WIR SCHAFFEN WISSEN – HEUTE FÜR MORGEN
Lorenz Gubler :: Paul Scherrer Institut Membranes for Water Electrolysis - Target-Oriented Choice and Design of Materials
Durability and Degradation Issues in PEM Electrolysis Cells and its Components February 16, 2016 :: Fraunhofer ISE, Freiburg, Germany Acknowledgement
Norway Sintef United Kingdom Johnson Matthey Fuel Cells Teer Coatings
Germany Fraunhofer ISE France Areva H2Gen CEA
Switzerland PSI
The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n°303484. Page 2 Table of Contents
Introduction . Loss terms in PEM electrolysis Selection Criteria for Membranes . A simple performance model . Resistance - gas crossover tradeoff . Membrane development within NOVEL Durability Aspects . Radical-induced degradation . Thermal stress test
Conclusion
Page 3 Loss Terms in PEM Electrolysis
slope: 0.23 ∙cm2 (~Nafion 117)
Ohmic (mainly membrane) losses dominate at high current density K. Ayers et al., ECS Trans. 33/1 (2010) 3
Page 4 Simple Electrolysis Model
iR = 0.33 V @ 2 A·cm-2 2.0 10
)/h Ucell = U0 + b·log(i/i0) + iR : Ucell 2 8 1.8 Parameters: U = 1.23 V 6 0 b = 70 mV/decade 1.6 i = 10-5 A/cm2 U - iR 0 geom cell 4 2 R = 167 ·cm (Nafion 117 in water @ 80C) CellCell Voltage Voltage / / V V 1.4 2 required to make 1 kg(H 1 make to required 2
0 m 0.0 0.5 1.0 1.5 2.0 2.5 Current Density / A·cm-2 Increasing current density can reduce size of electrolyzer
Page 5 Reducing Membrane Thickness
R = R0 + Rm
2.4 non-membrane ohmic resistance: 2 R0 30 m·cm 2.2 R = / m conductivity * 2.0 membrane thickness
1.8 2 (m) R (mOhm·cm ) 1.6 Cell Voltage / V PFSA 200 m 200 167 PFSA 60 m 1.4 PFSA 25 m 60 71 25 47 012345678 Current Density / A·cm-2
* = 74% (HHV) Reducing membrane thickness allows much higher current density at given cell voltage (i.e., efficiency)
Page 6 Reducing Membrane Thickness
-2 Residual ohmic resistance R0 may 12 2 R = R0 30 m·cm increase over time due to passivation of Ti current collector and bipolar plate 10
8
6
4
2
0 Current Density @ 2.0Current Density V / A·cm 0 50 100 150 200 Membrane Thickness / m
Page 7 Performance reported by 3M
50 m PFSA membrane (825 EW)*, NSTF catalyst, 80C, 1 bara
50 W/cm2 !
*0.25 S/cm @ 80C Thermal management ?
Lewinski et al., ECS Transactions 69 (2015) 17, 893
Page 8 High Pressure Operation: Lab Test Data
3.0
2.5
2.0
Cell volatage (V) 1.5
1.0 0 1234 Current density (A/cm2) psmaller gas bubbles lower mass transport losses There is little influence of the pressure on the polarization behavior of the electrolysis cell A. Reiner, Siemens
Page 9 Area Resistance and Membrane Thickness
80C, 100% r.h. 200
Area resistance R 2 (Nafion with EW 1'100): ·cm
150 R = R0 + Rm Rm = / 100 fitting parameters: 2 R0 = 30 ·cm 50 = 146 mS/cm Area Resistance / m 0 0 50 100 150 200 Membrane Thickness / m
Page 10 Resistance - Gas Crossover Tradeoff
80C, 100% r.h., 2.5 bara fit H2 crossover ix: H
5 NR211 2 2 -2 x mol∙cm XL-100 → P(H ) = 4.5·10-13 4 2 cm2∙s∙kPa
3 NR212 fit area resistance R (EW 1'100): N1035 2 N105 R = R0 + / N115 N117 2 N1135 → R0 = 30 ·cm , = 146 mS/cm Crossover / mA·cm
2 1 H
0 0 50 100 150 200 250 Area Resistance / m·cm2
A. Albert, ACS Appl. Mater. Interf. 7 (2015) 22203
Page 11 Gas Crossover - Minimum Current Density
Nafion gas permeabilities
-2 -2 1 bar P(H2) P(O2) Ref. 10 bar mol∙cm mol∙cm cm2∙s∙kPa cm2∙s∙kPa 10 100 bar 5.3210-13 2.5210-13 1 / A·cm / A·cm 2 2 -13 -13 i @ 2.0 V 5.1010 2.7010 2 -13 -14 in O in O 1 1.810 8.010 3 2 2 4.5010-13 n/a 4
0.1 P(H2) ≈ 2P(O2) for 2% H 2% for for 2% H 2% for min min i i O 2 O in H ∙ ∙ 0 50 100 150 200 2 2 Membrane Thickness / m H 4 H in O ∙ ∙ 2 2 1 M. Schalenbach et al., J. Phys. Chem. C 119 (2015) 25145 4 O in H 2 T. Sakai et al., J. Electrochem. Soc. 132 (1985) 1328 2 2 3 Z. Zhang et al., J. Membr. Sci. 472 (2014) 55 4 fit of H2 crossover data Page 12 Operational Range (Turndown Ratio)
8 imax = i @ 2.0V 7.55 p = 20 bar -2 7 a 6.95 6 5.20 5
4 imin = limit of 2% H2 in O2* 3 (proportional to p !) 2.35 2.90 2 * ~equivalent to 0.5% O2 in H2 1 Current Density / A·cm Current Density 0.87 0 200 60 25 Operational range with thin membranes is limited due to Membrane Thickness / m crossover issue → Need to increase gas barrier properties of membrane
Page 13 Membrane Development Strategies
80C, 100% r.h., 2.5 bara 1. PFSA membranes
5 improve gas barrier properties -2 Nafion membranes 4 2. Alternative membranes choose materials with intrinsically 3 better combination of resistance and gas permeability 2 Crossover / mA·cm
2 1
H 2. 1. 0 0 50 100 150 200 250 Area Resistance / m·cm2
Page 14 Reinforced PFSA Membranes with Recombination Catalyst
60C, 1 bara 2.2
2.0 JM PFSA membrane: 1.8 (adapted automotive membrane) • thickness ~60 m 1.6 • reinforced 1.4 Nafion 117 • containing PGM-type H -O Cell Voltage / V Cell Voltage JM PFSA 2 2 recombination catalyst 1.0 JM PFSA with recom cat 0.8
/ % 0.6 2 0.4 → O 2
H 0.2 0.0 • Improved performance 0.00.51.01.52.0 • lower gas crossover Current Density / A·cm-2
Page 15 Ion Conducting Graft Copolymer Membranes
barrier comonomer crosslinker
base polymer
• partially fluorinated film (ETFE, ECTFE) • chemically stable • mechanically robust polyelectrolyte grafts • ion exchange groups • additional functionalities (e.g. crosslinker, barrier groups)
L. Gubler, Adv. Energy Mater. 4 (2014) 1300827
Page 16 Gas Permeation Properties
80C 20 Gas permeation measured Nafion 212 (60 m)
) Grafted membrane (50 m) by mass spectrometry method* -1 15 ·kPa -2
·cm 10 -1 Permeation 2 ·mol·s O 5 -15 (10
0 050100 Relative Humidity (%) Grafted membrane shows much lower gas crossover
* Z. Zhang et al., J. Membr. Sci. 472 (2014) 55
Page 17 Property Map
80C, 100% r.h., 2.5 bara wet state, machining direction 40 grafted 5 Grafted -2 membranes grafted 4 30 XL-100 X-linked
N115 3 Nafion 20 membranes N117 2 10 Crossover / mA·cm Crossover Tensile Stress / MPa
2 1 H
0 0 0 50 100 150 200 250 0 50 100 150 200 250 Area Resistance / m·cm2 Strain / %
A. Albert et al., ACS Appl. Mater. Interf. 7 (2015) 22203
Page 18 Cell Performance with Grafted Membrane
60C, 1 bara (tested @ JMFC) Grafted membrane vs. Nafion 117 2.2 • similar performance 2.0 • lower gas crossover 1.8 (factor ~2)
1.6 Nafion 117 Grafted 1.4 membrane Cell Voltage / V
0.6 0.5 0.4 / % 2 0.3 → O 0.2 2
H 0.1 0.0 Further MEA (CCM) 0.00.51.01.52.0 development required Current Density / A·cm-2
Page 19 Can We Do Better Than That ?
80C, 100% r.h., 2.5 bara
5 -2-2
4 ETFE (50 m)
3 Nafion membranes 2 ECTFE-25 ECTFE (25 m) based Crossover Crossover / / mA·cm mA·cm
22 1 HH ETFE-50 based 2 R = 70 m·cm ~ Nafion 212 0 -2 0 50 100 150 200 250 ix = 0.9 mA·cm < Nafion 117 Area Resistance / m·cm2 Thin membrane based on ECTFE-25 shows promising properties
Page 20 Operational Range (Turndown Ratio)
8 7.55 p = 20 bar • low Ohmic resistance -2 7 a 6.95 • low crossover 6 5.20 5.17 5
4 • wider range of operating current density 3 2.35 2.90 • suitable for dynamic 2 operation
1 Current Density / A·cm 0.90 0.87 0 200 60 25 grafted PFSA Membranes / m ECTFE-25 imin = limit of 2% H2 in O2 Cell tests to be done
Page 21 Durability Membrane Degradation Mechanisms
Metal ion contamination (reversible) . Water supply issue . Corrosion of bipolar plates . Core shell / alloy catalysts
Loss of mechanical integrity: . Mechanical stress, creep . Overall membrane thinning . Local thinning
Chemical degradation:
. H2, O2 crossover . H2O2 and radical formation . Fluoride release
Page 23 Membrane Degradation Mechanisms
Need accelerated stress tests
K. Ayers et al., ECS Trans. 33/1 (2010) 3
Page 24 Contributions within NOVEL
Investigation of chemical degradation by measuring fluoride emission rate (FER) coupled to a degradation model
FER in the fuel cell*:
FC under load: 0.01 - 0.1 g·cm-2·h-1 FC under OCV: 1 - 10 g·cm-2·h-1
Shape of curve with maximum at intermediate current density well- reproduced by model
M. Chandesris et al., Int. J. Hydrogen Energy 40 (2015) 1353 * FER compilation in L. Gubler et al., J. Electrochem. Soc. 158 (2011) B755
Page 25 Contributions within NOVEL
Thermal Stress Test (TST): exposure of membrane to 90C for 5 days
post-test analysis of
membrane solution
• FTIR • UV-Vis • IEC • Ion chromato- membrane • SEM/EDX graphy
Membrane IEC loss (%) grafted, 44.2 0.8 initial design down-selected 4.2 0.5 grafted Nafion 117 < 1 A. Albert et al., in preparation
Page 26 Conclusions
Operation over large current density range desired Resistance ‐ crossover tradeoff for a given ionomer type Strategies for improved membranes in NOVEL . Modified PFSA membranes (reinforced, with recombination catalyst) . Other ionomer classes with superior combination of resistance and gas barrier properties Durability aspects tackled within NOVEL . Radical induced degradation: model and experiment . Thermal stress test at 90C in water
Page 27 Thank You
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