PEM Mechanical Characterization Compression-Creep ...
Transcript of PEM Mechanical Characterization Compression-Creep ...
LAWRENCE BERKELEY NATIONAL LABORATORY
PEM Mechanical Characterization
Compression-Creep Investigation
Claire Arthurs, Ahmet Kusoglu, Chris Capuano
Energy Conversion Group | ESDR
Lawrence Berkeley National Lab
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University of California, Berkeley
240th ECS Meeting | October 10-14, 2021
PEM Mechanical Response in Electrolyzers
Understanding Mechanical Durability
C. Arthurs and A. Kusoglu, A. (2021). Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers. ACS Applied Energy Materials, 4, 4, 3249–3254
• The PEM in an electrolyzer is subjected to
various types of mechanical loads:
• Design / Assembly loads:
Assembly (pressure), gaskets (stress)
• Operational loads:
Cell operation (pressure, temperature, swelling)
Resulting a distribution of stresses
• Our mechanical understanding of “PEM”
relies on studies on fuel cells
What is similar & different for PEMWEs?
PEM Mechanical Response in Electrolyzers
Understanding Mechanical Durability
C. Arthurs and A. Kusoglu, A. (2021). Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers. ACS Applied Energy Materials, 4, 4, 3249–3254
• The PEM in an electrolyzer is subjected to
various types of mechanical loads:
• Design / Assembly loads:
Assembly (pressure), gaskets (stress)
• Operational loads:
Cell operation (pressure, temperature, swelling)
Resulting a distribution of stresses
• Our mechanical understanding of “PEM”
relies on studies on fuel cells
What is similar & different for PEMWEs?
• Higher degree of hydration
• Pressure as a key parameter (“load”)
• Long-term operation (“creep”)
Key for mechanical durability is probing
Membrane mechanical response under:
- Hydrated conditions (liquid)
- With pressure control (compression)
- Ability to monitor Creep (time)
• Tensile testing is not representative of in-
situ behavior of PEM in electrolyzers
• We have developed a compression creep
fixture and test procedure
• Polymer is compressed using Instron
equipped with a custom-made in-situ
compression stage
Samples can be tested in ambient and in liquid
water, at controlled temperature
Its thickness change over time is recorded via
external LVDTs
Creep strain can be measured as a function of
applied stress
Characterizing PEMs under Compression
Compression (Creep) in controlled environment
4C. Arthurs and A. Kusoglu, A. (2021). Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers. ACS Applied Energy Materials, 4, 4, 3249–3254
PEM Mechanics: Compression vs. Tension
Tension and Compression response of Nafion is different in dry and wet
C. Arthurs and A. Kusoglu, A. (2021). Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers. ACS Applied Energy Materials, 4, 4, 3249–3254
• Tensile and compression behavior of a
Nafion ionomer membrane is strikingly
different
• Tensile testing is the most commonly used
method to assess mechanical stability
For almost all applications
It is a key stress mode for failure in PEMFCs
• It might not be an ideal representative of
operando behavior of PEMs, especially in
electrolyzers
PEM Mechanics: Uniaxial Test vs. Creep Test
Creep refers to change in thickness in a material over time under a constant force
C. Arthurs and A. Kusoglu, A. (2021). Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers. ACS Applied Energy Materials, 4, 4, 3249–3254
• In tension: Creep results in an increase in
length (+ , positive strain)
• In compression: Creep results in a decrease
in length ( – , negative strain)
• Strain decomposition:
𝜺𝒕𝒐𝒕𝒂𝒍 = 𝜺instantaneous + 𝜺𝒄𝒓𝒆𝒆𝒑
• Creep Strain = f(load, environment, time)
PEM Mechanics: Creep
Creep refers to change in thickness in a material over time under a constant force
C. Arthurs and A. Kusoglu, A. (2021). Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers. ACS Applied Energy Materials, 4, 4, 3249–3254
• In tension: Creep results in an increase in
length (+ , positive strain)
• In compression: Creep results in a decrease
in length ( – , negative strain)
• Strain decomposition:
𝜺𝒕𝒐𝒕𝒂𝒍 = 𝜺instantaneous + 𝜺𝒄𝒓𝒆𝒆𝒑
• Creep Strain = f(load, environment, time)
• What is creep?
• When a material is held under a prescribed
load (stress), it tends to deform slowly
• Strain is a result of long-term stress
P = 35 MPa
• Input: stress hold at 35 Mpa
5000 psi (higher than normal
operation pressure)
• Output: reduced thickness
Initial strain (initial elastic +
plastic deformation)
Creep strain (time dependent)
• Creep strain defined positive
An order-of-magnitude smaller
than the elastic (initial) strain
Compression Creep: A new PEM Protocol
Nafion 117 Membrane exhibits compression creep over 24 hours (shown here in dry state)
8C. Arthurs and A. Kusoglu, A. (2021). Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers. ACS Applied Energy Materials, 4, 4, 3249–3254
Initial Strain
creep
Compression Creep: Dry vs. Wet State (25°C)
Both Hydration and Pressure impacts the creep response
9C. Arthurs and A. Kusoglu, A. (2021). Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers. ACS Applied Energy Materials, 4, 4, 3249–3254
• 5 MPa (725 psi) for 24 hours
• 35 MPa (5000 psi) for 24 hours
• Hydrated Nafion creeps more (in water)
• It slows down after the first hour
Compression Creep: Effect of Pressure
Nafion 117 in water: Creep strain decreases with increasing stress (pressure)
10C. Arthurs and A. Kusoglu, A. (2021). Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers. ACS Applied Energy Materials, 4, 4, 3249–3254
• Increasing compression increases the
initial strain (as expected)
• Increasing compression reduces the
subsequent creep strain, perhaps by
restricting the mobility of polymer chains
• Nafion 11x series exhibit 2-3% creep
strain in water after 24 hours
• Corresponds to ~0.1% thickness
change per hour (under pressure)
Compared to nominal thickness
But creep is nonlinear over time
• For the same creep rate, an initially thicker
PEM exhibits more displacement reduction
Relative (strain) vs. Absolute (thickness) change
• In 24 hours, Nafion membrane thickness in
water could reduce 3-6 micron due to creep
With a dependence on pressure
Compression Creep: Implications
All Nafion membranes investigated exhibited comparable creep response
11C. Arthurs and A. Kusoglu, A. (2021). Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers. ACS Applied Energy Materials, 4, 4, 3249–3254
Values w.r.t
Compressed
thickness
Conclusions and Key Accomplishments
• A compression-creep setup was developed and implemented for PEMs
• In-situ creep testing in dry and wet conditions is accomplished with pressure control
A new membrane creep protocol was developed (started with HydroGEN consortium)
• Compression Creep in Electrolyzer PEMs occurs over the course of days
• Key findings for PEM (electrolyzers):
Compression response differs significantly than tension ➔ important for durability considerations
In-situ hydration control accomplished ➔ Hydration alters the nature of creep
High-pressure compression reduces thickness more, but also suppresses creep to a certain extent
• Ongoing Work
The effect of membrane thickness is perhaps conflated with the other factors, studying other PFSAs
Temperature control: Currently testing the setup at 50oC (to be completed this year)
A metric for stability: Creep analysis and mechanics modeling for creep rates and compliance values
• Chris Capuano of Proton (Nelhydrogen)
• Douglas Kushner, Grace Lau, Ahmet
Kusoglu
• Kusoglu Group
• Energy Conversion Group
Acknowledgements
Thank you!!!!
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Kusoglu Group