, 2003
Electrochemical Impedance Spectroscopy Sinusoidal perturbation (potential or current) applied to sample that covers a wide range of frequencies Multi-frequency excitation allows: measurement of several electrochemical reactions (steps) with different rates (time constants) measurement of the capacitance of an electrode Related to AC impedance theory, response of a circuit to alternating current or voltage as a function of frequency Analog of AC circuit theory used to characterize the electrochemical system in terms of its equivalent circuit Equivalent circuit models frequently good approximations, data can often be fitted to yield accurate results Electrochemical Impedance Spectroscopy Principal advantage of EIS: a purely electronic model can be employed to represent an electrochemical cell Small excitation amplitudes are typically used 5 to 10 mv peak-to-peak Excitation waveforms of this amplitude cause only minimal perturbation of the electrochemical test system Localized Electrochemical Impedance Spectroscopy (LEIS) uses a precision scanning electrochemistry probe for spatially resolved EIS
Electrochemical Impedance Spectroscopy Apply potential as sine wave with varying frequency Overall Measurement Circuit Wheatstone Bridge Null Circuit Adjustable R 4, C 4 Electrochemical Impedance Spectroscopy Measurement Arrangement I-V Characteristics w/ impedance perturbation around operating point From: Ivers-Tiffée, Weber, and Schichlein, Univ. Karlsruhe
Electrochemical Impedance Spectroscopy Nyquist Plot Re(Z) and Im(Z) vs. log(freq.) From: Ivers-Tiffée, Weber, and Schichlein, Univ. Karlsruhe Electrochemical Impedance Spectroscopy Graphical Evaluation of EIS Oversimplified model of fuel cell From: Ivers-Tiffée, Weber, and Schichlein, Univ. Karlsruhe
Electrochemical Impedance Spectroscopy EIS for varying oxygen partial pressures From: Ivers-Tiffée, Weber, and Schichlein, Univ. Karlsruhe Microstructural Analyses SEM of anode supported SOFC YSZ LSM Anode: NiO/YSZ
Microstructural Analyses SEM of anode supported SOFC Microstructural Analyses SEM with energy- and angle-selective backscattered (ESB) electron imaging: high resolution at low-kv
Combinatorial Methods for Catalysts Unfolding of a 3-D 3 D 220 member composition map into two dimensions From: Mallouk (Ill. Inst. of Tech.) and Smotkin (Penn. State Univ.) Combinatorial Methods for Catalysts Robotic delivery of salt solutions to 715-well array Array of 28 Pt/Rh Rh/Os catalysts in 6M aqueous methanol, ph 6, quanine indicator From: Mallouk (Ill. Inst. of Tech.) and Smotkin (Penn. State Univ.)