bluuebunny/crossref_metadata_2025 huggingface dataset:
High Performance AlGaN/GaN Hemts on GaN Substrates
Authors: Lei Lei , Yang Song , Yu Kun , Zhang Yong , Makhdoom Shahid , Wang Xuhui , Zhou Ting , Lu Xing , Zhang Anping
Published: February 2020

AlGaN/GaN HEMTs grown on GaN substrates have the potential to achieve significant improvement in device performance and reliability. We present a comprehensive study of high performance AlGaN/GaN HEMTs on GaN substrate with an emphasis on correlation of defects with device performance and reliability. Three AlGaN/GaN HEMT structures (AlGaN/GaN-HEMTs-on-GaN, AlGaN/GaN-HEMTs-on-Si and AlGaN/GaN-HEMTs-on-sapphire) were grown and used for comparison. Temperature-dependent electrical performance of AlGaN/GaN HEMTs were analyzed and correlated with defects. The results show that very smooth surface morphology of AlGaN/GaN-HEMTs-on-GaN is demonstrated with significantly reduced structural defects (characterized by HR-TEM) and surface defects (characterized by AFM). Leakage current reduction of more than four orders of magnitude is achieved with AlGaN/GaN-HEMTs-on-GaN structure. The effects of defects on surface passivation and dynamic on-resistance of AlGaN/GaN HEMTs will also be presented and analyzed.
(Invited) Fundamental Differences Between the Traditional III-V Compounds and Nitride Semiconductors
Authors: Moustakas Theodore D.
Published: February 2020

In this talk I will address the fundamental question as to why the performance of emitters based on nitride semiconductors, grown heteroepitaxially on foreign substrates, is insensitive to high concentration of extended defects, which is not the case for traditional III-V compounds. I will discuss the following important differences between these two families of materials, which contribute to this finding: (a) the growth of Nitride semiconductors occurs at temperatures much lower than the half of their melting temperature, where the brittle to ductile transition occurs. In traditional III-V compounds the growth temperature is generally higher than ½ TM; (b) the chemical bonding in nitrides is strongly ionic while is mostly covalent in traditional III-V compounds. This leads to the bunching of the intrinsic surface states as well as the states associated with dangling bonds in edge dislocations near the band edges where they act as traps rather than recombination centers. Furthermore, the surface states have less effect on the surface Fermi level position; (c) the nitrides can exist in the wurtzite structure (equilibrium) and the cubic structure (metastable) and the enthalpy of formation of the two allotropic forms differs by only a few meV. Thus, conversion between the two phases occurs easily by creation of stacking faults along the close-packed (0001) and (111) planes, which due to the smaller energy gap of the cubic phase leads to strong band structure potential fluctuations in the wurtzite matrix; (d) additional band structure potential fluctuations exist in the nitride alloys due to various forms of partial alloy ordering or to compositional inhomogeneities introduced by phase separation or the growth mode, These potential fluctuations contribute to exciton localization and thus efficient radiative recombination.
(Invited) Semiconductor Nanowires for Optoelectronics and Energy Applications
Authors: Jagadish Chennupati
Published: February 2020

Semiconductors have played an important role in the development of information and communications technology, solar cells, solid state lighting.  Nanowires are considered as building blocks for the next generation electronics and optoelectronics.  In this talk, I will introduce the importance of nanowires and their potential applications and discuss about how these nanowires can be synthesized and how the shape, size and composition of the nanowires influence their structural and optical properties. I will present results on axial and radial heterostructures and how one can engineer the optical properties to obtain high performance optoelectronic devices such as lasers, THz detectors, solar cells.  Future prospects of the semiconductor nanowires will be discussed.
Luminescence of Eu3+ in a Mullite-Type Host Material Bi2Ga2Al2O9
Authors: Tiwari Hansnath , Varadaraju U V
Published: February 2020

The escalating energy demand coupled with limited conventional energy resources led to a widening gap between supply and demand, making it inevitable to look for novel energy efficient technologies. Solid state lighting is one of the promising areas where a significant amount of energy consumed can be conserved without compromising on the sophistications of conventional lighting. Solid state lighting kindled the interest of researchers owing to their impressive energy efficiency, longer life time, greater reliability, eco-friendly nature and the reasonable ease of fabricating the white light emitting diodes (WLEDs). Typically blue light emitting chip in combination with yellow light emitting phosphor1, Y3Al5O12:Ce3+ (YAG:Ce3+) have been used to produce the white light. However, the obtained white light has poor light quality and low color rendering index (CRI >80), due to lack of red component in the obtained white light spectrum. Tricolor phosphors (red, green and blue) in combination with near ultra violet (NUV) emitting source (InGaN chip) were considered to be an alternative to get better lighting properties and the generated white light by this technique has similarity to sun light and a good CRI value2
,
3. The commercially used red phosphor Y2O2S:Eu3+ is sensitive towards moisture and decomposed under high temperature which is harmful to the environment. Nitrides, CaAlSiN3:Eu2+ can be used as a substitute but its preparation needs greater care and highly controlled preparation conditions. Therefore, the red phosphors with thermal, chemical stability and high luminous intensity became an essential need for lighting industries.

Recently, mullite and mullite-type ceramics have attracted attention because of their interesting optical properties4, high thermal and chemical stability5. In this context, we report Eu3+ activated mullite-type Bi(1-x)2Ga2Al2O9:xEu3+ (x = 0.00, 0.025, 0.05, 0.10, 0.20 and 0.30) solid solution which were prepared by simple solid state method. The prepared samples were characterized by x-ray diffraction and luminescence properties were measured under near UV and blue light excitation. The europium dopped Bi2Ga2Al2O9 (BGAO) showed characteristic excitation peaks between 250 nm and 500 nm. The chromaticity coordinates (0.61, 0.37) calculated under blue light excitation for 20% Eu3+ doped BGAO, located in orange-red region of chromaticity graph. The coordinates were close to the reference red phosphors and the results indicate that it have potential application in the field of lighting and displays under blue LED excitation.

References:

1. S. Nakamura, MRS Bull., 22, 29–35 (1997).

2. J. S. Kim et al., Appl. Phys. Lett., 85, 3696–3698 (2004).

3. C. K. Chang and T. M. Chen, Appl. Phys. Lett., 91, 081902 (2007).

4. G. Blasse and O. B. Ho, J. Lumin., 21, 165–168 (1980).

5. M. M. Murshed and T. M. Gesing, Mater. Res. Bull., 48, 3284–3291 (2013).

Figure 1
Recent Advances on the Optical Properties of Zinc-Doped Antimony Sulphide (Sb2S3) Thin Films
Authors: Nwofe Patrick AKATA , Ozibo Cyril Ozibo , Agbo Peter Ekuma
Published: February 2020

The influence of zinc (Zn) impurities at different concentrations on the properties of antimony sulphide (Sb2S3) thin films grown using the solution growth technique is reported. The films were grown at room temperature of 28oC,  a deposition time of  2 h, and an initial pH of 4.40. The films were then doped with different concentrations of Zinc impurities (0.25 -1.0M) and then annealed at annealing  temperatures  ≤ 400 oC. The films were characterised using optical spectroscopy and Fourier transform infrared spectroscopy (FTIR)  to investigate the transmittance, absorbance and reflectance versus wavelength measurements, X-ray diffractometry (XRD) to study the structural properties, Scanning electron microscopy (SEM) to investigate  the morphological properties, and Rutherford Backscattering (RBS) techniques for the compositional analysis.  The results indicate that the film thickness was in the range 0.3 -1.5μm, with an increase in the film thicknesses of the doped layers compared to the as-deposited films.  The transmittances of the doped layers were found to be reduced compared to the as-deposited films. The results of the optical analysis show that the films had direct energy band gap with optical absorption coefficient (α) > 104 cm-1 in both the doped and as-grown  layers. The refractive index was typically ≤ 3.6, with higher values obtained from the doped layers. The values of the energy band gap were in the range suitable for application in solar photovoltaic and photonic devices.
Top Seeded Solution Growth of 4H-SiC Bulk Crystal Using Graphite Block for Long-Term Growth
Authors: Ha Minh-Tan , Byeon Dae-Seop , Yoon Ji-Young , Lee Myung-Hyun , Seo Won Seon , Lee Won-Jae , Kim Cheol-Jin , Jeong Seong Min
Published: February 2020

Solution growth method of SiC is assumed to be a promising technique to grow high quality SiC bulk crystal. Solution growth of SiC are based on the liquid phase growth from an incongruent melt composed of Si and C. Among various solution growth techniques, top seeded solution growth (TSSG) method for SiC has been widely studied for a next generation SiC growing technique. In TSSG method, the crystal growth on seed crystals is accomplished by the precipitation of solid SiC from C dissolved a Si melt on a seed crystal located at the top of the liquid. Because of the low solubility of C in liquid Si, SiC crystal growth on seed crystals using TSSG involves the use of a Si-rich Si-C solution. Therefore, in the TSSG method, high C solubility is required to increase the growth rate of SiC. Hence, there have been lots of studies how to increase the C solubility of Si melt. Eventually, several metallic solvents were proved to enhance the carbon solubility, so that the growth rate of SiC using TSSG were remarkably increased up to 2 mm/h. So, it is very optimistic to increase the thickness of grown crystal.

However, TSSG has critical issue to increase crystal thickness because the C is supplied from the graphite crucible which is also the reservoir of Si melt. As growing time goes, therefore, the crucible will be dissolved to Si melt and the dissolved carbon will be consumed to grow single crystalline SiC crystal. Eventually, the thickness of SiC grown layer will be limited the volume size of graphite crucible. The reduction of graphite crucible will bring about the change of temperature distribution in the solid and liquid inside the reactor. Mechanical failure during crystal growth could be happened.

Therefore, in this study, we applied a graphite block inside graphite crucible for elongate the growing time limit using graphite crucible for TSSG. Several graphite blocks with different design and materials properties were evaluated using finite element analysis using COMSOL multiphysics. The simulated results were then applied to the real experimental growth using small seed samples. Finally, the durability of graphite crucible and the crystal quality were comparatively evaluated with and without graphite blocks using several characterizations techniques.
Optical and Electric Properties of Cu-Doped ZnO Films Grown on MgO (100) Substrate
Authors: Ho Ming-Yuan , Wen Meng Chieh , Chang Liuwen , Chou Mitch
Published: February 2020

This work studied the luminescent and electric properties of Cu-doped ZnO films grown on MgO (100) substrate by plasma-assisted molecular beam epitaxy. The doping quantity was controlled by regulating Cu Kundsen cell temperature. The crystallinity and surface morphology of the Cu-doped ZnO films were characterized by high resolution X-ray diffraction, atomic force microscopy, and scanning electron microscopy. The chemical state and elemental compositions was analyzed by X-ray photoelectron spectroscopy (XPS). The near-band-edge emission (NBE) and defect-related emission (DLE) were analyzed by photoluminescence spectroscopy. The electric properties were determined by Hall measurement and four-point probe measurement.

The experimental results revealed that ZnO films grown epitaxially on MgO substrate with (10-10)ZnO//(100)MgO. X-ray rocking curve measurements showed that the (10-10)ZnO reflection exhibits a full width at half maximum value of about 0.7o-1.0o. The XPS analysis showed that the doped copper atoms have a monovalent state in the ZnO films grown at Cu Kundsen cell temperatures lower than 900oC. The amount of Cu incorporation was estimated to be in the range of 1x1019 to 1x1021 atoms/cm3. The NBE peak of the Cu-doped ZnO films can be deconvoluted into three sub-peaks: a free exciton emission at 3.29 eV, a basal-plane stacking fault emission at 3.19 eV, and a third peak at 3.25 eV. The third peak might be related to an acceptor-bound excitons due to the doping of copper, since it disappeared after annealing. Hall measurement indicated that both pure and Cu-doped ZnO films are n-type semiconductor. The electron concentration of undoped ZnO film is 1x1018 cm-3, whereas that of the Cu-doped ZnO films are in the range of 5x1015~5x1016 cm-3. Obviously, the electron concentration of ZnO is compensated by the substitution of Cu+ to Zn2+ which generates acceptor levels. However, the electron concentration of the Cu-doped ZnO films rose back to the order of magnitudes 1018 cm-3 after annealed in oxygen or hygrogen, indicating the acceptor states were eliminated. Moreover, in this study, the incorporation of copper in ZnO does not result in an increase of the yellow-green luminescence.
Thermal-Structural Optimization of Light with LED Packaging
Authors: Zhang Liqiang , Li Yongli , Ge Daohan , Zhu X.D.
Published: February 2020

At present, the light-emitting-diode (LED) has been a tremendous surge in its application as a new type of generation lighting source. The thermal performance and structural reliability of LED system have been investigated by different conditions in electronic package [1]. With the minimization of the LED packaging, the reliability problem, especially, the thermal reliability becomes more and more seriously. Hence the heat dissipation for LED system becomes much crucial. Lots of heat management methods have been investigated for LED system designing and manufacturing. The most popular cooling method used in the LED system is the passive cooling, however, its cooling efficiency is very low. It is very significant to develop the heat dissipation method [2, 3].

A systematic numerical and experimental method is developed to optimize a caution lamp with multichips LEDs in this paper. Here we intend to describe the thermal performance of LED caution lamp with multiple LED chips through the structural optimization by the finite element method. And to understand the thermal characteristics of the caution lamp with difference structural parameters. After the optimization, one type of caution lamp was made and the highest temperatures of LED chips have been test. This optimization method is useful for design and manufacturing of caution lamps with multiple LED chips.

After numerical simulation, we prepared the caution lamp sample with six LED chips according to the results. The Thermalcouples are used to test the temperature of the lamp. Compared with the simulation and the experiment data, the high temperature of LEDs were reduced by choosing fitting parameters. It indicates that the results and methods used in this paper can provide the guidance in the understanding of thermal management for LED lamps design.

Figure 1 show temperature distribution of light structures before and after thermal-structural optimization.

Figure 2 show the test principle connection and test system.

Figure 1
(Invited) 2D Semiconductor Electronics: Advances, Challenges and Opportunities
Authors: Javey Ali
Published: February 2020

Two-dimensional (2-D) semiconductors exhibit excellent device characteristics, as well as novel optical, electrical, and optoelectronic characteristics. In this talk, I will present our recent advancements in defect passivation, contact engineering, surface charge transfer doping, and heterostructure devices of layered chalcogenides.  We have developed a defect repair/passivation technique that allows for observation of near-unity quantum yield in monolayer MoS2. The work presents the first demonstration of an optoelectronically perfect monolayer. Forming Ohmic contacts for both electrons and holes is necessary in order to exploit the performance limits of enabled devices while shedding light on the intrinsic properties of a material system. In this regard, we have developed different strategies, including the use of surface charge transfer doping at the contacts to thin down the Schottky barriers, thereby, enabling efficient injection of electrons or holes.  We have been able to show high performance n- and p-FETs with various 2D materials.  Additionally, I will discuss the use of layered chalcogenides for various heterostructure device applications, exploiting charge transfer at the van der Waals heterointerfaces. I will also present progress towards achieving tunnel transistors using layered semiconductors.
(Invited) Phosphorene Transistors - a Brief Review
Authors: Xiong Kuanchen , Luo Xi , Hwang James C. M.
Published: February 2020

Phosphorene, despite its instability when directly exposed to ambient oxygen or water, has been successfully processed into stable field-effect transistors (FETs). This paper reviews the emergence and progress of phosphorene FETs, all in little over a year. In such a short time, back-gated FETs evolved into front-gated FETs, gate length was reduced to the sub-micron range, passivation by high-k dielectrics or hexagonal boron nitride was demonstrated with temporal, thermal and mechanical stability, ohmic contact was achieved down to cryogenic temperatures, and cutoff frequencies was pushed above 10 GHz. These and other attractive characteristics promise the phosphorene FET to be a viable candidate for current-generation thin-film electronics, as well as future-generation ultra-thin-body low-power-consumption high-speed and high-frequency transistors.

Table I lists chronologically the pioneering papers on phosphorene FETs, which experimentally demonstrated promising performance characteristics, despite crude construction so that the top surface of the phosphorene channel was either exposed or protected only by polymer. Additionally, although the carrier mobility, current capacity, and on/off ratio were adequate, further improvement in the subthreshold slope was needed to improve the energy efficiency of these devices. To passivate the phosphorene surface, atomic-layer deposited aluminum oxide and exfoliated hexagonal boron nitride (hBN) have been used to demonstrate phosphorene FETs that were stable under ambient conditions up to several months and over a temperature range of −263 °C to 150 °C as listed in Table II. Longer-term stability could not be assessed because phosphorene FETs exist for just little over a year.

The attractive characteristics of phosphorene FETs demonstrated to date, such as carrier mobility, current capacity, on/off ratio, cut-off frequencies, temporal, thermal and mechanical stabilities indicate that they are viable candidates for thin-film flexible electronics operating in the gigahertz frequency range. However, for higher-speed and higher-frequency applications, they need to be further improved especially in reducing the interface state density to the 1011-cm−2 range and the gate length to the 10-nm range. For such an ultrathin-body low-power consumption nano-FET, not only the gate stack, but also the entire stack from the front gate, gate insulator, phosphorene channel, back insulator, back gate, to substrate needs to be optimized together, in addition to optimization of ohmic contacts and proper scaling of all other dimensions. This daunting task may be accomplished in much shorter time than the decades it took to develop Si or III-V FETs, judging from the extremely rapid development of phosphorene FETs to date. Ultimately, the same as for all 2D devices, uniform, reproducible and large-area growth or synthesis of phosphorene will be required for low-cost high-yield manufacture.

Figure 1
(Invited) Strain Engineering in Ultrathin Black Phosphorus
Authors: Li Yanyong , Lau Shu Ping
Published: February 2020

We report the giant anisotropic strain response in vibrational modes of ultrathin black phosphorus (BP). A modified bending technique was employed to apply precise uniaxial tensile strain to zigzag or armchair directions of the BP flakes. The Raman shift rates of A1
g, B2g and A2
g  modes of BP are significantly distinct for strain applied along zigzag or armchair directions. For the applied strain along zigzag direction, the Raman shift rate and the Grüneisen parameter of the B2g mode can reach a remarkable value of ~-11 cm-1/% strain and ~3.7 respectively, which are the largest among all the reported values in two-dimensional materials. The density functional theory calculations were performed to understand the exceptional anisotropic strain response in the vibrational modes of BP. It is found that not only the bond lengths but also the bond angels are changed in BP upon applied strain which lead to giant anisotropic response in mode B2g . Furthermore, we demonstrate a simple method based entirely on the strained BP and conventional Raman spectroscopy to determine the crystallographic orientations of BP without the aid of polarization Raman spectroscopy. This work may pave a way to study the strain-induced anisotropic electrical conductance and the magnetotransport properties of BP.
(Invited) Novel Electronic and Photonic Applications of Anisotropic Black Phosphorus
Authors: Wang Han
Published: February 2020

In this talk, I will discuss our recent work in introducing black phosphorus (BP) to the layered-material family as a novel anisotropic 2D material for electronic and optoelectronic applications. Narrow gap BP thin film (0.3 eV in bulk) serendipitously fill the energy space between zero-gap graphene and large-gap TMDCs, making it an promising material for near and mid-infrared optoelectronics. BP thin films show high mobility above 650 cm2/V.s at room temperature along the light effective mass (x) direction, implying its promising potential for high frequency, thin-film electronics. Furthermore, its anisotropic nature within the plane of the layers may allow for the realization of conceptually new electronic and photonic devices impossible in other 2D materials. In the talk, I will also present our work in demonstrating 20 GHz black phosphorus radio-frequency transistors and the ballistic avalanche phenomenon in vertical black phosphorus devices. Our recent progress in characterizing the optical properties of monolayer BP and in achieving large-area synthesis of black phosphorus thin film will also be discussed. I will conclude with remarks on promising future directions of black phosphorus research and how this new material is expected to benefit the next-generation electronics and photonics technologies.
Simultaneous Intercalation and Oxidant-Free Polymerization of Pyrrole Between 2D Titanium Carbide (MXene) for High Performance Pseudocapacitive Electrodes
Authors: Boota Muhammad , Anasori Babak , Voigt Cooper , Zhao Meng-Qiang , Barsoum Michel W. , Gogotsi Yury
Published: February 2020

Electrochemically active organic materials which offer pseudocapacitve characteristics are attractive for energy storage applications because they are economical, environmentally benign, lightweight, and their functionalities can be tuned to achieve the required electrochemical properties.1 Despite aforementioned advantages, they tend to degrade upon cycling mainly because of their poor conductivity. One option to improve the cycling and the overall electrochemical performance of the pseudocapacitve organic materials is to deposit/graft them onto conductive carbon particles.2 Two-dimensional carbides and carbonitirides called MXenes, which generally offer metallic conductivity, may become a suitable option to replace carbon in pseudocapacitve electrodes to improve the electrochemical performance of the organic materials.3,4

Here, we report a facile approach to simultaneously align and intercalate heterocyclic organic molecule pyrrole between the Ti3C2Tx MXene layers. On contrary to the conventional approaches, pyrrole monomer was polymerized without addition of any oxidant when mixed with the Ti3C2Tx colloidal solution, which was confirmed by various spectroscopic techniques. When the optimized composition was tested as a pseudocapacitive electrode, it exhibited high volumetric and gravimetric capacitances of 1000 F/cm3 and 416 F/g, respectively, at 5 mV/s. Most important, excellent capacitance retention of 92% was observed after 25,000 cycles at 100 mV/s. 

1. M. Boota et al., Adv. Mater. (2015).  DOI (10.1002/adma.201504705)

2. M. Boota et al., ChemSusChem, 8, 3576–3581 (2015)

3. M. R. Lukatskaya et al., Science., 341, 1502–1505 (2013)

4. M. Ghidiu, M. R. Lukatskaya, M.-Q. Zhao, Y. Gogotsi, and M. W. Barsoum, Nature, 516, 78–81 (2014)
Low-Resistance Contacts for Single-Layered MoS2 n-FETs By Hydrogen Plasma
Authors: Lu Ang-Yu , Cheng Chia-Chin , Chen Min-Cheng , Li Ming-Yang , Chiu Ming-Hui , Hsu Chang-Lung , Huang Jing-Kai , Shi Yumeng , Li Lain-Jong
Published: February 2020

Achieving low Schottky barriers interface is important for future electronic devices based on transition metal dichalcogenides. In this study, we demonstrate that the electronic properties of single-layer molybdenum disulfide (MoS2) can be tuned from semiconducting to metallic via controlling its defect level (sulfur vacancy) by using hydrogen plasma. Raman, Photoluminescence, Atomic Force Microscopy, and X-ray photoelectron spectra suggest that the top layer sulfur in SL-MoS2 can be totally removed and stabilized in ambient environment. Field-effect transistors (FETs) with highly sulfur vacancies/Ti electrodes fabricated and exhibited high performance n-FETs. The results provide a promising route for low Schottky barrier contacts devices with graded junction interface.
Electrodeposition and Capacitance of WS2 Thin Films Onto ITO and Glassy Carbon from Aqueous and Non-Aqueous Electrolytes
Authors: Fan Li , Suni Ian Ivar
Published: February 2020

Two-dimensional (2D) layered materials, including graphene, metal oxides, metal chalcogenides and their composites have shown to be efficient and promising materials for high performance supercapacitor electrodes, with high surface area and large in-plane conductivity. Among the 2D layered supercapacitors, MoS2, MoSe2, WS2, and WSe2 are semiconductors with intriguing electronic, electrochemical and electrocatalytic properties.

We have synthesized WS2 thin films onto both ITO and glassy carbon electrodes by the electro-reduction of (NH4)2WS4 from both aqueous and acetonitrile electrolytes.  During cyclic voltammetry, a cathodic peak that overlaps with H2 evolution is observed at at -1.2V vs Ag/AgCl in aqueous solutions.  In acetonitrile, no cathodic peak is observed on ITO, but a cathodic peak that overlaps with a background cathodic peak is observed on glassy carbon. The background peak appears to arise form TEA-BF4 reduction, since the current density scales with TEA-BF4 concentration. 

The standard reduction potential, effective diffusion coefficient and transfer coefficient for WS2 electrodeposition in acetonitrile are determined are estimated from cyclic voltammetry at different scan rates. Results will also be presented for the specific capacitance of WS2 films deposited atop glassy carbon electrodes (GCE) following application of potential sweep methods for activing GCE.
The ARPA-E REBELS Program: Advances and Challenges after 1.5 Years of Research
Authors: Albertus Paul S. , Soloveichik Grigorii L. , Tuttle John R. , Litzelman Scott Joseph , Pouy Mark J.
Published: February 2020

In 2014, the Advanced Research Projects Agency-Energy (ARPA-E) started the Reliable Electricity Based on ELectrochemical Systems (REBELS) program, focusing on fuel cells utilizing natural gas for stationary power production and value-added products generation. The intent of REBELS was twofold: (1) demonstrate that operating fuel cells in an “intermediate” temperature range of 200-500 oC could reduce system cost compared to polymer electrolyte membrane (PEM) fuel cells and higher temperature solid oxide fuel cells (SOFCs), and (2) create new electrochemical functionality such as charge storage in the stack and liquid fuel production to benefit grid applications. REBELS was enabled by more than 15 years of materials research on new electrolytes, such as solid acids, proton-conducting ceramics, and alternative oxygen-conducting ceramics to yttria-stabilized zirconia. This presentation describes the rationale behind the REBELS program, including changes in the U.S. electrical grid and the evolving opportunities for distributed power generation. Technical barriers to intermediate temperature operation such as methane activation at lower temperatures and reduced power density due to electrode overpotentials and electrolyte resistance will be highlighted.

In the year and a half since the REBELS program began, teams working on a variety of material systems have made progress on several fronts. New anode compositions and microstructures enabled fuel cells to operate directly on methane at 500 oC without coking. New cathode materials mitigated the increase in electrode overpotential as temperature decreased from higher temperature SOFC operation. New electrocatalyst materials and morphologies allowed solid acid fuel cells to operate around 250 oC with reduced platinum loadings. New electrochemical functions have also been demonstrated, such as power production with internal charge storage (no external fuel) and conversion of methane to products such as methanol and ethylene.

This presentation will summarize the performance of technologies within the REBELS program with respect to PEM and SOFCs, and it will revisit the original assumptions that motivated the program. Components of the techno-economic models required to rigorously evaluate the program, such as electrochemical performance, materials cost, stack lifetime, efficiency etc. will be discussed.
Reaction Distribution in Mixed-Conducting SOFC Cathodes Investigated with Patterned Thin Film Electrodes
Authors: Amezawa Koji , Fujimaki Yoshinobu , Shindo Yusuke , Nakamura Takashi , Iguchi Fumitada , Yashiro Keiji , Yugami Hiroo , Kawada Tatsuya , Nitta Kiyofumi , Terada Yasuko
Published: February 2020

Solid oxide fuel cells (SOFCs) recently attract attentions as one of promising high-efficiency energy-conversion devices. For enhancement of the cell performance or low temperature operation, oxygen reduction kinetics on the cathode should be improved. In a mixed ionic-electronic conducting (MIEC) cathode, such as (La,Sr)CoO3
-

d
and (La,Sr)(Co,Fe)O3
-

d
, reaction area for the oxygen reduction reaction is not limited to only the electrode/electrolyte/gas interface (the triple phase boundary) but also distributes to the oxide surfaces within a certain distance from the electrode/electrolyte interface, i.e. within an effective reaction area. However, there are no reports which directly observe such a reaction distribution by experimental manners, as far as the authors know.

For experimental and quantitative observation of the reaction distribution in an MIEC SOFC electrode, a patterned thin film electrode, which is a kind of the columnar electrode simplifying a porous electrode, was used in this work [1]. As a typical MEIC electrode material, La0.6Sr0.4CoO3-

d
was chosen and deposited on a Ce0.9Gd0.1O1.95 electrolyte. In an MEIC cathode, it is believed the oxygen chemical potential decreases in the area where the electrochemical oxygen reduction reaction takes place. Thus, reaction distribution can be understood if the distribution of oxygen potential is revealed. In this work, operando micro X-ray absorption spectroscopy (XAS) technique was applied to evaluate the oxygen potential was evaluated as a function of the distance from the electrode/electrolyte interface. In this operando technique, X-ray absorption spectra could be obtained with a spatial resolution higher than 1 mm at elevated temperature (~1073 K) while controlling atmospheric condition and passing the electronic current through the cell. The effective reaction area could be estimated, for instance, 20 mm from the electrode/electrolyte interface in the case of the La0.6Sr0.4CoO3-

d
patterned thin film electrode having a thickness of 400 nm at 973 K under 1 bar of p(O2) and -220 mV of cathodic overpotential. The effective reaction area tended to expand with decreasing ambient p(O2) and increasing cathodic overpotential.

References

[1] K. Amezawa, Y. Fujimaki, T. Nakamura, K. D. Bagarinao, K. Yamaji, K. Nitta, Y. Terada, F. Iguchi, K. Yashiro, H. Yugami, T. Kawada, Electrochem. Soc. Trans., 66(2), 129 (2015).
Tailoring Electrode Materials for Proton-Conducting Solid Oxide Fuel/Electrolysis Cells with Chemically Stable BaZrO3 Electrolyte
Authors: Bi Lei , Traversa Enrico
Published: February 2020

Proton-conducting oxides offer an alternative solution for solid oxide cells with its unique advantages, such as low activation energy, high ionic conductivity at intermediate temperatures (1, 2). In the community, it is generally realized that the use of chemically stable electrolyte material BaZrO3 is the most promising way for the development of proton-conducting solid oxide cells. Although the use of BaZrO3 has been achieved with desirable fuel cell performance (2), the design of its air electrode is still a challenge, especially for the proton-conducting solid oxide electrolysis cells. In this presentation, we report our recent work on the tailor of the air electrode materials for proton-conducting solid oxide cell, including the use of traditional Sr-doped LaMnO3 (LSM) cathode for BaZrO3-based fuel cells with desirable cell performance and the design of mixed proton-electronic mixing conducing air electrode for proton-conducting electrolysis cells.

LSM, which is one of the most classical air electrode materials for solid oxide cells, is regarded can be only used above 700 oC due to its poor catalytic activity at intermediate temperatures (around 600 oC). In our study, we successfully employed the inkjet printing technique, enabling LSM work properly as the air electrode for proton-conducting solid oxide fuel cells with BaZrO3 electrolyte. After the optimization, the fuel cell performance reached 200 mW cm-2 at 600 oC, which is encouraging considering the air electrode material is LSM. A further improvement of fuel cell performance was achieved by using mixed electronic-oxygen conductor air electrode material, reaching the highest power output for the BaZrO3-based cells up to now.

In addition to the work on fuel cells, research on the air electrode material for solid oxide electrolysis cells was also carried out. Following to our previous work using proton-electronic mixed conductor for proton-conducting fuel cells (3, 4), a similar strategy was proposed for electrolysis cells. Coupling BaZr0.5Pr0.3Y0.2O3 (BZPY30, a mixed protonic-electronic conductor) with La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) as the air electrode, the cell showed a dramatic electrolysis cell improvement compared with the LSCF- BaZr0.7Pr0.1Y0.2O3 (BZPY10, a pure protonic conductor). This result suggests the extension of proton conduction at the air electrode is critical for the transportation of proton at air electrode side and thus facilitates the electrochemical reactions for the proton-conducting electrolysis cells.

References

1. L. Bi, S. Boulfrad and E. Traversa, Chem. Soc. Rev., 43, 8255 (2014).

2. E. Fabbri, L. Bi, D. Pergolesi and E. Traversa, Adv. Mater., 24, 195 (2012).

3. E. Fabbri, I. Markus, L. Bi, D. Pergolesi and E. Traversa, Solid State Ionics, 202, 30 (2011).

4. E. Fabbri, L. Bi, D. Pergolesi and E. Traversa, Energ. Environ. Sci., 4, 4984 (2011).
Ba0.5Sr0.5Co0.8Fe0.2O3 Cathode for Intermediate Temperature Proton-Conducting Solid Oxide Fuel Cells
Authors: Sun Shichen , Cheng Zhe
Published: February 2020

Comparing with conventional solid oxide fuel cells (SOFCs) that typically operate at ~750 oC and above, intermediate temperature SOFCs (IT-SOFCs) that operate at temperature of ~400-700 oC offer advantages such as the possibility for the use of lower cost sealing and interconnect materials and the potential for improved reliability and robustness due to the expected slower degradation at reduced temperature.  For IT-SOFCs, proton-conducting oxides such as acceptor-doped Ba(Ce, Zr)O3 are often preferred as the electrolyte materials for their high ionic conductivity and low electronic transport number while Ni is almost always used for the anode.  In comparison, researchers have not settled on a proper cathode or oxygen electrode, and various materials such as LSCF and SSC have been explored and show different degree of success.  Here, we report the use of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) as cathode material for proton conducting IT-SOFC with BaZr0.1Ce0.7Y0.1Yb0.1O3 (BZCYYb) electrolyte.  In particular, the chemical stability of BSCF cathode subjecting to different atmospheres containing water (H2O) vapor and CO2 and the compatibility of BSCF with BZCYYb electrolyte will be examined.  In addition, the impacts of cell operating conditions including cathodic bias voltage, moisture and CO2 on the electrochemical performance of cathode including bulk and interfacial resistance will be reported using cathode symmetrical cells and/or anode-supported button cells.  Preliminary results seem to suggest that BSCF cathode displays reasonable stability and compatibility under typical proton conducting IT-SOFC operating conditions, and cathode performance increases in humidified air compared with dry air.  Finally, the directions for future design of cathode for proton conducting IT-SOFC will be discussed.
(Invited) Studies of Hydride Ion Conduction
Authors: Irvine John T. S.
Published: February 2020

We have recently confirmed that barium hydride shows fast pure ionic transport of hydride ions (H-) in a high temperature, high symmetry phase. BaH2 gives rise to hydride ion conductivity of 0.2 S/cm at 630oC. This is an order of magnitude larger than that of state-of-the-art proton conducting perovskites or oxide ion conductors at this temperature. Alkaline earth hydride (AEH2) compounds also show remarkable exchange capability with nitrogen gas, forming a variety of nitrided compounds. Here we will present combined in situ measurements of conductivity and structure as functions of temperature and amosphere.
Time Resolved DXAS Study on Micro and Nano NiO/Ce0.9Gd0.1O1.95 Cermets for Intermediate Temperature Solid Oxide Fuel Cells
Authors: Fernandez Zuvich Afra , Larrondo Susana , Saleta Martin , Napolitano Federico , Caneiro Alberto , Troiani Horacio , Lamas Diego German , Arce Mauricio Damian , Serquis Adriana , Soldati Analia
Published: February 2020

The development of Intermediate-Temperature Solid Oxide Fuel Cells (IT-SOFC) as high efficient energy conversion systems requires the optimization of its components: cathode, anode and electrolyte.  CeO2-based materials present excellent catalytic properties for the oxidation of H2 and CH4 fuels. Through the incorporation of metals oxides (Gd2O3, Sm2O3, Y2O3) in the CeO2 lattice the ionic conductivity can be significantly improved, becoming good electrolytes, particularly (Ce,Gd)O2-
d (CGO) compound. Furthermore, the addition of metals such as Ni or Cu enhances the material electronic conductivity, thus enabling them as efficient IT-SOFC anodes [1].

It is recognized that the polarization characteristics of Ni-CGO cermet anodes are strongly affected by their preparation conditions, which determine the microstructure and morphology of these materials. For that reason, further exploration of new microstructures has to be done in order to improve anode performance [2]. In a previous work we presented a new modified sol-gel method to incorporate Ni into the precursors solution, which produced a powder with a very homogeneous Ni distribution resulted in smaller particle sizes with narrower distribution than the commercial cermet [3]. In this work, we present a study of the performance of these anodes sintered at different temperatures for the hydrogen oxidation reaction. The electrode polarization resistance is evaluated through Electrochemical Impedance Spectroscopy (EIS) on symmetric cells, while the cell power of Ni-CGO/CGO/LSCF configuration is measured in a home-made equipment. The structural and morphological characterization are analyzed by X-Ray Diffraction and Transmission and Scanning Electron Microscopy, respectively. The study of the oxidation state and coordination of Ce and Ni in these cermets, simulating in-operando conditions, was performed using synchrotron Dispersive X-ray absorption spectroscopy (DXAS) technique. The correlation between the electrochemical behavior, electronic properties and microstructural characteristics will be discussed.

[1] W.C. Chueh, Y. Hao, W. Jung and S.M. Haile, Nature Materials 11 (2012) 155-161.

[2] Z. Liu, B. Liu, D. Ding, M.F. Liu, F.L. Chen, C. R. Xia, Journal of Power Sources 237 (2013) 243-259.

[3] A. Fernandez Zuvich, A. Soldati, S. Larrondo, M. Saleta, D.G. Lamas, L. B. Baqué, A. Caneiro and A. Serquis, ECS Transactions  64 (2014) 233-240.
Effect of Cobalt-Doped Electrolyte on the Electrochemical Performance of LSCFO/CGO Interfaces
Authors: Baqué Laura Cecilia , Padmasree Karinjilottu , Ceniceros Reyes Mónica Aimee , Troiani Horacio , Arce Mauricio Damian , Serquis Adriana , Soldati Analía
Published: February 2020

Solid oxide fuel cells (SOFCs) are efficient and environmental friendly devices which directly convert hydrogen and fosil fuels into electric energy. The operation temperature of these devices is mainly determined by the nature of its electrolyte. Conventional SOFCs are composed of yttria based electrolytes requiring operation temperatures around 1000ºC, whereas ceria based electrolytes have allowed reducing the temperature down to the 500-700ºC range. Nevertheless, these materials require sinterization temperatures in the 1300-1600ºC range to achieve complete densification, which produces very large grain sizes and poor mechanical properties. In the last years, several research works have demonstrated that the addition of transition metals oxides is effective in improving the densification and thus, lowering the sinterization temperatures of ceria based electrolytes. Such research works, generally focalize on how the addition of these sintering aids affect the properties (density, sintering temperature, conductivity) of the electrolyte. Nonetheless, it is well established that SOFC performance depends not only on the characteristics and performance of its individual components (i.e. cathode, electrolyte and anode) but also on their interfaces. Hence, a comprehensive study on how the addition of electrolyte sintering aids affects the cell performance is still missing.

In this work, we have studied La0.4Sr0.6Co0.8Fe0.2O3-d/Ce0.8Gd0.2O2-d/La0.4Sr0.6Co0.8Fe0.2O3-d symmetrical cells containing Ce0.8Gd0.2O2-d electrolytes synthesized by a molten salt route [1] with and without Co oxide doping. La0.4Sr0.6Co0.8Fe0.2O3-d nanostructured cathodes with high electrochemical performance [2,3] were deposited by spin coating techniques on both sides of the electrolyte pellets. The assemblies were studied by Electrochemical Impedance Spectroscopy (EIS), X-Ray Diffraction and Transmission and Scanning Electron Microscopy. EIS results indicate that both electrolyte and cathode impedance response are affected by the addition of the sintering aid to the electrolyte, despite using cathodes with similar microstructure according to SEM observations. A possible explanation for such behavior was founded during TEM characterization which reveals the formation of columnar structures at the cathode/electrolyte interface of the cell with cobalt-doped electrolyte. These columnar structures would negatively affect the cell performance hindering the diffusion of oxide ions from the cathode to the electrolyte.

[1] Mendoza-Mendoza E., Padmasree K., Montemayor S. and Fuentes A., J Mater Sci 47 (2012) 6076-6085.

[2] Baqué L, Caneiro A, Moreno MS, Serquis A., Electrochem Commun 10 (2008) 1905-1908.

[3] Soldati A.,  Baqué L.,  Troiani H.,  Cotaro C., Schreiber A., Caneiro A, Serquis A. Int J Hyd Energy 36 ( 2011) 9180-9188.
(Invited) Potential Energy Landscape for Oxygen Vacancy Dynamics in Doped Ceria
Authors: Kim Sangtae
Published: February 2020

Ionic transport plays a key role in controlling a wide range of properties and processes important for applications such as catalysis, batteries, fuel cells, environmental sensors, and resistive random-access memory devices. For most of these applications, an outstanding need exists for ionically conducting materials with enhanced transport of charge carriers and consequently high conductivities at lower temperatures. The two most important factors that control the hopping transport of charge carriers are their concentration and mobility in the host structure. The effect of charge-carrier concentration on electrical conductivity has been investigated extensively over the last several decades in relation to transport in the bulk as well as near interfaces, and it is well-understood within the framework of microscopic and mesoscopic models of ionic conduction. In contrast, a fundamental understanding of the controlling factors for charge-carrier mobility in ionic conductors remains rudimentary at best, and it necessitates a detailed knowledge of the interactions between the mobile charge carriers and the potential energy landscape (PEL) provided by the immobile host lattice. In this contribution, we provide a comprehensive picture of the PEL for oxygen-vacancy migration in acceptor doped CeO2 based on the results from a combined application of nuclear magnetic resonance (NMR) and electrochemical impedance spectroscopy (EIS). We also present the results of our investigation on the effect of crystallite size on cation coordination environments and oxygen vacancy ordering in micro- and nanocrystalline doped ZrO2 and CeO2 by using high-resolution magic-angle-spinning NMR spectroscopy.