Molecular Dynamics Simulation to Reveal Effects of Binder Content on

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Molecular dynamics simulation to reveal effects of binder content on Pt/C catalyst coverage in a high temperature polymer electrolyte membrane fuel cell Sung Hyun Kwon, So Young Lee, Hyoung-Juhn Kim, Hee-Tak Kim, and Seung Geol Lee ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00484 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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ACS Applied Nano Materials

Molecular dynamics simulation to reveal effects of binder content on Pt/C catalyst coverage in a high temperature polymer electrolyte membrane fuel cell

Sung Hyun Kwon,1 So Young Lee,2 Hyoung-Juhn Kim,2,* Hee-Tak Kim3 and Seung Geol Lee1,*

1

Department of Organic Material Science and Engineering, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea 2

3

Fuel Cell Research Center, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea

*E-mail: [email protected] (S.G. Lee) *E-mail: [email protected] (H. Kim) 1

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Abstract Full atomistic molecular dynamics (MD) simulations were performed in order to provide detailed information on the morphologies of Pt/C catalyst with varying PTFE-binder contents. Changes in the surface configuration and PTFE coverage on Pt particles with changing binder content were examined on the molecular level; this coverage can affect the catalytic performance of Pt particles and PTFE binding. The PTFE-binder content in the prepared solutions ranged from 4.0 to 35.1 wt%. From Pt-PTFE pair correlation analysis, the coordination number of this pair increased from 0.43 to 1.23 as the PTFE-binder content increased from 4.0 to 35.1 wt%, with a concomitant 40.0 to 84.0% change in coverage over the Pt surface. At low PTFE content, the PTFE binder was dispersed between Pt particles and the carbons on the Pt/C surface to form a triple-phase boundary (TPB). Subsequently, Pt particles become increasingly covered by PTFE with increasing binder content. However, no significant changes were observed when the PTFE content exceeded 20.0 wt%; we expect that the catalytic performance of Pt will significantly decrease at PTFE-binder contents greater than 20.0 wt%. Considering the Pt-retaining role of the binder, we conclude that the optimum PTFE-binder content is less than 20.0 wt% for the ~2.6-nm-diameter Pt particle used in this study. This investigation can be provided detailed information on polymer properties and electrode morphologies for high-temperature polymer electrolyte membrane fuel cells applications at various PTFE-binder contents.

Keywords: fuel cell; high temperature PEM; molecular dynamics; catalyst; PTFE; binder; Pt/C

2


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1. Introduction Fuel cells are good environmentally friendly energy devices because they offer several benefits that help solve environmental problems.1 Research and development into fuel cells continue to unlock the advantages eco-friendly energy. Among various types of fuel cells, polymer electrolyte membrane fuel cells (PEMFCs) are potential renewable energy devices due to advantages that include high energy efficiencies, low operating temperatures, and short startup times.2 In general, PEMFCs are categorized into low-temperature and hightemperature PEMFCs (LT-PEMFCs and HT-PEMFCs, respectively), depending on their operating temperatures. LT-PEMFCs operate at low temperatures, below 100 °C, due to presence of water in the polymers.3-4 In spite of the limitations of LT-PEMFCs, they exhibit good power densities when compared to other fuel cells.1 However, a critical impediment of an LT-PEMFC is the susceptibility of the Pt particles in its electrode layers to carbon monoxide poisoning.5-6 On the other hand, HT-PEMFCs operate between 100 and 200 °C; these PEMFCs are less affected by carbon monoxide poisoning due to their high operating temperatures when compared to those of LT-PEMFCs. In addition, HT-PEMFCs are good alternative energy-generation systems because they not only overcome the limitations of the LT-PEMFCs, but they also exhibit several advantages associated with their high operating temperatures through the reuse of high amounts of heat energy.7-8 However, the energy efficiencies of HT-PEMFCs are lower than those of LT-PEMFCs due to low oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) kinetics, and low proton conductivities.9-12 Therefore, from an economic and commercial perspective, improving energy efficiency is a key research objective that will ultimately improve HT-PEMFC performance. HT-PEMFC is composed of a gas-diffusion layer (GDL), the electrode, and polymer 3



membranes. The polymer membranes are mainly fabricated from polybenzimidazole (PBI) and poly(2,5-benzimidazole) (ab-PBI), and are doped with phosphoric acid (H3PO4) as the proton-conduction medium, in order to transfer protons across them. Most electrode catalyst layers are manufactured using catalyst inks that are deposited on the gas-diffusion layer. These catalyst inks include various components, such as the catalyst itself (mainly Pt), carbon black, a polymer binder, and a solvent to disperse these components. Therefore, the polymer binder in the catalyst layer is deeply connected to the catalyst and the carbon black. Control of the binder distribution is necessary in order to understand and improve the performance of the electrode.13-14 These binders are typically polymers such as polytetrafluoroethylene (PTFE)15-20, PBI18, 21-24, Nafion18, 25, poly(vinylidene difluoride) (PVDF)18, and PBI-PVDF18, 26-27

, and exhibit a variety of mechanical properties, gas-transfer features, and solubilities that

determine the performance of the cell. PTFE binders have been widely used in previous researches 15-20 that investigated the optimized PTFE-binder content on HT-PEMFC-electrode performance. Notably, the PTFE binder acts as an electrical insulator in the electrode; consequently cell performance decreases with increasing PTFE-binder content because the binder obstructs reaction pathways, such as those involving the movement of phosphoric acid and oxygen to the Pt particles. Jeong et al.20 reported that the optimum binder distribution, which depends on the PTFE content, needs to be improved in order to optimize cell performance. Moreover, Mack el al.15 showed that the electrode morphology influences its performance in an HT-PEMFC. Hence, controlling the distribution of the PTFE binder on the Pt/C is key to improving cell performance.15 Consequently, understanding how the morphology changes in response to changes in PTFE-binder content on the Pt/C catalyst, is crucial. In this context, the molecular dynamics (MD) simulation technique has been used to provide 4


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detailed information on polymer properties and electrode morphologies at various PTFE contents in this study. In addition, MD simulations are useful tools for elucidating detailed atomistic information when designing PEMFC components.28-31 It should be noted that most of MD simulation used in HT-PEMFC studies have focused on the phosphoric acid and PBI in the membrane system, while the HT-PEMFC electrode has not been thoroughly simulated, even though establishing the structures and morphologies of the contents of the PTFE binder at the molecular level is important. Therefore, in this study, we investigated how the PTFE distribution changes with changing PTFE content on a Pt/C catalyst using full atomistic MD simulations. We simulated the evaporations of solutions containing eight different PTFEbinder contents from the Pt/C surface. We also investigated changes in the surface configuration and Pt-particle coverage with changing PTFE-binder contents at the molecular level; these changes can affect the catalytic performance of the Pt particles.

2. Computational Details All MD simulations were performed using full atomistic models. In order to analyze the influence of the PTFE binder on the Pt/C, we simulated the evaporation of eight solutions from the Pt/C surface, with PTFE-binder contents in the 4.0–35.1 wt% range.

2.1 Model Preparation 2.1.1. Construction of the Pt particle and the Pt/C structure First, we determined the size of the Pt particle by referring to the TEC10E50E material available from Tanaka Kikinzoku Kogyo (TKK). Shao et al.32 measured the average Pt 5



particle size in the commercial TEC10E50E Pt/C catalyst to be 2.5 ± 0.4 nm and found that the Pt particles were cuboctahedral in shape. Fig. 1(a) shows the size and morphology of a Pt particle modeled on that measured by Shao et al.32 The cuboctahedral shape of the Pt particle forms eight (111) planes and six (100) planes; this particle contains 586 Pt atoms. In order to match the ratio of Pt to C in the commercial Pt/C (TKK catalyst with 45.9 wt% Pt), 1792 carbon atoms were used to form one graphite layer, and a total of six layers were used to simulate the carbonaceous structure.

Fig. 1. (a) Size and morphology of the Pt particle used in this study (b) Vapor-liquid equilibrium for an IPA/water system at 1 atm, and the mole fractions of water and IPA with changing IPA liquid composition.

2.1.2. Establishing the PTFE/solvent model In general, water and isopropyl alcohol (IPA) have been used as solvents for the dispersion of the PTFE binder and the Pt/C during the preparation of catalyst inks.15, 19-20 The catalystink composition was chosen to match that used experimentally.20 The initial solvent was a 70:30 (w/w) IPA:water mixture, and the catalyst ink contained Pt/C and the PTFE binder in a 94.6:5.4 (w/w) ratio. Catalyst inks are generally sprayed onto the gas-diffusion layer to form 6


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the catalyst layer of the electrode. The water:IPA evaporation ratio with changing IPA-liquid composition is an important parameter required when building an electrode system. We relied on the data reported by Brunjes et al. that determined the IPA-vapor composition with changing IPA-liquid composition.33 Fig. 1(b) displays the vapor-liquid equilibrium for an IPA/water system; the black dots and red line in Fig. 1(b) indicate experimental 33 and fitted data, respectively. The IPA/water vapor-liquid equilibrium reveals that IPA molecules are easier to evaporate than water molecules in PTFE/solvent systems. The blue and green dashed lines in Fig. 1(b) are the water- and IPA-content profiles, respectively. For a mixture with an initial 70:30 (w/w) IPA:water composition, water remains in the catalyst ink despite the complete evaporation of IPA. In this context, water mainly affects the distribution of the PTFE binder on the Pt/C surface during the last stages of evaporation. Hence, in order to study the distribution of the PTFE binder, IPA was not considered and our simulations were performed with only the PTFE binder and water molecules on the Pt/C matrix. All of PTFE/solvent and PTFE-chain models were prepared with a 100 degrees of polymerization (DP).

2.2 Simulation Details 2.2.1. Force fields and MD parameters A modified DREIDING force field34 was applied to the PTFE binder and the carbon structure, and the F3C force field was used for the water molecules.35 The DREIDING force field has been successfully used to study for various organic materials36-40 including PEMFC systems.41-44 The embedded atom method (EAM) force field45 was used to describe interatomic potentials in the Pt particles because this force field is capable of describing the 7



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details of metallic species. One of the main issues associated with describing energy profiles is the requirement to set up vdW parameters for interactions between metals (Pt particles) and non-metals (carbon, water, and the PTFE binder). We used the parameters reported by Brunello et al. as the vdW parameters that describe metal and non-metal electrode systems.46 The total potential energy is given by:

‫ܧ‬௧௢௧௔௟ = ‫ܧ‬௩ௗௐ + ‫ܧ‬ொ + ‫ܧ‬௕௢௡ௗ + ‫ܧ‬௔௡௚௟௘ + ‫ܧ‬௧௢௥௦௜௢௡ + ‫ܧ‬௜௡௩௘௥௦௜௢௡

(1)

where ‫ܧ‬௧௢௧௔௟ , ‫ܧ‬௩ௗௐ , ‫ܧ‬ொ , ‫ܧ‬௕௢௡ௗ , ‫ܧ‬௔௡௚௟௘ , ‫ܧ‬௧௢௥௦௜௢௡ , and ‫ܧ‬௜௡௩௘௥௦௜௢௡ are the total, vdW, electrostatic, bond-stretching, bending, torsion, and inversion energy components, respectively. All MD simulations used the velocity-Velet algorithm47 to integrate equations of atomic motion with time, in 1.0-fs steps for equilibrium simulations, and 0.5-fs steps for evaporation simulations. The atomic charges on the PTFE binder in the electrode were assigned on the basis of Mulliken charge analyses48 from DFT calculations at the DNP level with the GGA-PBE functional49, while F3C-water-model charges were used for the water molecules.35 The largescale atomic/molecular massively parallel simulator (LAMMPS) code, developed by Plimpton et al.,50-51 was used for the MD simulations in this study. The particle-particle particle-mesh (PPPM) method52 was used to calculate electrostatic interactions during the MD simulations. 2.2.2. Model construction and simulations of the solvent and PTFE binder on Pt/C To generate the Pt/C surface, we construct systems of 68.180 × 68.880 × 500 Å in size, with periodic boundary conditions (PBC). A z-direction of 500 Å was used to prevent interactions beyond the PBC. The initial PTFE-solvent models were prepared by Monte-Carlo simulation 8


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53

. The PTFE-binder contents on the Pt/C surface were prepared such that they ranged

between 4.0 and 35.1 wt%. The characteristics of the PTFE-binder on the Pt/C surface are described in Table 1.

Table 1. Properties of the Pt/C and the PTFE binder in the model systems. Number of Pt atoms (47.0 wt% in Pt/C)

586

Number of C atoms (53.0 wt% in Pt/C)

10752

Pt/C

6687

Number of water molecules Number of PTFE chains (DP = 100) PTFE contents (wt %)

1

3

4

6

8

10

13

4.0

11.1

14.2

20.0

25.0

29.4

35.1

2.2.3. Model equilibration After the model structures were built, Canonical ensemble (NVT) MD simulations were performed for 10 ns at 298.15 K in order to finalize equilibration. Fig. 2 displays the modelpreparation concept with a 20 wt% PTFE content on the Pt/C surface prior to the waterevaporation simulation. Following equilibration, evaporation of the water molecules in the PTFE solvent models were simulated by NVT MD for 7 ns, after which data for analysis were collected over a further 5-ns NVT-MD-simulation run.

9



Fig. 2. The model-preparation concept with water molecules at a PTFE-content of 20 wt%. White, gray, red, cyan and blue color represent hydrogen, carbon, oxygen, fluorine, and platinum atoms, respectively. Pt and PTFE atoms are represented by van der Waals radius scheme for clarity.

3. Results and Discussion 3.1 Equilibrated Structures Three-dimensional (3D) snapshots of the equilibrated structures from the data collected over the final 5 ns of the MD simulations are displayed in Fig. 3; the PTFE binder increasingly covers the Pt/C surface with increasing PTFE content. As expected, PTFE-binder molecules are dispersed near to the Pt particle and the carbon interface where they form triple-phase boundaries (TPBs); the PTFE gradually covers the Pt particle and the carbon surface with increasing PTFE-binder content. The distribution of the PTFE binder on the Pt surface has a positive effect that fixes the Pt particle to the surface and enhances Pt durability; however, on 10


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the other hand, the negative electrical-insulating effect of the PTFE binder results in a loss of the catalytic Pt surface in the electrode. Therefore, we predict that excessive PTFE-binder content will decrease the performance of the catalyst. To quantitatively analyze the MD data, we examined the structure by constructing radial distribution functions (RDFs) of the model systems.

Fig. 3. Equilibrated Pt/C-surface structures with PTFE contents of: (a) 4.0 wt%, (b) 7.7 wt%, (c) 11.1 wt%, (d) 14.2 wt%, (e) 20.0 wt%, (f) 25.0 wt%, (g) 29.4 wt%, and (h) 35.1 wt%. Gray, cyan, and blue color represent carbon, fluorine and platinum atoms, respectively. Pt and PTFE atoms are represented by van der Waals radius scheme for clarity.

3.2 Distribution of PTFE on Pt/C Understanding the PTFE distribution on the Pt/C surface is necessary in order to balance the binder and insulator characteristics of the PTFE on the Pt/C surface and for efficient electrode design. To characterize the PTFE distribution on the Pt and carbon surfaces in the electrode, 11



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we analyzed the PTFE and Pt particle using a pair correlation function; this function,

g ୅ି୆ (‫ )ݎ‬, describes the probability of finding A at distance r from B averaged over equilibrium trajectories in MD simulations, so that:

g ୅ି୆ (‫( = )ݎ‬

௡ಳ

ସగ௥మ ∆௥

)/(

ேಳ ௏

)

(2)

where ݊஻ is the number of B particles located at distance r in a shell of thickness ∆r from particle A, ܰ஻ is the number of B particles in the system, and V is the total volume of the system. Using this pair correlation function, it is possible to determine the characteristics of the PTFE-binder distributed on the Pt/C surface. Fig. 4(a) clearly reveals that the ρg ୔୲ି୊ (‫)ݎ‬ intensities did not significantly increase as the PTFE content was increased from 20.0 to 35.1 wt%. This means that PTFE molecules at contents in excess of 20.0 wt% are similarly distributed on the Pt/C surface. To quantitatively analyze the PTFE distribution on the Pt/C with changing PTFE content, we determined the coordination number (CN) by integrating the first peaks of the pair-correlation functions. Since the distance of the first minimum does not change significantly with changing PTFE content, we set the CN cutoff distance to 3.85 Å. The CNs of Pt-F(PTFE) pairs are summarized in Fig. 4(b) and Table 2. The Pt-F(PTFE) CN increased from 0.43 to 1.12 as the PTFE-content on the Pt/C surface increased from 4.0 to 14.2 wt%; however it did not significantly change as the PTFE content was increased from 20.0 to 35.1 wt%, with values of 1.22 and 1.23, respectively. As shown in Fig. 3(a), the PTFE molecules at low content (4.0 wt% with 1 PTFE chain) tended to surround the Pt particle to form a TPB with the Pt particle and the carbon interface. After that, as shown in Figs. 3(b) and (c), the PTFE binder becomes dispersed over both the Pt particle and the carbon surface. Finally, as shown in Figs. 3(d)–(g), the PTFE binder covers most of the Pt particle and the carbon surface. The total Pt-F(PTFE)-pair CN exhibited no significant change in the 20.0– 12


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35.1 wt% PTFE-content range. This means that the PTFE binder almost entirely covers the Pt/C at contents above 20.0 wt%, and its property as an electrical insulator decreases the catalytic performance of the Pt, despite exhibiting the best binding performance. It should be noted that low activity of ORR in HT-PEMFC is also significantly affected by the surface poisoning by H3PO4 that is immersed in PBI. (a)

0.025 4.0 wt % PTFE 7.7 wt % PTFE 11.1 wt % PTFE 14.2 wt % PTFE 20.0 wt % PTFE 25.0 wt % PTFE 29.4 wt % PTFE 35.1 wt % PTFE

࣋ࢍࡼ࢚ିࡲ ࢘

0.020

0.015

0.010

0.005

0.000 0

1

2

3

4

5

6

7

8

Distance ( ) (b)

2.0 1.6

Coordination number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2 0.8 0.4 0.0

0

10

20

30

40

PTFE content (wt%)

Fig. 4. (a) Pair correlation functions (ρgPt-F(r)) of Pt-F(PTFE) pairs on the Pt/C surface at different PTFE contents. (b) The coordination number of fluorine atoms in PTFE to platinum as a function of PTFE content. 13



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Table 2. Coordination number of Pt-F(PTFE) pairs according to PTFE content. PTFE contents (wt %)

4.0

7.7

11.1

14.2

20.0

25.0

29.4

35.1

Coordination number (CN)

0.43

0.71

0.93

1.12

1.22

1.17

1.23

1.23

3.3 Surface Analysis Fig. 5(a) displays the surface-analysis concept involving the surface of the Pt particle, the PTFE binder, and the carbon surface; Pt atoms in contact or not with the carbon surface through the PTFE binder, based on vdW-radius calculations, are shown in blue and red, respectively. As shown in Fig. 5(b), initially, the Pt atoms in contact with the atoms of the PTFE binder and the carbon surface are located in the lower regions of the Pt particle at low PTFE content (4.0 wt%) because the PTFE binder is initially dispersed near to the TPB of the Pt particle and the carbon surface. Fig. 6 displays quantitative analyses of the contact between the Pt surface, the PTFE binder, and the carbon surface, with changes in PTFE-binder content.

14


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Fig. 5. (a) Analyzing the contact surface involving the Pt particle, PTFE, and the carbon surface. Pt atoms on the Pt surface in contact with the PTFE binder and the carbon surface are depicted as red balls at PTFE-binder contents of: (b) 4.0 wt%, (c) 7.7 wt%, (d) 11.1 wt%, (e) 14.2 wt%, (f) 20.0, (g) 25.0 wt%, (h) 29.4 wt%, and (i) 35.1 wt%.

The Pt surface coverage is calculated by: Pt surface coverage (%) = (Ptcontact / Ptsurface) ൈ 100 (%)

(3)

where Ptcontact represents the number of Pt atom is contact with the PTFE binder based on the vdW radius, and Ptsurface represents the number of Pt atom in the first layer of the Pt surface. As shown in Fig. 6 and Table 3, the total Pt surface coverage, which is the sum of the upper and lower parts of the Pt surface that are in contact with PTFE and the carbon surface, gradually increased from 40.4 to 84.3% as the PTFE content was increased from 4.0 to 35.1 wt%. At low content (4.0 wt%), the lower part of the Pt surface is predominantly covered by the PTFE binder; at a content of 4.0 wt%, the PTFE binder covers the lower part of the Pt to a 15



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level of 30.3%. This means that the PTFE binder is initially distributed mainly near to the lower part of the Pt surface; this initial distribution forms a TPB between the Pt particle and the carbon surface. With increasing binder content (to 20 wt%), the upper part of the Pt surface becomes increasing covered with the PTFE binder; however, no significant change is observed as the PTFE-binder content exceeds 20.0 wt%. In same manner, we also calculated the carbon surface coverage by: Carbon surface coverage (%) = (Ccontact / Csurface) ൈ 100 (%)

(4)

where Ccontact represents the number C atom is contact with the PTFE binder based on the vdW radius. Csurface represents the number of C atom in the first layer of the carbon surface. As shown in Fig. 7, the total carbon surface coverage is gradually increased from 6.6 to 29.4% as the PTFE content was increased from 4.0 to 20.0 wt%. The carbon surface coverage is not significantly changed with over 20.0 wt% of PTFE content. We expect that the catalytic performance of the Pt may decrease at PTFE-binder contents above 20 wt%. Considering that the role of the PTFE is to bind the Pt to the carbon surface, we conclude that the optimum PTFE-binder content is around 20 wt% which is reasonable well agreed with the experimental results20. However, we expect that increasing the Ptparticle size will result in an increase in the optimum PTFE-binder content.

16


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Fig. 6. Coverages of the upper and lower parts of the Pt surface with the PTFE binder on the carbon surface as a function of PTFE-binder content.

Table 3. PTFE-binder coverage over the Pt and carbon surfaces at various PTFE-binder contents. PTFE contents (wt %)

4.0

7.7

11.1

14.2

20.0

25.0

29.4

35.1

Number of PTFE chains

1

2

3

4

6

8

10

13

Total Pt surface coverage (%) (PTFE binder and carbon surface)

40.4

59.9

72.3

79.7

82.4

80.3

82.6

84.3

Pt surface coverage (%) (at carbon surface)

13.6

Pt surface coverage (%) at upper part of Pt surface

10.3

20.1

28.3

38.4

40.1

40.0

40.4

41.4

Pt surface coverage (%) at lower part of Pt surface

30.1

39.8

44.0

41.3

42.3

40.3

42.2

42.9

17



Fig. 7. Analyzing the contact surface involving the PTFE contents and the carbon surface. Carbon atoms in C surface contact with the PTFE binders are depicted as blue balls at PTFEbinder contents of: (a) 4.0 wt%, (b) 7.7 wt%, (c) 11.1 wt%, (d) 14.2 wt%, (e) 20.0, (f) 25.0 wt%, (g) 29.4 wt%, and (h) 35.1 wt%. (i) Carbon surface coverage with PTFE binder as a function of PTFE-binder content.

4. Conclusions Pt/C models with PTFE-binder contents in the 4.0–35.1 wt% range, and water, have been simulated using full atomistic MD simulations. As the water in the PTFE/solvent on the Pt/C surface evaporates, the PTFE binder disperses over the Pt particle and the carbon surface. Equilibrated models reveal that the PTFE binder at low content (4.0 wt%) forms a TPB with the Pt particle and the carbon surface. With increasing binder content, the PTFE increasingly covers the Pt particle and the carbon surface. Analyses of pair correlation functions reveals 18


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that the Pt-F(PTFE)-pair CN first increased from 0.43 to 1.23 as the PTFE-binder content increased from 4.0 to 35.1 wt%. However, the Pt-F(PTFE)-pair CN did not significantly increase at binder contents greater than 20 wt%. PTFE coverage over the Pt surface at the lower part of the Pt surface was 31.3% at a PTFE content of 4.0 wt%, which means that the PTFE binder is initially distributed mainly at the lower part of the Pt surface to form a TPB between the Pt particle and the carbon surface. Similar coverages of the upper part of the Pt surface were observed in the 20.0–35.1 wt% PTFE-binder-content range, which indicates that the Pt surface was almost completely covered by the binder at a PTFE content of 20.0 wt%. This study can be provided detailed information for the electrode design guidelines regarding to the PTFE-binder content for HT-PEMFC applications.

Acknowledgement This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Nos. NRF-2016M1A2A2937151 and NRF2015M1A2A2057129).

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References 1.

Stambouli, A. B., Fuel cells: The expectations for an environmental-friendly and

sustainable source of energy. Renewable and Sustainable Energy Reviews 2011, 15 (9), 4507-4520. 2.

Yang, C.; Costamagna, P.; Srinivasan, S.; Benziger, J.; Bocarsly, A. B., Approaches and

technical challenges to high temperature operation of proton exchange membrane fuel cells.

Journal of Power Sources 2001, 103 (1), 1-9. 3.

Li, H.; Tang, Y.; Wang, Z.; Shi, Z.; Wu, S.; Song, D.; Zhang, J.; Fatih, K.; Zhang, J.; Wang, H.;

Liu, Z.; Abouatallah, R.; Mazza, A., A review of water flooding issues in the proton exchange membrane fuel cell. Journal of Power Sources 2008, 178 (1), 103-117. 4.

Yan, W.-M.; Chen, F.; Wu, H.-Y.; Soong, C.-Y.; Chu, H.-S., Analysis of thermal and water

management with temperature-dependent diffusion effects in membrane of proton exchange membrane fuel cells. Journal of Power Sources 2004, 129 (2), 127-137. 5.

Adams, W. A.; Blair, J.; Bullock, K. R.; Gardner, C. L., Enhancement of the performance and

reliability of CO poisoned PEM fuel cells. Journal of Power Sources 2005, 145 (1), 55-61. 6.

Qi, Z.; He, C.; Kaufman, A., Effect of CO in the anode fuel on the performance of PEM fuel

cell cathode. Journal of Power Sources 2002, 111 (2), 239-247. 7.

Das, S. K.; Reis, A.; Berry, K. J., Experimental evaluation of CO poisoning on the

performance of a high temperature proton exchange membrane fuel cell. Journal of Power

Sources 2009, 193 (2), 691-698. 8.

Kwon, K.; Yoo, D. Y.; Park, J. O., Experimental factors that influence carbon monoxide

tolerance of high-temperature proton-exchange membrane fuel cells. Journal of Power Sources

2008, 185 (1), 202-206. 9.

Liu, Z.; Wainright, J. S.; Savinell, R. F., High-temperature polymer electrolytes for PEM fuel

cells: study of the oxygen reduction reaction (ORR) at a Pt–polymer electrolyte interface. Chemical

Engineering Science 2004, 59 (22), 4833-4838. 10.

Li, Q.; Jensen, J. O.; Savinell, R. F.; Bjerrum, N. J., High temperature proton exchange

membranes based on polybenzimidazoles for fuel cells. Progress in Polymer Science 2009, 34 (5), 449-477. 11.

Zeis, R., Materials and characterization techniques for high-temperature polymer

electrolyte membrane fuel cells. Beilstein Journal of Nanotechnology 2015, 6, 68-83. 12.

Liu, Z.; Wainright, J. S.; Litt, M. H.; Savinell, R. F., Study of the oxygen reduction reaction

(ORR) at Pt interfaced with phosphoric acid doped polybenzimidazole at elevated temperature and low relative humidity. Electrochimica Acta 2006, 51 (19), 3914-3923. 13.

Qingfeng, L.; Hjuler, H. A.; Bjerrum, N. J., Oxygen reduction on carbon supported platinum

catalysts in high temperature polymer electrolytes. Electrochimica Acta 2000, 45 (25), 4219-4226. 14.

Fishman, Z.; Hinebaugh, J.; Bazylak, A., Microscale Tomography Investigations of

Heterogeneous Porosity Distributions of PEMFC GDLs. Journal of the Electrochemical Society 2010,

157 (11), B1643-B1650. 20


Page 20 of 24

Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15.

Mack, F.; Klages, M.; Scholta, J.; Jörissen, L.; Morawietz, T.; Hiesgen, R.; Kramer, D.; Zeis, R.,

Morphology studies on high-temperature polymer electrolyte membrane fuel cell electrodes.

Journal of Power Sources 2014, 255, 431-438. 16.

Wannek, C.; Konradi, I.; Mergel, J.; Lehnert, W., Redistribution of phosphoric acid in

membrane electrode assemblies for high-temperature polymer electrolyte fuel cells. International

Journal of Hydrogen Energy 2009, 34 (23), 9479-9485. 17.

Wannek, C.; Lehnert, W.; Mergel, J., Membrane electrode assemblies for high-temperature

polymer electrolyte fuel cells based on poly(2,5-benzimidazole) membranes with phosphoric acid impregnation via the catalyst layers. Journal of Power Sources 2009, 192 (2), 258-266. 18.

Su, H.; Pasupathi, S.; Bladergroen, B.; Linkov, V.; Pollet, B. G., Optimization of gas diffusion

electrode for polybenzimidazole-based high temperature proton exchange membrane fuel cell: Evaluation of polymer binders in catalyst layer. International Journal of Hydrogen Energy 2013, 38 (26), 11370-11378. 19.

Lee, H.-J.; Kim, B. G.; Lee, D. H.; Park, S. J.; Kim, Y.; Lee, J. W.; Henkensmeier, D.; Nam, S. W.;

Kim, H.-J.; Kim, H.; Kim, J.-Y., Demonstration of a 20 W class high-temperature polymer electrolyte fuel cell stack with novel fabrication of a membrane electrode assembly. International Journal of

Hydrogen Energy 2011, 36 (9), 5521-5526. 20.

Jeong, G.; Kim, M.; Han, J.; Kim, H.-J.; Shul, Y.-G.; Cho, E., High-performance membrane-

electrode assembly with an optimal polytetrafluoroethylene content for high-temperature polymer electrolyte membrane fuel cells. Journal of Power Sources 2016, 323, 142-146. 21.

Matsumoto, K.; Fujigaya, T.; Sasaki, K.; Nakashima, N., Bottom-up design of carbon

nanotube-based electrocatalysts and their application in high temperature operating polymer electrolyte fuel cells. Journal of Materials Chemistry 2011, 21 (4), 1187-1190. 22.

Lobato, J.; Cañizares, P.; Rodrigo, M. A.; Linares, J. J.; Pinar, F. J., Study of the influence of

the amount of PBI–H3PO4 in the catalytic layer of a high temperature PEMFC. International


Seland, F.; Berning, T.; Børresen, B.; Tunold, R., Improving the performance of high-

temperature PEM fuel cells based on PBI electrolyte. Journal of Power Sources 2006, 160 (1), 2736. 24.

Ong, A.-L.; Jung, G.-B.; Wu, C.-C.; Yan, W.-M., Single-step fabrication of ABPBI-based GDE

and study of its MEA characteristics for high-temperature PEM fuel cells. International Journal of

Hydrogen Energy 2010, 35 (15), 7866-7873. 25.

Kim, H. J.; An, S. J.; Kim, J. Y.; Moon, J. K.; Cho, S. Y.; Eun, Y. C.; Yoon, H. K.; Park, Y.; Kweon,

H. J.; Shin, E. M., Polybenzimidazoles for high temperature fuel cell applications. Macromolecular

Rapid Communications 2004, 25 (15), 1410-1413. 26.

Zhai, Y.; Zhang, H.; Xing, D.; Shao, Z.-G., The stability of Pt/C catalyst in H3PO4/PBI PEMFC

during high temperature life test. Journal of Power Sources 2007, 164 (1), 126-133. 27.

Liu, G.; Zhang, H.; Hu, J.; Zhai, Y.; Xu, D.; Shao, Z.-g., Studies of performance degradation 21



Page 22 of 24

of a high temperature PEMFC based on H3PO4-doped PBI. Journal of Power Sources 2006, 162 (1), 547-552. 28.

Sun, H.; Yu, M.; Zhao, X.; Almheiri, S., Molecular simulation of mass transport in

phosphoric acid doped poly(2,5-benzimidazole) polymer electrolyte membranes. International


Spieser, S. A. H.; Leeflang, B. R.; Kroon-Batenburg, L. M. J.; Kroon, J., A Force Field for

Phosphoric Acid: Comparison of Simulated with Experimental Data in the Solid and Liquid State.

The Journal of Physical Chemistry A 2000, 104 (31), 7333-7338. 30.

Zhu, S.; Yan, L.; Zhang, D.; Feng, Q., Molecular dynamics simulation of microscopic

structure

and

hydrogen

bond

network

of

the

pristine

and

phosphoric

acid

doped

polybenzimidazole. Polymer 2011, 52 (3), 881-892. 31.

Yan, L.; Zhu, S.; Ji, X.; Lu, W., Proton Hopping in Phosphoric Acid Solvated Nafion

Membrane: A Molecular Simulation Study. The Journal of Physical Chemistry B 2007, 111 (23), 6357-6363. 32.

Shao, M.; Peles, A.; Shoemaker, K., Electrocatalysis on Platinum Nanoparticles: Particle Size

Effect on Oxygen Reduction Reaction Activity. Nano Letters 2011, 11 (9), 3714-3719. 33.

Brunjes, A. S.; Bogart, M. J. P., Vapor-Liquid Equilibria for Commercially Important Systems

of Organic Solvents: The Binary Systems Ethanol-n-Butanol, Acetone-Water and Isopropanol-Water.

Industrial & Engineering Chemistry 1943, 35 (2), 255-260. 34.

Mayo, S. L.; Olafson, B. D.; Goddard, W. A., DREIDING: a generic force field for molecular

simulations. The Journal of Physical Chemistry 1990, 94 (26), 8897-8909. 35.

Levitt, M.; Hirshberg, M.; Sharon, R.; Laidig, K. E.; Daggett, V., Calibration and Testing of a

Water Model for Simulation of the Molecular Dynamics of Proteins and Nucleic Acids in Solution.

The Journal of Physical Chemistry B 1997, 101 (25), 5051-5061. 36.

Lee, S. G.; Brunello, G. F.; Jang, S. S.; Bucknall, D. G., Molecular dynamics simulation study

of P (VP-co-HEMA) hydrogels: Effect of water content on equilibrium structures and mechanical properties. Biomaterials 2009, 30 (30), 6130-6141. 37.

Lee, S. G.; Choi, J. I.; Koh, W.; Jang, S. S.; Kim, J.; Kim, G., Effect of Temperature on Water

Molecules in a Model Epoxy Molding Compound: Molecular Dynamics Simulation Approach. Ieee

T Comp Pack Man 2011, 1 (10), 1533-1542. 38.

Lee, S. G.; Pascal, T. A.; Koh, W.; Brunello, G. F.; Goddard, W. A.; Jang, S. S., Deswelling

Mechanisms of Surface-Grafted Poly(NIPAAm) Brush: Molecular Dynamics Simulation Approach. J

Phys Chem C 2012, 116 (30), 15974-15985. 39.

Lee, S. G.; Koh, W.; Brunello, G. F.; Choi, J. I.; Bucknall, D. G.; Jang, S. S., Effect of

monomeric sequence on transport properties of D-glucose and ascorbic acid in poly(VP-co-HEMA) hydrogels with various water contents: molecular dynamics simulation approach. Theor Chem Acc

2012, 131 (4). 40.

Lee, J.; Song, J.; Lee, H.; Noh, H.; Kim, Y. J.; Kwon, S. H.; Lee, S. G.; Kim, H. T., A Nanophase22


Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Separated, Quasi-Solid-State Polymeric Single-Ion Conductor: Polysulfide Exclusion for LithiumSulfur Batteries. ACS Energy Lett 2017, 2 (5), 1232-1239. 41.

Cheng, C. H.; Malek, K.; Sui, P. C.; Djilali, N., Effect of Pt nano-particle size on the

microstructure of PEM fuel cell catalyst layers: Insights from molecular dynamics simulations.

Electrochimica Acta 2010, 55 (5), 1588-1597. 42.

Jang, S. S.; Molinero, V.; Çaǧın, T.; Goddard, W. A., Nanophase-Segregation and Transport

in Nafion 117 from Molecular Dynamics Simulations: Effect of Monomeric Sequence. The Journal

of Physical Chemistry B 2004, 108 (10), 3149-3157. 43.

Brunello, G.; Lee, S. G.; Jang, S. S.; Qi, Y., A molecular dynamics simulation study of

hydrated sulfonated poly(ether ether ketone) for application to polymer electrolyte membrane fuel cells: Effect of water content. J Renew Sustain Ener 2009, 1 (3). 44.

Brunello, G. F.; Mateker, W. R.; Lee, S. G.; Il Choi, J.; Jang, S. S., Effect of temperature on

structure and water transport of hydrated sulfonated poly(ether ether ketone): A molecular dynamics simulation approach. J Renew Sustain Ener 2011, 3 (4). 45.

Zhou, X. W.; Johnson, R. A.; Wadley, H. N. G., Misfit-energy-increasing dislocations in

vapor-deposited CoFe/NiFe multilayers. Physical Review B 2004, 69 (14). 46.

Brunello, G. F.; Lee, J. H.; Lee, S. G.; Choi, J. I.; Harvey, D.; Jang, S. S., Interactions of Pt

nanoparticles with molecular components in polymer electrolyte membrane fuel cells: multi-scale modeling approach. RSC Advances 2016, 6 (74), 69670-69676. 47.

Swope, W. C.; Andersen, H. C.; Berens, P. H.; Wilson, K. R., A computer simulation method

for the calculation of equilibrium constants for the formation of physical clusters of molecules: Application to small water clusters. The Journal of Chemical Physics 1982, 76 (1), 637-649. 48.

Mulliken, R. S., Electronic Population Analysis on LCAO–MO Molecular Wave Functions. I.

The Journal of Chemical Physics 1955, 23 (10), 1833-1840. 49.

Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple.

Physical Review Letters 1996, 77 (18), 3865-3868. 50.

Plimpton, S., Fast Parallel Algorithms for Short-Range Molecular-Dynamics. Journal of

Computational Physics 1995, 117 (1), 1-19. 51.

Plimpton, S.; Pollock, R.; Stevens, M. In Particle-Mesh Ewald and rRESPA for Parallel

Molecular Dynamics Simulations, The Eighth SIAM Conference on Parallel Processing for Scientific Computing, PPSC Minneapolis, MN, 1997; pp 1-13. 52.

Hockney, R. W.; Eastwood, J. W., Computer simulation using particles. CRC Press: Boca

Raton, FL, 1988. 53.

BIOVIA, D. S., Materials Studio, v8.0. San Diego, Dassault Systèmes, 2014.

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Effect of binder content on Pt/C catalyst coverage 134x114mm (150 x 150 DPI)


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Molecular Dynamics Simulation to Reveal Effects of Binder Content on

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