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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
3
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
12
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
13
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
14
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)
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.
16