bluuebunny/crossref_metadata_2025_split huggingface dataset:
Catalyst Particle Scale Analysis By Transport and Reaction Simulation on 3D Nano-Scale Images for PEMFC
Authors: Ogawa Shohei , Padgett Elliot , Muller David A. , Kongkanand Anusorn , Litster Shawn 
Published: February 2020

Optimizing the structure of the catalyst layer is critical to improving the proton exchange membrane fuel cell (PEMFC) performance at low Platinum (Pt) loading. A cell with reduced cathode Pt-loading for the cost reduction suffers from a limited performance at high current density, and it is a barrier to practical applications of PEMFC [1]. In addition to Pt-loadings, the types of carbon supports affect cell performance [2]. In the prior work, the nano-scale scanning transmission electron microscopy computed tomography (STEM-CT) was applied to visualize the individual Pt particle distributions in 3D at two Pt-loadings on two types of carbon supports [3]. This imaging showed that with medium surface area, low porosity carbon support (e.g., Vulcan), a large majority of Pt particles are on the surface of the carbon support. In contrast, STEM-CT showed that with a high-surface-area carbon (HSC), a majority of Pt particles are within the porous carbon support. These internal Pt particles are not in contact with the acidic electrolyte, therefore it is unclear how protons are transported to the Pt surface and how the oxygen reduction reaction (ORR) occurs on these Pt particles. In addition, prior experimental results suggest that the utilization of inner Pt particles is improved at high relative humidity with HSC since the micropores in the carbon support are likely to be filled with condensed water [3].
Here, to investigate the characteristics of those catalyst materials, we developed a transport and ORR model on 3D structures obtained from the STEM-CT images. We solve the Poisson-Nernst-Planck equations to model the electromigration and surface charge effects on the proton concentration in the porous carbon supports with a continuum approximation. Figure 1 shows the ORR current density on the Pt particles in the HSC support at the electrode potential, 0.7 V. The simulation result suggests that negative surface charge on Pt particles may be necessary for a high proton concentration and activity of inner particles in water-filled micropores at high relative humidity. This model can be used to characterize the performance of the catalyst layers in different configurations.
Acknowledgement
This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.
References
[1] A. Kongkanand and M. F. Mathias, “The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells,” J. Phys. Chem. Lett., vol. 7, no. 7, pp. 1127–1137, Apr. 2016.
[2] V. Yarlagadda et al., “Boosting Fuel Cell Performance with Accessible Carbon Mesopores,” ACS Energy Lett., vol. 3, no. 3, pp. 618–621, 2018.
[3] E. Padgett et al., “Editors’ Choice—Connecting Fuel Cell Catalyst Nanostructure and Accessibility Using Quantitative Cryo-STEM Tomography,” J. Electrochem. Soc., vol. 165, no. 3, pp. F173–F180, 2018.

Figure 1
Macroscopic Modeling of Liquid Water in Channel and GDL Based on Detailed Two-Phase CFD
Authors: Tanaka Shoichi , Takayama Ryo , Takayama Tsutomu , Motegi Haruki , Tsukamoto Takayuki , Yoneda Masakazu 
Published: February 2020

In order to perform efficient simulations of full-stack PEFC systems, the behavior of liquid water in channels and GDL is modeled using detailed two-phase CFD with VOF method. The relationship between liquid water volume fraction, pressure drop and liquid water velocity are evaluated in a macroscopic control volume of well­-resolved two-phase CFD of the flow channel, resulting in the database of gas and liquid relative permeability for water saturation ratio. The exhaust velocity of liquid water from GDL to channel is estimated by another detailed CFD, where the structure of GDL is measured by X-ray CT and directly converted to computational mesh. The effect of contact angle of separator attached to GDL is also studied from the viewpoint of characteristics about water exhaust.
For evaluating the models obtained, simulations of PEFC are performed. Coupled with electrochemical reactions, all relevant transport phenomena (mass, chemical species and heat) are solved on the coarse-grained mesh. The mass transport equation is solved by Darcy’s law, using the distribution of the equivalent hydraulic parameter in flow channels estimated by detailed gas phase CFD. Here mass transport is coupled with heat and chemical species transport equations. Those all transport phenomena are coupled with electrochemical reactions in the MEA. Transport of chemical species and water through the MEA are also considered. Many engineering models are employed in order to consider various transport phenomena, such as water uptake into electrolytes and effective oxygen transport resistance from gas phase to reaction sites in catalyst layers, etc. Electrochemical reactions are modeled by Butler-Volmer equation, in the manner of lumped parameter models. The results of simulations of 25 cm2 serpentine flow cell under several operating conditions are compared with experimental data, showing that the model we propose is useful for the purpose of PEFC design.
Membrane Electrode Assembly Fabrication Method Effects on Catalyst Layer Structure, Interfaces, and Performance
Authors: Medina Samantha , Wang Min , Van Cleve Timothy , Mauger Scott A , Ulsh Michael , Neyerlin K C , Pylypenko Svitlana 
Published: February 2020

The membrane electrode assembly (MEA) is a critical component in polymer electrolyte membrane fuel cells. MEAs are often prepared by depositing the catalyst layer directly on to the membrane. In lab-scale research, catalyst coated membranes or CCMs can be prepared by hand painting or ultrasonic spraying. Catalyst-coated diffusion media (CCDM), also known as gas diffusion electrodes (GDE) have the catalyst layer deposited on the gas diffusion media. For the large-scale manufacturability of MEAs, GDEs may be easier to manufacture in a roll-to-roll (R2R) fashion because the challenges associated with membrane swelling are avoided. In addition, for R2R fabrication, there are many ink deposition methods. For example, in gravure coating, a patterned gravure roller can be used in combination with a doctor blade and a pressure roller to coat the catalyst layer. The coating quality can be optimized by modifying the rotation speed of the gravure roller.1 Alternatively, in slot die coating modification of the distance from the slot die to the substrate can be used to improve coating quality, effectively changing the sheer rate experienced by the ink. With all fabrication methods (ultrasonically sprayed, slot die R2R, micro gravure R2R, etc.) several processing parameters can be varied, impacting the structure of the electrode and its performance. Additionally, ink composition (solvent ratio, ionomer to catalyst ratio) will also have significant impact on the catalyst layer structure.2 Thus, the MEA fabrication method, the processing conditions, ink composition, and chemistry and morphology of the catalyst will all affect the surfaces and interfaces that exist in an MEA.
It is important to understand these material interfaces and the structure, property, performance relationship between the catalyst layer components. Electron microscopy plays a key role in these studies. This work will highlight the importance of assessing MEAs at different scales and from different perspectives: catalyst morphology, top-down electrode view, cross-sectional MEA view. Microscopy can be a powerful tool for visualizing all the components within an electrode structure. Extraction of quantifiable parameters from digital image processing provides direct correlations with performance metrics, furthering the utility of electron microcopy for electrode studies.
[1] S.A. Mauger et. al, Gravure Coating for Roll-to-Roll Manufacturing of Proton-Exchange-Membrane Fuel Cell Catalyst Layers, J. Electrochem. Soc. 165 (2018) F1012–F1018. doi:10.1149/2.0091813jes.
[2] Khandavalli, S. et al. Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks. ACS Appl. Mater. Interfaces (2018). doi:10.1021/acsami.8b15039

Figure 1
(Invited) A New View of Fuel Cell Electrocatalysts through Multimodal Analytical Electron Microscopy
Authors: Cullen David A. , Cao Shaohong , Chi Miaofang , More Karren L. 
Published: February 2020

Significant advancements have been made in the performance of oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cells. State-of-the-art Pt-alloy catalysts show exceptionally high mass activity and recent research into ordered nanoparticles shows promising improvements in durability. The activity of platinum group metal-free (PGM-free) catalysts, which will be required as world-wide demand for fuel cell stacks increases, have also improved substantially, although significant durability issues persist. Accelerated materials discovery and development is required to keep pace with ever-changing market and performance demands, such as the shift in focus towards heavy duty transportation where highly durable, cost-effective catalysts are required.
Scanning transmission electron microscopy (STEM) has long played an important role in assessing the quality and durability of electrocatalysts and electrode structures, and the increasing demands on catalyst performance require a corresponding improvement in both the resolution and quantitative precision of analytical methods. Fortunately, recent advances in high-speed cameras and spectrometers allow for an unprecedented view of catalyst structure and composition in two, and in some cases, three dimensions. The newly installed JEOL NEOARM atomic-resolution STEM at the Center for Nanophase Materials Sciences combines the latest generation spectrometers with new imaging modes such as electron ptychography. The ability to acquire multiple signals simultaneously at high data acquisition speeds and across a range of accelerating voltages (30-200 kV) allows for a fresh look at the atomic structure of both Pt-based and PGM-free catalysts.
The simultaneous application of electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy to study the beam sensitive surfaces of sub-5nm Pt-alloy nanoparticles (<5nm) will be discussed. The combination of dual EELS-EDS analysis using ultra-low voltage (30kV) electron beams under cryogenic cooling will also be demonstrated on atomically dispersed transition metal sites in PGM-free electrodes. Finally, recent developments in pixelated cameras and ptychographic analysis of 2D materials will be extended to both Pt-based and PGM-free electrocatalyst systems to explore the opportunities and limits of low-dose, high-speed imaging.

Acknowledgements

Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE), as part of the FC-PAD and ElectroCat Consortia, which is part of the Energy Materials Network. Microscopy performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility.
Pairing Asymmetric Gas Diffusion Media for High-Power Fuel Cell Operation
Authors: Atkinson Robert W. , Garsany Yannick , Bethune Keith , St-Pierre Jean , Gould Benjamin D. , Swider-Lyons Karen 
Published: February 2020

Proton exchange membrane fuel cell (PEMFC) power production is highly influenced by the properties of the gas diffusion media (GDM). The GDM typically comprise a mesoporous carbon layer (MPL) on top of a porous carbon-fiber gas diffusion layer (GDL) which are both wetted with polytetrafluoroethylene (PTFE) to modify their water retention. In a PEMFC, the GDM are compressed to either side of the catalyst coated membrane. The GDM are active in the fuel cell transport processes including electron and heat conduction, water removal, and distributing reactant gases to the catalyst layers, which affects the electrochemistry in the catalyst layers. These transport phenomena rely on both the properties of the GDM solid phase and its voids, making optimization of GDM material properties challenging. The majority of reports in the PEMFC literature addressing GDM focus solely on the cathode, since water is produced at this electrode and its ineffective removal can block active sites and occlude O2 diffusion. Most research reports use symmetrical anode and cathode GDM. To date, there has been less discussion on the role of the anode GDM or how anode and cathode GDM properties can be selected in concert to increase fuel cell power output, despite that a significant amount of water in a fuel cell is rejected to the anode.
We consider that anode and cathode GDM properties, such as air permeability, must be chosen in unison to improve cell water management. In our previous work, we used X-ray computed tomography to observe that dry-laid, non-woven, Freudenberg GDM maintain large void volume while permitting high compressive stress, ultimately resulting in lower contact and ohmic resistances and better mass transport compared to another distinct class of GDM (wet-laid, SGL).1

In this work, we make a range of PEMFCs using a suite of Freudenberg GDM to study the influence of anode and cathode GDM air permeability and how these properties may be paired to improve cell hydration and increase fuel cell power production in a broad range of operating conditions. Fuel cells are characterized with cyclic voltammetry, electrochemical impedance spectroscopy (EIS), gravimetric analysis, and limiting current measurements to quantify the sources of mass transport resistance. Gradients of GDM material properties from anode to cathode improve cell hydration and facilitate water removal to reduce mass transport resistances and enable higher power operation.

1. “The Role of Compressive Stress on Gas Diffusion Media Morphology and Fuel Cell Performance,” R. W. Atkinson III, Y. Garsany, B. D. Gould, K. E. Swider-Lyons, I. V Zenyuk, ACS Applied Energy Materials, 2017, 1 (1), 191–201.
Limiting Current Measurements of Oxygen Transport Resistance in Platinum Group Metal-Free Fuel Cell Cathodes
Authors: Langhorst Lisa , Uddin Aman , Zhang Hanguang , Wu Gang , Litster Shawn 
Published: February 2020

Polymer electrolyte fuel cells (PEFCs) currently have a significant commercialization cost due to the price of the platinum or platinum-alloy catalysts they use on the cathode. Platinum group-metal (PGM) free catalysts have the potential to reduce this cost, but have lower volumetric activity, requiring the use of thicker catalyst layers. The increased catalyst layer thickness results in an increase in through-plane oxygen-transport resistance. In addition to that resistance, there are additional non-resistance due to diffusion through water filled pores and across ionomer films. Here, we elucidated the relative magnitude of the various oxygen-transport resistances from limiting current density measurements. To extract the various resistances, we measured the limiting current density for three PGM-free fuel cells with varied catalyst loading, testing each with varied total pressure and carrier gas (Helox, dry air, and 5% O2). Measuring limiting current density at various pressure separates pressure-dependent resistances from pressure-independent resistances. Pressure-dependent resistances include intermolecular gas diffusion while pressure-independent resistances include both Knudsen diffusion and diffusion through thin ionomer films that cover the catalyst. For increased loading and reduced local fluxes at the active site, local microstructure and ionomer thin-film diffusion resistances are less significant. Also, because of the lower volumetric activity and local oxygen flux, those resistances are generally lower within PGM-free cathodes. Varying the carrier gas influences the bulk gas diffusion. From this, and comparison to literature, the component of oxygen-transport-resistance due to the catalyst layer was determined apart from the channel and gas diffusion media of the cathode. Such information can be used to advance the morphology and materials of PGM-free cathodes for enhanced O2 transport and fuel cell power density.
DOE ACKNOWLEDGEMENT
This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under Award Number DE-EE0008076. The authors gratefully acknowledge research support from the the Electrocatalysis Consortium (ElectroCat), established as part of the Energy Materials Network under theU.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under Contract Number DE-EE0008076.
Towards a High-Performance DMFC Membrane Electrode Assembly Using Iron-Doped Metal Organic Framework Catalysts on the Cathode
Authors: Abdelrahman Mohamed Elfadil , Velez Daniel , Zhang Hanguang , Wu Gang , Li Xianglin , Litster Shawn 
Published: February 2020

Direct methanol fuel cells (DMFC) have several advantages for a broad spectrum of energy density demanding applications, ranging from portable devices to stationary backup power. DMFCs deliver power at high efficiencies compared to internal combustion technologies and use high energy density methanol as the fuel. However, DMFCs face several challenges, including methanol crossover at the cathode and slow methanol oxidation reaction (MOR) kinetics at anode that impede its performance. Hence, high loadings of a platinum group metal (PGM) catalyst are normally needed on both the anode and cathode, leading to high raw material catalyst costs. In this work, a PGM-free iron catalyst derived from metal organic framework precursors (Fe-MOF) is used on the cathode side to catalyze the oxygen reduction reaction (ORR). The PGM-free cathode has shown a high tolerance to methanol crossover relative to standard PGM catalysts and high performance relative to previously reported DMFC PGM-free cathode performances in the field. A wide range of operating conditions were tested, and we evaluated the maximum power density using pure oxygen and air with varying methanol concentration. We conducted our tests with a custom, integrated H2 reference electrode that separately provided the anode and cathode overpotentials. This is necessary for cell and electrode optimization since DMFC anode overpotentials are large relative to that of H2 PEMFCs and cannot be used as a quasi-reference electrode. Furthermore, we performed long term durability tests to evaluate PGM-free catalyst stability in the DMFC environment.
This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under Award Number DE-EE0008440.
Investigation of Concentration Overvoltage Increase in Low Pt-Loading PEFC
Authors: Takei Shota , Uda Keisuke , Nagayama Mayumi , Sasaki Kazunari , Hayashi Akari 
Published: February 2020

Introduction

Although fuel cell vehicles (FCVs) have already been commercialized, the cost reduction of PEFCs is required for full-scale distribution of FCVs, and so it is essential to reduce the amount of Pt, which is an expensive precious metal used as an electrode catalyst. So far, our research group has been working on preparation and performance evaluation of membrane electrode assembles (MEAs) with different cathode Pt-loading in order to determine the minimum requirement of Pt1. Then, we have found that concentration overvoltage increases in the very low Pt-loading region, lower than 0.20 mgPt/cm2. This is considered that resulting low carbon content (thin cathode layer) has led to increase in the water volume fraction in the catalyst layer under the same current density, even though the gas transfer distance is rather shorten. Therefore, in this study, we are aiming to investigate the mechanism of increase in concentration overvoltage in detail in order to reduce concentration overvoltage.

Experimental

The 1 cm2 MEAs were fabricated by a spray printing method2 using the Pt/C catalyst and Nafion® electrolyte. Here, the amount of anode Pt/C was fixed to 0.30 mgPt/cm2 (0.35 mgC/cm2), and the amount of cathode Pt/C was varied from 0.05 to 0.60 mgPt/cm2 (0.06-0.70 mgC/cm2). For the Pt/C catalyst, on the purpose of examining the influence of hydrophilic/ hydrophobic characters of carbon, TEC10E50E (46.5%Pt/KB) and TEC10E50EA (46.6%Pt/Graphitized KB (GKB)) were used. Then, current-voltage (I-V) characteristics and ECSA of MEAs were evaluated. Furthermore, impedance was also measured, and ohmic, activation, and concentration overvoltages were separately analyzed.

Results and discussion

For successful drain of water from the cathode layer in order to reduce concentration overvoltage, two approaches, such as changing evaluation conditions of MEAs and controlling the surface character of carbon supports in MEAs.
First, the humidity condition of MEA evaluation was changed from RH100% to 57%, and I-V performance was evaluated. As a result, no change in concentration overvoltage was observed on changing relative humidity. This is probably because the generated water is rather effective on concentration overvoltage than relative humidity of MEA evaluating condition.
Next, MEAs prepared using Pt/KB and Pt/GKB for the cathode and I-V performance was evaluated. As a result, as shown in Fig. 1, reduction of concentration overvoltage was confirmed by using Pt/GKB with 0.23 mgC/cm2, where increase in concentration overvoltage was already seen in case of Pt/KB. This is most likely because successful water drain derived by the hydrophobic character of GKB in comparison to KB. In other words, this result has suggested that controlling the hydrophobic character of carbon supports can reduce the concentration overvoltage even for low Pt-loading MEAs. Detailed discussion will be reported through further changing the amount of carbon.

Figure 1