Abstract
For large output of polymer electrolyte fuel cells (PEFCs), the electrode reaction
activity of the catalyst layer (CL) consisting of carbon supports, Pt nanoparticles,
Nafion chains, and water should be improved. Experimentally, it is reported that when
Ketjen black (KB) with meso pores is used as the carbon support, the output of PEFC
increases and that the pore size of the KB support affects the electrode reaction activity
of the Pt nanoparticles. Therefore, in the present study, to clarify the effect of pore
size on the electrode reaction activity of the Pt nanoparticles, we constructed catalyst
particle (CP) models in which the Pt nanoparticles are supported and Nafion chains are
coated on the KB model and investigated the CP structures with a different pore size of
the KB support by reactive molecular dynamics method. Regardless of the pore size, the Pt
nanoparticles on the exterior of the pore are fully covered with the Nafion chains and the
Pt nanoparticles in the interior of the pore are not covered with the Nafion chains. This
result suggests that the Pt nanoparticles in the interior of the pore show high oxygen
transport property that does not depend on the pore size. Furthermore, we evaluated the
connectivity of the Nafion chains to H2O molecules absorbed on the Pt
nanoparticles on the exterior and in the interior of the pores because the Nafion chains
conduct the protons to the H2O molecules on the Pt nanoparticles. As the pore
size increases, more Nafion chains penetrate the interior of the pore and contact with
H2O molecules on the Pt nanoparticles, because more Nafion chains are
vertically distributed above the larger pore. Finally, these results propose that both
high oxygen transport property and high electrode reaction activity are achieved over the
Pt nanoparticles in the interior of the large pore of the KB support because the oxygen
diffusion in the pore is not blocked by the Nafion chains and the large pore size promotes
the formation of a proton conducting path composed of the Nafion chains, H2O,
and Pt nanoparticles.
1 Introduction
Fuel cells that use hydrogen fuel and emit no carbon dioxide, are strongly required to
realize a clean energy society. Polymer electrolyte fuel cells (PEFCs) are utilized for
vehicles at present and intended to be applied to large mobilities such as buses, trains,
and ships. Thus, it is necessary to increase the output of PEFC, which is achieved by
improving the electrode reaction activity of the catalyst layer (CL) in PEFC. The CL
consists of carbon supports, Pt nanoparticles, ionomer, and water, and the activity for
electrode reactions on the Pt nanoparticles such as oxygen reduction reaction (ORR) depends
on the structure of the carbon support surface and the composition of ionomer and water in
the CL. Then, many researchers have investigated the effect of the surface structure of the
carbon supports on the electrode reaction activity of the Pt nanoparticles on the CL. For
example, Yarlagadda et al. showed that the cathode CL with Ketjen black
(KB) support has higher electrode reaction activity compared to that with carbon black
supports without pores [1]. Moreover, the KB support
with accessible pores was found to show higher electrode reaction activity than that with
curved pores. Kobayashi et al. reported that in the case of the KB support
with large pores at the cathode, the Pt nanoparticles in the large pores showed a higher
effective surface area and a higher electrode reaction activity than those on other sites
[2]. Therefore, elucidating the effect of the pore
structure of the KB support on the electrode reaction activity of the CL and optimizing the
carbon support structure will contribute to the improvement of the output of PEFC. In the
present study, to improve ORR activity at the cathode, we analyzed the ionomer and water
coverage over the Pt nanoparticles on KB support with different pore sizes to clarify the
effect of the pore size on the electrode reaction activity of the Pt nanoparticles in the CL
from a nanoscale perspective.
2 Method and Model
To investigate distributions of the Nafion chains and water molecules around Pt
nanoparticles in the CL, we used our developed molecular dynamics simulator "Laich" [3]. ReaxFF developed by Duin et al. was
used as the force field to handle bond generation and dissociation during electrode chemical
reactions [4]. We calculated the charge of each atom
by electron equilibration method [5]. To construct the
KB model, six pores were created in an amorphous carbon sphere with a diameter of 20 nm.
Matsuhashi, et al. showed that the pore size in the commercial KB particles
of EC300J and ECP600JD is distributed in the range of 3.0-10.0 nm [6]. Then in this study, two models with different pore sizes of 4 and 6 nm
were constructed. These pore sizes are in the range observed experimentally. The shape of
the pore is a cone with a depth of 7.5-8.0 nm. To stabilize the KB models, the KB models
were relaxed with the NVT ensemble at a temperature of 400 K for 5 ps. In
addition, ten and four Pt nanoparticles with a diameter of 1 nm were put on the KB outer
surface and inside pores, respectively. To promote the adhesion of Pt nanoparticles to the
KB models, the Pt-supported KB models were relaxed with the NVT ensemble at
a temperature of 400 K for 10 ps. Next, to stabilize the Pt-supported KB models, the
Pt-supported KB models were relaxed with the NVT ensemble at a temperature
of 300 K for 15 ps. The pore sizes of the KB models and Pt-supported KB models did not
change before and after the relaxations. Then, the dangling bonds on the KB support were
terminated with OH and H groups. Figure 1 presents
the Pt-supported KB model with a pore of 6 nm in size. For an ionomer, a Nafion chain with
equivalent content of 1150 g/mol and six side chains was used. The Nafion chains were coated
over the Pt-supported KB model. The Pt-supported KB structure over which Nafion chains were
coated is called catalyst particle (CP) model in this study. Ionomer/carbon ratio are 1.03
and 1.14 for the Pt-supported KB models with 4 and 6 nm pore sizes, respectively. To
stabilize the CP structures, molecular dynamics calculations were performed at a temperature
of 300 K for 100 ps in the NVT ensemble. In these calculations, there are
no fixed parts.
3 Results and Discussion
To clarify the effect of the pore size of the KB support on the coverage states of the
Nafion chain and water molecules over the Pt nanoparticles, we compared two CP structures
with a different pore size of 4 and 6 nm. Figure 2
(a) shows the CP model with pore sizes of 6 nm after MD calculation at 100 ps. We
discuss the coverage states of water molecules and Nafion chains over the Pt nanoparticles
on the exterior and in the interior of the pore of the KB supports marked by black and blue
boxes as shown in Figure 2 (b), respectively.
Figure 3 shows the comparison of coverage states
of the Nafion chains and water molecules on the exterior and in the interior of the pore of
the KB supports with different pore sizes. Here, the ideal coverage states of the Pt
nanoparticles in the CL for high electrode reaction activity at the cathode are considered
as followings: (i) not full coverage states of the Nafion chains over the Pt nanoparticle to
allow smooth diffusion of oxygen molecules to the Pt nanoparticles for oxygen reduction
reaction and (ii) a lot of contacts of the Nafion chains with H2O molecules
adsorbed on the Pt nanoparticles for proton conduction to the Pt nanoparticles. Figures. 3
(a) and (b) show the coverage states of the Nafion chains over the Pt nanoparticles on the
exterior of the pore of 4 and 6 nm in size, respectively. The Pt nanoparticles on both
models are fully covered with the Nafion chains, indicating the low oxygen transport
property because the oxygen diffusion to the Pt nanoparticles is blocked by the Nafion
chains. Figure 3 (c) shows that Pt nanoparticles
in the interior of the pore are not covered by the Nafion chains when the pore size is 4 nm,
leading to high oxygen transport property. However, the Nafion chains do not connect with
the H2O molecules on the Pt nanoparticles, indicating low proton conductivity. In
the case of a pore size of 6 nm, Pt nanoparticles are not covered by the Nafion chains and
the Nafion chains partially connect with the H2O molecules on the Pt
nanoparticles in the interior of the pore (Figure 3
(d)). These results proposed that the coverage state of Pt nanoparticles as shown
in Figure 3 (d) has both high electrode reaction
activity and high oxygen transport property because protons can be conducted from the Nafion
chains to the Pt surface and the oxygen diffusion is not blocked by the Nafion chains. To
evaluate the connectivity of the Nafion chains to the H2O molecules on the Pt
nanoparticles in the interior of the pore, the number of carbon atoms in the Nafion chains
that penetrated the pores of the KB supports with different pore sizes were analyzed (Figure 4). Figure
4 shows that the carbon atoms of the Nafion chains penetrated more in the pore of 6
nm than those in the pore of 4 nm, indicating that the Nafion chains more easily penetrate
the pore of 6 nm than that of 4 nm in size. Then, as the pore size increases, more Nafion
chains penetrate the interior of the pore and contact with H2O molecules on the
Pt nanoparticles.
Furthermore, to discuss the reason why more Nafion chains penetrate the larger pore, we
analyzed the angle distributions of the Nafion chains above the pore of 4 and 6 nm in size.
The angle (θ) is defined as the angle between
a
→
and
b
→
in Figure 5 (a).
a
→
is from the center of the gravity of the Nafion chains to the
tip of the Nafion chains and
b
→
is from the center of the gravity of the Nafion chains to the
center of the KB support. In addition, we subtracted 90° from θ and took
the absolute value to evaluate the orientation of the Nafion chains above the pore. The
large and small angles indicate the vertical and horizontal orientations of the Nafion
chains to the pore, respectively. The angles of the Nafion chains distributed within 2 nm
from the KB support surface were evaluated and these were classified into 15 ° (Figure 5 (b)). In the case of the pore of 6 nm in
size, the Nafion chain over the angle of 45 ° or more is larger than that of 4 nm in size,
indicating the dominant distribution of the vertical Nafion chains. Here, the horizontal
Nafion chains block the entrance of the pore and cannot penetrate the pore. Therefore, the
larger the pore size is, the more Nafion chains above the pore are vertically distributed
toward the pore and can easily penetrate the pore.
These results suggest that both high oxygen transport property and high electrode reaction
activity are achieved over the Pt nanoparticles in the interior of the large pore of the KB
support because the oxygen diffusion in the pore is not blocked by the Nafion chains and the
large pore size promotes the formation of a proton conducting path composed of the Nafion
chains, H2O, and Pt nanoparticles.
4 Conclusion
To clarify the effect of the pore size of the KB support on the electrode reaction activity
of the CL in PEFC, we investigated the coverage states of the Nafion chains and water
molecules over the Pt nanoparticles on the KB support with the pore of 4 and 6 nm in size
using reactive molecular dynamics method. Regardless of the pore size, the Pt nanoparticles
on the exterior of the pore are fully covered with the Nafion chains and the Pt
nanoparticles in the interior of the pore are not covered with the Nafion chains. This
result suggests that the Pt nanoparticles in the interior of the pore are not blocked by the
Nafion chains for oxygen diffusion, indicating high oxygen transport property that does not
depend on the pore size. To evaluate the connectivity of the Nafion chains to the
H2O molecules on the Pt nanoparticles in the interior of the pore, the number
of carbon atoms in the Nafion chains that penetrated the pores of the KB supports with
different pore sizes were analyzed. As the pore size increases, more Nafion chains
penetrated the interior of the pore and contacted with H2O molecules on the Pt
nanoparticles. We also proposed that this is because the Nafion chains above the large pore
can vertically distribute toward the pore. Finally, these results propose that both high
oxygen transport property and high electrode reaction activity are achieved over the Pt
nanoparticles in the interior of the large pore of the KB support because the oxygen
diffusion in the pore is not blocked by the Nafion chains and the large pore size promotes
the formation of a proton conducting path composed of the Nafion chains, H2O, and
Pt nanoparticles.
Acknowledgment
This research was supported by the FC Platform Program (Grant No. JPNP20003): Development
of design-for-purpose numerical simulators for attaining long life and high performance
project (FY 2020–FY 2022) conducted by the New Energy and Industrial Technology Development
Organization (NEDO), Japan. We acknowledge Center for Computational Materials Science,
Institute for Materials Research, Tohoku University for the use of supercomputing system
MASAMUNE-IMR (202012-SCKXX-0502).
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