Fuel Cell Catalyst Layers Research Articles

Continuous Graded Catalyst Layers for PEM Fuel Cells with Improved Humidity Range Tolerance

Abstract Grading electrodes is a promising approach to reduce ohmic and mass transport-related voltage losses in proton exchange membrane fuel cells. In graded electrodes, the ionomer and/or catalyst are spatially non-uniformly distributed to optimize performance over a wide operating range. In this study, through-plane ionomer gradients were fabricated with a simple wet-layer deposition technique that can readily be adapted in a roll-to-roll process. The presence of ionomer gradients was verified using scanning transmission electron microscopy and the profiles were found to be continuous despite using only two coating steps. Single cell tests revealed that the gradients outperform conventional electrodes and maintain the optimal performance for different relative humidities. These improvements were traced back to enhanced mass transport and protonic conduction properties identified by detailed electrochemical analysis. This manufacturing approach for electrodes offers an accessible toolbox to produce versatile and specialized multi-layered catalyst layers for different applications.

Insights into the pore structure effect on the mass transfer of fuel cell catalyst layer via combining machine learning and multiphysics simulation

Analysis of the catalytic layer (CL) pore structure and three-phase (electron phase, proton phase, and reactant phase) mass transfer in high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) is critical for improving concentration polarization and optimizing performance. However, there is a lack of studies on the pore structure’s impact on the catalytic layer three-phase mass transfer. In this work, the influence of pore structure on catalytic layer three-phase mass transfer and multiphysics distribution was investigated by combining machine learning and multiphysics simulation. Among many key factors investigated by machine learning that impact the performance of catalytic layer, porosity emerges as the primary influencing factor. Porosity accounts for 32.7 % of the electric potential drop, with macro/micro porosity contributing 27.6 % and 5.1 %, and 32.8 % of the current density, with macro/micro porosity contributing 15.6 % and 17.2 %. Furthermore, the macropore structure has a much greater influence on the performance of the catalytic layer compared to the micropore structure. Moreover, the simulation results demonstrated that porosity has a great influence on three-phase mass transfer, the oxygen molarity increases with increasing porosity. The electric potential difference increases exponentially as porosity increases. The relationship between current density and porosity is a volcanic relationship, which can be expressed as a quadratic polynomial function. The mesoscopic pore-scale model (PSM) with an εeff of 0.502 (εmic of 0.2 and εmac of 0.378) exhibits the highest mean current density at 619.526 mA/cm2@0.5 V, attributed to a more uniform and efficient three-phase transfer path. The accuracy of the mesoscopic pore-scale model and the polynomial equation is further confirmed through corresponding experiments. The experiment results show that the current density decreased as the εeff increased from 0.682 to 0.702, which aligns with the theoretical predictions of the PSM model and polynomial equation. These studies are valuable for comprehending the internal three-phase mass transfer behavior and optimizing the catalytic layer pore structure to enhance HT-PEMFC performance.

Fuel Cell Catalyst Layers with Platinum Nanoparticles Synthesized by Sputtering onto Liquid Substrates.

Platinum (Pt) nanoparticles are widely used as catalysts in proton exchange membrane fuel cells. In recent decades, sputter deposition onto liquid substrates has emerged as a potential alternative for nanoparticle synthesis, offering a synthesis process free of contaminant oxygen, capping agents, and chemical precursors. Here, we present a method for the synthesis of supported nanoparticles based on magnetron sputtering onto liquid poly(ethylene glycol) (PEG) combined with a heat-treatment step for attachment of nanoparticles to a carbon support. Transmission electron microscopy imaging reveals Pt nanoparticle growth during the heat-treatment process, facilitated by the carbon support and the reducing properties of PEG. Following the heat treatment, a bimodal size distribution of Pt nanoparticles is observed, with sizes of 2.5 ± 0.8 and 6.7 ± 1.8 nm, compared to 1.8 ± 0.4 nm after sputtering. Synthesized Pt nanoparticles display excellent specific and mass activities for the oxygen reduction reaction, with 1.75 mA/cm2 Pt and 0.27 A/mgPt respectively, measured at 0.9 V vs the reversible hydrogen electrode. The specific activities reported herein outperform literature values of commercial Pt/C catalysts with similar loading and are on par with values of bulk Pt and mass-selected nanoparticles of comparable size. Also, the mass activities agree well with the literature values. The results provide new insights into the growth processes of SoL-synthesized carbon-supported Pt catalyst nanoparticles, and most crucially, the high performance of the synthesized catalyst layers, along with the possibility of nanoparticle growth through a straightforward heat-treatment step at relatively low temperatures, offer a scalable new approach for producing fuel cell catalysts with more efficient material utilization and new material combinations.

Coarse-Grained Molecular Dynamics Simulation of Ionomer-Mediated Carbon Cluster Bonding in Polymer Electrolyte Fuel Cell Catalyst Layers

This study examines the impact of ionomer to carbon mass ratio and the number of platinum particle clusters on the structural dynamics and organization of Pt-C-Nafion catalyst layers during the evaporation process. Using Coarse-Grained Molecular Dynamics (CGMD) simulations, we analyzed the center of mass position and ionomer coverage on Pt surface. The results show that a higher ionic polymer to carbon mass ratio (0.6) increases lateral mobility and short-range ordering along with the coverage of the Pt surface with ionomers, whereas increasing the number of Pt particle clusters (from 1 to 4 ) enhances lateral diffusion as well as leading to more difficult adsorption of ionomers. These findings highlight the importance of optimizing these parameters to obtain a homogeneous and stable catalyst layer, which is critical for fuel cell performance. Future work will include tuning the Pt/C mass ratio, hydrophilicity, and IC ratio in conjunction with energy and force analyses to further understand the effect of Nafion on Pt/C clusters.

Macroporous Structures of Nb-SnO2 Particles as a Catalyst Support Induce High Porosity and Performance in Polymer Electrolyte Fuel Cell Catalyst Layers.

Macroporous niobium-doped tin oxide (NTO) is introduced as a robust alternative to conventional carbon-based catalyst supports to improve the durability and performance of polymer electrolyte fuel cells (PEFCs). Metal oxides like NTO are more stable than carbon under PEFC operational conditions, but they can compromise gas diffusion and water management because of their denser structures. To address this tradeoff, we synthesized macroporous NTO particles using a flame-assisted spray-drying technique employing poly(methyl methacrylate) as a templating agent. X-ray diffraction analysis and scanning electron microscopy confirmed the preservation of crystallinity and revealed a macroporous morphology with larger pore volumes and diameters than those in flame-made NTO nanoparticles, as revealed by mercury porosimetry. The macroporous NTO particles exhibited enhanced maximum current density and reduced gas diffusion resistance relative to commercial carbon supports. Our findings establish a foundation for integrating macroporous NTO structures into PEFCs to optimize durability and performance.

Revealing the Path-Dependence of Catalyst Degradation in Polymer Electrolyte Fuel Cells

Electro-catalyst plays a central role in many clean energy technology applications including water electrolysis for green hydrogen generation, emission free fuel cells for transportation, electrochemical CO2 reduction and combine heat and power generation. Electro-catalysts degrade via multiple mechanisms, depending on the exact operating conditions in its life cycles. In this presentation, we demonstrate the dependance of the electro-catalyst degradation in polymer electrolyte fuel cell (PEFC) on the specific “paths” that the fuel cell takes to age. Specifically, differently ordered aging protocols with the same total number of accelerated stress tests (ASTs) cycles (i.e., 31,000 total cycles including 30,000 cycles of square-wave load/unload ASTs and 1000 cycles of triangular-wave carbon corrosion ASTs) were used to degrade membrane electrode assemblies (MEAs). At the end-of-life, performances were distinct, indicating that depending on the specific aging route taken, the fuel cell catalyst layer degrades differently after same number of total AST cycles. The load/unload aging protocol led to more severe losses of performance when preceding after the carbon corrosion aging ASTs

Characterizing PEM fuel cell catalyst layer properties from high resolution three-dimensional digital images, part I: A numerical procedure for the ionomer distribution reconstruction

In this work, a numerical approach is presented to reconstruct the ionomer three-dimensional distribution in the microstructure of the cathode catalyst layer (CCL) of a polymer electrolyte membrane fuel cell (PEMFC) from a 3D segmented image of the CCL obtained by focused ion beam-scanning electron microscopy (FIB-SEM). Three forms of ionomer are distinguished: the ionomer filaments, the coating ionomer and the inter-carbon grains ionomer. The ionomer filaments are identified by applying a filtering procedure to the size distribution in the segmented solid phase obtained via a maximum sphere inscription algorithm. The coating ionomer is reconstructed via a filtering procedure of the curvature distribution computed over the interface between the two segmented phases from the consideration that concave regions are preferential ionomer accumulation zones during the drying step of the CCL fabrication procedure. This procedure of coating controlled by the local curvature is applied until a desired value of coverage is reached. In a last step, catalyst nanoparticles, also not visible in the FIB-SEM image, are reconstructed in the image.

Generative Artificial Intelligence for Designing Multi-Scale Hydrogen Fuel Cell Catalyst Layer Nanostructures.

Multiscale design of catalyst layers (CLs) is important to advancing hydrogen electrochemical conversion devices toward commercialized deployment, which has nevertheless been greatly hampered by the complex interplay among multiscale CL components, high synthesis cost and vast design space. We lack rational design and optimization techniques that can accurately reflect the nanostructure-performance relationship and cost-effectively search the design space. Here, we fill this gap with a deep generative artificial intelligence (AI) framework, GLIDER, that integrates recent generative AI, data-driven surrogate techniques and collective intelligence to efficiently search the optimal CL nanostructures driven by their electrochemical performance. GLIDER achieves realistic multiscale CL digital generation by leveraging the dimensionality-reduction ability of quantized vector-variational autoencoder. The powerful generative capability of GLIDER allows the efficient search of the optimal design parameters for the Pt-carbon-ionomer nanostructures of CLs. We also demonstrate that GLIDER is transferable to other fuel cell electrode microstructure generation, e.g., fibrous gas diffusion layers and solid oxide fuel cell anode. GLIDER is of potential as a digital tool for the design and optimization of broad electrochemical energy devices.

Characterizing PEM fuel cell catalyst layer properties from high resolution three-dimensional digital images, Part II: Oxygen effective diffusion and proton effective conductivity tensors

The oxygen effective diffusion and proton effective conductivity tensors of a cathode catalyst layer (CCL) are computed using a two-scale approach on CCL microstructure three-dimensional images obtained by focused ion beam-scanning electron microscopy (FIB-SEM) after reconstruction of the ionomer distribution. The results show that the obtained tensors are not isotropic. It is also found that some off-diagonal components are not negligible in the case of the proton effective conductivity tensor. The simulations confirm that Knudsen diffusion has a significant impact on oxygen diffusion. The results are comparable to those obtained in previous works on synthetic images as regards the oxygen effective diffusion through-plane component but differ as regards the proton conductivity through-plane component. Contrary to previous works using synthetic images which tend to underestimate the proton conductivity, they are globally in better agreement with experimental data. The results also show that the effective proton conductivity is heterogeneous at the scale of the computational domains typically considered in several previous works due to the variability in the ionomer distribution at this scale. The impact of liquid water in the primary pores on the proton transport is explored and found to have a significant impact.

Gas Transport Resistance of Hydrocarbon-Based Catalyst Layers in Proton-Exchange Membrane Fuel Cells

Recent developments in hydrocarbon-based proton exchange membrane fuel cells have significantly narrowed the performance gap compared to state-of-the-art cells using perfluorosulfonic acid ionomers (PFSA). However, balancing protonic resistance and gas transport resistance in the catalyst layer remains a challenge at low humidity. This study investigates gas transport resistance and its components in sulfonated phenylated polyphenylene-based catalyst layers using various limiting current methods. Results show that increasing the dry ionomer to carbon (I/C) ratio from 0.2 to 0.4, a measure to catch up with protonic resistance of PFSA-based catalyst layers, significantly increases gas transport resistance in the cathode catalyst layer by 28 %. The data suggest a strong correlation between local gas transport resistance and IEC. A high IEC is beneficial for the gas transport through the ionomer film. However, at low ionomer volume fractions the local gas transport resistance is dominated by the I/C independent interfacial resistance. Furthermore, a low IEC hydrocarbon ionomer, such as Pemion® PP1-HNN4–00-X (IEC = 2.5 meq g−1), not only exhibits a beneficial interfacial resistance, but also suppresses excessive ionomer swelling, which typically occurs during operating conditions where liquid water is forming.

Impacts of Catalyst Ink Composition and Wet Film Thickness on Fuel Cell Catalyst Layers Fabricated by Direct Film Coating Method

Utilizing a direct film coating method (DFCM), such as doctor blade coating, offers a promising approach for efficient and scalable catalyst layer (CL) production for fuel cells. To further widen the understanding of lab-scale DFCM, the present research investigates how different Pt-based catalyst ink formulations coated via doctor blade coating with varying blade gap thickness (BGT) affect the CL quality and catalyst loading. In total, 120 CL samples were prepared by coating 20 different catalyst ink formulations with varying solids content, ionomer-to-carbon (I/C) ratio, and water-to-isopropanol solvent ratio with BGTs of 75, 125, and 200 μm. Inspection of these samples showed that the solvent ratio affects the coating uniformity, with the most uniform films achieved with a ratio of 1.67 or greater. Furthermore, increasing the I/C ratio for a given solids content ink formulation decreases the Pt loading, whereas an I/C ratio above or below 1.0 reduces cell performance due to mass transport or proton conductivity impacts, respectively. In addition, a relationship factor and equations are presented to estimate the solid weight and catalyst loading of the fabricated CL based on the ink formulation and BGT. Overall, this work provides important guidance for lab-scale DFCM fabrication of industrially relevant CLs.

Probing multiphase reactive transport interactions in the polymer electrolyte fuel cell catalyst layer degradation

The multiphase, multicomponent reactive transport plays a critical role in the degradation of the cathode catalyst layer in Polymer Electrolyte Fuel Cells. This warrants a fundamental understanding of the pore-scale, mechanistic interactions in the catalyst layer. Herein, the interfacial and transport interactions due to the flooding dynamics in the presence of carbon support corrosion, which is a primary degradation mode in the cathode catalyst layer, are delineated based on a comprehensive mesoscale model. The mechanistic interrogation of the degradation-transport interactions reveals the spatio-temporal heterogeneity of the pristine and aged microstructures. An electrode-averaged saturation front of 80 % unveils a critical limit of percolation which results in an onset of limiting current density. Further, it is observed that the reactant severity is amplified at progressively higher stages of corrosion owing to the appearance of dead pores and a thicker ionomer film. We also shed light on the fluid displacement patterns which reveal the existence of a capillary fingering regime. The interplay of the operating current density and carbon corrosion in governing the substrate-ionomer interaction is described through an electrochemical Damköhler number.

Colloidal Stability of PFSA-Ionomer Dispersions Part II: Determination of Suspension pH Using Single-Ion Potential Energies.

Perfluorosulfonic acid (PFSA) ionomers serve a vital role in the performance and stability of fuel-cell catalyst layers. These properties, in turn, depend on the colloidal processing of precursor inks. To understand the colloidal structure of fuel-cell catalyst layers, we explore the aggregation of PFSA ionomers dissolved in water/alcohol solutions and relate the predicted aggregation to experimental measurements of solution pH. Not all side chains contribute to measured pH because of burying inside particle aggregates. To account for the measured degree of dissociation, a new description is developed for how PFSA aggregates interact with each other. The developed single-counterion electrostatic repulsive pair potential from Part I is incorporated into the Smoluchowski collision-based kinetics of interacting aggregates with buried side chains. We demonstrate that the surrounding solvent mixture affects the degree of aggregation as well as the pH of the system primarily through the solution dielectric permittivity, which drives the strength of the interparticle repulsive energies. Successful pH prediction of Nafion ionomer dispersions in water/n-propanol solutions validates the numerical calculations. Nafion-dispersion pH measurements serve as a surrogate for Nafion particle-size distributions. The model and framework can be leveraged to explore different ink formulations.

Stabilizing the Catalyst Layer for Durable and High Performance Alkaline Membrane Fuel Cells and Water Electrolyzers.

Anion exchange membrane (AEM) fuel cells (AEMFCs) and water electrolyzers (AEMWEs) suffer from insufficient performance and durability compared with commercialized energy conversion systems. Great efforts have been devoted to designing high-quality AEMs and catalysts. However, the significance of the stability of the catalyst layer has been largely disregarded. Here, an in situ cross-linking strategy was developed to promote the interactions within the catalyst layer and the interactions between catalyst layer and AEM. The adhesion strength of the catalyst layer after cross-linking was improved 7 times compared with the uncross-linked catalyst layer due to the formation of covalent bonds between the catalyst layer and AEM. The AEMFC can be operated under 0.6 A cm-2 for 1000 h with a voltage decay rate of 20 μV h-1. The related AEMWE achieved an unprecedented current density of 15.17 A cm-2 at 2.0 V and was operated at 0.5, 1.0, and 1.5 A cm-2 for 1000 h.

The Lifetime of Hydroxyl Radical in Realistic Fuel Cell Catalyst Layer.

The lifetime of hydroxyl radicals (⋅OH) in the fuel cell catalyst layer remains uncertain, which hampers the comprehension of radical-induced degradation mechanisms and the development of longevity strategies for proton-exchange membrane fuel cells (PEMFCs). In this study, we have precisely determined that the lifetime of ⋅OH radicals can extend up to several seconds in realistic fuel cell catalyst layers. This finding reveals that ⋅OH radicals are capable of carrying out long-range attacks spanning at least a few centimeters during PEMFCs operation. Such insights hold great potential for enhancing our understanding of radical-mediated fuel cell degradation processes and promoting the development of durable fuel cell devices.

Morphology of Thin-Film Nafion on Carbon as an Analogue of Fuel Cell Catalyst Layers.

Species transport in thin-film Nafion heavily influences proton-exchange membrane (PEMFC) performance, particularly in low-platinum-loaded cells. Literature suggests that phase-segregated nanostructures in hydrated Nafion thin films can reduce species mobility and increase transport losses in cathode catalyst layers. However, these structures have primarily been observed at silicon-Nafion interfaces rather than at more relevant material (e.g., Pt and carbon black) interfaces. In this work, we use neutron reflectometry and X-ray photoelectron spectroscopy to investigate carbon-supported Nafion thin films. Measurements were taken in humidified environments for Nafion thin films (≈30-80 nm) on four different carbon substrates. Results show a variety of interfacial morphologies in carbon-supported Nafion. Differences in carbon samples' roughness, surface chemistry, and hydrophilicity suggest that thin-film Nafion phase segregation is impacted by multiple substrate characteristics. For instance, hydrophilic substrates with smooth surfaces correlate with a high likelihood of lamellar phase segregation parallel to the substrate. When present, the lamellar structures are less pronounced than those observed at silicon oxide interfaces. Local oscillations in water volume fraction for the lamellae were less severe, and the lamellae were thinner and were not observed when the water was removed, all in contrast to Nafion-silicon interfaces. For hydrophobic and rough samples, phase segregation was more isotropic rather than lamellar. Results suggest that Nafion in PEMFC catalyst layers is less influenced by the interface compared with thin films on silicon. Despite this, our results demonstrate that neutron reflectometry measurements of silicon-Nafion interfaces are valuable for PEMFC performance predictions, as water uptake in the majority Nafion layers (i.e., the uniformly hydrated region beyond the lamellar region) trends similarly with thickness, regardless of support material.

Comprehensive Molecular Dynamics Study of Oxygen Diffusion in Carbon Mesopores: Insights for Designing Fuel-Cell Catalyst Supports.

Mesoporous carbon is often used as a support for platinum catalysts in polymer electrolyte fuel-cell catalyst layers. Mesopores in the carbon support improve the performance of fuel cells by inhibiting the adsorption of ionomer onto the catalyst particles. However, the mesopores may impair mass transport. Hence, understanding molecular behaviors in the pores is essential to optimizing the mesopore structures. Specifically, it is crucial to understand the oxygen transport in the high-current region. In this study, the diffusion coefficients of oxygen molecules in carbon mesopores were calculated for various pore lengths, pore diameters, filling rates, and water contents in the ionomer via molecular dynamics simulations. The results show that oxygen diffusion slows by 2 orders of magnitude because of pore occlusion, and it slows down by an additional 1 or 2 orders of magnitude if ionomers are present in the pores. The occlusion can be theoretically predicted by considering the surface free energy. This theory provides some insight into mesoporous carbon designs; for instance, the theory suggests that narrow pores should be shortened to prevent occlusion. Slow diffusion in the presence of ionomers was attributed to the localization of oxygen at the dense ionomer-carbon interface. Thus, to improve oxygen transport properties, carbon surfaces and ionomer structures may be designed in such a manner as to prevent densification at the interface.

In situ investigation of moisture sorption mechanism in fuel cell catalyst layers

Research focusing on catalyst layers is critical for enhancing the performance and durability of proton exchange membrane fuel cells.

Microwave Drying of Catalyst Layers for Fuel Cells and Electrolysers

Due to an increasing demand for sustainable energy, the polymer electrolyte membrane fuel cell (PEMFC) is considered a key technology in energy conversion with hydrogen as an energy carrier. The performance characteristics of a PEMFC is significantly influenced by the morphology of the catalyst layer[1], which is formed predominantly during the drying step in the manufacturing process.In order to improve the performance of PEMFC and hence reducing production costs, research effort as well as industrial interest has focused on the drying process. Moreover, energy efficiency of the drying step in the production of thin-film catalyst layers is considered a key driver of cost perspective for the mass-market introduction of PEM fuel cells and electrolyzers due to increasing energy costs and the need for reduction of environmental impact.In the presented work, an innovative drying approach for thin-film catalyst layers, used in PEMFC and electrolyzer applications, is investigated. The thin-film is either coated on a substrate and transferred onto a polymer electrolyte membrane in a second process step or is directly coated onto the polymer electrolyte membrane by doctor blade coating. Drying is performed by coupling of microwave radiation into the wet film, leading to a homogeneous and energy-efficient heating of the coated film. Energy losses of the microwave drying process are significantly reduced compared to conventional convective drying. Additionally, the microwave induced drying step offers maximum control during the process.Dried thin-films, produced in the lab scale, are transferred to a polymer electrolyte membrane by a hot-pressing step and are completed with gas diffusion layers to membrane electrode assemblies (MEA). Electrochemical performance of the MEA is characterized on a fuel cell test bench in single cell configuration. The morphology of the structure obtained from microwave drying is analyzed and compared to CCM samples dried in a conventional convective oven.The effect of a microwave-based drying process is discussed with regard to fuel cell performance and structure-property relations of the electrodes.

(Invited) Characterization of PEM Fuel Cell Catalyst Layers – from Powder to End of Life

Increasing the long-term stability of polymer electrolyte membrane fuel cells in general and under dynamic operation, in particular, is crucial for its commercialization. This stability is not only affected by operation parameters, but also the components and their design. In this talk, we discuss our ongoing activities at Fraunhofer ISE to fully understand the cell’s behavior and stability through experimental and numerical methods. To do so, the catalyst layers are produced in house and characterized through a fully automated and standardized protocol. Hence, we look at the effect of catalyst layer design on the overall life of the cell. In this talk, we will discuss the ageing of catalyst layers in relation to their design using both in-situ and ex-situ experimental analysis as well as numerical modeling.