bluuebunny/crossref_metadata_2025 huggingface dataset:
Terahertz Spectroscopy: Studying Carrier Dynamics in Semiconductor Nanostructures
Authors: Titova Lyubov V. , Xu Sijia , Baribeau Jean-Marc , Lockwood David J , Hegmann Frank
Published: February 2020

Understanding the ultrafast dynamics of photoexcited carriers in semiconductor nanostructures and their dependence on sample morphology is crucial for their incorporation into photonic devices. Time-resolved terahertz (THz) spectroscopy is an all-optical, contact free technique that allows directly measuring the transient mobile carrier dynamics and terahertz conductivity over picosecond time scales, and is uniquely suitable as a probe of conductivity in nanomaterials.

We have applied time-resolved THz spectroscopy to investigate ultrafast carrier dynamics to a variety of nanostructured semiconducting systems such as silicon nanocrystal films, nanogranular iron pyrate (FeS2) and vanadium dioxide (VO2). Furthermore, terahertz emission spectroscopy of nanomaterials, where the sample itself emits THz radiation in response to optical excitation, provides important insights into carrier dynamics.  We have recently generated THz pulses from optically excited macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs).  We propose that top-bottom asymmetry present in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized THz radiation by a photocurrent surge.
Cathodoluminescence Studies of Gamma-Irradiation Effects on AlGaN/GaN High Electron Mobility Transistors (HEMTs)
Authors: Yadav Anupama , Antia Michael , Flitsiyan Elena , Chernyak Leonid , Lubomirsky Igor , Salzman Joseph
Published: February 2020

AlGaN/GaN High Electron Mobility Transistors (HEMTs) are the most advanced electronic nitride devices to be used at high temperatures and in harshly radiated environments. A clear understanding of radiation effects on these devices is essential in order to predict their behavior in space and terrestrial applications including military, nuclear and high energy physics. Only a limited amount of insight exists into the behavior of III-N based junction transistor after exposure to gamma irradiation. Investigation of gamma-irradiated device properties will provide important information about the defects in semiconductor heterostructures. Therefore, it is important to understand how defects are created by gamma-rays and how they can be healed by annealing. Here we present the impact of gamma-irradiation on AlGaN/GaN heterostructures fundamental properties including carrier transport and recombination. We also report the effect of annealing at 200o C over 25 min on gamma-irradiated samples through the Electron Beam Induced Current (EBIC) method.
Measurement of Recombination Lifetimes in Gesn Alloys
Authors: Erdman Emily , Senaratne Charutha , Menendez Jose , Kouvetakis John , Mathews Jay
Published: February 2020

Silicon photonics requires the development of Si-based lasers, modulators, waveguides, and detectors operating in the infrared beyond the band gap of silicon. A number of approaches to solving this problem have been pursued, including epitaxial growth of lower band gap materials on Si. The SiGe alloy system was explored for many years, but limitations in quality due the large lattice mismatch between Si and SiGe with high amounts of Ge, as well as the band gap (0.66 eV at its lowest for pure Ge) and its indirect nature, have prevented its widespread use. Integration of III-V materials such as InGaAs has also been tried extensively, but the problems of phase segregation and film quality have not been overcome.

The GeSn alloy system has been gaining attention in the photonics world due to its unique optical properties, and the growth of these alloys on Si platforms has jumpstarted efforts to employ this technology for the fabrication of CMOS-compatible infrared optoelectronics. These crystalline alloy films are grown directly on Si or Ge-buffered Si, have band gaps below that of Si or Ge, and are grown to thicknesses of up to 1 µm. Additionally, the band gap of GeSn should be direct for concentrations greater than about 10% Sn.

Much work has been done on the development of GeSn materials, and theoretical designs for lasers based on these alloys have been appearing in the literature frequently. However, the realization of complex optoelectronic devices like laser diodes requires materials with long recombination lifetimes to achieve population inversion. Thus, the study of recombination lifetimes in GeSn alloys is of paramount importance to the actualization of GeSn lasers. Unfortunately, the standard method of time-resolved photoluminescence (TRPL) is not well-suited to the study of GeSn alloys. For low Sn contents (<10% Sn), the band gap is indirect, so the intensity of optical emission is not high. Such a measurement would require single photon counting, but single photon detectors are not available at the emission wavelengths for the materials of interest. Conductivity-based methods should be more suitable, but the standard techniques of microwave photoconductive decay (µ-PCD) and RF-coupled photoconductive decay (RFPCD) suffer from low sensitivity in thin films or low temporal resolution.

In this work, we employ a new conductivity-based method for measurement of recombination lifetimes in thin film GeSn alloys. The method of transmission-modulated photoconductive decay (TMPCD) uses the transverse transmission of microwaves through a thin film, allowing for a very large interaction volume between the free carriers and the microwaves, thereby increasing the sensitivity for thin film materials. We will discuss this new method and show preliminary results on GeSn films with up to 7% Sn.
A Npg-Based Ultra-Thin Anode Catalyst Layer for Spewe
Authors: Zeng Yachao , Guo Xiaoqian , Wang Xunying , Wang Zhiqiang , Yu Hongmei , Shao Zhigang , Yi Baolian
Published: February 2020

Nanoporous gold (NPG) has a long history of application. By electrochemical or chemical removal of Ag from AgAu alloy, the remaining gold undergoes a self-organization process forming a 3D bicontinuous network with interconnected ligaments. Due to its relatively high surface area, high conductivity, small size ligament and 3D bicontinuous porous structure, NPG has found its way in electrocatalysis, bio-sensors, and fuel cells. By extending dealloy time or thermal coarsening temperature; the structure of NPG can be easily tuned.

In this paper, a facile method was proposed for a novel MEA applied in solid polymer electrolyte water electrolysis (SPEWE). NPG with a thickness of 100 nm dealloyed from 12K gold foil was employed as substrate.  IrO2 was deposited onto its surface via a conventional thermal decomposition method. A loading of IrO2 as low as 50 μg cm-2 was achieved in this experiment while keeping an acceptable performance. With a IrO2 loading of 0.2 mg cm-2，the performance was 1.686 V@ 1000 mA cm-2 at 80 ºC. Scanning electron microscopy measurement showed that IrO2 formed a uniform thin layer on the surface of NPG. The maximum thickness is 500 nm, which is only 1/20 to the conventional catalyst layer.  The ultra-thin catalyst layer eased the transport of reactant and product, which reasonably explicated the remarkable performance. During the stability test, the cell voltage was constant over 200 h at 250 mA cm-2. The result here demonstrated that an ultra-thin catalyst layer with a low IrO2 loading could applied in real solid polymer electrolyte water electrolyzer.
CO2 Electrochemical Reduction to Hydrocarbon Fuels on Carbon-Supported Copper Nanoparticles: Support Effect
Authors: Lu Qin , Purdy Andrew , Dyatkin Boris , Gogotsi Yury , Baturina Olga A
Published: February 2020

Previously, we reported activity and selectivity of Cu nanoparticles towards CO2 electroreduction to hydrocarbon fuels (CH4 and C2H4) on three different supports (Vulcan Carbon (VC), Ketjenblack (KB) and singled walled carbon nanotubes (SWCNT))1. We found that carbon materials not only serve as inert supports, but are actively involved in electrochemical CO2 reduction likely due to hydrogen spillover from the support to the surface of Cu nanoparticles.  Here we explore the effect of carbon support on the product distribution in CO2 electroreduction reaction in more detail expanding the number of nanostructered supports to reduced graphene oxide (rGO), and onion-like carbon (OLC)3.  These five carbon materials (VC, KB, SWCNT, rGO and OLC) have different surface area, electronic conductivity, morphology, and inherent size.

Cu nanoparticles of 15-25 nm diameters on different carbon supports were synthesized by the reduction of CuCl2 using NaBH4 in aqueous ethylene glycol solution (20% v/v). X-Ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) were used for nanoparticles ex-situ analysis. The catalytic activities and selectivity of supported Cu nanoparticles towards CO2 electrochemical reduction to hydrocarbon fuels were evaluated using a sealed rotating disk electrode (RDE) setup connected to a gas chromatograph1.

Cu nanoparticles supported on VC and OLC exhibited higher catalytic activity vs other catalysts towards CH4 and C2H4 generation, respectively, between -1.4 and -1.6 V (vs Ag/AgCl reference electrode). At a more negative potential of -1.6 V, all five Cu catalysts promoted generation of both CH4 and C2H4, with the higher selectivity in terms of Faradaic efficiencies towards C2H4 formation. The C2H4/CH4  ratio increased by almost an order of magnitude in the following order: Cu/VC (2.6:1) <Cu/KB (4.5:1) < Cu/SWNT (5.8:1) < Cu/GPO (10.8:1)< Cu/OLC (33:1). Cu/OLC showed both the lowest onset potential and the greatest selectivity towards C2H4 formation. 

This presentation will discuss the reasoning behind the different catalytic activity and selectivity of supported Cu nanoparticles in catalyzing the CO2 electrochemical reduction reaction.

[1] O. A. Baturina, Q. Lu, M. A. Padilla, L. Xin, W. Li, A. Serov, K. Artyushkova, P. Atanassov, F. Xu, A. Epshteyn, T. Brintlinger, M. Schuette, and G. E. Collins, ACS Catal. 2014, 4, 3682.

[2] P. V. Kamat, J. Phys. Chem. Lett. 1 (2010), 520.

[3] J. K. McDonoug, Y Gogotsi,
Interface,   22 (2013), 61.
Exploring the Non-Innocence of Inorganic Complex Ligands in (photo-) Electrochemical CO2 Reductions
Authors: Saravanan Karthikeyan , Oyeyemi Victor B , Keith John A.
Published: February 2020

Carbon neutral energy cycles necessitate the conversion of CO2 into other compounds.1 However, the unfavorable thermodynamics and slow kinetics of multiple proton and electron transfers makes CO2 conversion difficult. Improved insight into these processes would help elucidate reaction mechanisms that guide the development of improved and versatile photo- and electro-catalysts.2 Recent experiments have demonstrated that inorganic Ru complexes can catalyze the photo- and electro-chemical reduction of CO2 into methanol.3 These reactions involve aqueous phase aromatic N-heterocycles (ANH) in addition to complexes that contain ANH ligands themselves (e.g. phenanthroline). We previously developed4 a set of thermodynamic descriptors from quantum chemistry calculations to describe electrochemical conditions at which ANH molecules participate in energetically efficient proton and electron transfers. In this study, we report our latest efforts to provide insight into the mechanism of Ru catalyzed CO2 reduction using Pourbaix diagram descriptors. Although transition metal complexes are normally assumed to be electrochemically stable, our calculations indicate that certain environments may result in hydride transfers facilitated via the ligands of the inorganic complexes.

References:

1.      Olah, G. A., Prakash, G. K. S. & Goeppert, A. Anthropogenic Chemical Carbon Cycle for a Sustainable Future. J. Am. Chem. Soc. 133, 12881–12898 (2011).

2.      Qiao, J., Liu, Y., Hong, F. & Zhang, J. A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels. Chem. Soc. Rev. 43, 631–675 (2013).

3.      Boston, D. J., Pachón, Y. M. F., Lezna, R. O., de Tacconi, N. R. & MacDonnell, F. M. Electrocatalytic and Photocatalytic Conversion of CO2 to Methanol using Ruthenium Complexes with Internal Pyridyl Cocatalysts. Inorg. Chem. 53, 6544–6553 (2014).

4.      Marjolin, A. & Keith, J. A. Thermodynamic Descriptors for Molecules That Catalyze Efficient CO2 Electroreductions. ACS Catal. 5, 1123–1130 (2015).
Precise Viscosity Analysis of Alkali Metal Molten Salts at High Temperature
Authors: Kim Sun Woog , Uematsu Kazuyoshi , Toda Kenji , Sato Mineo
Published: February 2020

Recently, solar thermal energy has been attracted considerable attention as a renewable energy alternative to fossil fuels. The operation and efficiency of solar thermal energy plants employing thermal energy storage techniques have been proved at both experimental and commercial scales.1 In solar thermal energy plants, alkali metal molten salts have recently attracted as thermal storage media and heat transfer fluids, because these salts have a low melting point, high heat capacity, and moderate energy density.2 In addition, alkali metal molten salts are less expensive and more environmentally friendly than currently available high-temperature oils. However, many investigations are scarce focusing on thermophysical properties of molten salts at high temperature. In particular, the viscosity of alkali metal molten salts is few reported and the reported viscosities are shown larger difference by analysis method or other one. In this study, therefore, we are measured the viscosity of alkali metal molten salts at high temperature above 500 °C by rotation method using high temperature rheometer system (MCR 502, Anton Paar) and reliability of obtained viscosity values is evaluated.3

This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), "energy carrier" (Funding agency: JST).

References

Q. Peng, X. Yang, J. Ding, X. Wei, and J. Yang, Appl. Energy, 112 (2013) 682.

S.M. Flueckiger and S.V. Garimella, Sol. Energy, 86 (2012) 1631.

S.W. Kim, K. Uematsu, K. Toda, and M. Sato, J. Ceram. Soc. Jpn., (2015) in press.
Defect Chemistry of Perovskite Titanates: From Bulk Materials to Nanostructured Materials
Authors: Guo Xin
Published: February 2020

To describe the electrical conductivities of perovskite titanates (e.g. SrTiO3 and BaTiO3) and the relation to parameters like temperature, oxygen partial pressure or acceptor- or donor-type dopants, the defect chemistry has been extensively studied since the late 1960s. The studies finally lead to the formulation of the point defect chemistry on the basis of defect chemical reactions, exchange processes with the atmosphere and changes of the valence state regarding their individual thermal activation and thermo­dynamic equilibriums. Comparing with microstructured counterparts, nanostructured materials have significantly higher interface (surface and/or grain boundary) to bulk fraction, which leads to some unique effects in nanostructured perovskite titanates. One effect is that the space charge layer becomes increasingly important. Owing to the high surface-to-volume ratio, the surface space charge layer plays an important role in thin films. For example, the p-type conductivity of the epitaxial SrTiO3 thin films decreases, and the n-type conductivity increases with decreasing film thickness. This phenomenon is attributed to the charge carrier redistribution in the space charge layer. Moreover, the defect thermodynamics at nanometer-scale is different. E.g. the oxidation enthalpy of nanocrystalline BaTiO3 ceramics is found to be about one-third of that of microcrystalline BaTiO3. As a result, the p-type electrical conductivity of nanocrystalline BaTiO3 is about 2 orders of magnitude higher than that of microcrystalline BaTiO3.
Disorder and Ionic Transport in Pyrochlore/Defect Fluorite Oxide Thin Films
Authors: Kreller Cortney R , Holesinger Terry , Valdez James , Wang Yongqiang , Uberuaga Blas
Published: February 2020

Numerous avenues are currently being explored to enhance ionic conductivity in solid oxide materials including dopants, nano-structuring, and strain engineering.  Their effect on material properties, specifically mobile carrier concentration and mobility, needs to be understood and ultimately exploited to develop advanced functional materials.  In this work, we seek to explore the relationship between intrinsic disorder and oxygen ion conduction.  The pyrochlore compounds, A2B2O7, consist of both ordered cation and anion sublattices.  The introduction of anti-site disorder, where the A and B cations switch positions, is accompanied by increasing disorder on the anion sublattice, and complete randomization of the A- and B-site cations results in the defect fluorite phase.  Several workers have previously studied the influence of structural disorder on ionic conductivity by introducing disorder through dopants, temperature or pressure.  In the present work, we are using irradiation as a tool to introduce, and control the extent of, structural disorder in pyrochlore oxide thin films. We have chosen to investigate the end members of the Gd2(ZrxTi1-x)2O7 system as Gd2Zr2O7 is an intrinsic fast-ion conductor that is also very rad tolerant whereas Gd2Ti2O7 is a comparatively poor ionic conductor that is much more susceptible to amorphization under irradiation than the zirconates.  Electrochemical Impedance spectroscopy is used to measure the conductivity of materials with varying degrees of structural disorder in order to better understand how the underlying transport mechanisms depend on crystalline disorder.
Stress Engineering in Developing Mixed Conducting Si-Ge Solutions for Lithium Ion Batteries
Authors: Mishra Kuber , Geppert Mark , Zhou Xiao-Dong
Published: February 2020

Despite having the highest theoretical specific capacity (3579 mAh/g), silicon has been known to undergo massive anisotropic volume expansion (~280%), and consequently, causing the fracture of the particles and loss of the specific capacity upon further cycling. Germanium has been reported to undergo isotropic expansion and observed in the in-situ studies to sustain lithiation induced fracture up to the particle size of ~680 nm. In this work, solid solution of Si and Ge of various compositions were synthesized. The formation of a single phase solid solution was confirmed by the X-ray diffraction and the electrode morphology were studied using Scanning electron microscopy.  The solid solution of equimolar Si and Ge (Si0.5Ge0.5) displayed excellent electrochemical performance both in the cyclic stability as well as the total specific capacity.
Effect of Alloy Elements on the Corrosion Resistance and Electrical Conductivity of Austenitic Stainless Steel for MCFC
Authors: Kim Kwangmin , Kim JongHee , Jo KiHoon
Published: February 2020

Bipolar plate and current collector are important stainless steel components of the molten carbonate fuel cell (MCFC) because they supply the pathway of the electron flow between each unit cell. However, these stainless steel components are prone to corrosion in the hot corrosion environment and corrosion products forming on the surface increase the ohmic resistance. In addition, hot corrosion reduces the mechanical strength of the current collector, accelerates electrolyte loss, and finally leads to degradation in cell performance. Therefore, corrosion of stainless steel components is one of the main problems in MCFC systems. To solve these problems, various methods have been studied, such as addition of alloy elements, surface coatings and replacement with higher grade materials. Corrosion resistance of stainless steels is evaluated by potentiodynamic polarization test and weight loss measurements. Electrical conductivity of specimens is measured in molten carbonate at 650 oC. Results from electrochemical tests show that high Cr content causes the formation of a dense and thin oxide layer, subsequently improving the corrosion resistance. However, high Cr content gives rise to inner oxide penetration. Mn enrichment occurs in the oxide layer and may lead to an increase in electrical conductivity. In addition, minor elements such as Si, Nb and N may contribute to the improvement of corrosion resistance and electrical conductivity.
Combined Pore Network and Finite Volume Modeling of Thermal and Water Transport in Porous Transport Layers of PEM Fuel Cells Using Openfoam
Authors: Pharoah Jon G. , French Andrew , Konduru Vinaykumar , Medici Ezequiel , Allen Jeffrey S
Published: February 2020

Modeling the pore-scale transport in PTL using computational fluid dynamics (CFD) is computationally expensive. A pore network model is an excellent tool that allows computation of simpler physical models quickly. In the current work, a pore network model is implemented in the finite volume CFD code, OpenFOAM, to simulate the thermal and mass transport in the Porous Transport Layers (PTL) of Polymer Electrolyte Membrane (PEM) fuel cells. In the model, a network of pores are generated from experimentally determined pore size distributions. This pore network is coupled to the continuum model to study the transient water transport through the PTL. In the paper, a description of the model will be presented and the results of the OpenFOAM model will be compared with those from the in-house pore network model.
Conjugate Analysis of Heat-Species-Charge Transport for Evaluating Effects of the Temperature Gradient on Cell Performance
Authors: Miyagawa Kiyofumi , Araki Takuto
Published: February 2020

1. Introduction

Management of water in the PEMFC is very important to improve the cell performance. However, the water transport mechanism in the PEMFC is complicated and it is difficult to understand by only experimental methods because of the thinness of the components of the PEMFC. Thus, numerical analyses are very useful methods. 

The two-dimensional analytical model considering distributions of temperature, spices and reaction in components of the PEMFC is developed to estimate the unsteady performance of the PEMFC. The absolute value of the temperature of the cell impacts on the cell performance. However, the gradient of temperature has also great impacts on the cell performance. Moreover, temperature, chemical reaction and transport phenomena have interaction, for example the phase change and impeding the transport of the reactant gases by liquid water. So, it is important to consider not only temperature but also water transport. MPL was reported to moisturize CL and PEM and at the same time MPL enhances liquid water drainage to Channel. However, the detailed mechanism how MPL works on the cell performance is yet unclear. Finally, in this study, the effects of the gradient of temperature are investigated, and the models with MPL and without MPL are compared to validate the work of MPL.

2. Analytical methods

The unsteady cell performance has been studied with two-dimensional analytical model considered heat, species and charge transport in components of the PEMFC, such as GDL, MPL, CL and PEM. The impact of temperature gradients has been investigated by comparison of non-isothermal and isothermal condition at 72 deg. C.

3. Results and discussions

In Fig. 1, the distribution of temperature was shown. From this, the temperature of the area nearby CL and PEM was the highest and the value was 72 deg. C. And the temperature around separators was lowest and the value was 70 deg. C. The difference of temperature is only 2 deg. C, however, the current density had more than 5 times difference between non-isothermal and isothermal condition as shown in Fig. 2. The distributions of the liquid water saturation of each condition were shown in Fig. 3 and Fig. 4, respectively. Under isothermal condition, the value of liquid water saturation in CL was about approximately s=1, on the other hand, there is little liquid water in CL under non-isothermal condition. Then, current density drastically decreased from 40 s under isothermal condition with the liquid water accumulation in the CL.

Figure 1
Correlating Site Density and Catalytic Activity for Non-Precious ORR Electrocatalysts
Authors: Strasser Peter , Ranjbar Nastaran , Steinberg Julian , Kramm Ulrike
Published: February 2020

Carbon materials doped with iron and nitrogen (Fe-N-C) are highly active, precious metal-free catalysts for the electrochemical conversion of molecular oxygen in fuel cells, metal air batteries, and electrolytic processes1-4. However, accurate measurement of their intrinsic turn-over-frequency (TOF) and active site density based on metal centers in bulk and surface has remained difficult to date, which has hampered a more rational catalyst design. Here, we report a successful quantification of bulk and surface-based active site density and associated TOF values of mono- and bimetallic Fe/N-doped carbons using a combination of chemisorption, desorption, and 57Fe Mößbauer spectroscopy techniques. Our general approach yields novel experimental descriptors for intrinsic activity and active site utilization, aiding in the catalyst development process and enabling a previously unachieved level of understanding of reactivity trends owing to a deconvolution of site density and intrinsic activity.
             

1.            Wu, G.; Zelenay, P., Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 2013, 46, 1878-1889.

2.            Sahraie, N. R.; Paraknowitsch, J. P.; Göbel, C.; Thomas, A.; Strasser, P., Noble-Metal-Free Electrocatalysts with Enhanced ORR Performance by Task-Specific Functionalization of Carbon using   Ionic Liquid Precursor Systems. J. Am. Chem. Soc. 2014, 136, 14486-14497.

3.            Chung, H. T.; Won, J. H.; Zelenay, P., Active and stable carbon nanotube/nanoparticle composite electrocatalyst for oxygen reduction. Nature Communications 2013, 4.

4.            Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 2011, 332, 443-447.
Synthesis of Non-PGM ORR Catalysts: Control of Chemical, Morphological and Catalytic Properties
Authors: Serov Alexey , Stariha Sarah , Artyushkova Kateryna , Andersen Nalin , Atanassov Plamen
Published: February 2020

Development of non-platinum catalysts for both the anode [1] and cathode [2] attracts significant attention from researchers around the globe. Catalysts consisting of transition metal, nitrogen and carbon (M-N-C) where M=Fe, Co, Ni etc)framework possess reasonable stability and activity in both acidic and alkaline media. The use of inexpensive organic precursors doped with low molecular weight metal salts is proposed in the present research to provide a high surface area framework consisting of C-N decorated with M species.

All catalysts presented in this work were synthesized using a modified sacrificial silica support method which has been widely adopted in our research group [4]. Appropriate amounts of metal precursor (iron nitrate or cobalt nitrate) and nitrogen-containing, precursors (e.g. polyethyleneimine (PEI)) were deposited on the surface of fumed silica (surface area ~380 m-2 g-1) by Mechanochemical approach. The composite material was pyrolyzed in a nitrogen atmosphere at various temperatures, to form the M-N-C-SiO2framework. The pyrolysis conditions were determined from a MS-TGA analysis, where MS was used to identify the precursor’s decomposition products as a function of thermal treatment.  After heat treatment, the fumed silica was removed by etching in HF acid and washed appropriately, revealing a high surface area M-N-C matrix

The catalysts were comprehensively characterized by BET, SEM, TEM, XRD and XPS. Catalytic activity for ORR was measured in acidic media (0.5M H2SO4) using the RRDE method and fuel cell test.

Figure 1.  Mechanochemical synthesis

The broad XRD peak for Fe indicates small metallic iron particles exist on the order of several nanometers (Fig. 1). It can be assumed that the homogeneous distribution of precursors on the high surface of fumed silica prevents metal particles from agglomeration.

Analysis of material morphology by SEM reveals that the Fe-DAAPyr catalyst yields a highly porous surface structure. BET analysis of the adsorption isotherm reveals a surface area of ~700m-2 g-1.

Synthesized materials were found to be active in ORR in both RRDE and MEA fuel cell tests conditions.

We would like to acknowledge funding support from DOE-EERE Fuel Cell Technology Program: “Development of Novel Non Pt Group Metal Electrocatalysts for PEM Fuel Cell Applications” (S. Mukerjee, NEU, PI).

References:

M. Min, C. Park, H. Kim, C. Kwak, A. A. Serov, H. Kweon, S. Lee, Electrochimica Acta, 52, (2006) 1670-1675.

A. Serov, K. Artyushkova, P. Atanassov, Adv. Energy Mater., 4: 1301735 (2014). doi: 10.1002/aenm.201301735..

U. Tylus, Q. Jia, K. Strickland, N. Ramaswamy, A. Serov, P. Atanassov, S. Mukerjee, J. Phys. Chem. C, 118 (17) (2014) pp 8999–9008.

A. Serov,A. Aziznia, P. H. Benhangi, K. Artyushkova, P. Atanassov, E. Gyenge  J. Mater. Chem. A 1 (2013) 14384-14391.

Figure 1
Fuel Cell Technology to Market Activity
Authors: Satyapal Sunita
Published: February 2020

This talk will give a brief overview of the Fuel Cell Technology to Market Activities and posters that will be occurring throughout the week around the Polymer Electrolyte Fuel Cells 15 (PEFC 15) session rooms
Nafion Nanothin Films
Authors: Karan Kunal
Published: February 2020

In the conventional catalyst layer of polymer electrolyte fuel cells, the ionomer (e.g. Nafion) exists as an ultra-thin film (4-10 nm thick) coating the Pt/C catalyst aggregates. Recent studies [1-12], including those from our group, have begun to investigate the properties and structure of such thin ionomer films. Thickness-dependent properties including proton conduction (7, 11), wettability of free-surface(5), water uptake (2, 6) and water diffusivity (9) of such ultra-thin ionomer films have been observed. The influence of substrate and processing method has also been observed (12). A number of the properties of the thin ionomer films differ significantly from the much thicker counterpart, i.e. the free film or bulk ionomer membrane. The nanostructure of these films are less understood. And, although indirect methods such as x-ray reflectivity (1) and grazing incidence (2) small angle x-ray scattering (GISAXS) have been employed a clear picture of the nanostructure of these thin films has not emerged.

The presentation will summarize the current understanding of ionomer thin film structure and properties. Results of recent and ongoing work on direct imaging of thin Nafion films to probe the nano-features of the phase-segregated morphology will be presented. Results of thermal behavior of thin films investigated using in-situ thermal ellipsometry will also be presented.

 Reference:

A. Dura et al.,  Macromolecules 42 (2009), 4769-4774.

M.A. Modestino et al.,  Macromolecules 2013, 46 (3), pp 867–873, 2013

M. Bass et al., Macromolecules 44 (2011), 2893–2899.

Dishari, K. S.; Hickner, A. M. ACS Macro Lett., 1 (2012), 291-295.

D.K Paul et al., Macromolecules, 46 (2013), 3461–3475

A. Kongkanand J. Phys. Chem. C, 115 (2011) 11318–11325.

D.K. Paul, D. K Electrochem. Comm., 13 (2011), 774-777.

A. Modestino et al., Macromolecules, 45 (2012), 4681–4688.

S.A. Eastman, Macromolecules, 45 (2012), 7920−7930.

D.K. Paul and K. Karan J. Phys. Chem. C, 118 (2014), 1828–1835.

D.K. Paul et al., J. Electrochem. Soc., 161 (2014), F1395-F1402

A. Kusoglu et al
Adv. Funct. Mater. 2014, 24 (30), 4763-4774
Enhanced Durability with Cu-Containing Chemically Ordered Pt-Alloy Catalyst for Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cell
Authors: Arumugam Balamurugan , Tamaki Takanori , Yamaguchi Takeo
Published: February 2020

Large-scale commercialization of polymer electrolyte fuel cells (PEFCs) requires great improvement in activity and durability of the cathode catalyst [1]. We have reported enhanced activity and durability of the Pt-based intermetallics, Pt2FeCo/C, with L10 ordered face-centered tetragonal (fct) structure both in acidic solution [2] and as the cathode catalyst of a membrane-electrode assembly [3]. In this work, Cu with higher standard reduction potential was employed to form ordered fct-Pt2FeCu/C, and its activity of oxygen reduction reaction (ORR) and durability were compared with its counterpart bimetallic catalyst, fct-PtFe/C, and the commercial catalyst from Tanaka Kikinzoku Kogyo (TKK-Pt/C).

Pt2FeCu/C and PtFe/C were fabricated by the heat treatment of the mixture of metallic precursors and carbon black at 800 °C under 20% H2 in N2 for 2 h. The metal content of the catalyst was around 40 wt%. ORR activity of the catalysts was evaluated in 0.1 M HClO4solution using the rotating disk electrode technique. The durability of the catalysts was examined in the same solution at 60 °C by applying square wave potential cycling, 0.6 V for 3 s and 1.0 V for 3 s, according to load cycle durability testing protocol recommended by the Fuel Cell Commercialization Conference of Japan (FCCJ).

XRD patterns of Pt2FeCu/C and PtFe/C showed the formation of chemically ordered fct structures. Both the fct catalysts initially showed high mass activity above 0.5 A/mg-Pt. Figure 1 shows the change in electrochemical surface area (ECSA) and mass activity during load-cycle durability test at 60 °C. Cu-containing fct-Pt2FeCu/C retained more than 70% of mass activity and ECSA at 10K durability cycles, while fct-PtFe/C and TKK-Pt/C catalysts showed a significant loss of activity and ECSA: retention was about 50% for TKK-Pt/C and 40% for fct-PtFe/C. The mass activity of fct-Pt2FeCu/C catalyst was enhanced by more than twice with respect to TKK-Pt/C catalyst, both at the initial state and after 10K durability cycles. STEM-EDX line-scans of the catalysts before and after durability cycles showed that dissolution of Fe and Cu was much less in fct-Pt2FeCu/C than in fct-PtFe/C. The enhancement in the durability of the fct-Pt2FeCu/C catalyst may be ascribed to the synergistic effects of the presence of Cu with higher redox potential and the ordered structure of catalyst.

 References

[1] M.K. Debe, Nature,
486, 43–51 (2012).

[2] B. Arumugam, B. A. Kakade, T. Tamaki, M. Arao, H. Imai, T. Yamaguchi, RSC Advances, 4, 27510–27517 (2014)

[3] T. Tamaki, A. Minagawa, B. Arumugam, B.A. Kakade, T. Yamaguchi, J. Power Sources, 271, 346–353 (2014)

Figure 1
Making Economical MOF-Based Precious-Metal Free Oxygen Reduction Reaction Catalysts
Authors: McCool Geoff , Li Jingkun , De Castro Emory , Mukerjee Sanjeev , Halevi Barr
Published: February 2020

The performance and promise of precious metal free catalysts for the Oxygen reduction Reaction (ORR) has increased steadily with several synthesis methods approaches developed. Catalysts based on Metal-Organic-Framework(MOF) precursors have been at the forefront of non-Precious Metal Catalyst development, with a number of groups having demonstrated well performing catalysts. A key limitation to these catalysts is the high cost of commercial MOF precursors. In an effort to reduce the cost of MOF-based ORR catalysts several groups have developed low-cost MOF synthesis.

In this presentation, we describe the scale-up and engineering for air operations of inexpensive MOF-based ORR catalysts as developed by Northesastern University and scaled to 50x by Pajarito Powder.
High Surface Area Perovskite Oxide-Based Oxygen Reduction Reaction Electrocatalysts
Authors: Pylypenko Svitlana , Christ Jason M , Batson Tim , Cadigan Christopher A. , Tong Jianhua , O'Hayre Ryan
Published: February 2020

Development of effective precious-metal free oxygen reduction reaction (ORR) electrocatalysts is highly desirable as projections indicate that the costs associated with Pt can contribute up to half of the overall cost of low-T fuel cells and electrolyzers. Development of inexpensive and effective catalysts for the ORR also has ramifications for other applications, including metal-air batteries and water splitting.There exists a variety of chemistries and classes of non-precious catalyst materials used for the ORR in alkaline media, including non-precious metal alloys, perovskite oxides, spinels and pyrolyzed metal macrocycles. For perovskite oxide (ABO3) catalysts, activity is strongly affected by the B cation and activity can be tuned by A and/or B-site doping. Therefore, identification of doping strategies to tune the perovskite catalyst structure and chemistry to achieve maximum intrinsic catalytic activity is of major importance. Performance of these materials is also limited by the very low surface areas yielded by current synthesis methods. Current soft-chemistry approaches that allow high surface areas are generally only successful for the production of simple (e.g. binary) oxides.

Our catalyst development approach involves i) doping strategies to tune the perovskite catalyst structure and chemistry to achieve maximum intrinsic catalytic activity; ii) novel synthesis processes for the production of compositionally-complex perovskite oxide catalysts with magnitude higher surface areas than those reported in the literature using current techniques; and iii) functionalization of the carbon support to create composite C/oxide catalysts with optimized performance. Combined, these advances allow for greatly improved ORR performance as demonstrated in Figure 1, which shows the evolution of ORR activity for a Ca0.9La0.1Al0.1Mn0.9O3-
δcatalyst (as quantified by rotating disc electrode) after successive implementation of the strategies discussed above. Structure-property correlations to address the interdependence between catalytic activity and variables related to materials composition, structure, and morphology will be presented. 

Figure 1. Oxygen reduction mass activities of Ca0.9La0.1Al0.1Mn0.9O3-
δ

Figure 1
Ordered Hierarchically Porous Carbon Codoped with Iron and Nitrogen As Electrocatalyst for the Oxygen Reduction Reaction
Authors: Deng Chengwei , Zhong Hexiang , Yao Lan , Liu Sisi , Xu Zhuang , Zhang Huamin
Published: February 2020

Nitrogen-doped carbon catalysts have attracted great attention as potential alternatives to the expensive Pt-based catalysts used in fuel cells. Herein, a novel catalyst based on nitrogen and iron co-doped ordered hierarchically porous carbon (Fe-NOHPC) is prepared via an evaporation-induced self-assembly process, followed by carbonization under an ammonia atmosphere. The soft template and the Fe species are found to promote the formation of the NOHPC structure and facilitate the oxygen reduction reaction (ORR) process. The catalyst possesses an ordered hierarchically porous structure with a large surface area (1172.5 cm2 g-1) and pore volume of 1.03 cm3 g-1. Compared to the commercial 20 % Pt/C, it has better ORR catalytic activity and higher stability, as well as higher methanol tolerance in an alkaline medium, demonstrating its potential for use in direct methanol alkaline fuel cells as a non-precious cathode catalyst. The nitrogen configuration, Fe species and pore structure of the catalysts are believed to correlate with its high catalytic activity.
Nanoengineered Catalysts for Fuel Cells
Authors: Fu Shaofang , Zhu Chengzhou , Zhou Yazhou , Du Dan , Lin Yuehe
Published: February 2020

The rising energy demand and environmental problems require the transition from traditional fossil fuels to clean and sustainable energy sources. Proton exchange membrane fuel cells (PEMFCs) are promising candidates due to their high efficiency and zero emission. However, the large-scale implantation of FCs is hindered by the sluggish cathodic reaction. Pt-based nanoparticles, which are supported on carbon materials, are traditional catalysts for oxygen reduction reaction (ORR). Unfortunately, the high cost, low activity and stability limit the commercialization of PEMFCs. Therefore, efforts are need to explore novel catalysts with low cost, high activity and stability. To this end, some strategies have been applied in our group to develop catalysts for ORR. In order to reduce the usage of Pt, we synthesized Pt-M alloys with core-shell structures, which are demonstrated to be more active and stable, compared to commercial Pt/C catalysts. For the supporting materials, 3D carbon materials are good candidates for ORR due to their high surface area and porosity. On the other hand, we also focus on the non-Pt catalysts, such as heteroatom-doped carbon materials, which attract more attentions because of the elimination of Pt.
Investigation of PtCo/Graphene and PtCoRu/Graphene Catalysts for Oxygen Reduction Reaction
Authors: Kepeniene Virginija , Stagniunaite Raminta , Tamasauskaite-Tamasiunaite Loreta , Norkus Eugenijus
Published: February 2020

The electrocatalytic oxygen reduction reaction (ORR) is an important process in energy converting systems such as low temperature proton exchange membrane fuel cells (PEMFCs). Therefore, understanding the oxygen reduction mechanistic pathways in terms of developing an efficient and stable catalyst has been the main research interest over several decades.

In the present study a rapid microwave heating method was used to prepare the graphene supported PtCo and PtCoRu catalysts with the different Pt:Co and Pt:Co:Ru molar ratios. The catalytic activity of the PtCo/graphene and PtCoRu/graphene catalysts, for oxygen reduction reaction was investigated by linear scan voltammetry in the cathodic direction at 5 mV s–1 in the 0.5 M H2SO4 solution saturated with O2 at a temperature of 25 oC. Physical characterization of the catalysts was performed by X-ray Diffraction, High Resolution Transmission Electron Microscopy, Energy Dispersive X-ray Analysis and Inductively Coupled Plasma Optical Emission Spectroscopy techniques. The cyclic voltammetry has been used to estimate the electrochemically active surface areas of the catalysts.

It was found that the PtCo/graphene and PtCoRu/graphene catalysts prepared by means of microwave synthesis show enhanced activity towards oxygen reduction as compared with that at bare Pt/graphene catalyst. The PtCo/graphene and PtCo /graphene catalysts show higher onset potentials, as well as higher current densities towards oxygen reduction reaction as compared with those at the bare Pt/graphene catalyst.
Third-Body Effects of Surfactants Adsorbed on Pt Electrocatalysts in Proton Exchange Membrane Fuel Cells
Authors: Chung Young-Hoon , Jang Jong Hyun , Kim Soo Jin , Chung Dong Young , Park Hee Young , Cha In Young , Sung Yung-Eun , Yoo Sung Jong , Kim Hyoung-Juhn , Henkensmeier Dirk , Kim Jin Young
Published: February 2020

In the high-temperature PEMFCs that utilize H3PO4-doped polybenzimidazole (PBI) membranes, Pt poisoning by H3PO4 is an important technical issue, while higher ORR kinetics are expected at elevated operating temperatures (~ 180oC).1 In this study, the surface of Pt nanoparticles was modified by surfactant adsorption, in order to enhance ORR activity and decrease H3PO4 poisoning. Previously, the Conway group reported so-called the “third-body (or ensemble) effect”, which refers to the blocking of poisonous spectator species on the electrocatalyst.2

When commercial Pt/C catalysts were modified by oleylamine (OA), ORR activity could be significantly enhanced, due to the variation in the electronic structure of the Pt nanoparticles.3 The ORR activity of the OA-adsorbed Pt/C was decreased by 29% when H3PO4 was added to the electrolyte solution, whereas the activity decay was much larger for untreated Pt/C (53%). This can be understood as a result of the third-body effect by the adsorbed oleylamine molecules.

In addition, a direct synthesis procedure was studied to produce OA-adsorbed Pt/C.4 The Pt/C nanoparticle electrocatalysts were prepared by conventional colloidal-reduction method, but the surfactant removal step by heat-treatment or acid treatment was eliminated. The morphology and crystal structure were analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The surface modification by surfactant was confirmed by near-edge X-ray absorption fine structure (NEXAFS) and cyclic voltammetry (CV).

The synthesized Pt/C electrocatalysts were characterized by half-cell test, and the expected third-body effect of adsorbed surfactants could be confirmed. A membrane electrode assembly (MEA) was fabricated with synthesized Pt/C and a homemade H3PO4-doped para-polybenzimidazole (p-PBI) membrane, and its single cell performance was evaluated at 160oC. As the MEA compression during cell assembly is an important factor, catalyst layer thickness were similarly controlled. In the low-current density region, the cell voltage of the MEA with synthesized OA-adsorbed Pt/C was higher than that with untreated commercial Pt/C. From extrapolation in Tafel plots, the kinetic current densities were measured to confirm the third-body effect for Pt nanoparticles supported on carbon substrate. Cell voltage was stable over 150 h, indicating that the adsorbed surfactants are durable under high-temperature conditions.

References

1. R. Gisbert, G. Garcı´a and M. T. M. Koper, Electrochim. Acta 55 (2010) 55 7961.

2. H. Angerstein-Kozlowska, B. MacDougall and B. E. Conway, J. Electrochem. Soc. 120 (1973) 756.

3. Y.-H. Chung, D. Y. Chung, N. Jung, Y.-E. Sung, J. Phys. Chem. Lett. 4 (2013) 1304.

4. Y.-H. Chung, S. J. Kim, D. Y. Chung, H. Y. Park, Y.-E. Sung, S. J. Yoo, J. H. Jang 51 (2015) 2968.
A FIB-SEM Study on Correlations between PEFC Electrocatalyst Microstructure and Cell Performance
Authors: Okumura Makito , Nagamatsu Yohei , Noda Zhiyun , Tachikawa Yuya , Daio Takeshi , Hayashi Akari , Sasaki Kazunari
Published: February 2020

The electrocatalyst layer of the polymer electrolyte fuel cells (PEFCs) has a complicated porous microstructure. Since the structure allows gas transport, proton transport, and water management, cell performance depends strongly on their 3-dimensional microstructure. Understanding on the correlations between the electrocatalyst microstructure and the cell performance is therefore essential in improving the PEFC electrochemical performance. The purpose of this study is thus to clarify these correlations using focused-ion-beam coupled scanning electron microscopy (FIB-SEM).

We used the standard electrocatalyst (TEC10E50E)and prepared model cells with the electrode area of 1cm2. We then varied the Nafion ratio in the electrocatalyst layer to systematically modify the electrocatalyst microstructure. After model cell preparations, IV performance was measured and each electrode overvoltage was separated. We then observed electrocatalyst microstructure by the FIB-SEM technique to examine the correlations between the 3D microstructure and the cell performance. In addition, we analyzed cell performance and electrocatalyst microstructure of Pt/SnO2-based electrocatalysts to examine the structural difference from the standard Pt/C electrocatalyst layer.

Figure 1 shows IV characteristics of several cells with different Nafion contents. In case Nafion ratio was 28wt.%, IV performance was the highest. In case Nafion ratio was less than 28wt.%, whilst IV performance was lower than that of 28wt.%, the cells can still generate high current densities. On the other hand, in case Nafion ratio was higher than 28wt.%, the cells can exhibit a lower cell voltage due to a much higher concentration overvoltage. This is because gas transport pathway in the electrocatalyst layer is filled with Nafion ionomer with increasing Nafion ratio. In order to examine the correlations between the cell performance and the microstructure, 3D microstructure of the electrocatalyst layers was systematically examined by FIB-SEM. As an example, Figure 2 shows 3D microstructure of the electrocatalyst layer with the Nafion ratio of 28wt.%. Similarly, we observed other Pt/C electrocatalyst samples with different Nafion contents. The Pt/SnO2-based electrocatalyst layer was also analyzed for the comparison. Details on the FIB-SEM observation conditions and the microstructure-performance correlations are presented at the symposium.

Figure 1