bluuebunny/crossref_metadata_2025_split huggingface dataset:
Comparing Different Cross-Section Cutting Methods for SEM Analysis of Membrane-Electrodes Assemblies
Authors: Merino Susana , Novillo Carlos , de Diego Gonzalo , Conde Julio J , Folgado María Antonia , Ferreira-Aparicio Paloma , Chaparro Antonio M 
Published: February 2020

The observation of membrane-electrodes assemblies (MEAs) in cross-section is a basic tool for their morphological characterization, and the study of changes occurring in the different layers by operation in a fuel cell. But the preparation of a well-defined and representative cross-section plane in the MEAs is difficult due to a layered structure with very different mechanical properties among the layers, including a Nafion membrane, two catalyst layers, and two gas diffusion layers. For this reason, the preparation of samples for cross-sectional observation requires appropriate cutting techniques able to leave a cross-sectional plane showing the morphology of each layer with minor changes, ie. avoiding uneven surfaces, mixing of layers, porosity and morphology alterations, and/or delamination. It is possible that no single method may accomplish with all requirements, so different cross sectioning procedures must be used to have a complete characterization. With this aim, different preparation methods for cross-section analysis of MEAs have been tested in order to make a comparative analysis, showing advantages and disadvantages for each of the methods.
The preparation procedures analyzed are cutting with sharp-edge, laser-cutting, embedded-mechanical polishing, and focused ion beam (FIB). Results are shown in the figure. Cutting with a sharp edge (Fig. a) is a quick and easy method that may provide good cross-sectional observation and morphology preservation if an appropriate tool is used and careful handling. However, the probability for a successful preparation is not high due to frequent intermixing of layers, delamination, and uneven surface. Laser-cutting with a CO2 laser (Fig. b)) occurs by burning and vaporization of the material, leaving a plane with a clear damage at a micrometric scale. Especially Nafion membranes and carbon clothes showed severe alteration of the phases in the cutting plane, so this method cannot be used for a micrometric study of the morphology of the layers. Mechanical polishing of an embedded MEA a resin (Fig.c) provides, on the other hand, better preserved layers and more reproducible results; the morphology of powder layers, like the catalyst layer and microporous layer, may be altered because of the rearrangement of particles during the polishing process. Better preservation of multilayered samples can be obtained with the FIB technique (Fig.d), although the observation profile cannot be extended beyond a few millimeters. The FIB cross-cutting technique, that may be combined with a cross section polishing with argon gas, appears to better preserve the morphology of the layers and in a reproducible way.
Acknowledgements.
This work was supported by the Ministerio de Economía y Competitividad of Spain, and Fondo Europeo de Desarrollo Regional (FEDER), Project E-LIG-E, ENE2015-70417-P (MINECO/FEDER).

Figure 1
Advanced Electron Microscopy Methods to Evaluate Thin Ionomer Films in PEM Fuel Cell Catalyst Layers
Authors: More Karren L. , Cullen David A. 
Published: February 2020

The ability to characterize the structure and chemistry of ionomer films within the catalyst layers in membrane electrode assemblies (MEAs) at the Å-scale via analytical electron microscopy presents many challenges. The ionomer degrades rapidly during analysis with high-current electron beams, which can be mitigated to some extent when experiments are conducted using higher operating voltages and under cryogenic temperatures.1These microscopy operational parameters have proven most useful for imaging, mapping, and analyzing ionomer aggregates that are typically tens-to-hundreds of nms in size but have not been particularly useful for understanding the nature of thin ionomer films that percolate through the thickness of the catalysts layers.
Recently, Oak Ridge National Laboratory has acquired a new aberration-(probe)-corrected analytical scanning transmission electron microscope (STEM) that can be operated under conditions significantly more favorable for interrogating “soft matter” at very high resolution, e.g., structure and chemistry of the nm-scale thin ionomer films within intact catalyst layers. This new STEM, the JEOL-NEOARM, can be operated between 30kV-200kV under low-electron-dose conditions. The NEOARM is equipped with state-of-the-art analytical capabilities for the simultaneous acquisition of electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS). Most notably, the new STEM is equipped with a novel pixelated detector that enables ultrafast image/diffraction (ptychographic) data acquisition that will minimize exposure of the ionomer films to the electron beam, preventing damage accumulation, and allowing for complete data reconstruction following data collection.
We will present imaging and spectroscopic results obtained by varying the STEM operating conditions on the NEOARM (voltage, electron dose, sample temperature, ptychography reconstruction methods) and exploiting its unique combination of low-dose and ultrafast capabilities. The high-resolution structural and chemical information will provide new insight regarding thin ionomer films within MEA catalyst layers
Research supported by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, as part of the Fuel Cell Performance and Durability (FC-PAD) Consortium. Microscopy research performed as part of a user project at the Center for Nanophase Materials Sciences, which is a U. S. Department of Energy, Office of Science User Facility.

References

D.A. Cullen, R. Koestner, R.S. Kukreja, Z.Y. Liu, S. Minko, O. Trotsenko, A. Tokarev, L. Guetaz, H.M. Meyer, C.M. Parish, and K.L. More, “Imaging and Microanalysis of Thin Ionomer Layers by STEM,” Journal of The Electrochemical Society
161F1111-17 (2014).
Influence of Pore Size Distribution on Operando Gas Diffusion Layer Liquid Saturation
Authors: Xu Hong , Bührer Minna , Marone Federica , Schmidt Thomas J. , Büchi Felix N. , Eller Jens 
Published: February 2020

Hydrogen fed polymer electrolyte fuel cells (PEFCs) have drawn attention as key technology for future decarbonized energy systems [1], in particular in the mobility sector, due to their high current density at low operating temperatures and rapid start-up [2]. The liquid water generated during high current density operation may saturate the pores of gas diffusion layers (GDLs), hence impede the gas transport to the catalyst layer, leading to performance losses. Higher performance requires, improved water management in GDLs. In this work, the liquid water distribution in GDLs has been studied with an operando X-ray tomographic microscopy (XTM) setup at the TOMCAT beamline of the Swiss Light Source (SLS) at Paul Scherrer Institut (PSI) [3,4]. XTM scans with a duration of 0.1 s and a voxel edge length of 2.75 um were employed to investigate PEFCs assembled with three types of GDL materials (Freudenberg I6, Toray TGPH-060 and SGL 24BA) with increasing pore size (see Figure 1). Properties of the operando liquid saturation were determined between OCV and 1.25 A/cm2 at current steps of 0.25 A/cm2 under H2/air operation with 100%/0% and 100%/100% humidified gases at start-up temperature of 25 oC. The influence of the GDL substrate pore size, current density and gas humidity on liquid water saturation and connectivity in the GDL will be presented.
References:
[1] Int. Energy Agency, Advanced Fuel Cells Implementing Agreement Annual Report (2009).
[2] J. W. Pratt, L. E. Klebanoff, K. Munoz-Ramos et al., Sandia Report, SAND2011-3119 (2011).
[3] J. Eller, F. Marone, F. N. Büchi, ECS Trans. 69(17), 523–531 (2015).
[4] J. Eller, J. Roth, F. Marone, M. Stampanoni, F. N. Büchi, J. Electrochem. Soc. 164(2), 115-126 (2017).

Figure 1
Mechanistic Understanding of the Role of Ionomer in the PEM Fuel Cell Catalyst Layer
Authors: Goswami Navneet , Mistry Aashutosh N , Verma Ankit , Grunewald Jonathan B , Fuller Thomas F. , Mukherjee Partha P. 
Published: February 2020

With an attempt to design fuel cell catalyst layers with low Pt loading capable of delivering suitable performance and life expectancy, interfacial interactions between the different phases need to be improved. In this aspect, modulating the distribution of ionomers in the cathode catalyst layer is an interesting direction that is worth exploring as it can provide a highly connected proton conducting network in addition to reducing transport limitations.
In the present work, microstructural realisations are achieved through tomography data based on focused ion beam scanning electron microscopy (FIB-SEM) to obtain the Pt/C phase and an interfacial energy based erosion model to generate the ionomer phase. These microstructural features are then deployed to a catalyst layer based transport model to address the interplay of geometrical stochasticity and operating conditions such as relative humidity. The microstructural heterogeneity is thus investigated to unravel the structure-transport-property interactions of Pt-C and ionomer phases to provide a comprehensive understanding of the electrochemical performance.
Cathode Catalyst Layer Design with Graded Porous Structure for Proton Exchange Membrane Fuel Cells
Authors: Garsany Yannick , Atkinson Robert W. , Bethune Keith , St-Pierre Jean , Gould Benjamin D. , Swider-Lyons Karen 
Published: February 2020

Cathode catalyst layer (CL) design is important for improved performance, durability and stability of proton exchange membrane fuel cells (PEMFCs). An effective cathode CL must serve multiple functions simultaneously: electron and H+ conduction, O2 or H2 supply, and effective water management. The reaction in the CL requires three-phase boundaries (or interfaces) among ionomer (for proton transfer), platinum (for catalysis), and carbon (for electron transfer), as well as voids (reactant diffusion); an optimized cathode CL structure should have micropores and mesopores to balance water egress and O2 diffusion.
Our previous research showed that increasing microporosity in the cathode CL increases the power production of the PEMFCs [1]. Modeling indicates that high CL microporosity enhances water evaporation, which reduces mass transport resistances to enable higher power production [2]. Meanwhile, low microporosity encourages liquid water retention.
It is known that catalyst support type has a profound effect on electrode porosity, influencing both micro- and mesoporosity [3, 4]. By constructing electrodes with layers of different carbon supported catalyst types, we demonstrate that we can create porosity gradients in the CLs that vary the electrode water management properties and the power of the PEMFC. Two different types of commercial platinum/carbon electrocatalysts are used in dual cathode CLs – 40 wt. % Pt on Vulcan carbon (Pt/VC) having low micropores and low porosity and 40 wt. % Pt on Ketjen black (Pt/KB) having large micropores and high porosity. CLs are created with both homogeneous porosity in the Z direction (Figure 1a), and compared to ones with non-uniform porosity across the Z-direction (Figure 1b).
The microstructure of these cathode CLs is characterized using N2-sorption porosimetry. The CLs are assembled into PEMFCs and probed with polarization curves (at 100, 50 and 25% RH), cyclic voltammetry, O2 gain, electrochemical impedance spectroscopy, and limiting current measurements to quantify the sources of mass transport resistance.
The results indicate that the regions with low microporosity encourage liquid water retention, while high microporosity encourages water evaporation. These countering actions are balanced for high CL hydration and gas diffusion, and better PEMFC performance.

References

[1] Y. Garsany, R.W. Atkinson, M.B. Sassin, R.M.E. Hjelm, B.D. Gould, K.E. Swider-Lyons, J. Electrochem. Soc., 165 (2018) F381-F391.
[2] M. Eikerling, J. Electrochem. Soc., 153 (2006) E58-E70.
[3] T. Soboleva, K. Malek, Z. Xie, T. Navessin, S. Holdcroft, ACS Appl. Mater. Interfaces, 3 (2011) 1827-1837.
[4] Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, Journal of Power Sources, 315 (2016) 179-191.

Figure 1
Low Cost Gas Diffusion Layer Materials and Treatments for Durable High-Performance PEM Fuel Cells
Authors: Leonard Daniel Philip , Borup Rod L. 
Published: February 2020

The gas diffusion layer (GDL) is a critical component of proton exchange membrane fuel cells (PEMFC). It serves as the primary mediator in electrical and thermal conductivity as well as gas and water transport within the cell. Currently, the production of GDLs is dependent on the use of high-cost materials and manufacturing methods. The most common base material for GDLs is polyacrylonitrile (PAN). PAN, in addition to its high base cost ($15 - 20/kg), requires high graphitization temperatures (> 1700°C) in order attain sufficient conductivity and post-carbonization treatment with Teflon to increase the hydrophobicity for improved water transport. The high strength of PAN based GDLs has been required to prevent fiber intrusion into the channel of the flow field. Some state-of-the-art flow fields no longer utilize the land/channel design, and as such may not require the use of high strength materials like PAN in order to be functional. This presents the opportunity to create novel GDLs with equivalent or superior performance at lower cost.
We are investigating methods to significantly reduce the cost of GDL production while maintaining durability and performance characteristics necessary to PEM fuel cells. First, we are developing the use of inexpensive, natural fiber materials, such as cotton, jute, and bamboo. Second, we are developing methods to reduce the carbonization temperatures and eliminate manufacturing steps, such as the need for a microporous layer.
Figure. SEM images of commercial (A) (without microporous layer), cotton (B), and jute (C) based GDLs. The cotton and jute GDLs had been carbonized at 1100°C in N2 atmosphere.

Figure 1
Increase in Oxygen Transport Resistance Caused By Degradation in the Porous Layers in a Proton Exchange Membrane Fuel Cell
Authors: Zhang Xu , Liu Hongtan , Zhang Xuyang , Alrashidi Abdullah 
Published: February 2020

The increase in mass transport loss accounts for important performance loss when various degradation mechanisms occur in the proton exchange membrane fuel cell (PEMFC)[1-5]. The deterioration of mass transport could be ascribed to two aspects: the increase of liquid water saturation, due to the worse water transport, and increased oxygen diffusion resistance in the pore space. The loss of hydrophobicity has been identified in catalyst layer (CL) [1, 6], gas diffusion layer (GDL) [7, 8], and micro porous layer (MPL) [9, 10], which leads to worse water management. Microscopic characterizations also confirm severe damages to the microstructure, which may hinder the oxygen diffusion in the multiscale porous components[4, 11]. In this study, the oxygen transport resistance at the cathode side of a PEMFC is measured to unravel the increase in oxygen transport resistance associated with microstructure damage. Limiting current measurements are conducted under different dry oxygen fraction conditions before and after an accelerated stress testing (AST). Both GDLs with and without MPL are employed at the cathode side. The results show the presence of MPL increase the total oxygen transport resistance due to the additional layer and its lower porosity. After the aging testing, the oxygen transport resistances in both cells increases significantly. These agree with the severe pore microstructure damage caused by aging testing. The cell with MPL has even larger increases. Further decouple of the total resistance indicates that the cell with MPL has smaller increases in molecular diffusion resistance, but presents significantly greater increases in the other resistance. This implies that pore structure damage occurs in MPL itself and results in a severe increase in Knudsen diffusion resistance.
Fig.1 (a) Oxygen transport resistance under different pressure; (b) Decouple of oxygen transport resistance without and with MPL at the cathode.

Figure 1
A High-Power Non-Enzymatic Glucose Biofuel Cell Based on a Nano/Micro Hybrid-Structured Gold Anode
Authors: Chu Tien-Fu , Wang Gou-Jen 
Published: February 2020

Biofuel cell (BFC) that has attracted considerable attentions recently is a green energy generator that can generate electricity directly from the oxidation of biological organic matter. Glucose biofuel cells (GBFCs) that uses glucose as fuel has emerging need due to the rapid growth of wearable or implantable medical devices. In general, enzyme based GBFCs that use enzymes as the catalyst are still the mainstream of current development due to the excellent selectivity and reaction rate of specific enzymes. However, the enzyme based GBFCs suffer from the disadvantages of prone to poisoning, limited enzyme stability and cumbersome enzyme immobilization procedures. Therefore, non-enzymatic technologies that use nanostructures of noble metals such platinum, gold, and silver as the catalyst has become a new development trend.
In this study, a high-power non-enzymatic glucose biofuel cells based on a nano/micro hybrid-structured gold anode is proposed. In this novel electrode, a uniformly distributed micro hemisphere array of polycarbonate was fabricated by hot embossing. Then a self-assembled monolayer of gold nanoparticle was formed on the micro hemisphere array as the anode. The cathode was composed of a graphene film coated glassy carbon electrode. The anode and cathode are separated by a proton exchange membrane (nafion) to complete the battery assembly. Experimental results demonstrated that the proposed non-enzymatic GBFC has a high power density of 9.459 mWcm-2, a high current density of 8 mAcm-2, and an open circuit voltage (Voc) of 0.7128 V at room temperature. In addition, the proposed GBFC can be easily produced on a large scale, the cost of fabrication is low, repeatability is high, and is easy to preserve on a long-term basis. Therefore, it is highly feasible to be commercialized and used in practical applications.

Figure 1
Phosphonic Acid-Functionalized Polyaromatic Electrolytes for High Temperature Proton Exchange Membrane Fuel Cells
Authors: Park Eun Joo , Lee Albert S , Kim Yu Seung 
Published: February 2020

Proton exchange membrane fuel cells (PEMFCs) are one of the most promising environmentally friendly energy conversion devices. The high temperature PEMFC system requires simplified heat and water management, which would provide substantial cost benefits and high potential for commercialization. Phosphonated polyaromatic electrolytes are promising candidates as proton conductors at high temperatures due to their ability to conduct protons under the full range of humidity levels and their covalently bound phosphonic acid structure, which eliminates the risk of leaching of the acid group at high humidity conditions. Despite their synthesis of research for more than decades, only few successful examples of phosphonated polymers in the PEMFCs have been reported.
Here we present the new synthesis of phosphonic acid-functionalized polyaromatic electrolytes as electrode ionomers of high temperature PEMFCs. The polyaromatic electrolytes were synthesized by acid-catalyzed Friedel-Crafts polycondensation and followed by the SN
Ar Michaelis-Arbuzov reaction of aryl pentafluorides. The electrolytes with the ion exchange capacity ranged 1.9-2.2 meq/g have good solubility in polar aprotic solvents, which eases the fabrication of materials as electrode binders. We could obtain the peak power density up to 700 mW/cm2 at current density of 2.1 A/cm2 at 200 °C with the membrane electrode assembly using the phosphonic acid functionalized ionomer. We will compare the fuel cell performance to that of ion-pair coordinated electrode binders1 and discuss the high applicability of the phosphonated polymer electrolytes in high temperature systems.
Reference

K.-S. Lee, J.S. Spendelow, Y.K. Choe, C. Fujimoto, Y. S. Kim, Nature Energy, 2016, 1, 16120-16124.
Phosphoric Acid Loss Mechanism of Acid-Base and Ion-Pair Coordinated Proton Exchange Membranes
Authors: Kim Yu Seung , Lee Albert S , Choe Yoong-Kee , Matanovic Ivana 
Published: February 2020

Acid retention of phosphoric acid-doped proton exchange membranes (PEMs) is one of the most critical factors that determine the performance and durability of high-temperature PEM fuel cells. However, the mechanism of phosphoric acid loss in the PEMs in the presence of water has not been discussed in terms of the energetics of phosphoric acid. In this study, we investigate the interactions of the phosphoric acid cluster by 31P NMR experiments and density functional theory calculations to propose new phosphoric acid loss mechanism of the phosphoric acid-polybenzimidazole PEM and phosphate-quaternary ammonium ion-pair PEM.
The small molecule study suggests that the removal of the phosphoric acid from the phosphoric acid cluster does not occur due to the stronger interaction of phosphoric acid and water. On the contrary, adding water molecules to the PA cluster increases the cluster interaction energy by creating hydrogen bonding. In the phosphoric acid incorporated PEMs, we observed a consistent result that adding water increases the PA cluster volume rather than losing phosphoric acid. However, with excessive water activity, the polymer loses phosphoric acid due to the incapability to hold the water and phosphoric acid beyond a certain level.1 In the phosphoric acid loss process, the equilibrium phosphoric acid composition in the cluster plays a critical role.
In the latter part of this presentation, we explain how the acid-base interaction and ion-pair interaction impact the equilibrium phosphoric acid composition the phosphoric acid cluster. We also explain how the equilibrium phosphoric acid composition affects the proton conductivity at different RH level. Finally, we demonstrate the high-temperature membrane fuel cell durability in the presence of water to prove the newly proposed acid loss mechanism. This presentation provides critical insights to design high-temperature proton exchange membranes.
Reference
1. A. S. Lee, Y. -K. Choe, I. Matanovic, Y. S. Kim, The energetic of phosphoric acid interactions reveals a new acid loss mechanism, J. Mater. Chem. A. DOI: 10.1039/C9TA01756A (2019).