Phenomenological Model of Electrochemically Active Surface Area Loss Mechanisms in the Cathode Catalyst Layer

Steven G. Rinaldo,a,b Wendy Lee,b Jürgen Stumper,b and Michael Eikerling.a

Department of Chemistry, Simon Fraser University,a and the Automotive Fuel Cell Cooperation Corporation,b Burnaby, BC, Canada.

1. Introduction Durability and lifetime continue to be foremost issues in polymer electrolyte fuel cell (PEFC) development. Performance issues arise due to the instability of functional materials found in the fuel cell. These include, but are not limited to, the catalyst support, polymer electrolyte membrane (PEM) and dispersed nanoparticle catalyst. Platinum, the hitherto irreplaceable catalyst is thermodynamically unstable at high potential and low pH as suggested by the potential-pH diagrams developed by Pourbaix; it is also well known that potential cycling exacerbates this instability. Mechanisms of Pt mass loss and redistribution in the cathode catalyst layer (CCL) are sensitive to materials properties, CCL structure as well as conditions such as pH, temperature and potential. The primary effect of Pt mass loss and redistribution is the loss of electrochemically active surface area. The cartoon below illustrates the relative importance of various ECSA loss mechanisms under normal operating conditions

decreasing influence on ECSA loss under normal conditions

dissolution redeposition agglomeration detachment

2. General Degradation Model (illustrative) particle radius distribution (PRD)

in the cathode catalyst layer

degradation stressor1

PRD temporal evolution (imaging)ECSA temporal evolution(voltammetry)decoupling mechanisms is difficult

3. General Degradation Model (mathematical) spatially invariant PRD continuity equation (PRD temporal evolution)2,3,4

0 0

, 1, ', d ' , ', d ' ,

2 agg agg det

d- ', ' , '

d

rK r r r K r r K t

t

rf r t

f r t f r t r f r t f r t r f r tt r

Pt mass balance (closed CCL) and ECSA temporal evolution

particles interact via mean-field concentration

3Pt Pt Pt CCL

0

4, d

3

M C t r f r t r

2ECSA

0

4 , d

S r f r t r

4. Simplifications (minor support corrosion)

(i) PRD/ECSA temporal evolution determined by dissolution and redeposition

(ii) particle radius change controlled by Gibbs-Thomson interfacial kinetics

dep diss flat 0Pt Pt Pt Pt exp

d

d

r

t R

k C t k Cr

diss flat

dep dissPt Pt0 Pt Pt Ptflat

0 0 Pt

k Crt T t C t k k

R R C

flat 4Pt Pt Pt 0

CCL CCL

4

3

M C R

, 1, exp 0

ff 3

0

, d 1

f

(iii) dimensionless governing equations

5. Main Effective Parameters

6. Dissolution Limit (Potentiostatic)5 1

7. Dissolution Limit (Potential Cycling)6

model evaluation via an in-situ transmission electron microscopy study extracted surface tension and dissolution rate suggest cathodic dissolution mechanism of dissolution changes; potentiostatic vs. potential cycling

8. Redeposition (Parametric Study) 1

9. Conclusions and Outlook • good agreement with experimental data (potentiostatic and cycling)• predict PRD evolution based on ECSA loss and vice-versa• relate observable phenomena (PRD, ECSA) to material properties (surface tension)• fundamental link between Pt oxide formation/reduction and Pt mass balance• key aspect is the relation between oxide mediated Pt dissolution and ECSA loss

outlook• evaluate surface tension of nanoparticles under electrochemical conditions• deconvolution of ECSA loss curves insight into mechanism(s)• refine sub-model of particle radius change dissolution mechanism

effPt

0

2RRT

(A) surface tension ( ) strong function of oxide coverage (PtO)

(B) extended surface dissolution rate ( ) Pt/PtO dissolution mechanisms

(C) redeposition coefficient ( ), Pt mass/electrode volume ( )

dissPtk

effPt

CCL

3322

33

main factors:• particle size • electrode potential• oxide growth/reductionagglomeration

dissolution/redeposition

detachment

main factors: • temperature• support properties• particle movement

main factors: • support corrosion

22

11

11

11 22 33

1. K.J.J. Mayrhofer, Electrochem. Commun., 10, 1144 (2008).2. M. Smoluchowski, Physik. Zeitschr., 17 (1916), pp. 557–599.3. I. M. Lifshitz and V. V. Slyozov, J. Phys. Chem. Solids., 19, 35 (1961).4. C. Wagner, Z. Elektrochem., 65, 581 (1961).5. S.G. Rinaldo, J. Stumper, and M. Eikerling, J. Phys. Chem. C., 114, 5773 (2010).6. S.G. Rinaldo, W. Lee, J. Stumper and M. Eikerling, ESL, 14, B47 (2011).

inte

rrela

ted

mech

an

ism

s

,, 0d

d

r

t

f r tf r t

t r 3

Pt Pt Pt CCL

0

4, d

3

M C t r f r t r 2ECSA

0

4 , d

S r f r t r

small particles dissolve faster effective dissolution rate

analytical parametric solution via method of characteristics parametric study revealed strong influence of surface tension on ECSA loss dissolution rate dispersion could result in mean radius increases with ECSA loss

1.0 0.8 0.6 0.4 0.2 0.01.0

1.5

2.0

2.5

mean r

adiu

s (n

m)

normalized ECSA

Pt = 0.2 J m-2

Pt = 0.7 J m-2

Pt = 1.2 J m-2

Pt = 1.7 J m-2

0 1 2 30.0

0.4

0.8

1.2

1.6

0 1 2 30.0

0.4

0.8

1.2

1.6

0 1 2 30.0

0.4

0.8

1.2

1.6

0 1 2 30.0

0.4

0.8

1.2

1.6

0 h 60 h 200 h 400 h 800 h

Pt = 0.2 J m-2

(d)(c)

(b)

(a) 0 h 0.6 h 1.6 h 16 h 40 h

Pt = 0.7 J m-2

0 h 0.1 h 0.5 h 1.4 h 4 h

Pt = 1.2 J m-2

Pt particle radius (nm)

num

ber

of p

art

icle

s (a

rb. u

nits

)

0 h 0.01 h 0.04 h 0.2 h 1 h

Pt = 1.7 J m-2

10-410-310-210-1100 101 102 1030.00

0.25

0.50

0.75

1.00

Pt = 0.2 J m-2

Pt = 0.7 J m-2

Pt = 1.2 J m-2

Pt = 1.7 J m-2n

orm

aliz

ed E

CS

A

time (h)

0 1 2 3 4 50.00

0.25

0.50

0.75

1.00

0 1 2 3 4 5 60.00

0.25

0.50

0.75

1.00 TEM, Shao et al. 0 h model, 0 h

num

ber

of par

ticl

es (ar

b. units) TEM, Shao et al. 192 h

model, 192 h

Pt particle radius (nm)

0 50 100 150 2000.4

0.5

0.6

0.7

0.8

0.9

1.0 model Shao et al.

norm

aliz

ed E

CS

A

time (h)

comparison with potentiostatic experiments in high/low potential regions fitted surface tension values indicate dissolution occurs at a PtO interface ratio of surface tension (high/low potential) agree with experimental ratios extended surface dissolution rate trends follow experiment

0 2 4

0.5

0.6

0.7

0.8

0.9

1.0(b)(a)

Ref. 25, CO-stripping 0 h Ref. 25, CO-stripping 2 h Ref. 25, CO-stripping 4 h this work, Eq. 4

norm

aliz

ed E

CS

A

potential cycling treatment time (h)

1 2 3 4 5 6

0

20

40

60

80

100

120 this work, 0 h Ref. 25, IL-TEM, 0 h this work, 2 h Ref. 25, IL-TEM, 2 h this work, 4 h Ref. 25, IL-TEM, 4 h

num

ber

of p

artic

les

per

dr in

terv

al

particle radius (nm)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40

1

2

3

4

effective dissolution region

Ref. 8, potentiostatic ECSA Ref. 32, anodic DST Ref. 32, cathodic DST Ref. 33, Pt CV

this work, potential cycling ECSA

Ref. 30, PtO DFT

Ref. 29, Pt DFT

Ref. 31, Pt ZCM

surf

ace

tens

ion (

J m

-2)

potential (V vs. SHE)

Ref. 28, Pt DFT

-100

-80

-60

-40

-20

0

20

40

60

80

cur

rent de

nsi

ty (A

cm

-2)

1 2 3 4 5 6 7 80

20

40

60

80

100

120

1 2 3 4 5 6 7 80

20

40

60

80

100

120

1 2 3 4 5 6 7 80

20

40

60

80

100

120

1 2 3 4 5 6 7 80

20

40

60

80

100

120

= 1 x 10-4

= 10-1 = 2 x 10-4

= 1 x 10-6

num

ber

of par

ticl

es (ar

b. units)

particle radius (nm)

0.0 5.0x10-3 1.0x10-2

0.0

0.2

0.4

0.6

0.8

1.0 = 10-1

= 2 x 10-4

= 1 x 10-4

= 1 x 10-6

analytical, = 0

norm

aliz

ed E

CS

A

dimensionless time

0.0 5.0x10-3 1.0x10-20

25

50

75

100

125

150

175

= 10-1

= 2 x 10-4

= 1 x 10-4

= 1 x 10-6

dim

ensio

nle

ss c

oncentr

ation

dimensionless time

10-6 10-4 10-2 100 1020.0

0.2

0.4

0.6

0.8

1.0 R

0 = 13.2; = 0.0001

norm

aliz

ed E

CSA

log

transition

dissolution

redeposition

parametric study on the effect of redeposition loss of small particles and growth of large particles mean-field concentration approaches asymptotic value two ECSA loss mechanisms identified

• dissolution induced ECSA loss• redeposition induced ECSA loss