Chemical Membrane Degradation Research Articles

Effect of Contamination in the Carbon Catalyst Support on Chemical Degradation of Polymer Electrolyte Membranes in Fuel Cells

Chemical degradation of polymer electrolyte membranes (PEMs) is an important issue related to the durability of polymer electrolyte fuel cells (PEFCs). Although various studies have been conducted on the chemical degradation of PEMs, the relationship between cathode catalyst materials and chemical degradation of the PEM has not yet been fully understood. Here, we conduct accelerated chemical degradation tests of the PEM and investigate the effect of using cathode electrocatalysts with different carbon support materials. This aims to clarify the effect of the choice of carbon support on the chemical degradation of PEMs. It is confirmed that contaminants in the carbon support material can have a significant impact on chemical degradation of the PEM.

Insight into the evolution of membrane chemical degradation in proton exchange membrane fuel cells:From theoretical analysis to model developing

The chemical degradation of the proton exchange membrane (PEM) causes the changes in membrane properties and morphology, which directly affects the cell performance and membrane degradation. In this work, the changes of PEM during the chemical degradation are theoretically analyzed at both microscopic and mesoscopic levels. Based on the theoretical study, a bidirectionally coupled physical model of PEM fuel cell (PEMFC) performance and PEM chemical degradation is developed, which is used to investigate the evolutions of cell performance and chemical degradation. It can be found that the open circuit voltage first increases slightly and then decreases with time, and the ohmic loss increases rapidly under accelerated degradation conditions. The membrane chemical degradation is analyzed in terms of four aspects: the mass loss rate of the membrane, the region of pronounced degradation, the contribution of different degradation mechanisms and the proportion of ionomer species remaining. Furthermore, the effect of operating conditions on the degradation is examined. The degradation is most sensitive to voltage and has the fastest rate at high voltage, high temperature, high gas pressure and relative humidity of 60%. This work provides a deep insight into the PEM chemical degradation, which is instructive for the long-term prediction of PEMFC lifetime.

Dynamic modeling of chemical membrane degradation in polymer electrolyte membrane fuel cells: Effect of precipitated Pt particles

Chemical membrane degradation in polymer electrolyte membrane fuel cells operated under real conditions is often accompanied and affected by Pt catalyst degradation. This study investigates the effects of both artificially impregnated Pt particles and Pt migration on chemical membrane degradation are numerically investigated. For the first time, the impact of Pt migration is numerically analyzed. Membrane structural changes with coupled chemical membrane and Pt degradations are proposed. For Pt particles that are artificially impregnated in the membrane, yielding a Pt content of up to 50 mol% Pt, the membrane degrades at the fastest rate at moderate Pt contents (2 mol% Pt). The impregnated Pt particles promote the conversion of Fe3+ to Fe2+, which adversely impacts the membrane durability. The decrease in Pt content leads to reduction in both the Fe2+ conversion time and trend. In comparison, the naturally precipitated Pt particles in the membrane from the degraded Pt catalysts in the CL, have demonstrated a Pt content consistently lower than 2 mol% Pt. As a result, the chemical degradation rate of real fuel cells accelerates significantly with increasing Pt particle concentration. The chemical degradation rate increases with the gas solubility at the pore/electrolyte interface. With higher gas solubility, the peak location of chemical membrane degradation shifts from the anode side to the cathode side.

A new insight into the chemical degradation of proton exchange membranes in water electrolyzers

A new insight to interpret the PEM degradation process in both electrodes of a PEMWE.

Coupled Simulation of Chemical Membrane Degradation and Oxygen Crossover in PEM Water Electrolysis

The membrane of polymer electrolyte membrane water electrolysis (PEMWE), as one of the lifetime limiting components, does not only have an impact on performance, but also plays an important role for safety issues over time. Membrane thinning is a key phenomenon caused by chemical membrane degradation and directly affects the conductivity and gas crossover. Therefore, a chemical membrane degradation model based on Fenton equations [1,2] coupled with a gas crossover model [3] is presented in this work. The presence of Oxygen at cathode side is considered by oxygen crossover, which is strongly enforced by supersaturation effects. Specifically, this cathode-side oxygen is conducive to hydrogen peroxide formation. In interaction with metal ions, e.g. iron, from the feed water, which partially migrate through the membrane, hydroxyl radicals can be formed. These attack the polyfluorinated sulfonicacid (PFSA) membrane at different points, releasing sulfon acid head groups (SO3H-) and fluride ions (F-), which can be measured as sulfur release rates (SRR) and fluride release rates (FRR) in the effluent water, respectively. The degradation model provides concentration profiles for the relevant species involved in the process, which, e.g., influence the thickness and conductivity of the membrane via structure-function relations. In particular, the changed thickness again influences the crossover effect, which itself represents an input for the degradation model. Through this coupling, both models can be updated at specific times. Implications of the model-based analysis on the mechanistic understanding of chemical membrane degradation by radical attack in PEMWE are discussed. Furthermore, the model results can be verified and calibrated by experimental data in combination with adapted accelerated stress tests (AST) and also encourage their enhancement.

Comparative Study of Degradation Features in Proton Exchange Membrane Water Electrolysis: Irregular Renewable Energy Vs Regular Steady-State/Dynamic Loads

Green hydrogen production is currently on the rise, and it is reshaping the future of the energy sector. Numerous hydrogen roadmaps and projects are scaling up this technology in various target sectors, which necessitates cost reduction for green hydrogen implementation. Proton exchange membrane water electrolysis coupled with the renewable energy source (PEMWE-ReS) is a promising technology that offers notable advantages to decrease the electricity cost by using wind generation and solar photovoltaic system. However, to meet the growth of green hydrogen technology, accurate lifetime assessment and subsequent development of durable materials should be considered. In this regard, this presentation will discuss a comparative analysis of stability test results based on various load-ranges. In particular, to consider the PEMWE-ReS coupling, a realistic solar profile was simulated and applied for PEMWE cell measurement. The results were then compared to previous steady-state and dynamic loads to gain insight into the differences in degradation patterns at field and laboratory scale. For this purpose, the degradation features were systematically analyzed with electrochemical, microscopic, and spectrochemical analyses to see the dynamic of overpotential changes, membrane chemical degradation, catalyst dissolution/migration/diffusion, and Ti porous transport layer passivation. Notably, a realistic solar profile that simulates PEMWE-ReS conditions is featured by the increased fluoride release rate due to cycling to low load, and Pt dissolution/diffusion to the anode. Given the degradation features of the realistic solar operation, it was determined that a combined pattern of specific steady-state and dynamic cycling loads can induce the degradation features of solar operation the most. The findings can provide useful insights for establishing accelerated stress test to diagnose system durability and for setting design guidance for durable materials.

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

Suppression of Chemical Degradation By Gas Barrier Polymer Electrolyte Membranes

[Introduction]Polymer electrolyte fuel cell (PEFC) is one of the promising technologies for decarbonization. Especially, PEFC is suitable to power heavy-duty vehicles (HDV) because of its high efficiency and high fuel capacity. However, there are some serious technical challenges for PEFC application for HDV such as the durability. When we focus on the chemical degradation of polymer electrolyte membranes (PEMs), it happens due to attack by radical species. One of the radical formation mechanisms is the reaction at the anode side by the penetrated oxygen from cathode. Therefore, a promising mitigation strategy against chemical degradation is the suppression of oxygen permeability through the PEM.To verify this concept, we developed high gas barrier PEM. As a model gas barrier PEM, we made a sandwich type PEM with high oxygen barrier property. The sandwich PEM was prepared by depositing a thin interlayer consisting blended poly(vinyl alcohol) (PVA) as a gas barrier material and poly(vinyl sulfonic acid) (PVS) as a proton conductor in between two layers of Nafion 211 membranes. We evaluate chemical durability using the sandwich gas barrier PEM and discuss about the mechanism.[Experimental]To fabricate thin sandwich PEMs (15-20 µm), ethanol diluted Nafion dispersion solution (20 mg/ml) and PVA/PVS solution (10 mg/ml) were sprayed on a polytetrafluoroethylene (PTFE) sheet by a spray gun (Tamiya HG wide airbrush). The thickness of each layer was controlled by controlling the deposited weight. Then, membrane characterization procedures such as surface roughness, proton conductivity, and dimensional stability were performed. To evaluate fuel cell performance, the PEMs were mounted to a JARI cell with 1 cm2 active area. Chemically accelerated stress test was performed by open circuit voltage (OCV) holding test following NEDO protocol.[Results and Discussion]Several areal densities of PVA/PVS interlayer were successfully incorporated into Nafion membrane to make sandwich PEM. The sandwich PEM shows considerable fuel cell performance, and thin sandwich PEMs show higher performance than the 50 µm sandwich PEM, although it is lower than the thin sprayed Nafion. For example, the peak power density of 15 µm sandwich PEM was 0.44 W cm-2 compared to 0.30 W cm-2 and 0.50 W cm-2 for 50 µm sandwich PEM and 15 µm Nafion, respectively. Furthermore, the incorporation of interlayer to thin PEM can suppress the hydrogen crossover current density from 7.8 mA cm-2 to 5.5 mA cm-2 indicating superior gas barrier property. From the higher gas barrier property, it is predicted thin sandwich PEMs will have higher chemical durability than Nafion with similar thickness.

Suppression of PEFC Membrane Degradation By Using SnO2 As Electrocatalyst Support

Introduction Long-term durability of polymer electrolyte fuel cells (PEFCs) is required for various applications such as automobiles. It has been suggested that certain ions dissolved from electrocatalyst materials accelerate radical formation and subsequent chemical degradation of polymer electrolyte membrane (PEM) by the Fenton reaction [1]. Membrane degradation increases hydrogen and oxygen permeability and decreases proton conductivity and mechanical strength [2]. These phenomena significantly affect the electrochemical performance and durability of PEFCs. Therefore, in case metal oxides are used as catalyst support, it is necessary to check the dissolution of metal ions and to evaluate their effects on membranes. Our research group has been studying Nb-doped SnO2 (Sn(Nb)O2), showing improved durability of the electrocatalysts [3]. Here in this study, we evaluate the effects of Sn(Nb)O2 on the chemical degradation of PEMs. Experimental A commercial Pt/C catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo, Japan) was used as the anode catalyst, while three types of cathode catalysts were applied and compared: 1) the commercial Pt/C; 2) Pt/MC where Pt was directly deposited on mesoporous carbon (MC); and 3) Pt/Sn(Nb)O2/MC where Pt was deposited on the Sn(Nb)O2 decorated on the MC. The durability test at open circuit voltage (OCV) was performed for 100 h. Before and after the durability test, cell performance, hydrogen crossover current density, and electrochemical surface area (ECSA) were characterized. The degree of membrane degradation was evaluated by measuring fluorine emission rate (FER) by ion chromatography, and membrane thickness and elemental distribution by energy-dispersive x-ray spectroscopy (EDS) coupled with field-emission scanning electron microscopy (FESEM). Results and discussion It was found that the use of Pt/Sn(Nb)O2/MC led to better durability in the OCV durability test. The decrease in OCV using Pt/Sn(Nb)O2/MC was smaller than that using Pt/C and Pt/MC, less than half of that Pt/C, as shown in Figure 1. The membrane thickness with Pt/Sn(Nb)O2/MC after the durability test was the same as that before the durability test, indicating that SnO2 actually suppressed the membrane degradation as shown in Figure 2. EDS elemental analysis confirmed no Sn ion dissolution from the cathode side as shown in Figure 3, also suggesting that SnO2 at the cathode suppressed the membrane degradation. In this presentation, we will quantitatively analyze the membrane degradation in using SnO2 and discuss possible mechanisms. Acknowledgment This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Analysis of Cerium Ion Transport in Anode Side Catalyst Layer for Improving Polymer Electrolyte Membrane Durability of Polymer Electrolyte Fuel Cells

This study deals with Polymer Electrolyte Fuel Cells (PEFC), which are used in automobiles and household fuel cells. Currently, there are two main challenges for the practical application of PEFCs: durability and cost reduction. The target value for durability is required to be 40,000 hours or more. However, the current durability is about 10,000 hours. Therefore, there is an urgent need to investigate the causes of deterioration and to take countermeasures. One of the causes of degradation is chemical degradation of the polymer electrolyte membrane. When the byproduct hydrogen peroxide encounters impurities such as iron ions, hydroxyl radicals are generated, which attack and degrade the polymer electrolyte membrane. To prevent this degradation, a substance that inactivates hydroxy radicals (radical scavengers) has been featured. Radical scavengers have been added to the polymer electrolyte membranes to prevent this degradation and are now being used in practical applications. One of the most useful radical scavengers is Ce ion. However, it has been observed experimentally that Ce ions in the PEM migrate to the cathode catalyst layer during cell operation. As a result, the Ce ions in the electrolyte membrane are reduced and the electrolyte membrane is degraded by radicals. Conversely, ceria added to the anode microporous layer ionizes and diffuses into the PEM. Because the ceria is ionized, it provides the Ce ions for the electrolyte membrane. However, how Ceion is transported in the catalyst layer and under what conditions it is transported have not been clarified. If Ce ion is not transported sufficiently from the microporous layer to the PEM, it becomes less effective as a radical scavenger and degradation progress. In addition, excessive Ce ion transport also interferes with proton transport in the PEM and reduces the output capacity of the PEFC. However, the cost of running experiments for over 40,000 hours is quite expensive, and the phenomena occurring inside the nanostructured electrolyte membrane are difficult to analyze experimentally. Therefore, simulation analysis is useful to understand the Ce ion transport mechanism in the catalyst layer for controlling Ce ion transport toward improvements of the durability of the electrolyte membrane while maintaining fuel cell performance.In this study, molecular dynamics (MD) simulations have been performed for analysis of structure of Ce ions within ionomers and dynamic properties of Ce ions in catalyst layer. Catalyst layer consists of carbon, polymer chains, Ce ions and solvent molecules, which are water molecules and hydronium ions. A Nafion chain, which has the chemical structure with equivalent weight =1114 was employed. The modified version of the DREIDING force field reported by Mabuchi et al. was used in this study. We have adopted the BOND potential of the COMPASS force field, which is based on the Anharmonic aSPC/Fw Water Model for water molecules and does not consider the Grotthuss mechanism to analyze the effect of cerium ions due to water convection. Water content λ, which indicates the ratio of solvent molecules to sulfonic groups in Nafion, was set at 5, 9 and 13. Twenty polymer chains, 140 hydronium ions and 20 Ce ions were placed in the simulation box with the two-dimensional periodic boundary conditions in the in-plane directions to ensure charge neutrality. The concentration of Ce ions in this case corresponds to 1.1 wt % of the Nafion mass in the membrane. This is a typical concentration for Ce-Nafion composite membranes. We applied to include ionic dipole interaction. For the other molecules, the general 12-6 LJ-type was used, and the arithmetic mixing rule was applied to calculate interaction between them and cerium ions, water oxygen and oxygen of hydronium. The annealing process was applied in order to equilibrate the system. After annealing, the production run was 10 ns using NVE ensemble. The time step was set at 1 fs, and the sampling interval was 5,000 steps. The temperature of the system was set at 323 K, 363 K and 403 K. We will determine where the Ce ions are transported in the catalyst layer by examining the density distribution and diffusion coefficient of Ce ions while changing the initial position of Ce ions. We will also report transport factors by analyzing the structural properties around Ce ions in the presentation. Figure 1

Simulation of Proton Exchange Membrane Durability Under Fuel Cell Vehicle Operation – a Fundamental Study

Proton exchange membrane fuel cells (PEMFCs) have been proven to be a promising candidate to replace combustion engines due to their zero-carbon emission and high power densities. Despite the recent success in PEMFC commercialization, a number of challenges such as high cost and difficulty in lifetime estimation still hinder their further development. PEMFC durability tests require a long time to complete; therefore, durability predicting models are increasingly important as a supporting tool for further development and implementation.Fuel cell membranes undergo a variety of dynamic conditions during regular operation such as varying temperature, humidity, current density, and cell potential. The cyclic variations of humidity and temperature (hygrothermal variations) during dynamic operation lead to swelling and contraction of the membrane. The fluctuating stress caused by the continuous expansion and contraction of the membrane when confined within the cell leads to mechanical membrane degradation. The recurring swelling and contraction of the membrane which stem from water content changes in the membrane cause fatigue and creep and ultimately the formation of pinholes, cracks, and tears [1-2].Chemical membrane degradation occurs when radicals such as hydroxyl and hydroperoxyl are formed and attack the membrane and is escalated by high operating cell potentials. Chemical degradation results in decay in the ionomer chemical structure, membrane thinning, increased gas crossover, and potential electrode shorting [3]. The outcomes of such degradation are aggravated in the presence of mechanical degradation [4]. Hence, the study of combined chemo-mechanical membrane degradation is crucial for overall fuel cell membrane durability [5].The objective of the present work is to establish a predictive membrane lifetime simulation tool designed to represent chemo-mechanical membrane degradation under various operating conditions for automotive fuel cell applications. To this end, a statistical model is developed based on the membrane fibrillar morphology presented by El Hannach et al. [6-7]. A network of fibre bundles is generated to represent the membrane structure where any bundle is assumed as the primary building block of the membrane [8-10]. Each bundle contains an aggregate of backbone chains with typical mechanical and chemical properties. Mechanical and chemical degradation rates are separately studied and coupled in the model. The thermally activated breaking rate of each fibre is calculated to evaluate the mechanical degradation rate at any given time. In order to calibrate the mechanical degradation rate in the model, the mechanical degradation of a reinforced membrane under cross-pressure accelerated mechanical stress tests ( p-AMST) [11] is investigated to obtain the membrane fatigue lifetime for a variety of cross pressures and temperatures. Then, the genetic algorithm is successfully applied to optimize the experimental parameters considering the least squares method. Furthermore, accelerated membrane durability tests (AMDT) carried out by Macauley et al. [12] are considered to calibrate the chemical degradation rate parameter in the model. Finally, the fully calibrated model is used to estimate the membrane lifetime under realistic fuel cell vehicle operating conditions (e.g. drive cycles).Fuel cell components experience different degradation mechanisms. In this regard, this modelling framework on the membrane lifetime can be used in conjunction with other methodologies on the cathode catalyst lifetime such as [13-14] to scrutinize the key factors and their impacts on fuel cell durability under similar operating conditions. Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Research Chairs, and Simon Fraser University Community Trust Endowment Fund.

Effect of Accelerated Stress Testing Conditions on Combined Chemical and Mechanical Membrane Durability in Fuel Cells

Understanding membrane degradation induced by combined chemical and mechanical stresses is critical to designing durable polymer electrolyte membrane fuel cells. Accelerated stress tests (ASTs) are usually designed and carried out to study membrane degradation and identify stresses leading to it. In this work, a customized small-scale fuel cell fixture designed for in situ X-ray computed tomography (XCT) imaging is utilized to study the impact of different AST conditions on combined chemical and mechanical membrane durability. The XCT imaging technique allows the acquisition of a tomographic dataset yielding an integrated 3D image stack, which in turn, is used to analyze and compare global membrane degradation mechanisms. It was identified that cell temperature and relative humidity (RH) strongly influence the chemical membrane degradation rate, whereas the mechanical degradation rate was promoted by RH cycles with high amplitude and short period, which were dynamically diagnosed through a single frequency electrochemical impedance spectroscopy technique developed to track membrane hydration. When applied consecutively, the high chemical and mechanical stress intensities produced a joint chemo-mechanical failure mode with distinct evidence of chemical (thinning) and mechanical (fatigue-fracture) contributions in a relatively short time. The proposed AST is thus recommended for chemo-mechanical membrane durability evaluation in fuel cells.

Proton-exchange polymer composite membrane of Nafion and microcrystalline cellulose for performance improvement of direct glycerol fuel cell

In this research, glycerol was used as fuel for a direct glycerol fuel cell (DGFC). Nafion (NF) was used to fabricate a proton exchange membrane and microcrystalline cellulose (MCC) was used as a filler for fabricating the NF composite membranes with different MCC loadings (0, 5, 10, or 15 wt%). The composite membranes were characterized using X-ray diffraction XRD for study of the crystallinity index (CI), SEM-EDS was applied to observe the atom dispersion of membranes, and FTIR-ATR was used to identify the chemical bonds of samples. The results showed that the 15 % MCC/NF membrane had a greater CI than the 5 % MCC/NF composite membrane, while the atom dispersion of the 5 % MCC/NF membrane was better than the others. The composite membrane properties were tested based on water uptake, solubility, ion exchange capacity, chemical degradation, 1 M and 2 M glycerol permeability, proton conductivity, selectivity, and fuel cell performance. The 5 wt% MCC loading increased the water uptake and proton conductivity and decreased the glycerol crossover compared to the other membranes. However, 15 wt% MCC had more durable chemical membrane degradation than the 5 wt% MCC membrane and the solubility test identified that the 5 %, 10 %, and 15 % MCC/NF composite membranes had 2.97, 3.08, and 4.92 times, respectively, that of the recast NF membrane. The highest selectivity (1.54 × 105 Sscm−1) of the membranes was recorded for the 5 % MCC/NF composite membrane at 50 °C. The DGFC performance with 1 M glycerol at 70 °C of the 5 % MCC/NF composite membrane had a maximum power density 18.07 mW/cm2 and a current density 0.139 mA/cm2.

Suppression of PEFC Membrane Degradation By Using SnO2 As Electrocatalyst Support

Chemical degradation of polymer electrolyte membranes (PEMs) is a significant issue related to the durability of polymer electrolyte fuel cells (PEFCs). Previous studies have reported that certain ions dissolved from e.g. electrocatalyst materials can accelerate the chemical degradation of PEMs by promoting radical formation via the Fenton reaction. In this study, the influence of using an SnO2 support within the cathode catalyst layers on chemical degradation of the PEMs is evaluated. Accelerated chemical degradation tests were conducted by holding the cell at open circuit voltage (OCV) for 100 h, and the change in thickness of the PEMs were recorded. The results suggest that the use of SnO2 as an alternative metal oxide support for cathode catalysts can effectively suppress chemical degradation in PEMs.

Elucidating mechanistic background of the origin and rates of peroxide formation in low temperature proton exchange fuel cells

Degradation of electrode-membrane assembly of the low-temperature hydrogen fuel cells represents one of the main obstacles in wider adoption of these clean and efficient electrochemical sources of electrical energy. Chemical degradation of proton exchange membrane is initiated by hydrogen peroxide formation, which forms in the fuel cell as a by-product to water in oxygen reduction reaction and decomposes to reactive radical species, damaging to the membrane chemical structure. Depending on the operating conditions of the fuel cell, the source of hydrogen peroxide can be either cathode, anode, or, as we argue in the paper, also the Pt particles in the membrane, which originate from the cathode catalyst dissolution, diffusion into the membrane and redeposition of Pt ions inside the membrane. In the paper we propose a mathematical model of intertwined physical processes in membrane and catalyst layer, aimed at unifying the description of hydrogen peroxide formation throughout entire membrane-electrode assembly at any fuel cell operating conditions. The model results, compared to experimental data, indicate that Pt particles inside the membrane can indeed be an important source of hydrogen peroxide in aged fuel cells. For a fresh fuel cell, numerical simulation using proposed model show that hydrogen peroxide can be formed at either cathode or anode, depending on the fuel cell operating condition, but with anode production being more prominent in standard fuel cell operating conditions.

Polyphenol synergistic cerium oxide surface engineering constructed core-shell nanostructures as antioxidants for durable and high-performance proton exchange membrane fuel cells

The chemical degradation of proton exchange membranes (PEMs) suffering from radical attack is a crucial issue in the development of proton exchange membrane fuel cells (PEMFCs), while incorporating cerium oxide (CeO2) nanoparticles (NPs) with regenerative redox properties to eliminate radicals could effectively alleviate degradation. However, the low stability and restricted activity of CeO2 in the PEMFC operating condition and the poor compatibility with PEM due to CeO2 agglomeration stimulate the urgency for structural modulation of CeO2. Achieving a balance of membrane performance and oxidation resistance through a rational surface modification strategy of CeO2 is important for expanding PEMFC applications. Herein, polyphenols surface functionalized CeO2 core-shell structure (CeO2@TP) were constructed via assembling oxidation-induced coupling tea polyphenols (TP) on CeO2 NPs. The TP oligomeric shell as a protective layer enhances the stability of CeO2, mitigating radical scavenging activity degradation and improving compatibility with PEMs. Physicochemical characterisation shows that the overall performance of the nanocomposite membrane is improved due to the interaction of CeO2@TP nanoparticles with the polymer matrix. Gratifyingly, the phenolic hydroxyl-rich reductive TP oligomeric shell accelerates the regeneration of Ce(IV) to Ce(III), increasing the proportion of Ce(III) and oxygen vacancies on the CeO2 surface, thus boosting antioxidation efficiency. As a result, the CeO2@TP-based PEMs exhibited an OCV decay rate of 0.22 mV h−1, a maximum power density of 1.06 W cm−2, an H2 crossover value of 2.18 mA cm−2, thickness retention (91.1%), and negligible Ce migration after 404 h of accelerated degradation testing. It is proven that the desired consequences of doping antioxidants in PFSA matrix can be intensified by surface modulation of the physicochemical properties of CeO2.

Design of proton exchange membranes with high durability for fuel cells: From the perspective of machine learning

Proton exchange membranes (PEMs)（） with high chemical durability have been generally researched in recent years. Free radical scavengers such as ceria can significantly suppress the chemical degradation of proton exchange membranes when added to proton exchange membranes. However, proton conductivity may decline in the presence of ceria. Thus, a trade-off between performance and durability in ceria-containing membranes emerges. To address this challenge, we developed a novel machine learning methodology called SPARK (Smart Prediction of Advanced Research on PEMs using Knowledge-based machine learning) to quickly predict the performance and durability of proton exchange membranes in fuel cell applications with high precision. Moreover, with the definition of the mixed index, the performance and durability were coupled, and the trade-off of performance and durability could be easily made. The SPARK methodology not only streamlines the design process for ceria-containing proton exchange membranes but also has broader implications for the development of other proton exchange membrane systems.

Synergistic Utilization of a CeO2-Anchored Bifunctionalized Metal–Organic Framework in a Polymer Nanocomposite toward Achieving High Power Density and Durability of PEMFC

The free radicals produced during the long-term operation of fuel cells can accelerate the chemical degradation of the proton exchange membrane (PEM). In the present work, the widely used free radical scavenger CeO2 was anchored on amino-functionalized metal–organic frameworks, and flexible alkyl sulfonic acid side chains were tethered onto the surface of inorganic nanoparticles. The prepared CeO2-anchored bifunctionalized metal–organic framework (CeO2-MNCS) was used as a promising synergistic filler to modify the Nafion matrix for addressing the detrimental effect of pristine CeO2 on the performance and durability of PEMs, including decreased proton conductivity and the migration problem of CeO2. The obtained hybrid membranes exhibited a high proton conductivity up to 0.239 S cm–1, enabling them to achieve a high power density of 591.47 mW cm–2 in a H2/air PEMFC single cell, almost 1.59 times higher than that of recast Nafion. After 115 h of acceleration testing, the OCV decay ratio of the hybrid membrane was decreased to 0.54 mV h–1, which was significantly lower than that of recast Nafion (2.18 mV h–1). The hybrid membrane still maintained high power density, low hydrogen crossover, and unabated catalytic activity of the catalyst layer after the durability test. This study provides an effective one-stone-two-birds strategy to develop highly durable PEMs by immobilizing CeO2 without sacrificing proton conductivity, allowing for the realization of improvement on the performance and sustained durability of PEMFC simultaneously.

Analysis of PEM Water Electrolyzer Failure Due to Induced Hydrogen Crossover in Catalyst-Coated PFSA Membranes

Polymer electrolyte membrane water electrolysis (PEMWE) is a leading candidate for the development of a sustainable hydrogen infrastructure. The heart of a PEMWE cell is represented by the membrane electrode assembly (MEA), which consists of a polymer electrolyte membrane (PEM) with catalyst layers (CLs), flow fields, and bipolar plates (BPPs). The weakest component of the system is the PEM, as it is prone to chemical and mechanical degradation. Membrane chemical degradation is associated with the formation of hydrogen peroxide due to the crossover of product gases (H2 and O2). In this paper, membrane failure due to H2 crossover was addressed in a membrane-focused accelerated stress test (AST). Asymmetric H2O and gas supply were applied to a test cell in OCV mode at two temperatures (60 °C and 80 °C). Electrochemical characterization at the beginning and at the end of testing revealed a 1.6-fold higher increase in the high-frequency resistance (HFR) at 80 °C. The hydrogen crossover was measured with a micro-GC, and the fluoride emission rate (FER) was monitored during the ASTs. A direct correlation between the FER and H2 crossover was identified, and accelerated membrane degradation at higher temperatures was detected.

Photometric Method to Determine Membrane Degradation in Polymer Electrolyte Fuel Cells

A new method for measuring membrane degradation in polymer electrolyte fuel cells (PEFCs) is proposed. The method is based on the detection of fluoride ions in effluent water from the cathode- and anode outlet of the PEFC using photometry (PM). The fluoride emission rate (FER) is an indicator of the membrane’s state of health (SoH) and can be used to measure the chemical membrane degradation. Commercial catalyst-coated membranes (CCMs) have been tested at 80 °C and 90 °C at 30% relative humidity (RH) to investigate the reliability of the developed method for fuel cell effluent samples. To verify the measurement, a mean-difference plot was created by measuring the same data with a fluorine selective electrode. The average difference was at ±0.13 nmol h−1 cm−2, which indicates good agreement between the two methods. These new findings imply that PM is a promising method for quick and simple assessment of membrane degradation in PEM technology.