catalyst-ionomer interactions/amfc durability

Los Alamos National Laboratory

Catalyst-ionomer Interactions/AMFC Durability

Yu Seung Kim

MPA-11: Materials Synthesis and Integrated Devices, Los Alamos National Laboratory, Los

Alamos, New Mexico, 87545 USA

Operated by Los Alamos National Security, LLC for the U.S. Department of Energy's NNSA

2019 Anion Exchange Membrane Workshop∣ May 30, 2019 ∣ Sheraton Dallas Hotel, Dallas, TX


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Acknowledgment

Cy Fujimoto (Polymer synthesis,SNL)

Dongguo Li (Electrochemistry)

Albert Lee(Characterization)

Sandip Maurya(Device testing)

Sarah Park(Synthesis)

CollaboratorsChulsung Bae (RPI)Ivana Gonzales (UNM)Michael Hibbs (SNL)Daniel Leonard (LANL)Joseph Dumont (LANL)Rex Hjelm (former LANL)

NIST neutron centerLANSCE neutron center

FundingDOE EERE FCTO


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LANL Project funded by DOE FCTOResearch Focus Major Founding

AEMStability(2016)

Polymer backbone degradation1

AEMFCPerformance(2017-2019)

Phenyl group adsorption2

Cation-hydroxide-water co-adsorption3

AEMFCDurability

(2019)

Oxidation of phenyl group (in the cathode ionomer)Degradation of ammonium group (in the anode ionomer)

1Fujimoto et al. J. Memb. Sci. 423-424, 438-449, 2012; 2Matanovic et al. J. Phy. Chem. Let. 8, 4918-4924, 2017; 3Li et al. Cur. Opinion in Electrochem., 12, 189-195, 2018; 4S. Murya, S. Energy & Environ.

Sci. 11, 3283-3291, 2019


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Catalyst-Ionomer Interactions


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Adsorption of ionomer fragment on electrocatalysts

Li et al. Curr. Opin. Electrochem. 12, 189-195 (2018)

QA OH

Anion exchange ionomer

Backbone phenyl

Sidechain phenyl

Electrocatalyst

Cationic group

Adsorption energy calculated by DFT

Those adsorptions are unique phenomena for alkaline ionomer because acid system

uses perfluorosulfonic acid.


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Impact of phenyl adsorption on HOR catalyst on AMFC performance


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Identification of phenyl group adsorption on Pt at HOR potential

Matanovic et al. J. Phys. Chem. Lett. 8, 4918 (2017)

TMAOH BTMAOH

NOH N OH

E / V vs. RHE-0.05 0.00 0.05 0.10 0.15 0.20

i / m

A cm

-2 ge

o

-2

-1

0

1

2

0.1 M TMAOH0.1 M BTMAOH

HOR voltammogram of Pt/C (RDE experiment)

Electric field [V/A]-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Ead

,fiel

d [eV

]

-4

-3

-2

-1

0

ΔE=-2.30 eV ΔE=-2.15 eV ΔE=-1.51 eV ΔE=-2.15 eV

Adsorption Position (side view) of BTMA on Pt(111) at U = 0 V

Effect of Applied Potential on the Interaction

Conformational change

U = 3Å x Efield


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Impact of backbone phenyl adsorption on HOR of Pt

Potential (V vs. RHE)0.0 0.2 0.4 0.6 0.8 1.0

Cur

rent

Den

sity

* (m

A cm

-2ge

o)

-2

-1

0

1

2

N OH

Potential (V vs. RHE)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cur

rent

Den

sity

* (m

A cm

-2)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

N

N

Phenyl adsorption

Phenyl Desorption due tocation adsorption

Phenyl adsorption from polymer backbone

BTMAOH phenyl adsorption Ionomer phenyl adsorption

• Backbone phenyl impacts the HOR up to 0.8 V vs. RHE, more significant than ammonium

functionalized phenyl group. Maurya et al. Chem. Mater. 30, 2188 (2018)Matanovic et al. J. Phys. Chem. Lett. 8, 4918 (2017)


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Impact of ionomer type on HOR of Pt

Poly(phenylene)(2012 SNL)

F2C

F2CO

FC CF3

OCF2

CF2

COHN N C

N

N

F2C

F2C

OH

N

N

Perfluorinated(2013 LANL)

Polyolefinic(2018 LANL)

Potential (V vs. RHE)

0.0 0.2 0.4 0.6 0.8 1.0

Cur

rent

den

sity

(mA

cm

-2no

rm)

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

Loss due to the backbone phenylgroup adsorption

Pt/C

N NH

n

OH

• As the number of ammonium unsubstituted phenyl group increased, the HOR activity

decreases.

Pt microelectrode in contact with quaternized polymers

Maurya et al. Chem. Mater. 30, 2188 (2018)


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The adsorption energy of polymer backbone phenyl fragment on Pt(111) by DFT calculation

Fragment Ead (eV) rank

fluorene -1.38 1o-terphenyl -1.52 2benzene* -1.95 3biphenyl ether -2.19 4biphenyl -2.87 5m-terphenyl -3.61 6p-terphenyl -3.94 7

Adsorption energies (in eV) using optPBE-vdW

Benzene

BiphenylO

Biphenyl ether

p-terphenyl

m-terphenyl

o-terphenylFluorene

Matanovic et al. Chem. Mater. in press (2019)


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Impact of phenyl group adsorption on AMFC performance

CF3 CF3

N

n 1-n

OH

Quaternized poly(biphenyl alkylene)

n = 1BPN100

n = 0.45BPN45

Quaternized poly(terphenyl alkylene)

CF3

NOHp-TPN

F3C

NOHm-TPN

CF3CF3

.45 .55

Quaternized polyfluorene

FLN55

N N OHOH

Matanovic et al. Chem. Mater. in press (2019)

AMFC performance of MEAs using different ionomers

Courtesy: Prof. Bae at RPI


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Impact of phenyl adsorption on ORR and OER catalysts on AMFC and

AEM electrolyzer durability


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Discovery of the impact of phenyl adsorption on ORR or OER durability

• An MEA using less phenyl adsorbing ionomer (FLN) showed better AMFC durability

AMFC current density change of

two MEAs (only cathode ionomer

is different)


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Identification of phenyl group adsorption on Pt at ORR potential

N+

OH- N+

OH-

OHHO

Ar

BCC

A

1.0 V vs. RHE

ORR current changes, CV, ORR voltammograms and 1H NMR analysis of BTMAOH


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Detection of the phenol group formation from cathode ionomer1H NMR analysis of cathode ionomer (BPN) after 75 hours of AMFC operation at 0.9 V

CF3

N+ OH-

Oxidation CF3

N+OH-

OH*

* *

a)

b)

Ar

ab

cd

e

f

g

BPN

1H NMR analysis of cathode ionomer (FLN) after 230 h hours of AMFC operation at 0.9 V

N+ N+OH

-OH-

CF3 CF3

OH * * *

N+ N+OH

-OH-

CF3 CF3 Oxidation

4 44 4

ga

DMFH2O

a

bce

d

f ddd

g

g

DMSO-d6

• Phenyl oxidation for BPN: 1.3% after 75 h vs. 0.34% after 230 hMaurya et al. Manuscript under review, J. Power Sources (2019)


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Phenyl oxidation at OER potential (AEM electrolysis)

a)

(1) parallel (2) lying (3) standing

Ar

DMSO-d6

cb

dfeH2O

a

F3C

N+ OH-

Ara

b

dc

f

e

b)

Initial

After 100 h

g

F3C

N+OH

-

OH

* *g

5.8 5.7 5.6

ppm

Initial

After 100 h

a)

Inset figure

at 2.1 V

Phenyl adsorption energy (DFT calculation) iV curves of IrO2 catalyzed AEM electrolyzer

• PGM catalysts have high phenyl adsorption energy and the AEM electrolyzer using the

OER catalysts showed performance decay.Li et al. ACS Appl. Mater. Interf. 11, 9696-9701 (2019)

3% conversion


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Take-home message for phenyl oxidation

OER catalyst

Electrochemical oxidation &

phenol formation

At 2.1 V

Performance decay of AEM electrolyzer over time

Step 2

Effect of pH on Pt ORR activityDegradation mechanism

• The concentration of phenol at the catalyst-ionomer interface is much higherthan that in the bulk ionomer.

• The impact is similar to carbonation but this brings permanent damage to the cellmore significant than carbonation.


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Impact of cation adsorption on HOR catalysts on AMFC performance


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Identification of cation adsorption on Pt at HOR potential

E /V vs. RHE

-0.1 0.0 0.1 0.2 0.3 0.4 0.5

i /m

A cm

-2

-0.5

0.0

0.5

1.0

1.5

2.0

initial

TMAOH after 0.1 V / 120 min

Time / min

0 20 40 60 80 100 120

i / m

A cm

-2ge

o

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

TMAOH

HClO4

HOR voltammogram of Pt/C in 0.1 M TMAOH

Chronoamperometry at 0.1 V vs. RHE

• HOR activity of Pt in TMAOH decreases after exposure the electrode at 0.1 V vs. RHEChung et al. J. Electrochem. Soc. 163, F1503-F1509 (2019)


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Detection of TMA-water co-adsorption on Pt at 0.1 V vs. RHE

IRRAS in the (a) fingerprint and (b) O-H stretching regions duringchronoamperometry of Pt in 0.1 M TMAOH

• Time-dependent cation adsorption on Pt at 0.1 V vs. RHE.• OH stretching region indicates that the co-adsorbed layer

has high concentration of cationic group vs. water.

Chung et al. J. Phys. Chem. Lett. 7, 4464-4469 (2016)

Wavenumber / cm-1

9001000110012001300140015001600

Abso

rban

ce

TMAOH soln.

Initial

15 min

55 min

as(CH3)

s(CH3)

as(C-N)

Pt surfacea)

Wavenumber / cm-1

2400260028003000320034003600

Abso

rban

ce

as,s(OHwater) as,s(OHhdroxide)

b)

TMAOH soln.

Initial

15 min

55 minPt surface

0.1 M TMAOH0.1 M TMAOH

Harmon et al. J. Mol. Struc. 159, 255-263 (1987)

TMAOH: pentahydrate

trihydrate

dihydrate

monohydrate

FTIR of highly conc. TMAOH


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Impact of co-adsorption on hydrogen diffusionH2 permeability in H2SO4 and KOH as afunction of the concentration.

Ruetschi J. Electrochem. Soc.114, 301 (1967)

Concentration (M)

0 2 4 6 8 10 12 14 16

H2 p

erm

eabi

lity

(S*D

)

0.1

1

10

100

H2SO4

KOH

AEM (thickness): quaternized poly(terphenylene) (35 µm), ionomer:BPN, anode catalyst: PtRu/C (0.75 mgmetal cm-2) cathode catalyst: Pt/C(0.6 mgmetal cm-2), cathode flow rate: 300 sccm of O2.

Impact of anode flow rate on AMFCperformance

Li et al. Curr. Opin. Electrochem. 12, 189-195 (2018)


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Anode ionomer design aspect towards highly-performing AMFCs

Park et al. unpublished (2019)

n

CF3

N

(b) o-BTN (c) TEA-o-BTN

n

CF3

N

n

CF3

N

(a) BPN

Current density (A cm-2)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Cel

l vol

tage

(V)

0.0

0.2

0.4

0.6

0.8

1.0 TEA-o-BTN (2,000 sccm H2)TEA-o-BTN (500 sccm H2)o-BTN (500 sccm H2)BTN (500 sccm H2)

80 °C

Increase fractional

free volume

Decrease adsorption

energy

Temperature (°C)

Diffusion coefficient / (10-9 m2 s-1)BPN o-BPN Fold increase

25 2.69 8.00 3.0

40 3.24 12.45 3.8

65 3.78 17.58 4.7

80 4.45 22.30 5.0

Table. Self-diffusion coefficients of H2 in BPN and o-BTN dispersions by PFG 1H NMR

Impact of anode ionomer structure on AMFC performance

• Impact of the cationic group ismore significant Researchneed


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Impact of cation adsorption on HOR catalysts on AMFC Durability


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Motivation of cation co-adsorption on AMFC durability

• The concentration of the TMAOH in the co-adsorbed layer calculated from scattering length density is unusually high: the ratio of TMAOH to water = 5:1

(The highest TMAOH to water ratio in the aqueous solution is 1:1)

Schematic illustration of co-adsorbed layer structure on Pt after 12 h exposure at 0.1 V vs. RHE

Neutron reflectometry electrochemical cell*

Dumont et al. unpublished data (2019)


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Hour (h)

0 20 40 60 80 100 120 140

Curr

ent d

ensi

ty (A

cm

-2)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

Hour (h)0 20 40 60 80

HFR

( c

m2 )

0.0

0.1

0.2

0.3

0.4

at 80 °C, 0.9 V

Impact of coadsorption on AMFC lifeCurrent density change at 0.9 V

Anode: FLN, Cathode: FLNboth are phenyl non-adsorbing


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The ionomer degradation due to the cation co-adsorption

CF3

N+ OH-

Oxidation

N

Hoffman Elimination

CF3

N+ OH-

* *

HO *

* *

CF3

f

Ar

ab

cd

e

g

hi

j

Note: * are also possible oxidation sites

Anode Side

Cathode

g

f

Ar

a bcde

DMSO-d6

hij• Cathode ionomer degradation = 30% of the

aromatic groups via phenyl oxidation.• Anode ionomer degradation = 27% of the TMA

cationic groups via Hoffman Elimination.

After AMFC running for 100 h at 0.9 V

Lee et al. unpublished data (2019)


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Water management issue driven by the co-adsorption

Maurya et al. unpublished data (2019)

n

CF3

N

(b) o-BTN (c) TEA-o-BTN

n

CF3

Nn

CF3

N

(a) BPN

80 °C, 600 mA/cm2

Time (hour)

0 20 40 60 80 100

HFR

(Ohm

cm

2 )

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14BTNo-BTNTEA-o-BTN

80 °C, 600 mA/cm2

Time (hour)

0 20 40 60 80 100

Cur

rent

den

sity

(A/c

m2 )

0.0

0.2

0.4

0.6

0.8

1.0

BPNo-BTNTEA-o-BTN

Cell voltage change at 0.6 A/cm2 HFR change at 0.6 A/cm2

Cel

l vol

tage

(V)


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Summary

AMFC anode performance

AMFC cathodedurability

AMFC anode performance

AMFC anodedurability

AEM electrolyzer

anodedurability


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Electrode performance discussion• Subject

• Ionomer performance• Low PGM performance• Non PGM performance• Electrode processing• Liquid electrolyte

• Area• Fuel cells (HOR and ORR)• Electrolyzers (HER and OER)

catalyst-ionomer interactions/amfc durability

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