Accelerated Stress Test Protocols Research Articles

About the Necessity to Consider Membrane Electrolyte Degradation Statistically

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are subjected to aging under pure mechanical, pure chemical and the combination of both stressors. Progression of degradation leads to membrane failures. In this study, seven different accelerated stress test (AST) protocols are applied to degrade two types of commercially available membrane electrode assemblies (MEAs) to demonstrate the necessity to evaluate lifetime of this system with statistical methods. In total, data from 56 samples is reported. Membrane lifetime is derived from hydrogen crossover and open circuit voltage (OCV) which is tracked over the course of degradation. The characteristic membrane lifetime distribution of each AST protocol is described via a cumulative Weibull distribution function (CDF). As a result, the scatter of lifetime distribution correlates with the lifetime itself and thus conclude that less repeats are required for ASTs creating short lifetimes compared to those causing long lifetimes. As the latter is required especially for membrane lifetime prediction, these conclusions are relevant for anybody designing the lifetime of PEMFC stacks.

Screening reversal tolerance through rotating disc electrode studies

Anode reversal is a detrimental phenomenon that occurs in proton exchange membrane fuel cells (PEMFCs) when the anode is subject to fuel starvation, leading to irreversible damage. This is a critical issue for the commercialization of PEMFCs. However, the development of more robust anode catalysts has been limited by the need for membrane electrode assembly (MEA)-level testing, which is costly and time-consuming. Here, we propose two accelerated stress test (AST) protocols based on widely available rotating disc electrode (RDE) methods to determine their suitability for screening anode catalysts and predicting MEA-level durability. One method is based on a carbon corrosion (cc) voltage cycling protocol between 1.0 and 1.5 V vs. RHE, while the other is a chronopotentiometry (cp) method. We evaluate three model catalysts (Pt30, Pt50, and Pt70) at the MEA-level through standard reversal testing and at the RDE-level using the two AST protocols (RDE-cc and RDE-cp). We find that both RDE-level methods can reliably predict MEA-level anode durability, and can be used by materials scientists to screen the relative durability of new anode catalysts. Our results also suggest Pt70 is a promising candidate for anode reversal tolerant PEMFCs, as it shows superior durability under both MEA-level and RDE-level tests.

Accelerated stress test protocol for solid oxide cell based on ex-situ artificial aging of the fuel electrode via redox-cycling - In memory of Prof. A. Milchev

Accelerated stress test protocol for solid oxide cell based on ex-situ artificial aging of the fuel electrode via redox-cycling - In memory of Prof. A. Milchev

Degradation of Sb-Doped SnO2 Supported IrRu-Based Electrocatalysts for PEM Water Electrolysis Via Accelerated Stress Protocols

Proton exchange membrane (PEM) water electrolyzers offer a very promising option to offset the environmental burden of conventional hydrogen production by splitting water into high purity hydrogen and oxygen without releasing carbon dioxide as by-product. However, the corrosive nature of the process makes it dependent on expensive materials, such as iridium (Ir), ruthenium (Ru) or titanium (Ti), which makes the upscaling of the technology considerably more difficult [1, 2]. To ensure industrial success of this technology, the study of the mechanisms involved in the degradation of the cell components is of great importance. Particularly, the development of accelerated stress test protocols has attracted attention in recent years, due to their potential to yield representative data pertaining degradation without incurring long testing times that would meet the typical industrial standards of 20,000 to up to 50,000 hours [3].Several strategies to investigate the various degradation mechanisms have been discussed in recent years in the literature. Particularly, Spöri pointed out the difference between transient and static operation and the degradation mechanisms triggered by each of these modes, while also investigating the intrinsic mechanisms of the catalyst via rotating disk electrode analysis [3]. Meanwhile, Frensch explored in their work the relation of cathodic fluoride release with different current density profiles and different temperatures [4]. Furthermore, attempts have been made to increase the electrochemical stability of oxygen evolution reaction (OER) electrocatalysts by adding a support material to the catalyst layer. This was described by Saveleva in their work, as they found evidence that supporting Ir on antimony-doped tin oxide (Sb-doped SnO2) resulted in a preferential dissolution of Sb and Sn, thus noticeably reducing the dissolution rate of Ir [2].In this work, the electrochemical activity and stability of newly developed IrRu OER catalysts supported on Sb-doped SnO2 is studied in a single cell setup. The catalyst and support materials were synthetized as part of the study. For the preparation of the MEAs the fabricated materials were used with membranes and ionomers which are commercially available. The prepared MEAs were analyzed using both static and dynamic current profiles with a current range of up to 4 A cm-2. To understand the different mechanisms and material dependencies at play during characterization, the MEA configuration (membrane type, material ratios, ionomer) was varied. The first screenings with different MEA configurations not only suggest a strong stability with the catalyst loading, as it was previously observed by Alia [5], but also a strong dependence on the anodic ionomer content, which reflects in the degradation rate in the first hundred hours of test.

Accelerated Stress Testing of Solid Oxide Electrolysis Cells in a Symmetric Steam-Rich Atmosphere

Temperature, current density, steam utilization variability, and local steam starvation or excess may cause diverse degradation processes to occur in solid oxide electrolysis cells. Typical degradation mechanisms for SOEC with nickel-based fuel electrode materials include nickel migration/evaporation and agglomeration, which may lead to reduced active sites at the triple-phase boundary. However, the phenomenological cause of such degradation mechanisms is still actively debated, making the selection of a durable fuel electrode material challenging. It is desirable to understand the process by which these phenomena occur not only to minimize it, but to intentionally induce it to develop accelerated stress test protocols for cell lifetime prognostics. The degradation of solid oxide electrolyzers has been reported to be accelerated by certain stressful operating conditions. Brisse and Mocotuguy conducted a review of the degradation mechanisms of solid oxide button cells; they reported that high steam partial pressures can lead to the formation of nickel hydroxide at the cathode, which then diffuse to the surface of YSZ1. Sun et al. hypothesize that this is the main mechanism for nickel migration, due to the positive oxidation state of nickel and the direction of the electric potential2. Changes in the surface morphology of the fuel electrode due to redox cycle-induced degradation has been reported, showing the dynamics of nickel migration and agglomeration under these conditions3 . Redox cycling was found to be a destructive test protocol; efforts are currently being made by the group of Daria Vladikova to prevent fracturing of the electrodes and electrolyte during such stressful testing4. Königshofer et al. have reported through multiple investigations that protocols inducing operation at high steam conversion rates (>90%) and large current densities (0.8 A/cm2) have been effective at reproducing long-term SOEC voltage degradation5,6.The objective of this research is to establish a protocol that will help determine the causes that trigger Ni migration or evaporation, and that will mimic the electrode degradation mechanism to standardize the procedure for comparing the long-term performance of solid oxide electrolysis cells. Symmetric button cells of various fuel electrode chemistries with a diameter of 20 mm (active area 1.23 cm2) are tested under symmetric atmospheres with steam/H2 blends. The following two accelerated stress tests (AST) protocols are employed: cycling of steam partial pressure and cycling of current density. In the former, the steam molar fraction is varied from 75% to 95% in a 20-minute cycle for at least 400 cycles (i.e., 1,176 h). In the latter protocol, current density is controlled using a galvanodynamic 10-minute long cyclic ramp profile between 0.8 A/cm2 and 1.5 A/cm2 at a constant steam molar fraction between 75-95% for at least 800 cycles (i.e., 1,176 h).For all tests, the cells are operated at a constant furnace temperature between 700°C and 850°C. The performance characteristics of the various cells are expressed in terms of electrochemical impedance spectroscopy (EIS) and polarization curves every 12 h during the AST. X-ray diffraction (XRD) and fluorescence (XRF) are used to analyze the cells at beginning of life and post-mortem to identify phase changes which occur during operation. Scanning Electron Microscopy - Energy Dispersion Spectroscopy (SEM-EDS) and Glow-discharge spectroscopy (GD) are used to identify and map the nickel concentration on the cathode surface and cross-section. Preliminary results suggest that the high frequency resistance may increase by nearly 500% under a constant high-steam condition (> 90%) in a Ni-YSZ|YSZ|Ni-YSZ cell during a 200 h operation. P. Moçoteguy and A. Brisse, Int J Hydrogen Energy, 38, 15887–15902 (2013).X. Sun, T. L. Skafte, and S. H. Jensen, Fuel Cells (2021).S. Yang et al., J Alloys Compd, 921, 166085 (2022).D. Vladikova et al., Energies (Basel), 15 (2022).B. Königshofer et al., J Power Sources, 523 (2022).B. Königshofer et al., J Power Sources, 497 (2021).

Accelerated stress test of polymer electrolyte membrane water electrolyzer via solar power generation condition

Hydrogen has emerged as a promising energy carrier to mitigate the imbalance between renewable energy supply and demand. Developing high-performance, highly durable, low-cost electrolysis systems is essential for sustainable hydrogen production with reduced installation and operational costs. Practical accelerated stress test protocols are crucial to assess the performance durability of electrolyzers, which need a lifespan of over a decade. Additionally, investigating performance durability under diverse operating conditions is necessary. This research employs an accelerated stress test protocol, compressing 24 h of solar power generation into a minute and operating continuously for five days. The impact of anode structure on performance durability is analyzed using the accelerated stress test protocol, revealing performance degradation under the rainy-day protocol. Finally, better dispersibility led to lower current density reduction (0.38 A/cm2) in rainy-day protocol, compared to poorly dispersed water-based electrode's reduction (0.57 A/cm2), indicating improved durability.

Experimental assessment of proton exchange membrane fuel cell performance degradations during emulated start-up/shut-down phases

Start-Up/Shut-Down phases in PEM Fuel Cell system has paid particular attention because of the well-identified reverse current mechanism leading to harsh corrosion environment within the Cathode Catalyst Layer. In this work, an experimental ageing test is designed to assess a comprehensive investigation of electrochemical properties degradations and thus influence on cell performances during the Start-Up phase. An Accelerated Stress Test protocol is designed to mimic and cycle the cathodic potential profile during the H2|Air front propagation along anodic channel. A sensitivity analysis is performed on the maximum cathodic potential and the exposure time parameters. Results divulge that lowering maximum potential by a dummy load application and lowering exposure time by accelerating H2 feeding during Start-Up phases are promising mitigation strategies. Moreover, this paper reports a promising effect by scanning the cathodic potential above 1.0 V for conditioning carbon support and optimizing cell performance at high current densities.

Investigating the Particle Growth in Bimodal Pt/C Catalysts by In-Situ Small-Angle X-ray Scattering: Challenges in the Evaluation of Stress Test Protocol-Dependent Degradation Mechanisms

The influence of different combinations of accelerated stress test (AST) protocols simulating load-cycle and start/stop conditions of a proton exchange membrane fuel cell (PEMFC) vehicle is investigated on a bimodal Pt/C catalyst. The bimodal Pt/C catalyst, prepared by mixing two commercial catalysts, serves as a model system and consists of two distinguishable size populations. The change in mean particle size was investigated by in situ small-angle X-ray scattering (SAXS). The comparison to the reference catalysts, i.e., the two single-size population catalysts, uncovers the presence of electrochemical Ostwald ripening as a degradation mechanism in the bimodal catalyst. Increasing the harshness of the applied AST protocol combinations by faster changing between load-cycle or start/stop conditions, the particle size of the larger population of the bimodal catalyst increases faster than expected. Surprisingly, the change in mean particle size of the smaller size population indicates a smaller increase for harsher AST protocols, which might be explained by a substantial electrochemical Ostwald ripening.

Relation between half‐cell and fuel cell activity and stability of FeNC catalysts for the oxygen reduction reaction

AbstractFeNC catalysts are promising substitutes of platinum‐type catalysts for the oxygen reduction reaction (ORR). While previous research disclosed that high pyrolysis temperatures are required to achieve good stability, it was identified that a trade‐off needs to be made regarding the active site density. The central question is, if a good stability can also be reached at milder pyrolysis conditions but longer duration retaining more active sites, while enabling the defect‐rich carbon to heal during a long residence time? To address this, a variation of pyrolysis temperatures and durations is used in FeNC fabrication. Carbon morphology and iron species are characterized by Raman spectroscopy and Mössbauer spectroscopy, respectively. Fuel cell (FC) activity and stability data are acquired. The results are compared to ORR activity and selectivity data from rotating ring disc electrode experiments and resulting durability in accelerated stress tests mimicking the load cycle and start‐up and shut‐down cycle conditions. It is discussed how pyrolysis temperature and duration affect FC activity and stability. But, more important, the results connect the pyrolysis conditions to the required accelerated stress test protocol combination to enable a prediction of the catalyst stability in fuel cells.

Evaluation of the durability of ZrC as support material for Pt electrocatalysts in PEMFCs: Experimental and computational studies

Zirconium carbide (ZrC) is evaluated as support material for corrosion resistance to replace carbon in Pt electrocatalyst for the first time. The commercial ZrC (1 m2g-1) is activated using solid sodium carbonate producing high surface area activated ZrC (a-ZrC, 134 m2g-1). Both ZrC and a-ZrC exhibit good corrosion resistance properties during carbon corrosion tests. As supports in Pt/ZrC and Pt/a-ZrC electrocatalysts, the ECSA losses are below 40% after standard accelerated stress test protocols satisfying the standard set targets of DoE. The Pt/ZrC does not show significant shift in the onset potential in the linear sweep voltammetry after start-up and shut-down protocol tests. The shift in E1/2 potentials for Pt/ZrC and Pt/a-ZrC catalysts, respectively, are 13 mV and 24 mV, which are less compared to 30 mV for Pt/C. The computational analysis shows strong-metal-support-interaction between ZrC and Pt which is responsible for catalyst durability because of higher dissolution potential.

Universal Correlation between the Roughness Factor and PEMFC Performance Losses in Voltage Cycling Based Accelerated Stress Tests

During operational lifetime, proton exchange membrane fuel cells (PEMFCs) suffer from high performance losses. For the year 2025, the U.S. Department of Energy (DoE) set the durability target for PEMFC light-duty transportation applications at 5,000 hours.1 To meet the automotive target, different aging protocols were established in order to simulate load cycle variations.2 It is well known that load or voltage cycling induces substantial catalyst degradation due to an increasing loss of electrochemically active surface area (ECSA), which is known to be a main driver for the resulting performance penalties.3 Thus, an increasing interest exists in developing accelerated stress test (AST) protocols that can sufficiently describe the degradation of the different components of a membrane electrode assembly (MEA) during automotive application using shorter measurement times.In this study, we report on a strategy to predict the performance degradation of an MEA, making use of a strongly accelerated voltage cycling based ASTs. For this, voltage cycling based ASTs were performed using 5 cm2 MEAs with a 0.1 mgPt cmMEA -2 loaded Pt/C catalyst (TEC10V20E, Tanaka) for both cathode and anode. Voltage cycling was done under H2/N2 (200/75 nccm) at 80 °C, 95% RH, and ambient pressure using square wave profiles with a constant lower potential limit (LPL) of 0.6 V and different upper potential limits (UPLs) of 0.85, 0.95, and 1.0 V, with LPL/UPL hold times of 1, 2, or 8 s. Full characterization of the MEA at beginning-of-life and after each set of voltage cycling intervals was performed by: i) measuring H2/O2 and H2/air polarization curves; ii) determining the ECSA by cyclic voltammetry and CO stripping; iii) conducting limiting current measurement to calculate the O2 transport resistance (R O2 total); and, iv) conducting electrochemical impedance spectroscopy (EIS) measurements under blocking conditions to quantify the proton conduction resistance in the cathode catalyst layer (R H+ cath).When the ECSA loss ranges between ≈20-85%, it was found that the ECSA decreases approximately linearly when plotted against the logarithm of the number of cycles (see fig. 1), exhibiting higher slopes for procedures with higher UPLs and longer hold times, in agreement with the results from previous studies.3, 4 In addition, a direct and AST protocol independent correlation between the loss in total available surface area, i.e., the roughness factor (rf ≡ ECSA × Pt-loading), and the individual loss contributions in H2/air performance curves, namely kinetic (mass/specific activity for the oxygen reduction reaction (ORR)) and O2 transport resistance contributions, was shown. This is due to the fact, that these individual voltage losses are not only highly affected by the cathode rf,3, 5 but that the Pt dissolution/redeposition mechanism seems to be identical for all of the here investigated voltage cycling ASTs, despite the largely varying UPLs and hold times. As a result, a universal correlation between the H2/air performance losses and the rf deterioration was found throughout all ASTs. The important corollary of this finding is that the data from quickly degrading voltage cycling ASTs (i.e., with high UPL) can be used to project the H2/air performance losses when cycling under less degrading conditions.

Effect of low voltage limit on degradation mechanism during high‐frequency dynamic load in proton exchange membrane water electrolysis

With regard to the hydrogen economy, setting the criteria for the durability evaluation of renewable energy-powered proton exchange membrane water electrolysis (PEMWE) systems is an important milestone. In this study, accelerated stress test (AST) protocols that simulate the fluctuating power supply of renewable energy were explored. The average load was varied by changing the low voltage limit (LVL = 1.4, 1.5, 1.7, and 1.9 V) with fixation of a high voltage limit of 2.2 V. AST protocols can accurately reflect the real solar profile with various loads and high ramp rates. Protocols with the LVL = 1.4 V and LVL = 1.5 V demonstrated opposite trends even with minimal difference in LVL, resulting in positive and negative degradation slopes, respectively. The positive degradation slope (meaning performance decrease) for LVL 1.4 V is attributed to the reversal current, resulting in degradation of the cathode catalyst, which is characteristic of the fuel cell operating mode. On the contrary, a slight performance increase was observed for LVL 1.5 V, presumably caused by the thinning of the membrane and the creation of a rougher surface in the anode/membrane interface. Meanwhile, protocols with higher LVL (LVL 1.7 V and LVL 1.9 V) showed significant mass transport loss observed at voltages larger than 1.8 V. This may be attributed to severe delamination of the catalyst/membrane and/or diffuse layer/catalyst interface, presumably due to the changes in bubble nucleation and the resulting stress. The findings suggest a rational guideline for establishing a unified AST protocol applicable to renewable energy-powered PEMWE systems, and subsequent material development strategies to minimize degradation processes.

Visualization Guided Mitigation of Mechanical Membrane Degradation in Fuel Cells

Dynamic operational power demands from a fuel cell, especially in transportation applications, are known to causefluctuations in its relative humidity (RH) and temperature [1]. This leads to repetitive in-plane compressive and tensile stress build-up in the confined perfluorosulfonic acid (PFSA) ionomer membranes due to constraining of their cyclic swelling and shrinkage under hydrated and dehydrated states, respectively. The ionomer’s dynamic water sorption characteristics during these hygrothermal variations lead to catalyst coated membrane (CCM) deformation and cyclic mechanical stresses resulting in structural fatigue and micro-crack development at high stress accumulation sites [2]. Lately, laboratory-based X-ray computed tomography (XCT) was used for non-destructive four-dimensional (4D) visualization and microstructural analysis of fuel cell parts showing novel insights on their failure processes [3,4]. The objective of the present work is to leverage 4D XCT visualization to design and demonstrate novel mitigation strategies for mechanical membrane degradation in fuel cells. A custom designed X-ray transparent fixture housing a small-scale single cell fuel cell was subjected to 2 min. each of wet and dry cycles with N2 gas at anode and cathode compartments, thereby causing fatigue-based membrane damage [5]. 3D tomographic data sets of same membrane electrode assembly (MEA) locations were acquired as a function of degradation time, which facilitated to study the damage development and growth. As a baseline for this work, a non-bonded MEA with high surface roughness gas diffusion layers (GDLs) demonstrated that the primary membrane failure mode was CCM buckling into GDL voids/pores [6]. To overcome this failure mode, two separate mitigation approaches are explored in the present work: (i) a commercial GDL with low surface roughness; and (ii) bonding of MEA to arrest buckling. Furthermore, a multi-physics finite element method (FEM) model is developed to simulate the contact interaction at the catalyst layer | GDL interface and non-uniformity in the GDL microstructure. The XCT findings are correlated with the numerical model to perceive the specific impact of GDL surface features on the development and distribution of membrane failures.Similar to the baseline MEA, membrane crack development was observed to be the key failure mode for both mitigation approaches. When the two mitigation designs are compared at similar cycling stages with the baseline (Fig. 1), however, the size and density of membrane cracks is drastically reduced. Since both mitigation strategies are tested using the same accelerated stress test protocol as the baseline, it is hereby established that each approach provides substantial mitigation against fatigue driven mechanical membrane degradation, predominantly due to reduced severity of CCM buckling deformation, resulting in a doubling of the membrane lifetime in each case. Complementary finite element simulations corroborate the experimental findings and further estimate the critical GDL void sizes to prevent CCM buckling and the required interfacial MEA adhesion quality to stabilize the MEA for overall membrane durability. Acknowledgements Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program.

Effect of Commercial Gas Diffusion Layers on Catalyst Durability of Polymer Electrolyte Fuel Cells in Varied Cathode Gas Environment

Improvement in durability of polymer electrolyte fuel cells (PEFCs) accelerates the commercialization of the technology in both light-duty and heavy-duty commercial vehicle applications. The Department of Energy (DOE) has adopted an accelerated stress test (AST) protocol to evaluate the durability of catalyst in PEFCs1. The protocol simulates a typical drive cycle and consists of potential cycling (30,000 square wave cycles) in nitrogen gas environment on cathode. The potential is cycled from lower potential limit (LPL) of 0.6 V to upper potential limit (UPL) of 0.95 V with a hold time of 3 seconds at each potential. Multiple studies have shown UPL, relative humidity (RH) and temperature as the primary stressors of catalyst degradation2,3. Gas diffusion layers (GDLs) play a key role of water management in the cathode catalyst layer and can therefore directly affect the local RH4. Currently, properties such as porosity, micro porous layer (MPL) thickness, PTFE content and thermal conductivity of commercially available GDLs have been optimized for peak power density at beginning of life under varying operating conditions including wet (high RH) and dry (low RH). Little is known about the effect of GDL on the durability of catalyst in ASTs.In this study we compare Pt catalyst aging in membrane electrode assemblies using three different commercially available GDLs, with distinct thermal, mechanical, and electrical properties. In the first part of the study, the DOE adopted AST protocol is used at 80°C operating temperature and 100% RH. In situ electrochemical characterization at regular intervals is utilized to identify electrochemical surface area loss (ECSA) trends and polarization performance. Figure 1 A shows before and after AST polarization curves for different GDLs and Figure 1 B shows ECSA loss as a function of AST cycles. Measurements were taken before AST, after 1,000, 5,000, 15,000 and 30,000 cycles. It was observed that for AST conducted in nitrogen cathode gas environment, catalyst durability is independent of GDL used. This is also confirmed by the ECSA loss trend. The indifference in Pt catalyst degradation can be attributed to lack of water produced due to absence of oxygen reduction reaction in nitrogen environment. All the GDLs lost approximately 80% of their ECSA from before AST. The second part of this study aims to elucidate effect of GDL properties on catalyst durability by conducting AST in air environment with open circuit potential as UPL. Figure 1 A. Before and After AST polarization curves collected at 80°C, 100% RH and 150kPa (a) backpressure in differential conditions B. ECSA loss as function of AST cycles.

An Identical-Location STEM Study of the Degradation of Oer Electrocatalysts for PEM Electrolyzers

The slow kinetics of the oxygen evolution reaction (OER) and high cost of OER electrocatalysts are among the main barriers for proton exchange membrane water electrolysis (PEMWE) technology [1]. Improving the design of active and low-loaded OER catalysts requires a fundamental understanding of catalyst degradation mechanisms.Current state-of-the-art OER catalysts for PEM electrolyzers are iridium-based, such as pure iridium oxide or mixed oxides of iridium and ruthenium. The dissolution of iridium catalyst in acidic environment during OER has been reported extensively in both liquid electrolyte [2] and membrane electrode assemblies (MEAs) [3]. Current understanding of the iridium catalyst degradation reveals a trade-off between stability and activity. Hydrous iridium oxide (IrOx) shows higher activity, but less stability compared to rutile iridium oxide (IrO2) [4, 5]. Mechanistic studies reveal Ir (IV) oxide being the stable oxide and Ir (III) oxide being the intermediate species during OER and dissolving rapidly [4, 5]. Furthermore, density functional theory (DFT) and experimental work show that oxygen binding energy is affected by surface facets and sub-surface structures for iridium-based catalysts [6]. Surface oxide skins can exhibit unique structural features, including different coordination, compression or expansion related to lattice mismatch, and weaker or stronger binding relative to the pure metal or metal oxide, which govern the OER kinetics at end-of-life (EOL). [6]Obtaining an electrocatalyst structure that provides both high activity and high stability, particularly at high current densities, remains a challenge. Stabilizing the weak iridium intermediate caused by oxygen vacancies will be of great importance [5] and requires a fundamental understanding of iridium surface oxides to address this challenge. It is unclear how the surface oxides are developed as degradation progresses and what roles of other accompanied phenomena, such as surface reconstruction and segregation [6], are playing during degradation. Therefore, the formation and structural change of iridium surface oxide and subsequent impact on dissolution rates during accelerated stress test (AST) is the focus of this work.Identical-location scanning transmission electron microscopy (IL-STEM) is a powerful approach for tracking the evolving morphology and chemistry of a catalyst particle during degradation [7, 8]. In this work, commercially available iridium oxide catalyst will be studied using IL-STEM. Aberration-corrected STEM coupled with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) will be used to study the catalyst surface structure and composition at the atomic scale. The effect of different upper potential limits and potential scanning profiles will also be investigated. The dissolution rates of iridium under these different AST protocols will be determined using time-resolved inductively coupled plasma-mass spectrometry (ICP-MS). Changes in the composition and structure of iridium surface oxide before and after ASTs will be analyzed with EDS and EELS and corroborated with results from X-ray photoelectron spectroscopy (XPS) and, further down the road, operando X-ray absorption spectroscopy (XAS). The outcome of this work can shed light on the strategies to maintain high activity for precious metal catalysts.[9]References[1] Buttler, A., Spliethoff, H., Renewable and Sustainable Energy Reviews, 2018, 82, 2440-2454.[2] Cherevko S., Geiger S., Kasian O., et al., J. Electroanal. Chem. 2016, 774, 102–110.[3] Yu, H., Bonville, L., Jankovic, J., et al. Appl. Catal. B: Environ., 2020, 260, 118194.[4] Pfeifer V., Jones T.E., Velasco Velez J.J., et al., Phys. Chem. Chem. Phys. 2016, 18, 2292–2296.[5] Geiger, S., Kasian, O., Ledendecker, M. et al. Nat. Catal. 2018, 1, 508–515.[6] Alia, S.M., Ha, M-A., Anderson, G.C., et al. J. Electrochem. Soc. 2019, 166, F1243-F1252.[7] Hartl, K., Hanzlik, M, Arenz, M., Energy Environ. Sci., 2011, 4, 234-238.[8] Rasouli, S., Myers, D., Kariuki N., et al., Nano Lett. 2019, 19, 46−53.[9] This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the H2NEW Consortium. Electron microscopy was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Pulsed vs. galvanostatic accelerated stress test protocols: Comparing predictions for anode reversal tolerance in proton exchange membrane fuel cells

Fuel starvation events occur in proton exchange membrane fuel cells (PEMFCs) when the flow fields become blocked by ice or dust/debris from the environment. When this occurs, the electrons at the anode catalyst layer (ACL) must be generated by either water oxidation or carbon corrosion which forces the cell into a ‘reversal’ event. Such events rapidly degrade the anode catalyst layer, but can be mitigated through the use of reversal tolerant catalysts (RTCs) such as IrOx. While widely used in industry, this topic has not received significant attention in the literature. At the membrane electrode assembly (MEA)-level, there are two main approaches to evaluate reversal tolerance: 1) Galvanostatic and 2) Pulsing. In galvanostatic accelerated stress tests (ASTs), the current is held at a constant value until the cell voltage reaches a predetermined value. For pulsing ASTs, the MEA is put through short term ‘reversal’ conditions before going back to normal operating mode. These ASTs are in fact quite different in terms of the stress they put on the ACL. Here, we directly compare these two ASTs, showing that, when normalized for the same amount of total reversal time, the pulsing experiments result in more severe anode damage than galvanostatic testing.

Mitigated Start-Up of PEMFC in Real Automotive Conditions: Local Experimental Investigation and Development of a New Accelerated Stress Test Protocol

This study combines local electrochemical diagnostics with ex situ analysis to investigate degradation mechanism associated to start-up/shut-down (SU/SD) of PEMFC and mitigation strategies adopted in automotive stacks. Local degradation resulting from repeated SU/SD was analyzed with and without mitigation strategies by means of a macro-segmented cell setup provided with Reference Hydrogen Electrodes (RHEs) at both anode and cathode to measure local electrodes potential and current. Accelerated Stress Test (AST) for start-up with and without mitigation strategies are proposed and validated. A ten-fold acceleration of performance loss due to un-mitigated SU/SD has been calculated with respect to AST for catalyst support. Under mitigated SU/SD, instead, a strong degradation was observed as localized at cathode inlet region (i.e. −38% ECSA loss and −22 mV voltage loss after 200 cycles) due to local potentials transient reaching up to 1.5 V vs RHE. These localized stress conditions were additionally reproduced in a zero-gradient and a new AST protocol (named start-up AST) was proposed to mimic the potential profile observed upon SU/SD cycling. Representativeness of the start-up AST for real world degradation was confirmed up to 200 cycles. Platinum dissolution and diffusion/precipitation within the polymer electrolyte was shown to be the dominant mechanism affecting performance loss.

Development of mathematical transfer functions correlating Solid Oxide Fuel Cell degradation to operating conditions for Accelerated Stress Test protocols design

This work presents an innovative model-based approach for the development of mathematical transfer functions capable of correlating Solid Oxide Fuel Cells (SOFCs) degradation rate to the applied operating conditions. Such functions can be used for fuel cell lifetime prediction and Accelerated Stress Test (AST) protocols design. The proposed approach relies on multiscale modelling methodology and links local degradation to high level performance models to evaluate the key operating variables accelerating voltage decay over time. A thorough simulation analysis is performed to convey the correlation among operating variables and degradation rate into mathematical transfer functions. To better illustrate the overall design and application process of such functions, a case study accounting for Ni agglomeration is addressed. The multiscale modelling framework is applied to correlate microscale (i.e., Ni particles size change) and macroscale (i.e., SOFC voltage reduction) levels through the most affected mesoscale parameters. The model is then used to simulate voltage decay over time and link degradation rates to the applied operating conditions. Afterwards, a parametric analysis is performed to investigate the influence exerted by each operating variable on the degradation rate and derive the transfer functions. An example of application for Accelerated Stress Test (AST) protocols design is then given.

Iron Contamination and Local Distribution within a 50 Cm Single-Channel Proton Exchange Membrane Water Electrolysis Cell

Proton exchange membrane water electrolysis (PEMWE) will play a major role in the clean and versatile energy conversion in the near future.[1] Offering scalable solutions of PEMWE are of high industrial interest and have to provide a sufficient durability and reliability.[2] To achieve this, a better understanding of the impact of contamination on the PEMWE performance is very crucial. In this context, the membrane electrode assembly (MEA) is susceptible to several degradation processes like metallic cations (Fe3+/2+ stemmed from tubing). In particular, soluble and reduced iron species forming by Fenton reaction might attack via radicals the MEA.[3,4] Hence, analyzing impurities and their resulting impacts on e.g. the proton transport resistance and degradation mechanisms at different length scales and local distributions are of large interest.In this work, the influence of spatially resolved iron contamination in MEAs on the cell performance has been investigated using a 50 cm single-channel PEMWE cell equipped with local current density measurement setup.[5] Adding defined amounts of FeSO4 via the anode inlet feed, the changes on the three main regimes of overpotentials, i.e. kinetic, ohmic and mass transport, is correlated with the current density mapping as well as local and global electrochemical impedance spectroscopy (EIS) measurements. Ex-situ scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) are used to detect the distribution and agglomeration of iron species within the MEA.With this combined approach of electrochemical and ex-situ physical methods, the contamination pathways and their effects of (spatially resolved) iron contamination on the overall performance of MEA are investigated as a function of degree of contamination and operating conditions. Especially, the (spatially resolved) overpotential-specific degradation processes along the 0.5 m channel, which mimic a technically relevant stack size, allow us to conclude on the optimum design of MEA, targeting use of mitigation strategies and development of accelerated stress test protocols.[1] M.A. Pellow, C.J.M. Emmott, C.J. Barnhart, S.M. Benson, Energy Environ. Sci. 8 (2015) 1938-1952.[2] P.K. Shen, C.-Y. Wang, S.P. Jiang, X. Sun, J. Zhang, Electrochemical Energy: Advanced Materials and Technologies, CRC Press, 2015.[3] X. Wang, L. Zhang, G. Li, G. Zhang, Z.-G. Shao, B. Yi, Electrochim. Acta 158(2015) 253-257.[4] C. Rozain, P. Millet, Electrochim. Acta 131 (2014) 160e167.[5] C. Immerz, B. Bensmann, P. Trinke, M. Suermann, R. Hanke-Rauschenbach, JECS 165 (16) (2018)

Status and challenges for the application of platinum group metal-free catalysts in proton-exchange membrane fuel cells

Platinum group metal-free electrocatalysts are an important class of materials for the sustainable energy economy and significant effort has been made toward developing platinum group metal-free catalysts to replace their costly Pt counterparts in fuel cell electric vehicles. The progress made in activity over the last decade is reviewed, along with synthesis strategies for more controlled active site formation and performance gains achieved through electrode engineering. The increased focus on durability, the growing understanding of degradation mechanisms, and the need for standard performance and accelerated stress test protocols are discussed.