Oxygen Transport Properties Research Articles

Anionic and cationic co-doping to enhance the oxygen transport property of Bi0.5Sr0.5FeO3-δ as a high-performance air electrode for reversible solid oxide cells

Reversible solid oxide cells (RSOCs) have attracted considerable interest due to their efficient energy conversion. However, the low catalytic activity of air electrodes limits the application of RSOCs. Bi0.5Sr0.5Fe0.9Ta0.1O3-δ developed in our previous work exhibits a comparatively desirable performance and is expected to be further optimized. Herein, a novel strategy of anionic F and cationic Ta co-doped Bi0.5Sr0.5Fe0.9Ta0.1O3-δ-xFx (x = 0, 0.05, 0.10, 0.15, denoted as, BSFTF0, BSFTF5, BSFTF10, BSFTF15) materials are obtained to enhance the oxygen transport property of Bi0.5Sr0.5FeO3-δ for RSOCs. The polarization resistance (Rp) of BSFTF10 (0.017 Ω cm2) decreases by 66 % compared with that of BSFTF0 (0.051 Ω cm2) at 800 °C. In fuel cell mode, the single cell with BSTF10 reaches the maximum power density of 908 mW cm−2, which is approximately 72 % higher than the only Ta-doped BSFTF0 (527 mW cm−2) at 800 °C. In electrolysis mode, the single cell with BSFTF10 exhibits a high electrolysis current density of 1679 mA cm−2, indicating an approximately 90 % increase compared to that of BSFTF0 at 800 °C and 1.5 V with 70 % CO2-30 % CO feed gas. The bulk chemical diffusion (Dchem) of BSFTF10 is up to 5.12 × 10−4 cm2 s−1, which is 4.3 times higher than that of BSFTF0. The superior oxygen transport property of BSFTF10 is attributed to the high electronegativity of F− (4.0) and low electronegativity of Ta5+(1.8), which may weaken the chemical bonding between B-site ions and O2−, generating more oxygen vacancies and accelerating the rate of oxygen transfer. The results indicate that F and Ta co-doping is an effective strategy to develop a highly catalytically active air electrode for RSOCs featuring oxygen transport properties.

Commentary: COVID-19 impairs oxygen delivery by altering red blood cell hematological, hemorheological, and oxygen transport properties.

Commentary: COVID-19 impairs oxygen delivery by altering red blood cell hematological, hemorheological, and oxygen transport properties.

Optimizing oxygen transport properties in Al-substituted La0.5Sr0.5Co0.8Fe0.2-xAlxO3-δ (x = 0–0.20) perovskites

In recent years, there has been a surge in the investigation of perovskite-type complex oxides, with a particular focus on La1-xSrxCo1-yFeyO3, renowned for their exceptional mixed ionic-electronic conducting (MIEC) properties. La0.6Sr0.4Co1-yFeyO3 compositions have prominently emerged in this research domain. The hypothesis of enhancing redox stability and mitigating oxygen variations by incorporating trivalent aluminum (Al3+) into the B-site of perovskite structures has gained attention. In this study, single-phase La0.5Sr0.5Co0.8Fe0.2-xAlxO3-δ (x = 0–0.20) perovskite oxides were synthesized using the solution combustion technique. The physicochemical properties of the synthesized materials were thoroughly characterized, including their morphology, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) analysis, oxygen nonstoichiometry, and electrical transport properties. Results revealed lattice parameter variations associated with oxygen nonstoichiometry and B-site cation size, exhibiting a decline up to x = 0.10 followed by an increase. Al-substitution significantly influenced surface morphology, oxygen nonstoichiometry, and electrical conductivity. Notably, La0.5Sr0.5Co0.8Fe0.1Al0.1O3-δ displayed lower electrical conductivity (676 S cm−1 at 300 °C) among the studied oxides. Oxygen nonstoichiometry had a significant impact on oxygen transport parameters, with these perovskites demonstrating improved chemical diffusion coefficient, Dchem (5.5 × 10-5 cm2 s−1), and the oxygen surface exchange coefficient, Kchem (2.32 × 10-5 cm s−1) at 900 °C, suggesting its potential as an oxygen transport membrane. These findings underscore the promising role of Al-substituted perovskite oxides in various advanced applications.

Effects of Ink Water/Isopropanol Ratio on Structural and Oxygen Transport Properties of Pt/Ionomer Interface: Insights from Molecular Dynamics Simulations

Effects of Ink Water/Isopropanol Ratio on Structural and Oxygen Transport Properties of Pt/Ionomer Interface: Insights from Molecular Dynamics Simulations

Towards the development of textured polycrystalline La2NiO4+δ membranes and their oxygen-transporting properties

Towards the development of textured polycrystalline La2NiO4+δ membranes and their oxygen-transporting properties

Structural, surface and oxygen transport properties of Sm-doped Nd nickelates

Ruddlesden – Popper phases are promising materials for solid oxide fuel cell/electrolyzer air electrodes, oxygen separation membranes and other electrochemical devices due to their high oxygen mobility provided by a cooperative mechanism of oxygen migration involving both regular and highly-mobile interstitial oxygen. This work aims at studying structural, surface and oxygen transport properties of Sm-doped Nd nickelates sintered in a furnace at the temperatures in the range of 800–1250 °C and using electron beams at the temperatures in the range of 1150–1250 °C. Nd2-xSmxNiO4+δ, x = 0.2 and 0.4 are synthesized by a modified Pechini technique and characterized by X-ray diffraction, X-ray photoelectron spectroscopy and temperature-programmed isotope exchange of oxygen with C18O2 in a flow reactor. The phase composition studies show the ability to obtain single-phase materials at certain sintering conditions. Nd/Sm ratio on the surface is close to stoichiometric one, while (Nd + Sm)/Ni ratio on the surface is below stoichiometric. Such materials possess a high oxygen mobility (D⁎ up to ∼10−7 cm2/s at 700 °C).

Optimizing the lyophilization of Lumbricus terrestris erythrocruorin

Haemorrhagic shock is a leading cause of death worldwide. Blood transfusions can be used to treat patients suffering severe blood loss but donated red blood cells (RBCs) have several limitations that limit their availability and use. To solve the problems associated with donated RBCs, several acellular haemoglobin-based oxygen carriers (HBOCs) have been developed to restore the most important function of blood: oxygen transport. One promising HBOC is the naturally extracellular haemoglobin (i.e. erythrocruorin) of Lumbricus terrestris (LtEc). The goal of this study was to maximise the portability of LtEc by lyophilising it and then testing its stability at elevated temperatures. To prevent oxidation, several cryoprotectants were screened to determine the optimum formulation for lyophilisation that could minimise oxidation of the haem iron and maximise recovery. Furthermore, samples were also deoxygenated prior to storage to decrease auto-oxidation, while resuspension in a solution containing ascorbic acid was shown to partially reduce LtEc that had oxidised during storage (e.g. from 42% Fe3+ to 11% Fe3+). Analysis of the oxygen equilibria and size of the resuspended LtEc showed that the lyophilisation, storage, and resuspension processes did not affect the oxygen transport properties or the structure of the LtEc, even after 6 months of storage at 40 °C. Altogether, these efforts have yielded a shelf-stable LtEc powder that can be stored for long periods at high temperatures, but future animal studies will be necessary to prove that the resuspended product is a safe and effective oxygen transporter in vivo.

Comparison of the performances and aging mechanisms of XLPE and EVA insulation during their thermal oxidative aging

The thermal oxidation of low-density polyethylene and poly(ethylene-co-vinyl acetate) both cross-linked with triallyl isocyanurate (respectively denoted XLPE and EVA) was studied at 165 °C in air by Fourier transform infrared (FTIR) spectroscopy, dielectric spectroscopy and uniaxial tension. Monitoring the carbonyl index throughout the exposure allowed showing that these two polymers have similar oxidation reactions, taking place on their ethylene monomers, and therefore similar oxidation kinetics. In addition, in EVA, the acetate side groups thermally decompose into several degradation products during the exposure, such as: trans-vinylene double bond, acetic acid, saturated and unsaturated γ-lactone, and methane, but without interacting with the oxidation reaction. As oxidation is diffusion controlled, it leads to the development of oxidation profiles across the sheets of about 1 mm thick of these two polymers. In addition, as the oxygen transport properties (both solubility and diffusivity) are higher in EVA than in XLPE, oxidation affects the center of the EVA sheets from the early periods of exposure, whereas it is necessary to wait for high conversion ratios of the oxidation reaction to observe the same types of chain-shaped profiles in XLPE sheets. Oxidation results in dramatic changes in functional properties (both electrical and mechanical properties) for both polymers. Before thermal aging, EVA is already a poor insulating material compared to XLPE, due to the presence of polar side groups (i.e. acetates). In contrast, EVA exhibits a higher ductility, which justifies its use as nanofillers in PE-based insulating materials in order to improve their mechanical properties. The establishment of correlations between the changes in functional properties and the progress of oxidation allows the proposal of structural, electrical and mechanical end-of-life criteria for both polymers, corresponding to critical values of carbonyl index, dielectric constant, and elongation at break, respectively.

Estimation of oxygen transport properties in proton exchange membrane fuel cells under dry and wet operating conditions

Quantifying the mass transport in the proton exchange membrane fuel cells at different working conditions are important to further improve their performances. A transport model is established and the oxygen total transport resistance (Rtot) at different working current densities in the membrane electrode assembly is measured by the limiting current densities. The Rtot is almost fixed at the dry region at low current density. However, the Rtot significantly increases at the wet region at high current density due to the flooding. It is found that the flooding mainly influences the gas diffusion layer, and hinders molecular diffusion. An empirical relationship between the Rtot and current density is proposed. Based on the calculated Rtot at certain current density, the cell voltages are estimated, which are reasonably close to the measured cell voltages. These results provide a simple method to determine the mass transport resistance in wet region in fuel cells.

Comprehensive Molecular Dynamics Study of Oxygen Diffusion in Carbon Mesopores: Insights for Designing Fuel-Cell Catalyst Supports.

Mesoporous carbon is often used as a support for platinum catalysts in polymer electrolyte fuel-cell catalyst layers. Mesopores in the carbon support improve the performance of fuel cells by inhibiting the adsorption of ionomer onto the catalyst particles. However, the mesopores may impair mass transport. Hence, understanding molecular behaviors in the pores is essential to optimizing the mesopore structures. Specifically, it is crucial to understand the oxygen transport in the high-current region. In this study, the diffusion coefficients of oxygen molecules in carbon mesopores were calculated for various pore lengths, pore diameters, filling rates, and water contents in the ionomer via molecular dynamics simulations. The results show that oxygen diffusion slows by 2 orders of magnitude because of pore occlusion, and it slows down by an additional 1 or 2 orders of magnitude if ionomers are present in the pores. The occlusion can be theoretically predicted by considering the surface free energy. This theory provides some insight into mesoporous carbon designs; for instance, the theory suggests that narrow pores should be shortened to prevent occlusion. Slow diffusion in the presence of ionomers was attributed to the localization of oxygen at the dense ionomer-carbon interface. Thus, to improve oxygen transport properties, carbon surfaces and ionomer structures may be designed in such a manner as to prevent densification at the interface.

COVID-19 impairs oxygen delivery by altering red blood cell hematological, hemorheological, and oxygen transport properties.

Introduction: Coronavirus disease 2019 (COVID-19) is characterized by impaired oxygen (O2) homeostasis, including O2 sensing, uptake, transport/delivery, and consumption. Red blood cells (RBCs) are central to maintaining O2 homeostasis and undergo direct exposure to coronavirus in vivo. We thus hypothesized that COVID-19 alters RBC properties relevant to O2 homeostasis, including the hematological profile, Hb O2 transport characteristics, rheology, and the hypoxic vasodilatory (HVD) reflex. Methods: RBCs from 18 hospitalized COVID-19 subjects and 20 healthy controls were analyzed as follows: (i) clinical hematological parameters (complete blood count; hematology analyzer); (ii) O2 dissociation curves (p50, Hill number, and Bohr plot; Hemox-Analyzer); (iii) rheological properties (osmotic fragility, deformability, and aggregation; laser-assisted optical rotational cell analyzer (LORRCA) ektacytometry); and (iv) vasoactivity (the RBC HVD; vascular ring bioassay). Results: Compared to age- and gender-matched healthy controls, COVID-19 subjects demonstrated 1) significant hematological differences (increased WBC count-with a higher percentage of neutrophils); RBC distribution width (RDW); and reduced hematocrit (HCT), Hb concentration, mean corpuscular volume (MCV), and mean corpuscular hemoglobin concentration (MCHC); 2) impaired O2-carrying capacity and O2 capacitance (resulting from anemia) without difference in p50 or Hb-O2 cooperativity; 3) compromised regulation of RBC volume (altered osmotic fragility); 4) reduced RBC deformability; 5) accelerated RBC aggregation kinetics; and (6) no change in the RBC HVD reflex. Discussion: When considered collectively, homeostatic compensation for these RBC impairments requires that the cardiac output in the COVID cohort would need to increase by ∼135% to maintain O2 delivery similar to that in the control cohort. Additionally, the COVID-19 disease RBC properties were found to be exaggerated in blood-type O hospitalized COVID-19 subjects compared to blood-type A. These data indicate that altered RBC features in hospitalized COVID-19 subjects burden the cardiovascular system to maintain O2 delivery homeostasis, which appears exaggerated by blood type (more pronounced with blood-type O) and likely plays a role in disease pathogenesis.

Linking the ionomer film morphology and nanoscale oxygen transport properties in fuel cells

Transport processes are crucial for the performance of electrochemical energy conversion devices and attract wide attentions. This work focuses on the critical oxygen transport process in the ionomer electrolyte film on a Pt electrode, which highly limits the performance of low Pt-loading fuel cells. Reduction of oxygen transport resistance may be achieved by optimizing the ionomer film morphology. Therefore, the relationship between ionomer film morphology and oxygen transport characteristics is explored by altering ionomer side chain lengths in this work. The results show that the swollen structure with larger water agglomerates in the ionomer film with shorter ionomer side chains is detrimental to the formation of oxygen transport paths. However, the multilamellar structure with an alternating alignment of water agglomerates and PFSA ionomer agglomerates in the ionomer film with longer ionomer side chains has a larger water-PFSA interface, which provides more oxygen transport paths and thus reduces the oxygen transport resistance. This work inspires the novel design concept of the ionomer electrolyte film with low local oxygen transport resistance, i.e., enlarging the water-PFSA interface parallel to the oxygen transport direction via altering the ionomer material properties, which is valuable for the development of low Pt-loading fuel cells.

The changes in the oxygen transport capability of erythrocyte hemoglobin in patients with ischemic stroke

The statistics of acute cerebrovascular accident (ACC) has been disappointing for decades. It is especially interesting to study the oxygen transport properties of blood under stroke as a probable mechanism for progression of acute hypoxia of the nervous tissue. The aim of this research was to study changes in the structure and oxygen transport capacity of hemoglobin in patients with varying degrees of ischemic stroke severity using RAMAN spectroscopy. Materials and methods. There was a clinical cross-sectional observational study of patients (n=60) with stroke of ischemic origin. We did standard diagnostic procedures, RAMAN spectroscopy of blood smears (inVia™ Basis spectrometer (Renishaw, UK, laser wavelength 532 nm, power 1.5 mW), statistical analysis of the data. Results. The conformational structure of hemoporphyrin in erythrocytes of patients with stroke has significant differences from the results of relatively healthy donors, corresponding to the severity of the pathology. The content of oxyhemoglobin decreases by 7.1–16.1 % (p<0.05) relative to the control. The ability of hemoglobin to bind and release ligands increases by an average of 34.7–47.2 % (p<0.05) and 9.9–23.9 % (p<0.05), respectively. Ligand affinity increases by 30.1 %, 29.0 % and 47.3 % (p<0.05) under mild, moderate and severe forms of the disease. Conclusion. In the erythrocytes of patients with stroke of ischemic origin, the conformation of hemoglobin changes, its affinity for oxygen increases, and its oxygen transport function is modified. The revealed changes correlate with the severity of the stroke.

Molecular Dynamics Analysis of the Scattering Phenomena of Oxygen Molecules on an Ionomer Surface in Catalyst Layer of Fuel Cell

As energy demand increases and global warming progresses, expectations for fuel cells, which generate electricity through chemical reactions between hydrogen and oxygen, are rising. There are various types of fuel cells, which are classified according to the electrolyte. Polymer electrolyte fuel cells (PEFCs) are expected to be used in stationary power sources for home use and fuel cell vehicles because of their low operating temperature, short start-up time, and ease of miniaturization. The cost has been an issue in the widespread use of fuel cells. The amount of platinum used in the catalyst layer (CL) must be reduced to reduce costs. However, it increases the current density per unit area of platinum. There are three factors that contribute to voltage drop: ohmic polarization, activation polarization, and diffusion polarization. At high current densities, the voltage drop due to diffusion polarization is dominant. The main cause of diffusion polarization is the transport resistance of oxygen molecules in the CL, and improving the oxygen transport properties in the CL will lead to lower cost PEFCs. In this study, we analyzed the transport properties of oxygen molecules in the CLs of PEFCs using the Monte Carlo (MC) method. The effect of oxygen scattering on the overall oxygen transport properties was investigated by considering the behavior on the surface of the ionomer film (surface diffusion) into the MC method, which simulates the overall transport in the PEFC CL. The objective of this study is to reproduce accurate oxygen transport by comparing the results of MC and molecular dynamics (MD) methods. First, we used the MC method to analyze oxygen scattering phenomena. In this study, we used the 3D structure data of CL in the MC simulations. The three-dimensional structure of the sample was reconstructed from sequential slice images. Molecules were assumed to reflect specularly on the boundaries of the simulation domain. Oxygen molecules were randomly placed in the simulation system. The time step was set at 5 ps and the simulation time was set at 4 μs. In order to examine the effect of surface diffusion on the transport properties of oxygen molecules, the simulation was performed using various conditions of surface diffusion. We calculated the effective diffusion coefficient of oxygen in the CL as the transport property of oxygen molecules in the MC method. As the surface diffusion coefficient increases, the effective diffusion coefficient of oxygen increases. When the surface diffusion coefficient is small, the effective diffusion coefficient of oxygen increases due to a decrease in the time constant, which determines the probability distribution of the surface residence time. However, when the surface diffusion coefficient is large, a different trend emerges. As the time constant increases, the effective diffusion coefficient has a peak value and then decreases. Next, to verify the model of surface diffusion in the MC method, we analyzed the scattering phenomenon of oxygen molecules in a simulation system that simulates the pores in the CL using the MD method. We modeled the nano pores in the CL as a slit between the ionomer walls in the MD method. Nafion with the equivalent weight (EW) of 1146 was used as polymer model. The number of Nafion chains is set at 4, and the water content λ is set at 7. Three-dimensional periodic boundary conditions were applied to the simulation domain. After the system was equilibrated, 1 ns of NVT simulation was performed for production. The temperature was set at 297 K. The behavior of oxygen molecules impacting on the ionomer wall was analyzed. We analyzed the behavior of oxygen molecules colliding with the ionomer membrane using the MD method. The average residence time of surface diffusion on the ionomer thin film was of the order of 10-10s. When the calculation results were compared with the MC results, it was found that the average residence time obtained by the MD method was the value where the surface diffusion coefficient affects much on the effective diffusion coefficient. Comparing the results of the MD method with the surface diffusion model used in the MC method, there are some discrepancies between the results and the model. For example, the MC method used a model in which oxygen molecules reflect diffusively on the ionomer surface. However, in the MD method, it was found that oxygen molecules tend to reflect in the direction of travel (Fig. 1). Therefore, a more accurate model of the MC method needs to be developed in the future. Figure 1

Comparison of Different Carbon Supports By Simulation of Agglomeration in Polymer Electrolyte Fuel Cell Catalyst Ink

Polymer electrolyte fuel cells (PEFCs) are one of the most possible power sources for heavy-duty vehicles like trucks due to their high conversion efficiency and low environmental impact. Since platinum which is often used as a catalyst is expensive, it is essential to reduce the amount of platinum by increasing the power density. Regarding materials, it was reported that the activity is increased by supporting platinum on porous carbon [1]. However, there is a voltage drop in the high current density region comparing non-porous carbon. This is caused by oxygen transport properties, but the relationship with the carbon structure is unknown. Oxygen moves through the voids in the catalyst layer, so it is necessary to control the porous structure that forms the voids. This porous structure depends on the conditions of the catalyst layer fabrication process. In particular, the dispersion process is important to determine the size of the carbon agglomerate, or the backbone of the catalyst layer. In this study, an agglomeration model in catalyst ink was developed, and the dispersion process was verified by simulation under various conditions to elucidate the factors that affect agglomeration.The discrete element method (DEM) was used for the simulation. In this study, aggregates were approximated into spheres, and the forces between the particles were the drag force from the fluid, Brownian motion, and particle-particle interactions. Based on DLVO theory, the particle-particle interactions were given as an attractive force due to the van der Waals interaction and a repulsive force due to the overlapping of the electric double layer. In addition, the effect of ionomer was considered. The ionomer effect was introduced as a repulsive force and modeled with the same equation as the repulsion due to the electric double layer. The dielectric dependence equation of the ionomer shape that Magnus et al. modeled [2] was incorporated. As the amount of ionomer added increases, a binder effect is observed between the particles. This effect was also introduced using the adsorption data of ionomers on carbon. The carbon was assumed to be non-porous (Vulcan) and porous (Ketjen). Considering the difference in the platinum ratio on the surface depending on the carbon porosity, the Hamaker constant, which affects the van der Waals force, was determined. Also, the zeta potential, which affects the electric double layer force, was used at different values for carbons. In this study, the calculations were started with the particles uniformly dispersed in the ink. Periodic boundary conditions were used as boundary conditions. Using the above numerical model, simulations were conducted with different water-alcohol ratios and ionomer carbon ratios (I/C).As a result, it was confirmed that differences in the agglomerate size for two carbon supports. This result implies that the composition of the ink and the type of carbon affect the catalyst layer structure and cell performance.AcknowledgmentThis study was carried out under the project of Development of Advanced PEFC Utilization Technologies supported by the NEDO.Reference[1] Y. C. Park et al., J. Pow. Sou r., 315, 179 (2016).[2] M. So et al., Int. J. Hydrog. Energy, 44, 28984 (2019).

Visualization of Oxygen Bubbles on a Flat Ionomer-Coated Platinum Electrode

To improve the performance of polymer electrode membrane electrolyzers, it is necessary to grasp oxygen transport from the reaction site. This paper presents a visualization study which examines the effect of ionomers on the behavior of oxygen bubbles generated from the catalyst covered with ionomers. For the sake of visualization, a flat electrode was fabricated on a glass plate as shown Fig.1 (a). The reaction area was plated with platinum and covered entirely with ionomer using a spin coater.As shown Fig.1 (b), oxygen bubbles generated on ionomer surface were successfully captured with a high-speed camera. As shown Fig.1 (c), (d) the post visualization by electron microscopy with EDX showed that there were no significant cracks and damages on the ionomers. In other words, oxygen bubbles are generated on the ionomer surface without an apparent three-phase boundary (i.e., catalyst-ionomer-water). This suggests oxygen generated on the Pt surface was transported through the ionomer layer. The three-phase boundary may not be mandatory for the catalyst layer. The results presented in this research can provide better understanding of oxygen transport properties inside catalyst layer, thereby contributing to the future optimization of the structural design of catalyst layers. Acknowledgement This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (P14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).Fig.1 (a) Flat electrode on a glass plate, (b) Figure of the oxygen bubbles when electrolyzing, Element mapping of (c) Platinum and (d) Fluorine Figure 1

Molecular Dynamics Analysis of the Scattering Phenomena of Oxygen Molecules on an Ionomer Surface in Catalyst Layer of Fuel Cell

The purpose of this study is to clarify the effect of surface diffusion, which is the behavior of gas molecules on ionomer thin films, on the transport properties of oxygen in fuel cell catalyst layers. To this end, the direct simulation Monte Carlo (DSMC) and molecular dynamics (MD) methods were used in our simulations. The results of the DSMC method showed that the overall transport is strongly affected by the behavior of surface diffusion. It was found that the diffuse reflection model for surface diffusion used in the DSMC method has room for improvement as the MD results show a different trend that oxygen molecules tend to reflect in the direction of travel.

Comparison of MEA Degradation through FCV Actual Driving Test and Load-Cycle Durability Test

The degradation of a membrane electrode assembly (MEA) after an actual driving test was analyzed, and its mechanism was compared with that of a load-cycle durability test in a laboratory. MEA samples were extracted from an FC stack of a Toyota MIRAI 2014th model which had been operated in actual driving of 200,000 km for approximately 6000 h. The degradation distribution across the MEA plane was found to be insignificant. The electrolyte membrane did not show significant degradation affecting its properties following the durability tests. The MEA performance degradation is primarily attributed to the deterioration of the catalyst activity and oxygen transport properties in the catalyst layer, which is mainly caused by catalyst coarsening. The degradation was well simulated by the NEDO load-cycle durability test of 30,000 cycles. The evaluation results are applicable to validation of AST protocols and model-simulation of MEA degradation.

Ultrafast carbothermal shock strategy enabled highly graphitic porous carbon supports for fuel cells

The electronic conductivity and durability of porous carbon supports can be improved by increasing the degree of graphitization in the material; however, the preparation of highly graphitic porous carbon using conventional furnaces remains a significant challenge. Herein, we demonstrate a universal and highly efficient carbothermal shock strategy that significantly improves the degree of graphitization of porous carbon supports, including bowl-like carbon, hollow carbon spheres, ZIF8-derived carbon, BP2000, and Ketjen EC 300J. Taking bowl-like carbon as an example, we illustrate the synthesis of a graphitized bowl-like carbon (G-BC-S) support and evaluate the performance of PtCo/G-BC-S in the oxygen reduction reaction (ORR) in rotating disk electrodes (RDE) and H2/air PEM single cells. PtCo/G-BC-S exhibits faster ORR kinetics than PtCo/BC and Pt/C, with little loss of activity (25%) and only 13 mV of E1/2 decay after 20000 cycles accelerated stress testing under 1.0–1.5 V vs. a reversible hydrogen electrode (RHE).</a> The significantly enhanced performance of the PtCo/G-BC-S catalyst arises from the high activity and chemical/structural stability of the PtCo intermetallic nanoparticles and from the high degree of graphitization and well-defined porous structure of the bowl-like carbon support, which confers excellent electrical conductivity and oxygen transport properties. This study provides a reliable and universal strategy for the development of high-performance porous carbon supports for practical applications in fuel cells.

A Post-Treatment with Facile Thermal Control to Prepare Pt/C Catalysts with Accessible Carbon Supports

Mass market adoption of PEM fuel cells (PEMFCs) for emission-free propulsion still hinges on catalyst materials that remain highly active at low platinum loadings without comprising durability of the PEMFC system. A promising path towards this goal is the development of novel carbon support morphologies, as research efforts throughout the past decade have shone light on their importance for determining overall cell performance and for mitigating catalyst degradation.[ 1-2] Briefly, porous carbon supports such as Ketjenblack accommodate Pt nanoparticles in internal pores, protecting the Pt surface from contact with the ionomers’ sulfonate endgroups at the expense of hindered oxygen diffusion into these pores. Conversely, platinum deposited onto solid carbons without internal mesoporosity (e.g., Vulcan) necessarily displays depressed oxygen reduction reaction kinetics due to ionomer poisoning, but is characterized by superior oxygen transport properties.‘Accessible’ porous carbons, a concept coined by Yarlagadda et al., get around this trade-off by protecting Pt nanoparticles in carbon pores that do not impede oxygen diffusion to the Pt active site by virtue of their pore size and absence of bottlenecking elements.[ 3] Such accessible carbon morphologies may be obtained by tailored carbon-templating techniques, as increasingly featured in industry research.[ 4-5] A more economic route is the oxidative post-treatment of commercially available Pt catalysts on porous carbon supports, which was first patented by General Motors, and subsequently studied by our group.[ 6-7] Essentially, this leverages the Pt-catalyzed oxidation of the carbon for targeted pore widening, producing catalysts that enable outstanding performance at both low and high current density. A crucial disadvantage of this process is, however, the high exothermicity of the carbon oxidation reaction, requiring careful thermal control and small batch sizes to selectively etch the carbon structures without uncontrolled combustion of the entire carbon support.In this work, we present a post-treatment that achieves Pt-catalyzed pore opening by an alternative route, allowing a more controlled etching of the carbon support structure. This enables >90% shorter reaction times and larger batch sizes, potentially paving the way to a larger-scale implementation. Changes to the thus-obtained materials are characterized by N2 physisorption and transmission electron microscopy. The modified catalysts are then fabricated into low-loaded membrane electrode assemblies (MEAs) and tested comprehensively in 5 cm2 single cells. We demonstrate substantially increased high current density performance in H2/air compared to the pristine catalyst (up to 60 mV at 2 A cm-2, Figure 1), which we ascribe to improved catalyst accessibility as evidenced by oxygen limiting current measurements. REFERENCES N. Ramaswamy, W. Gu, J. M. Ziegelbauer and S. Kumaraguru, J. Electrochem. Soc., 167, 6, 64515 (2020).E. Padgett, V. Yarlagadda, M. E. Holtz, M. Ko, B. D. A. Levin, R. S. Kukreja, J. M. Ziegelbauer, R. N. Andrews, J. Ilavsky, A. Kongkanand and D. A. Muller, J. Electrochem. Soc., 166, 4, F198-F207 (2019).V. Yarlagadda, M. K. Carpenter, T. E. Moylan, R. S. Kukreja, R. Koestner, W. Gu, L. Thompson and A. Kongkanand, ACS Energy Lett., 3, 3, 618–621 (2018).Y. Kamitaka, T. Takeshita and Y. Morimoto, Catalysts, 8, 6, 230 (2018).V. Yarlagadda, N. Ramaswamy, R. S. Kukreja and S. Kumaraguru, Journal of Power Sources, 532, 231349 (2022).A. Kongkanand and M. K. Carpenter, US 9,947,935 B1, filed Sep. 30, 2016, and published Apr. 17, 2018.T. Lazaridis and H. A. Gasteiger, J. Electrochem. Soc., 168, 11, 114517 (2021). Figure 1