bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the Controversial Role of the Metal in Fe/N/C or Co/N/C Electrocatalysts for the Reduction of Oxygen in the Acidic Medium of PEM Fuel Cells
Authors: Dodelet Jean-Pol 
Published: February 2020

The proton-exchange membrane hydrogen/air (PEM) fuel cell constitutes an efficient and environmentally-harmless alternative to the gasoline-dependent technology used today for transportation. However, the replacement of platinum-based catalysts, used at their electrodes and accounting for a large fraction of the fuel cell stack cost, is one of their challenges to reach full commercialization.
Today, Fe/N/C-catalysts and perhaps also Co/N/C-catalysts obtained from the pyrolysis of molecular precursors are the most promising non-platinum-group-metal (non-PGM) catalysts for the electrochemical reduction of oxygen (air) to water in an acidic medium such as that of PEM fuel cells.
Despite decades of research on Fe- (or Co) -based electrocatalysts for the oxygen reduction reaction (ORR), the role of the metal is still one that raises a great deal of controversy. Consequently, the nature of the catalytic site in these non-noble metal ORR catalysts is still a topic of debate. One camp within the scientific community believes that the metal is an integral and electrochemically active part of the catalytic site, while the other believes that the metal is merely a chemical catalyst for the formation of special oxygen-reducing N-doped carbon structures.
After presenting the case for the importance of non-noble catalysts at the cathode of PEM fuel cells, we will introduce the three models of active sites that were advocated during the 1980’s by van Veen, Yeager, and Wiesener, and discuss how they have evolved, especially that of Yeager. Wiesener’s model will be analyzed in detail through the work of several research groups that have been staunch supporters.
It will be shown that all the active sites proposed by van Veen, Yeager and Wiesener in the 1980’s, while different, are in fact simultaneously present in Fe- (or Co)-based catalysts active for ORR in acidic medium, except that their activity and relative population in these catalysts is different, depending on the choice of the metal precursor, nitrogen precursor, structural properties of the carbon support and the synthesis procedure.
A good knowledge of the nature and structure of the catalytic sites is important to further improve catalyst activity and site stability, especially for the Fe/N/C cathode catalysts that lead to the highest power densities reported to date for non-PGM-based catalysts able to reduce oxygen at meaningful cell voltages in PEM fuel cells.
Identifying Water Saturation of Various Layers in PEMFCs through EIS and X-ray Radiography
Authors: Antonacci Patrick , Lee Jongmin , Yip Ronnie , Ge Nan , Kotaka Toshikazu , Tabuchi Yuichiro , Bazylak Aimy 
Published: February 2020

Polymer electrolyte membrane fuel cells (PEMFCs) are a growing technology that makes use of electrochemical energy for many practical applications. Energy is converted from electrochemical to electrical and thermal energy through the process of oxygen-reduction and hydrogen-oxidation, with water as a by-product. The performance of the fuel cell is heavily influenced by the mass transport capabilities of the water produced through this electrochemical reaction [1]. In order to understand the performance of specific fuel cell builds, it is important to develop a diagnostic tool which offers an accurate depiction of the amount of liquid water in the fuel cell during operation. Two of these diagnostic tools are electronic impedance spectroscopy (EIS) and synchrotron X-ray radiography. For this study, X-ray radiography was performed at the Biomedical Imaging and Therapy Bending Magnet (05B1-1) beamline at the Canadian Light Source Inc. (Saskatoon, Canada).
In order to quantify the amount of liquid water in a PEMFC during operation, X-ray radiography was applied in the through-plane direction of the cell. Using the Beer-Lambert law, the processed images allowed for the liquid water to become visible in the anode and cathode flow channels, the anode and cathode gas diffusion layers (GDLs), the microporous layers (MPLs), as well as the membrane electrode assembly (MEA) [2]. EIS was used as a non-invasive diagnostic tool in quantifying the equivalent resistances of the fuel cell. This study aims to quantify the water saturation of various layers of the fuel cell during EIS measurements, and demonstrate the role of the water saturation of each layer to the overall mass transport resistance of the fuel cell, measured through EIS. The fuel cells analyzed have varying MPL thicknesses, as well as fuel cells without MPLs, at increasing constant current densities. Nyquist plots will be discussed with relation to amount of liquid water visualized.

Figure Captions

Figure 1. Subtracted in-plane image of the fuel cell with 150 μm-thick MPL. The image only shows 3 flow channels of 20.
Figure 2. Nyquist plot of the fuel cell observed simultaneously with x-ray radiography 1.0 A/cm2.
Figure 3. Water thickness profile plot at 1.0 A/cm2, from the image in Figure 1. Labeled from left to right: Anode GDL, Anode MPL, MEA, Cathode MPL and Cathode GDL.

References

[1] D. Malevich, E. Halliop, B. Peppley, J. Pharoah, and K. Karan, Journal of the Electrochemical Society 156 (2) B216 – B224 (2009)
[2] J. Hinebaugh, J. Lee, and A. Bazylak, Journal of Electrochemical Society 159 (12) F826-F830 (2012)
Comparison of Water Transport Mechanisms Between Commercial and Experimental Virgin and Aged Diffusion Media Using High Resolution Neutron Radiography
Authors: LaManna Jacob M , Mench Matthew M. 
Published: February 2020

The motivation of this work is to characterize water transport for a new high tortuosity anode diffusion media (DM) and to provide the modeling community with valuable benchmark validation data. The high resolution neutron images in this work were taken at the Neutron Imaging Facility at the National Institute of Standards and Technology (NIST) Center for Neutron Research. Tests were conducted with well-defined boundary conditions and publically available materials. Gradients in pressure, inlet relative humidity, and temperature were used to isolate hydraulic, diffusion, and temperature driven flow through the membrane and porous media.
A custom designed 4.8 cm2fuel cell with liquid coolant for precise thermal boundary conditions was used to acquire the neutron images. Test conditions were selected to produce extreme gradients within the cell to push model prediction capabilities. The high resolution neutron images allow for the determination of through-plane water profiles and water transfer rates as a function of current density as shown in Figure 1. Additionally, real-time water balance and net water drag measurements were made with dew point sensors.
This talk will discuss the differences in performance and water content of the experimental DM and baseline commercial DM presented previously, and attempt to draw conclusions relating the structural properties of the porous media and resulting performance and water transport and distribution. Additionally, partially degraded materials (e.g. electrodes used for hundreds of hours) will be compared to virgin materials to understand the impact of age and lifetime on water transport and performance. Data from this work will build upon an online database of benchmark data for the modeling community made available through this Department of Energy EREE supported program.

Acknowledgements

This work is funded by the United States Department of Energy Efficiency and Renewable Energy (EREE) Program through General Motors under Award Number DE-EE0000470. The authors would like to thank Dr. Dan Hussy and Dr. David Jacobson of NIST for their guidance.
Figure 1: Liquid water profiles for all 1.5 A/cm2 test conditions for baseline DM (upper) and high tortuosity DM (lower). The high tortuosity DM significantly reduces anode water content for identical operating conditions to baseline.
In Situ Observation of Epitaxial Li-Si-Nanostructure Formation on Si(111)
Authors: Grosse Frank , Proessdorf Andre , Hanke Michael , Bierwagen Oliver 
Published: February 2020

Silicon is considered as anode material for Li-ion batteries due to its high theoretical capacity with a value of 4200 mAh/g for Li22Si5. The associated volume change exceeding 300 % during de-/lithiation of silicon is considered as the major problem for application with a possible solution in using nanostructured material. Various details of the formation of Li-Si phases are presently under discussion like the formation of amorphous vs. crystalline structures or the Li-Si phase order during lithiation.
To shed light on the relevant processes during the Li-Si phase formation a clean Si substrate was exposed under ultra-high condition in a molecular beam epitaxy (MBE) environment to an atomic lithium flux. During the experiment the Si substrate temperature was gradually reduced from initial 700°C down to room temperature. Simultaneously, the onset of the nucleation and the formation of Li-Si phases were observed by in situ synchrotron x-ray diffraction (XRD), azimuthal reflection high-energy electron diffraction (ARHEED), and line-of-sight quadrupole mass spectrometry (QMS) of the desorbing Li. Initial only Si surface reconstructions are observed involving only the top most atomic layer. At lower temperatures, Li-Si nanostructures are formed. First, crystalline Li12Si7 appears, followed by the formation of Li21Si5 phases. Both phases grow epitaxially. The epitaxial relationship is Si[111]||Li12Si7[010] and Si[11-2]||Li12Si7[100] as well as Si[111]||Li21Si5[111] and Si[01-1] ||Li21Si5[01-1]. Further confirmation besides the XRD data is gained by investigating the Li-Si surfaces. The Li21Si5(111) surface exhibits an approximately nine times larger (1x1) surface unit cell than the unreconstructed Si(111) surface. Ex situ investigations of various intermediate stages of the Li-Si phase formation by atomic force microscopy show nanostructures which formed on the substrate surface with a height of about 10 nm and 100 nm diameter much smaller than usually reported in Si anode material. It demonstrates that the formation of the Li-Si phases depends on the Li kinetics as well as the Si release mechanism from the crystalline substrate and its diffusion.

Acknowledgements: This work was supported by the Leibniz-Gemeinschaft under the project No. SAW-2011-PDI-230. The experiments are carried out at the synchrotron radiation facility Bessy II (Helmholtz-Zentrum Berlin, Germany).
Development of Redox-Switchable Organocatalysts
Authors: Suga Seiji , Takasuka Yuki , Onishi Yuki , Okamura Yuuya , Fujiwara Ikumi , Kurihara Yuusuke , Kawakami Mai , Mitsudo Koichi 
Published: February 2020

Redox-switchable catalysis is an emerging research field that seeks to regulate chemical reactions by the simple reduction-oxidation manipulation. The modulation of electron density of redox-responsible ligands in the transition metal catalyzed reactions by changing oxidation state was usually used to control catalytic activity. Meanwhile, we have developed the cation pool method which involves generation and accumulation of organic cations in relatively high concentration.For example, the accumulation of an organo-dication 2 has been accomplished by the electrochemical oxidative C-C bond cleavage of compound 1 at low temperature (Scheme 1).1) The carbocations having two electron deficient carbon centers are expected to serve as effective Lewis acids, and we revealed that several reactions such as Mukaiyama-Aldol reaction,2) Diels-Alder reaction, and Nazarov cyclization, were significantly promoted by a catalytic amount of the organo-dications in a highly efficient manner (Scheme 2). We are highly motivated to investigate the utility of the dications as redox-switchable catalysts, and revealed the possibility to modulate Mukaiyama-Aldol reaction by using the reaction system (Scheme 3).
1) Okajima, M.; Suga, S.; Itami, K. Yoshida, J. J. Am. Chem. Soc.
2005, 127, 6930-6931.
2) Mukaiyama, T.; Kobayashi, S.; Murakami, M. Chem. Lett.
1984, 1759-1762.
Electrochemical Characteristics of Li4Ti5O12 - Coated MCMB as an Anode Material for Hybrid Capacitor
Authors: Lee Jong Dae , Choi Jeong Eun 
Published: February 2020

In recent years, the over consumption of fossil fuels leads to critical environmental problems and increasing attentions are being paid on energy security issues. The green energy sources, such as solar and wind, are typically periodic or intermittent and therefore requires an electrical energy storage for utilization purposes. Great research effort has been focused on the energy storage systems of capacitors and batteries. The hybrid capacitor incorporates a faradaic lithium ion battery system in combination with a non faradaic electric double layer capacitor system in a unit cell. This combination called hybrid capacitors can be used to obtain better performance in power/energy demands.
In this research, we coated Li4Ti5O12 onto meso - carbon micro beads (MCMB) to form a core - shell structure by a sol–gel process followed by calcinations. This MCMB-Li4Ti5O12 of core-shell structure was prepared to improve low electric conductivity of Li4Ti5O12 and cycling behavior. Prepared MCMB-Li4Ti5O12 composite was analyzed by XRD and SEM. The electrochemical performances of hybrid capacitor were investigated using MCMB-Li4Ti5O12 as the negative electrode and LiMn2O4, Active Carbon Fiber (ACF) as the positive electrode. The anodes were prepared by coating slurries containing the active materials (76wt%), Super P (17wt%), and polytetrafluroethylene (PTFE, 7wt%) on Al foil. The cathodes were made by dispersing 85wt% LiMn2O4, 10wt% Super P and 5wt% PTFE binder. The electrolyte consisted of 1M LiPF6in a mixture of ethylene carbonate(EC) / dimethyl carbonate(DMC) / ethyl-methyl carbonate(EMC) with a volume ratio of 1:1:1.
The electrochemical behaviors of hybrid capacitor using organic electrolytes were characterized by charge/discharge, cyclic voltammetry, impedence and cycle tests. The hybrid capacitor using MCMB-Li4Ti5O12/LiMn2O4 electrodes had better capacitance than MCMB hybrid systems and was able to deliver a specific energy as high as 67 Wh/kg at a specific power of 781 W/kg. Also, it was found that MCMB-Li4Ti5O12 composite exhibited very good cycle life performance.
VASP Calculation on Lithium Reaction to Tin Phosphides
Authors: Doh Chil-Hoon , Lee Won-Jae , Pervez Syed Atif , Song Jae-Sung 
Published: February 2020

Studies on electrochemical reaction of SnPx materials were carried out by VASP ab initio calculation, where. SnPx having four different crystalline compositions, SnP0.75(Sn4P3), SnP0.94(SnP17/18), SnP and SnP3. The reaction potential of the lithiation of SnP0.94(SnP17/18) material was compared between intercalation and conversion reaction. The total-energy calculation and full structural optimization of SnPx, SnPx related materials and Li materials were performed using the Vienna ab initio Simulation Package (VASP)[1]. The possibility of intercalation of lithium to SnP0.94 was analyzed to obtain reaction potential of 1.853 V (Li/Li+) accommodating lithium to 6i site. 6i site is a vacant site in Sn18P17 (SnP0.94). Through further intercalation of lithium, reaction potential of Li0.5SnP0.94, Li1.5SnP1.56 and Li6.06SnP0.94 can also be calculated as 0.9026, 0.705 and -1.5 V(Li/Li+), respectively.Considering conversion reaction, SnP0.94 can react to produce 2 kinds of products such as LiP and Li3P as shown in Eq. 1.
SnP0.94 + (0.94z+y) = 0.94LizP + LiySn ---------(1)
At LiySn formation, y of zero means that only conversion reaction takes place without the formation of LiySn. Reaction potential of y=0 were 1.35 V and 0.948 V for the formation of LiP and Li3P. Reaction potential of the hole reaction [SnP0.94 + xLi = products], including intercalation and conversion, was plotted against reacted lithium extent of x in Fig. 1. It is observed that the reaction potential of conversion is higher than that of the lithium intercalation. In case of conversion reaction to form LiP with reacted lithium extent of 0.94 was 1.35 V which was higher than 0.937 V of lithium intercalation to form Li0.56SnP0.94 . Therefore, conversion reaction is labile compared to intercalation. Moreover, the reaction potential of conversion to form Li3P with reacted lithium extent of 2.833 was 0.948 V which is higher than that of the conversion reaction potential of 0.704 V to form LiP with reacted lithium extent of 3.28. Therefore conversion reaction to form Li3P is easier than to form LiP. We will discuss VASP calculation on lithium reaction to Tin Phosphides in details
[1] Kresse,G.; Hafner, J. Phys. Rev. B
,
47 (1993) 558; 49(1994) 14251.
Vibrational Spectroscopic Study of Nafion Ionomer and Its Model Molecules on Platinum Surfaces
Authors: Yagi Ichizo , Notsu Hideo 
Published: February 2020

In polymer electrolyte fuel cells (PEFCs), perfluorosulfonated polymers such as Nafion are usually utilized as electrolyte membrane and ionomers in catalyst layers. Recently, molecular structure and ion-conducting mechanism in the ultrathin ionomer films covering catalysts’ surfaces have been interested and various measurements at ultrathin Nafion films were carried out by means of electrochemical methods,[1-3] in situ vibrational spectroscopy,[3,4] and DFT calculations. However, the evaluation of the molecular conformation and orientation of perfluorosulfonated polymers are very difficult, since the molecular weights of these molecules are much larger and the vibrations of C-F bonds included in both the backbone and side-chains overlap with those of sulfonic acid groups, which can provide useful information to speculate the orientation of side-chains.
Recently, we have performed homodyne vibrational sum frequency generation (VSFG) spectroscopic measurements at Nafion-covered Pt surfaces. By comparing the VSFG spectra and ATR-IRRAS spectra of the same samples, where the thicknesses of Nafion film were controlled, VSFG spectroscopy selectively detect the vibrational bands, especially the stretching band of sulfonate terminal in Nafion at the Pt/Nafion or Nafion/air interface. Since the relative hydrophobicity was confirmed at the top most layer of the samples, sulfonate groups were considered not to be adsorbed at Nafion/air interface. However, the picture is just a speculation from the indirect experimental results. Ex situ heterodyne VSFG [5.6] measurements have proved that the direction of sulfonate groups in Nafion ultrathin films is downward toward Pt surfaces. However, in situ and dynamic behaviors of sulfonate terminals
In the present study, in situ surface enhanced infrared absorption spectroscopy (SEIRAS) at Pt electrode[7] immersed in diluted Nafion dispersion and Nafion-model electrolyte solutions are applied to follow the dynamic behaviors of sulfonates.

Acknowledgement

This work was supported by New Energy, Industrial Technology Development Organization (NEDO), Fuel Cell Promotion Office of the Agency of Natural Resources and Energy, Ministry of Economy, Trade and Industry (METI).

References

[1] R. Subbaraman et al., J. Phys. Chem. C, 114, 8414 (2010)
[2] M. Ahmed et al., J. Phys. Chem. C, 115, 17020 (2011)
[3] A.N. Gomez-Marin et al., J. Phys. Chem. C, 114, 20130 (2010)
[4] J. Zeng et al., Langmuir, 28, 957 (2012)
[5] S. Nihonyanagi et al., J. Chem. Phys., 130, 204704 (2009)
[6] S. Nihonyanagi et al., J. Am. Chem. Soc., 132, 6867 (2010)
[7] M. Osawa et al., J. Phys. Chem. C, 112, 4248 (2008)
Shape-Controlled Nanocrystals and Core-Shell for Oxygen Reduction Reaction
Authors: Shao Minhua 
Published: February 2020

The slow kinetics of oxygen reduction reaction (ORR) is one of the main obstacles hindering the commercialization of low-temperature fuel cells.1 Platinum is by far the most effective electrocatalyst for ORR, but it is an extremely rare and expensive element. One way to address this problem is to increase the ORR activity of a metallic catalyst by optimizing the shape of constituent nanoparticles and thus maximizing the expression of facets most reactive towards ORR reaction.2-3 Another approach is to increase the utilization of expensive noble metal atoms by developing core-shell catalysts.4-5

In this study, the structural dependence of palladium nanocrystals for ORR, as well as the impact of the structure of the Pd core on the ORR activity of Pt shells in core-shell catalysts was studied in details. The carbon supported Pd nanocubes and octahedra with an edge length of 5-6 nm were synthesized and their ORR activities were measured using a rotating disk electrode technique.6 The activity enhancement of cubic Pd in a HClO4 solution was about 10 and 6 times over octahedral and conventional Pd, respectively. Interestingly, no difference in ORR activity was observed in an alkaline solution among the three samples (Figure 1). Two (octahedral and conventional) of the three Pd samples showed an increase in activity when an atomic thick Pt shell was deposited on their surfaces. Surprisingly, the activity decreased slightly for the Pd cubes after the deposition of a Pt shell.
The density functional theory (DFT) calculations were conducted to explain this observation and will be discussed in the meeting.
References

Electrocatalysis in Fuel Cells: A Non- and Low- Platinum Approach; Shao, M., Ed.; Springer: London, 2013.

N. M. Markovic, R. R. Adzic, B. D. Cahan and E. B. Yeager, J. Electroanal. Chem. 1994, 377, 249.

V. R. Stamenkovic, B. Fowler, B. S. Mun, G. Wang, P. N. Ross, C. A. Lucas and N. M. Markovic, Science 2007, 315, 493.

Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Topics in Catalysis 2007, 46, (3-4), 249.

Shao, M. H.; Shoemaker, K.; Peles, A.; Kaneko, K.; Protsailo, L. J. Am. Chem. Soc. 2010, 132, (27), 9253.

Shao, M.; Yu, T.; Odell, J.; Jin, M.; Xia, Y. Chem
.
Commun. 2011,
47, 6566.