Perfluorosulfonic Acid Research Articles

Recent Progress in Materials Design and Fabrication Techniques for Membrane Electrode Assembly in Proton Exchange Membrane Fuel Cells

Proton Exchange Membrane Fuel Cells (PEMFCs) are widely regarded as promising clean energy technologies due to their high energy conversion efficiency, low environmental impact, and versatile application potential in transportation, stationary power, and portable devices. Central to the operation and performance of PEMFCs are advancements in materials and manufacturing processes that directly influence their efficiency, durability, and scalability. This review provides a comprehensive overview of recent progress in these areas, emphasizing the critical role of membrane electrode assembly (MEA) technology and its constituent components, including catalyst layers, membranes, and gas diffusion layers (GDLs). The MEA, as the heart of PEMFCs, has seen significant innovations in its structure and manufacturing methodologies to ensure optimal performance and durability. At the material level, catalyst layer advancements, including the development of platinum-group metal catalysts and cost-effective non-precious alternatives, have focused on improving catalytic activity, durability, and mass transport. Similarly, the evolution of membranes, particularly advancements in perfluorosulfonic acid membranes and alternative hydrocarbon-based or composite materials, has addressed challenges related to proton conductivity, mechanical stability, and operation under harsh conditions such as low humidity or high temperature. Additionally, innovations in gas diffusion layers have optimized their porosity, hydrophobicity, and structural properties, ensuring efficient reactant and product transport within the cell. By examining these interrelated aspects of PEMFC development, this review aims to provide a holistic understanding of the state of the art in PEMFC materials and manufacturing technologies, offering insights for future research and the practical implementation of high-performance fuel cells.

Coupled cation–electron transfer at the Pt(111)/perfluoro-sulfonic acid ionomer interface and its impact on the oxygen reduction reaction kinetics

Coupled cation–electron transfer at the Pt(111)/perfluoro-sulfonic acid ionomer interface and its impact on the oxygen reduction reaction kinetics

Poly- and per-fluoroalkyl substances (PFAS) in the African environments: progress, challenges, and future perspectives.

Per- or poly-fluoroalkyl substances (PFAS) are a group of anthropogenic compounds that are used in a variety of industrial processes and consumer products with their ubiquitous presence in the environment recently gaining relevant attention. Progress and milestones on PFAS contamination within multiple environments from African continent are highlighted in this review. Identification and quantitation of PFAS within African environments is important to the public at large because of their toxicity and possible ecotoxicological risk. Two most studied classes of PFAS are perfluoro carboxylic acid (PFCA) (i.e., perfluorooctanoic acid (PFOA)) and perfluoro sulfonic acid (PFSA) (i.e., perfluoro sulfonic acid (PFOS)) with many more classes of PFAS been created by industry. Within the African continent, studies reported PFAS in water, sediments, soils, fish, dust, breastmilk, infant formulae, dust, atmosphere, marine species and wildlife. Southern Africa contributed more studies on the presence of PFAS in the environment with Central Africa contributing the least. Despite growing awareness of PFAS contamination in Africa, the number of studies, studied compounds, and concentration levels vary significantly across regions and matrices. While some countries in Southern and Western Africa have made progress in PFAS research, the overall disparity in research output highlights the urgency for increased attention, resources, and concerted efforts to comprehensively address PFAS contamination. This review also revealed PFAS contamination within freshwater environments, with non-existent data from marine water environments. Collaboration among scientists, policymakers, industry players as well as regional and international communities are essential to mitigate the impact of PFAS in the African environment.

A non-target evaluation of drinking water contaminants in pilot scale activated carbon and anion exchange resin treatments

This study evaluates the effectiveness of five types of Granular Activated Carbon (GAC) and one anion exchange resin in a pilot plant for treating groundwater for drinking water production, specifically targeting the removal of persistent compounds like PFAS. Using liquid chromatography and supercritical fluid chromatography coupled with high-resolution mass spectrometry, hundreds of features (i.e. peak at specific mass and retention time) were detected in the groundwater by non-target analysis. Initially, after treating <3200 bed volumes (BV), the GAC filter materials showed < 6 % breakthrough for all features from the groundwater, with decreasing efficiency down to 79 % breakthrough after seven month (69,000 treated BV for µGAC). Using resin as a lag filter after GAC did not improve the removal of compounds detected in positive electrospray ionization mode. However, it enhanced removal by up to 35% for compounds detected in negative electrospray ionization mode, indicating higher selectivity of resin for acidic compounds like PFAS. The shortest detected PFAS (PFBA and PFPeA) broke through completely for all GAC and the resin material except the proprietary blended GAC (at 15,700 treated BV), which had only 19% breakthrough for PFPeA. The so far rarely detected perfluoro(4-ethylcyclohexane)sulfonic acid (PFECHS) was well adsorbed by GAC coupled to resin and by the proprietary blended GAC. Pesticides were effectively removed by GACs, but not by the resin filter. Contaminants not previously detected in groundwater, 2,4,5-trichlorobenzenesulfonic acid (TCBS) and 2-amino-4-chloro-5-methylbenzenesulfonic acid (ACMBS), were effectively removed (>92 %), but high ACMBS concentrations (360 ng/L) in groundwater are of concern. The drinking water after the resin filter revealed 20 new contaminants, such as tributylamine derivatives and monobutyl phthalate, indicating resin filters contribution to drinking water contamination. Accelerated migration experiments of the resin revealed additional contaminants, such as NDBA and further phthalates, highlighting the need for continued monitoring and evaluation of resin materials in water treatment systems.

Assessing the Unique Degradation Mechanisms of Hydrocarbon-Based Membranes in Conventional MEA Design Using 4D in-Situ X-Ray Computed Tomography

The degradation of proton exchange membrane fuel cells (PEMFCs) stands as a critical determinant in evaluating the robustness of fuel cell systems, crucial for the commercial-scale transition to sustainable energy. For zero-emission vehicles, fuel cell stacks need to demonstrate lifespans exceeding 8,000 (light-duty vehicles) and 30,000 hours (heavy-duty vehicles)[1]. For achieving robust PEMFC systems at commercial scale, it is crucial to precisely understand the degradation pathways to effectively pinpoint and mitigate degradation issues. To achieve this, customized small-scale fuel cell fixtures are utilized for accelerated stress testing (AST) and X-ray computed tomography (XCT) in-situ and ex-situ visualization [2,3]. This approach enables comprehensive analysis of membrane electrode assembly (MEA) aging processes. Our group previously employed this approach to identify chemical, mechanical, and chemo-mechanical degradation mechanisms in commonly used perfluorosulfonic acid (PFSA) ionomer membranes[4,5]. In recent years, transitioning from PFSA membranes to hydrocarbon-based (HC) membranes in PEMFCs has gained significant attention. This shift eliminates fluorinated compounds typically found in PFSA membranes, aligning with sustainability goals, and reducing environmental impact. This transition signifies both technological advancement and innovation, driving forward the quest for efficient, cost-effective, and environmentally friendly energy solutions. Despite their promising characteristics, HC membranes are more susceptible to mechanical degradation than PFSA ones[6], presenting unique compatibility challenges due to their distinct chemical composition. Historically, PEMFC systems intentionally developed and optimized for PFSA may face performance issues and premature failure with HC membranes. This research therefore investigates the unique aspects of HC membrane degradation within a conventional fuel cell design with PFSA ionomer catalyst layers, using 4D in-situ XCT to understand chemo-mechanical degradation, enhancing fuel cell technology.Herein, ZEISS Xradia® 520 Versa micro XCT system was used to visualize samples. Repetitive scans were taken at room temperature in a dry state of the MEA. The first set of MEAs consisted of reinforced HC membranes (Pemion-PF1-HLF8-15-X) of 15 µm thickness, spray-coated with PFSA catalyst ink to form catalyst-coated membranes (CCMs), and assembled with gas diffusion layers (Freudenberg H14C15, 190 µm) featuring smooth and crack-free microporous layer. The AST protocol was custom-designed for chemo-mechanical membrane degradation and included open circuit voltage hold for chemical degradation and relative humidity cycling for mechanical degradation, which were applied consecutively. It has been demonstrated that MEAs undergo dimensional changes during wet-dry AST cycles due to alternating conditions of hydration and dehydration[5,7]. It is apparent from the degraded XCT image (Figures 1a-d) that cracks were initiated and propagated between the membrane and catalyst layers, with cracks prominently visible near agglomerated electrode particles. Differential swelling between the membrane and catalyst layer, compounded by varying rates of expansion or contraction, resulted in magnified mechanical stresses at the membrane-catalyst layer interfaces.The second set of cells consisted of the same reinforced HC membrane assembled with commercial gas diffusion electrodes (GDEs; 0.5 mg/cm² 60% Pt on Vulcan, Sigracet 22 BB, 215 µm). The existence of interfacial voids (Figure 1e) was observed in the pristine sample at both anode and cathode membrane-GDE interfaces, likely due to interfacial adhesion fatigue (IAF)[8]. Uneven compression and interfacial incompatibility during fuel cell assembly results in IAF. When these GDE based MEAs were subjected to wet-dry AST cycles, mechanical stress was induced at the membrane-GDE interfaces[9, 10]. The uneven mechanical stress can aggravate creep propagation in the membrane. Due to creep propagation, membrane thinning at the edges was evident from the XCT images, and resulted in electrode shorting, as shown in Figure 1(f and g). It is believed that constraints during creep failure hindered membrane's return to original thickness after swelling, causing permanent dimensional changes. The combined effects of cyclic dimensional changes and membrane thinning due to the creep mechanism contributed to performance degradation and reduced durability of GDE-based MEAs. Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada, Ionomr Innovations Inc., Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Pacific Economic Development Canada, and Canada Research Chairs. References C.S. Gittleman et al., Joule, 5 (2021) 1660–1677.Y. Singh et al., J Power Sources, 412 (2019) 224–237.D. Ramani et al., Electrochim Acta, 380 (2021) 138194.Y. Chen et al., J Power Sources, 520 (2022) 230674.Y. Chen et al., J Electrochem Soc, 170 (2023) 114526.S.H. Mirfarsi et al., Int J Hydrogen Energy, 47 (2022) 13460–13489.A. Sadeghi Alavijeh et al., J Power Sources, 427 (2019) 207–214.X. Huang et al., J Polym Sci B Polym Phys, 44 (2006) 2346–2357.V.A. Sethuraman et al., J Electrochem Soc, 155 (2008) B50.S.H. Mirfarsi et al., Int J Hydrogen Energy, 50 (2024) 1507–1522. Figure 1

Organic-Inorganic Composite Membrane Containing Low-Cost Functionalized Organosilane (FOSi) Polymer for Vanadium Redox Flow Batteries

Even though the perfluorinated polymer membranes show the promising performance in various type of energy device, they are suffered from their high cost, high permeability, and difficulty in synthesis. Composite membranes prepared by adding either organic polymer or inorganic materials can reduce the amount of perfluorosulfonic acid (PFSA) and its permeability without compromising conductivity.A new composite membrane containing low-cost and unique functionalized organosilane (FOSi) with various PFSA (long & short side chain PFSA) as a hybrid organic-inorganic composite membrane (OICM) in large-scale production has shown in this study. The OICM solution is fabricated by varying the amount of PFSA and FOSi using Roll to Roll (R2R) film casting machine. The OICM’s proton conductivity and performance are comparable to the Nafion membrane, owing to the additional hydrophilic functionalized group in the membrane. The OICM possesses high proton conductivity in the range of 0.10 to 0.135 S/cm and an ion exchange capacity in the range of 1.00 -1.45 meq/g, even though the water uptake has been lowered than the PFSA owing to better dispersion of FOSi. The prepared OICM has been used for vanadium redox flow battery (VRFB) and showed a low vanadium permeability and remarkable selectivity, which is a significant benefit. Consequently, the OICM performs comparably to Nafion in energy efficiency combined with coulombic and voltage efficiency. The unit cell with OICM has a normalized coulombic efficiency due to low permeability than PFSA and high voltage efficiency due to high proton conductivity.

How Do Hydrocarbon (HC)-Based Membrane Electrode Assemblies (MEAs) Compare with Perfluorosulfonic Acid (PFSA)-MEAs Above 80 °C?

The ongoing debate around the sustainability of per- and polyfluoroalkyl substances (PFASs) might restrict their use and the production in many sectors [1]. Particularly, in the field of proton exchange membrane fuel cells (PEMFCs), perfluorosulfonic acid (PFSA) ionomers, such as Nafion, represent the current state-of-the-art for membranes and ionomer in the catalyst layers (CLs). In fact, they provide an overall unmatched combination of properties such as ionic conductivity and stability [2,3]. In contrast, hydrocarbon (HC)-based polymers are an attractive alternative to PFSA-based materials, as they have been proven to be valid choices from a performance-standpoint and could potentially overcome the thermal stability limitations of PFSAs [4,5]. However, it has been reported that the conductivity of HC-based polymers is not surpassing that of PFSA-based polymers, especially in the CL, even upon optimization of the formulation [4,6]. Additionally, only a few studies report characterization of HC-MEAs above 80 °C, which is the targeted temperature range for some fuel cell applications [4,7].In this work we used perfluorosulfonic acid (PFSA)- and sulfonated polyphenylene-based (HC)-polymers as ionomer in the CL and membrane in a fuel cell run above 80 °C. By spray-coating HC-based CLs onto either PFSA- or HC-membranes, we separated the contribution of the membrane from those of the electrodes and we analyzed the behaviour of each component individually. The electrochemical tests were performed between 80 and 120 °C, in a wide range of humidity and pressure levels. They included polarization curves and electrochemical impedance spectroscopy (EIS) in H2/air, similarly to our previous publications [8]. Additionally, we performed cyclic voltammetry and EIS to characterize the activity and conductivity of the cathode CL. Finally, by using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), we analyzed cross-sections of sprayed samples before and after the test, to investigate possible changes in the structure.A SEM/EDX cross-section of a sample is shown in Figure 1. The MEA consisted of a PFSA-based membrane (with a mechanical reinforcement) and HC-based CLs. Results suggested that the membrane thickness is unchanged after spraying and that the catalyst layers had a quite compact structure and a thickness of about 9 and 6 μm, for respectively the CL on the left and the one on the right. The EDX elemental analysis confirmed that the fluoride signal was solely present in the membrane, where a PFSA-based polymer was employed, while no appreciable signal was detected in the electrodes given the fluoride-free polymer chosen as ionomer.By methodically analyzing the performance losses of different polymers in the CL and in the membrane, we contribute to directing the research where the greatest limitations are still present, above 80 °C.Figure 1 SEM/EDX cross-section of an in-house sprayed sample (left) and a zoom on the line scan with the corresponding EDX spectra for Pt, F and C (right). While the membrane is PFSA-based, the CL is HC-based.[1] European Chemical Agency, Per- and polyfluoroalkyl substances (PFAS), Regist. Restrict. Intentions until Outcome. (2023). https://echa.europa.eu/registry-of-restriction-intentions/-/dislist/details/0b0236e18663449b (accessed April 10, 2024).[2] Y. Prykhodko, K. Fatyeyeva, L. Hespel, S. Marais, Progress in hybrid composite Nafion®-based membranes for proton exchange fuel cell application, Chem. Eng. J. 409 (2021) 127329. https://doi.org/10.1016/j.cej.2020.127329.[3] D.A. Cullen, K.C. Neyerlin, R.K. Ahluwalia, R. Mukundan, K.L. More, R.L. Borup, A.Z. Weber, D.J. Myers, A. Kusoglu, New roads and challenges for fuel cells in heavy-duty transportation, Nat. Energy. 6 (2021) 462–474. https://doi.org/10.1038/s41560-021-00775-z.[4] H. Nguyen, F. Lombeck, C. Schwarz, P.A. Heizmann, M. Adamski, H.F. Lee, B. Britton, S. Holdcroft, S. Vierrath, M. Breitwieser, Hydrocarbon-based PemionTM proton exchange membrane fuel cells with state-of-the-art performance, Sustain. Energy Fuels. 5 (2021) 3687–3699. https://doi.org/10.1039/d1se00556a.[5] I. Innovations, Ionomr Innovations’ Pemion® hydrocarbon-based proton exchange membrane and polymer exceed industry durability targets, (n.d.). https://ionomr.com/wp-content/uploads/2023/01/Pemion-Durability-Data_News-Release-and-Technical-Backgrounder_For-Release-Jan19.pdf (accessed April 8, 2024).[6] A. Strong, B. Britton, D. Edwards, T.J. Peckham, H.-F. Lee, W.Y. Huang, S. Holdcroft, Alcohol-Soluble, Sulfonated Poly(arylene ether)s: Investigation of Hydrocarbon Ionomers for Proton Exchange Membrane Fuel Cell Catalyst Layers, J. Electrochem. Soc. 162 (2015) F513–F518. https://doi.org/10.1149/2.0251506jes.[7] Hydrogen and Fuel Cell Technologies Office Energy.gov, Fuel Cell 2016 Multi-Year Research, Development and Demonstration Plan, 2016. https://www.energy.gov/sites/default/files/2017/05/f34/fcto_myrdd_fuel_cells.pdf.[8] M. Butori, B. Eriksson, N. Nikolić, C. Lagergren, G. Lindbergh, R. Wreland Lindström, R.W. Lindström, The Effect of Oxygen Partial Pressure and Humidification in Proton Exchange Membrane Fuel Cells at Intermediate Temperature (80 - 120 ◦C), J. Power Sources. 563 (2023). https://doi.org/10.1016/j.jpowsour.2023.232803. Figure 1

A New Archetype: Opportunities of Pemion® Fully Hydrocarbon Ionomer and Composite Membranes in Proton-Exchange Membrane Fuel Cell Applications

Hydrogen fuel cells are a critical technology for the renewable energy transition, pairing with green hydrogen to represent the most economic and scalable power generation for many applications, in particular ‘heavy duty’ systems such as commercial trucking, aviation, shipping, remote operations such as mining, and the replacement of diesel generators. Perfluorosulfonic acid (PFSA) polymers as membrane and ionomer in the catalyst layer today represent the fundamental building blocks of proton-exchange membrane fuel cells (PEMFCs) and a broad range of electrochemical systems including PEM water electrolyzers (PEMWE). However, potential regulation and liabilities related to the production of environmentally mobile fluorinated materials represent a potential bottleneck to the scaling of these critical systems for the renewable energy economy. Acid-functionalized small molecules that are used as process aides and expelled as degradation products are particularly concerning and warrant direct or indirect replacement by non-fluorinated or ‘hydrocarbon’ chemistries. Sulfonated poly(arylene) ionomers exhibit exemplary stability and a the only presently commercialized material of this category, Pemion®, promises to enable to a long-sought step-change in system-level temperature hitherto limited by the physical properties of PFSAs as well as increase durability due to preferential chemical properties.This presentation highlights the similarities and major divergences of the properties of hydrocarbon PEM Pemion®, comparing the physical and electrochemical properties of an ePTFE-reinforced product PF1-HLF8-15-X with those of hydrocarbon-reinforced membranes of various chemistries and production methods, as well as comparing PFSA- and hydrocarbon-based ionomers. Preliminary sub-scale work determining the structure-property relationships within Pemion® ‘fully hydrocarbon’ systems preliminarily demonstrate industrially relevant performances (i.e. sub-scale peak power densities >1 W/cm²) even in ‘hot and dry’ conditions, with a substantial (>50%) reduction in measured hydrogen permeability. The potential for system impacts of higher operational temperatures, increased liquid permeation, non-annealable electrode structures, and reduced gas permeation will be discussed, and preliminary data including multi-day comparisons with PFSA-based systems at operational temperatures ≥110 °C presented.

Perpendicularly Stacked Array of PTFE Nanofibers As a Reinforcement for Highly Durable Composite Membrane in Proton Exchange Membrane Fuel Cells

The configuration of reinforced composite membrane (RCM), composed of porous polytetrafluoroethylene (PTFE) as a mechanical reinforcement and perfluorosulfonic acid (PFSA) as a proton conductive polymer has gained a large interest due to its promisingly high performance for polymer electrolyte membrane (PEM) fuel cells. However, the inaccessible polymeric nanocomposites in preparing RCMs are still faced with critical challenges associated with immiscible interactions between hydrophilic sulfonate groups in PFSA and the hydrophobic nanoporous PTFE matrix. Herein, we report a well-refined and facile fabrication strategy for producing a cross-aligned PTFE (CA-PTFE) framework. The electric-field guided electrospinning enables the creation of unique micron-scale, grid-type PTFE matrix, which is synthesized by annealing of electrospun conjugated polymers resulting in the removal of carrier polymer and the formation of continuous fibrious structure via fusion of PTFE particles. The CA-PTFE RCM embodying uniformly impregnated PFSA in a grid-type PTFE matrix, facilitates hydration of the membranes, with minimal swelling and efficient diffusion of protons through concentrated sulfonate groups. The CA-PTFE RCM adopted cell showed outstanding fuel cell currents during both low and high humidity operation, with a current density of 1.38 A cm−2 at 0.6 V and maximum power density of 0.85 W cm−2 under RH 100% condition. Furthermore, the CA-PTFE RCM was able to achieve a long-lasting single-cell operation with a significantly low hydrogen crossover (less than 5 mA cm−2 at 0.4 V) even after 21,000 wet/dry cycles, which surpasses the standard of membrane durability for transportation application. The rational design of fibrous PTFE reinforcements opens up new engineering opportunities for the future development of high-stability PEM fuel cells.

Non-Fluorinated Sulfonated Poly(phenylene sulfone)-Based Polymers As Membranes and Binders in Proton Exchange Membrane Fuel Cells

Sulfonated poly-phenylene-sulfone (sPPS) polymers [1,2] are emerging as a promising alternative to traditional fluorinated membranes in proton exchange membrane fuel cells (PEMFCs), offering advantages such as lower gas crossover and reduced production costs. In addition, accelerated stress tests at open circuit voltage (OCV) demonstrate that sPPS membranes exhibit superior stability compared to perfluorosulfonic acid (PFSA) membranes under extreme conditions, such as 90°C and 30% relative humidity (RH) at 50 kPa g [3], and 80°C at 100% RH and 150 kPa g . Moreover, Figure 1a highlights that sPPS membranes, when used with PFSA-bonded electrodes, slightly outperform conventional PFSA membranes in terms of performance as well as maximum power density under the tested conditions. Notably, the area-specific resistance (HFR) recorded during the polarization curve is significantly lower than that of its PFSA counterpart. This is attributed to the exceptional high conductivity of sPPS membranes and their ability to be manufactured with thicknesses < 10 µm.Our research extends their use in the catalyst layers of membrane electrode assemblies (MEAs). More specifically, we adopted two strategies to enhance the MEA performance: At the electrode architecture level, we varied the catalyst dispersion composition by adjusting the ionomer content and the nature of the solvent (see Figure 1b). At the electrochemical level, we optimized the break-in procedure. While no significant changes were observed with variations in solvent type and composition, modifications to the electrochemical break-in procedure significantly improved performance, as evidenced by an increase in the electrochemical active surface area. Figure 1c reveals that although cathodes bonded with sPPS experience initial kinetic losses at lower current densities, their performance gradually approaches that of PFSA-bonded electrodes at higher current densities. These initial kinetic losses are associated to interactions between platinum and sPPS, as identified through X-ray photoelectron spectroscopy (XPS).In conclusion, sPPS polymers hold significant potential as both membranes and binders, paving the way toward reducing dependency on fluorinated compounds for more sustainable PEMFC.

Mitigating Anodic Reactions in Water-in-Salt Electrolytes for Refractory Metal Electrodeposition

The use of water-in-salt electrolytes has been a transformational advancement in electrochemistry. Water-in-salt electrolytes rely on a super high concentration of salts in aqueous electrolytes to lower the activity of water. The water is occupied in the solvation of the salts, leaving minimal ‘free water’ in the system. In the context of electrodeposition, this results in a widened cathodic window as the limited quantity of protons suppresses the hydrogen evolution reaction (HER).Seldom discussed is the impact on anodic reactions that are also limited by the transport of water for the oxygen evolution reaction (OER). Lithium chloride (LiCl) has been used as a cost effective and readily available support salt in water-in-salt electrolytes. However, in this system OER competes with chlorine evolution. At low pH chlorine evolution is more thermodynamically and kinetically favorable. This reaction depletes the system of chlorine atoms required to solvate excess water and saturates the electrolyte with dissolved chlorine. As a result, the electrolyte has a limited lifetime.One solution to this problem is the use of proton exchange membranes to separate the anode from the cathode. The use of these membranes will be discussed focused on our interest in rhenium (Re) electrodeposition. Rhenium has traditionally been difficult to deposit by electrolysis from aqueous solutions due to the over potential for hydrogen evolution and complex electrochemical reactions. Further, many intermediate rhenium oxides catalyze HER, so maintaining a stable water-in-salt electrolyte is crucial.Herein, we present systematic studies investigating various proton exchange membranes in an H-cell confirmation on the electrodeposition of rhenium. Perfluorosulfonic acid and hydrocarbon polymer backbones are explored, both with sulfonic acid functional groups. As these membranes are semipermeable, membrane thickness is also explored. Both volume changes in the two H-cell components and pH are monitored as a function of time. The impact on the morphology and electrodeposition kinetics of rhenium will be presented as a function of membrane properties.

Regulating Assembly of Porous Electrode Catalyst Layers

Polymer-electrolyte fuel cells (PEFCs) demonstrate remarkable potential in replacing non-renewable energy resources in paving the way for a sustainable future. PEFCs have fairly high efficiencies and energy densities, making them feasible for applications such as transportation and energy storage. The most expensive units of the PEFCs are the catalyst layers, traditionally composed of platinum interspersed on a carbon support with a perfluorosulfonic acid (PFSA) which stabilizes the ink dispersions as well acts as a binder for the dried catalyst layers. Despite the great importance of optimizing the composition of the catalyst layer to improve the practicality of PEFCs, predicting the performance of the final structure of the fuel cell remains a challenge.In this talk, we outline an approach to predict key properties of the catalyst layers such as the porosity and particle sizes that compose it through mathematical modeling of the ink properties. We discuss a kinetics-based algorithm based on interacting particles that simultaneously solves for the size distributions of the particles and the pH of the ink, a parameter that can be used for verification. The results of the aggregation formulation will be fed into calculations to determine fundamental properties of the catalyst layer, such as porosity through a void fraction calculation. To verify the accuracy of this approach, we will compare against experimental data for varying concentrations and solvents. The insights gained from these considerations will enable efficient design of future catalyst layers.

Ionomer Impact on Oxygen Evolution Reaction Activity and Kinetics for Ir-Based Catalysts

Renewable hydrogen generation using the low-temperature proton-exchange membrane water electrolyzer (PEMWE) represents a crucial technology for achieving global zero-carbon emissions. Iridium oxide (IrOx) stands out as a primary platinum group metal catalyst for the anode in electrochemical water splitting due to a balance in activity and stability in acidic environments.1 However, the anode catalyst performance is still predominately hampered by the sluggish kinetics of the oxygen evolution reaction (OER) compared to the hydrogen evolution reaction (HER) at the Pt cathode.2 Therefore,understanding the factors influencing the OER kinetics of Ir-based catalysts is essential for developing strategies to achieve high OER activity in the PEMWE anode electrocatalysts. This presentation will delve into our investigation of how the perfluorosulfonic acid (PFSA) binder affects the anode catalyst OER behaviors, specifically the corresponding kinetics and activity of two commercial Ir-based OER catalysts, IrOx from Alfa Aesar and TiO2-decorated IrOx (IrTiOx) from Umicore, as a function of the PFSA ionomer-to-catalyst (I/C) ratio. We combined the cavity microelectrode (CME) technique, which offers a direct approach to study OER kinetics without the ionomer,3 with the rotating disc electrode (RDE) technique, which allows to assess the impact of PFSA on OER kinetics and activity at varied I/C ratios in an acidic electrolyte. The study provided valuable insights into ionomer dependent OER mechanisms, elucidating the intricate interactions among the ionomer, catalyst, and reaction intermediates.AcknowledgementsThis research is supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office under the auspices of the H2NEW Consortium. Argonne National Laboratory is managed by the U.S Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357.Reference S. M. Alia, M.-A. Ha, G. C. Anderson, C. Ngo, S. Pylypenko and R. E. Larsen, Journal of The Electrochemical Society, 2019, 166, F1243.E. Oakton, D. Lebedev, M. Povia, D. F. Abbott, E. Fabbri, A. Fedorov, M. Nachtegaal, C. Copéret and T. J. Schmidt, Acs Catalysis, 2017, 7, 2346-2352.J. Behnken, M. Yu, X. Deng, H. Tüysüz, C. Harms, A. Dyck and G. Wittstock, ChemElectroChem, 2019, 6, 3460-3467.

Protonic Vs. Gas Transport Resistance in Hydrocarbon-Based Catalyst Layers: A Trade-Off Analysis for PEM Fuel Cells Under Low Humidity Conditions

Recent developments in hydrocarbon-based proton exchange membrane (PEM) fuel cells have significantly narrowed the performance gap compared to state-of-the-art cells using perfluorosulfonic acid (PFSA) ionomers. However, at low relative humidity (RH), challenges persist in balancing sufficient proton conductivity and minimizing gas transport resistance in the electrodes. Understanding these limitations is essential to increase performance and enable future applications of hydrocarbon-based PEM fuel cells.This study addresses these challenges by systematically investigating both the gas transport resistance and the influence of protonic conductivity on the performance of sulfonated phenylated polyphenylene-based electrodes. Results reveal an increased protonic resistance stemming from reduced thin film conductivity and enhanced cathode tortuosity at ionomer-to-carbon (I/C) ratios lower than 0.4. Reducing the I/C ratio and operating at relative humidities approaching automotive conditions (e.g., below I/C = 0.4 and 65% RH) led to a significant drop in electrode protonic conductivity. Subsequently, a highly non-homogeneous current distribution within the cathode catalyst layer is formed, resulting in modeled and experimentally verified low platinum (Pt) utilization.At I/C ratios higher than or equal to 0.4, the electrode's protonic resistance approaches values of typical PFSA-based catalyst layers. Nevertheless, this leads to a notable decline in performance due to the rise in local and pore network-related gas transport resistance. Additionally, the local transport resistance was found to strongly positively correlate with the ion exchange capacity of the ionomer, complicating the trade-off between low gas transport resistance and high electrode conductivity. Reducing the catalyst layer thickness (e.g., by increasing the platinum-on-carbon ratio) offers an experimentally verified opportunity to increase overall hydrocarbon-based fuel cell performance by minimizing both gas transport resistance and protonic resistance in the electrode.The observed trends offer valuable insights for guiding future optimization in the development of novel HC ionomers.

(Invited) Stable Catalysts, Functionalized Supports and Fluorocarbon Additives for Fuel Cell Electrodes

Proton Exchange Membrane (PEM) fuel cell operating on hydrogen is an alternative for the diesel-powered internal combustion engines for medium- and heavy-duty vehicle applications due to their higher efficiency and zero-local greenhouse gases emission. To achieve this prospect, PEM fuel cells need to demonstrate improved durability over a much longer operational lifetime of ~30,000 hours (or an equivalent of a million miles of driving distance) and lower total cost of ownership (sum of capital and operational expenses) compared to the diesel-based incumbent. In this regard, the United States Department of Energy (DOE) has set an end-of-test platinum specific power density target of 2.5 kW/gPGM at a rated voltage of 0.7 V after running a 25,000-hour equivalent accelerated stress test (AST). Japanese New Energy & Industrial Development Organization (NEDO) has set an even more aggressive 2030 ultimate target of 5.3 kW/gPGM (Figure 1).Fuel cell cathode electrodes are composed of high surface area carbon black supported platinum or platinum-alloy nanoparticles for electrocatalytic Oxygen Reduction Reaction (ORR) dispersed along with perfluorosulfonic acid (PFSA) ionomer to serve both as proton conducting and binding agent. The continuous loss in power delivered by a fuel cell could be attributed to the i) loss in electrochemical surface area of Pt, ii) loss in kinetic activity of Pt/PtCo catalyst and iii) contamination effects of dissolved cobalt alloying element in the ionomer phase. Of this, the loss in electrochemical surface area is the single largest contributor to the fuel cell voltage degradation which is due to the coarsening of platinum nanoparticles either via migration-coalescence on the carbon support and/or via dissolution-reprecipitation (electrochemical Ostwald ripening).In this context, recent developments in fuel cell cathode materials from General Motors focusing on improving durability by modifying the platinum/carbon support interface and re-designing platinum/ionomer-water interface will be presented. These include the development of i) durable Pt catalysts via surface protection with ZrO2-x nanoclusters,[1] ii) sulfate functionalized carbon supports for alternate electrode structures,[2] and iii) small molecule fluorocarbon modified catalysts interfaces for improved durability. Acknowledgements: This work was partially supported by U.S Department of Energy (DOE), Hydrogen and Fuel Cell Technologies Office (HFTO) under grants DE-EE0008821 and DE-EE0010749.

Ionomer Adsorption Study during Electrochemical Reaction of Pt Electrode Surfaces Using Electrochemical Quartz Crystal Microbalance

Introduction: In polymer electrolyte fuel cells (PEFCs), the platinum (Pt) catalyst is covered by the solid polymer electrolyte ionomer. The ionomer used as an electrolyte at both the anode and cathode serves the function of transporting protons and dissolved oxygen to the surface of the Pt catalyst. Therefore, it is considered that the adsorption/desorption behavior of the ionomer during PEFC operation affects the catalytic activity of Pt. While perfluorosulfonic acid (PFSA) based ionomers are now widely used, previous studies have shown that the sulfonic acid groups on the side chains strongly adsorb to the Pt surface at thr cathode and suppress the oxygen reduction reaction (ORR) [1]. In this study, the relationship between the adsorption/desorption of several PFSA-based ionomers, such as NafionⓇ and AquivionⓇ, and the redox behavior of Pt electrodes was investigated by in situ monitoring with an electrochemical quartz crystal microbalance (EQCM), which is a method that can measure the potential-dependent, minute mass of 10-9 g (ng) changes on the electrode surface. Experiment: Electrochemical measurements were performed with a three-electrode cell, which was composed of Pt-QCM electrode as a working electrode, Pt wire as a control electrode, and Ag/AgCl as a reference electrode. Prior to the EQCM experiment, electrochemical cleaning of the electrode surface was carried out by using 50 mM H2SO4 solution. We used the aqueous dispersion solutions containing NafionⓇ and AquivionⓇ, and then the cyclic voltammograms (CVs) at scan rate of 50 mV s-1 were acquired at room temperature. Results and Discussion: In both aqueous dispersion solutions, we observed the mass changes during Pt oxide formation/removal and hydrogen adsorption/desorption reactions. NafionⓇ exhibits specific adsorption/desorption behavior during the Pt oxide formation/removal reaction process. At the potential approximately 0.5 V in the negative-going scan, cathodic current due to the removal of the surface Pt oxide flowed accompanied by mass increased and then decreased on Pt surface. This result suggested that OH adsorbed on the surface of the platinum oxide is reductively removed from the surface accompanied with NafionⓇ adsorption, and after that the surface Pt oxide is reductively removed accompanied with NafionⓇ desorption. On the other hand, AquivionⓇ showed adsorption/desorption behavior dependent on the reaction process of hydrogen adsorption/desorption on the Pt surface. At the potential range of 0 V or more negative, when hydrogen absorbed Pt surface in negative-going scan, the mass was greatly decreased and when hydrogen removed from surface in the positive-going scan, the mass increased again. Comparing the side chain structures of NafionⓇ and AquivionⓇ, the former has long side chain and one more ether group. It is considered that the difference in the length of these side chains corresponds to the difference in the adsorption/desorption activity on the Pt surface.

Fitting Scattering Profiles of Perfluorinated Sulfonic Acid Ionomer Dispersions

Proton exchange membrane fuel cells (PEMFC) provide a path to wider adoption of hydrogen energy systems. This is because the energy density afforded by hydrogen makes PEMFC a strong candidate for the medium- and heavy-duty transportation sectors, where high allowable costs can reduce the barrier to adoption. As such, the focus of PEMFC research is toward greater efficiency and durability to reduce lifetime vehicle costs. To this end, high oxygen permeability ionomers (HOPI) are of interest, as the material has already been shown to increase mass activity in the cathode catalyst layer due to reduced anion poisoning from shorter side chains, in addition to enabling high power density through reduced oxygen transport resistance. While the performance of electrodes cast from HOPI and HOPI blended with commercial perfluorosulfonic acid (PFSA) has been documented, questions remain regarding the behavior of the novel ionomer in the dispersions and inks from which catalyst layers are fabricated. The literature on conventional PFSA ionomers devotes much attention to the effects of solvent media on interactions of ionomer backbone and side chains to form aggregates, motivated by the possible relationship between aggregates in dispersion and the structure of the cast catalyst layer, and thereby cell performance. In the present work, we probe the characteristic sizes and interactions among ionomer aggregates in dispersions by small angle x-ray scattering (SAXS). We report properties of aggregates in both HOPI and a commercial PFSA ionomer over a series of ionomer concentrations and at different ratios of water and propanol in the solvent medium. From these dispersions, aggregate spacings are estimated by fitting to the interference peak in the scattering intensity profile, and sizes are determined by form factor fitting. Next, series of blended HOPI and PFSA ionomer are prepared and studied by SAXS to understand the morphology of ionomer blends in solution. We further evaluate ionomer blends with different durations of ultrasonication, which can provide insight into the effect of sonication on ionomer blend aggregates during ink preparation.

Load Cycling Durability Protocols for Predictive MEA Lifetime Modeling for Long-Life PEMFC Applications

As the large-scale commercialization of proton exchange membrane fuel cells (PEMFC) for numerous heavy-duty applications draws nearer, there is an increasing emphasis on the delivery of highly robust membrane electrode assemblies (MEAs) capable of achieving upwards of 25,000 hours of on-road durability (1). A particular area of MEA durability focus is that of the proton conducting perfluorosulfonic acid (PFSA) membrane which is subjected to varying degrees of oxidative stress during fuel cell operation. The development of highly durable membranes necessitates the implementation of accelerated testing protocols that can be used to determine the sensitivity of membrane lifetime to a number of operational parameters such as cell temperature and membrane humidification. The objective of sensitivity studies is to develop robust and predictive degradation models using state-of-the-art (SOA) MEAs under the accelerated operational space that can be subsequently applied to the milder operational space of a long-lifetime application. Traditionally, open circuit voltage (OCV) degradation studies have been employed to assess membrane durability. While OCV degradation experiments can be highly accelerating from a chemical degradation perspective, producing high fluoride release and membrane thinning rates, the operating conditions do not sufficiently emulate the dynamic chemical stresses generated under load nor do they provide RH cycling and concomitant mechanical stresses that accompanies load cycling (2). At the other end of the cell load continuum, constant current durability tests have consistently failed to produce high degradation rates, even in the presence of Fe accelerants.In response to the recognized shortcomings of OCV and related accelerated durability tests, we have developed a suite of flexible, single-cell, load-cycling durability tests that more closely emulate stresses experienced in variety of real-world, on-load applications. Test protocols enable the variation numerous operational parameters including temperature, inlet RH, high and low cycling current densities and the ratio of time spent at the high and low current densities. In-situ degradation rates are assessed by monitoring emitted fluoride concentrations at regular intervals. Cerium-mitigated (3), SOA PFSA membranes fabricated with PtCo/HSC cathode electrodes are quite robust even under the harshest load cycling conditions (110 °C/ 30% RH), producing fluoride release rate (FRR) values less than 1x10-8 gF/cm-2h-1. The creation of predictive chemical degradation models requires significant and quantifiable chemical degradation signals to occur across accelerated the reaction space continuum. Therefore, given the high stability of SOA membranes, the degradation rates are further accelerated by the addition of Fe2+ Fenton’s catalysts. The Figure below shows the FRR profiles obtained from a series of four MEAs, differing only in the amount of added Fe2+ prior to the durability test conducted at 95 °C/30% RH with load cycling between 0.02-1.5 A/cm². In addition to Fe sensitivity findings, we will present data regarding sensitivities of degradation rates with respect to temperature, inlet RH, high current density values and time ratios at high and low current densities.(1) D. A. Cullen, K. C. Neyerlin, R. K. Ahluwalia, R. Mukundan, K. L. More, R. L. Borup, A. Z. Weber, D. J. Myers, A. Kusoglu, Nature Energy, 6, 462–474 (2021)(2) C. S. Gittleman, F. D. Coms, Y.-H. Lai, In Polymer Electrolyte Fuel Cell Degradation, M. Matthew, K. E. Caglan, T. N. Veziroglu, Editors, p. 15, Academic Press, Boston (2012)(3) F. D. Coms, H. Liu, and J. E. Owejan, ECS Transactions, 16, 1735 (2008) Figure 1

Time-Resolved X-Ray Fluorescence Analysis of Cerium Ion Transport in Polymer Electrolyte Membranes Under Humidity Gradient

The durability of more than 50,000 hours is required for the widespread application of proton exchange membrane fuel cells [1]. Chemical degradation of perfluorosulfonic acid (PFSA) membrane electrolytes by radicals is one of the degradation factors [2]. To quench the radicals, cerium ions are incorporated into the membrane electrode assembly (MEA) as radical scavengers. However, cerium ions move within the plane of MEA due to humidity gradients [3]. The chemical degradation by the radicals occurs in the depleted areas of cerium ion. Understanding the in-plane transport mechanism of cerium ions under operational conditions, including quantifying parameters like diffusion coefficients, is crucial for developing MEAs with enhanced durability. This study introduces an operando measurement technique employing a custom-built cell to monitor the spatial distribution changes of cerium ions over time. This technique utilizes X-ray fluorescence spectroscopy (XRF) with synchrotron high-energy X-rays, allowing for the analysis of cerium ion distribution under operational conditions while controlling the in-plane humidity gradient. By analyzing the data, we explore the phenomena governing the in-plane transport of cerium ions, driven by humidity and concentration gradients.Two gas inlets were connected to each end plate with an X-ray transmission window to control two types of humidity environments on the left and right side with the in-plane direction of the MEA. The humidity at each gas line was monitored by using dew point hygrometers placed on gas outlets. For the catalyst-coated membranes (CCMs), a platinum catalyst layer was applied to a PFSA-based PEM with approximately 20% sulfonic acid substituted with cerium ions. Gas diffusion layers (GDLs) with microporous layer (MPL) were used. XRF measurements were performed at beamline BL37XU on SPring-8. The incident energy was 60 keV, and Ce-Kα fluorescence X-rays were counted. The beam size was 0.5×0.5 mm2. The cell temperature was maintained at 80°C, and different relative humidity (RH) in the in-plane direction were established using N2 gas (80% on left side and 50% on right side), and then changed to the entire RH 50% to observe cerium ion relaxation behavior.Figure 1 shows the Ce-Kα intensity change of the MEA under the condition of humidity difference with in-plane direction. The x-axis corresponds to the horizontal position of the MEA, with RH 80% on the left side and RH 50% on the right side. The y-axis corresponds to the time under the humidity gradient. The darker blue indicates weaker intensity of the Ce-Kα fluorescence X-ray and the red indicates stronger intensity. This suggests that cerium ions move from the high humidity side to the low humidity side with the humidity gradient as the driving force. The time scale for this transport of a few millimeters is shown to be about tens of minutes. Figure 2 compares the Ce-Kα intensity after the humidity gradient test and the relaxation in homogeneous RH 50% for 10 hours. After the humidity gradient test, cerium ion is concentrated on the right side closed to the center. The concentrated cerium ion is slightly diffused into the left part after the relaxation for 10 hours. However, the initial distribution state does not completely disappear, indicating that the driving force of the humidity gradient exceeds that of the concentration gradient at RH 50%. Reference [1] Y. Fujii, ECS Trans., 111(6), 21 (2023).[2] R. Borup, et al., Chem. Rev., 107(10), 3904 (2007).[3] Y.-H. Lai, et al., J. Electrochem. Soc., 165(6), F3217 (2018).[4] K. Morita, et al., ECS Trans., submitted. Acknowledgements: The authors would like to thank M. Toida and T. Tambo (Toyota Motor Corporation) for providing samples and for in-depth discussions. Figure 1

Cellulose-Based Crosslinked and Reinforced Sulfonated Poly(ether ether ketone) Membranes for Robust Proton Exchange Membranes in Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) are a promising clean energy solution due to their high efficiency and eco-friendly operation. Central to their function is the proton exchange membrane (PEM), typically composed of perfluorosulfonic acid (PFSA) ionomers like Nafion and Aquivion. However, challenges such as cost, gas crossover, and limited temperature range persist. To address these, researchers explore hydrocarbon (HC)-based polymers like sulfonated poly(ether ether ketone) (SPEEK), prized for cost-effectiveness and stability. However, HC-based polymers suffer from issues like unstable mechanical behavior and reduced proton conductivity under low humidity, necessitating further development for practical application.In this study, we propose two approaches to overcome these drawbacks of HC-based PEMs. Firstly, we prepare crosslinked SPEEK membranes with cellulose acetate to enhance proton conduction under low RH conditions and stabilize membrane properties. The crosslinked structure of SPEEK/cellulose membranes is investigated using small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy. Additionally, we incorporate cellulose-based nanofibers obtained by electrospinning as a reinforcing substrate, enhancing the interaction between the SPEEK matrix and cellulose nanofiber filler. This results in a dense membrane without voids, effectively preventing gas crossover and improving membrane robustness. These investigations using cellulose-containing SPEEK membranes lead to enhanced power density and durable operation of fuel cells, particularly at low RH conditions.