Advantages of polymer wrapping on to carbon black to improve Pt utilization efficiency in polymer electrolyte membrane fuel cells

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Advantages of polymer wrapping on to carbon black to improve Pt
utilization efficiency in polymer electrolyte membrane fuel cells
1Samindi Jayawickrama a and Tsuyohiko Fujigaya a-d
1Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka, 819-0395, Japan
2WPI-I2CNER, Kyushu University, Fukuoka, 819-0395, Japan 3JST-PRESTO, 4CMS, Kyushu University, Fukuoka, 819-0395, Japan
Division - Electrochemical Energy Conversion Division
The Electrochemical Energy Conversion Section is
focused on the development and in-depth
understanding of materials, processes and devices for
the conversion of renewable energy into electricity or
chemical energy carriers. Especially in the context of a
sustainable energy system utilizing hydrogen as an
energy carrier and its electrochemical energy
conversion is of particular importance.
Milestones
~ 2020 < 0.5 g kW-1
e-
H＋
electrode
catalyst
electrolyte
membrane
electrode
Current situation
The efficiency of the fuel cell
thoroughly depends on the MEA
quality. The deposition of Pt nano-
particles into the pores of carbon
support and inhomogeneous
Nafion distribution hinders effective
utilization of Pt nano-particles in
the MEA.
Nafion ionomer
Reduced Pt utilization
(catalyst layer)
Pt Carbon black (CB)
H+
O2
Research Approach
Micropore
In this study we used
polybenzimidazole (PBI) as the
wrapping material. A thin layer of PBI
can cap the micropore gates in CB,
preventing Pt deposition into pores
and can achieve homogeneous
Nafion layer around Pt.
H+
O2
Micropore
capping
Homogeneous
Nafion distribution
Interaction
PBI Wrapping
v The maximum Pt utilization efficiency obtained is 0.32 g kW-1
1.Prevention of Pt deposition into pores
2. Reduced H+ resistance in the catalyst layer due the
homogeneous distribution of Nafion
3. Homogeneously dispersed Pt particles
v We successfully improved the Pt utilization efficiency in all
polymer wrapped CBs by,
Summary Future plans
q Preparation of Poison free electrocatalysts
Having a Nafion coverage on the Pt hinders the effective
O2 diffusion.
Sulfonated PBI will facilitate H+ conduction in the catalyst
layer without Nafion. Since there is no Nafion barrier for
O2 diffusion, mass transfer limitation will be minimum,
improving Pt utilization efficiency further
CB
Nafion
Pt
O2 diffusion
resistance
Sulfonated PBI
Recent progress
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dV
p
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Pore size / nm
q N2 Adsorption – Specific surface area
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0 10 20 30 40 50
1336 m2g-1
1126 m2g-1
206 m2g-1
83 m2g-1
60 m2g-1
53 m2g-1
Vulcan
KB/
PBI
Vulcan/
PBI
AB/
PBI
KB
AB
Micropore
reduction
-22 %
Micropore
reduction
-83 %
Micropore
Reduction: ND
Ketjen black; KB Vulcan Acetylene black; AB
T. Fujigaya et al., ACS Appl. Mater. Interfaces, 2016, 8, 14494.
Three types of CBs were used for the PBI wrapping study.
Micropore capping phenomena is observed in high surface area
CB; ketjen black and Vulcan. There is no micropore capping
occurred in acetylene black; low surface area CB.
KB Vulcan AB
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Voltage / V
q Cyclic voltammetry - ECSA
-80
-60
-40
-20
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Current
/
A
g
-1
PBI wrapped
+15 %
PBI wrapped
+27 %
PBI wrapped
+16 %
49.2 m2g-1 62.4 m2g-1
87.4 m2g-1
49.3 m2g-1
42.1 m2g-1 73.0 m2g-1
2.34 ± 0.53 3.06 ± 0.50 3.49 ± 0.68
2.33 ± 0.57 4.39 ± 1.21
3.10 ± 0.60
Our research efforts are focused on improving
Pt utilization in membrane electrode assembly
(MEA) by controlling the interface properties
around Pt nano -particles.
Polymer electrolyte fuel cells
Improved Pt utilization
(catalyst layer)
q Fuel cell test – Polarization losses
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Current density / A cm
-2
+19 %
+25 %
KB/PBI/Pt
0.59 Wcm-2
Vulcan/PBI/Pt
0.70 Wcm-2
Vulcan/Pt
0.59 Wcm-2
KB/Pt
0.47 Wcm-2
KB Vulcan AB
0 0.5 1 1.5 2
+10 %
AB/Pt
0.71 Wcm-2
AB/PBI/Pt
0.78 Wcm-2
The Electrochemical surface area (ECSA) of PBI wrapped system
is higher compared to that of non-wrapped system. For ketjen
black and vulcan, improved ECSA is only coming from the pore
capping phenomena; no effect from Pt particle size.
Interestingly, PBI wrapping aids the homogeneous Pt dispersion
in acetylene black, improving ECSA.
v Micropore capping and
homogenous Nafion
distribution are the main
reasons for improved power
density of PBI wrapped high
surface area CB systems as
we expected.
v There are other advantages of
PBI wrapping method. One of
them are catalyst ink stability
which ultimately result in
smooth catalyst layer without
large agglomerates.
Anode Cathode
Target
~6 gPt/vehicle
A. Kongkanand et al., J. Phys.
Chem. Lett., 2016, 7, 11272. ~ 2025 < 0.1 g kW-1