Membrane Fuel Cell Catalyst Layers Research Articles

Nanoscale in-Situ Characterization of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers for Improved Water Management

Polymer electrolyte membrane fuel cells (PEMFCs) facilitate a sustainable energy infrastructure by offering emission-free electricity generation using renewably sourced hydrogen gas. The electrochemical reactions in a fuel cell occur at platinum-loaded catalyst layers (CLs), which are susceptible to degradation. Obstructing catalyst sites (triple phase boundaries1) with reaction by-products, such as water2,3, is the primary degradation mechanism in PEMFCs. Although various CL designs have been tested for improved electrochemical performance, in situ nanoscale visualization of platinum degradation and water accumulation in the CL at controlled temperature and relative humidity (RH) values are required to understand fundamental water mechanics through platinum and carbon nanostructures.In this work, we employed scanning transmission X-ray microscopy (STXM) and X-ray absorption fine structure spectroscopy (XANES) to evaluate the effect of temperature on pristine and broken-in fuel cell CLs. We developed a novel CL in-situ sample cell to enable nanostructured characterization of catalyst layers using STXM in a controlled temperature environment. Carbon 1s, Fluorine 1s, and Oxygen 1s spectral edges were probed utilizing near-edge X-ray absorption fine structure spectroscopy (NEXAFS) to reveal chemical degradation mechanisms and differentiate material structure. Through our custom in-situ cell, we demonstrate that the chemical composition and water accumulation throughout the PEMFC CLs at low, intermediate, and high operating temperatures (25°C, 40°C, and 60°C, respectively) can be quantified, along with thermal expansion analyses of the CL and ionomer membrane. Moreover, we reveal realistic structural and chemical characteristics of nanoscale catalysts by distinguishing regions of Nafion® Ionomer, catalyst carbon support, and platinum throughout the PEMFC’s membrane electrode assembly at industrially relevant fuel cell temperatures. The insights from this work will inform strategies to mitigate water flooding from a nanoscale perspective in addition to the novel in-situ spectromicroscopy characterization of current CL features.1. Fouzaï, I., Gentil, S., Bassetto, V. C., Silva, W. O., Maher, R., & Girault, H. H. (2021). 9(18), 11096-11123.2. Li, H., Tang, Y., Wang, Z., Shi, Z., Wu, S., Song, D., ... & Mazza, A. (2008). 178(1), 103-117.3. Kumsa, D. W., Bhadra, N., Hudak, E. M., Kelley, S. C., Untereker, D. F., & Mortimer, J. T. (2016). 13(5), 052001.

PGM Recovery: Maximizing PGM Dissolution through Minimizing Ostwald Ripening

Efficient recovery of platinum group metals (PGMs) through electrochemical means using mild conditions is of significant impact from both the industrial and the environmental points of view. Owing to their large surface-to-volume ratio, fast dissolution of PGM nanoparticles is possible through potential cycling between oxidizing and reducing potentials, leading to formation and dissolution of surface-oxide layer (transient dissolution). However, at reducing potentials, reduction of the dissolved metal species on the source nanoparticles leads to enhance the Ostwald ripening process1 and hence to decrease the overall dissolution efficiency significantly. In this talk, we present strategies to accelerate dissolution of PGM nanoparticles by limiting Ostwald ripening through (1) use of surface switching species (SSS), which selectively block the PGM surface to inhibit redeposition induced particle growth at reducing condition and expose the PGM surface to facilitate dissolution at oxidation condition, respectively2. (2) use of an auxiliary deposition system to continuously remove the dissolved PGM species through reaction kinetic manipulation3. Such surface relegation property to depress unwanted process and assist desired reaction documents an alternative and efficient approach for precious metal electrochemical recovery. References Sharma, R.; Andersen, S. M., Quantification on Degradation Mechanisms of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers during an Accelerated Stress Test. ACS Catal. 2018, 8 (4), 3424-3434.Sharma, R.; Simonsen, S. B.; Morgen, P.; Andersen, S. M., Inhibition of Ostwald ripening through surface switching species during potentiodynamic dissolution of platinum nanoparticles as an efficient strategy for platinum group metal (PGM) recovery. Electrochim. Acta 2019, 321, 11.Sharma, R.; Gyergyek, S.; Andersen, S. M., Environmentally and Industrially Friendly Recycling of Platinum Nanoparticles Through Electrochemical Dissolution-Electrodeposition in Acid-Free/Dilute Acidic Electrolytes. ChemSusChem 2018, 11 (21), 3742-3750.

Investigating the effect of solvent composition on ink structure and crack formation in polymer electrolyte membrane fuel cell catalyst layers

To improve the performance of polymer electrolyte membrane fuel cells (PEMFCs), controlling the microstructure of the membrane electrode assembly (MEA) catalyst layer is crucial. Ink design, which includes a catalyst, an ionomer, and a solvent, serves as the starting point for controlling the microstructure of the catalyst layer. However, there is a significant lack of understanding of the ink structure required for this purpose. In this study, we investigated the effect of the solvent, a key component that determines the ink structure. The ink comprises 20 wt% Pt/C, short-side-chain (SSC) Aquivion ionomer, and a solvent mixture of 1-propanol (NPA) and water. Three types of inks with different compositions of NPA and water are manufactured, and their stability and rheological properties were measured to infer and compare the ink structures. Furthermore, the crack characteristics of the catalyst layer were compared by directly coating the ink onto the electrolyte membrane using the doctor-blade method. In the ink with a high water content, we observed a gel-like elastic behavior dominated by network structures formed by ionomers adsorbed between catalyst particles. In contrast, the ink with a high NPA content exhibited a liquid-like viscous behavior dominated by well-dispersed catalyst particles and ionomers. These properties of the inks directly influence the crack formation characteristics after coating. Specifically, the strong liquid properties of the NPA-rich ink were found to suppress crack formation in the catalyst layer. These findings provide important insights into how the solvent composition affects ink structure and how it, in turn, influences crack formation in the catalyst layer, which can help optimize the ink design to improve the performance of PEMFCs.

Quantification and evolution on degradation mechanisms of proton exchange membrane fuel cell catalyst layer under dynamic testing conditions

The catalyst layer is one of the core components of the Proton Exchange Membrane Fuel Cell (PEMFC). Studying its degradation mechanism is very important to improve the durability of PEMFC. Therefore, the degradation of the catalyst layer after the 100 h, 225 h, and 650 h durability tests are studied quantitatively in this paper. The results show that the agglomeration of Pt particles is the main reason for the degradation of the catalyst layer in the first 100 h, accounting for 80%. Catalyst dissolution and the loss of proton connectivity caused by the ionomer are the main factors for the degradation of the catalyst layer between 225 and 650 h, which increases from 23% in the initial stage to 46%. In addition, the corrosion degree of the carbon support inside the catalyst layer gradually intensified with the test time. This study reveals the dominant degradation mechanism of the catalyst layer in different degradation stages and the evolution of the whole degradation process, which has important guiding significance for the structural design of the catalyst layer for long-life fuel cells.

Effect of micro-cracks on the in-plane electronic conductivity of proton exchange membrane fuel cell catalyst layers based on lattice Boltzmann method

Micro-cracks commonly occur on the catalyst layers (CLs) during the manufacturing of catalyst coated membranes (CCMs). However, the crack shape parameters effect on CLs in-plane (IP) electronic conductivity λs is not clear. In this work, the relationship between crack parameters and the λs is obtained based on the two-dimensional (2D) multiple-relaxation time (MRT) lattice Boltzmann method (LBM). The LBM numerical model is validated by the normalized λs experiment applied on three different home-made cracked CLs, and the parameter study focus on crack width, length, quantity and phase angle are carried out. The results show that the decrease of λs has different sensitivity |k| to the parameters above. The crack width has little effect on λs decrease, and the |kw| is 0.038. However, crack arm length and quantity show more significant impact, which |kl| and |kN| are 0.753 and 0.725, respectively. The CLs with different crack propagation directions show significant anisotropy on λs, and a 53.53% decrease in λs is observed between 0° and 90° crack phase angle change. To manufacture a high electronic conductivity CL, crack initiation and migration mitigation are highly encouraged.

Ionomer Distribution Strategy of Anion Exchange Membrane Fuel Cell Catalyst Layer in Terms of Interaction between Catalyst Slurry Components

Anion exchange membrane fuel cells (AEMFCs) have been intensively studied in recent years to replace proton exchange membrane fuel cells (PEMFCs). The acid-to-alkali transitions have the potential to lower overall system costs because it allows the non-precious metal catalysts and inexpensive metal stack hardware. Significant strides have been made in materials science in the last few decades, particularly the development of high IEC-containing anion exchange membrane (AEM) and ionomer (AEI) possessing high OH- conductivity, enabling comparable cell performance to that of PEMFC. In addition, the discovery of high HOR activity of PtRu by oxophilic and/or electronic effects, and reducing ionomer poisoning, provided an opportunity to further advance the cell performance of AEMFC. However, despite the remarkable development of materials, modest AEMFCs still have inadequate power performance (< 0.5 W cm-2) even using the precious catalysts, which stems from a lack of understanding of the catalyst layer (CL) design.CL consists of a catalyst and an ionomer, where the chemical energy of the fuel is converted into electrical energy in triple-phase-boundaries (TPBs). TPB is an electrochemically active site where catalyst (electrons), ionomer (H+ or OH-), and reactant gases were concurrent. In order to realize a high-performance fuel cell, the TPB should be high enough to utilize the capabilities of the catalyst, and the ideal state would be to maximize the contact area between the ionomer and the catalyst while minimizing the loss of CL porosity. Accordingly, the structure of the CL is one of the key factors that directly affect the performance of the fuel cell and is in great account.CL is fabricated from a slurry containing a catalyst, ionomer, and dispersing solvent, whose structure is determined by the complex interactions between slurry components. Therefore, fine-tuning of comprehensive interaction (catalyst/ionomer, catalyst/solvent, and ionomer/solvent) is the core technology in CL design, and an in-depth understanding of each interaction is also essential. In this regard, we recently presented the rational design of the CL by controlling AEI size, distribution via a solvent selection of the catalyst slurry. Specifically, the larger the solubility parameter (excluding the hydrogen bonding term) of the organic solvent mixed with water, the smaller the dispersed AEI size, leading to an even distribution of the AEI in the CL. This induces a high electrochemical surface area of the CL, making it possible to achieve high performance AEMFC from the low current region. However, nevertheless, we found that the distribution of AEI in the AEMFC catalyst layer was still not completely uniform through various types of electron microscopy analysis. In particular, when the morphology of the AEMFC CL was compared with the Nafion ionomer (commonly used in PEMFC field) as a reference, AEI caused lower porosity by clogging the pores of Pt/C nanoparticles. For this reason, we found that AEMFCs had lower performance than Nafion-based PEMFCs, despite their high ORR activity under alkaline conditions.Molecular dynamics (MD) and density functional theory (DFT) simulation analyzes show that AEI has a particularly low interaction with carbon compared to Nafion, and for this reason, we found that AEI aggregates with each other rather than evenly distributed on the Pt/C surface. We used QPC-TMA as AEI in this study and confirmed that commercialized AEI (FAA-3 and XA-9) also form low CL porosity and pore-closing characteristics. This result suggests that low interaction between AEI/carbon is a universal property of current levels of AEI. Therefore, when designing an ionomer to improve the performance of AEMFC, high interaction with carbon should be considered as another important variable.

Nonconformal Particles of Hyperbranched Sulfonated Phenylated Poly(phenylene) Ionomers as Proton-Conducting Pathways in Proton Exchange Membrane Fuel Cell Catalyst Layers

Characteristic poor electrochemical kinetics, high ionic resistance, and high mass transport resistance within the catalyst layer (CL) are chief among parameters that cause poor performance of proton exchange membrane fuel cells (PEMFCs) utilizing hydrocarbon-based proton-conducting ionomers. Herein, the design and addition of nondimensionally swellable, nonconformal, hyperbranched sulfo-phenylated poly(phenylene) ionomer particles (HB-sPPT-H+) are reported to introduce a direct pathway for proton conduction in hydrocarbon ionomer-based CLs, resulting in an eight times reduction in ionic resistance of the CL, a 71% increase in catalyst mass activity, and a >90% increase in power at 0.6 V (H2/air) compared to state-of-the-art hydrocarbon ionomer-based CLs. The benefits of incorporating HB-sPPT-H+ ionomer particles are also shown when employed in perfluorosulfonic acid (PFSA) ionomer-based PEMFCs. These results dispel a commonly held conception that hydrocarbon ionomers possess limitations of gas permeability and electrochemical activity and open up previously unexplored avenues of ionomer development for nonfluorous, wholly hydrocarbon PEMFCs.

Electrospray Deposition: A Breakthrough Technique for Proton Exchange Membrane Fuel Cell Catalyst Layer Fabrication

This Spotlight article presents the state-of-the-art of electrospray deposition technique applied to the fabrication of proton exchange membrane fuel cell (PEMFC) components, mainly focusing on catalyst layers in gas diffusion electrodes. The atomization of a suspension of particles over a substrate under the influence of a strong electric field results in the growth of a film with macroporous morphology and many interesting properties. This so-called electrospray deposition has reported many noteworthy beneficial effects for the fabrication of the catalyst layers of gas diffusion electrodes of PEMFCs. The electrosprayed catalyst layers prepared from suspensions of catalyst particles and ionomers present a dendritic macroporous morphology with superhydrophobic character that improves the water management inside the cell and increases the performance by ∼20% with respect to standard electrodes prepared by airbrushing. Other interesting effects observed with electrosprayed catalyst layers are increased catalyst utilization and water absorption capabilities of the ionomer, improved performance under nonhumidified conditions, and a reduction in catalyst degradation. In addition, the electrospray deposition decreases platinum losses during fabrication thanks to the attractive electrostatic forces between the ion mist and the substrate compared with regular ink-based spray methods.

Towards the understanding of transport limitations in a proton-exchange membrane fuel cell catalyst layer: Performing agglomerate scale direct numerical simulations on electron-microscopy-based geometries

Improving the cathode catalyst layer design requires understanding the sources of transport limitations in proton-exchange membrane fuel cells. For the purpose, a framework consisting on an electron microscopy characterization setup in couple with a numerical modeling software is proposed. The latter integrates high-performing geometry building capabilities which ensure full phase discretization (carbon, platinum, ionomer and pore phase) and freedom when designing the structure morphology and meshing. The 3D structure of the carbon phase is extracted from a focused ion beam scanning electron microscopy analysis having a 2 nm isotropic resolution. The platinum phase is built according to a nanoparticle size histogram determined from high angle annular dark field scanning transmission electron microscopy images. To add the ionomer phase, the thickness of the layer is measured on high resolution transmission electron microscopy images. The multi-physics model includes gas transport in the pores, and gas and ionic transport in the Nafion. A 4-step reaction mechanism is used to solve the electrochemistry. Numerical simulations are performed on two catalyst layer portions. The results show that structural heterogeneities can deeply impact performance. Such impact is mainly linked to oxygen diffusion limitations through the Nafion film and, to a certain extent, to interparticle competition effects.

Effects of ionomer and dispersion methods on rheological behavior of proton exchange membrane fuel cell catalyst layer ink

The catalyst layers of proton exchange membrane fuel cells are usually fabricated by (1) mixing catalyzed carbon black, ionomer solution, and solvents to form an ink, (2) coating this ink by spraying or transferring from decal to a membrane or gas diffusion layer, and (3) drying. During the mixing stage, dispersion methods, such as ball milling and sonication, are employed to homogenize the ink and prevent agglomeration of solid ingredients. The present study focused on the first step of fabricating the catalyst layer and conducting a rheological investigation on catalyst inks. The viscosity and thixotropy of inks that were prepared by ball milling and sonication, respectively, were measured, and some major factors affecting the change in the viscosity were analyzed. The catalyst layer inks exhibited pseudoplastic fluid behavior, which was clearly non-Newtonian, and their viscosity decreased with stress rate. The results indicated that the ionomer solution plays a decisive role in the dispersion of carbon-supported catalysts. Hence, homogeneous ink cannot be formed without the ionomer solution. The viscosity of the ink was dependent on the solid content ratio and the dispersion method used, while the dispersion time and amplitude were less critical. Ball milling and sonication methods result in inks with different rheological characteristics. Rheological data indicate that the thixotropic recovery of the ink prepared by ball milling is faster in comparison to that prepared by sonication.

A practical method to characterize proton exchange membrane fuel cell catalyst layer topography: Application to two coating techniques and two carbon supports

The coating technique and the type of carbon support can strongly affect the surface topography, the homogeneity and the thickness of the catalyst layers of Proton Exchange Membrane fuel cells. These features can in turn strongly modify the local diffusion properties of the layer and the final cell performance, but literature about catalytic layers does not often address such issues. This work aims at studying the topography of proton exchange membrane fuel cell catalyst layers by means of 3D characterization methods. For that purpose, two types of slurries, made of NafionⓇ and carbon black or a carbon xerogel, have been coated on a KaptonⓇ sheet by film-casting and by spray-coating. The surface topography of the samples was characterized by contact profilometry and 3D laser scanning microscopy. The data obtained with both methods were processed in order to obtain accurate 3D representations of the surface of the layers. Contact profilometry allows to perform measurements over the entire surface of the sample (5 cm × 5 cm). Layers obtained by film-casting turn out to be very smooth, but also thinner at the edges of the coating. For layers prepared by spray-coating, the reproducibility and the homogeneity are improved; however, the surface displays a higher roughness. With 3D laser scanning microscopy measurements, only a small part of the sample was analyzed (0.25 cm2) with a high definition, allowing to characterize more precisely the surface topography. The comparison of both techniques has been enriched by the simulation of the profilometer tip motion at the surface of the observed 3D microscopy profile, giving insight into the observed differences. The 3D microscopy measurements proved to be more accurate, but not representative of the whole sample, especially for less homogeneous layers prepared by film-casting.

Pore-scale study of pore-ionomer interfacial reactive transport processes in proton exchange membrane fuel cell catalyst layer

Abstract Understanding interactions between constituent distributions and reactive transport processes in catalyst layer (CL) of proton exchange membrane fuel cell is crucial for improving cell performance and reducing cell cost. In this study, high-resolution porous structures of cathode CL are reconstructed, where all the constituents in CLs are resolved. A pore-scale model based on the lattice Boltzmann method is developed for oxygen diffusion in pores and ionomer, as well as electrochemical reaction at the Pt surfaces. Particularly the model considers the pore-ionomer interfacial transport processes with distinct characteristics of sharp concentration drop, large diffusivity ratio and interfacial dissolution reaction. After validated by interfacial transport processes with analytical solutions, the pore-scale model is applied to reactive transport processes inside complex CL nanoscale structures. Pore-scale results reveal that pore-ionomer interfacial transport processes generate extremely high local transport resistance, significantly reducing the total reaction rate. As volume fraction of carbon increases, the value of the optimum ionomer content generating the best cell performance decreases, while the value of the optimum ionomer content resulting in the lowest performance loss under low Pt loading reduces. The two values generally are different. The pore-scale model helps to understand reactive transport processes and to optimize the CL structures.

The ex-situ and in-situ gas diffusivities of polymer electrolyte membrane fuel cell catalyst layer and contribution of primary pores, secondary pores, ionomer and water to the total oxygen diffusion resistance

Oxygen transfer resistance in catalyst layer (CL) of polymer electrolyte membrane fuel cells (PEMFCs) is the limiting factor under high current density operation. The gas diffusivity of the CL has been studied ex-situ and in-situ in the literature, however, an order of magnitude difference between values of ex-situ and in-situ diffusivities and contribution of pores, ionomer and water to the total mass transfer are yet remain to be investigated. In this study ex-situ and in-situ gas diffusion of ten different CL designs are measured. The ex-situ measured resistance is a part of the total resistance measured in-situ, and having that, the constituting parts of total diffusion may be obtained. The diffusion resistance of ionomer-water accounts for about 80% of the total resistance, and the rest is almost all due to secondary pores. Although, primary pores’ direct contribution to the diffusion resistance is negligible, their size has a notable impact on the ionomer-water relative diffusivity and increasing their diameter can linearly increase the ionomer-water diffusivity. Moreover, porosity of CL affects ionomer-water diffusivity by power of three. This study shows the efforts to reduce the total oxygen diffusion resistance should focus on increasing the CL porosity and size of primary pores.

Liquid Water Uptake and Contact Angle Measurement of Proton Exchange Membrane Fuel Cell (PEMFC) Electrodes

In this work, capillary rise experiments were performed to assess the wetting properties of free-standing proton exchange membrane fuel cell catalyst layers (CLs) with several ionomer chemistries (equivalent weight -EW- ranging from 540 to 1115). The samples were attached to a micro-balance and then immersed into liquid water to (i) measure the mass gain from the liquid uptake and (ii) estimate the (external) contact angle to water. The results showed that the ionomer EW had an impact on the CL wettability. Specifically, a highly hydrophobic behavior was observed at high ionomer EW (i.e., 1115): 173° contact angle to water. More hydrophilic behavior was observed at low ionomer EW (i.e., 540): 107° contact angle to water. For an intermediate range of ionomer EW (i.e., from 620 to 1100), minimal variation in the CLs wetting properties was observed at an average contact angle of 160°.

Correction to Quantification on Degradation Mechanisms of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers during Accelerated Stress Test

RETURN TO ISSUEPREVCorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to Quantification on Degradation Mechanisms of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers during Accelerated Stress TestRaghunandan SharmaRaghunandan SharmaMore by Raghunandan Sharma and Shuang Ma Andersen*Shuang Ma AndersenMore by Shuang Ma Andersenhttp://orcid.org/0000-0003-1474-0395Cite this: ACS Catal. 2018, 8, 6, 4979Publication Date (Web):May 1, 2018Publication History Published online1 May 2018Published inissue 1 June 2018https://doi.org/10.1021/acscatal.8b01558Copyright © 2018 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views652Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (198 KB) Get e-AlertsSUBJECTS:Membranes,Transmission electron microscopy,Fuel cells,Materials Get e-Alerts

Quantification on Degradation Mechanisms of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers during an Accelerated Stress Test

The long-term durability of the catalyst layers of a low-working temperature fuel cell such as a polymer electrolyte membrane fuel cell (PEMFC) is of significant scientific interest because of their operation criteria and high initial cost. Identification of degradation mechanisms quantitatively during an accelerated stress test (AST) is essential for assessing and improving the durability of such catalyst layers. In this study, we present a quantitative analysis of the degradation mechanisms such as (i) electronic connectivity loss due to carbon support corrosion, (ii) proton connectivity loss due to ionomer/catalyst interface loss, (iii) catalyst loss due to dissolution or detachment, and (iv) physical surface area loss due to particle growth that is responsible for the electrochemical surface area (ECSA) loss in Pt-based catalyst layers for PEMFCs during an AST performed through potential cycling (linear sweep cyclic voltammetry) between 0.4 and 1.6 V for 7000 cycles in Ar-saturated 1 M H2SO4. Using a ...

4D imaging of polymer electrolyte membrane fuel cell catalyst layers by soft X-ray spectro-tomography

4D imaging - the three-dimensional distributions of chemical species determined using multi-energy X-ray tomography - of cathode catalyst layers of polymer electrolyte membrane fuel cells (PEM-FC) has been measured by scanning transmission x-ray microscopy (STXM) spectro-tomography at the C 1s and F 1s edges. In order to monitor the effects of radiation damage on the composition and 3D structure of the perfluorosulfonic acid (PFSA) ionomer, the same volume was measured 3 times sequentially, with spectral characterization of that same volume at several time points during the measurements. The changes in the average F 1s spectrum of the ionomer in the cathode as the measurements progressed gave insights into the degree of chemical modification, fluorine mass loss, and changes in the 3D distributions of ionomer that accompanied the spectro-tomographic measurement. The PFSA ionomer-in-cathode is modified both chemically and physically by radiation damage. The 3D volume decreases anisotropically. By reducing the incident flux, partial defocusing (50 nm spot size), limiting the number of tilt angles to 14, and using compressed sensing reconstruction, we show it is possible to reproducibly measure the 3D structure of ionomer in PEM-FC cathodes at ambient temperature while causing minimal radiation damage.

A multiscale approach to accelerate pore-scale simulation of porous electrodes

Abstract A new method to accelerate pore-scale simulation of porous electrodes is presented. The method combines the macroscopic approach with pore-scale simulation by decomposing a physical quantity into macroscopic and local variations. The multiscale method is applied to the potential equation in pore-scale simulation of a Proton Exchange Membrane Fuel Cell (PEMFC) catalyst layer, and validated with the conventional approach for pore-scale simulation. Results show that the multiscale scheme substantially reduces the computational cost without sacrificing accuracy.

Optimization of polymer electrolyte membrane fuel cell catalyst layer with bidirectionally-graded composition

The catalyst layer (CL) of the polymer electrolyte membrane (PEM) fuel cell is comprised of multiple constituent materials including platinum catalyst, carbon support and ionomer. The loading and spatial distribution of these materials must be optimized to improve fuel cell performance and reduce cost. In this study, the CL composition is optimized simultaneously in both the in-plane and through-thickness directions for the first time. An improved agglomerate approach is employed within a 2D cathode model and optimal CL constituent (Pt, C, and ionomer) distributions are obtained to maximize the current density under different operating regimes. The optimum distributions are found to depend significantly on the operating regime, with regions of high constituent loading located in the high reaction-rate zones. In comparison to unidirectional grading (either in the in-plane or through-thickness direction), higher performance is obtained with a bidirectionally-graded CL composition. We also conducted two-component optimization by considering the joint distributions of Pt with Nafion®, and C with Nafion®. Higher performance improvements are obtained for two-component optimization.

Effect of blending carbon nanoparticles and nanotubes on the formation of porous structure and the performance of proton exchange membrane fuel cell catalyst layers

Different kinds of sub-micron structured carbon, namely carbon black and multi-walled carbon nanotubes (MWCNTs), were blended in catalyst ink in order to fabricate catalyst layers having differing porous structures. Carbon black is used as catalyst support in the conventional catalyst layers; the pore structure depends strongly on its aggregate structure. MWCNTs have a structure completely different from the carbon black at sub-micron scale. Two kinds of MWCNT, of differing diameters, were used as blending materials in the catalyst ink with the platinum-supported carbon, and their effects on the porous structure and cell performance were investigated. The catalyst layer containing thick MWCNT had lower porosity, fewer micro-cracks and larger pores than the conventional catalyst layer blended with carbon black. The MWCNT diameter influenced the distribution of pore sizes in the catalyst layers. The catalyst layer containing the smaller diameter MWCNTs displayed a shift in the peak of the pore size distribution, toward smaller size. These characteristic porous structures affected the performance of the resulting cell and caused a large voltage drop at high current densities in the polarization curve. This is because of the porosity and also the pore size.