Monitoring of membrane failure due to pinhole formation Viktor - PowerPoint PPT Presentation

Monitoring of membrane failure due to pinhole formation Viktor Hacker, Eva Wallnöfer Department of Chemical Engineering and Environmental Technology, TU GRAZ, Austria Peter Prenninger AVL List GmbH, Austria Trondheim, June 23 rd ,2009

Overview 1. Introduction degradation of PEMFC membranes � influence of operating conditions � 2. In-situ Degradation Studies operating conditions � characterization methods � 3. Conclusions Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 2

Degradation of PEMFC Membranes – impacts of degradation � membrane decomposition (release of HF, SO 2 , CO 2 , CO, C-F-compounds) � membrane thinning � higher gas permeability � platinum particle deposition in the membrane � performance loss of the MEA � decrease of life time Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 3

Degradation of PEMFC Membranes – causes � thermal degradation � mechanical degradation � chemical degradation Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 4

Degradation of PEMFC Membranes – influencing factors o material � membrane thickness � gas pressure properties � temperature gas humidity � o cell assembling � cell potential o operating conditions Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 5

In Situ Degradation Studies Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 6

Operating Conditions 5 to 1 fuel cells (in series), 25 cm 2 each � operated under the same conditions up to 1300 h permanent operating 24h/day, 7 days/week � interruptions only for electrochemical � characterizations every one or two weeks, one cell was removed � (SEM analysis) � gas flow: H2: λ = 1.5 (at OCV: 300 ml min -1 ) � Air: λ = 2.2 (at OCV: 300 ml min -1 ) � � MEAs: pt loading: A: 0.4 mg cm -2 , C: 0.6 mg cm -2 � membrane: bilayer membrane, reinforced with � PTFE, thickness: 35 µm activation of the MEAs ( 6 h at 0.4 and 0.6 V) � Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 7

Characterisation Methods � performance (UI) � cell potential (CP) � membrane resistance (MR) � fluoride emission rate (FER) � pinhole detection (PD) � membrane thickness and condition (SEM) Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 8

Hydrogen Diffusion anode: H 2 flow / cathode: N 2 flow � standard conditions H 2 diffuses through the membrane and gets oxidised � with an increasing potential the hydrogen diffusion current is limited by diffusion to < 5 mA cm -2 � if there is a pinhole, the current increases with increasing potential � 14 14 14 14 14 14 0 h 110 h 280 h 450 h 620 h 790 h 960 h 1130 h Po t e n t a l i V / -2 -2 -2 -2 -2 -2 12 12 12 12 12 12 Current Density / mA cm Current Density / mA cm Current Density / mA cm Current Density / mA cm Current Density / mA cm Current Density / mA cm Hydrogen Diffusion Hydrogen Diffusion Hydrogen Diffusion Hydrogen Diffusion Hydrogen Diffusion Hydrogen Diffusion 10 10 10 10 10 10 8 8 8 8 8 8 6 6 6 6 6 6 4 4 4 4 4 4 2 2 2 2 2 2 0 0 0 0 0 0 0.00 0.00 0.00 0.05 0.05 0.05 0.10 0.10 0.10 0.15 0.15 0.15 0.20 0.20 0.20 0.25 0.25 0.25 0.30 0.30 0.30 0.35 0.35 0.35 0.40 0.40 0.40 0.45 0.45 0.45 0.50 0.50 0.50 0.00 0.00 0.00 0.05 0.05 0.05 0.10 0.10 0.10 0.15 0.15 0.15 0.20 0.20 0.20 0.25 0.25 0.25 0.30 0.30 0.30 0.35 0.35 0.35 0.40 0.40 0.40 0.45 0.45 0.45 0.50 0.50 0.50 Potential / V Potential / V Potential / V Potential / V Potential / V Potential / V Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 9

Hydrogen Diffusion - Results � the formation of a pinhole can not be forecasted � end of membrane lifetime: time interval, at which the hydrogen diffusion current 90 density is in the range 80 -2 hydrogen diffusion current density between 4 and 5 mA cm -2 between 0.2 amd 0.5 V / mA cm 70 60 50 40 standard 30 low humidity low temperature 20 high pressure 45 mA 10 90 mA 0 135 mA 0 100 200 300 400 500 600 700 800 900 1000 1100 405 mA time / h 90 mA, low humidity Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 10

Cell Performance � the cumulated performance of the stack was investigated; H 2 : λ = 1.5 / Air: λ = 2.2 � no gas pressure; T: 70 °C � polarisation curves: changes with operating time 0 h 110 h 280 h 450 h 620 h 790 h 960 h 1130 h Po t en t V al i / low temperature (40 °C) low humidity (40 % rH) standard -2 -2 1 1 1 1 1 135 mA cm 45 mA cm 0.9 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.8 0.8 Potential / V Potential / V Potential / V Potential / V Potential / V 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.0 0.0 0.0 0.0 0.0 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.4 0.4 0.4 0.6 0.6 0.6 0.6 0.6 0.8 0.8 0.8 0.8 0.8 1.0 1.0 1.0 1.0 1.0 1.2 1.2 1.2 1.2 1.2 -2 -2 -2 -2 -2 Current Density / A cm Current Density / A cm Current Density / A cm Current Density / A cm Current Density / A cm Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 11

Cell Performance - Results standard low humidity low temperature high pressure 45 mA 90 mA 135 mA 405 mA 90 mA, low humidity maximum power density ▫ changes with operating time ▫ the performance loss is related to the membrane degradation, but also influenced by the electrode degradation ▫ the formation of the first pinhole is not exactly related to a certain loss of performance Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 12

Cell Performance - Results standard low humidity low temperature high pressure 45 mA 90 mA 135 mA 405 mA 90 mA, low humidity OCV ▫ OCV decreases with the operating time (non-linear) ▫ normalised OCV (to 0.95 V) ▫ the formation of the first pinhole is related to a loss of 30 - 70 mV (mostly ~ 50 mV) of the initial OCV of the “stack” because of the increasing gas diffusion through the degradated membrane ▫ a certain influence of the electrode degradation may exist Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 13

Fluoride Emission Rate - Results 0.010 Anode Standard ▫ FER in the anode and cathode Cathode Standard exhaust water was nearly the 0.008 same, even though the degradation FER / mg F - cm -2 h -1 of the anode side was higher ( → SEM investigations) 0.006 ▫ FER was slightly decreasing with 0.004 operating time 0.002 0.000 0 100 200 300 400 500 600 700 800 Time / h Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 14

Fluoride Emission Rate - Results standard low humidity low temperature high pressure 45 mA 90 mA 135 mA 405 mA ▫ accumulation of the total anode 90 mA, low humidity and cathode FER: ▫ the formation of the first pinhole is related to a certain cumulated FER: Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 15

Membrane Resistance - Results ▫ change of the resistance with 1.2E-02 the operating time 1.1E-02 ▫ the resistance does not correlate clearly to 1.0E-02 membrane resistance / Ω ▫ operating conditions 9.0E-03 ▫ operating time 8.0E-03 ▫ pinhole formation ▫ membrane thickness 7.0E-03 (SEM) 6.0E-03 ▫ a slight increase of the resistance could be observed at higher 5.0E-03 degradation/ long operating 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 time in most cases time / h standard ▫ the resistance is influenced by low humidity low temperature ▫ the loss of proton conducting, high pressure hydrophilic functional groups 45 mA 90 mA ▫ the structural changes of the 135 mA hydrophobic phase 405 mA ▫ the thinning of the membrane 90 mA, low humidity Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 16

Scanning Electron Microscopy - Results standard low humidity low temperature high pressure membrane thickness 45 mA ▫ the thinning of the 90 mA membrane is not clearly 135 mA related to operating 405 mA conditions 35 ▫ membrane thinning occurs under moderate and harsh membrane thickness / µm 30 conditions ▫ pinhole formation is not 25 related to a significant membrane thinning 20 15 10 5 0 100 200 300 400 500 600 700 800 900 1000 1100 time Diagnostic Tools for Fuel Cell Technologies Professor Horst Cerjak, 19.12.2005 17