Oxygen Mass Transport Research Articles

Influence of Particle Size on Mass Transport during the Oxygen Reduction Reaction at Single Silver Particles Using Scanning Electrochemical Cell Microscopy.

Single entity electrochemical measurements enable insight into the electrocatalytic activity of individual particles based on composition, shape, and crystallographic orientation. In addition to structural effects, particle size can further influence electrocatalytic activity and reaction mechanisms through mass transport effects. In this work, electrodeposition was used to grow well-separated silver particles of varying sizes from 100 to 500 nm in radius. Using a multimicroscopy approach of scanning electrochemical cell microscopy combined with scanning electron microscopy, the electrocatalytic current of individual silver particles toward the oxygen reduction reaction was evaluated as a function of their size. It was found that the current density increased with decreasing particle radius, which was correlated to the mass transport of oxygen to the silver particle, demonstrating the importance of size dependent mass transport effects that can occur at the single particle level using scanning electrochemical cell microscopy and opening new opportunities for quantitative electrocatalysis measurements.

Machine Learning Accelerated Identification of Bifunctional Active Sites in Metal-Organic Framework Derived Metal Oxide Heterostructures for High-Performance Zinc-Air Batteries

The demand for electrochemical energy storage (ESS) systems and their market size has drastically increased in many applications ranging from portable devices through electric vehicles to large-scale grid systems over the past decades. Yet, a more advanced ESS is required due to the safety issues and limited energy density of currently commercialized lithium-ion batteries (LIBs) for wide and matured applications. As a solution to this challenge, zinc-air batteries (ZABs) have gained tremendous attention due to their high energy density, low cost, environmental friendliness, and much lower fire risk characteristics. ZABs utilize oxygen from the atmosphere at their cathode during discharge, where the atmospheric oxygen is reduced to hydroxide via an oxygen reduction reaction (ORR). On the other hand, hydroxide is released as oxygen via oxygen evolution reaction (OER) during charge. The fact that oxygen is drawn directly from the air enables high gravimetric and volumetric energy densities of ZABs compared to those of commercial LIBs. However, the sluggish kinetics of the four-electron transfer steps in ORR and the mass transport of oxygen in OER remain major challenges for the practical application of ZABs. Meanwhile, precious metal-based catalysts such as Pt/C and RuO2 are known to have good catalytic activity towards ORR and OER. However, the integration of different catalysts into the same cathode is very difficult. Additionally, the high cost and natural scarcity of novel metals limit their usage for practical applications.We synthesized a bifunctional CoO/Mn3O4 heterostructure (CMH) OER/ORR electrocatalyst which was derived from metal-organic framework (MOF) for excellent activities in ZABs. The CMH cathode exhibited high ORR and OER performances, as demonstrated by the higher ORR current density than that of the Pt/C catalyst as well as the low OER overpotential exceeding those of the RuO2 catalyst. Remarkably, atomic structures of the CMH were elucidated using global optimization of genetic algorithm integrated with the machine learning force field. By adopting first-principles electronic structure calculations, we further revealed the charge transfer and active sites for both reactions in the identified heterojunction. Furthermore, assembling CMH cathode and Zn metal anode into a ZAB full-cell configuration achieves a ~5.5-fold higher energy density of 879 Wh kg-1 exceeding those of commercial lithium-ion batteries (LIBs), a high-discharge outstanding power density that is 1.39 times higher than novel metal catalysts, and robust cycle stability with negligible voltage decay which is far superior to that of benchmark Pt/C+RuO2 electrocatalysts.

Which Insights Can Gas Diffusion Electrode Half-Cell Experiments Give into Activity Trends and Transport Phenomena of Membrane Electrode Assemblies?

In order to push the energy turnaround forward, research and development on technologies enabling storage and release of renewable energies, such as water electrolysis and fuel cells is vital in the current decade. For both processes it is essential to collect meaningful data for catalyst activity in terms of transferability to the industrial application already on the lab-scale and at an early stage of material development. Gas diffusion electrode (GDE) half-cell setups were recently presented as a fast and cost-effective tool to characterize oxygen reduction reaction (ORR) catalyst layers at fuel cell relevant potentials and current densities (3 A cm-2) [1]. An Inter-lab comparison demonstrated that GDE testing with various, slightly differing setups can deliver data that is both reliable and comparable if standardized measurement protocols are followed [2]. On the way to widespread application, it is essential to evaluate to which extend GDE half-cell measurements can provide reasonable trends of how catalyst layer properties influence ORR activity in membrane electrode assembly (MEA) applications, despite the major influence of the active metal itself. Elucidating possibilities and limitations of GDE half-cell measurements to study catalyst layer influences is scope of this study.The transferability of trends was studied for i) an ionic liquid modified and unmodified Pt/C catalyst (following our previous work [3]) and ii) employing nitrogen modified carbon support at different ionomer to carbon ratios (following the work of Ott et al. [4]). Regarding the activity increase due to IL modification we will show that rotating disk electrode (RDE) and GDE results at low current densities show the same trend. GDE testing also shows a maximum activity depending on the IL loading at slightly elevated current densities. In the high current density regime, however, MEA testing showed a negative effect of the IL modification, which was also obtained correctly within the GDE testing at the same current densities. For the N-doped Pt/C catalyst a positive effect was observed in MEA characterization with low ionomer content under dry conditions, where proton transport losses are dominant for the unmodified catalyst. However, at higher I/C ratio and under wet conditions, where proton transport is sufficient for both unmodified and N-doped catalyst, worse performance after N modification is observed in the high current density regime. During GDE evaluation only the latter phenomenon could be capture and the unmodified catalyst showed a superior performance over the N-doped material at any I/C ratio in the ohmic and in the mass transport limiting regime.In summary, we showed that GDE half-cell experiments, where the catalyst layer is in direct contact with the liquid electrolyte, can be used to reliably predict trends in catalytic activity for real MEAs at high current densities that are related to differences in oxygen mass transport [5]. However, differences in catalytic activity being related to proton accessibility especially at lower relative humidities cannot be captured completely, as the catalyst layer is always wetted fully. An introduction of an ionomer membrane between catalyst layer and liquid electrolyte might be necessary during GDE evaluation for a reliable description of all transport phenomena in real fuel cells. Nevertheless, our results indicate that GDE measurements without the utilization of an ionomer membrane allow for the study of oxygen mass transport properties independent of other transport phenomena at fuel cell relevant current densities.Literature:[1] Schmitt et al. Electrochemistry Communications 2022, 141, 107362 [2] Ehelebe et al. ACS Energy Lett. 2022, 7, 816-826. [3] Zhang et al. Advanced Functional Materials 31.28 (2021): 2010977 [4] Ott et al. Nature materials 19.1 (2020): 77-85. [5] Schmitt et al. Energy Adv. 2023, 2, 854.

Diagnosis of Polymer Electrolyte Fuel Cell: Should We Rely Only on Electrical Quantities?

Polymer electrolyte fuel cells (PEMFC) will play a central role in the establishment of a de-carbonized society. They are seen as a key technology to empower electric heavy-duty vehicles. However, the lifespan is still below the fixed standards for their commercialization. A way to increase their durability is to complement them during operation with a mitigation strategy enabling to avoid the arise of faulty states accelerating degradation processes. Nowadays, most of the proposed fault identification and isolation architectures rely on monitoring of the performance through electrochemical impedance spectroscopy (EIS). Nevertheless, the analysis of impedance data is often complicated, since the contribution of many processes in the EIS spectra are coupled with each other and, for this reason, difficult to identify. This constitutes a limiting factor for its employment as diagnostic tool. In the last years, in order to overcome such issues, the use of nonelectrical periodic inputs and/or outputs has been advanced. The idea motivating this research direction is the possibility to detect the contribution of selected processes on the performance of PEMFCs separating them from the others. Novel frequency response techniques involving back pressure, temperature and partial pressure have been developed [1].In this contribution, the so-called concentration frequency response analysis (CFRA) is considered [2, 3]. It consists on perturbing the cathode side of the cell with a feed characterized by a periodic partial pressure of oxygen and water, and the detection of the electric response, voltage or current. A test station has been designed and constructed in order to apply this technique and investigate its capabilities. A model based analysis of the measured CFRA spectra indicates the possibility to decouple the contributions of the water transport into Nafion and oxygen mass transport on the performance of the cell depending on the chosen concentration input and electrical output. In this way, the analysis and identification of the related power losses is simplified with respect to classic EIS.Moreover, the performance of a fault identification and isolation architecture employing CFRA as monitoring technique have been evaluated and compared to EIS. For such purpose, an identifiability analysis of parameters related to faulty states and obtained by model fitting to spectra of each technique has been performed. The results clearly show that the employment of CFRA based on water pressure input offers the most reliable parameter estimation and, consequently, constitutes the most preferable diagnosis method among the others analyzed.

Understanding How Porous Transport Layer Features Affect Proton Exchange Membrane Electrolyzer Performance

PEM Water electrolysis has emerged as a promising technology for the production of green hydrogen. A key component of PEM electrolyzer is the porous transport layer (PTL). The porous transport layer has numerous important roles in water electrolysis. In addition to enabling electrical conduction and water and gas transport, the PTL is necessary for maintaining excellent contact with the cell components. Cell performance is anticipated to be affected by PTL properties such as structure, composition, and thickness.This study delves into the critical contribution of porous transport layer properties to mass transport resistance, activation losses, catalyst layer resistance, and ohmic losses. I will present our experimental framework to deconvolute the impact of PTL properties such as thickness, porosity, and microporous layer on various overpotentials. Voltage loss breakdown analysis was conducted to determine kinetic, ohmic, mass transport, and catalyst layer losses. Theoretical calculations of thermodynamic voltage typically rely on standard reaction potentials adjusted for reactant and product concentrations, operating temperature, and pressure via Nernst corrections. Ohmic voltage, which accounts for contact resistances, bulk resistances, and ohmic losses from the membrane, was determined using high-frequency impedance spectroscopy measurements. To calculate losses from OER kinetics, a Tafel fit is subsequently implemented on the HFR-free voltage. Typically, this process tends to result in residual voltage at elevated current densities, including losses arising from mass transport of water and oxygen, resistance within the catalyst layer, and impacts stemming from ionic impurities. Ionic contamination is omitted from our analysis as a result of the short duration of the experiment.Our detailed analysis reveals that the properties of PTL can have a substantial impact on the performance of the water electrolyzer, which is crucial for enhancing the performance and durability of PEMWE. This study presents a novel framework for forecasting the performance of diverse PTL materials, which may be effectively used to drive the development and optimization of PTL for water electrolyzers. Figure 1

Enhancement of Mass Transport in Catalyst Layers of HT-PEMFC with Tetrafluorophenyl Phosphonic Acid Binder.

The design and development of new and efficient catalyst binder materials are important for improving cell performance in high-temperature proton-exchange membrane fuel cells (HT-PEMFCs). In this study, a series of tetrafluorophenyl phosphonic acid-based binder materials (PF-y-P, y = 1, 0.83, and 0.67) with rigid structures and controllable degrees of phosphonation were prepared and used in HT-PEMFCs using the ultra-strong acid-catalyzed Friedel-Crafts reaction and the combined Michaelis-Arbuzov reaction. The samples exhibited high stability, low water uptake, superior proton conductivity, and cell performance. In addition, the oxygen mass transport properties of the PF-1-P binder were investigated using high-temperature microelectrode electrochemical testing techniques. Compared with the phosphoric acid-doped polybenzimidazole (PBI) binder, the O2 solubility of PF-1-P binder material increased by 30% (5.36 × 10-6 mol cm-3) and the PF-1-P binder material exhibited better cell stability in HT-PEMFCs. After 10.5 h of discharge at a constant current of 0.12 A cm-2, the MEA voltage decreased by 7.1% and 20.8% in case of the PF-1-P and PBI binders, respectively.

In Situ-Grown Ultrathin Catalyst Layers for Improving both Proton Exchange Membrane Fuel Cell and Anion Exchange Membrane Fuel Cell Performances.

The mass transport and ion conductivity in the catalyst layer are important for fuel cell performances. Here, we report an in situ-grown ultrathin catalyst layer (UTCL) to reduce the oxygen mass transport and a surface ionomer-coated gas diffusion layer method to reduce the ion conducting resistance. A significantly reduced catalyst layer thickness (ca. 1 μm) is achieved, and coupled with the ionomer introduction method, the ultrathin catalyst layer is in good contact with the membrane, resulting in high ion conductivity and high Pt utilization. This ultrathin catalyst layer is suitable for both proton exchange membrane fuel cells and anion exchange membrane fuel cells, giving peak power densities of 2.24 and 1.11 W cm-2, respectively, which represent an increase of more than 30% compared with the membrane electrode assembly (MEA) fabricated by using traditional Pt/C power catalysts. Electrochemical impedance spectra and limiting current tests demonstrate the reduced charge transfer, mass transfer, and ohmic resistances in the ultrathin catalyst layer membrane electrode assembly, resulting in the promoted fuel cell performances.

Probing the Origin of Overpotential for Sodium‐Oxygen Batteries with Distribution of Relaxation Time

AbstractSodium‐oxygen batteries are emerging as new battery systems. Deep understanding of the origin of overpotential and the kinetic process in sodium‐oxygen batteries remain challenging yet critical. We apply a method of distribution of relaxation time (DRT) to decipher the electrochemical impedance spectroscopy (EIS), allowing us to monitor the changes of different kinetic processes during the discharging and charging. The origin of the overpotential in a battery was further comprehensively investigated combining DRT analysis with differential electrochemical mass spectrometry, Raman and other characterizations. Overpotential is found to primarily stem from oxygen mass transport during discharging, and from poor solid‐solid contact at the electrode surface during charging. Our work demonstrates the study of kinetic processes using DRT analysis, and suggests effective ways to improve the performances of sodium‐oxygen batteries.

1D One-Phase Modeling of the Anode Catalyst Layer/Porous Transport Layer Interface Affecting Proton Exchange Membrane Water Electrolysis

Optimizing the structure of the porous transport layer (PTL) is crucial for improving the performance of proton exchange membrane water electrolysis (PEMWE), particularly for cells with low anode catalyst loadings. A growing number of studies reveal that catalysts are normally not entirely utilized in PEMWE, whereas the degree of catalyst utilization strongly depends on the structure of the PTL. As the oxygen evolution reaction (OER) at the anode is the limiting reaction step in PEMWE, inefficient use of the anode catalyst layer (aCL) would result in higher activation losses and, thus, cause a waste of unused catalysts [1–3]. This phenomenon becomes more serious when catalyst loading is reduced to meet the eco-friendly requirement of $2 per kgH2 while conserving noble material resources [4].Schuler et al. [1, 5] combined commercial CCMs with different PTL configurations, and found that with different aCL/PTL interface properties, the activation overpotential could have a maximum deviation of 20 mA at 0.1 A/cm2. Peng et al. [2] manufactured an ultra-low Ir loading CCM with 0.03 mgIr/cm2. Optimizing the aCL/PTL interface showed that the activation overpotential can have a maximum deviation of 40 mV at 0.75 A/cm2. Previous studies concluded that the aCL/PTL interface is crucial for improving the catalyst utilization. Consequently, the triple-phase boundary (TPB) site concept at the aCL/PTL interface is gaining more and more attention: the catalysts can only be entirely activated at reaction sites with sufficient water supply, ionic and electronic conductivities, and oxygen removal pathways [3]. According to TPB, not all reaction sites on the aCL meet these requirements. Figure 1 shows the corresponding physical picture. At the contact point of aCL and PTL (S) offers much greater electronic conductivity compared to the pore area (P), where electron removal is limited by the aCL in-plane conductivity (i.e., 1.85×106 S/m vs. 65.8 S/m) [3]. However, beneath the contact area (S), mass transport of oxygen is limited, indicating the necessity of optimizing the aCL/PTL interface.Figure 1: Illustration of physical picture at aCL/PTL interface.In our contribution, we implement a one-dimensional, one-phase model to investigate the interplay between PTL geometry at the aCL/PTL interface and catalyst utilization. The PTL transports water toward the aCL and electrons and oxygen toward the cathode and anode bipolar plates, respectively [6]. To simplify the physical conditions, we assume sufficient water supply and ion removal for the reaction sites on the aCL. Therefore, mass transport and electron transport limitations are only influenced by the aCL/PTL interface properties. Oxygen is assumed to dissolve in water only so that bubble formation does not disturb the reaction sites. Additionally, an ideal heat management condition is assumed in the model to achieve an isothermal condition. Our simulation results are in good agreement with literature, enabling the prediction of activation overpotentials corresponding to the PTL structure at aCL/PTL interface. Moreover, our results illustrate the local aCL behavior, which can be used to optimize the porous structure at the aCL/PTL interface, maximizing catalyst utilization. With our model, we aim to screen for an optimum PTL structure for a given aCL, allowing us to improve PEMWE cell performance by minimization of activation losses.

Accelerated Stress Tests to Project PEM Fuel Cell Durability

One of the major degradation mechanisms limiting the long-term durability of proton exchange membrane fuel cells (PEMFCs) is the loss of platinum electrochemically active surface area (ECSA) of the carbon-supported platinum (Pt/C) cathode catalyst, caused by Pt dissolution that is followed by both Ostwald ripening of the Pt nanoparticles and loss of Pt into the ionomer phase [1]. The Pt ECSA loss is accelerated when subjecting PEMFCs to extended load-cycling inducing concomitant cycling of the cathode potential. To this end, accelerated stress tests (ASTs) can be conducted either by controlling the cell/stack current (“load-cycling” AST) under H2/air (anode/cathode) or the potential (“voltage-cycling” AST) under H2/N2 (anode/cathode).Most of the experiments studying the effect of load-cycling on catalyst durability have been based on voltage-cycling in a H2/N2 configuration, showing that Pt ECSA loss is aggravated with increasing upper potential limit (UPL), temperature, and relative humidity (RH) [2, 3, 4]. Comparing voltage-cycling induced degradation under H2/N2 versus H2/air, a recent study has found an essentially identical Pt ECSA loss, but a slightly higher H2/air performance decay when cycling under H2/N2 [4]. This study by General Motors Corporation indicates that Pt ECSA loss may not be a unique descriptor for H2/air performance loss, contrary to what was observed in recent work by Toyota Motor Corporation and Kyushu University [5] as well as in our own studies [6].In this talk, we will discuss the correlation between H2/air performance loss and Pt ECSA loss during voltage-cycling ASTs at different conditions (UPL, RH, and gas-feed) conducted in 5 cm2 active area single-cell PEMFCs, complemented by a voltage-loss analysis to deconvolute oxygen reduction reaction (ORR) activity losses and mass transport losses (due to oxygen mass transport and proton conduction in the cathode catalyst layer). We will also discuss whether the H2/air performance loss is governed by the Pt ECSA loss (independent of catalyst loading) or, as we had proposed previously, by the cathode electrode roughness factor (rf) loss (in cm2 Pt/cm2 cathode, i.e., the product of ECSA and Pt loading) [7]. As roughly 100,000 [8] or even more voltage-cycles are expected for heavy-duty applications, requiring very long measurement times, an approach to relate the degradation under harsh AST conditions with those under application-relevant conditions will be discussed. Experiments are conducted with cathode catalysts based on different carbon supports (Vulcan, Ketjenblack, or so-called accessible carbon supports [9, 10]), on catalysts with different initial Pt ECSAs (i.e., different Pt nanoparticle sizes) and with different initial Pt-loadings (i.e. different initial rf). Finally, exploratory experiments to evaluate the effect of start-up/shut-down on the correlation between electrode rf and H2/air performance decay will be discussed.

Understanding the Increase in H2/Air PEM Fuel Cell Performance of Pt Based Cathode Catalysts with the Tailored Pore Structure of Mesoporous Graphitic Spheres

To reach its climate neutrality goal by 2050, the European Union has agreed on ceasing the sale of new petrol and diesel-driven vehicles by the end of 2035. Therefore, interest in carbon-neutral transport options, such as battery-electric vehicles (BEV) or fuel-cell-electric vehicles (FCEV) has tremendously increased. Especially for heavy-duty applications, proton exchange membrane fuel cells (PEMFCs) can be a viable solution, as they exhibit a higher gravimetric power density compared to BEVs.[1] However, cost and supply constraints of the catalyst active material, i.e. platinum, are still a major hurdle to make PEMFCs competitive with incumbent technologies. While this has been addressed by a reduction of the Pt loading, this induces some major drawbacks regarding the overall performance due to the decrease in catalyst activity towards the oxygen reduction reaction (ORR) and the increase in proton and oxygen mass transport resistances to the Pt surface.[2] It has been shown that those issues can be mitigated by using cathode catalyst materials with a mesoporous carbon support, as they exhibit a high ORR activity while additionally facilitating the transport of protons and gaseous oxygen to Pt nanoparticles that are located inside the pores of the carbon support particles.[3] Previous studies have demonstrated that by using a multi-step catalyst synthesis procedure where Pt nanoparticles are incorporated into a mesoporous graphitic sphere (MGS) support and subjected to a pore-confined growth by a subsequent high-temperature treatment, a precisely defined catalyst structure is achieved with a good H2/air performance as well as high stability towards catalyst degradation.[4] In order to correlate the impact of a precisely defined mesoporous carbon structure on the overall H2/air performance, single cell tests were performed using 5 cm2 membrane electrode assemblies (MEAs) with cathode loadings of 0.1 mgPt cmMEA -2 using different commercially available 20 wt.% Pt/C catalysts as well as a 28 wt.% Pt/C catalyst based on lab-scale synthesized carbon supports: i) Ketjen black (KB, TEC10E20E, TKK); ii) Vulcan (Vu, TEC10V20E, TKK); and, iii) mesoporous graphitic spheres (MGS). The latter was produced following the procedure by Knosalla et al.[5] A full MEA characterization was included to measure the electrochemically active surface area (ECSA) by CO-stripping, the H2/air performance at different operating conditions (high and low relative humidity and high pressure), the H2/O2 performance, and the O2 and H+ transport resistance in the cathode by limiting current measurements and electrochemical impedance spectroscopy (EIS), respectively. Finally, the catalyst morphology has been studied ex/situ using three/dimensional reconstruction by electron tomography and by gas adsorption measurements (argon at 87 K and water at 298 K).It was found, that the mesoporous carbon support significantly improves the H+ and O2 transport, resulting in an improved H2/air performance compared to both commercial catalyst materials (see. Fig. 1 MGS vs. KB or Vu). Interestingly, the MGS retains also a high ORR catalyst mass activity (imass ≈ 400 A/gPt) which is similar to the one of Pt/KB cathode catalysts (with higher internal microporous structure), leading to the good H2/air performance also at low current densities. The overall optimal performance of Pt/MGS suggests that these materials have a defined porous internal network structure which combines the beneficial effect of a facilitated O2 and H+ transport through larger mesopores while still shielding the Pt surface from ionomer contact by incorporation of Pt nanoparticles into smaller micropores.

Improving Water Management and Reactant Distribution in PEM Fuel Cells Via Flow Fields with Biomimetic Auxiliary Channels

The bipolar plate, a component of the polymer electrolyte membrane (PEM) fuel cell is responsible for reactant delivery, product removal, and mechanical stability of PEM fuel cell stacks. Bipolar plates also account for 70-90% of the weight and volume, and 18-28% of the production cost of stacks (1). Improving the function of the flow fields embedded in the bipolar plates can significantly impact the energy density of PEM fuel cells by improving liquid water management and reactant distribution. Previous works show that water preferentially accumulates under the lands of flow fields in the gas diffusion layer (GDL) for various GDL materials, creating a heterogeneous distribution of water and reactants, thereby increasing cathode mass transport overpotential(2–4). Biomimetic channel architectures have been shown to enhance preferential water flow; however, these designs have not been tailored to control water accumulation for improved reactant distribution (5,6). Targeting areas of known water accumulation such as the under-land regions of GDLs, could have a profound impact on reducing mass transport losses and improving reactant homogeneity, thereby improving the power density and efficiency of the PEM fuel cell.In this work, biomimetic auxiliary channels were laser-cut into the lands of a parallel PEM fuel cell flow field to enhance the liquid water removal and reactant distribution in the under-land region of the GDL. Constant-current electrochemical testing and electrochemical impedance spectroscopy revealed a 29% increase in peak power density resulting from a 54% decrease in oxygen mass transport overpotential using the biomimetic flow field (BFF) compared to a baseline trapezoidal flow field (TFF). Operando synchrotron X-ray radiography revealed reduced GDL water accumulation when using the BFF compared to the TFF. Water accumulation under the BFF flow fields was more homogenous especially near the catalyst layer (CL) – GDL interface, indicating enhanced reactant distribution near the CL reaction sites. Therefore, the reduced mass transport overpotential and corresponding power density increase were attributed to enhanced liquid water removal and reactant distribution due to the biomimetic auxiliary channels. These results demonstrate that significant performance enhancements can be realized by embedding alternate pathways for air and water in the lands of flow field components. Ultimately, this work can be used to further optimize the design of bipolar plates for more efficient stacks and accelerate the commercialization of PEM fuel cells. Y. Wang, D. F. Ruiz Diaz, K. S. Chen, Z. Wang, and X. C. Adroher, Materials Today, 32, 178–203 (2020).N. Ge et al., Electrochim Acta, 328, 135001 (2019).D. Muirhead et al., Int J Hydrogen Energy, 42, 29472–29483 (2017).S. Chevalier et al., J Electrochem Soc, 164, F107–F114 (2017)N. Guo, M. C. Leu, and U. O. Koylu, Int J Hydrogen Energy, 39, 21185–21195 (2014).S. Feng et al., Science, 373, 1344–1348 (2021).

Influence of Sidechain Length in Perfluoro-Sulfonic Acid Ionomer on Electrode Slurry Interactions and Finalized Microstructure in the Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells (PEFCs) have emerged as one of the most promising next-generation energy devices for both passenger-owned and heavy-duty vehicles due to their high energy efficiency, low emissions, and quiet operation. However, the performance of the fuel cell electrode is often limited by the strong specific adsorption of long sidechain (LSC) perfluoro-sulfonic acid (PFSA) ionomer onto the platinum (Pt) catalyst2, which results in a decrease in the electrocatalytic activity1 as well as increase oxygen mass transport resistenace at the MEA level. To address this issue, a short sidechain (SSC) ionomer has been developed, which has been found to exhibit excellent efficiency over LSC ionomer3 The reduced specific adsorption of SSC ionomer onto the Pt catalyst results in a higher available surface area for the electrochemical reaction, which improves the overall fuel cell performance. The sidechain length of ionomers plays a crucial role in determining the overall cell performance4 however, due to the differences between them, the interactions among slurry components are not yet fully understood.Unlike the previous literatures, we investigates the details of ionomer sidechain length and its impact on the interaction among slurry components and resulting electrode microstructure, through a comparative analysis of LSC and SSC ionomers.5 Our hypothesis propose that the physical interactions between components are considerably impacted by the sidechains with a specific emphasis on the mobility and ion-pair association in the slurry. To probe the influence of ionomer sidechains, we conducted a thorough investigation wherein incrementally varied its concentration from 0 to 1.5 mmolSO3-gC -1, at intervals of 0.3 mmolSO3-gC -1. The main purpose of this evaluation was to observe the evolution of particle aggregation according to the correlation between adsorbed ionomer and free ionomer in electrode slurry. Drawing from the U-shaped viscosity profile obtained through rheological measurements, we propose an optimized ionomer concentration of 0.6 mmolSO3-gC -1 for the SSC ionomer-based slurry and 0.9 mmolSO3-gC -1 for the LSC ionomer-based slurry. To characterize the microstructure of the as-prepared electrode, we employed scanning electron microscopy (SEM) and confocal microscopy, with a focus on particle agglomeration and the homogeneous ionomer distribution. Finally, the electrochemical analysis was performed on the fabricated membrane electrode assembly (MEA) in a 25 cm² single cell for a cathode Pt loading of 0.1 mgPtcm⁻². As a result, the optimized SSC ionomer-based electrode exhibited low local O₂ transport resistance and high Pt utilization, leading to over a 40 % enhancement in fuel cell performance at 0.6 V with a high proton accessibility value above 0.85 when compared to the optimized LSC ionomer-based electrode. This study highlights the crucial importance of understanding the effects of ionomer sidechain length on electrode slurry interactions and microstructure and establishes a significant correlation between them. Our findings contribute to the ongoing development of high-performance PEFCs, offering valuable insights into optimizing electrode composition and structure for enhanced fuel cell performance.(1) Ahn, C.-Y.; Park, J. E.; Kim, S.; Kim, O.-H.; Hwang, W.; Her, M.; Kang, S. Y.; Park, S.; Kwon, O. J.; Park, H. S. Differences in the electrochemical performance of Pt-based catalysts used for polymer electrolyte membrane fuel cells in liquid half-and full-cells. Chemical Reviews 2021, 121 (24), 15075-15140.(2)Kodama, K.; Motobayashi, K.; Shinohara, A.; Hasegawa, N.; Kudo, K.; Jinnouchi, R.; Osawa, M.; Morimoto, Y. Effect of the side-chain structure of perfluoro-sulfonic acid ionomers on the oxygen reduction reaction on the surface of Pt. ACS Catalysis 2018, 8 (1), 694-700. (2) Garsany, Y.; Atkinson, R. W.; Sassin, M. B.; Hjelm, R. M.; (3) Gould, B. D.; Swider-Lyons, K. E. Improving PEMFC Performance Using Short-Side-Chain Low-Equivalent-Weight PFSA Ionomer in the Cathode Catalyst Layer. Journal of The Electrochemical Society 2018, 165 (5), F381-F391.(4) Ramaswamy, N.; Kumaraguru, S.; Koestner, R.; Fuller, T.; Gu, W.; Kariuki, N.; Myers, D.; Dudenas, P. J.; Kusoglu, A. Editors’ Choice—Ionomer Side Chain Length and Equivalent Weight Impact on High Current Density Transport Resistances in PEMFC Cathodes. Journal of The Electrochemical Society 2021, 168 (2), 024518.(5) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chemical Reviews 2004, 104 (10), 4535-4586. DOI: 10.1021/cr0207123.

Experimental and numerical efforts to improve oxygen mass transport in porous catalyst layer of proton exchange membrane fuel cells

Experimental and numerical efforts to improve oxygen mass transport in porous catalyst layer of proton exchange membrane fuel cells

Exploring the Potential of Hierarchical Zeolite-Templated Carbon Materials for High-Performance Li-O2 Batteries: Insights from Molecular Simulations.

The commercialization of ultrahigh capacity lithium-oxygen (Li-O2) batteries is highly dependent on the cathode architecture, and a better understanding of its role in species transport and solid discharge product (i.e., Li2O2) formation is critical to improving the discharge capacity. Tailoring the pore size distribution in the cathode structure can enhance the ion mobility and increase the number of reaction sites to improve the formation of solid Li2O2. In this work, the potential of hierarchical zeolite-templated carbon (ZTC) structures as novel electrodes for Li-O2 batteries was investigated by using reactive force field molecular dynamics simulation (reaxFF-MD). Initially, 47 microporous zeolite-templated carbon morphologies were screened based on microporosity and specific area. Among them, four structures (i.e., RHO-, BEA-, MFI-, and FAU-ZTCs) were selected for further investigation including hierarchical features in their structures. Discharge product cluster analysis, self-diffusivities, and density number profiles of Li+, O2, and dimethyl sulfoxide (DMSO) electrolyte were obtained to find that the RHO-type ZTC exhibited enhanced mass transfer compared to conventional microporous ZTC (approximately 31% for O2, 44% for Li+, and 91% for DMSO) electrodes. This is due to the promoted formation of small-sized product clusters, creating more accessible sites for oxygen reduction reaction and mass transport. These findings indicate how hierarchical ZTC electrodes with micro- and mesopores can enhance the discharge performance of aprotic Li-O2 batteries, providing molecular insights into the underlying phenomena.

Model-Driven Membrane Electrode Assembly Design for High-Performing Open-Cathode Polymer Electrolyte Membrane Fuel Cells

Open-cathode fuel cells use air cooling to effectively reduce system cost. However, due to the challenging hygrothermal environment, they generally suffer from low performance compared to conventional, liquid-cooled cells. A pre-validated, three-dimensional computational model is used in the present work to determine the effects of different sub-component designs, namely the polymeric membrane, composition of the cathode catalyst layer (CCL), and structure of the cathode microporous layer (CMPL), on the performance of an open-cathode fuel cell. This comprehensive parametric study performed on a total of 90 cases shows the increment in current density to be 7% and 31% by improvising the membrane and CCL design, respectively, at 0.6 V. A steep increase of 87% is also achieved by strategically modifying the CMPL design at 0.4 V operation. An overall increment of 119% and 131% in current density is achieved for the best membrane electrode assembly (MEA) design at 0.6 and 0.4 V, respectively, as compared to the baseline design. These improvements are achieved by collective improvements in kinetics, oxygen mass transport, ohmic resistance, self-heating, and water retention in the ionomer phase. The proposed MEA design could facilitate open-cathode fuel cell stacks with 2× higher power output or 56% lower weight and materials cost for a given power demand.

Stability analysis and numerical simulation of non-steady partial differential model in the human pulmonary capillaries using finite differences technique

In the present study, a mathematical model of non-steady partial differential equation from the process of oxygen mass transport in the human pulmonary circulation is proposed. Mathematical modeling of this kind of problems lead to a non-steady partial differential equation and for its numerical simulation, we have used finite differences. The aim of the process is the exact numerical analysis of the study, wherein consistency, stability and convergence is proposed. The necessity of doing the process is that, we would like to increase the order of numerical solution to a higher order scheme. An increment in the order of numerical solution makes the numerical simulation more accurate, also makes the numerical simulation being more complicated. In addition, the process of numerical analysis of the study in this order of solution needs more research work.

Radiative ternary nanoliquid flow over an exponentially stretching sheet with motile oxytactic microorganisms

This article examines the mixed convection ternary nanoliquid flow containing a suspension of oxytactic microorganisms under the impact of nonlinear thermal radiation. The potential applications include microbiology, biotechnology, biosensors, cell therapy, medication release system, and protein expression. The governing equations that handle the problem are partial differential equations, they are dimensional, nonlinear in nature, and are nondimensionalized by using nonsimilar transformations. Further, for the computation of mathematical analysis, the Quasilinearization technique and numerical method such as the implicit finite difference scheme are used. The impacts of varying relevant factors on velocity, temperature, and microbe density are thoroughly examined. Results designate that the radiation factor favorably affects the velocity and temperature profiles. The rate of mass transport of liquid oxygen rises by about 45% when the Schmidt number grows from 1 to 2. The coefficient of skin friction is found to be more for ternary nanofluid than hybrid nanoliquid and nanoliquid. The current analysis compares the solution to earlier literature and finds that there is a good agreement.

Insights into Oxygen Mass Transport Mechanisms in High-Temperature Proton Exchange Membrane Fuel Cells with Microelectrode

The mass transport of oxygen in the cathode catalyst layer of high-temperature proton exchange membrane fuel cells (HT-PEMFCs) has a great impact on cell performance. However, some bulk average methods are unable to study the permeation properties of oxygen near the triple phase boundary of HT-PEMFCs due to the size mismatch. Here, we develop a microelectrode integrated system to quantitatively study the O2 mass transport behavior at the Pt/binder interface under actual operating temperatures (100 °C–180 °C). The oxygen diffusion coefficients and solubility obtain from potential-step chronoamperometry and a modified Cottrell equation follow the diffusion “ball-cage” model and the dual-mode solubility model. Subsequently, molecular dynamics simulations are used to describe the key structural elements and diffusion behavior of oxygen molecules from the microscopic perspective. These results provide a scientific approach to study the mass transfer process of oxygen at the local environment, endowing with insightful strategies for future improvement and applications of HT-PEMFCs.

Numerical investigation of tapered flow field configuration to improve mass transport and performance of proton exchange membrane fuel cell

To increase the oxygen mass transport, water removal, and cell performance of proton exchange membrane fuel cells, tapered flow field configurations (FFCs) considering porous medium thickness (PMT) and thermal contact resistance (TCR) are proposed. In this work, the impact of the tapered FFC on the internal electrochemical process and overall cell performance is quantitatively explored using a three-dimensional multiphase fuel cell model. The tapered FFC is compared without and with account for both the PMT and the TCR between bipolar plates and gas diffusion layers. Compared to the conventional FFC, an increased ratio of the side length for the inlet to that for the outlet (LI/O) for the tapered FFC, addressing both the PMT and the TCR improves hydrogen, and oxygen delivery, water removal, and cell performance. The tapered FFC design with a LI/O of 1.2 is superior to all other tapered FFC designs in terms of uniformity of reactants, with an improvement in pressure drop by 68.74%, current density by 7.57%, and power density by 12.63%. These improvements result in an overall enhancement of cell performance.