Bipolar Membrane Development toEnable Regenerative Fuel Cells Co-PIs: Todd Deutsch, KC Neyerlin Key contributors: Yingying Chen, SadiaKabir, Ellis Klein National Renewable Energy Laboratory May 2020

DOE Hydrogen and Fuel Cells Program 2020 Annual Merit Review and Peer Evaluation Meeting

Project ID: fc182

This presentation does not contain any proprietary, confidential, or otherwise restricted information.

Overview

Timeline and Budget • Project start date: 01/01/18 • Project end date: 03/31/20 • Total project budget: $400k

– Total DOE funds spent*: $400k

* As of 3/31/20

Barriers • A – Cost • B – Durability • C – Performance

Partners • Anion ionomer development

program at NREL • MEA fabrication group at NREL • Fuel cell and electrolysis testing

groups at NREL

NREL | 2

Relevance • This project directly addresses DOE FCTO’s interest in developing Reversible Fuel Cells (RFCs) – From MYRD&D,

“Advantages of reversible fuel cell technology include high round-trip efficiency (60–90%), decoupled power and energy capacity, long cycle life, low self-discharge rate, and reliable and stable performance.”

• A BPM with an electrospun junction has never been integrated into a fuel cell or water electrolysis MEA, much less a unitized RFC. These studies represent a completely new field with significant promise to ameliorate some of the key challenges in RFC development, as well as provide significant gains to the BPM understanding.

• Objectives

– Fabrication of a bipolar membrane (BPM) with a dual-fiber electrospun junction that can be employed in a stable, high-performance RFC membrane electrode assembly (MEA).

– The key technical aspects of the project are focused on fabricating/optimizing electrospun 3D junction for implementation into MEAs for fuel cell, electrolyzer, and RFC devices. Membrane characteristics such as composition, fiber diameter, and the incorporation of catalysts/particulates at the interfacial/junction will be tested first in either individual fuel cell or electrolyzer devices.

NREL | 3

Approach Electrospinning • Electrostatic voltage (4-50 kV) between a blunt

tip needle and grounded substrate • Charged polymer jet from a mixture of

Nafion®/perfluorinated AEM (PFAEM) ionomer and/or catalyst in carrier polymer (e.g. PEO)

• Solvent (IPA and water) evaporates from fiber as it travels from tip to substrate, filament also elongates during transit, narrowing diameter – Relative humidity in chamber is a critical

experimental variable • 300-500 nm diameter nanofiber threads • Randomly aligned nanofibers collected as mat

of uniform thickness and fiber volume fraction on a membrane

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• Unique aspect of our approach: Dual head electrospinning results in 3D interface of interpenetrating cation/anion membrane fibers

• Polymer dispersions electrospun concurrently on programmable, rotating, translating drum

• Substrate attached to drum – Glass, membrane, conductive carbon

tape, TEM grid, etc. • Water dissociation catalysts introduced to

polymer dispersions

15 wt.% PFAEM+ PEO

15 wt.% Nafion® +PEO

A high surface area 3D BPM interface should have improved properties over a planar, 2D BPM

NREL | 5

Approach – 2D BPM Fabrication

Step 1:Ultrasonic spray ofjunction material on Nafion

Step 3:Hot Pressing

Nafion

Step 2:Spread AEM layerSqueeze out bubbles

Materials

NREL’s proprietary GEN 2 – PFAEM

AEM

Nafion

Final BPM structure:

PFAEM ~ 30 µm

Sprayed catalyst layerNafion 25 µm

NREL | 6

Approach – 3D BPM Fabrication

Step 1a: Electrospin Nafion nanofiber mat Step 2: Nanofiber mat exposed to IPA vapor for 15 min each side; Hot pressed for 2 min

PFAEM ~ 30 µm

3D Junction ~ 3 µmNafion ~ 30 µm

Step 1c: Electrospin AEM nanofiber matStep 1b: Co-electrospin Nafion & AEM + GO spray

AEM+PEO

Nafion+ PEO

Graphene oxide (GO)

A

C

B

3D Junction

50 µm

50 µm

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Approach – 3D BPM Fabrication

0%

20%

40%

60%

80%

100%

0-50 50-100 100-150 150-200 200-250 250-300 300-350Diameter (nm)

AEM NF

Nafion NF

• 83% of Nafion nanofibers (NF) and 56% AEM NF diameters: 100 ~ 150 nm

• AEM NF diameter ranges slightly broader due to occasionally beaded fibers

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Approach

FY19/FY20 Milestones

Complete

Complete

Complete

Partially met

Milestone Name/Description Criteria End Date Type

Break down the ensemble BPM resistance into its discrete components and report the resistance contributions due to the PEM, AEM, and interfacial junction

Using AC impedance analysis, determine the individual component resistances.

6/30/2019

Quarterly Progress Measure (Regular)

RFC unitized cell hardware

Develop hardware that allows operation of an MEA in both fuel cell and electrolyzer mode without having to remove the MEA.

9/30/2019

Quarterly Progress Measure (Regular)

Evaluate the stability and performance in fuel cell and electrolysis mode at elevated temperature.

Determine the degradation profile of BPM MEAs while operating continuously for 8 hours in fuel cell and electrolysis mode at 80°C.

12/31/2019

Quarterly Progress Measure (Regular)

Demonstrate >1A/cm2 in both fuel cell and electrolysis mode using BPM RFC.

Measure current densities >1 A/cm2 in both fuel cell and electrolysis mode with a BPM MEA.

3/31/2020Annual Milestone (Regular)

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Accomplishments and Progress Flow Cell Screening

𝐑𝐛 = R"!#$" + R%&$" + R'() + R*()RW: Resistance of water dissociation reactionCPE: Electrical double layer (heterogeneity)

HFR: high-frequency resistanceLFR: low-frequency ResistanceCPE: constant phaseelement

+++++

-----

BPM

Junction

CPE

Rw

RCEL

CEL

Anod

e RNaOH RAEL

AEL

Reference electrode

Reference electrode

Cathode

RH2SO4

NREL | 10

Accomplishments and Progress

• Graphene oxide (GO) as water dissociation catalyst helps significantly lower the potential drop across BPM

PFAEM MembraneGO Spray

Nafion Membrane2DCatalyzed

[1] Martínez, Rodrigo J., and James Farrell. "Water splitting activity of oxygen-containing groups in graphene oxide catalyst in bipolar membranes." Computational and Theoretical Chemistry 1164 (2019): 112556.[2] McDonald, Michael B., and Michael S. Freund. "Graphene oxide as a water dissociation catalyst in the bipolar membrane interfacial layer." ACS applied materials & interfaces 6.16 (2014): 13790-13797.

80

60

40

20

0R

W [

ohm

cm

2 ]50250

i [mA/cm2]

Uncatalyzed Catalyzed with 2 µg/cm

2 GO

6.0

5.0

4.0

3.0

2.0

1.0

0.0

E [V

]

100806040200i [mA/cm2]

Uncatalyzed Catalyzed with 2 µg/cm

2 GO

E ele

ctro

lyte

-free

[V]

Flow Cell Screening

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Ø Catalytic sites increaseØ Junction conductance & Electric field & Water dissociation rate decrease

0.4

0.6

0.8

1

1.2

1.4

0 50 100 150

E [V

]

i [mA/cm2]

2 µg/cm2

20 µg/cm2

200 µg/cm2

1000 µg/cm2

[GO Loading]

𝐄 𝐢𝐑𝐞𝐥𝐞#𝐟𝐫𝐞𝐞

[V]

0123

456

00.20.40.60.8

11.21.4

0 200 400 600 800 1000

Rw [o

hm]

E [V

]

GO Loading [µg/cm2]

Accomplishments and Progress Flow Cell Screening

2 1000100µg/cm2 GO

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-2%-1%0%1%2%3%4%5%6%

1 2 3 4 5 6ΔV @

500

mA/

cm2

[%]

Scan Number

00.20.40.60.8

11.21.41.61.8

0 100 200 300 400 500

R w[o

hm]

i [mA cm-2]

Commercial FBM

2D BPM, 100 µg/cm2 GO

3D BPM, 100 µg/cm2 GO

3D electrospun BPMØ Lower voltage and lower water dissociation

resistance than commercial BPM, thanks to its 3D high surface area of catalyst

Ø Best stability under repeated high current scan§ Strong adhesion from interpenetrating

fibers in both 3D junction and the interface of ion exchange layer

0

0.5

1

1.5

2

0 100 200 300 400 500

E [V

]

i [mA cm-2]

Commercial FBM

2D BPM, 100 µg/cm2 GO

3D BPM, 100 µg/cm2 GO

EiR_electrolyte-free

Water dissociation resistance

E change@500 mA/cm2

Commercial FBM2D BPM, 100 µg/cm2 GO3D BPM, 100 µg/cm2 GO

Accomplishments and Progress Flow Cell Screening

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Accomplishments and Progress BPM MEA Electrolysis

Ø Anode GDE: 0.45 mg/cm2 PtIr on 20% PTFE treated Toray carbon paper

Cathode GDE: 0.4 mg/cm2 Pt/HSC on Sigracet 29BC

Ø 1 M H2SO4 was fed to Nafion side and 1 M NaOH was fed to AEM side

at 10 ml/minØ Room temperature, ~20 oC

Ø Galvanodynamic scan at 1 mA/cm2*s

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Accomplishments and Progress BPM MEA Electrolysis

0

1

2

3

4

5

0 100 200 300 400 500

E [V

]i [mA/cm2]

Commercial FBM2D BPM3D Catalyzed w/ Spray

200 µm

FBM

• NREL fabricated 3D Catalyzed w/ Spray BPM & 2D BPM

outperforms commercial BPM in MEA electrolysis test

• Commercial FBM is not designed for MEA testing – uneven

surface due to reinforcing fiber causes high contact resistance

PFAEM MembraneGO Spray

Nafion Membrane2D Catalyzed100 µg/cm2

GO

Electrospun PFAEM

3D Junction w/ GO

Electrospun Nafion3D Catalyzed w/ Spray

100 µg/cm2

GO

NREL | 15

Accomplishments and Progress BPM MEA Fuel CellØ AEM & Cathode GDE was ion-exchanged

from I-/Cl- to OH-

Ø Cathode GDE: 0.8 mg/cm2 Pt/HSC on 29BCØ Anode GDE: 0.4 mg/cm2 Pt/HSC on 29BCØ H2/O2 for anode/cathode flowØ Temperature 60 oCØ Relative humidity ranged from 50% ~ 100%

High Pt loadings used:§ Minimize kinetic loss on

electrodes§ Focus on BPM junction

optimization for fuel cell performance

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Accomplishments and Progress BPM MEA Fuel CellBPM = Nafion + 20 µg/cm2 C + AEM

Ø What does Vulcan carbon do?

§ Lower contact resistance

§ Higher dielectric constant

§ Water reservoir

§ Mechanical strength

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8

E HFR

-free

[V]

i [A/cm2]

60 oC, 121 kPa, 100%RH, H2/O2

40 µg/cm2 C

20 µg/cm2 C2 µg/cm2 C

Nafion

AEM

C

0

0.2

0.4

0.6

0.8

1

No C 2 ug/cm2 C 20 ug/cm2 C 40

HFR

[ohm

cm

2 ]

None 20 µg/cm2

C40 µg/cm2

C2 µg/cm2

C

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Accomplishments and Progress

01234567

0 20 40 60 80 100

E [V

]

i [mA/cm2]

BPM= Nafion + AEMBPM= Nafion + GO + AEM

0

0.2

0.4

0.6

0.8

1

0 0.3 0.6 0.9

E [V

]

i [A/cm2]

Nafion+AEM+2 µg/cm2 C+20 µg/cm2 C

Graphene Oxide(Electrolysis Performance)

Carbon(Fuel Cell Performance)

BPM Junction Optimization for RFC

Nafion(Binding)

Optimize the recipe for BPM junction to achieve BPM reversible fuel cell operation:

Carbon + GO + Nafion

Ø With ionomer in junction as binder when C/GO >20 µg/cm2

Ø Achieve mechanical strength in junction

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Accomplishments and Progress

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6

E [V

]

i [A/cm2]

100% RH

95% RH

90% RH

2D BPM

Fuel Cell (FC)

AEM Membrane (OH-)

40 µg/cm2 (C+GO+Nafion)8:2:2

Nafion Membrane (H+)

2DBPM

Electrolysis (EC)60 oC, 100 kPa, H2/O2 25 oC

02468

101214

0 0.2 0.4 0.6 0.8 1

E [V

]

i [A/cm2]

2D BPMFBM

Same BPM in FC and EC

NREL | 19

Accomplishments and Progress: Responses to Previous Year Reviewers’ Comments

• Modeling would help with the fundamental understanding of BPM performance and its optimization.– The relatively small budget does not permit inclusion of experiment,

characterization, and modeling components of this study. We have incorporated finite element modeling of 3D electrospun BPM junctions in another HFTO-supported BPM project on CO2 reduction.

• The project team needs to measure the round-trip efficiency and cycle life of the most optimized 3D BPMs.– From the data presented on slide 18, the round-trip efficiency is 21% at 100 mA/cm2

and drops to 12% at 200 mA/cm2. The difficulty of finding a common catalyst optimized for both water dissociation and recombination as well as the challenge of removing halide from the electrospun AEM fibers results in a mismatch between the best performing fuel cell and electrolyzer BPMs.

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Remaining Challenges

Remaining Challenges:

• The gas diffusion electrode is not optimized for reversible fuel cell operation

– MPL works for fuel cell, but causes the membrane to dry out in electrolyzer mode

– Carbon paper is prone to corrosion in electrolyzer mode

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Collaboration and Coordination

• NREL’s anion ionomer development program– Federal lab – Within DOE FCTO– Provide this project PFAEM polymer, we provide characterization results

• NREL’s MEA fabrication, fuel cell and electrolysis characterization groups– Federal lab– Within DOE FCTO– Maintain equipment for MEA fabrication as well as fuel cell and

electrolyzer test stands that enable performance evaluation of BPM devices

• This project relies on a great working relationships that leverage materials and capabilities previously developed within NREL’s fuel cell and electrolysis group to achieve its objectives

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Proposed Future Work

• Optimize the 3D junction (C+GO+N) in electrospun BPM for RFC

• Exchange the AEM layer in electrospun BPM from I- to OH- by in-situ conditioning in electrolyzer

• Test durability and performance under reversible operation in repeated cycles

End of project milestone: 3/31/20

• Demonstrate >1 A/cm2 in both fuel cell and electrolysis mode using BPM RFC approach – Achieved 1 A/cm2 in electrolysis mode but only 550 mA/cm2 in fuel cell

mode.

This project ended 3/31/20

NREL | 23

Technology Transfer Activities

• We are responding to FOAs that use BPMs for other applications, such as CO2 electrolyzers and electrodialysis.

• We are in discussions with companies with a potential for funding 3D electrospun BPMs for water and energy applications

NREL | 24

Summary

• We are able to make electrospun BPMs with 3D junction showing high performance and high durability in electrolysis mode

• We were not able to condition BPMs (exchanging to H+/OH-

ionic form) due to the fast corrosion of carbon paper in electrolysis mode. This prevented us from testing the commercial BPM or electrospun BPM in fuel cell mode.

• A 2D BPM with carbon in the junction was optimized for fuel cell operation.

• We are able to achieve 550 mA/cm2 in fuel cell mode and 1000 mA/cm2 in electrolysis mode with the same 2D BPM

• Future work– Optimizing the 3D junction of electrospun BPM for RFC– Optimizing the electrode and hardware for continuous RFC operation

www.nrel.gov

Thank You

This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

NREL/PR-5900--76878