Perfluorosulfonic Acid Ionomer Research Articles

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.

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.

Modulating Perfluorinated Ionomer Functionality via Sidechain Chemistry

AbstractIonomers are important to electrode function in energy conversion devices, such as fuel cells and electrolyzers, as the catalyst‐binding nanometer‐thick films under confinement, which compromises mass transport of species. Mitigating confinement effects is necessary for improved performance of polymer electrolyte fuel cells. Studies on perfluorosulfonic acid (PFSA) ionomers provided insights into origins of transport resistances, but limited improvements by current strategies necessitate a need for alternative chemistries to enhance film function. This work investigates new ionomers based on sidechain modifications of PFSA to tune local acidity and structure. Their application‐relevant properties (hydration, thermal‐transition) and nanomorphology are systematically characterized using environmental ellipsometry and grazing‐incidence X‐ray scattering, respectively, to develop chemistry‐structure‐property relationships in thin‐film motif (≤100 nm). Introducing acid sites to sidechains improves hydration and nano‐phase‐separation but reduce chain mobility. Replacing the hydrophilic end‐groups with hydrophobic perfluoro‐groups enhances mobility but disrupts phase‐separation. Although modifying sidechain functional groups alters ionomer‐substrate interactions (Pt/C), chemistry has greater influence on structure‐properties. Proton conductivity correlates with hydration, regardless of chemistry. This work improves the fundamental understanding of chemistry‐function relationships of ionomers by providing a novel means to independently control their side‐chain vs. end‐group and observe resulting effects on interfacial properties to modulate electrode function in electrochemical technologies.

All-Perfluorosulfonated-Ionomer Composite Membranes Containing Blow-Spun Fibers: Effect of a Thin Fiber Framework on Proton Conductivity and Mechanical Properties.

In this study, thin fiber composite polymer electrolyte membranes (PEMs) were prepared using short side-chain perfluorosulfonic acid (PFSA) ionomers, Aquivion, to create composite PEMs with improved proton conductivity and improved mechanical properties. PFSA thin fiber webs prepared by blow spinning and successive hot pressing were used as the porous substrate. Herein, PFSA ionomers were used for both the substrate and the matrix of the composite PEMs, and their structures, properties, and fuel cell performance were characterized. By adding the PFSA thin fiber webs to the matrix, the proton conductivity was enhanced and the mechanical properties were slightly improved. The prepared PFSA thin fiber composite PEM showed better FC performance than that of the pristine PFSA one for the high-temperature low-humidity condition in addition to the low-temperature high-humidity one. To the best of our knowledge, this is the first report on the all PFSA composite membranes containing a PFSA thin fiber framework.

Study on ammonia transport and separation in Aquivion® perfluoro sulfonated acid membranes

The present study reports the results of a series of sorption and permeation tests of pure ammonia as well as nitrogen and hydrogen carried out on Aquivion C87-05 (short-side chain perfluoro sulfonic acid ionomer). Such material is indeed of interest for possible applications in sustainable processes for ammonia production, either as base material for polymer electrolyte membranes in low-temperature electrochemical ammonia synthesis or as a membrane for effective product separation.NH3, N2, and H2 permeation tests are performed at different temperatures (20, 35, and 50 °C) and both in dry and humid conditions (R.H. up to 80%), aiming to assess the influence of these parameters on the resulting permeabilities, while ammonia sorption is inspected at the same temperatures, and pressures up to near saturation conditions.Pure ammonia permeability reached outstanding values around 7000 Barrer in dry Aquivion membranes, revealing an increasing trend with upstream pressure, while it decreased with temperature. The same behavior is recorded for NH3 solubility, indicating that sorption drives the ammonia transport through the membrane.The obtained separation performances are found to be significantly better than those of other polymeric membranes proposed for the same separations, as compared to a permeability-selectivity plot.

Cross-Aligned PTFE Nanofiber Reinforced Composite Membrane for PEM Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) have widely been considered environmental friendly alternative energy technologies due to their high energy conversion efficiency and zero emission. In the recent, the importance of performance and durability of electrolyte membrane, interfacial layer separating two electrodes, prohibiting fuel cross-over, while transporting only protons, has become much crucial for utilizing PEMFCs to the automobile applications such as fuel cell electric vehicles (FCEVs) and drones.Unfortunately, the current state membranes, mainly composed of perfluorosulfonic acid (PFSA) ionomers, have suffered from the high cost and low proton conductivity, and low dimensional stability during dynamic humidity changes during fuel cell operation. In order to deal with these issues, recently, a reinforced composite membranes (RCMs) have been developed in which a PFSA ionomer is impregnated into a porous support matrix consisted with nanofibrous polymer materials such as polytetrafluoroethylene (PTFE) due to their thermal/mechanical robustness. The PTFE based RCMs could provide enhanced mechanical stability and electrochemical performance with a reduced use of PFSA ionomer, but low impregnation of ionomers into nanosized hydrophobic PTFE networks, resulting in phase separation and irreversible creep deformation.Herein, we report a novel preparation of vertically alined PTFE aggregated fibers (PTFE AFs) as a reinforcement by using facile electrospinning hetero-polymer solutions containing PTFE nanoparticles and PEO (Polyethylene oxide) carrier polymer and subsequent thermal removal of PEO. Developed PTFE AF were effectively soaked by PFSA ionomers, and PTFE AF based RCMs delivered outstanding dimensional stability and proton conductivity compared to commercial RCMs. Furthermore, PTFE AF based RCMs used MEA exhibited exceptional single-cell performance and long-term stability. The enhanced performance was attributed to the facile proton conduction through the one dimensional channels between aligned fibers without fiber scattering, and effective suppression of volume expansion of hydrated PFSA ionomer by rectangular shaped PTFE fiber network. We believe the widespread use of novel PTFE fiber based membranes with an unique pattern in a variety of membrane applications where porous supports are used. Figure 1

Influence of Sidechain Length in Perfluoro-Sulfonic Acid Ionomer on Electrode Slurry Interactions and Finalized Microstructure in the Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells (PEFCs) have emerged as one of the most promising next-generation energy devices for both passenger-owned and heavy-duty vehicles due to their high energy efficiency, low emissions, and quiet operation. However, the performance of the fuel cell electrode is often limited by the strong specific adsorption of long sidechain (LSC) perfluoro-sulfonic acid (PFSA) ionomer onto the platinum (Pt) catalyst2, which results in a decrease in the electrocatalytic activity1 as well as increase oxygen mass transport resistenace at the MEA level. To address this issue, a short sidechain (SSC) ionomer has been developed, which has been found to exhibit excellent efficiency over LSC ionomer3 The reduced specific adsorption of SSC ionomer onto the Pt catalyst results in a higher available surface area for the electrochemical reaction, which improves the overall fuel cell performance. The sidechain length of ionomers plays a crucial role in determining the overall cell performance4 however, due to the differences between them, the interactions among slurry components are not yet fully understood.Unlike the previous literatures, we investigates the details of ionomer sidechain length and its impact on the interaction among slurry components and resulting electrode microstructure, through a comparative analysis of LSC and SSC ionomers.5 Our hypothesis propose that the physical interactions between components are considerably impacted by the sidechains with a specific emphasis on the mobility and ion-pair association in the slurry. To probe the influence of ionomer sidechains, we conducted a thorough investigation wherein incrementally varied its concentration from 0 to 1.5 mmolSO3-gC -1, at intervals of 0.3 mmolSO3-gC -1. The main purpose of this evaluation was to observe the evolution of particle aggregation according to the correlation between adsorbed ionomer and free ionomer in electrode slurry. Drawing from the U-shaped viscosity profile obtained through rheological measurements, we propose an optimized ionomer concentration of 0.6 mmolSO3-gC -1 for the SSC ionomer-based slurry and 0.9 mmolSO3-gC -1 for the LSC ionomer-based slurry. To characterize the microstructure of the as-prepared electrode, we employed scanning electron microscopy (SEM) and confocal microscopy, with a focus on particle agglomeration and the homogeneous ionomer distribution. Finally, the electrochemical analysis was performed on the fabricated membrane electrode assembly (MEA) in a 25 cm² single cell for a cathode Pt loading of 0.1 mgPtcm⁻². As a result, the optimized SSC ionomer-based electrode exhibited low local O₂ transport resistance and high Pt utilization, leading to over a 40 % enhancement in fuel cell performance at 0.6 V with a high proton accessibility value above 0.85 when compared to the optimized LSC ionomer-based electrode. This study highlights the crucial importance of understanding the effects of ionomer sidechain length on electrode slurry interactions and microstructure and establishes a significant correlation between them. Our findings contribute to the ongoing development of high-performance PEFCs, offering valuable insights into optimizing electrode composition and structure for enhanced fuel cell performance.(1) Ahn, C.-Y.; Park, J. E.; Kim, S.; Kim, O.-H.; Hwang, W.; Her, M.; Kang, S. Y.; Park, S.; Kwon, O. J.; Park, H. S. Differences in the electrochemical performance of Pt-based catalysts used for polymer electrolyte membrane fuel cells in liquid half-and full-cells. Chemical Reviews 2021, 121 (24), 15075-15140.(2)Kodama, K.; Motobayashi, K.; Shinohara, A.; Hasegawa, N.; Kudo, K.; Jinnouchi, R.; Osawa, M.; Morimoto, Y. Effect of the side-chain structure of perfluoro-sulfonic acid ionomers on the oxygen reduction reaction on the surface of Pt. ACS Catalysis 2018, 8 (1), 694-700. (2) Garsany, Y.; Atkinson, R. W.; Sassin, M. B.; Hjelm, R. M.; (3) Gould, B. D.; Swider-Lyons, K. E. Improving PEMFC Performance Using Short-Side-Chain Low-Equivalent-Weight PFSA Ionomer in the Cathode Catalyst Layer. Journal of The Electrochemical Society 2018, 165 (5), F381-F391.(4) Ramaswamy, N.; Kumaraguru, S.; Koestner, R.; Fuller, T.; Gu, W.; Kariuki, N.; Myers, D.; Dudenas, P. J.; Kusoglu, A. Editors’ Choice—Ionomer Side Chain Length and Equivalent Weight Impact on High Current Density Transport Resistances in PEMFC Cathodes. Journal of The Electrochemical Society 2021, 168 (2), 024518.(5) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chemical Reviews 2004, 104 (10), 4535-4586. DOI: 10.1021/cr0207123.

Thin Membranes Using PFSA-Vinylon Intermediate Layer for PEM Fuel Cells

Fuel cells are attracting attention as one of the key energy devices for achieving carbon neutrality in 2050. Currently, fuel cells are being improved for versatile mobile applications. Among the components of fuel cell devices, polymer electrolyte membranes for proton exchange, in particular, are required to have high proton conductivity at high temperature, high and low humidification, and thin membranes. Proton-exchange fuel cells utilize perfluorosulfonic acid (PFSA) ionomers and membranes from Chemours and 3M and Solvay [1]. Nafion membranes with a thickness of 178-25 μm and Gore select membranes (with reinforcement) with a thickness of 20-5 μm have been commercialized [2,3]. On the other hand, thinner polymer electrolyte membranes can reduce the ohmic voltage drop in fuel cells to improve performance and lower costs. However, it is difficult to obtain thin membranes of less than 20 μm without reinforcement using PFSA ionomers.In this report, a thin PFSA electrolyte membrane (10 μm) without reinforcement was successfully developed by introducing a PFSA-vinilon composite layer as an intermediate layer of the PFSA membrane. The fuel cell evaluation revealed that it has a high current density.The ionomers used were 5% Nafion solution (DE520 CS type, EW=1100) and 6% Aquivion solution (D83-06A, EW=830). PVA (polyvinyl alcohol) was synthesized from polyvinyl acetate (PVAc, Mw=100,000). The laminated membranes (10 μm thickness) was obtained by preparing a 2 μm PFSA layer, followed by a 6 μm PFSA-PVA composite layer, then a 2 μm PFSA layer was coated. Finally, laminated membranes with PFSA-Vinylon composite layers were obtained by Formalization reaction the PFSA-PVA composite layers.FTIR evaluation of the PFSA/ PFSA-Viynylon/ PFSA multilayers confirmed the presence of an intermediate layer by observing Vinylon-derived peaks. Conductivities of about 4.5 mS/cm and 2.5 mS/cm in the thickness direction were obtained at 80°C, 90%RH and 20%RH, respectively. On the other hand, I-V evaluation was performed at 80°C, 100%RH and 35%RH using hydrogen, oxygen and air. Nafion 212 was used as a comparison membrane. The I-V characteristics of the laminated membranes showed higher current density than that of the Nafion membrane. Maximum current densities of 3 A/cm2 at 0.6 V and 3 A/cm2 at 0.4 V were obtained at 80°C, 100% RH and 35% RH with hydrogen and oxygen, respectively. Regarding the comparison of PFSA ionomers, Aquivion showed higher performance at higher current densities than Nafion [4].

High Durability of Pemion® Proton Exchange Membranes in Cross-Pressure Accelerated Mechanical Stress Tests

Polymer electrolyte membrane fuel cells (PEMFC) are the dominant technology for hydrogen-powered fuel cell electric vehicles in clean transportation systems. To be suitable for commercialization and applicability in real-world use-cases, light and heavy-duty fuel cell vehicles require lifetimes of over 8,000 and 30,000 hours, respectively [1]. Hence, enhancing the durability of all fuel cell components, particularly the proton exchange membrane (PEM), is of great importance.Recently, fuel cell membranes based on hydrocarbon (HC) chemistries have become increasingly common in the literature [2]. Materials with polyaromatic backbones, tunable electrochemical properties, and low reactant permeability are increasingly seen as potential alternatives to incumbent perfluorosulfonic acid (PFSA) materials [2]. Additionally, as restrictions on the use of fluorinated materials in various industries continue to grow, the importance of HC chemistries will as well. Sulfo-phenylated polyphenylenes (sPPPs) are a particular class of HC materials that show promise [3]. However, the phase separation between hydrophilic and hydrophobic domains within sPPPs may not be as discrete as in PFSAs [3], requiring higher ion exchange capacity (IEC) values than PFSAs to achieve similar protonic conductivity. High IEC typically results in greater material hydrophilicity, which can render membranes dimensionally unstable in a fuel cell [4].State-of-the-art commercial PEMs are now manufactured as thin films (≤ 18 µm), offering small ohmic loss and therefore high fuel cell performance. To improve dimensional stability and eliminate the risks of electrical shorting, PEMs are commonly mechanically reinforced using a porous, inert, and non-ionic substrate, such as expanded polytetrafluoroethylene (ePTFE). In a work by Miyake et al. [5], sulfonated polyphenylene-based ionomer membranes with flexible polyethylene mechanical reinforcement were prepared and the results indicated promising mechanical properties and improved longevity in RH cycling tests. However, there is still a lack of data about the fatigue durability of HC membranes in the literature, and evaluating and comparing the mechanical durability of numerous PEMs with a mechanical reinforcement layer in a traditional wet-dry cycling accelerated stress test would be a time-consuming and thus costly process [6].In our previous work, [7] we combined constant pressure differential across an ePTFE-reinforced perfluorosulfonic acid (PFSA) ionomer membrane with in-situ RH cycles (ΔP-AMST) to simulate the actual mechanical stresses in the fuel cell environment, while accelerating the durability testing for reinforced PEMs. In this study, ΔP-AMST is used to benchmark the fatigue lifetime curves for Pemion® membranes and compare it with a commercial reinforced PFSA-based PEM. The test temperature was set to 90 °C, and dry and wet (90% RH) phases were 60s and 30s, respectively. The hardware and semi-MEA specifications used for the test are shown in Figure 1a-c. A semitransparent polycarbonate spacer plate with 20 mm thickness and a circular aperture (25.4 mm) in the middle was used to allow free expansion of the membrane (Figure 1d). The cathode side was pressurized to create a cross pressure between the two sides and intensify the stress on the membrane during the RH cycling process. The ultimate failure of the membrane is shown in Figure 1e. The radial stress at the center of the deformed membrane is estimated by Hencky’s solution, as reported in our previous work [7]. Figure 1f demonstrates the estimated nominal stress on the membranes as a function of their lifetimes in terms of RH cycles. According to the fatigue S-N curves, reinforced Pemion® membranes afford longer lifetime than incumbent PFSA materials if the membrane edges are well protected. In addition, the impact of IEC on the fatigue durability of Pemion® membranes is investigated and the results are evaluated against stress of dehydration and dynamic mechanical analysis measurements. Acknowledgements This project was financially supported by Natural Sciences and Engineering Research Council of Canada (NSERC), Ionomr Innovations Inc, Canada Foundation for Innovation (CFI), British Columbia Knowledge Development Fund (BCKDF), Western Economic Diversification Canada (WD), and Canada Research Chairs (CRC).

Present and Future Perspectives on Fuel Cell Membrane Chemical Stability

Durability is key to the value proposition of proton exchange membrane (PEM) fuel cells for transportation applications. Ultimately, fuel cell vehicles need to match the lifetimes of diesel powered trucks, and last longer than lithium batteries, for widespread adoption[1]. The burden of reaching these aggressive targets falls on all components of the system, stack, and membrane electrode assembly. Of these, membrane durability remains an important area of research, in particular the chemical stability of the ionomer. The recognition of the ‘simple unzipping’ of the perfluorosulfonic acid (PFSA) backbone by hydroxyl radicals (HO·)[2],[3] and the subsequent introduction of cerium radical scavengers[4],[5] mark significant advancements in fuel cell lifetimes. Yet, even with this technology, it is unclear if this class of membranes can meet the 25,000+ hours required for heavy duty applications without additional improvements. Furthermore, recent proposals to restrict usage of fluorochemicals in many applications[6] presents a risk to the rapidly growing PEM fuel cell and water electrolysis markets. This talk will outline recent work that looks beyond simple peroxide degradation of PFSA ionomers and highlight the role of other reaction mechanisms suspected in accelerated fuel cell durability testing. Analysis of effluent water for ionomer fragments during these tests provides a greater understanding of the degradation mechanisms and suggests additional pathways beyond back-bone unzipping. Not only are these fragments useful in inferring reaction mechanisms, but there is also evidence they adsorb to the platinum surface, ultimately adding to performance decay. In other words, increased membrane durability may also result in increased catalyst stability. Finally, chemical stability of hydrocarbon membranes will be considered in light of the advances over the last two decades in understanding PFSA stability. Similarities and differences will be discussed between these two classes of PEM membrane with suggestions on how the techniques used to understand PFSA stability could be applied to hydrocarbon ionomers and advance the understanding of chemical stability in these systems.[1] Cullen, D. A.; Neyerlin, K. C.; Ahluwalia, R. K.; Mukundan, R.; More, K. L.; Borup, R. L.; Weber, A. Z.; Myers, D. J.; Kusoglu, A. New Roads and Challenges for Fuel Cells in Heavy-Duty Transportation. Nat Energy 2021, 6 (5), 462–474. .[2] Curtin, D. E.; Lousenberg, R. D.; Henry, T. J.; Tangeman, P. C.; Tisack, M. E. Advanced Materials for Improved PEMFC Performance and Life. Journal of Power Sources 2004, 131 (1), 41–48.[3] Coms, F. D. The Chemistry of Fuel Cell Membrane Chemical Degradation. ECS Trans. 2008, 16 (2), 235.[4] Frey, M. H., Pierpont, D. M., Hamrock, S. J., US 8,367,267 and US 9,431,670[5] Coms, F. D.; Liu, H.; Owejan, J. E. Mitigation of Perfluorosulfonic Acid Membrane Chemical Degradation Using Cerium and Manganese Ions. ECS Trans. 2008, 16 (2), 1735.[6] European Chemicals Agency (ECHA) proposal submitted 7th February 2023

Investigating Nano-Sized Dispersity and High Crystallinity of Perfluoro-Sulfonic Acidionomer in Polymer Electrolyte Fuel Cell Electrodes Via Electrospray Deposition

Polymer electrolyte fuel cells (PEFCs) hold great promise for mitigating the worldwide climate crisis by tapping into the potential of producing clean energy from renewable sources. However, widespread commercial adoption of PEFCs requires addressing the challenges associated with PEFC performance and durability, particularly concerning electrode materials and design. The PEFC electrode is a crucial component of PEFC performance, with the ionomer playing a vital role as both an electrode binder and proton conductor. Nano-sized dispersity and high crystallinity of the ionomer are vital properties that are crucial for achieving enhanced performance and durability. However, existing methods for fabricating PEFC electrodes have been unable to attain these desirable ionomer characteristics. In this work, we propose a novel approach to fabricating PEFC electrode architectures with ionomers possessing nano-sized dispersity and high crystallinity using the electrospray deposition (ESD) technique. In the ESD process, a precursor solution is atomized into charged droplets, while evaporating the solvent, and results in the formation of advanced electrode with vertical architecture that exhibits high crystallinity of ionomer, in contrast to much lower crystallinity of those fabricated by conventional decal methods. The ionomer characteristics significantly impact the overall fuel cell performance and durability. Therefore, controlling the residual solvent content during the ESD process is a crucial aspect that is vital for achieving the desired ionomer characteristics. The residual solvent content is tuned by investigating the initial mixed solvent ratio, substrate temperature, and distance between the nozzle to substrate. By fine-tuning the residual solvent content and optimizing the deposition parameters, the ESD technique can overcome the limitations of traditional fabrication methods, resulting in PEFC electrodes that offer enhanced performance, increased durability, and improved energy efficiency. The electrospray deposition (ESD) technique provides precise control over the amount of residual solvent required for deposition, making it a scalable technique. This work shows that the residual solvent content of PEFC electrodes significantly impacts the ionomer crystallinity and the electrode architecture using a well-controlled and optimized electrospray technique, without the use of any additives or special ionomer. In summary, this work demonstrates a novel approach to fabricating PEFC electrode architectures with ionomers possessing nano-sized dispersity and high crystallinity using the electrospray deposition technique. This approach provides precise control over the residual solvent content, allowing for enhanced performance and increased durability of PEFC electrodes due to the high-crystallinity of the ionomer. These findings have important implications for the development of sustainable energy technologies and contribute to the ongoing efforts towards mitigating the worldwide climate crisis.

High-performance and durable membrane electrode assemblies based on CeO2 and ePTFE double-reinforcement strategy for proton exchange membrane fuel cells operating at elevated temperatures

The membrane electrode assemblies (MEAs) play a decisive role in operating conditions and lifetime of fuel cell. In this study, MEAs that could work stably at elevated temperatures were prepared. With this method, a short-side chain perfluorosulfonic acid ionomer coating with CeO2 nanoparticles and two layers of expanded polytetrafluoroethylene reinforcement, to be used as a proton exchange membrane, was directly deposited between the cathode and anode gas diffusion electrodes, forming a double-reinforced integrated MEA. The effects of CeO2 doping amount on performance and other electrochemical properties of MEAs were systematically studied. Under the coupling effect of the water-retention and non-proton-conducting properties of CeO2, the 0.5 wt% CeO2-doped MEA (MEA-S0.5) had optimal performance above 100 °C and under low relative humidity (RH) conditions. At 120 °C and 30% RH, MEA-S0.5 exhibited peak power density of 0.460 W cm−2 (H2/air operation, 100 kPa), which was about 1.28 times that of MEA without CeO2 doping (MEA-S0). Furthermore, CeO2 effectively enhanced the durability of the MEA. Compared with MEA-S0, MEA-S0.5 did not exhibit significant degradation in output power, H2 crossover, or proton conductivity in the 100-h open-circuit voltage-holding test. This work paves the way for developing high-performance, durable MEAs that are suitable for elevated temperatures.

Sub-20 nm ultrathin perfluorosulfonic acid-grafted graphene oxide composite membranes for vanadium redox flow batteries

Perfluorosulfonic acid (PFSA) membranes, such as Nafion, are widely used in vanadium redox flow batteries (VRFBs) because of their high proton conduction through the ion channels and excellent chemical stability. However, the high vanadium permeability of PFSA membranes induced by the randomly interconnected channels limits efficient cell operation. In this work, we demonstrate a sub-20 nm ultrathin PFSA-grafted graphene oxide/PFSA (PFSA-g-GO/PFSA) composite membrane with highly aligned ion channel morphology, which results in a 100-fold improvement in proton/vanadium ion selectivity compared to 25 μm-thick Nafion 211. In addition, the PFSA-g-GO nanosheets physically reinforce the ultrathin membrane while enabling the proton transport through the grafted PFSA ionomers, leading to stable cell operation at overall current densities from 40 to 200 mA cm−2. Especially, at a high current density of 200 mA cm−2, the PFSA-g-GO/PFSA composite membrane shows an energy efficiency (EE) of 78%, which is higher than that of Nafion 211, indicating its potential as an ion-selective membrane for VRFB.

Do Melamine and Melamine-Formaldehyde Surface Modifiers Improve Fuel Cell Activity and Durability?

Precious group metal-based catalysts based on Pt- and Pt-alloys are currently used as catalysts for the kinetically sluggish Oxygen Reduction Reaction (ORR) on the cathode of fuel cells. The activity of Pt or Pt-alloy catalysts towards ORR is severely compromised by the adsorption of spectator-species such as sulfonate anions from perfluorosulfonic acid (PFSA) ionomer or Pt-O(H)ad via water activation. Considering recent evidence pointing to the improvement in ORR activity via the use of melamine organic molecule or melamine-formaldehyde copolymeric additives as surface blocking agents for the prevention of sulfonate or Pt-OHad adsorption, we evaluated these additives in fuel cells. The catalysts were modified with either of these additives and characterized using ex situ and in situ methods to verify successful adsorption and eventual performance in fuel cells. These surface modifying agents were found to suppress Pt-OHad formation as well as suppress ORR activity. There was no evidence to point out that these additives improved ORR activity and H2/air polarization performance in a fuel cell MEA on supported, polycrystalline Pt catalyst.

Molecular Weight Dependent Ionomer Volume Expansion and Its Effect on the Performance of PEM Fuel Cell Electrodes

Within the past years, proton exchange membrane (PEM) fuel cells have become more and more attractive due to their potential for the transition towards an environmentally friendly hydrogen economy. Especially by reducing the platinum catalyst loading, significant system cost reductions could be achieved, but low Pt loadings still lead to unassigned voltage losses during operation.[1,2] In order to overcome those losses, well-designed catalyst layers with optimized ionomer content and distribution are indispensable. The used perfluorosulfonic acid (PFSA) ionomers for PEM fuel cells are commonly characterized by the chemical structure of the ionomer and the equivalent weight (EW), which strongly affects the proton conductivity.[3,4] Current research focusses on the synthesis of modified ionomers with higher oxygen permeability to reduce mass transport losses, but also investigates the influence of the ionomer’s molecular weight (MW).[1,5,6] A high MW minimizes the water uptake of the ionomer/membrane in liquid water, which is desired for the use in PEM fuel cells to reduce mechanical stress during relative humidity (RH) cycling.[5] The exact MW of an ionomer is not easily accessible, but can be estimated by measuring the melt flow index (MFI), whereas a high MW is commonly reflected by a lower MFI if the ionomer chemistry and EW are the same.Within this study, a 3M ionomer modified to have a high MFI of 156 g/10 min (measured at 265 °C with a mass of 5 kg) is investigated and compared to a 3M standard ionomer with a low MFI of 6 g/10 min. Both PFSA ionomers have the same chemical structure and a similar EW of ~800. The significantly higher MFI of the modified ionomer (i.e., comprising a lower MW) is clearly reflected by a ~2 times higher water uptake compared to the 3M standard ionomer (measured for solution-cast membranes in liquid water at 80 °C). To evaluate the impact of the MFI within the cathode electrode, 5 cm² active area membrane electrode assemblies (MEAs) with these two ionomers at various ionomer/carbon (I/C) mass ratios in the cathode electrode (based on a 40 wt.% Pt/Vulcan catalyst at loadings of 0.11 mgPt/cm²) are manufactured and electrochemically characterized. At sufficiently high I/C ratios and under humid operation (i.e., I/C = 0.90 and 90 % RH), differential flow H2/air polarization curves of the MEAs reveal that the higher liquid water uptake of the 3M high-MFI ionomer can be correlated with a lower cell voltage (see Figure 1). Contrary to the MEAs with the 3M low-MFI standard ionomer, the larger ionomer swelling in MEAs with the 3M high-MFI ionomer reduces the void volume fraction in the electrode structure during operation. Most pronounced at high current densities, where oxygen mass transport plays an important role, the MEAs containing the 3M high-MFI ionomer show a significant voltage loss of ~50 mV at 2.5 A/cm² compared to the MEAs with the 3M low-MFI ionomer. This study demonstrates that the effect of ionomer volume expansion upon changes in the MFI and thus the MW might not be crucial for the MEA performance when using electrodes with a low I/C ratio, but is critical when aiming for electrode compositions with a high ionomer content, in which case a low-MFI (i.e., a high MW) ionomer should be favorable.

Recycling of PFSA-Based Membranes and Their Re-Application in PEM Fuel Cells

The commercialization of PEM fuel cells offers a promising opportunity to overcome our dependence on fossil fuels and provides an attractive alternative for automotive and stationary power applications.[1] However, PEMFCs are still costly in production, with the main cost factor due to the fabrication of the perfluorosulfonic acid (PFSA)-based proton exchange membrane and the noble metal catalysts.[2,3] Recycling of these components could increase the economic attractiveness of PEMFCs. However, while the high resistance of the ionomer membrane to thermal and chemical degradation is favorable for their use in fuel cells, it also makes membrane recycling a major challenge.Here, we present a newly developed recycling strategy to recover PFSA-based membranes after operation by a hydrothermal treatment. Both short side chain (SSC) and long side chain (LSC) ionomers with EWs of 800 and 1000 were investigated. In contrast to previously published methods, we were able to recover the PFSA ionomer by using water instead of high-boiling organic solvents, such as DMF or DMSO.[4,5] The recycled membranes were obtained from the recycled PFSA ionomer solution by doctor-blading. They were investigated by in-situ Raman techniques for studying the water uptake and analyzed by IR and small-angle X-ray scattering (SAXS) techniques to get a more detailed insight into their morphology. In single cell tests at standard (80 °C) and enhanced (up to 130 °C) PEMFC operating temperatures, the recycled membranes showed an increased cell performance as compared to the pristine 3M-800EW membrane. Solid-state NMR measurements confirmed that no chemical changes in the membrane occurred due to the heat-treatment process.We assume that the observed performance boost can be attributed to an increased water uptake and retention. Furthermore, a change in the polymer matrix formation may be responsible for the observed behavior, which led to an expanded water filled channel structure, where water can accumulate to facilitate proton diffusion as H3O+ through the channels. To confirm this assumption, the morphology of the recycled membranes will be investigated by in situ SAXS measurements at different temperatures and degrees of hydration.[1] I. Staffell, D. Scamman, A. Velazquez Abad, P. Balcombe, P. E. Dodds, P. Ekins, N. Shah, K. R. Ward, Energy Environ. Sci. 2019, 12, 463–491.[2] J. Fan, M. Chen, Z. Zhao, Z. Zhang, S. Ye, S. Xu, H. Wang, H. Li, Nat. Energy 2021, 6, 475–486.[3] D. Papageorgopoulos, Fuel Cell R&D Overview. 2019 Annual Merit Review and Peer Evaluation Meeting, 2019.[4] Q. Xu, X. Chen, S. Wang, C. Guo, Y. Niu, R. Zuo, Z. Yang, Y. Zhou, C. Xu, Energies 2022, 15, 8717.[5] H. F. Xu, X. Wang, Z. G. Shao, I. M. Hsing, J. Appl. Electrochem. 2002, 32, 1337–1340.

Studying Crack Formation in Catalyst Layer Films for Proton Exchange Membrane Fuel Cells

The performance and durability of proton exchange membrane fuel cells (PEMFCs) can be affected by cracks formed in catalyst layers (CLs). While previous research has shown that CLs prepared with pre-commercial high oxygen permeability ionomer (HOPI) are more efficient and durable, they can lead to significantly more cracking features than CLs prepared with traditional perfluorosulfonic acid (PFSA) ionomer. Therefore, it is important to reduce cracking to further improve cathode performance, durability, and film quality. With this goal in mind, we analyzed ink drying and crack formation during the wet-film coating process via imaging with an optical microscope fixed above a film applicator. Transient quantitative crack growth studies were performed to evaluate the dynamic nature of the process. Alongside that, the crack density of post-coating films was quantified at higher resolutions using a compound microscope. Ink composition, Pt-based catalysts and relative humidity of drying environment were varied in order to understand which combinations resulted in fewer cracks.This material is based upon work supported by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office, Award Number DE-EE0008822.

Substitution of the Nafion® Ionomer with Hydroxypropyl Methylcellulose for Proton Exchange Membrane Water Electrolyzer

The use of green hydrogen as a clean energy carrier has become an essential method of storing renewable energy instead of fossil fuels because of environmental concerns[1,2]. Proton exchange membrane water electrolysis (PEMWE) is one of several methods of producing green hydrogen. A perfluorosulfonic acid ionomer called Nafion® was typically used as a binder in PEMWE for many years. It has excellent chemical stability and high ionic conductivity, but it is expensive and has negative environmental effects [3]. It is therefore necessary to investigate new binders for the anode of PEMWE.In this study, we present a new electrode manufacturing method using hydroxypropyl methylcellulose (HPMC) binder and demonstrate that HPMC binder can be used sufficiently instead of traditional Nafion® ionomer through single-cell evaluation under PEMWE conditions. Although it has the disadvantage of low proton conductivity, HPMC is cost-effective and eco-friendly. In the case of the PEMWE anode, because water moves into the cell, water can take over the role of hydrogen ion transport instead of the HPMC binder. By the electrode’s interfacial stability test, it was discovered that the HPMC electrode’s form was preserved for 24 hours without any peeling. Further, it can be seen that there is no significant difference in PEMWE performance even if an HPMC binder with relatively low ionic conductivity is used. Additionally, it was established that each electrode demonstrated a low degradation (less than 0.3 mV h-1) in the subsequent 72 h durability test. The proposed affordable new binder HPMC can aid the development of a new electrode and a cost-effective PEMWE system.

Aging gracefully? Investigating iridium oxide ink's impact on microstructure, catalyst/ionomer interface, and PEMWE performance

In this study, we conducted a thorough investigation of the impact of aging iridium oxide (IrO2) perfluorosulfonic acid ionomer ink for up to 14 days on the properties of the ink and the resulting catalyst layers. We examined ink properties, such as zeta potential, dynamic light scattering (DLS), density, surface tension, and rheology, as functions of ink aging time. To evaluate the microstructure and catalyst/ionomer interface, we employed transmission electron microscopy (TEM), X-ray scattering, and X-ray photoelectron spectroscopy (XPS) techniques. Furthermore, we assessed the effect of ink aging on the performance of proton exchange membrane water electrolyzers (PEMWEs). Our findings reveal that most ink properties remain stable for 14 days. The variations in PEMWE cell performance are minimal, and no clear trend is observed in relation to ink aging time. This study demonstrates that the effects of aging the inks for 14 days on ink properties, catalyst layer structure, catalyst/ionomer interface, and PEMWE performance are negligible, indicating a substantial time window after ink preparation without any significant changes in its properties. These insights provide crucial guidance for the commercial production and coating processes of ink, which is necessary for scaling up PEM technologies to meet future demand.

Viscosity mechanism of perfluorosulfonic acid-based materials and their application in proton exchange membrane fuel cells

Perfluorosulfonic acid (PFSA) ionomers and their composites are popular electrolyte materials for proton exchange membrane fuel cells (PEMFCs), which subject to the combined actions associated with humidity change, temperature cycling, and clamp force during long-time operation. Thus, their time-related mechanical behaviour can greatly affect their proton conductivity and material degradation. Because viscosity is a vital factor controlling the mechanical deformation in the entire life cycle of a material, it is important to accurately predict for guiding the structural designation and operation condition. Currently, most of the literature discusses the PFSA viscosity based on large-strain-rate experiments. Microscopic driving mechanisms and macroscopic responses to small strain rates that occur during PEMFC operation remain largely unclear. In this study, the characterises of PFSA viscosity were investigated in terms of molecular interactions and macroscopic rheological behaviour. A cross-scale model was developed to describe the shear thinning phenomenon to obtain a zero-shear viscosity, which captures the linear viscous behaviour for small strain rates. Subsequently, a modified generalised Maxwell model consisting of elastic-plastic elements and zero-shear viscosity-dependent viscous elements was proposed and calibrated by the tensile and stress relaxation experiments for Nafion 117 membranes. In addition, the proposed visco-elastoplastic model, after validated by small-strain rate experiments, was used to investigate the response to cyclic loading during start-up and shutdown, which provides insights on the mechanical damage due to the hydrothermal cycles.