pt-co/c catalysts: pemfc performance and durability · pt-co/c catalysts: pemfc performance and...
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Pt-Co/C Catalysts: PEMFC Performance and Durability Prasanna Mani, Harmeet Chhina, Emily Hopkins and Wendy Lee
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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?
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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
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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
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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
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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
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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
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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
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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
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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
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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
15
(111)
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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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