Tuning the Ionomer Distribution in the Fuel Cell Catalyst Layer with

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Energy, Environmental, and Catalysis Applications

Tuning the Ionomer Distribution in Fuel Cell Catalyst Layer with Scaling the Ionomer Aggregate Size in Dispersion Gisu Doo, Ji Hye Lee, Seongmin Yuk, Sungyu Choi, Dong-Hyun Lee, Dong Wook Lee, Hyun Gyu Kim, Sung Hyun Kwon, Seung Geol Lee, and Hee-Tak Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01751 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Materials & Interfaces

Tuning the Ionomer Distribution in Fuel Cell Catalyst Layer with Scaling the Ionomer Aggregate Size in Dispersion Gisu Doo,† Ji Hye Lee,§ Seongmin Yuk,† Sungyu Choi,† Dong-Hyun Lee,† Dong Wook Lee,† Hyun Gyu Kim,† Sung Hyun Kwon,§ Seung Geol Lee,*,§ and Hee-Tak Kim*,†,‡ †

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science

and Technology (KAIST), Daejeon 34141, Republic of Korea ‡

Advanced Battery Center, KAIST Institute for the NanoCentury, Korea Advanced Institute of

Science and Technology (KAIST), Daejeon 34141, Republic of Korea §

Department of Organic Material Science and Engineering, Pusan National University, Busan

46241, Republic of Korea *E-mails: (H.-T. Kim) [email protected]; (S.G. Lee) [email protected]

ACS Paragon Plus Environment

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ABSTRACT: With the demands for better performance of polymer electrolyte membrane fuel cells (PEMFCs), studies on controlling the distribution of ionomers have recently gained interest. Here, we present a tunable ionomer distribution in the catalyst layer (CL) with dipropylene glycol (DPG) and water mixtures as ionomer dispersion medium. Dynamic light scattering and molecular dynamics simulation demonstrate that, by increasing the DPG content in the dispersion, the size of the ionomer aggregates in the dispersion is exponentially reduced due to the higher affinity of DPG for Nafion ionomers. The ionomer distribution of the resulting CLs dictates the dimensional feature of the ionomer dispersion. Although the ionomer distribution becomes more uniform with increasing the DPG content, an optimal power performance is obtained at a DPG content of 50 wt% regardless of feed humidity due to balanced proton and mass transports. As a guide for tuning the ionomer distribution, we suggest that the ionomer aggregates in the dispersion with a size close to that of the Pt/C aggregates form a highly connected ionomer network and maintain a porosity in the catalyst/ionomer aggregate resulting in high power performance.

KEYWORDS: Ionomer distribution, catalyst layer, ionomer dispersion solvent, Nafion ionomer, polymer electrolyte membrane fuel cell


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1. INTRODUCTION Recently, the ionomer distribution in the catalyst layer (CL) of polymer electrolyte membrane fuel cells (PEMFCs) has attracted increasing attention due to its critical role in power performance and in the durability of the CL.1-3 Ionomers in the CL cover the Pt surfaces providing proton transport between the Pt surfaces and the membrane as well as the mechanical integrity of the CL. However, the ionomer layers on Pt particles impose additional oxygen transport resistance, which is more pronounced for low Pt-loaded CLs.4-5 In search of a balance between these two opposite effects, the ionomer content was conventionally taken as a parameter for the CL optimization.6-9 At low ionomer contents, both ionic contact with Pt particles and the connectivity of the ionomer phase are insufficient. An increase in the ionomer content can lead to a larger ionic contact area and higher connectivity; however, gas transport becomes more retarded due to the thick ionomer film. For those reasons, conventional CL design based on ionomer content optimization often faces limitations in fuel cell performance. In addition to the ionomer content, ionomer distribution is an important design parameter. In conventional CLs derived from commercial Nafion ionomer solutions, the ionomer coverage is rather inhomogeneous10-11 due to the aggregation of the Nafion ionomers in the dispersion solvents such as water and isopropyl alcohol. It leads to an insufficient Pt utilization at low ionomer contents and to a high mass transport resistance at high ionomer contents. In this regard, the distribution of the ionomer film over the catalyst particles should be tuned in a controlled manner to facilitate gas diffusion without losing its proton conductivity. There have been several attempts to control the ionomer distribution for high performance fuel cells. These can be divided into two major directions: material modification and ionomer dispersion solvent control. The material approach includes the modification of Pt and carbon


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surfaces and the introduction of additives. Park et al. recently reported that a more uniform ionomer distribution can be obtained by tuning the carbon surface structures.12 Yang et al. investigated the different interactions between ionomers and carbon in ink depending on the surface functional groups on the carbon by using SAXS and cryo-TEM analysis.13 Orfanidi et al. reported that a N group containing carbon surface can induce a uniform ionomer distribution decreasing mass transport resistances at a high current density.14 Yang et al. introduced a polybenzimidazole coated carbon fiber support to induce a more uniform ionomer distribution on the carbon fibers.15 Choo et al. suggested that the ionomer distribution can be modulated by expanding the ionomer thin film through PEG addition and the subsequent removal of PEG;16 the modulated ionomer distribution increased the electrochemical active area and proton transport property without loss in oxygen transport at a fixed ionomer content. The design of an ionomer dispersion solvent is based on the fact that the structure of the Nafion ionomer aggregates in the CL slurry is highly dependent on the interaction between the dispersion solvent and the ionomers, and it translates to the resulting ionomer distribution in the CL. Ngo et al. reported that for IPA/water mixture solvents, the solvent used for the CL preparation influences the ionomer distribution in the final electrode, which in turn affects the MEA performance.17 Recently, Kim et al. showed that the interface between the Nafion ionomers and the solvent molecules in the catalyst ink may be critical for both the performance and durability of PEMFCs due to the variation in the mechanical properties as a result of the different mobilities of the Nafion ionomers.18-20 Despite these efforts, tuning the ionomer distribution in a controllable and scalable manner still lacks a practical methodology and remains a challenge due to a limited understanding of the relationship between ionomer distribution and power performance.


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In this work, we present a new ionomer dispersion with a binary mixture of dipropylene glycol (DPG) and water for tuning the ionomer distribution in the CL and enhancing the power performance of the CL. DPG has a high solvating power for perfluorinated sulfonic acid (PFSA) ionomers and can form a nano-dispersion of PFSA ionomers, which is a superior ionomer solubility in comparison with propanol21, the conventionally used solvent in this technology sector. In order to vary the scale of the ionomer aggregate from nano to micron, DPG was chosen. In addition, due to its high viscosity, the viscosity of the CL slurry can be increased, which is greatly required for fabricating high quality, defect-free CLs with conventional wet coating processes such as slot die, knife coating, and comma bar coating. Water, which is one of the conventional dispersing solvents for PFSA-based CLs, was selected as a co-solvent to control the size of the PFSA aggregates in the dispersion. We will demonstrate in this work that the binary solvent provides a way to control the ionomer distribution and improves the power performance by tuning the ionomer distribution.


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2. EXPERIMENTAL SECTION Preparation of the Nafion dispersions. To prepare the Nafion ionomer dispersions with the desired solvents and concentrations, Nafion powder was prepared beforehand from a commercial Nafion dispersion (D521, ion exchange capacity (IEC) = 0.95–1.03 mequiv g–1, Du pont) with a spray dryer (Mini Spray Dryer B-290, Büchi). The Nafion powder obtained was dispersed in DPG (Sigma-Aldrich)/water mixtures with various DPG contents (0, 30, 50, 70, and 100 wt%) at 10 wt%. Nafion powder was readily dissolved in the mixed solvents by simple stirring except for in pure water. A milky-white colloidal dispersion was obtained for pure water. The dimension of the ionomer aggregates in each dispersion was determined by DLS (Zetasizer nano ZS90, Malvern Co.) measurement at 25 oC and at a scattering angle of 90 o. The ionomer dispersions were diluted to 1 wt% for the DLS measurement. Molecular dynamics simulations part. Classical molecular dynamics (MD) simulations using the MD code LAMMPS22 (Large-scale Atomic/Molecular Massively Parallel Simulator) were performed for the Nafion ionomers in solvents. MD simulations were performed with the canonical (NVT) ensemble for 20 ns at 1 atm and 298.15 K, and then, the systems were equilibrated with subsequent 10 ns isothermal-isobaric (NPT) MD simulations. Data were collected from the last 5 ns of the NPT simulation. The generic DREIDING force field23 was used for the description of the intra- and intermolecular interactions of organic molecules, which has successfully been tested in various organic systems.24-27 The F3C force field28 was used for water molecules. The total potential energy, ‫ܧ‬௧௢௧௔௟ , has the following form: ‫ܧ‬௧௢௧௔௟ = ‫ܧ‬௩ௗௐ + ‫ܧ‬ொ + ‫ܧ‬௕௢௡ௗ + ‫ܧ‬௔௡௚௟௘ + ‫ܧ‬௧௢௥௦௜௢௡ + ‫ܧ‬௜௡௩௘௥௦௜௢௡

(1)


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where ‫ܧ‬௩ௗௐ , ‫ܧ‬ொ , ‫ܧ‬௕௢௡ௗ , ‫ܧ‬௔௡௚௟௘ , ‫ܧ‬௧௢௥௦௜௢௡ , ‫ܧ‬௜௡௩௘௥௦௜௢௡ denote the energies contributed from the van der Waals, electrostatic, bond stretching, angle bending, torsion and inversion components, respectively. The velocity-Verlet algorithm29 was used to integrate the equations of motion with a time step of 1 fs. Individual atomic charges were assigned by quantum mechanical calculations using the Mulliken population analysis.30 The Particle-Particle-Mesh (PPPM) method31 was used to handle the electrostatic interactions. To investigate the effect of the DPG/water ratio for the morphology of the Nafion ionomers in the dispersions, we considered three different systems, in which a single Nafion ionomer chain (20 wt%) was dispersed in three solvent mediums for which the DPG contents were 0, 50, and 100 wt%. All simulations were studied using fully atomistic models of Nafion ionomers (IEC corresponds to D521 with a degree of polymerization of 10), DPG, water and hydronium ion shown in Figure S1. Each initial structure was generated using the Monte Carlo (MC) code to search for stable configurations of the system. The solvation energy, ‫ܧ‬௦௢௟௩௔௧௜௢௡ , of the system was defined as ‫ܧ‬௦௢௟௩௔௧௜௢௡ = ‫ܧ‬௦௬௦௧௘௠ − (‫ܧ‬ே௔௙௜௢௡ + ‫ܧ‬௦௢௟௩௘௡௧ )

(2)

where ‫ܧ‬௦௬௦௧௘௠ is the total energy of the system, ‫ܧ‬ே௔௙௜௢௡ is the energy of the Nafion ionomer, and ‫ܧ‬௦௢௟௩௘௡௧ is the energy of the solvent, respectively. Fabrication of the Membrane Electrode Assemblies (MEAs). All the CLs were prepared in the same manner as described below. Commercial carbon-supported Pt catalyst (TEC10F50E, TKK) and the Nafion ionomer dispersions with various DPG contents (0, 30, 50, 70, and 100 wt%) were mixed by ball milling to prepare the CLs with different slurry solvents. The Nafion


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content of the CL slurries was fixed at an ionomer to carbon ratio of 1.0. The homogeneously mixed slurries were coated onto a polyimide film with a doctor-knife and dried at 60 oC overnight. To fabricate the MEAs, cathode and anode CLs were laminated onto both sides of a membrane, NRE211 (25 µm, Dupont), at 20 atm and 130 oC. The CL prepared with the DPG 50 wt% dispersion solvent was used as the anode for all the MEAs to investigate the effect of the ionomer dispersion solvent on the performance of the cathode CL. The Pt loading for the anode and cathode CL was 0.15 ± 0.01 and 0.25 ± 0.01 mg cm-2, respectively. To determine the pore size distributions of CLs, N2 adsorption-desorption measurements were conducted at 77 K and the Barrett-Joyner-Halenda (BJH) method was applied for the calculations. The ionomer distribution and thickness of each CL were investigated with field emission scanning electron microscopy (FE-SEM, Hitachi, S-4700) at the Korea Basic Science Institute (Jeonju, South Korea). Electrochemical analysis. Single cells with an active area of 25 cm2 were assembled using the MEAs: a pair of gas diffusion layers (30-A3, JNTG) and a pair of three-layered silicon gaskets and a pair of flow plates with a single serpentine flow field. For the single cells, IV polarization curves were obtained in the galvanostatic mode at 65 oC at a stoichiometric ratio of H2/air=1.5/2.0 and at three relative humidity (RH) conditions (50, 80, 100%) without any backpressure with a fuel cell test station (Scitech Korea Inc.). Impedance analysis was conducted using an AC impedance analyzer (HCP-803, Biologic Science Instrument) with a voltage amplitude of 10 mV and a frequency range from 100 kHz to 100 mHz.


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3. RESULTS AND DISCUSSION Scaling the size of Nafion ionomer aggregate in dispersion. How to scale the size of the ionomer aggregates in the CL slurry is the first challenge of this work. The key idea of this work is that, by adjusting the relative ratio of DPG and water with different affinities for the Nafion, which is a typical PFSA ionomer, the size of the ionomer aggregates can be controlled. To assess the feasibility of controlling the size with the DPG/water binary solvent, the hydrodynamic diameters (DH) of the Nafion ionomer aggregates in diluted ionomer dispersions in the DPG/water mixtures were measured by the dynamic light scattering (DLS) technique. Figure 1 shows marked differences in the z-averaged DH among the dispersions with different DPG contents (0, 30, 50, 70, and 100 wt%); the DH decreased exponentially as the DPG content was increased. In pure DPG, the ionomers form a nano-scale aggregate with a DH of 17 nm. This value is quite close to the dimension of the Nafion ionomer aggregates in other good solvents, such as glycerol or N-methylpyrrolidone, from the previous studies18, 21, indicating that DPG can be regarded as a good solvent for Nafion ionomers. When the DPG content was reduced from 100 to 50 wt%, the size of the ionomer aggregates was increased to a submicron scale (115 nm) close to the size of the Pt/C catalyst aggregates (100-300 nm).32 In pure water (DPG content of 0 wt%), the size was on a micron scale (2 µm) with the formation of a large ionomer aggregate. Therefore, the DPG/water binary solvent system is quite an effective means to induce a large difference in the ionomer aggregate size in the dispersion. To get a better comprehension about the size dependency of the ionomer aggregate on the DPG/water ratio, a molecular dynamics simulation was conducted for a single Nafion chain dropped in three solvent mediums (DPG content: 0, 50, and 100 wt%) at a fixed solid content of 20 wt%. The equilibrated structures and solvation energies of the Nafion ionomers in the three


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different solvents are shown in Figure 2a-c. In pure water, the backbone (-CF2CF2-) of the Nafion ionomer had a tendency to aggregate because hydrophilic water functions as an aggregate reagent for the hydrophobic backbone of the Nafion ionomer. At the same time, the sulfonic acid groups (SO3-) of the Nafion ionomer tended to be extended due to their hydrophilic and ionic characteristics, which impart a high affinity for the water molecules. It was previously reported that water prefers to interact with the sulfonic acid groups so that the backbones of the Nafion ionomers form enormous aggregates in the colloidal dispersion state due to the predominant intermolecular interactions of the backbones.33 In contrast, in pure DPG, the backbone (-CF2CF2) of the Nafion ionomer appeared to extend out because of the favorable hydrophobic interactions with the DPG molecules. In the DPG/water mixture, the equilibrium structure of the Nafion ionomer exhibited an intermediate aggregation for the backbone parts and sulfonic acid groups. For a more quantitative analysis, we calculated the solvation energies (‫ܧ‬௦௢௟௩௔௧௜௢௡ ) of the simulated systems. The calculated solvation energies of the Nafion ionomer in all three solvents are shown in Figure 2d. A negative value for the solvation energy indicates a good dispersion of the Nafion ionomer. The solvation energy in pure DPG was -344.59 ± 18.06 kcal mol-1, which is the lowest value among the three solvents. This result implies that the Nafion ionomer was well dispersed in DPG. The solvation energy of the Nafion ionomer was increased with an increasing the ratio of water, which was in good agreement with the experimental DH results mentioned above. To observe the details of the Nafion microstructure in the DPG/water mixture, we analyzed the structure using the radial distribution function (RDF). The RDF indicates the probability of


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finding a particle species from a reference particle as a function of the distance averaged over the equilibrium trajectory as follows: ௡

ே

g ஺ି஻ (‫ = )ݎ‬ቀସగ௥ಳమ ௗ௥ቁൗቀ ௏ಳ ቁ

(3)

where ݊஻ is the number of B particles located at a distance r in a shell of thickness dr from the A particle; ܰ஻ is the number of B particles in the system, and V is the total volume of the system, respectively. To directly compare the intensities, the product of the pair correlation and the number density, ρg(r), is used instead of the g(r). The RDFs of the DPG/water mixture are shown in Figure 2e. Surprisingly, the equilibrium structure in the DPG/water mixture (Figure 2b) shows that the Nafion ionomer was mainly surrounded by the DPG molecules. Meanwhile, most of the water molecules were placed on the outside of the DPG molecules. It is interesting that the intensity of the first peak of the S(Nafion)-O(water) pairs was noticeably closer and stronger than that of the S(Nafion)-O(DPG) pairs. This result indicates that the water molecules were dominantly and locally located near the sulfonic acid groups of the Nafion ionomer. Moreover, it is clear from the RDFs that the probability of finding the O of the DPG molecules around the F of the Nafion backbone is higher than that of finding the O of the water molecules around the F of the Nafion. These results mean that the backbone of the Nafion ionomer was dominantly surrounded by the DPG molecules rather than by the water molecules because the backbone of the Nafion ionomer is hydrophobic and is likely to have more affinity with the hydrophobic DPG molecules than with the water molecules. Taken together, the MD simulation suggests that the higher affinity of the Nafion ionomer backbone for DPG is responsible for the resulting nanoscale ionomer dispersion and that the increase in the water content destabilizes the Nafion ionomer backbone resulting in the aggregation of the backbones.


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Catalyst layer characterization and fuel cell performance. The next challenge is to ensure whether the size difference of the ionomer aggregates in the ionomer dispersions translates into a difference in the ionomer distribution in the CLs. The effects of solvent on the CL structure and performance were demonstrated in several papers;17, 19 however, these works did not provide clear evidence for a change in the ionomer distribution. However, as S. Holdcroft emphasized in the perspective,2 catalyst particles and ionomers independently form heterogeneous structures (Pt/C aggregates and ionomer aggregates) in the dispersion, and the final morphology of the CL is dictated by the heterogeneous structures of the individual components. The Pt/C aggregates are known to have a size of 100-300 nm diameter and contain intrinsic micropores in the carbon support and micro/mesopores (< 20 nm) from the interstitial void of the Pt/C particles in the aggregates.22 In this regard, the spatial arrangement of the ionomer and catalyst aggregates would be dependent on the relative size of the two aggregates and the pore structure of the catalyst aggregates. Taking into consideration the structural features of the ionomer and catalyst aggregates, a model for the ionomer distribution in CLs is proposed in Figure 3. In the case of the nanometerscale ionomer aggregates such as that in pure DPG, the size of the ionomer aggregates (17 nm) is much smaller than that of the catalyst aggregates and is sufficiently small to fill up the mesopores (2~20 nm) in the catalyst aggregates (Figure 3a). Therefore, it is expected that the ionomer aggregates can uniformly coat the surfaces of the catalyst aggregates, which may facilitate proton conduction; however, it can clog or narrow the pores in the interior of the catalyst aggregates, which may inhibit mass transport in the internal Pt particles. For the micronsized ionomer aggregates like that in pure water (DPG 0%), the ionomer aggregates are too bulky to cover the external surface of the catalyst aggregates; therefore, it rather forms isolated


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ionomer phases in the CL (Figure 3c). In fact, the CL from the slurry with pure water was disintegrated with immersion into water shown in Figure S2c, indicating that the ionomer phases did not properly function as a binder for the catalyst aggregates. The isolated ionomer phases would have difficulty in forming a highly integrated ionomer network causing the problem of poor proton transport though the CL. When the size of ionomer aggregates is comparable to that of the catalyst aggregates, which is the case for the DPG content of 50%, the ionomer aggregates cover the external surface of the catalyst aggregates, not significantly clogging or narrowing the mesopores in the catalyst aggregates (Figure 3b). In such an ionomer distribution, the ionomer phases can properly function as a binder and provide facile proton transport with the CL, not hindering the mass transport in the catalyst aggregates. To demonstrate a tunable ionomer distribution with the DPG/water solvent system, the morphologies of the resulting CLs were observed with FE-SEM (Figure 4). For the pure DPG solvent, Pt nano-particles (white dots) were barely exposed to the surface due to a homogeneous ionomer coverage on the catalysts shown in Figure 4a. Because the size of the ionomer aggregates in pure DPG is comparable to that of the Pt/C primary particles (~20 nm), they can cover the surfaces of the catalyst aggregates resulting in a homogeneous ionomer distribution. For the CL from the DPG content of 70 wt%, the Pt nano-particles were observed for some portion of the CL surface, which corresponds to the uncovered catalyst surface (Figure 4b). With a decrease in the DPG content from 70 to 30 wt%, the amount of uncovered Pt particles appears to increase demonstrating a more inhomogeneous ionomer distribution with a decreasing DPG content (Figure 4b-d). For the pure water solvent, an isolated bulky ionomer phase on a micron scale was clearly seen in the SEM image (Figure 4e-f). The SEM results clearly show that the ionomer distribution in the CL can be tuned by adjusting the size of ionomer aggregates in the


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dispersion. Additionally, to our interest, the morphological features strongly support the model suggested based on the relative sizes of the ionomer and catalyst aggregates. The influence of ionomer distribution change in the CL structure was further supported with BET analysis and apparent porosity calculation. The pore size distributions determined by BET analysis (Figure S3) demonstrate that the mesopore volume was decreased with increasing the DPG content in the dispersion solvents. It indicates that, for the dispersion solvents with high DPG contents, ionomers readily occupy the mesopores inside or between Pt/C agglomerates. In addition, the porosity of CL derived from the different dispersion solvents was determined from the CL thickness and was described in detail in the supporting information.34 As shown in Figure S4, the apparent porosity values show a clear tendency of decreasing porosity with DPG content. Since the SEM images of the CLs (Figure S5) do not show any micron-sized macro-pores, the differences in ε is attributed to a decreased mesopore volume with DPG content. It supports that more uniform Nafion ionomer distribution leads to a more compact packing of the solid components in the CLs, resulting in a lower porosity. To understand how the ionomer distribution can influence cell performances, IV polarizations of four single cells, for which the cathode CLs were derived from dispersions with different DPG contents (30, 50, 70, and 100 wt%), were compared. These cells are denoted as DPG-30, -50, 70, and -100, depending on the DPG content in the slurry. For the slurry with a DPG content of 0%, the mechanical integrity of the CL was quite poor due to significant aggregation of the Nafion ionomers; thus, the resulting MEA was excluded from the comparison. Figure 5a-c show the IV polarization curves for the single cells at 65 oC and at three different relative humidities (RHs) of 50, 80, and 100%, respectively. The investigation of the RH dependency on the power performance was intended to investigate the proton transport and mass transport properties of


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these CLs. A power density of 0.4 V was selected as a measure of the power performance for an easier comparison and was compared in Figure 5d. As shown in Figure 5, the power performances of the CLs are highly dependent on the dispersion solvent, and the dependency of the dispersion solvent is different depending on the operating RH. It should be noted that the polarizations at low current densities (< 0.2 A cm-2) are indifferent among the cells with different DPG contents for all the RHs investigated. It indicates that the kinetic polarizations of the CLs are nearly identical, which corresponds to the small differences in the ECSA as in the Figure S7, and the observed power performances are mainly attributed to any differences in the proton and/or mass transport property of the CLs. Regarding the low differences in the ECSA among the dispersion solvent, it is notable that the dimension of the ionomer aggregate is not a sole factor determining ECSA; it can be also affected by the adsorbed water on Pt surface and electrical disconnection between the catalyst particles by ionomer. Furthermore, even when large ionomer aggregates are formed in the dispersion, some Nafion chains which are not involved in the large ionomer aggregates, can be adsorbed on Pt surface due to their strong adsorbing power.35 We expect these various factors contribute to the low sensitivity of ECSA on the dispersion solvent composition. At the low RH of 50%, the power performance of DPG-30 was notably lower than those of the other MEAs (Figure 5a and d). Because proton transport through the CL considerably influences cell performance at such a low RH condition, the much lower power performance of DPG-30 can be understood as a slow proton conduction through its poorly connected ionomer phases based on the ionomer distribution results. In contrast, at the high RH of 80 and 100%, the cells with the higher DPG contents (DPG-70 and -100), which have a more uniform ionomer distribution, showed relatively poor power performances compared to the cells with the lowest DPG content


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(DPG-30). At such high RH conditions, power performances are more influenced by mass transport than by proton transport. Due to the porous CL structures as a result of the inhomogeneous ionomer distribution, the mass transport properties of the cells with lower DPG contents would be superior to those of the cells with higher DPG contents. Interestingly, regardless of the RH conditions, the DPG-50 cell, which corresponds to the case of comparable sizes of ionomer and catalyst aggregates, had the best power performances. It indicates that the ionomer distribution from the DPG content 50 wt% can be considered as an optimum structure, which provides a balance between proton and mass transport in a wide range of RH conditions. The RH dependency on power performance more clearly demonstrates how the ionomer distribution affects the power performance. As shown in Figure 5d, the power performance enhancement with the increase in RH from 50 to 100% was largest for the DPG-30 cell. According to the suggested model, it can be explained as follows: the disconnected and isolated ionomer phases are connected by the water adsorbed on the catalysts with increasing RH; whereas, mass transport is not significantly limited with the increase in RH due to its highly porous CL structure. On the other hand, the DPG-100 showed the smallest change in power performance with the increase in RH. Because the pores of the catalyst aggregates are clogged or narrowed by the ionomers, the mass transport is sensitively reduced with the reduction in oxygen concentration and an increase in water flooding. As a result of the compensated gain from the proton and mass transport, a weak dependency on RH can be found in the DPG-100 cell. To further elucidate the variation of the mass transport properties driven by the ionomer distribution, the impedances of the cells measured at 1.5 A cm-2 and RH 80% were compared in Figure S6. The high frequency and low frequency semi-circles originate from charge transfer and mass transfer process, respectively. The low frequency semi-circle became larger with the


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increase of the DPG content, indicating that the mass transport was more inhibited for the higher DPG content. It again demonstrates that homogeneous ionomer distribution in nanoscale can lead to a pore clogging and a consequent inhibition of mass transport.


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4. CONCLUSION In summary, we successfully demonstrated that the ionomer distribution in the CL can be tuned by controlling the size of the ionomer aggregates in the dispersion with a model system of DPG/water solvents. Due to the high affinity of DPG for the perfluorinated backbone of the Nafion ionomer, the hydrodynamic diameter of the ionomer aggregates was reduced from a micron to a nano size with the increase in the DPG content from 0 to 100 wt%. As the size of the ionomer aggregates was reduced, a more homogeneous ionomer distribution was observed, indicating that the structural features of the ionomer aggregates translate into the ionomer distribution of the corresponding CL. The increased homogeneity in the ionomer distribution is beneficial in enhancing proton transport in the CL at a low RH condition; however, it leads to a performance loss at high RH conditions in terms of the mass transport property, which infers that a certain degree of inhomogeneity in the ionomer distribution is needed to prevent pore clogging or narrowing. The morphological and electrochemical characterizations indicate that there is an optimum ionomer distribution for balanced proton and mass transports, and it can be achieved by tuning the property of the dispersion solvent. Therefore, the DPG/water solvent system presented in this work can be an effective platform to tune the ionomer distribution in CLs and motivate the development of advanced ionomer dispersions for high performance PEMFCs.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Chemical structures of molecules for simulations; Optical images of catalyst layers; Nyquist plots for impedance analysis (PDF).

AUTHOR INFORMATION Corresponding Author *E-mails : [email protected] (H.-T.K.) and [email protected] (S.G.L) Author Contributions G. Doo, S. Yuk, S. Choi, D.-H. Lee, D.W. Lee, and H.G. Kim conducted the experiments; J.H. Lee and S.H. Kwon performed the simulation part; H.-T. Kim and S. G. Lee supervised the research. ACKNOWLEDGMENT This work was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015M1A2A2057127 and NRF-2015M1A2A2057129).

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Figures

Figure 1. The z-averaged hydrodynamic diameter of the Nafion ionomer aggregates in the water/DPG mixtures with various DPG contents (0, 30, 50, 70, and 100 wt%).


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Figure 2. Equilibrium structures with a 2 x 2 x 2 supercell of Nafion ionomer in (a) water, (b) water/DPG (50/50) mixture and (c) DPG solvent. The red, blue and cyan colors denote the Nafion ionomer, DPG and water molecules, respectively. Sulfur atoms of the Nafion ionomer are represented by yellow colors for clarity. (d) The solvation energies of the Nafion ionomers in each solvent system and (e) radial distribution functions of the Nafion ionomer in water/DPG (50/50) mixture solvent.


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Figure 3. Suggested models in cross sectional view to account for the ionomer distribution in the catalyst layer depending on the dimension of the ionomer aggregates in the dispersion. (a) represents the case that the ionomer aggregate size (Dion) is small compare to the catalyst particle aggregate size (Dcat), (b) is for the similar dimension of ionomer and catalyst particle aggregate, and (c) describes the case when the ionomer aggregate is larger than the catalyst particle aggregates.


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Figure 4. SEM images of the catalyst layer fabricated with (a) DPG 100 wt% (b) DPG 70 wt% (c) DPG 50 wt% (d) DPG 30 wt%, and (e-f) DPG 0wt% aqueous dispersions.


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Figure 5. IV polarization curves of cells with various DPG ratios at 65 oC with a gas flow of (a) RH 50%, (b) RH 80%, and (c) RH 100% and (d) the power density comparison at 0.4 V.


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Table of Contents Graphic


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Tuning the Ionomer Distribution in the Fuel Cell Catalyst Layer with

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