XL Membrane Research Articles

Embrittlement of PFSA Reinforced Membrane Interfaces after Ex-Situ Chemical-Mechanical Aging

Perfluorosulfonic acid (PFSA) membrane degradation has been identified as one of the main factors responsible for the short lifetime of PEMFCs [1,2]. The aging mechanisms of these membranes are complex due to mechanical fatigue combined to chemical aggressions. Mechanical fatigue results from the wetting/drying cycles while the chemical aggressions are induced by radicals formed during fuel cell operation. Although it is commonly accepted that chemical and mechanical stresses can interact to accelerate membrane degradation [3], coupled studies are rarely conducted. Thus, an original experimental set-up to perform ex-situ aging tests combining membrane exposure to cyclic mechanical stresses to a free radical environment under conditions close to those observed during cell operation has been developed [4]. Ex-situ aging was performed on PFSA membranes reinforced (Nafion™ XL) with a cyclic compression oscillating between 0 - 10 MPa (0.1 Hz frequency) and a Fenton solution containing 1 ppm Fe2+ and 3 %vol H2O2. Such stresses revealed a significant morphological alteration with the appearance of “bubbles” close to the surface membrane (Figure 1, left). No chemical structure modification was detected within the three different layers. Nevertheless, SEM-EDX, FTIR and Raman microscopic analyses highlight delamination close to the reinforcement/PFSA interface (Figure 1, rigth). Additional peeling tests argued this failure point in the Nafion™ XL membrane. Figure 1

Proton-conductive aromatic membranes reinforced with poly(vinylidene fluoride) nanofibers for high-performance durable fuel cells.

Durability and ion conductivity are counteracting properties of proton-conductive membranes that are challenging to achieve simultaneously and determine the lifetime and performance of proton exchange membrane fuel cells. Here, we developed aromatic ionomers reinforced with nonwoven poly(vinylidene fluoride) (PVDF) nanofibers. Because of the right combination of an isotropic nonwoven PVDF with high porosity (78%) and partially fluorinated aromatic ionomers (SPP-TFP-4.0), the resulting composite membrane (SPP-TFP-4.0-PVDF) outperformed state-of-the-art chemically stabilized and physically reinforced perfluorinated Nafion XL membrane, in terms of fuel cell operation and in situ chemical stability at a high temperature (120°C) and low relative humidity (30%). The SPP-TFP-4.0-PVDF membrane exhibited excellent chemical stability and stable rupture energy at high and low RH levels, allowing it to be an alternative proton-conductive membrane to meet the U.S. Department of Energy target to be used in automobile fuel cells in 2025.

Mitigating cerium migration for perfluorosulfonic acid membranes using organic ligands

Improving the electrochemical stability of proton exchange membranes is a pressing priority for heavy-duty fuel cell vehicles. The lifetime of the most widely used perfluorosulfonic acid membranes is limited by reactive free radicals generated inside the system. Cerium has been found to reduce the chemical degradation of the membranes. However, cerium migration during fuel cell operation limits the chemical durability enhancement effect expected from the radical scavenging activity of cerium. Here we investigate a wide range of organic immobilizers for cerium, measuring their suitability concerning cerium retention, radical scavenging activity, and fuel cell performance. We report that partially fluorinated phosphonic acids enhance cerium retention up to 45 times and reduce fluoride emission rate by 38% compared to the commercial Nafion™ XL membrane pre-impregnated with cerium. The energetics of cerium-phosphonic acid complex systems by density functional theory calculations rationalizes effective cerium immobilization.

Insights into the Influence of Different Pre-Treatments on Physicochemical Properties of Nafion XL Membrane and Fuel Cell Performance.

Perfluorosulfonic acid (PFSA) polymers such as Nafion are the most frequently used Proton Exchange Membrane (PEM) in PEM fuel cells. Nafion XL is one of the most recently developed membranes designed to enhance performance by employing a mechanically reinforced layer in the architecture and a chemical stabilizer. The influence of the water and acid pre-treatment process on the physicochemical properties of Nafion XL membrane and Membrane Electrode Assembly (MEA) was investigated. The obtained results indicate that the pre-treated membranes have higher water uptake and dimensional swelling ratios, i.e., higher hydrophilicity, while the untreated membrane demonstrated a higher ionic exchange capacity. Furthermore, the conductivity of the acid pre-treated Nafion XL membrane was ~ 9.7% higher compared to the untreated membrane. Additionally, the maximum power densities obtained at 80 °C using acid pre-treatment were ~ 0.8 and 0.93 W/cm2 for re-cast Nafion and Nafion XL, respectively. However, the maximum generated powers for untreated membranes at the same condition were 0.36 and 0.66 W/cm2 for re-cast Nafion and Nafion XL, respectively. The overall results indicated that the PEM’s pre-treatment process is essential to enhance performance.

Impact of Sulfonated Poly(Ether Ether Ketone) Pretreatments on Proton Exchange Membrane Fuel Cells Performances and Durability

Many works have been devoted to the development of low cost ionomers as alternative to perfluorosulfonic acid membranes for Proton Exchange Membrane Fuel Cells (PEMFC) applications. Among them, sulfonated Poly(Ether Ether Ketone) (sPEEK) membranes. Before use, sPEEK membranes must be pretreated to ensure a complete protonic substitution and removal of residual reagents/solvent. This is generally ensured by soaking the membrane in an acid solution -at room temperature or higher, usually at 80°C1- followed by a rinsing step in water at room temperature. In addition to these pretreatment steps, it was reported2 that an hydrothermal treatment in water at high temperature for a few hours to a few days can improve the membrane nanostructure, water uptake and proton conductivity with expected positive effects on performances. Herein, we studied the impact of sPEEK membrane pretreatment on fuel cell performances and durability using two different batches of Fumapem E730 from Fumatech, acquired in 2019 and in 2020. Five different protocols were tested. The first consists only in membrane acidification and rinsing, while the four others also include a hydrothermal treatment in water at 80°C from 1 hour to 72 hours, and possibly an additional hydro alcoholic treatment. The 2020 batch membranes subjected to a hydro alcoholic pretreatment followed by a 72-hours hydrothermal pretreatment achieved the best performances and the lowest high frequency resistance (Rhf) when tested in a fuel cell. Their performances even exceeded those of a Nafion XL membrane. This could be explained by a better ionomer nano-structuration and therefore a better protonic conductivity. These performances are compared to those of the 2019 batch. In addition, the impact of pretreatment is investigated via Accelerated Stress Tests (AST) combining load (hence potential) and load-driven humidity cycling, and open-circuit voltage.

Development of WO3-Nafion Based Membranes for Enabling Higher Water Retention at Low Humidity and Enhancing PEMFC Performance at Intermediate Temperature Operation.

The proton exchange membrane (PEM) represents a pivotal material and a key challenge in developing fuel cell science and hydrogen technology. Nafion is the most promising polymer which will lead to its commercialisation. Hybrid membranes of nanosized tungsten oxide (WO3) and Nafion were fabricated, characterised, and tested in a single cell. The incorporation of 10 wt% WO3 resulted in 21% higher water uptake, 11.7% lower swelling ratio, almost doubling the hydration degree, and 13% higher mechanical stability of the hybrid membrane compared to the Nafion XL. Compared to commercial Nafion XL, the rNF–WO-10 hybrid membrane showed an 8.8% and 20% increase in current density of the cell at 0.4 V operating at 80 and 95 °C with 1.89 and 2.29 A/cm2, respectively. The maximum power density has increased by 9% (0.76 W/cm2) and 19.9% (0.922 W/cm2) when operating at the same temperatures compared to the commercial Nafion XL membrane. Generally, considering the particular structure of Nafion XL, our Nafion-based membrane with 10 wt% WO3 (rNF–WO-10) is a suitable PEM with a comparable performance at different operating conditions.

Parametric study on electrochemical reforming of glycerol for hydrogen production

Hydrogen production by electrochemical reforming of glycerol was investigated in this study. Within this scope, the performance of the system under different operating conditions was evaluated by parametric studies and optimum operating conditions were determined. The effects of membrane type, membrane pre-treatment procedure and temperature were investigated. System performance was examined also with long-term tests. The formation of hydrogen at the cathode was determined by analyzing the product gases by gas chromatography. Optimum condition for maximum hydrogen production was obtained with the Zn/Zn electrode pair in the presence of 0.4 M glycerol and 0.04 M H2SO4 at the anode side, 0.04 M H2SO4 at the cathode side and with pre-treated Nafion XL membrane. As the result of performance tests, room temperature and 2 V potential were found to be the most suitable operating conditions.

A chemical-mechanical ex-situ aging of perfluorosulfonic-acid membranes for fuel cells: Impact on the structure and the functional properties

Perfluorosulfonic acid (PFSA) membranes are key components for the operation of Polymer Electrolyte Membrane Fuel Cells (PEMFC). They undergo harsh conditions in terms of chemical environment and mechanical fatigue leading to irreversible degradations, up to their failure and to the Fuel Cell (FC) shutdown in worst cases. Through an ex-situ approach, this study aims to provide new insights about the impact of conjoint mechanical and chemical stresses on the structure and the functional properties of PFSA membranes. In this regard, various aging tests have been carried out using a custom-made device able to combine the application of a mechanical stress (sinusoidal profile) and the exposure to an environment containing free radicals ( i.e. Fenton's reagents). After about 20 h of accelerated aging test the analysis of the aged membranes permitted to demonstrate that the exposure to the combined stress significantly altered the morphology of Nafion™ XL membrane, with the appearance of bubbles close to the membrane surface and even delamination at the PFSA/reinforcement interface. On the other hand, and rather paradoxically, the water sorption and self-diffusion properties of the aged membranes as well as their cell performances remained satisfactory. • Nafion™ membranes were both exposed to chemical and mechanical stress simultaneously. • Coupling mechanical and chemical stresses induces important morphological changes. • Interfacial delamination of XL layers appeared after 20 h of conjoint stress. • Reinforcement layer in XL had little/lower effect on FER for severe solicitations.

Online Mass Spectrometric Investigation of Hydrogen Cross-over in PEM Water Electrolysis Cells

Polymer electrolyte membrane water electrolysers today mainly still use classic extrusion cast PFSA membranes of significant thickness such as Nafion® N117 or N115. In PEM fuel cells, much thinner solution cast membranes are used today in order to reduce ohmic resistance. In spite of the fact that reduced ohmic resistance would also benefit the goal to increase current density of operation for PEMEL so far it is not done for the sake of limiting hydrogen and oxygen cross-over. The necessity of this arises from the fact that hydrogen in oxygen concentrations exceeding certain limits can cause severe safety risk for the operation of PEMEL systems. An alternative approach to mitigate the issue is the introduction of recombination layers1, 2 which scavenge hydrogen and oxygen within the membrane and combine them back into water. In order to appropriately design such layer an online measurement of the hydrogen cross-over is required.In this contribution we will report on the use of online mass spectrometry to measure and quantify the hydrogen cross-over. The used set-up consists of a balticFuelCells cell holder with 4 cm2 cell with titanium flow fields as well as two pumps to supply water to anode and cathode compartment. Downstream of the cell on the anode side a water separator removes the remaining water from the product gas line. The capillary of a mass spectrometer with differential pump system is inserted into the exhaust gas line. Hydrogen concentration was measured by monitoring the m/z = 2 signal. For the calibration of the cell only a PTL was mounted into the cell holder. During the calibration measurement water was flown through the anode compartment while the anode side was fed with a 5% H2 in N2 mixture. The observed m/z = 2 signal was set for 5 vol% of hydrogen.For the tests, home-made CCMs were used with a Pt/C (JMFC) cathode layer and an IrO2 (Alfa Aesar) anode layer. A Freudenberg GDL was used as cathode PTL and a sintered Ti element as anode PTL. Tests were performed for CCM using Nafion® N115 or Nafion® XL electrolyte membrane. For some of the experiments a Nafion® overlayer was applied to the Nafion® XL to further determine the effect of Nafion® thickness. Also, a first test with a Pt black layer as recombination layer insert between the Nafion® XL membrane and the Nafion® overlayer was performed.Results reveal that the hydrogen cross-over for the Nafion® XL membrane is significantly higher than for the Nafion® N115 membrane as has to be expected. Already a rather thin Nafion® over cast to the Nafion® XL membrane can reduce the cross over to the Nafion® XL level without the need to achieve the full thickness of the Nafion® N115 membrane. The addition of the Pt layer is very effective to reduce H2 cross-over and will be further investigated. Of further importance is the strong spike of H2 cross over during current increase slopes which can be observed with this technique. These spikes bear the risk of the transient existence of flammable mixture. The observation is only possible due to the high time resolution of the MS. In the presentation further analysis will be presented.The presented work received financial support by the Fraunhofer-Gesellschaft as part of the Fraunhofer Cluster of Excellence Integrated Energy Systems (CINES), dimension technical components, grant number 007-601144. The support is gratefully acknowledged.References D. Bessarabov, E. K. and S. S.C., Editors, p. 17, Ceres Power Ltd, Horsham, RH135PX, United Kingdom, Electrochemical Society Inc (2019).S. Garbe, U. Babic, E. Nilsson, T. J. Schmidt and L. Gubler, Journal of the Electrochemical Society, 166(13), F873‐F875 (2019). Figure 1

High Surface Area Pt/C Electrocatalyst Modification with Ionic Liquids for Improved Ionic Conductivity in Polymer Electrolyte Fuel Cell Catalyst Layer

Oxygen reduction reaction (ORR), specifically in technological case of platinum (Pt) electrocatalyst, is one of the determining factors for commercialization and mass production of affordable fuel cell electric vehicles. ORR takes place at the triple phase boundary of the Pt electrocatalyst, the ionomer (Nafion) or water, and oxygen gas. In order to maximize the electrochemical surface area (ECSA) of the Pt nanoparticles, they are dispersed on high surface area (HSA) carbon supports and a large portion of them are buried inside the micro (< 2nm) and mesopores (< 50nm). Because of the size exclusion, ionomer cannot penetrate pores below 20 nm and water acts as the proton conducting media in these pores1. In addition, the dense polytetrafluoroethylene (PTFE) backbone blocks the oxygen transport. Therefore, the interface design principle in this work for improving catalyst layer ORR performance is to infiltrate micropores with ionic liquids (ILs) to facilitate sufficient proton and oxygen delivery to the Pt active sites. Ionic strength of ILs is within the range of 2.77 – 5.3×103 mol.m- 3 which is up to 7 orders of magnitude higher than that of water1. Higher ionic strength of electrolyte in micropores will ensure that electric double layers are thin and they do not exclude protons from entering the pores as fuel cells typically operate at higher potentials than the potential of zero charge of Pt. We implemented three imidazolium-derived ILs including 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim]+[NTf2]-), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim]+[NTf2]-), and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4dmim]+[NTf2]-) to modify 40 wt.% Pt on HSA Ketjenblack EC-300J catalyst (Pt/C). Our RDE prescreening showed improved proton conduction and accessibility of the electrocatalyst in all Pt/C-Nafion-IL catalyst layers compared to that of Pt/C-Nafion making them great candidates for catalyst modification2. After optimization of ink recipe including IL to carbon and water to IPA ratios, the MEAs with 5cm2 active area were fabricated via decal transfer of anode and cathode catalyst layers on Nafion XL membrane. Figure 1a represents the fuel cell polarization curves of the IL-modified catalysts compared to the baseline Pt/C. Pt/C-[C4mim]+[NTf2]- shows the best performance and highest kinetic activity compared to the baseline and other two IL-modified catalyst layers. The open circuit voltages (OCVs) were 0.942, 0.933, 0.948, and 0.929 V for baseline Pt/C, Pt/C-([C2mim]+[NTf2]-), Pt/C-([C4mim]+[NTf2]-), and Pt/C-([C4dmim]+[NTf2]-), respectively. Electrochemical impedance spectroscopy (EIS) data was used for measuring cathode catalyst layer ionic conductivity (Figure 1b), as Nyquist spectra represents a 45° line in the high frequency range where real and imaginary parts are equal, followed by a straight 90° line. A physical impedance model obtained from Obermaier et al.3 was fitted to EIS data, specifically at high frequencies to get the catalyst layer’s ionic conductivity. All the modified catalysts showed improved ionic conduction compared to the baseline Pt/C. Conductivity values for catalyst layers of Pt/C, Pt/C-([C2mim]+[NTf2]-), Pt/C-([C4mim]+[NTf2]-), and Pt/C-([C4dmim]+[NTf2]-) with approximately 10μm thickness were 1.93, 3.95, 2.34, 2.04 S/m, respectively. Our fuel cell results show that high conductivity of ([C4mim]+[NTf2]-) along with its optimum chemical structure is responsible for improved performance across kinetic and ohmic regions compared to the baseline Pt/C. References Avid, A. & Zenyuk, I. V. “Confinement effects for nano-electrocatalysts for oxygen reduction reaction”. Curr. Opin. Electrochem. 25, 100634 (2021). Avid, A. & Zenyuk, I. V. Ionic Liquid Modified Pt/C Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells. ECS Meet. Abstr. MA2020-02, 2155 (2020). Obermaier, M., Bandarenka, A. S. & Lohri-Tymozhynsky, C. A Comprehensive Physical Impedance Model of Polymer Electrolyte Fuel Cell Cathodes in Oxygen-free Atmosphere. Sci. Rep. 8, 4933 (2018). Figure 1

Temperature-dependent fatigue crack growth mechanisms of fuel cell membranes

The fatigue crack growth behaviors of two fuel membranes are investigated at different temperatures. Fatigue crack growth rate increases with temperature, and Nafion XL membrane is more sensitive to temperature. An Arrhenius-type expression for fatigue crack growth rate is proposed and good prediction results are obtained. For Nafion XL composite membrane, fatigue crack grows by forming micropores ahead of crack tip. These micropore are formed due to the combined action of hygrothermal environment and mechanical stress, which increase with temperature. Besides, the interior micropores and microstructure changes in reinforcement layer also account for higher temperature sensitivity of Nafion XL membrane.

Fatigue response of Nafion® XL membrane in biaxial tension: Temperature effects

Fatigue response of Nafion® XL membrane in biaxial tension: Temperature effects

Time-resolved monitoring of composite Nafion™ XL membrane degradation induced by Fenton's reaction

Although proton-exchange membrane fuel cells (PEMFC) are considered as a safe and clean energy technology, aging phenomena of the membrane-electrode assembly (MEA), and more particularly of the membrane, still restrict PEMFC lifetime. In this paper, ex-situ accelerated chemical degradation – via Fenton's reagents exposure – of reinforced Nafion™ XL membrane is performed in order to investigate the impact of chemical degradation on membrane structure and properties in comparison with unreinforced Nafion™ NR211 membrane. Results demonstrated that both XL and NR211 membranes undergo chemical decomposition after exposure to Fenton's reagents at different degradation rate. Emissions of two main degradation products released by the Nafion™ membranes were monitored as a function of exposure time, demonstrating the greater stability of the XL membrane compared to the NR211 membrane. However, the impact of chemical degradation on water sorption and water transport properties seems negligible for both membranes. This study aims at providing new insight on chemical degradation of composite Nafion™ XL membrane.

Influence of Flow Rate on Catalyst Layer Degradation in Polymer Electrolyte Fuel Cells

With the U.S. Department of Energy (DOE) 2020 durability target for transportation applications of 5000 hours in mind [1], several studies have been performed to understand the factors influencing load cycle durability of the cathode catalyst layer subjected to various operating conditions. In general, high upper potential limit (> 1 V vs RHE), high temperature (> 90 °C) and high relative humidity (RH) during potential cycling have been shown to exacerbate degradation [2]. It is known that Electrochemical surface area (ECSA) loss occurs by Pt dissolution attributed to particle size growth via modified Ostwald ripening, crystal migration, detachment from carbon support due to carbon corrosion, and precipitation in the membrane by chemical reduction due to hydrogen crossover [3].Accelerated stress tests (ASTs) are often used to mimic material component degradation similar to real automotive driving conditions. Different cycle profiles have been shown to cause varying rates of degradation depending on the operating conditions. The influence of flow rate of reactant gases on degradation, however, has received little attention. Stariha et al. [4], showed that a relatively high flow rate AST had the highest degradation acceleration factor and resulted in non-uniform decay rates when compared to low flow rate conditions of nitrogen gas (N2) at the cathode. However, this phenomenon has not been further studied in detail.The current study is aimed at elucidating the effect of flow rate on cathode catalyst layer degradation subjected to a standard DOE square-wave AST protocol. Single cell studies with 5-cm2 active area were performed using Nafion® XL membrane (Ion Power Inc.) and SGL-22 BB gas diffusion electrodes (GDEs) as MEA materials and 1-serpentine flow field. All the MEA samples used in the present study had nominally identical Pt loading and were subjected to a consistent conditioning procedure prior to AST. A DOE square-wave cycling protocol [4] was executed: H2/N2 flows at anode/cathode, voltage cycling between 0.6 V and 0.95 V, and 3 seconds hold at each voltage. 30,000 cycles (total test time of 50 hours) were performed at 100% RH, 80°C, and atmospheric pressure. Three different cathode N2 flow rates of 20 sccm, 40 sccm and 80 sccm were used during the AST with fixed H2 flow of 40 sccm at the anode. ECSA loss was tracked using cyclic voltammograms at several intervals during the AST. Complete electrochemical characterization at beginning and the end of life (BOL and EOL), along with ex-situ diagnostic techniques such as TEM and GIXRD, were used to obtain particle size distribution and spatial degradation profiles. The results indicate higher performance loss (at EOL) for 80 sccm flow of N2 at the cathode compared to 20 sccm and 40 sccm flow rates, most likely due to inhomogeneous Pt loss from inlet to the outlet due to variations in local flow conditions by fully saturated N2 flow at different flow rates.

Fatigue crack propagation behavior of fuel cell membranes after chemical degradation

The durability of fuel cells is significantly impaired by chemical and mechanical degradations of perfluorosulfonic-acid membranes. However, how the mechanical degradation, especially the fatigue crack propagation behavior, is impacted by chemical degradation is not clear. In this paper, the fatigue crack propagation behavior of Nafion 212 and Nafion XL composite membranes after chemical degradation are investigated. The fluoride release rates of Nafion 212 and Nafion XL membrane are 0.335 and 0.095 μmol cm−2 h−1, respectively. Likewise, the fatigue crack propagation rate of Nafion 212 membrane is also larger, which is attributed to two fatigue crack propagation mechanisms induced by chemical degradation, including bubble collapsing and pore interconnecting. By contrast, the fatigue crack propagation mechanism of Nafion XL membrane is not significantly changed where chemical degradation only accelerates crack growth in exterior surface layers. These findings provide new insights into the failure mechanisms under combined chemical and mechanical degradations.

Characterization of Ionomer Membranes with Confocal Raman Microscopy

Ionomer membranes are crucial components of electrochemical energy systems such as fuel cells, electrolyzers, or redox-flow batteries. They need to meet demanding requirements for high-performance and long-term stable devices, and a targeted optimization of the membranes requires exact knowledge of parameters such as membrane thickness, ion exchange capacity, or water uptake. We employ confocal Raman microscopy as a tool for in-depth characterization of membranes that are used in proton exchange membrane fuel cells and water electrolyzers. Raman spectroscopy as a vibrational spectroscopy technique is well suited to characterize ionomer membranes, and the combination of Raman spectroscopy with a confocal microscope additionally adds spatial resolution to this chemical analysis. Confocal Raman microscopy is a valuable tool to complement classical characterization of ionomer membranes that provides a plethora of chemical information about the investigated specimen on a single-digit micron scale.We present our latest results on application-relevant ionomer degradation and water management in membranes determined by confocal Raman microscopy. We found that a Nafion NR-211 membrane that was aged in an accelerated stress test in a fuel cell suffered from severe thinning and an anisotropic increase in its equivalent weight between the electrodes.1 Both are signs of chemical-mechanical degradation and were quantified with high spatial resolution using confocal Raman microscopy (Fig. 1A). Further, we used confocal Raman microscopy to characterize ionomer composite membranes. Composite membranes are a promising approach to optimize membrane properties by e.g. integrating mechanically stabilizing porous networks. We quantified the equivalent weight along the depth profile of a Nafion XL membrane, and also analyzed the influence of the reinforcement layer on water distribution within the hydrated membrane (Fig. 1B).2 Figure 1: Results of ionomer membrane characterization by Raman microscopy. A) Quantification of degradation of Nafion NR-211 in a fuel cell (open circuit voltage (OCV) hold as stress test). B) Through-plane Raman image of water distribution in a Nafion XL membrane. 1. T. Böhm, R. Moroni, M. Breitwieser, S. Thiele and S. Vierrath, J. Electrochem. Soc., 166(7), F3044-F3051 (2019).2. M. S. Mu’min, T. Böhm, R. Moroni, R. Zengerle, S. Thiele, S. Vierrath and M. Breitwieser, J. Memb. Sci., 585, 126–135 (2019). Figure 1

Effect of Pretreatment on Microstructure and Mechanical Properties of Nafion™ XL Composite Membrane

AbstractAs an ion‐conductive polymer electrolyte, Nafion™ XL composite membrane is used to meet the requirement for cost reduction, performance and durability improvement in polymer electrolyte fuel cells. While significant durability is observed for Nafion™ XL membrane, the mechanisms of improved mechanical durability, especially the impact induced by pretreatment, is still left unexplored. In this paper, ex situ mechanical tests of pretreated membranes are carried out, including tensile, fracture and fatigue crack propagation tests, to unravel the complex interplay between strength, fracture toughness and cyclic hygrothermal stress that controls the mechanical durability. Pretreatment increases strain‐hardening modulus at the expense of reduced ductility, while the break stress is not affected. Results from in situ SEM tensile tests reveal that pretreatment increases the orientation resistance during tension, leading to increased stress responses. In addition, the fracture resistance and fatigue crack propagation resistance increase after pretreatment, which is ascribed to increased membrane stiffness and reinforcement fiber reorientation, respectively. From a microscopic point of view, fibers along the cross‐sectional reinforcement layer collapse, corresponding to fiber reorientation at a smaller size scale and accounting for the above increased resistance. The findings reported here will provide more insights into the mechanical durability of composite ion‐conductive membranes.

Effect of Carbon Support on the Durability of d-PtCo Catalysts in PEM Fuel Cells

Catalyst support materials are an essential component of proton exchange membrane (PEM) fuel cells to decrease catalyst Pt loading. To be classified as a good catalyst support, a material must have high surface area, high electrical conductivity, be durable in harsh environments, and be economically feasible for large scale commercial production. In light of this, carbon is one such material that has widely been used in PEM fuel cells as a catalyst support. Various carbon materials of differing morphologies have been utilized as supports where high surface area is beneficial to greater Pt accessibility and higher performance, while graphitization and lower surface areas are beneficial to durability especially at high potentials. Here, we present a durability analysis of platinum cobalt catalyst supported on five structurally different carbon supports which are as follows: high surface area carbon TEC36E32 (29% Pt / 2.3% Co / 526.6 m2/g-Cat), high surface area graphitized carbon TEC36EA32 (29.3% Pt / 3.3% Co / 108.5 m2/g-Cat), Vulcan carbon TEC36V32 (29.1% Pt / 3.6% Co / 160 m2/g-Cat), graphitized Vulcan carbon TEC36VA32 (29.3% Pt / 3.4% Co / 70 m2/g-Cat), and acetylene black TEC36F32 (29.3% Pt / 3.4% Co / 523.6 m2/g-Cat). Each of the aforementioned carbon supported dealloyed-PtCo catalysts were spray coated onto one side of a DuPont® XL membrane at a loading of approximately 0.1 mgcat/cm2. The other side of the membrane, which served as the anode, was kept the same for all samples and spray coated in the same manner at the same loading with platinum supported on Vulcan carbon TEC10V20E (20% Pt). The membrane electrode assemblies (MEAs) were then incorporated into 5 cm2 differential cells with SGL Sigracet® Gas diffusion layers (GDLs). Subsequent to conditioning, catalyst performance was evaluated via the following characterizations: local oxygen transport resistance measurement, fuel cell polarization curves, sheet resistance measurement, impedance measurement in air and HelOx, mass activity measurement, and cyclic voltammetery measurements. DOE recommended square wave catalyst accelerated stress testing (AST) was utilized to degrade the catalysts with a recovery protocol and characterization performed after 15,000 and 30,000 cycles of the AST.1 Figure 1 illustrates the performance of TEC36V32 and TEC36E32 cathode catalyst MEAs where power densities of 0.643 W/cm2 and 0.716 W/cm2 were obtained respectively at 80oC, 100%RH and 150kPa pressure. In addition to this characterization, evolution of the resistances that arise from oxygen transport during the catalyst durability testing, both pressure dependent and pressure independent, will be presented for the various carbon supports. Further characterization of the catalysts after the testing will also be presented to provide insight on the influence of carbon support materials on catalyst performance with cell degradation. Acknowledgement: This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen).

Local hydration in ionomer composite membranes determined with confocal Raman microscopy

Water management in electrochemical energy applications like fuel cells has a crucial impact on performance, in particular on the ionic conduction of ionomer membranes. To strengthen the understanding of water management in such devices, we report a novel method for non-destructive measurements of the hydration of composite membranes based on confocal Raman microscopy. Composite membranes were produced by spray-coating of Nafion into a mesh of electrospun poly(vinylidene fluoride-co-hexafluoropropylene)/polyvinylpyrrolidone (PVDF-HFP/PVP) blend nanofibers. Hydration levels of several pure nanofiber meshes and nanofiber/Nafion composites were evaluated by linear least squares fitting of reference Raman spectra to hyperspectral images. We found that spectral contribution of water to nanofiber spectra depends on the PVDF-HFP/PVP ratio and is independent from fiber diameter. Further, we were able to reliably determine nanofiber polymer composition of single fibers based on Raman spectroscopy. Raman imaging of composite membranes was performed at ambient air and fully hydrated conditions to study the local hydration in PVDF-HFP/PVP/Nafion composites as well as in a Nafion XL membrane. 2D through-plane mappings revealed that the nanofiber hydration positively correlated with PVP content. In the Nafion XL membrane, the polytetrafluoroethylene-based reinforcement was verified as a hydrophobic layer sandwiched between Nafion ionomer, which showed a more than 10% reduced hydration compared to the outer Nafion layers. These results motivate the use of confocal Raman microscopy as a novel method to investigate the local water distribution in ionomer composite membranes that are widely used in electrochemical energy conversion.

30 μm thin hexamethyl-p-terphenyl poly(benzimidazolium) anion exchange membrane for vanadium redox flow batteries

We present the first results of an anion exchange ionomer membrane, hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI), in a vanadium redox flow battery. Anion exchange membranes exhibit superior vanadium crossover suppression compared to proton exchange membranes due to the Gibbs–Donnan effect. HMT-PMBI was benchmarked against a similarly thin Nafion XL membrane which allowed us to compare differences based solely on chemical properties of the ionomer materials. We report cycling data of 45 cycles at a current density of 150 mA/cm2 with excellent coulombic efficiency of >99.4%, energy efficiency of 80.6–74.2% and a low ohmic resistance of 0.219–0.255 Ω cm2. In addition, a three times lower self-discharge rate is obtained for the HMT-PMBI membrane compared to Nafion XL. HMT-PMBI is therefore a potential alternative for PFSA based ionomers in VRFB applications.