Accelerated Stress Test Research Articles

IrO2/Ir Composite Nanoparticles (IrO2@Ir) Supported on TiNxOy Coated TiN: Efficient and Robust Oxygen Evolution Reaction Catalyst for Water Electrolysis

AbstractIt is crucial but challenging to reduce the required noble‐metal loading without compromising the catalytic performance of oxygen evolution reaction (OER) catalysts. This study presents a highly active OER catalyst composed of IrO2 with Ir rich surface (IrO2@Ir) nanoparticles supported over nano TiN coated with TiOxNy (IrO2@Ir/TiN). The present approach demonstrates superior OER catalysts with high activity through small, uniformly dispersed IrO2@Ir nanoparticles, along with high durability owing to robust catalyst support and strong catalyst‐support interaction. The synthesized IrO2@Ir/TiN with an Ir loading of 40 wt % exhibits a mass‐normalized OER activity of 637 AgIr−1, which is 2.4 times that of the unsupported commercial benchmark IrO2 OER electrocatalyst. The fine nanoparticles and high activity enable significant (∼60 %) reduction in the Ir metal loading required to obtain equivalent OER performance. In addition, when evaluated through an accelerated stress test using potential cycling, the catalyst exhibits outstanding durability (79 % retention) compared to that of the commercial equivalent (66 % retention). The OER activity loss was attributed to the catalyst dissolution (30 % loss) and the catalyst particle growth (70 %), with no measurable loss due to the TiN support corrosion. The development of ultra‐fine IrO2@Ir nanoparticles and robust ceramic catalyst support significantly improved the Ir utilization and open a new perspective for supported OER catalyst.

Towards a realistic prediction of catalyst durability from liquid half-cell tests.

Liquid half-cell measurements provide a convenient laboratory method for determining relevant parameters of electro-catalysts applied in e.g. polymer electrolyte membrane fuel cells. While these measurements may be effective in certain contexts, their applicability to real-world systems, such as single-cells in a membrane electrode assembly (MEA) configuration, is not always clear. This is particularly true when assessing the stability of these systems through accelerated stress tests (ASTs). Due to different electrode compositions and operating conditions, nanoscale degradation proceeds differently. Nevertheless, given the high demands of MEA measurements in terms of time, testing equipment complexity, and amount of catalyst material, application-relevant predictions of catalyst durability from liquid half-cell tests are highly desirable. This study combines electrochemical and nanoparticle analysis based on transmission electron microscopy to conduct a typical voltage cycling AST for rotating disc electrode (RDE) measurements, showing that the loss of the electrochemically active surface area (ECSA) of the used Pt/Vulcan catalyst is strongly enhanced at 80 °C compared to room temperature, which goes along with increased nanoparticle coarsening. Additionally, a high ionomer/carbon mass ratio (I/C = 0.7) accelerates the ECSA loss, and further investigations of its influence suggest a combination of several factors, including the high local proton concentration and the presence of adsorbing anions. At the same temperature (80 °C) and I/C ratio (0.7), the ECSA loss vs. AST cycle number of the Pt/Vulcan catalyst is essentially identical for a voltage cycling AST conducted in either an RDE half-cell or an MEA configuration, suggesting that liquid electrolyte half-cell based ASTs can provide application-relevant results. Thus, our study points out a way for predicting the stability of electro-catalysts in MEAs based on RDE experiments that require less specialized equipment and only μg-quantities of catalysts.

Does the platinum-loading in proton-exchange membrane fuel cell cathodes influence the durability of the membrane-electrode assembly?

MEAs with various cathode Pt loadings were elaborated and aged using a multiple-stressor accelerated stress test (AST) in a segmented PEMFC.

Mercaptopropane-assisted synthesis of graphitic carbon-supported Pt nanoparticles for enhancing fuel cell start-stop performance

Although graphitic carbons, as a support for the cathode catalyst in proton exchange membrane fuel cells, have significant advantages in enhancing the corrosion resistance of the catalyst, the preparation of small-sized Pt particles on the graphitic carbon support often faces challenges due to its low porosity and lack of defect structures. Here, we report a mercaptopropane-assisted impregnation method to achieve size control of Pt nanoparticles on graphitic carbon. We show that mercaptopropane can coordinate with Pt during the impregnation process and transform into sulfur-doped carbon coatings through the subsequent thermal reduction process, which ensures the formation of small-sized Pt nanoparticles on graphitic carbon. Due to effective size control, the prepared cathode catalyst exhibited enhanced fuel cell performance compared to the catalyst prepared by the traditional impregnation method. We performed the accelerated stress test on the synthesized catalyst using the durability protocol recommended by the U.S. Department of Energy (DOE). After 5000 voltage cycles in the range of 1.0–1.5 V, the catalyst showed a negligible voltage loss of only 10 mV at a current density of 1.5 A·cm<sup>−2</sup>, meeting the DOE support durability target (30 mV).

High-performance atomic Co/N co–doped porous carbon catalysts derived from Co–doped metal–organic frameworks for oxygen reduction

Improving the activity and durability of carbon-based catalysts is a key challenge for their application in fuel cells. Herein, we report a highly active and durable Co/N co-doped carbon (CoNC) catalyst prepared via pyrolysis of Co–doped zeolitic–imidazolate framework–8 (ZIF–8), which was synthesized by controlling the feeding sequence to enable Co to replace Zn in the metal–organic framework (MOF). The catalyst exhibited excellent oxygen reduction reaction (ORR) performance, while the half-wave potential decreased by only 8 mV after 5,000 accelerated stress test (AST) cycles in an acidic solution. Furthermore, the catalyst exhibited satisfactory cathodic catalytic performance when utilized in a hydrogen/oxygen single proton exchange membrane (PEM) fuel cell and a Zn-air battery, yielding maximum power densities of 530 and 164 mW cm−2, respectively. X-ray absorption spectroscopy (XAS) and high-angle annular dark field-scanning transmission electron microscopy (HAAD–STEM) analyses revealed that Co was present in the catalyst as single atoms coordinated with N to form Co–N moieties, which results in the high catalytic performance. These results show that the reported catalyst is a promising material for inclusion into future fuel cell designs.

Prescriptive Lifetime Management for PEM fuel cell systems in transportation applications, Part I: State of the art and conceptual design

Proton exchange membrane (PEM) fuel cells represent a promising solution for a sustainable hydrogen-based mobility. Due to remaining challenges regarding costs and durability of PEM fuel cell systems, market activation programs as a precursor to market introduction are still required to accelerate the broad commercialization of PEM fuel cell systems in transportation applications. The recent shift by some original equipment manufacturers (OEMs) from passenger car (PC) and light-duty vehicle (LDV) applications to heavy-duty vehicle (HDV) applications is accompanied by higher lifetime and system power requirements that necessitate modified design and development approaches. Design-based approaches to increase the lifetime of PEM fuel cells usually concentrate on the use of advanced materials and the design of durable fuel cell stacks. Operation-based approaches aim to mitigate degradation through the development of advanced control strategies. However, the initial design of a PEM fuel cell system as well as the calibration of its control strategy to achieve a long service life is inevitably associated with a high degree of uncertainty. In addition, often challenging time-to-market specifications result in insufficient time for extensive durability testing, necessitating the use of accelerated stress tests (AST). Results of ASTs usually deviate to a greater or lesser extent from realistic driving conditions and exacerbate the issue of uncertainty regarding durability in field operation. Therefore, the coordinated development and deployment of on-board strategies for condition assessment, lifetime prediction and degradation mitigation are of huge importance to meet the high lifetime requirements in HDV applications.With this in mind, this paper presents an on-board Prescriptive Lifetime Management system that enables optimization-based post-prognostic decision-making to extend the service life of PEM fuel cell systems while considering the need to maximize system performance and efficiency. Based on the current state of the art, the requirements of the Prescriptive Lifetime Management are defined and its conceptual design as well as modeling and simulation framework are described.

Electrochemical characterization of evolving ionomer/electrocatalyst interactions throughout accelerated stress tests

The degradation of polymer electrolyte membrane fuel cells (PEMFCs) catalyst layers used for heavy-duty vehicles was examined using a catalyst-specific accelerated stress test (AST). High surface area carbon supported dispersed Pt (Pt/HSC), annealed Pt (a-Pt/HSC) and PtCo (PtCo/HSC) alloy catalysts were examined over the course of 90,000 cycles by measuring changes in mass activity, O2 transport resistance, electrochemical active surface area (ECSA), catalyst accessibility and ionomer-electrocatalyst interactions. Compared to a-Pt/HSC and Pt/HSC catalyst, the PtCo/HSC showed better initial mass activity, a larger initial mass transport loss, and faster degradation after the first 30k AST cycles, as a large portion of Co leached out during potential cycling. Pt/HSC showed higher initial performance relative to a-Pt/HSC but had faster degradation. STEM characterizations show that the ECSA losses are largely related to Pt dissolution resulting in either catalyst particle growth via the Ostwald ripening mechanism or redeposition in the membrane. Catalyst accessibility measurements showed decreased RH sensitivity for all three samples, while CO impedance measurements revealed a decrease in both Pt-water and carbon-water interactions. This implies that, Pt is either preferentially redepositing on the exterior of the carbon support, or that the ionomer is undergoing morphological changes enabling the enhanced intrusion of mesopores.

Low Temperature Curable Low Dk & Df Polyimide for Redistribution Layer

In this paper, we report low temperature curable (around 200～250℃), low Dk (2.7) and low Df (0.007) negative tone type photo-definable polyimides. Those materials, which were in states of liquid or B-stage sheet, were able to be patterned by photolithography process. Our novel polyimides showed excellent dielectric and mechanical properties, and good results on reliability tests. Our developed polyimides kept enough insulating performance after a biased HAST (High Accelerated Stress Test). Finally, in collaboration with IME, a test package which mounted our novel polyimide as redistribution layers was fabricated.

Influence of Titanium Additions on The Electrochemical Behaviour of NiCr/Ti Laser Cladded Coatings

In this research, the electrochemical evaluation of NiCr/Ti laser cladded coatings in simulated polymer electrolyte membrane fuel cells (PEMFCs) environment was investigated. The Laser Cladding technique was used to develop protective coatings on mild steel substrate using NiCr-based powders mixed with 12.5, 15, 17.5 and 20 wt.% Ti additions. The samples were tested at room temperature in Na2SO4 0.1M + 0.1 ppm F-. The potentiodynamic polarization curves are presented before and after the samples were subjected to accelerated stress tests, for 6 hours each, at +0.736 V (cathodic environment) and at -0.493 V (anodic environment). Afterwards, scanning electron microscopy (SEM) was used to investigate the effect of Ti addition in terms of morphology. Energy-dispersive X-ray spectroscopy (EDS) was performed for chemical evaluation of the surface after corrosion tests.

Lifetime prediction method of proton exchange membrane fuel cells based on current degradation law

Lifetime evaluation and prediction of proton exchange membrane fuel cells (PEMFCs) are essential for the lifetime extension and commercialization of fuel cells. Under the background that the accelerated stress test (AST) is difficult to evaluate the lifetime accurately, and the actual road test is time-consuming and costly, it is necessary to propose a quick lifetime prediction method of fuel cells. In this study, the current degradation law is analyzed based on the first-order kinetic model, and the lifetime prediction method based on the current degradation law is proposed. Moreover, the scale factor k is proposed to completely describe the fuel cell degradation due to complex degradation causes, and its calculation and selection methods are discussed. Finally, the accuracy of the proposed method is verified by the experimental results of single cells and fuel cell stacks, and the maximum relative error is less than 5% when the fuel cells conform to the first-order kinetic model. The lifetime prediction method of PEMFCs based on current degradation law improves the utilization of degradation information in current density, which is a feasible way for automotive fuel cell lifetime evaluation and prediction.

Durability study of direct methanol fuel cell under accelerated stress test

Durability of direct methanol fuel cell (DMFC) is one of the key issues for its commercial applications. This work proposes accelerated stress test (AST) protocols including high operation temperature, high current density, dry-wet cycle to resemble real-life operation conditions and shorten evaluation time. The cell with Nafion membrane is operated under a relatively high temperature of 88 °C with ambient pressure air, and an unprecedented constant current density of 400 mA cm−2. After 850 h of operation, the cell still survives with a permanent degradation rate of about 48 μV hr−1. From the in-situ electrochemical diagnostics, no pronounced ohmic loss occurs, indicating stable membrane/electrode interface. However, both anode and cathode performance degradation are found. The anode exhibits high mass transfer resistance at high current density due to degradation of gas diffusion layer (GDL). The GDL compensated anode potential loss, induced by the degradation of PtRu/C catalyst, is comparable to that of cathode at whole current density region. The degradation of cathode is primarily attributed to the loss of electrochemical surface area (ECSA) that is associated with the degradation of Pt/C and PtRu/C catalyst.

Understanding the Stability of Manganese Chromium Antimonate Electrocatalysts through Multimodal In Situ and Operando Measurements.

Improving electrocatalyst stability is critical for the development of electrocatalytic devices. Herein, we utilize an on-line electrochemical flow cell coupled with an inductively coupled plasma-mass spectrometer (ICP-MS) to characterize the impact of composition and reactant gas on the multielement dissolution of Mn(-Cr)-Sb-O electrocatalysts. Compared to Mn2O3 and Cr2O3 oxides, the antimonate framework stabilizes Mn at OER potentials and Cr at both ORR and OER potentials. Furthermore, dissolution of Mn and Cr from Mn(-Cr) -Sb-O is driven by the ORR reaction rate, with minimal dissolution under N2. We observe preferential dissolution of Cr totaling 13% over 10 min at 0.3, 0.6, and 0.9 V vs RHE, with only 1.5% loss of Mn, indicating an enrichment of Mn at the surface of the particles. Despite this asymmetric dissolution, operando X-ray absorption spectroscopy (XAS) showed no measurable changes in the Mn K-edge at comparable potentials. This could suggest that modification to the Mn oxidation state and/or phase in the surface layer is too small or that the layer is too thin to be measured with the bulk XAS measurement. Lastly, on-line ICP-MS was used to assess the effects of applied potential, scan rate, and current on Mn-Cr-Sb-O during cyclic voltammetry and accelerated stress tests. With this deeper understanding of the interplay between oxygen reduction and dissolution, testing procedures were identified to maximize both activity and stability. This work highlights the use of multimodal in situ characterization techniques in tandem to build a more complete model of stability and develop protocols for optimizing catalyst performance.

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.

Long-Term Temperature-Dependent Degradation of 175 W Chip-on-Board LED Modules

We report on the degradation dynamics and mechanisms of the commercially available chip-on-board (COB) high-power light-emitting diode (LED) modules with an electrical power of 175 W. Due to the associated thermal load, the temperature dependence of the aging processes is additionally analyzed within the scope of this work. The aging tests were performed for a period of 6000 h at four different case temperatures between 55 °C and 120 °C. The results of the accelerated stress tests indicate a temperature-activated aging process, which severely limits the lifetime of the modules. In addition, the following key findings can be reported: 1) a significant decrease in optical power occurs within 6000 h of operation; 2) depending on the stress test condition the accompanying color shifts exceed a limit of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta {u}\,'{v}\,'={0}.{007}$ </tex-math></inline-formula> ; and 3) the limiting degradation mechanism can be attributed to the package of the device and can be accelerated with temperature, current, and chemicals. Reported findings can be manifested by additional optical material inspections, allowing to use the results for optimizations of future module generations.

Short side chain perfluorosulfonic acid composite membrane with covalently grafted cup stacked carbon nanofibers for polymer electrolyte fuel cells

Efficient proton conduction through polymer electrolyte membrane is critical in the performance of low temperature polymer electrolyte fuel cells (LT-PEFC). Herein, polystyrene sulfonic acid grafted cup stacked carbon nanofiber (PSSA-g-CSNF) incorporated short side chain (SSC)-perfluorosulfonic acid (PFSA) ionomer composite polymer electrolyte is discussed. An optimum level of PSSA-g-CSNF introduced as an additive to SSC-PFSA matrix exhibit higher proton conductivity, improved mechanical, chemical stability, and limited hydrogen crossover towards PEFC application in comparison to recast pristine SSC-PFSA membranes (Aquivion®-720). Maximum current density of 2.5 A cm−2 at 0.5 V and peak power density of 1.4 W cm−2 under H2 and O2 condition is seen for the optimized nanocomposite membrane. Besides, the peak power density of the nanocomposite membrane shows an enhancement higher than 22% and 42% in relation to recast pristine Aquivion and Nafion212 respectively. In addition to this, nanocomposite membrane shows higher chemical stability and retention of 80% initial OCV even after 100 h of accelerated stress test (AST) as compared to 73% for pristine Aquivion. Uniform dispersion of PSSA-g-CSNF in Aquivion facilitates facile proton transport by limiting fuel cross-over via hydrogen bonding interaction with closely spaced hydrophilic groups in the membrane matrix in LT-PEFC.

Local ageing effects of polymer electrolyte fuel cell membrane electrode assemblies due to accelerated durability testing

AbstractAccelerated durability test (ADT) protocols are useful tools to reduce testing time and development costs of automotive polymer electrolyte fuel cell stacks. Established accelerated stress tests allow comparing individual cell components. However, such tests in general do not allow drawing reliable conclusions on the expected lifetime for a complete stack. In this work, we examine the influence of combined stressors on the ageing behavior of individual cell components operated on stack level. We combine known main stressors for mobile fuel‐cell operation such as dynamic load, temperature and humidity cycling or hydrogen/air fronts during start‐up to develop a new accelerated test protocol. It was applied to an automotive 5‐cell short stack for 460 operating hours (OpH). A second stack was operated in a reference long‐term test for 2300 OpH. Several in situ characterization techniques, such as cyclic voltammetry, and recording the local current density distribution were employed. In addition, post‐mortem analyses, such as focused ion beam scanning electron microscopy imaging, was applied in order to better understand the degradation mechanisms. The results presented here provide an excellent basis for the further development of a new ADT protocol, which will accelerate ageing processes in the fuel cell in a realistic way, i.e. leading to similar degradation characteristics as in long term testing.

Tailoring the Durability of Carbon-Coated Pd Catalysts Towards Hydrogen Oxidation Reaction (HOR) in Alkaline Media

For the hydrogen oxidation reaction (HOR), platinum group metal (PGM) catalysts are still the norm, and their large initial catalytic activity is counterbalanced by their uncertain durability in alkaline environments: harsh degradations proceed, like detachment and agglomeration of metallic nanoparticles from the carbon support, as witnessed from dedicated accelerated stress tests (AST). Herein, a strategy to increase such catalysts’ durability is provided, using carbon layers surrounding the Pd-based carbon-supported nanoparticles. The robustness of such catalysts, baring 0.5- or 0.7-nm-thick carbon cap over the Pd nanoparticles, is evaluated from AST combined with several pre/post-test characterization techniques: identical location transmission electron microscopy (IL-TEM), ex situ X-ray photoelectron spectroscopy (XPS) and ex situ inductively coupled plasma mass spectrometry (ICP-MS). The carbon layer protection limits the Pd-based nanoparticles’ agglomeration, detachment, and metal leaching, improving long-term catalyst durability in HOR-like operation. Thicker carbon layers surrounding the Pd nanoparticles lead to higher materials durability and lower degradation rate (larger performance stability) upon AST, compared to thinner carbon layers. In addition, the carbon-capped catalysts enable to maintain better the required Pd/PdO state of the surface that is essential for fast HOR, resulting in superior intrinsic HOR activity versus unprotected Pd/C. Overall, this work demonstrates that the activity-durability relationship can be tuned for carbon-capped catalysts.Graphical Abstract

Ordered CoPt oxygen reduction catalyst with high performance and durability

A high-performance and durable polymer electrolyte membrane fuel cell cathode catalyst composed of an ordered L1 0 –CoPt core and a thin Pt shell was designed and prepared. Under practical fuel cell membrane electrode assembly testing conditions, the cathode catalyst showed an outstanding initial mass activity of 0.6 A/mg Pt , which satisfies the U.S. Department of Energy performance target while also meeting the durability target of less than 40% loss in mass activity after 30,000 accelerated stress test voltage cycles. The high structural stability of the L1 0 –CoPt@Pt-shell catalyst was confirmed by postmortem materials characterization, and the origin of this robustness was revealed by density functional theory calculations, where the barriers for diffusion of Co atoms were observed to be significantly increased in the ordered intermetallic core. • An intermetallic ORR catalyst was prepared through a scalable impregnation method • Mass activity of 0.6 A/mg Pt and high durability were achieved in fuel cell testing • H 2 /air power density of 0.87 W/cm 2 at 0.67 V was achieved with only 0.1 mgPt/cm 2 • Order-dependent energetics of vacancy-mediated diffusion were evaluated using DFT Fuel cells with high power density and high durability could contribute to the realization of net zero carbon emissions in the transportation sector. The catalyst reported here, which uses atomic-level ordering to stabilize Co within the intermetallic CoPt lattice, achieves higher Co retention and better durability compared with conventional CoPt catalysts. The origins of this high durability were investigated using density functional theory calculations, which revealed significantly higher barriers for Co diffusion in ordered catalysts compared with disordered ones. This new intermetallic catalyst is a promising candidate for deployment in transportation and other clean-energy fuel cell applications. An oxygen reduction reaction catalyst composed of core-shell CoPt nanoparticles possessing an ordered Co–Pt core and Pt shell shows excellent performance and durability in fuel cell testing.

Silicon-doped niobium suboxide (NbOS): A fuel cell electrocatalyst support with enhanced conductivity and corrosion-resistance

A novel carbon-free silicon-doped niobium suboxide was synthesized and tested as an electrocatalyst support for fuel cell applications. The promising Nb0·24O0·46Si0.30 (NbOS) metal oxide support exhibits a low structural band-gap with significantly high electronic conductivity. Furthmore, the NbOS support shows high corrosion-resistance in acidic media at high potentials. Pt nanoparticles were dispersed over the NbOS support (Pt/NbOS) to create an electrocatalyst with excellent electroactivity towards the oxygen reduction reaction (ORR). Likewise, in situ electrochemical analysis, and impedance spectroscopy (EIS) confirmed remarkable durability of the Pt/NbOS catalyst when subjected to harsh accelerated stress testing (AST) condtions. The enhanced stability and electrocatalytic activity and of Pt/NbOS is attributed to the strong metal-support interaction (SMSI) between the Pt nanoparticles (NPs) and the conductive NbOS support, which were confirmed via XRD and XPS measurements.

Beware of cyclic voltammetry! Measurement artefact in accelerated stress test of fuel cell cathode revealed by operando X-ray diffraction

Efficient screening of proton exchange membrane fuel cell (PEMFC) cathode catalysts long-term stability at the laboratory scale requires the use of accelerated stress tests (ASTs), mostly consisting in cathode potential cycling in the device operational range. However, AST protocols must be complemented with auxiliary in situ electrochemical characterization techniques, such as cyclic voltammetry, at least at the beginning and at the end of test. Supposedly non-destructive, such techniques are necessary to interpret the AST in terms of catalyst performance stability.Here, by combining operando X-ray diffraction, on-line inductively coupled plasma mass spectrometry (ICP-MS) and ex situ characterization techniques, we provide experimental evidence that the cyclic voltammetry experiment performed at the end of AST controls the increase of the coherent domain size monitored on the aged catalyst. We propose this process originates from coalescence of crystallites during low potential excursions of the cathode far below the operational range of the device. This study reveals a ‘measurement artefact’ in typical AST protocols, where the investigated state of the aged catalyst is largely impacted by the cyclic voltammetry measurement itself, without relevance of fuel cell device practical operation.