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


• 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


• 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


• 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


• 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)


Initial Strain

creep

Compression Creep: Dry vs. Wet State (25°C)

Both Hydration and Pressure impacts the creep response


• 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)


• 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


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!!!!

13

Kusoglu Group

pem mechanical characterization compression-creep

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