Detection of the delayed condensation effect and determination of its impact on the accuracy of gas adsorption pore size distributions

Macroscopic, highly disordered, mesoporous materials present a continuing challenge for accurate pore structure characterization. The typical macroscopic variation in local average pore space descriptors means that methods capable of delivering statistically representative characterizations are required. Gas adsorption is a representative but indirect method, normally requiring assumptions about the correct model for data analysis. In this work we present a novel method to both expand the range, and obtain greater accuracy, for the information obtained from the main boundary adsorption isotherms by using a combination of data obtained for two adsorptives, namely nitrogen and argon, both before and after mercury porosimetry. The method makes use of the fact that nitrogen and argon apparently ‘see’ a different pore geometry following mercury entrapment, with argon, relatively, ‘ignoring’ new metal surfaces produced by mercury porosimetry. The new method permits the study of network and pore–pore co-operative effects during adsorption that substantially affect the accuracy of the characteristic parameters, such as modal pore size, obtained for disordered materials. These effects have been explicitly quantified, for a typical sol-gel silica catalyst support material as a case study. The technique allowed the large discrepancies between modal pore sizes obtained from standard gas adsorption and mercury thermoporometry methods to be attributed to the network-based delayed condensation effect, rather than spinodal adsorption. Once the network-based delayed condensation effect had been accounted for, the simple cylindrical pore model and macroscopic thermodynamic Kelvin-Cohan equation were then found sufficient to accurately describe adsorption in the material studied, rather than needing a more complex microscopic theory. Hence, for disordered mesoporous solids, a proper account of inter-pore interactions is more important than that of intra-pore adsorbate density distribution, to obtain accurate pore size distributions

Electrode Development and Characterization for Polymer Electrolyte Fuel Cell with Low to Zero Platinum Loading

<p>Electrification of vehicles could enable the transportation sector to be more efficient with reduced emissions. Polymer electrolyte fuel cell (PEFC) vehicles powered by sustainably generated hydrogen fuel would be a viable replacement for internal combustion engines. PEFC powered vehicles are highly efficient and offer zero tailpipe emissions. State-of-the-art PEFCs rely on platinum (Pt) and platinum alloy catalyst nanoparticles supported on high surface area carbon black bound by ionomer. The high Pt loadings currently needed for the low-temperature acidic oxygen reduction reaction (ORR) incur significant costs due to the Pt raw material. Degradation of conventional carbon supported Pt electrodes remains a challenge for the commercialization of PEFCs. In the cathode, the ORR is sluggish and results in a large overpotential loss and hence reducing the Pt utilization. The strategy to make PEFC commercially viable is by reducing the cost through either reducing the amount of Pt in the electrode or replacing the Pt catalyst with alternative low-cost catalysts. Ionomer binder in conventional electrodes needed for proton conduction introduces undesirable high oxygen transport resistance that further reduces the Pt efficacy. However, novel ionomer-free electrodes, which have an advantage of no ionomer film resistance, relies on water for proton conduction and thus hindering the performance and stability at dry conditions. Alternative electrode designs can potentially alleviate some of the problems in these high power density devices. This work presents an alternative composite Nafion nanofiber catalyst support electrode, in which the oriented nanofibers provide robust internal proton transport to a conformal Pt catalyst coating without impeding oxygen transport. The high-surface-area electrodes are prepared by solution casting Nafion onto a sacrificial template, and thin Pt films are deposited on the nanofibers using either physical vapor deposition or chemical vapor deposition. The electrochemical characterization of the nanofiber electrodes demonstrates the high current density and specific activity of this nanofiber approach relative to prior electrodes fabricated by depositing Pt directly onto other Nafion surfaces. Even with the improved electrode architecture, the Pt raw material cost is still an obstacle. Hence, Pt group metal-free (PGM-free) PEFC cathodes are of significant interest for low-temperature ORR since they have the potential to reduce PEFC costs dramatically. The activity and durability of PGM-free catalyst have significantly improved in vii the last 10 years. However, several challenges remain before they can become commercially viable. The PGM-free catalysts have lower volumetric activity and hence the PGM-free cathodes are thicker than Pt-based electrodes. Thus, they suffer from significantly greater gas and proton transport resistances that reduce the observed performance and robustness of operation. To better understand the efficacy of the catalyst and improve electrode performance, a detailed understanding of the correlation between electrode fabrication, morphology, and performance is crucial. This dissertation reports the characterization of PEFC cathodes featuring a PGM-free catalyst using nano-scale resolution X-ray computed tomography (nano-CT) and morphological analysis. In this work, the pore/solid structure and the Nafion distribution was resolved in three dimensions (3D) using nano-CT for three PGM-free electrodes of varying Nafion loading. The particular PGM-free cathode being studied feature two distinct length scales of interest and was resolved using multi-resolution imaging in nano-CT. The associated transport properties were evaluated from pore/particle-scale simulations within the nano-CT imaged structure. These characterizations are then used to elucidate the microstructural origins of the dramatic changes in fuel cell performance with varying Nafion loading. The results show that this is primarily a result of distinct changes in Nafions spatial distribution. The significant impact of electrode morphology on performance highlights the importance of PGM-free electrode development in concert with efforts to improve catalyst activity and durability. To understand the potential distribution in the thick electrodes we utilize a novel experimental technique to measure the electrolyte potential directly at discrete points across the thickness of the catalyst layer and evaluate the ORR along the thickness of the catalyst layer. Using that technique, the electrolyte potential drop, the through-thickness reaction distribution, and the proton conductivity is measured and correlated with the corresponding Nafion morphology and cell performance. At this stage of PGM-free catalyst development, it is also necessary to optimize these thick electrodes along with the catalyst. To address the significant transport losses in thick PGM-free cathodes (ca. > 60 m), we developed a two-dimensional (2D) hierarchical electrode model that resolves the unique structure of the PGM-free electrode. The 2D computational model is employed to correlate the morphology and the electrochemical performance of the PGM-free electrodes. The model is a complete cell, continuum model that includes an agglomerate model representation of the cathode. A unique feature of the approach is the integration of the model with morphology and transport parameter statistics extracted from nano-CT imaging of the electrodes. The model was validated with experimental results of the PGM-free electrode with three levels of Nafion loading. We discuss the sensitivity of the PGM-free catalyst layer on the operating conditions and the morphological parameters to identify improved architectures for PGM-free cathodes. We employ the model to evaluate the targets for the volumetric activity of the catalyst. A notable finding is the impact of the liquid water accumulation in the electrode and the significant performance improvement possible if electrode flooding is mitigated.</p

Meso-Structured Polymer Electrolyte Fuel Cell Electrode

Increasing the utilization of Pt and Pt alloy catalysts in polymer electrolyte fuel cell cathodes is critical to improving the high power density operation, particularly at low Pt loadings. State of the art electrodes are fabricated in an ink deposition process that leads to uncontrolled electrode architecture with random aggregates of functional domains (catalyst, ionomer, and pore volume) (1). The randomness in the domains induces high tortuosity transport pathways for ions and fluids, which cause severe transport resistance during high current density operation. Thin ionomer films cause additional transport resistance and poisoning of the Pt catalyst, which becomes more significant at low Pt loadings. Reducing the amount of ionomer in the catalyst domain without affecting the ionic transport resistance is key to improving the utilization of the Pt and reducing the transport resistance at low Pt loading. Rational design of the electrode structure with controlled low tortuous ionic transport pathways could improve performance. The introduction of the ionomer pathways could also enable reduction of the ionomer volume in the catalyst domain, reducing the transport resistance. Middelmen et al. proposed electrode structures consisting of aligned components in a low tortuosity configuration to improve performance (2). In this work, we present an alternative electrode structure based on a vertically aligned array of Nafion pillars in the cathode catalyst layer, as shown in Figure 1a. Figure 1b shows the SEM image of the Nafion pillars. Pt supported on carbon catalyst was deposited on the Nafion pillars to fabricate a meso-structured electrode. Nafion pillars provide high conductive and low tortuous pathways for protons, reducing the effective transport distance, and enabling reduction of the ionomer binder in the catalyst domain. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos. References 1. S. Litster and G. McLean, Journal of Power Sources, 130, 61 (2004). 2. E. Middelman, Improved PEM fuel cell electrodes by controlled self-assembly, in, p. 9 (2002). Figure

Spatially resolved degradation during startup and shutdown in polymer electrolyte membrane fuel cell operation

International audience• Degradation due air/air operation due to startup and shutdown in fuel cell studied. • The effect of platinum loading, and carbon support material is studied. • A segmented cathode hardware is utilized to study the effect along the flow field. • In-situ and ex-situ characterization were correlated to elucidate the degradation. • Limiting the anode's ability to reduce oxygen to water is key to mitigating loss. A B S T R A C T Polymer electrolyte membrane fuel cells have durability limitations associated with the startup and shutdown of the fuel cell, which is critical for real-world vehicle commercialization. During startup or shutdown, there exists an active region (hydrogen/air) and a passive region (air/air) between the cell inlet and outlet. Internal currents are generated in the passive region causing high-potential excursion in the cathode leading to accelerated carbon corrosion. In this study, a segmented cathode hardware is used to evaluate the effect of platinum loading on both cathode and anode, and carbon support material on degradation due to repeated series of startups or shutdowns. In situ losses in the performance and electrochemical surface area were measured spatially, and ex situ analysis of the catalyst layer thickness and platinum particle size was performed to understand the effect of startup or shutdown on different membrane electrode assembly materials. Startup degrades the region near anode outlet more, while shutdown degrades the region near anode inlet more compared to the rest of the electrode. While various system mitigation strategies have been reported in the literature to limit this degradation, one materials mitigation strategy is to limit the anode's ability to reduce oxygen to water through increasing the ratio of platinum loading in the cathode to the anode, or by using a bi-functional catalyst

A Viewpoint on X-ray Tomography Imaging in Electrocatalysis

With the emerging demands for clean energy and an economy with net-zero greenhouse gas emissions, electrocatalysis areas have attracted tremendous interest in recent years. The electrochemical devices that use electrocatalysis, such as fuel cells, electrolyzers, and flow batteries, consist of hierarchical structures, requiring comprehension and rational designs across scales from millimeter and micrometer all the way down to atomic scale. In past decades, electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been extensively utilized for imaging different scales of these devices in both two and three dimensions. However, electron-based techniques for high-resolution imaging require uninterrupted maintenance of a high-vacuum environment, leading to difficulties of sample preparation and lack of integrated observation without intrusion/disassembly. To overcome these disadvantages, more and more efforts have been dedicated to the development of X-ray imaging techniques recently, specifically absorption-based two-dimensional (2D) transmission X-ray microscopy and three-dimensional (3D) X-ray tomography, due to much better transmission behaviors of X-rays than electrons. X-ray tomography imaging mostly focuses on answering questions related to morphology and morphological changes at the microscale or near 1 μm resolution and nanoscale of 30 nm resolution. The method is nondestructive and it allows for the visualization of operando electrochemical devices, such as fuel cells, electrolyzers, and redox flow batteries. Operando X-ray microscopic tomography typically focuses on catalyst layers and morphology changes during degradation, as well as mass transport. Nanoscale tomography still predominantly is used for ex situ studies, as multiple challenges exist for operando studies implementation, including X-ray beam damage, sample holder design, and beamline availability. Both microscale and nanoscale tomography beamlines now couple various spectroscopic techniques, enabling electrocatalysis studies for both morphology and chemical transformations. This viewpoint highlights the recent advances in X-ray tomography for electrocatalysis, compares it to other tomographic techniques, and outlines key complementary techniques that can provide additional information during imaging. Lastly, it provides a perspective of what to anticipate in coming years regarding the method use for electrocatalysis studies.</p

Meso-Structured Polymer Electrolyte Fuel Cell Electrode

Increasing the utilization of Pt and Pt alloy catalysts in polymer electrolyte fuel cell cathodes is critical to improving the high power density operation, particularly at low Pt loadings. State of the art electrodes are fabricated in an ink deposition process that leads to uncontrolled electrode architecture with random aggregates of functional domains (catalyst, ionomer, and pore volume) (1). The randomness in the domains induces high tortuosity transport pathways for ions and fluids, which cause severe transport resistance during high current density operation. Thin ionomer films cause additional transport resistance and poisoning of the Pt catalyst, which becomes more significant at low Pt loadings. Reducing the amount of ionomer in the catalyst domain without affecting the ionic transport resistance is key to improving the utilization of the Pt and reducing the transport resistance at low Pt loading. Rational design of the electrode structure with controlled low tortuous ionic transport pathways could improve performance. The introduction of the ionomer pathways could also enable reduction of the ionomer volume in the catalyst domain, reducing the transport resistance. Middelmen et al. proposed electrode structures consisting of aligned components in a low tortuosity configuration to improve performance (2). In this work, we present an alternative electrode structure based on a vertically aligned array of Nafion pillars in the cathode catalyst layer, as shown in Figure 1a. Figure 1b shows the SEM image of the Nafion pillars. Pt supported on carbon catalyst was deposited on the Nafion pillars to fabricate a meso-structured electrode. Nafion pillars provide high conductive and low tortuous pathways for protons, reducing the effective transport distance, and enabling reduction of the ionomer binder in the catalyst domain. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos. References 1. S. Litster and G. McLean, Journal of Power Sources, 130, 61 (2004). 2. E. Middelman, Improved PEM fuel cell electrodes by controlled self-assembly, in, p. 9 (2002). Figure

(2021-2022 ECS Toyota Young Investigator Fellowship) Understanding and Suppression of Cation Transport into Polymer Electrolyte Membrane Fuel Cell

Polymer electrolyte membrane fuel cells (PEMFCs) are a viable zero-emissions option for the electrification of the heavy duty transportation sector. However, PEMFCs still suffer from degradation of materials over the fuel cell lifetime. Cation contaminants can be generated from corrosion of bipolar plates and balance of plant components, water contaminants, and environmental sources (e.g., Fe3+, Ca2+, Na+), making them present in the fuel or oxidant stream during operation(1). Cations have been shown to be detrimental to the performance of the PEMFC by reducing water uptake, ionic conductivity, and O2 transport, resulting in performance loss and degradation. Metal cations such as Fe3+ can also lead to chemical degradation of the membrane ionomer (2-4). It is critical to understand the mechanism and rate of cation transport from the bipolar plate channel to the membrane to develop mitigation strategy to suppress the cation transport. In this work, we present the study of the cation (Fe3+) transport mechanism through the gas diffusion layer (GDL) by introducing a cation solution in the cathode channel. Transport rates across the GDL are studied using an ex-situ GDL holder where Fe solution is introduced in the GDL substrate side with water transported through to the microporous layer side (MPL) and is collected and analyzed for Fe concentration, as shown in Figure 1a. Effect of the Fe concentration on transport rates is also studied using computational modeling. Understanding of the transport mechanism is then leveraged to identify mitigation solutions and suppress cation transport from the flow field to the electrode using a GDL with dual MPL architecture as shown in Figure 1b. Optimization of the dual MPL architecture for both durability and performance is also presented. Acknowledgements This research is supported by the 2021-2022 ECS Toyota Young Investigator fellowship and U.S. Department of Energy (DOE) Hydrogen and Fuel Cell Technologies Office, through the Million Mile Fuel Cell Truck Consortium (M2FCT). Authors acknowledge the Laboratory Directed Research and Development (LDRD) program at Los Alamos National Laboratory (LANL). References D. D. Papadias, R. K. Ahluwalia, J. K. Thomson, H. M. Meyer, M. P. Brady, H. L. Wang, J. A. Turner, R. Mukundan and R. Borup, Journal of Power Sources, 273, 1237 (2015). R. K. Ahluwalia, D. D. Papadias, N. N. Kariuki, J. K. Peng, X. P. Wang, Y. F. Tsai, D. G. Graczyk and D. J. Myers, Journal of the Electrochemical Society, 165, F3024 (2018). J. P. Braaten, X. M. Xu, Y. Cai, A. Kongkanand and S. Litster, Journal of the Electrochemical Society, 166, F1337 (2019). A. Kneer, J. Jankovic, D. Susac, A. Putz, N. Wagner, M. Sabharwal and M. Secanell, Journal of The Electrochemical Society, 165, F3241 (2018). Figure