Pt-Co/C Catalysts: PEMFC Performance and Durability Prasanna Mani, Harmeet Chhina, Emily Hopkins and Wendy Lee

Why are we looking at Pt-Co/C catalysts for PEM Fuel Cells even after so many years since the introduction? What trade offs in Pt-Co alloy characteristics can yield better performance and durability than Pt/C? – Pt:Co atomic ratio

– Particle size

– Metal loading

What causes the performance to drop during accelerated stress test?

2

Pt:Co atomic ratio – 2:1 to 9:1 Alloy particle size – 3 to 5 nm Metal loading – 30 & 50 wt%

Pt-Co/C catalysts

Catalyst Pt wt% Co wt% Pt/Co atomic ratio

XRD crystallite size, nm

Catalyst surface area, m2/g

Pt/C 52.5 - - 4.9 358 Pt2Co/C 47.0 6.9 2.1 4.2 333 Pt3Co/C 28.8 3.1 2.8 3.3 514 Pt4Co/C 31.5 2.3 4.1 3.2 438 Pt6Co/C 48.6 2.5 5.9 4.9 350 Pt7Co/C 30.1 1.3 7.0 3.9 521 Pt9Co/C 30.3 1.0 9.2 3.9 508

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All catalysts show disordered fcc structure

Positive shift in the (111) peak position for Pt-Co/C catalysts indicate alloying of Co with Pt

Pt2Co/C & Pt3Co/C show slightly broader distribution

XRD

36 37 38 39 40 41 42 43 44 45

Pt2Co/C

36 37 38 39 40 41 42 43 44 45

Pt3Co/C

36 37 38 39 40 41 42 43 44 45

Pt6Co/C

36 37 38 39 40 41 42 43 44 45

Pt7Co/C

36 37 38 39 40 41 42 43 44 45

Pt9Co/C

36 37 38 39 40 41 42 43 44 45

Pt/C

2 theta, o

4

Increased double layer capacitance for 30wt% catalysts compared to 50 wt% catalysts 30wt% catalysts show enhanced Hydrogen adsorption/desorption peaks

RDE

-4.E-04

-3.E-04

-2.E-04

-1.E-04

0.E+00

1.E-04

2.E-04

3.E-04

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Curr

ent d

ensi

ty (A

/cm

2)

Potential, V (RHE)

Pt/C

Pt2Co/C

Pt3Co/C

Pt4Co/C

Pt6Co/C

Pt7Co/C

Pt9Co/C

Scan rate: 20mV/s Pt loading on GC disk: ~10 µg/cm2

0.1M HClO4, 35 oC

5

Good co-relation between XRD crystallite sizes and ECSAs measured in RDE

ECSA

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0

10.0

20.0

30.0

40.0

50.0

60.0

Pt/C Pt2Co/C Pt3Co/C Pt4Co/C Pt6Co/C Pt7Co/C Pt9Co/C

Crys

talli

te s

ize,

nm

ECSA

, m2/

g

6

Pt-Co/C catalysts clearly show higher oxygen reduction activity over Pt/C Limiting current densities are close to theoretical calculation at 2000 RPM

ORR

Scan rate: 5mV/s (Anodic), 2000 RPM

-7.E-03

-6.E-03

-5.E-03

-4.E-03

-3.E-03

-2.E-03

-1.E-03

-1.E-170.0 0.2 0.4 0.6 0.8 1.0

Curr

ent d

ensi

ty (A

/cm

2)

Potential, V (RHE)

Pt/C

Pt2Co/C

Pt3Co/C

Pt4Co/C

Pt6Co/C

Pt7Co/C

Pt9Co/C

Pt loading on GC disk: ~10 µg/cm2

0.1M HClO4, 35 oC

7

Surprisingly catalysts with low Co (atomic ratios 6:1 to 9:1) show about 2.5x activity compared to Pt

Pt Mass activity

0.000

0.100

0.200

0.300

0.400

0.500

0.600

Pt/C Pt2Co/C Pt3Co/C Pt4Co/C Pt6Co/C Pt7Co/C Pt9Co/C

Mas

s act

ivity

, A/m

g-Pt

4.7x

8

Highest Pt specific activity of about 1000 µA/cm2-Pt is observed for Pt2Co/C & Pt3Co/C Alloying small amount of Co with Pt (Pt:Co atomic ratio of 6:1 to 9:1) increase specific

activity by 2x compared to Pt

Specific Activity

0

200

400

600

800

1000

1200

Pt/C Pt2Co/C Pt3Co/C Pt4Co/C Pt6Co/C Pt7Co/C Pt9Co/C

Specific activity, µA/cm2-Pt

3.4x

9

In fuel cells (~50cm2 active area), peak performance improvement observed for Pt4Co/C unlike Pt3Co/C in RDE

3.6x improvement in activity for Pt4Co/C over Pt in RDE translates to 35mV improvement in fuel cell performance

Fuel Cell Performance

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

Pt2Co/C Pt3Co/C Pt4Co/C Pt6Co/C Pt7Co/C Pt9Co/C

Cell

perf

orm

ance

gai

n, V

@ 0.1 A/cm2

10

Unexpectedly Pt2Co/C, Pt7Co/C & Pt9Co/C show smaller improvement over Pt/C at 1.5 A/cm2

Fuel Cell Performance

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

Pt2Co/C Pt3Co/C Pt4Co/C Pt6Co/C Pt7Co/C Pt9Co/C

Cell

perf

orm

ance

gai

n, V

@ 1.5 A/cm2

11

Catalysts with Pt:Co atomic ratios -2:1 to 4:1 show slightly higher voltage losses at 0.1 A/cm2

Also for these catalysts significantly additional voltage losses are observed at 1.5 A/cm2

Accelerated stress test

-0.010

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0.090

Pt/C Pt2Co/C Pt3Co/C Pt4Co/C Pt6Co/C Pt7Co/C Pt9Co/C

Cell

Volta

ge lo

ss, V

0.1 A/cm2 1.5 A/cm2

0.00.51.01.52.02.53.03.54.04.5

Degr

adat

ion

rate

, µV/

cycl

e @ 1.5 A/cm2 1.0 V, 2 sec

0.1 V, 2 sec

AST: 20,000 cycles

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BOL & EOL MEA samples

Catalyst layer thickness

Catalyst surface species

Particle size

Characterizations

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

BOL EOL BOL EOL BOL EOL BOL EOL

Pt2Co/C Pt3Co/C Pt4Co/C Pt6Co/C

Co/P

t Ato

mic

Rat

io

Ohmic resistance

PITM

Dissolution of Co

EDX analysis exhibits catalysts with higher Cobalt ratios show massive loss of Cobalt after 20,000 voltage cycles

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Structural characterization

35 40 45 50 55 60 65 70 75

Inte

nsity

2 Theta, o

Pt6Co/C-BOL

Pt6Co/C-EOL

35 40 45 50 55 60 65 70 75

Inte

nsity

2 Theta, o

Pt7Co/C-BOL

Pt7Co/C-EOL

Smaller negative shift in peak position observed for EOL samples show leaching of Co

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Significant shift (negative) in peak positions observed for Pt2Co/C & Pt3Co/C EOL samples show extensive dissolution of Co

Structural characterization

35 40 45 50 55 60 65 70 75

Inte

nsity

2 Theta, o

Pt2Co/C-BOL

Pt2Co/C-EOL

Pt3Co/C-BOL

Pt3Co/C-EOL

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(111)

Summary

35mV improvement in performance observed for Pt4Co/C over Pt/C at low

and high current densities

Of all the changes in catalyst properties, dissolution of Co seems to have a

key contribution in cell performance losses (AST: 20,000 cycles, 0.1-1.0V)

At 1.5 A/cm2, catalysts with higher cobalt ratios such as Pt2Co/C, Pt3Co/C

& Pt4Co/C show additional performance losses after voltage cycling; this is

attributed to leached Co related water management issues

Further optimization of alloy durability and performance is possible but

trade-offs are needed depending on the intended operating conditions. Pt-

Co/C catalysts are still a significant consideration for fuel cell vehicles.

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