Ionomer Thickness Research Articles

Mesoscopic process of multiphase reactive flow and heat transfer during direct cold start process in Pt/C particle coated in ionomer of PEMFC

To explore multiphase reactive flow and heat transfer process during direct cold start in catalyst layer of proton exchange membrane fuel cell, a pore scale model was established. Effects of released gaseous water particle number, platinum volume fraction, ionomer thickness, and carbon support diameter were studied. The results reveal that due to obstruction of ice at bottom, oxygen cannot fully enter the bottom. After motion of solid ice is completely melted, the obstruction of the bottom on oxygen transport weakens, and oxygen gradually enters the bottom of the catalyst layer. As the number of gaseous water particles increases, the frost layer becomes thicker, and the resistance of the frost layer to oxygen diffusion becomes more significant. The increase in platinum particles elevates temperature within the catalyst layer, accelerating melting of solid ice within the catalyst layer. The carbon support diameter only has an effect on the intermediate concentration region. When the carbon support diameter is 26 nm, more oxygen enters the ionomer due to the weakened obstruction of the frost layer. Increasing the carbon support diameter lowers the temperature within the catalyst layer.

Multiscale study of reactive transport and multiphase heat transfer processes in catalyst layers of proton exchange membrane fuel cells

Improving the performance of proton exchange membrane fuel cells (PEMFCs) requires deep understanding of the reactive transport processes inside the catalyst layers (CLs). In this study, a particle-overlapping model is developed for accurately describing the hierarchical structures and oxygen reactive transport processes in CLs. The analytical solutions derived from this model indicate that carbon particle overlap increases ionomer thickness, reduces specific surface areas of ionomer and carbon, and further intensifies the local oxygen transport resistance (Rother). The relationship between Rother and roughness factor predicted by the model in the range of 800-1600 s m-1 agrees well with the experiments. Then, a multiscale model is developed by coupling the particle-overlapping model with cell-scale models, which is validated by comparing with the polarization curves and local current density distribution obtained in experiments. The relative error of local current density distribution is below 15% in the ohmic polarization region. Finally, the multiscale model is employed to explore effects of CL structural parameters including Pt loading, I/C, ionomer coverage and carbon particle radius on the cell performance as well as the phase-change-induced (PCI) flow and capillary-driven (CD) flow in CL. The result demonstrates that the CL structural parameters have significant effects on the cell performance as well as the PCI and CD flows. Optimizing the CL structure can increase the current density and further enhance the heat-pipe effect within the CL, leading to overall higher PCI and CD rates. The maximum increase of PCI and CD rates can exceed 145%. Besides, the enhanced heat-pipe effect causes the reverse flow regions of PCI and CD near the CL/PEM interface, which can occupy about 30% of the CL. The multiscale model significantly contributes to a deep understanding of reactive transport and multiphase heat transfer processes inside PEMFCs.

Lattice Boltzmann study of the effect of catalyst layer structure on oxygen reduction reaction within a PEMFC

The catalyst layer (CL) structure has a great impact on proton exchange membrane fuel cell (PEMFC) performance. In this study, the microporous structure of a cathode CL is reconstructed via a stochastic algorithm. Subsequently, the lattice Boltzmann method (LBM) is used to investigate the influence of the CL structure regarding carbon carrier and platinum (Pt) particle sizes and ionomer thickness on oxygen reduction reaction (ORR) processes within the cathode CL. The LBM simulation results indicate that an increased carbon carrier size promotes oxygen transport through the CL pores owing to the decreased tortuosity of the porous CL but reduces the overall ORR rate. A larger Pt particle size leads to a reduction in the number of reaction sites but shortens the ionomer thickness for oxygen diffusion to the reaction sites. The oxygen transport resistance through the ionomer increases with increasing ionomer thickness and thus degrades the overall ORR rate. However, an excessively small ionomer thickness leads to the exposure and shedding of Pt, thereby reducing the number of reaction sites. The findings here demonstrate that proper design of the carbon carrier and Pt particle sizes and ionomer thickness are essential for achieving a CL with high oxygen transport performance and overall ORR rate.

Effect of cathode catalyst layer on proton exchange membrane fuel cell performance: Considering the spatially variable distribution

The effect of the ionomer spatial distribution in the cathode catalyst layer (CCL) of the proton exchange membrane fuel cell (PEMFC) on oxygen and proton transport has been extensively studied. However, models generally assume a mean spatial distribution (MSD) of ionomer, and consideration of the actual spatial distribution (ASD) is indeed insufficient. In this study, a multi-dimensional PEMFC model is developed to investigate the effect of ionomer ASD versus MSD on cell performance. In addition, we corrected the Damkohler number (Da) by taking into account the local oxygen transport interfacial resistance and catalyst parameters into δ (an effective ionomer thickness), which allows Da to more accurately reflect the oxygen supply. The results show that the MSD assumption for ionomer tends to underestimate cell performance at the same current density. The proton transport resistance of ASD is 3.17% higher than that of MSD, but the oxygen concentration is 42.8% larger. This is mainly attributed to the larger porosity and lower saturation near the CCL/CMPL interface in the ASD model, which facilitates oxygen transport. The Da sufficiently supports the results, and the Da at the CCL/MPL interface of the ASD model (0.34) is smaller than in the MSD model (0.82), indicating relative oxygen enrichment. This study provides guidance for future PEMFC models.

Molecular Dynamics Study of Adsorption Phenomenon of Aromatic Hydrocarbon Ionomer in Catalytic Layer of Polymer Electrolyte Fuel Cell

In response to the current global energy crisis and environmental problems, fuel cells are an optional solution. Among them, polymer electrolyte fuel cell (PEFC) has high expectations because of its low operating temperature and suitability for applications in households and mobility devices. Since PEFC requires the use of proton exchange membranes, the transmission efficiency and durability of the exchange membrane is a major concern. To improve the proton transport efficiency, which is generally required, a catalytic layer is integrated into the PEFC. The catalyst layer of PEFC generally contains platinum catalyst covered with the ionomer membrane. Proton conductivity increases with ionomer thickness, while oxygen permeability decreases with ionomer thickness. Therefore, in order to improve the power generation efficiency of PEFC, it is important to design the catalyst layer. In addition, the fluorinated ionomer membrane material (such as Nafion) used for PEFC has good proton conductivity and chemical stability, but the synthesis cost is relatively high. As a consequence, new materials such as composite materials and non-fluorine-based materials have received a lot of attention in recent years. In this study, we focus on a new ionomer material of aromatic hydrocarbon (named SPP-BP) and elucidate the adsorption state of the ionomer on the platinum surface on a nanoscale using molecular dynamics (MD) simulations. Then, the structural properties and water diffusion characteristics of the ionomers of new material (SPP-BP) and conventional Nafion were analyzed based on the MD simulation results.The degree of polymerization (m, p, s) of the monomers were given as 20, 5, and 23 (molecular weight 7.38 kDa), respectively, based on the reported experimental values of SPP-BP. For the polymer composition, molecular models of a random type (r-type), which shows a randomness of monomers, were used. A molecular model of Nafion was also created as the conventional fluorinated membrane materials for comparison with the SPP-BP. The platinum surface was built with a three layers of platinum atoms in a face-centered cubic lattice structure. Regarding the calculation potentials, the DREIDING forcefield was used for the interactions with the molecules. While for the intermolecular potential, the Spohr potential was used for Pt-H2O and Pt-H3O+ interactions, and the others were applied with the Lennard Jones (LJ). The parameters of those potentials were based on the previous studies.In the ionomer membrane, water content is an important indicator that affects the transfer of protons and substances. Based on the water absorption results obtained in the experiment, the water content of SPP-BP was calculated to be between 1.8 and 7.2. Therefore, the water content during the MD simulation was used 3, 5, and 7. The computational size was 55.5 Å × 52.9 Å in the transverse direction of the ionomer membrane (X and Y directions), and the thickness of the membrane adsorbed on the platinum surface was about 50-60 Å. A vacuum region of about 40 Å was set at the top of the system. To calculate the diffusion coefficient and the cluster character of water, multiple simulations with different initial structure were considered. Furthermore, in order to compare to the Nafion, the number of polymers and the computational size was adjusted for containing the same water content.Based on the simulation results, the density distribution of water, hydrophilic sulfo groups, and main body of polymer was analyzed. By the analysis of density distribution, the structure of the ionomer at the interface-vacuum, near the Pt surface and inside was studied. In addition, the orientation of the sulfo group of SPP-BP and Nafion near the Pt surface was analyzed. Besides, the water cluster characteristics and water diffusion properties of the ionomers of SPP-BP and Nafion were also investigated.Acknowledgements: This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan through the fund for ECCEED'30 Projects (No. JPNP20003). Numerical simulations were performed on the Supercomputer system "AFI-NITY" at the Advanced Fluid Information Research Center, Institute of Fluid Science, Tohoku University.

Analysis Method of Oxygen Permeation Resistance of Ionomer in Cathode Catalyst Layer in PEFC

Analysis Method of Oxygen Permeation Resistance of Ionomer in Cathode Catalyst Layer in PEFC

Predicted Impacts of Graded Catalyst Layer Ionomer and Pt Distributions on PEMFC Performance

As the world moves away from a dependence on fossil fuels, it is crucial to develop cleaner replacement technologies for transportation, electricity production, and more. In regards to the transportation sector, fuel cell electric vehicles (FCEVs) made with proton exchange membrane fuel cells (PEMFCs) provide much promise. These FCEVs offer high energy densities, long-distance ranges, and quick refueling times. Although companies like Toyota, Honda, and Hyundai already have some FCEVs in production, maintaining high levels of performance and durability in low-cost PEMFCs continue to limit the technology. Addressing this challenge has proven difficult due to complex coupled physiochemical resistances in the heterogeneous catalyst layer (CL).One method to further our understanding of PEMFC performance limitations is to model the device. These models should incorporate known physical processes and structures that occur in the CL, such as ionic and molecular transport, and electrochemical reactions. One complication of such a model is providing accurate parameters for species transport through nano-thin films of Nafion ionomer. Due to confinement, these thin-film polymers have been shown to have unique interfacial structuring and reduced water uptake with decreasing thickness [1,2]. Furthermore, the conductivity of these films has been shown to vary with local water content, temperature, and ionomer thickness – as the water absorption and composition change [2,3]. To capture these structure-property relationships, we developed numerical approximations for ionic conductivity and oxygen diffusion in our previous work [4]. The relationships are derived from quantitative measured structures from neutron reflectometry experiments along with separate conductivity measurements with thin-film Nafion.In this presentation, we extend our previous modeling efforts to incorporate two-phase transport. This allows for an analysis on the impacts of CL flooding. Additionally, the model has been updated to allow for novel CL microstructures – e.g. graded ionomer and Pt loadings. The graded designs concentrate higher loadings near the membrane and lower loadings near the gas diffusion layer in an attempt to lower molecular and ionic transport resistances. After validating the model against PEMFC data with low Pt loadings [5], physical processes are investigated to identify limiting phenomena. A parametric study on the performance of the graded microstructures is also completed. Preliminary results suggest that individually these graded CLs lead to reduced performance caused by higher levels of localized flooding or reduced ionomer conductivities. A microstructure with combined graded ionomer and graded Pt distributions however leads to improved performance over a uniformly loaded CL by simultaneously lowering ohmic overpotentials and reducing flooding. These predictions suggest modifications for future PEMFC designs that could accelerate the development and adoption of high-performance and low-cost FCEVs.[1] J.A. Dura, V.S. Murthi, M. Hartman, S.K. Satija, and C.F. Majkrzak, “Multilamellar Interface Structures in Nafion,” Macromolecules, vol. 42, no. 13, pp. 4769–4774, 2009.[2] S.C. DeCaluwe, A.M. Baker, P. Bhargava, J.E. Fischer, and J.A. Dura, “Structure-property Relationships at Nafion Thin-film Interfaces: Thickness Effects on Hydration and Anisotropic Ion Transport,” Nano Energy, vol. 46, pp. 91–100, 2018.[3] D.K. Paul, R. McCreery, and K. Karan, “Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy,” Journal of The Electrochemical Society, vol. 161, no. 14, 2014.[4] C.R. Randall and S.C. DeCaluwe, “Physically Based Modeling of PEMFC Cathode Catalyst Layers: Effective Microstructure and Ionomer Structure-Property Relationship Impacts,” Journal of Electrochemical Energy Conversion and Storage, vol. 17, no. 4, 2020.[5] J.P. Owejan, J.E. Owegan, and W. Gu, “Impact of Platinum Loading and Catalyst Layer Structure on PEMFC Performance,” Journal of The Electrochemical Society, vol. 160, no. 8, 2013.

Evaluation of ionomer distribution on porous carbon aggregates in catalyst layers of polymer electrolyte fuel cells

Understanding ionomer distribution properties that facilitate proton conduction and oxygen transfer to Pt particles in the cathode catalyst layer (CCL) of the polymer electrolyte fuel cell (PEFC) is essential for optimized design of CCL with high cell performance. In this study, the model structure of Ketjen black (KB) as porous carbon was numerically simulated. After validating the model, the relationship between the weight ratio of ionomer/carbon (I/C) and ionomer coverage was investigated. Moreover, relative proton conductivity of simulated KB was compared with the reference data of Vulcan XC-72 (VB) as non-porous carbon. Under the same I/C ratio conditions, ionomer coverage significantly differed depending on the carbon support. Moreover, under the same carbon volume ratio conditions, simulated KB exhibited lower relative proton conductivity than VB because simulated KB had the lower ionomer volume ratio than that of simulated VB. The relationship between ionomer content and ionomer properties differ depending on the carbon support. The results of our study can contribute to designing an optimal catalyst layer. • Actual ketjen black structure is numerically simulated and validated. • Ionomer thickness, coverage, conductivity on ketjen black are estimated. • At the same ionomer content, ionomer coverage differs depending on carbon support.

Evaluating the effect of ionomer chemical composition in silver-ionomer catalyst inks toward the oxygen evolution reaction by half-cell measurements and water electrolysis

The effect of anion exchange ionomer (AEI) chemistry on the kinetics of the oxygen evolution reaction (OER) was systematically studied in both half-cell and single-cell electrolysis experiments. OER was studied in 1 M K2CO3 at 50 °C using an array of ionomer-silver catalyst inks deposited on Ni foam. Different ionomer modifications were investigated to optimize the OER performance. The AEI used in this work features a block copolymer backbone of polychloromethylstyrene-b-polycyclooctene-b-polychloromethylstyrene which was functionalized with either benzyltrimethylammonium (TMA) or benzylmethylpiperidinium (MPRD) quaternary ammonium cations. Using an MPRD quaternary ammonium cation shifted the rate determining step at high overpotentials, ultimately providing enhanced performance. Increasing the catalyst ink dilution was also found to substantially improve mass activity and catalyst utilization (increase from 35 to 88 A g−1 Ag), likely by reducing the ionomer thickness and decreasing transport resistances. In the final ionomer modification, the Ni foam substrate was leveraged to partially hydrogenate the polycyclooctene midblock to be polyethylene-like at the Ni interface. A significant increase in the electrochemical surface area (Cdl increased from 8.8 to 20.8 mF cm−2) and performance (by 47 mA cm−2 or 29 A g−1 Ag) was observed with the incorporation of polyethylene. Kinetic results from half-cell experiments were validated via single-cell anion exchange membrane electrolysis experiments, where the optimized electrode displayed enhanced performance in both 1 M K2CO3 and DI H2O.

Multiscale Simulation of Proton Transport in the Catalyst Layer with Consideration of Ionomer Thickness Distribution

To spread polymer electrolyte fuel cells (PEFCs) widely, which have many advantages, such as low environment load and high energy conversion efficiency, design and optimization of cathode catalyst layers (CLs) are necessary. CLs have inhomogeneous microstructures where Pt catalysts covered with ionomer thin films are supported by carbon particles. Electrons and protons transport to Pt surface through carbon particles and ionomer thin films while oxygen molecules diffuse in void and permeate through ionomer thin film to reach the Pt surface. Therefore, analyzing and understanding of mass transport of proton and oxygen in CLs are important for improving fuel cell performance. However, since an ionomer thin film on the Pt surface has a thickness of about 4 to 10 nm,1 detail understanding of the relationship between the transport mechanism within the nanostructures of ionomers and the fuel cell performance remains an issue. Therefore, we have focused on the analysis of transport phenomena using mesoscale numerical simulations by considering the effects of nanostructures on mass transport obtained from molecular dynamics (MD) simulations. Kobayashi et al.2 have studied the thickness dependence of ionomer thin films on proton transport using MD simulations and introduced these effects into the mesoscale simulations. The authors found that the self-diffusion coefficients of proton (D H +) show a peak at the ionomer thickness of 7 nm, in which the value of D H + is almost two times larger than that in the bulk membrane, leading to the high proton conductivity that is 1.6 times larger than the bulk membrane. These results suggest that the influence of ionomer thickness at the nanometer scale on the performance of PEFCs is significant. However, in the mesoscale simulations, the resulting cell performance (i.e., I-V curve) show similar results between the systems with and without consideration of the thickness effects because the thickness distributions obtained from experiments was not sufficiently reproduced using the hydrophilic adhesion model for ionomer distribution in the mesoscale simulation. In this study, we develop a new method to improve the non-uniform ionomer distribution that reproduces the experimental thickness distribution. Furthermore, we take into account the oxygen transport results obtained from MD simulation in the mesoscale simulations to understand the effects of ionomer nanostructures on the oxygen transport at larger scale in CLs and the fuel cell performance.Reference Lopez-Haro, M. et al. Three-dimensional analysis of Nafion layers in fuel cell electrodes. Nat. Commun. 5, 1–6 (2014).Kobayashi, K., Mabuchi, T., Inoue, G. & Tokumasu, T. Nano/Microscale Simulation of Proton Transport in Catalyst Layer. ECS Trans. 92, 515–522 (2019).

Multiscale Simulation of Proton Transport in the Catalyst Layer with Consideration of Ionomer Thickness Distribution

To spread polymer electrolyte fuel cells, improving the cell performance is required. The cell performance depends on various factors. One of the factors that lowers cell performance is proton transport resistance in catalyst layers. Catalyst layers have multiscale structures which influence on proton transport resistance. In this study, to investigate the relationship between structures of catalyst layers and cell performances, mass transport and chemical reactions are calculated in the 3D catalyst layer models. The information about the ionomer thickness dependence on the diffusion coefficient of protons based on the molecular dynamics simulation is introduced to transport calculation in order to analyze nanoscale structure influence on the cell performance. As a result, we have found that the output voltage increases over the whole range of current density with increasing ionomer/carbon ratio considering ionomer thickness dependence. This result suggests that the nanoscale structure in catalyst layer has a large influence on the cell performance. Furthermore, cell performance analysis with ionomer thickness distribution based on experimental values is conduced in this study. The result suggests that contribution of nanoscale proton transport characteristic increases by improving the ionomer distribution model.

Physically Based Modeling of PEMFC Cathode Catalyst Layers: Effective Microstructure and Ionomer Structure–Property Relationship Impacts

AbstractThis work presents a pseudo-two-dimensional proton exchange membrane fuel cell (PEMFC) model incorporating Nafion ionomer structure–property relationships in the cathode catalyst layer (CL) to capture and explain losses at low Pt loading. Structural data from neutron reflectometry and thin film Nafion conductivity measurements predict variations in the oxygen diffusion coefficient and ionic conductivity with changing CL ionomer thickness and Pt loading. By including these structure–property relationships, predicted polarization curves agree closely with previously published experimental data from cells with Pt loadings between 0.025 and 0.2 mg/cm2. Results demonstrate that structure–property relationships based on physically measurable ionomer and CL properties provide a feasible interpretation of PEMFC CL phenomena for a range of Pt loadings and help explain previously unaccounted-for losses at low Pt. Results also show that simulations must account for surface species coverage variations in order to properly capture the kinetic losses. Finally, results suggest that an increase in ionomer thickness surrounding the C/Pt surfaces may lead to improved cell performance due to improved ionic conductivity.

Analysis of Ionomer Distribution and Pt/C Agglomerate Size in Catalyst Layers by Two-Stage Ion-Beam Processing

Ionomer distribution in catalyst layers (CLs) of polymer electrolyte fuel cells has garnered much attention because it affects proton and gas transfer. In this study, a novel visualization method of the overall through-plane ionomer and platinum-supported carbon (Pt/C) distributions in the CLs using two-stage ion-beam processing is proposed. The first stage is the formation of a flat and smooth cross-section using a broad ion beam. The second stage is the selective removal of the materials in the CL by a focused-ion beam. Scanning ion microscopic images were obtained after the first and second stages. The ionomer and Pt/C distributions were then obtained by image processing. CLs were prepared with the ionomer-to-carbon (I/C) ratio varied from 0.5 to 3.0. The effect of the dispersion process on the structure of the CL was also studied. With increasing I/C ratio, a thin ionomer layer was formed at the interface with the polymer electrolyte membrane. This behavior is attributed to deposition of ionomer during solvent evaporation. Ionomer thickness, agglomerate size of Pt/C, and pore size were evaluated. The agglomerate size of Pt/C was found to be affected by both I/C ratio and the dispersion process.

Nano/Microscale Simulation of Proton Transport in Catalyst Layer

Polymer Electrolyte Fuel Cells (PEFCs) are expected to be used for automobiles and various applications because of their high power density and low environmental load. To spread PEFCs, improving cell performance is required. One of the factors that lowers cell performance is proton transport resistance in catalyst layers (CLs) which depends on CLs structure. Typical CLs are constructed by carbon supported Pt nanoparticles covered with ionomer thin films. Ionomer thin films have two important roles. First, ionomer thin films work as a binder between carbon black particles. Second, ionomer thin films provide path way for protons, so that proton transport resistance decreases with increasing ionomer carbon ratio (I/C). Oxygen transport resistance is another reason to lower cell performance. In CLs, oxygen transport involves the transport in micro porous media and the permeation through ionomer thin film, thus oxygen transport resistance increases with increasing I/C. To improve cell performance, both protons and oxygen transport resistance need to be low by the optimization of CLs structure. In this study, to analyze the relationship between cathode CLs structure and proton transport property, three-dimensional catalyst layer model is constructed considering carbon black aggregate and ionomer thickness distribution. Mass transport is calculated based on multi-block model considering proton diffusion, oxygen diffusion, water diffusion, and electrical conduction (1). And cathode reaction is calculated by Butler-Volmer equation. Inoue et al. analyzed the relationship between cell performance and CLs structure using the same model. They reported that oxygen transport resistance tends to be dominant for cell performance. However, they didn’t consider ionomer thickness dependence on proton transport property. In our previous study, we analyzed thickness dependence on proton transport properties of ionomer thin films using molecular dynamics (MD) simulations (2). Ionomer thin film was modeled as Nafion and water molecules adsorbed on flat carbon surface. The thickness of films was changed systematically from four nm to ten nm by changing the number of Nafion chains. Diffusion coefficient of protons was calculated with different ionomer thicknesses. It was found that at low water content, proton diffusion coefficients show a peak at the ionomer thickness of around seven nm. In this study, to analyze proton transport phenomena in detail, information about the relationship between ionomer thickness and diffusion coefficient of protons obtained from MD simulations has been introduced to the multi-block model as a correction term. It was found that gradient of I-V curve decreases with increasing I/C considering the correction term. This results suggest that the contribution of proton transport resistance increases considering nanoscale phenomena. In the future work, we will analyze mass transport properties in CLs such as the details of the overvoltage and current density distribution to investigate the relationship between CLs structure and proton transport properties. Acknowledgment This research was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan and the Promotion of Science (JSPS) KAKENHI Grant no. 18H01364. We used the integrated supercomputation system at the Institute of Fluid Science.

(Invited) Design of Catalyst Layer Structure for High Fuel Cell Performance

The low Pt electrode is an important research topic for the commercialization of polymer electrolyte fuel cell (PEFC). Platinum nanoparticles act as a catalyst for the fuel cell electrode and promote the oxygen reduction reaction (ORR). In this process, the surface properties and structure of adjacent carbon supports and ionomer binders greatly affect the Pt catalyst. Therefore, the performance of the Pt/C catalyst in the fuel cell electrode is determined not only by the intrinsic activity of the catalyst itself, but also by the interface structure between the ionomer binder and the Pt/C catalyst. It has been reported that the ORR activity of the Pt/C catalyst in the fuel cell electrode is affected by the Pt poisoning of the sulfonic acid group tethered to the Nafion backbone as well as the oxygen permeability of the ionomer binder [1]. The difference in pore characteristics of the carbon support affects the optimum ionomer content [2] and Pt utilizaton [3] in the electrode. The surface properties of the carbon support also affect ionomer distribution and ionomer thickness on the catalyst surface, thereby altering the activity of the catalyst [4]. In spite of recent studies, understanding of complicated physicochemical phenomena in the microstructure of electrode is still very limited. In this talk, we will discuss the fuel cell performance characteristics based on several electrochemical analyses according to the microstructure control of the catalyst layer. Various functional groups were introduced on the surface of Pt/C catalyst by post treatment and the catalyst layer was prepared to observe the ionomer distribution and ion transport characteristics according to functional groups. In addition, a multilayer catalyst layer having different ionomer contents in the thickness direction of the catalyst layer was prepared using an ultrasonic spraying process, and changes in hydrogen ion transfer and oxygen diffusion transfer characteristics in these catalyst layers were observed in a fuel cell. Finally, the electrochemical properties of the catalyst layer made of the ionomer dispersed in the high-boiling organic solvent will be introduced. 1. V. Yarlagadda, M. K. Carpenter, T. E. Moylan, R. S. Kukreja, R. Koestner, W. Gu, L. Thompson, A. Kongkanand, ACS Energy Lett. 3 (2018) 618. 2. Y. Liu, C. Ji, W. Gu, J. Jorne, H. A. Gasteiger, J. Electrochem. Soc. 158(6) (2011) B614. 3. Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, J. Power Sources 315 (2016) 179. 4. A. Orfanidi, P. Madkikar, H. A. El-Sayed, G. S. Harzer, T. Kratky, H. A. Gasteiger, J. Electrochem. Soc. 164 (2017) F418.

Modeling Proton Exchange Membrane Fuel Cell Cathode Catalyst Layers with the Lattice-Boltzmann-Method Framework

In this work, a coupled Lattice-Boltzmann-Method and electrochemistry Direct-Numerical-Simulation model for a proton exchange membrane (PEMFC) electrode has been presented. One of the main challenges affecting study of the cathode catalyst layer (CCL) in PEMFCs is the lack of detailed understanding of species transport and how it affects electrochemical performance. Researchers have typically used high level approximations that oversimplify the microstructure of the CCL—these are known as macrohomogenous models. However, as the field has progressed, these idealizations have begun to show their flaws, especially in areas of improving catalytic performance with lower Pt-loadings and non-noble metal catalysts. Previously, the microstructure details needed to build an accurate mesoscale model have eluded researchers; however, with advances in tomography and focused-ion-beam scanning-electron-microscopy (FIB-SEM), creating these representations has become possible. Mesoscale modeling in the CCL has been traditionally approached through either the Lattice-Boltzmann-Method (LBM) or electrochemistry coupled Direct-Numerical-Simulation (DNS). These models have been underutilized in the fuel cell community due to their complexity and resource intensiveness; however, with advances in parallel computing, this has become not only a possibility, but a necessity for modeling phenomena such as low platinum loadings and interfacial effects. With these new advances, a synergistic modeling approach can be taken that combines the advantages of each method. This can shed light on the transport and degradation phenomena in PEMFCs, particularly catalyst layer considerations and carbon support corrosion. Here, the liquid water as well as gaseous oxygen, nitrogen, and water vapor profiles through segmented geometries made of tomography measurements from FIB-SEM were shown through LBM. This is shown in Figure 1, where the capillary pressure is varied in six stages from 1.8 MPa to 20 MPa, which shows a saturation increase from 5% to 60%, respectively. This diagram shows the capillary fingering regime we should expect through the porous media. Additionally, these same geometries were used to simulate electrochemical phenomena, such as reaction profiles, polarization curves, ionomer thickness, and many others. Additionally, to examine the effects of ionomer coverage on different interfaces, a physics-based model using erosion was used to add in the ionomer, as FIB-SEM is unable to differentiate between ionomer, Pt, or carbon support. Once in, on the LBM side, we can model the ionomer by changing its contact angle compared to the carbon support (hydrophilic or around 10° for the ionomer compared to hydrophobic or 110° to 140° for the support), whereas for DNS, it is taken into account by considering Henry’s law and other transport phenomena. Finally, to account for accurate liquid formation throughout the geometry according to the Pt distribution, a source term was included in the LBM model and then transferred to DNS to create one of the first numerically intensive frameworks for a PEMFC electrode available. Figure 1

Microscopic Analysis of PEMFC Catalyst Layers

Proton exchange membrane fuel cell (PEMFC) electrodes have a complex structure of carbon supported platinum (Pt) nanoparticles intermixed with proton conducting ionomer. This structures creates a porous network facilitating transport of electrons, protons, and reactants. These catalyst layers have been shown to exist with a non-homogenous distribution of ionomer, with aggregates and agglomerates of carbon and ionomer. To better understand the catalyst layer structure, we have applied atomic force microscopy (AFM) to investigate fuel cell catalyst layer properties at high resolution. Various ionomer-carbon (I/C) ratios were analyzed in order to identify correlations between carbon agglomerate size, pore distribution, and structure. Surface and cross-sectional images of topography, adhesion, and roughness were collected. Scanning electron microscopy (SEM) was performed to allow for additional depictions of the samples. Several other techniques are planned to further explore structures. Electrochemical capabilities of the AFM will be utilized to inspect the effects of current on the aforementioned properties and provide better control in measurements. Additionally, samples will be placed in a water-filled holder to study effects of swelling through in situ AFM. Transmission electron microscopy (TEM) will also be performed to complement these measurements. A Bruker AFM was used in PeakForce tapping mode with a ScanAsyst Air tip to obtain the images in Figure 1. Figure 1a shows the height map of a sample with an I/C ratio of 1. Figure 1b is the adhesion map of the same sample, showing clear spatial adhesion contrast. In this map, lighter colors correspond to the more adhesive ionomer. Darker areas signify hard materials like carbon. Figure 1c depicts the adhesion map overlaid on the height map, indicating how the adhesion and topography relate. The arrows in Figure 1c illustrate where the ionomer thickness can be measured between carbon aggregates. Figure 2a summarizes the average ionomer thickness calculated for a range of I/C ratios from 0.25 to 1.25. There is a positive correlation between I/C ratio and average ionomer thickness, suggesting that increased availability of ionomer leads to thicker surface coatings and more separation between the carbon aggregates. Figure 2b shows the relationship between I/C ratio and aggregate diameter. These values were measured by processing the images in ImageJ and reducing aggregates to approximated spheres. Although no clear relationship is apparent now, further investigation with differing I/C ratios may reveal a trend. Acknowledgements : This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Technical Development Manager: Greg Kleen and Fuel Cells Program Manager: Dimitrios Papageoropoulos). This work was also performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Figure 1

Microscopic Analysis of PEMFC Catalyst Layers

Atomic force microscopy (AFM) is used as a tool to analyze agglomerates in proton exchange membrane fuel cell electrodes. Catalyst layers were prepared with catalyst inks with varying ionomer/carbon (I/C) ratios. AFM is used to image the height and adhesion on the electrode surface and in cross-section to study the relationship between the I/C, carbon particle size, and agglomerate size. A positive correlation between ionomer coverage and particle diameter and I/C was observed. There was no identifiable trend observed between ionomer thickness in agglomerates and I/C, suggesting that ionomer coats the surface in the z direction at higher I/C ratios.

Nano/Microscale Simulation of Proton Transport in Catalyst Layer

To spread polymer electrolyte fuel cells, improving the cell performance is required. The cell performance depends on various factors. One of the factors that lowers cell performance is proton transport resistance in catalyst layers. Catalyst layers have Nano/Microscale structures which influence on proton transport resistance. In this study, to investigate the relationship between structures of catalyst layers and cell performances, mass transport and chemical reactions are calculated in the 3D catalyst layer models. The information about the ionomer thickness dependence on the diffusion coefficient of protons based on the molecular dynamics simulation studies, is introduced to transport calculation in order to analyze nanoscale structure influence on the cell performance. As a result, we have found that the output voltage increases over the whole range of current density with increasing ionomer/carbon ratio considering ionomer thickness dependence. This result suggests that the nanoscale structure in catalyst layer has a large influence on the cell performance.

Microstructure and Transport Analyses in Polymer Electrolyte Membrane Fuel Cells: Comparing Modeling Approaches

Despite the recent entrance of proton exchange membrane fuel cells (PEMFC) into the automotive industry, performance under low platinum loading is still hampered by prohibitive and poorly understood transport losses inside the heterogeneous catalyst layer (CL). Predictive models do not yet sufficiently capture the transport of oxygen, water, and protons in the CL ionomer phase, due to the complexity of the CL microstructure. These transport rates are highly dependent on the substrate, film thickness, and relative humidity. Confinement effects limit water uptake and transport rates below roughly 100 nm thickness, and substrate effects can induce ionomer structures (i.e. lamellae) that further impact transport rates. Implementing these complexities into computational models allows for the microstructural and transport effects on PEMFC performance to be studied and better understood, especially in devices with low platinum loading. Research presented here examines two pseudo-2d, Newman-type model approaches for examining microstructure and transport in PEMFC functional layers: particle-shell and flooded-agglomerate. Figures 1 and 2 show the differences in model domains for these two approaches, respectively. The particle-shell approach approximates a physically realistic structure by simulating a carbon particle at the center of a thin shell of Nafion. This thin shell is discretized to resolve the complex transport that takes place within it. At each vertical discretization, a single Nafion-wrapped carbon particle is taken to be representative of others at that same functional layer depth. The flooded-agglomerate model also simulates a representative spherical domain for each vertical discretization; however, the domain is made up of a cluster of Nafion-wrapped carbon particles enclosed in an additional layer of electrolyte. The models predict cell polarization to examine the effect of CL microstructure as a function of RH- and thickness-dependent transport properties. Figure 3 shows an example overpotential curve from the flooded-agglomerate model in which interesting behavior can be seen with respect to the Nafion thickness. It shows that the CL overpotentials decrease with decreasing ionomer thickness down to 7 nm. Below 7 nm thickness, decreased proton conductivity limits transport, leading to increasing overpotentials at higher currents. The trends indicate that the balance between proton and oxygen transport rates in the CL Nafion can lead to possible trade-offs in optimizing CL design. These and other results will be presented and discussed, to demonstrate the coupled impacts of Nafion thickness and transport on PEMFC performance, and the impact of modeling approach on CL microstructure design. Keywords – Nafion, Transport, PEM, Fuel Cell, Modeling, Electrochemistry, Flooded Agglomerate Figure 1