Durability Of Polymer Electrolyte Membrane Fuel Cells Research Articles

Defects in Membrane Electrode Assembly Environments: An Approach to Understanding Membrane Degradation Processes

The performance and long-term durability of polymer electrolyte membrane fuel cells (PEMFCs) are subject to different factors ––material properties, defects, assembly, maintenance and operational procedures.1 Among all the possible failure modes in a fuel cell, membrane electrode assembly (MEA) degradation is a determining factor for performance and lifetime.2 In the past few years, several studies have been developed to understand the degradation of membranes and electrocatalysts, and the corrosion of the carbon support; with a focus on the chemical aspects, rather than the physical features that could promote degradation. More studies are needed to evaluate physical factors that can compromise the MEA structure, especially the aspects that affect the membrane.3 The physical degradation of membranes can be effected by membrane intrinsic factors such as membrane creep, microcracks, and morphological changes.4 Furthermore, it is necessary to study the influence of membrane extrinsic factors ––such as roughness and imperfections at the interfaces between MEA components–– on the processes of membrane degradation. Whether morphological defects are generated before or during the assembly procedure or are intentionally introduced as strategy to improved water management, remains unclear if a defect can act as a precursor pit where membrane could start its degradation. In this sense, more efforts are needed to evaluate the influence of defects on potential and progressive membrane etching and pinholes formation.Our work is focused on understanding the influence of defects, specifically cracks present on gas diffusion layers (GDLs), on the performance and durability of PEMFCs. To control experimental variables associated with different production lots and suppliers, we designed a method to create artificial cracks on the mesoporous layers (MPLs) of commercial GDLs. This technique allows us to compare the cracked GDLs with the pristine GDL, ensuring our “network or pattern of cracks” as the only external variable introduced into the MEA environment. Also, we utilized other commercially available intentionally cracked GDLs as comparative elements. Accelerated stress test (ASTs) were performed and durability results were obtained from long-term (500 hours) fuel cell operation at elevated cell temperature (90 oC). Scanning electron microscopy (SEM) and laser profilometry, were used to characterize all the samples, before and after performance/durability tests. Mechanical and chemical durability of membranes are evaluated based on fluoride emission rates (FERs).This work enables a starting approach to understand the influence of cracks, existing on GDLs, on the membrane degradation processes. Results will give us insights for improving the membrane and GDLs technologies that we are developing in the Million Mile Fuel Cell Truck Consortium (M2FCT).Acknowledgement:This work was supported by the Hydrogen and Fuel Cell Technologies Office (HFTO), Office of Energy Efficiency and Renewable Energy, US Department of Energy (DOE) through the Million Mile Fuel Cell Truck (M2FCT) consortia, technology managers G. Kleen and D. Papageorgopoulos.

Robust Fuel Cell Electrodes: Pt Thin Film Catalysts for Anode Reversal Tolerance

In recent years, polymer electrolyte membrane fuel cells (PEMFCs) have garnered significant attention due to their remarkable attributes including high power density, energy efficiency, and zero-emission characteristics. This surge in interest has primarily been driven by the potential of PEMFC-powered fuel cell electric vehicles (FCEVs) to substantially reduce carbon dioxide emissions in the transportation sector, thereby propelling the advancement of the hydrogen economy on a global scale. However, the widespread commercialization of FCEVs hinges upon addressing key challenges surrounding the durability and resilience of fuel cell systems.One critical issue that has emerged is the phenomenon of hydrogen starvation in the anode, particularly during start-up/shutdown or cold start scenarios, which can lead to rapid and severe degradation of PEMFC performance and durability through cell reversal. This degradation manifests quickly, often within minutes, resulting in abrupt cell failure. Extensive prior research has been devoted to unraveling the mechanisms underlying degradation induced by hydrogen starvation and cell reversal in PEMFCs, with the goal of identifying effective mitigation strategies to minimize damage to membrane electrode assembly (MEA) components.In this study, we introduce an innovative materials approach aimed at mitigating cell damage by employing a thin film Pt anode catalyst supported on Nafion nanowires in a co-axial configuration, referred to as coaxial nanowire electrode (CANE). Leveraging the high roughness of Nafion nanowires, CANE eliminates the need for conventional carbon support and exhibits exceptional tolerance to reversal-induced stress. We present a comprehensive evaluation of CANE as a reversal-tolerant anode, comparing its performance to that of conventional supported catalysts (Pt/C). Furthermore, we explore the potential for further enhancing durability by integrating iridium with the CANE anode. Finally, we will delve into our efforts to evaluate the feasibility of implementing this concept in structures that can be manufactured at scale. This research contributes to the ongoing efforts to enhance the robustness and longevity of PEMFC systems, thereby advancing the prospects of FCEVs as a sustainable and viable solution for future transportation needs.AcknowledgementThis research was supported by the Hydrogen and Fuel Cell Technologies Office (HFTO), Office of Energy Efficiency and Renewable Energy, US Department of Energy (DOE) through the Million Mile Fuel Cell Truck (M2FCT) consortia, technology managers G. Kleen and D. Papageorgopoulos. Authors would also like to acknowledge support for this work from the Laboratory Directed Research and Development (LDRD) program at Los Alamos National Laboratory (LANL) (2020200DR).

Degradation Characteristics of Carbon-Supported Pt Via CO2 Detection during Startup/Shutdown of Polymer Electrolyte Membrane Fuel Cells

One of the major factors that reduces the durability of polymer electrolyte membrane fuel cells (PEMFCs) is degradation of the carbon-supported Pt in the catalyst layer, which can be attributed to the aggregation of Pt and the corrosion of carbon [1]. In this study, the degradation characteristics of a PEMFC during startup/shutdown were evaluated through detection of the CO2 generated during power generation tests. Non dispersive infrared-type (NDIR) analyzers have been shown to provide immediate and accurate measurements of carbon dioxide concentrations [2], and a pathway for the immediate measurement of CO2 concentration was established by connecting an NDIR analyzer to a test bench for the power generation tests. The degradation test of the PEMFC was repeated 100 times, involving the voltage sweep to 1.5 and 1.0 V and a voltage hold at 0.6 V. The PEMFC performance was evaluated using linear sweep voltammetry before the start of the tests and after every 10 sets of degradation tests, and the CO2 concentration was measured in parallel with the tests. As a result, the generation of CO2 and degradation of the PEMFC performance were confirmed. Figure 1 shows measured CO2 concentration during the degradation test, and approximately 3 ppm of the initial value is an offset derived from the measurement system. More CO2 was generated when the voltage was raised from 0.3 to 1.5 V than when it was raised from 0.6 to 1.5 V. Furthermore, the amount of CO2 generation decreased as the degradation tests were repeated, which can be explained by the passivation of some carbon atoms on the surface above 0.6 V. The study will be extended to investigate the effects of the voltage profile and carbon support type on degradation characteristics, contributing to the improvement of the durability of PEMFCs.

Impact of Electrode Reversal Method on Electrode Degradation By High-Potential Environment in Polymer Electrolyte Membrane Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) are key energy conversion devices, that address global energy challenges and climate change and are prized for various applications due to their high efficiency, low operating temperature, fast startup, and low toxic emissions. Enhancing PEMFC durability is crucial for advancing commercialization. Thus, prioritizing research on Catalyst Layer (CL) degradation mechanisms of Membrane Electrode Assembly (MEA) and developing analytical methods to understand extreme degradation conditions are becoming imperative.In PEMFCs’ operational scenarios, various issues deteriorate the membrane electrode assembly (MEA) of PEMFCs, leading to irreversible performance declines. These include Pt dissolution, ionomer degradation, carbon corrosion [1], and occasional membrane mechanical issues. Of these, the electrochemical carbon oxidation reaction is particularly detrimental, causing a significant reduction in electrochemical double layer capacitance (EDLC), the formation of oxygen functional groups in carbon supports, and substantial deterioration in electrochemical surface area (ECSA) [2, 3], ultimately leading to degraded PEMFC performance.On the other hand, reversible performance degradation occurs during fuel cell operation, which specific operational recovery procedures can rectify. Mechanisms contributing to reversible voltage degradation include the platinum oxide formation, adsorption of impurity ions and contaminants on the catalyst surface, water flooding/dehydration, and oxidation of the carbon support surface to create hydrophilic oxygen-containing groups [4]. Understanding reversible degradation mechanisms and recovery strategies is important for efficient operation, improved durability, and prolonged cell lifespan. Typically, the "electrode reversal" method is practical and economical, allowing power output during recovery without additional equipment, thereby minimizing recovery operation runtime.In this study, we examine the impact of the electrode reversal method on PEMFC performance and the deterioration tendency of MEA during carbon corrosion of cathode CL (Figure 1). Accelerated stress tests (AST) were conducted at high potentials exceeding 1.0 V (vs. RHE), specifically investigating the relationship between electrode degradation behaviors, porous carbon structural properties including ionic resistance and EDLC, and overall PEMFC performance.Particularly noteworthy after the electrode reversal recovery method were the discernible variations in degradation behaviors and diverse rates of performance decline observed through electrochemical impedance spectroscopy (EIS) analysis. EIS can be used to isolate the contribution of many processes to performance loss, allowing investigation of the effect of carbon corrosion-induced catalyst layer changes on fuel cell performance [5]. The EIS data underwent evaluation via parametric fitting utilizing the transmission line model (TLM) applied to the EIS spectrum. Furthermore, relaxation time distribution (DRT) analysis was employed to directly analyze internal resistance factors as a diagnostic tool for assessing electrode degradation by enhancing the TLM-based impedance analysis results. During the ASTs, electrochemical measurements (i-V, CV, LSV, and EIS) were performed to estimate the degree of PEMFC degradation, we also conducted ex-situ surface analysis of MEA using SEM, TEM, and XPS to elucidate the specific degradation components. Fig. 1 Electrode Reversal Recovery method during Anode cycling AST of the PEMFC single cell (a) polarization curves (i-V curves); 10 A constant current test (b)-(d): (b) fresh MEA, (c) after 250 cycles, (d) after 500 cycles of Anode cycling AST References [1] Sorrentino, Antonio, Kai Sundmacher, and Tanja Vidakovic-Koch. "Polymer electrolyte fuel cell degradation mechanisms and their diagnosis by frequency response analysis methods: a review." Energies 13.21 (2020): 5825.[2] Macauley, Natalia, et al. "Carbon corrosion in PEM fuel cells and the development of accelerated stress tests." Journal of The Electrochemical Society 165.6 (2018): F3148.[3] Kwon, JunHwa, et al. "Identification of electrode degradation by carbon corrosion in polymer electrolyte membrane fuel cells using the distribution of relaxation time analysis." Electrochimica Acta (2022): 140219.[4] Yang, Wenbin, et al. "Investigation of an electrode reversal method and degradation recovery mechanisms of PEM fuel cell." Electrochimica Acta 449 (2023): 142181.[5] Kwon, JunHwa, et al. "A Comparison Study on the Carbon Corrosion Reaction under Saturated and Low Relative Humidity Conditions via Transmission Line Model-Based Electrochemical Impedance Analysis." Journal of The Electrochemical Society 168.6 (2021): 064515. Figure 1

Hydrogen Limiting Current Test Diagnostic Tool for Analyzing PEMFC Durability in Accelerated Stress Tests

The durability of Polymer Electrolyte Membrane Fuel Cells (PEMFC) is a crucial factor for enhancing their competitiveness in the heavy-duty vehicle sector thus it is essential to assess the impact of the major degradation mechanisms on the performance and lifetime of the technology. To this end, accelerated stress tests have been standardized over the years with the aim of expediting the development of durable materials and being able to evaluate the durability in MEA. In this framework, it is extremely important to be able to separate different sources of voltage loss. Specifically, the degradation of mass transfer loss is extremely important because impacts the maximum power density available, efficiency and thermal management of the stack. Mass transfer loss is correlated at the material level, to the loss in electrocatalyst active surface (ECSA) being a consequence of the limitations in oxygen transport across the ionomer thin film. In this work mass transfer degradation is analyzed by ASTs that accelerate real-world ageing [1]. In this work, in addition to standard diagnostic tools, hydrogen limiting current test are investigated. Previous work reported that using hydrogen as reacting gas for limiting current test can give additional information on the sources of mass transfer loss compared to oxygen, specifically information on the ionomer thin film resistance [2]. In this work, limiting current tests were carried out as diagnostic tools during ASTs for electrocatalyst durability and novel ASTs that better mimick real-world vehicle operation within the IDFAST H2020 project and in the Italian project PERMANENT to investigate the degradation [2]. Correlation between mass transfer resistance in ionomer between oxygen and hydrogen limiting current test is found. Additional effects in the high potential region are observed and attributed to platinum oxide formation by means of a 1D MEA model that is adopted to investigate this effect.[1] Spingler F.B., Phillips A., Schuler T., Tucker M.C., Weber A.Z. (2017) International Journal of Hydrogen Energy, 13960-13969[2] Colombo E., Baricci A., Mora D., Guetaz L., Casalegno A., (2023) Journal of Power Sources, 580 , art. no. 233376

In-Situ Characterization of Cathode Catalyst Degradation in PEM Fuel Cells

The composition and morphology of the cathode catalyst layer (CCL) have a significant impact on the performance and stability of polymer electrolyte membrane fuel cells (PEMFC). Understanding the primary degradation mechanism of the CCL and its influencing factors is crucial for optimizing PEMFC performance and durability. Within this work, we present comprehensive in-situ characterization data focused on cathode catalyst degradation. The dataset consists of 36 unique durability tests with over 4000 testing hours, including variations in the cathode ionomer to carbon ratio, platinum on carbon ratio, ionomer equivalent weight, and carbon support type. The applied accelerated stress tests were conducted with different upper potential limits and relative humidities. Characterization techniques including IV-curves, limiting current measurements, electrochemical impedance spectroscopy, and cyclic voltammetry were employed to analyse changes in performance, charge and mass transfer, and electrochemically active surface area of the catalyst. The aim of the dataset is to improve the understanding of catalyst degradation by allowing comparisons across material variations and provide practical information for other researchers in the field.

Flow-field design of the bipolar plates in polymer electrolyte membrane fuel cell: Problem, progress, and perspective

As a promising carbon-neutral technology, the polymer electrolyte membrane fuel cell (PEMFC) is gaining considerable attention over the past decades. Many problems in PEMFC performance and durability can be ultimately ascribed to the flow-field design, which is a complex and systematic work owing to the inherent sophisticated nature of the PEMFC with multicomponent mass transportation and multiphysics field coupling. This paper presents a critical review of the state-of-the-art flow-field designs and an in-depth analysis of the key problems involved from a perspective of efficient mass transport within the PEMFC. In particular, flow-optimization principles are discussed specifically for the enhancement in reactant mass transfer, water management, optimized opening ratio, uniformity of flow distribution, and choice of appropriate numerical approaches assisting the flow-field design. The material formability and forming accuracy and their effects are also discussed for metallic bipolar plates. The objective of this review work is to present a comprehensive overview of the problems, progresses, and perspectives of the flow-field designs for bipolar plates in PEMFC and provide a general theoretical instruction for present and future relevant R&D activities that aim at high-performance, durable, and low-cost fuel cells.

Impact of Missing Cathode Catalyst Layer Areas on Performance and Durability in PEM Fuel Cells

High throughput and product quality are essential for the scalable production of cost-effective polymer electrolyte membrane fuel cells (PEMFCs). Therefore, it is important to understand the possible implications of various material defects that could appear in the manufacturing process. During the fabrication and assembly of catalyst-coated membranes (CCM)s, the catalyst layers (CL)s may contain void regions of partially missing material due to deficient deposition or unintentional removal, which could affect fuel cell performance and durability [1], [2]. For instance, CL cracks have been shown to expand and propagate into more harmful membrane cracks [3]–[5]. In addition, local thickness differences in CCMs arising from missing CL sections or deficiencies in other components could result in local stress concentration and susceptibility of membrane pinhole formation [6], [7]. Therefore, in general, a missing section of the CL area or other similar irregular features in the CL of a CCM may imply categorizing these CCMs as defective (i.e., scrap) and excluding them for fuel cell assembly. However, on the contrary, M. Kim et al. found increased cell performance for cracked CCM via tensile forces (within the elastic strain region) due to improved mass transport and decreased membrane resistance [8].Consequently, to improve the understanding of CCM defects, the present work investigates how different missing cathode catalyst layer (CCL) areas affect the PEMFC performance and durability. In more detail, we prepared via ultrasonic spray coating 25 cm2 active area MEAs with 4.0, 5.9, and 8.5% missing CCL areas obtained by masking (as shown in the Figure), and compared these to defect-free CCL baselines.The results of this study feature a comprehensive assessment of the fuel cell performance of the defective MEAs compared to the baseline MEAs. This includes extraction of the membrane and charge transfer resistances from the missing CCL areas via medium- and high-frequency equivalent circuit modeling of in-situ electrochemical impedance spectroscopy (EIS) data at a current density of 0.1 A/cm2 and contact, membrane, charge transfer, and mass transfer resistances at a current density of 1 A/cm2. Findings on the influences on the electrochemical surface area, hydrogen cross-over current density, and double-layer capacitance from such missing CCL area will also be presented. Results from chemical/mechanical and electrochemical cycling tests will also be shown regarding the effect on durability from the missing CCL area.The results from this work may benefit the economy of scale for high-volume fuel cell manufacturing by reducing unnecessary CCM replacements and materials waste by providing a threshold for an acceptable missing CCL area.AcknowledgementsFunding for this research was provided by National Research Council Canada’s Clean and Energy-efficient Transportation (CEET) program and Canada Research Chairs.

Ratcheting assessment of the catalyst layer in polymer electrolyte membrane fuel cells considering thermal-mechanical-humidity cycling

The properties of polymer electrolyte membrane fuel cells (PEMFCs) are highly affected by the catalyst layer (CL). However, the cyclic swelling and shrinking of the ionomer caused by the change of temperature and relative humidity (RH) combined with external random mechanical loads may lead to electrochemical activity degradation through cyclic plasticity accumulation. In this study, a theoretical model to assess the ratcheting limit of the CL for PEFMCs subjected to thermal-mechanical-humidity cycling is developed based on linear matching method (LMM). The influences of embedded depth of Pt/C particle, cyclic temperature or RH modes, as well as the random reaction force are considered by the proposed method. The ratcheting limits of CL are achieved under various thermal-mechanical-humidity cycling and verified by step-by-step simulation. Moreover, a simplified prediction model is established to evaluate the ratcheting limits under the combined temperature and RH cycling. It is of interest that the ratcheting limit under the pure normal force is almost 3 time of that under the pure tangential force under the same thermal-humidity cycling. Furthermore, the accumulated plastic strain occurs near the junction area causing the debonding of Pt/C particle from the ionomer under thermal-mechanical-humidity cycling, which agrees well with the microscopic observation. Based on ratcheting assessment results obtained by the proposed method, the reliability and durability of PEMFCs can be improved by increasing the affinity between Pt/C and ionomer, decreasing the size of gas pore and inhibiting the tangential force in the CL.

Ultrasound-Driven enhancement of Pt/C catalyst stability in oxygen reduction reaction

Polymer electrolyte membrane fuel cells (PEMFCs) have reached the commercialization phase, representing a promising approach to curbing carbon emissions. However, greater durability of PEMFCs is of paramount importance to ensure their long-term viability and effectiveness, and catalyst development has become a focal point of research. Pt nanoparticles supported on carbon materials (Pt/C) are the primary catalysts used in PEMFCs. Accomplishing both a high dispersion of uniform metal particles on the carbon support and robust adhesion between the metal particles and the carbon support is imperative for superior stability, and will thereby, advance the practical applications of PEMFCs in sustainable energy solutions. Ultrasound-assisted polyol synthesis (UPS) has emerged as a suitable method for synthesizing catalysts with a well-defined metal-support structure, characterized by the high dispersion and uniformity of metal nanoparticles. In this study, we focused on the effect of ultrasound on the synthesis of Pt/C via UPS and the resulting enhanced stability of Pt/C catalysts. Therefore, we compared Pt/C synthesized using a conventional polyol synthesis (Pt/C_P) and Pt/C synthesized via UPS (Pt/C_U) under similar synthesis conditions. The two catalysts had a similar Pt content and the average particle size of the Pt nanoparticles was similar; however, the uniformity and dispersion of Pt nanoparticles in Pt/C_U were better than those of Pt/C_P. Moreover, ex/in-situ analyses performed in a high-temperature environment, in which nanoparticles tend to agglomerate, have revealed that Pt/C_U exhibited a notable improvement in the adhesion of Pt particles to the carbon support compared with that of Pt/C_P. The enhanced adhesion is crucial for maintaining the stability of the catalyst, ultimately contributing to a better durability in practical applications. Ultrasound was applied to the carbon support without the Pt precursor under the same UPS conditions used to synthesize Pt/C_U to identify the reason for the increased adhesion between the Pt particles and the carbon support in Pt/C_U, and we discovered that oxygen functional groups (C-O, C = O, and O-C = O) for anchoring site of Pt particles were generated in the carbon support. Pt/C_U displayed an increase in stability in an electrochemical accelerated stress test (AST) in an acidic electrolyte. The physical and chemical effects of ultrasound on the synthesis of Pt/C via UPS were identified, and we concluded that UPS is suitable for synthesizing carbon supported electrocatalysts with high stability.

An open-source zero-gradient cell hardware to improve and accelerate durability testing of PEM fuel cells

The design of an open zero-gradient hardware is proposed in this work. The hardware is thought for characterizing single Polymer Electrolyte Membrane Fuel Cells (PEMFCs), specifically Membrane Electrode Assemblies (MEAs), with a focus on transport application. The objective of the proposed design is to minimize both operation and degradation heterogeneities over the cell active area in order to evaluate material properties only. It is indeed specifically intended to quantify the PEMFC materials performance without accounting for any interdependency between the layers of the MEA and the cell hardware design (e.g. heating/cooling system and flow field features). This is granted by combining high stoichiometry ratios of the gas reactants and limited pressure drops: they indeed keep uniform operating conditions (concentration in the gas phase, relative humidity, pressure and temperature), as verified by the electrochemical and operational characterization. With such characteristics, accurate information about both the performance and the degradation of the MEA materials can be provided. This tool is powerful for assessing the ranking in terms of performance and durability of different PEMFCs, as well as for application in the field of the materials local diagnostics.

Experimental study of homogeneity improvement and degradation mitigation in PEMFCs using improved reaction area utilization

The durability and degradation issues of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) utilized in high-humidity environments and small stack sizes are investigated in this study. To address the lack of uniformity in operation and the resulting degradation, a method for improving the uniformity of current density distribution and reducing degradation in PEMFCs is proposed. This is achieved by periodically controlling the flow direction using a reverse-flow configuration. The proposed method resulted in a delay of degradation by approximately 7% based on the end of degradation experiment. Furthermore, the uniformity of current density distribution and voltage differences between cells in the stack were improved. The proposed method exhibits potential for enhancing the durability and performance of PEMFCs in high-humidity environments and small stack sizes, thereby mitigating carbon or Pt degradation, particularly in applications such as micro mobilities.

Effects of Acid Leaching Post-Treatment Process on the Structural Stability of Pt-Based Intermetallic Catalysts for Fuel Cell Applications

Accelerating the commercialization of polymer electrolyte membrane fuel cells (PEMFCs) requires urgent improvement of a catalyst for oxygen reduction reaction (ORR) that uses expensive precious metals. It is also important to consider the durability of the catalyst, which can affect its performance even after long-term use.In this study, we synthesized a platinum-cobalt (Pt-Co) catalyst with a platinum (Pt) shell and an intermetallic inner core through the acid-leaching post-treatment process. By introducing a transition metal, we greatly improved the ORR activity, while controlling the intermetallic and core-shell structure greatly improved the catalyst's structural stability and durability. We confirmed through X-ray diffraction (XRD) that the Pt-Co compound was aligned in an intermetallic structure, and that the structure was maintained even after the acid leaching post-treatment. In the half-cell test, L10-PtCo@Pt/C, exposed to 0.5 M sulfuric acid solution for 6 hours, exhibited a 3.9-fold increase in mass activity and a 3.3-fold increase in specific activity compared to commercial Pt/C. Additionally, the electrochemical active surface area (ECSA) of L10-PtCo@Pt/C remained stable even after 30,000 cycles, with an insignificant change of 8% or less, while commercial Pt/C decreased by 30% or more. Nevertheless, when acid treatment was applied for longer than this condition, the increase in performance and its retention were significantly reduced. This is due to the fact that with longer acid leaching times, the Pt shell of the catalyst becomes thicker, resulting in a limitation of the alloying effect of the internal intermetallic core.Our study proposes an optimal combination of Pt-Co intermetallic core and Pt shell through precise control of acid leaching post-treatment process. Pt-Co catalysts with well-aligned intermetallic core and the moderate thickness of Pt shell structure can offer a promising solution for improving the performance and durability of PEMFCs, thereby paving the way for their widespread commercialization.

Stabilization of Platinum Catalyst Surface-Treated by Atomic Layer Deposition of Cobalt for Polymer Electrolyte Membrane Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) are attracting attention as next-generation energy-conversion devices because they are environmentally friendly and have high energy-conversion efficiencies. A membrane electrode assembly (MEA), composed of an anode, membrane, and cathode, is the most important part of a PEMFC where electrochemical reactions occur. A Pt/C catalyst, wherein Pt is supported on a carbon, is used as the catalyst layer for the anode and cathode. However, as Pt is a precious metal, current research is focused on reducing its loading and improving durability of PEMFCs. This study focused on improving the durability of catalysts by suppressing agglomeration, one of the degradation mechanisms of Pt catalysts. A protective layer of Co metal was deposited using the atomic layer deposition (ALD) process to improve the durability of catalysts. The ALD process is a type of chemical vapor deposition (CVD) process. This process is suitable for the PT/C catalyst as it allows for the deposition of a uniform thin film even on the porous catalyst layer. If the protective layer is thick, it may block the active site of the catalyst, resulting in decreased activity and reduced performance. However, ALD can form a thin protective layer because it can be controlled at the atomic level by adjusting the number of cycles. After fabricating a single cell using ALD Co-Pt/C as a catalyst for the cathode, The U.S. Department of Energy's electrocatalyst durability protocol was used for an accelerated degradation test (ADT). After Co-ALD was used as a cathode for 10 and 20 cycles, the active area of the Pt catalyst particles was reduced owing to the metal Co protective layer. The initial performance and electrochemical surface area (ECSA) of Co-ALD 10, 20 cycles decreased compared to the case of using Co-ALD 0 cycle. However, after ADT, the performance and ECSA reduction rate rather decreased, confirming that the co-protective layer through the plasma-enhanced atomic layer deposition (PEALD) process can improve the durability of the Pt/C catalyst. To identify the cause of the durability improvement, the cathode catalyst particles before and after ADT were compared through high resolution transmission electron microscopy (HR-TEM) analysis. As a result, in the HR-TEM images after ADT, the Pt catalyst particles were relatively uniformly distributed on the carbon support in the Co-Pt/C catalyst compared to Co-ALD 0 cycle. In addition, it was confirmed that the size of the Pt catalyst particles of Co-Pt/C was smaller than that of bare Pt/C (Co-ALD 0 cycle). It was concluded that when the Co protective layer was deposited on the Pt/C cathode, the protective layer suppressed the agglomeration of Pt catalyst particles, therefore, the increase in Pt particle size improved compared to the Co-ALD 0 cycle, and it was revealed that durability improved by suppressing catalyst degradation.

Shunt current protocol to eliminate reversible degradation of polymer electrolyte membrane fuel cells during constant load operations

The objective of this study is to determine the durability of polymer electrolyte membrane fuel cells (PEMFCs) in constant current operation incorporated with regular recovery protocol for eliminating reversible performance loss of membrane electrode assemblies. Effects of operation ‘shunt current protocol’ on PEMFC durability are studied through analyses of the main degradation mechanism based on results of electrochemical characterizations and post-mortem investigations. The voltage of the protected cell using the shunt current protocol is stably preserved under the applied current density for 700 h with less degradation (4.2% of decay ratio), while the performance of the unprotected cell steadily decreased with time (15.1% over 700 h). The substantial performance deterioration of the unprotected cell is mainly attributed to morphology deterioration of the cathode catalyst layer with oxidation of Pt catalysts and chemical degradation of ionomers caused by generation of excessive water from electrochemical oxygen reduction reactions under high-humidity operating conditions. In contrast, the shunt current protocol plays an important role in sustaining high oxygen activity at the cathode catalyst surface, detaching partially covered OH− species on Pt active sites from water oxidation during cell operation caused by periodically applied shunt current for a very short period of 1 s every 5 h. We hope to provide insight into the operation protocol to extend the lifetime of PEMFCs, minimizing conversion from recoverable performance loss to irreversible (permanent) degradation during operation.

Simulation of Cathode Catalyst Durability Under Fuel Cell Vehicle Operation - the Effect of Temperature

Fuel cell vehicles (FCVs) emerge to be promising candidates to produce clean power for the transportation sector in order to tackle climate change issues. Although the commercialization of polymer electrolyte membrane fuel cells (PEMFCs) has progressed in the past few decades, there are still obstacles to address, including high cost and limited hydrogen infrastructure. For heavy duty transportation applications, the PEMFC durability is also not yet proven, and extrapolating from lab data to real-world field operating conditions remains a significant challenge [1].In this work, the cathode catalyst degradation in PEMFCs is studied to make an estimation of fuel cell durability in the FCV application. The well-known Butler-Volmer approach is utilized to model platinum dissolution and redeposition, platinum oxidation and platinum ion formation during the fuel cell operation [2]. First, the model is calibrated with the results presented by Ferreira et al. [3] at 80℃; the calibrated model is then validated with Kocha’s results [4] to demonstrate the model’s capability of generating reliable results at different temperatures. In order to realistically predict the cathode degradation pattern and lifetime, the real-life fuel cell operating conditions should be obtained. Therefore, a drive cycle recorded based on a real-life transit bus operation in the city of Victoria is utilized to calculate the input fuel cell voltage profile. The procedure to calculate fuel cell operating voltage profile using the drive cycle has been thoroughly explained by Ahmadi and Kjeang [5]. This procedure employs Newton’s second law to calculate the required cell power density considering the air flow drag force counteracting the vehicle thrust. Finally, polarization curves representing the fuel cell performance are used to calculate the fuel cell voltage profile with the determined required cell power density. Polarization curves measured under a range of temperatures are employed to adequately reflect the effect of temperature on the fuel cell performance. The cell active area is considered to be 500 cm2 and the stack is assumed to contain 225 cells, representing a nominal stack power of 74 kW at 80℃. The fuel cell operation and degradation are simulated at three different temperatures: 60, 70, and 80℃. The change of remaining electrochemically active surface area (ECSA) with time is calculated as the output of the model.Temperature highly affects the electrochemical reactions of platinum dissolution in several ways. First, it affects the Tafel slope in the platinum dissolution reaction. Second, it substantially impacts on platinum degradation reaction rate constant [6]. The Arrhenius approach is taken in this study to apply the effect of temperature on the reaction rate constants. Fuel cell voltage loss over time is determined by assuming simple Tafel kinetics. The cathode lifetime is calculated at 0.6 A/cm2 and is estimated by considering 10% voltage drop as the failure criterion. Fig. 1 shows the simulated change of remaining ECSA over time and the resulting fuel cell lifetime for the three different cell temperatures. The results indicate that the cathode lifetime increases 168% when the cell temperature drops from 80℃ to 60℃ due to a significantly lower platinum degradation rate at 60℃. The results show relatively low cathode catalyst lifetimes for a transit bus. The reason is attributed to the bus drive cycle used as the model input. The drive cycle contains a great portion of deceleration and idling time which leads to a fuel cell operation close to open circuit voltage (OCV), thus experiencing a high degradation rate.In this regard, the present modeling framework could become a useful tool for fuel cell and FCV developers to predict lifetime for new products and systems prior to commercial release. Scientifically, the present model can also be used to further explore key influencing factors on fuel cell durability and develop more durable cell, stack, and system designs for a targeted FCV application. 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.

Simulation of Cathode Catalyst Durability Under Fuel Cell Vehicle Operation - the Effect of Fuel Cell Stack Size

The concerns regarding climate change have made the researchers seek a clean alternative for the fossil fuel vehicles. Fuel cell vehicles (FCVs) are considered to be promising candidates owing to their efficient energy conversion and zero-carbon emission. However, a number of obstacles such as high cost and limited hydrogen infrastructure have made the FCVs commercialization process challenging. Polymer electrolyte membrane fuel cells (PEMFCs) have been proven promising for transportation applications. For heavy duty transportation applications, the PEMFC durability is also not yet proven, and extrapolating from lab data to real-world field operating conditions remains a significant challenge [1].In this work, the cathode catalyst degradation in PEMFC is studied to estimate the effect of stack size on fuel cell durability in the FCV application. Platinum dissolution and redeposition, platinum oxidation and platinum ion formation during the fuel cell operation are modeled using the Butler-Volmer approach presented in [2]. A drive cycle recorded based on a real-life transit bus operation in the city of Victoria is utilized to calculate the input fuel cell voltage profile based on the methodology presented by Ahmadi and Kjeang [3]. According to this methodology, the required cell power density is calculated using Newton’s second law considering the air flow drag force as a counteracting force against the vehicle movement. Then, the required voltage cycle is obtained by employing a polarization curve characterizing the fuel cell performance. Finally, the change of remaining electrochemically active surface area (ECSA) with time is calculated as the output of the model. The fuel cell is assumed to operate at 80 ℃ and the cell active area is considered to be 500 cm2. Simple Tafel kinetics is then used to determine the fuel cell voltage loss. A 10% voltage drop at 0.6 A/cm2 is considered as the failure criterion for the cathode lifetime. Moreover, the effect of the fuel cell stack size is studied. By increasing fuel cell stack size, the required cell power density drops, leading to a decrease in the voltage cycle amplitude while the voltage cycle period remains the same. According to the empirical kinetic rate equation, the catalyst degradation exponentially increases with increasing the voltage. Therefore, a higher degradation rate is observed for a catalyst operating on a voltage cycle with a lower amplitude while the period and the upper potential limit (UPL) are maintained the same, causing a significant platinum ion generation. Fig. 1 shows the change of remaining ECSA over time and resulting fuel cell lifetime for three stack sizes which are represented by the stack nominal powers. The results show that the fuel cell lifetime will be roughly doubled when the stack size is reduced by half. Stack sizing is thus an important consideration for fuel cell durability in the FCV application.In this regard, predicting fuel cell lifetime is a crucial step in commercializing FCVs. The present modeling framework could be utilized by FCV developers to predict lifetime for new products instead of carrying out time-consuming lifetime experiments. The factors influencing fuel cell durability can also be investigated using the present model framework to develop durables cells and stacks for a targeted FCV application. 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.

Groovy Electrodes Enable Facile O2 and H+ Transport in PEMFCs

Polymer electrolyte membrane fuel cells (PEMFCs) are promising alternative to internal combustion engines, owing to their capability to produce on-demand power with zero local carbon emissions. However, PEMFCs suffer from limitations related to performance, particularly for heavy-duty vehicle applications [1]. A major contributor to the performance loss is the cathode electrode; uncontrolled conventional electrode structures lead to non-ideal transport pathways of O2 and H+ [2], subsequently leading to performance loss. Here, we report an alternative electrode structure termed groovy electrodes, fabricated via patterned Si templates. Si templates with desired patterns are fabricated through photolithography and deep-reactive ion etching techniques (Fig. 1a), and an electrode layer is coated onto the template and subsequently transferred to the membrane or the gas diffusion layer. The resulting electrode features ordered grooves with consistent depth and width (Fig. 1b-c). We will present how this alternative structure enables improved H+ transport (enabled by higher ionomer content) with grooves facilitating effective O2 transport to reaction sites. We will also demonstrate the benefits of a groovy electrode for the durability of PEMFCs. Reference Cullen et al., Nat. Energy, 6, 462 (2021).Ramaswamy et al., J. Electrochem. Soc., 167, 064515 (2020). Figure 1

The challenges in reliable determination of degradation rates and lifetime in polymer electrolyte membrane fuel cells

The durability of polymer electrolyte membrane fuel cells (PEMFCs) needs to be further improved to cope with application requirements and economic competitiveness. This article highlights the challenges in the reliable determination of degradation rates and lifetime. The reliable evaluation of performance degradation rates is fundamental to quantify and benchmark durability and to allow comparisons between PEMFC durability tests performed using different materials or in different laboratories. The use of efficient recovery procedures enables the discrimination of reversible and irreversible voltage losses and facilitates the understanding of recovery mechanisms. In the end, recent contributions about lifetime diagnoses and prediction are presented, which are promising to be implemented in PEMFC applications.

Sandwich‐like Nafion composite membrane with ultrathin ceria barriers for durable fuel cells

Improving the durability of the membrane electrode assembly (MEA) while obtaining high performance is required to further commercialize polymer electrolyte membrane fuel cells (PEMFCs). The durability of PEMFCs is improved by incorporating radical scavengers, such as CeO2 (ceria), into the MEA, especially the membrane. However, nanosized ceria particles are generally mixed with ionomers and are cast on substrates to fabricate composite membranes. In such a case, controlling their morphology and avoiding particle agglomeration is difficult. Herein, we report a novel method for constructing a robust membrane by incorporating ultrathin ceria barriers into the outermost sides of a commercial Nafion membrane to effectively alleviate radical attacks while ensuring the high uniformity and controllability of ceria layers. The improved durability of the composite membrane is confirmed via ex situ Fenton's test and in situ operation of a fuel cell. Moreover, we observe that the amount of fluoride ion emission and the loss of proton conductivity of the membranes decrease as the CeO2 density increases. The MEA comprises a modified membrane with CeO2 barriers that show proper areal density. It demonstrates excellent durability under accelerated environmental conditions (open circuit voltage test) and acceptable initial performance with an insignificant decrease in proton conductivity.