bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) Improving Conducting and Insulating Interfaces to 2D Materials
Authors: Franklin Aaron D
Published: February 2020

Bulk semiconductors remain the backbone of the ongoing technological revolution and continue to enhance the computational ability and accessibility of data. However, as the range of possible applications for electronics grows, so does the realization that there are distinct limits to what bulk materials can do. 2D layered materials, including graphene and transition metal dichalcogenides (TMDs), present nearly limitless possibilities for new electronic devices. While these materials are very encouraging based on their intrinsic properties, establishing conducting and insulating interfaces (needed to form useful 2D electronic devices) compromises the desirable intrinsic behavior. In this talk, recent advances in establishing scalable, high-performance conducting interfaces to semiconducting 2D materials will be presented. This includes the use of an in situ ion beam source to modify 2D materials during the deposition of contact metals in 2D field-effect transistors. The establishment of pure edge contacts using this approach is able to yield devices with ultimate scalability. In addition to the improvement of conducting interfaces, the ability to nucleate ultrathin insulating materials on the inert surface of various 2D materials will also be discussed. Using plasma-enhanced atomic layer deposition, sub-5 nm films can be realized without hampering the performance of few-layer thick 2D crystals or requiring the inclusion of a buffer layer. In some cases, the electronic properties of the 2D material are preserved and even enhanced at the device-level. Improvement of these critical conducting and insulating interfaces to 2D materials has wide-ranging value for their integration into new electronic and optoelectronic devices.
(Invited) Advancing Compound Semiconductors through Heterogeneous Integration
Authors: Douglas Erica A. , Hollowell Andrew E , Jordan Mathew , Friedmann Thomas , Wood Michael G , Michael Christopher , Arrington Christian L. , Musick Kate , McClain Jaime
Published: February 2020

Heterogeneous integration (HI) has been developed for Si-based industrial and R&D applications to minimize form factors while enabling such components as logic chiplets and memory into a single architecture. Sandia National Laboratories has a wide range of on-going research activities in developing HI of various semiconductor materials and devices such as optoelectronics and microelectromechanical systems, utilizing a variety of HI technologies, including fine pitch indium thermocompression bonding, solder reflow attachment, and Cu direct bond interconnects (DBI). This talk will provide an overview of present-day capabilities, technologies currently being worked on needed to enable future HI of compound semiconductors, as well as an overview of the progress to-date of Sandia’s research.

Acknowledgment: This work was supported in part by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
Model for Dislocation Pinning Interactions in Ingaas/GaAs (001) Heterostructures
Authors: Kujofsa Tedi , Ayers John E
Published: February 2020

Metamorphic semiconductor devices often contain abrupt mismatched interfaces with relatively high densities of misfit dislocations. In such cases there has been some experimental evidence for pinning interactions, whereby a gliding threading dislocation associated with a misfit dislocation along a <110> type direction is impeded by interaction with a misfit dislocation along the orthogonal <110> type direction. This impediment to glide will tend to reduce the extent of strain relaxation, and this effect is expected to be significant in thin layers for which the impediment is sufficient to arrest the motion of the gliding dislocation altogether. In this paper we present a practical model for pinning interactions which can be used to extend the usefulness of plastic flow models down to the pseudomorphic limit, and we compare this model against experimental results for InGaAs/GaAs (001) heterostructures.
Intercalation Pseudocapacitance in M-NbV-O Compounds as Anodes Enabling Fast-Charging in Lithium-Ion Batteries
Authors: Carneiro Frederico Augusto , Monteiro Robson
Published: February 2020

M-NbV-O compounds have attracted a lot of attention from the research community due to their merits in improving active material performance in lithium-ion batteries by enabling fast-charging due to the phenomenon of intercalation pseudocapacitance, where there are no solid-state diffusion limitations. TiNb2O7
and Ti2Nb10O29, both known as TNO, are an evolution of Li4Ti5O12 (LTO), a material known for its fast-charging capabilities and safety due to its operation above 1.0 V vs. Li/Li+. The higher operating potential prevents possible lithium deposition, resulting in the ability to fast charge without safety concerns. TNO overcomes LTO’s low energy density deficiency with a high theoretical capacity slightly larger than graphite’s (372 mAh/g), while retaining fast charge and safety benefits. Nb2O5, Nb16W5O55 and Nb18W16O93 are similar to TNO in that they exhibit high energy density and high rate capability but have the additional benefit of not requiring nanoscaling or nanostructuring for optimal performance.
Novel Nanocomposites for High-Performance Supercapacitors
Authors: Shim Jae-Jin , Mohapatra Debananda , Dhakal Ganesh
Published: February 2020

The use of renewable energy with unsteady supply has been gradually increased, requiring energy storage for steady supply of energy. Carbonaceous materials such as graphene, carbon nanotubes, and carbon fibers have been studied for capacitive energy storage purposes. On the other hand, materials based on metal oxides/sulfides have been studied for electrochemical charge storage. Though the carbonaceous materials carry large surface area and high electrical conductivity, they have limited energy density. However, the metal oxides/sulfides perform electrochemical reactions, resulting in high energy density. Composites of carbonaceous materials and metal oxides/sulfides exhibit the synergistic effects on energy density and power density, improving the supercapacitor performances tremendously. We have tried several different carbon materials, metals, and particle sizes, synthetic methods, morphologies, etc., for better performances up to 4000 F g-1. Recently, we have developed carbon nano-onions and doped carbon nano-onions in inexpensive way for making remarkably capacitive materials.

Figure 1
Pseudocapacitive Storage in Nanolayered Ti2NTxmxene Using Mg-Ion Electrolyte
Authors: Djire Abdoulaye , Bos Andre , Liu Jun , Zhang Hanyu , Miller Elisa M. , Neale Nathan
Published: February 2020

Electrochemical supercapacitors are hybrids of a capacitor and battery that rely on materials capable of storing charges via faradaic redox reactions or pseudocapacitive reactions in addition to conventional electrostatic double-layer charge storage. MXenes, a relatively new class of two-dimensional (2D) transition metal carbides and nitrides, are ideal candidates for supercapacitors due to their high electronic conductivity, high surface area, and ability to store charges viapseudocapacitive mechanisms. Nitride MXenes such as Ti2NTxare predicted to have higher pseudocapacitance than carbide MXenes but have not been explored experimentally. Here, we report on the synthesis, characterization, and pseudocapacitive charge storage mechanism in the Ti2NTxnitride MXene. Successful formation of nanolayered Ti2NTxMXene is characterized by XRD, SEM, and N2physisorption analyses. The identity of the surface terminating groups Txare assigned to primarily O and/or OH based on Raman, FTIR, and STEM-EELS. When tested in various electrolytes, the nanolayered Ti2NTxMXene exhibits pronounced reversible redox peaks and high areal capacitances (~1350 uF cm–2 in 1 M MgSO4aqueous electrolyte) well exceeding that expected from a double-layer charge storage (~50 uF cm–2) indicating that charge is stored in the material viaa pseudocapacitive mechanism. We report a trend in the capacitance as a function of cation as follows: Mg2+> Al3+> H+> Li+> Na+> K+, that matches theoretical predictions. Remarkably, nanolayered Ti2NTxMXene exhibits >200 F g–1 capacitance over a 1.0 V range in the Mg-ion electrolyte, and the capacitance increases to 160% of its initial value after 1000 cycles owing to the two-electron reaction and the unique multilayer adsorption behavior of the Mg2+cation on the Ti2NTxMXene. These findings identify Ti2NTxMXene as a new pseudocapacitive material that possesses high capacitance and wide working voltage in a safe and environmentally friendly Mg-ion electrolyte.
基于木质素磺酸盐衍生石墨烯量子点/石墨烯水凝胶的柔性超级电容器电极
Authors: Cheng Chen , Jin Xiaojuan
Published: February 2020

The emerging flexible supercapacitor has drawn great attentions, challenges still exist in the search for renewable, stable electrode materials. In this work, biomass lignosulfonates (LS) from waste liquor of sulfite pulping were used to produce graphene quantum dots (GQDs) by a two-step method. The LS-derived GQDs were synthesized with a large number of N, S, O-containing functional groups, which can participate in Faradaic reactions to provide improved pseudo capacitance and enhance structure stability of the supercapacitor electrode. Thus, the functionalized GQDs were used to decorated graphene hydrogels (GH-GQD). As expected, the prepared flexible GH-GQD electrode has excellent electrochemical performance and stability. It exhibits an impressive specific capacitance of 452 F g-1 at a current density of 0.5 A g-1, which is nearly double than that of pure GH (231 F g-1). Moreover, the GH-GQD electrode has a high cycle stability (89.0% capacitance retention after 10000 charge/discharge cycles) and mechanical flexibility (93.3% capacitance retention when bent to 180°). This study broadens the high-valued utilization of biomass waste and provides new ideas for the development of renewable and environment-friendly flexible energy storage devices.
Influence of Dopants on the Electrochemical Properties of Conducting Polymers As Electrodes for Supercapacitors
Authors: Zhang Miaomiao , Nautiyal Amit , Du Haishun , Zhang Xinyu
Published: February 2020

Conducting polymers are class of functional polymers having conjugated chain structures. Among them, polyaniline (PAni) and polypyrrole (PPy) are well known for their excellent electrochemical properties as electrode materials of supercapacitors. To achieve high conductivity in conducting polymers, doping is necessary. In conducting polymers, dopants tend to be randomly distributed within the conjugated polymer affecting the structural and electrical properties of conducting polymers and as a result the electrochemical performance are strongly affected by the resulting structural and electronic disorder.

In this paper, the influence of dopants: hydrochloric acid (HCl), perchloric acid (PrA), sulfuric acid (SA), p-toluene sulfonic acid (p-TSA), phosphoric acid (PhAc) and phytic acid (PA), on electrochemical performance of PAni and PPy were evaluated, respectively. For better comparation and understanding of the effect of dopants on the electrochemical properties of conducting polymers, the morphologies and structures of PAni and PPy obtained by using different dopants were characterized by SEM, TEM, XRD, TGA, etc. Their electrochemical performance as electrode materials for supercapacitors were evaluated by using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS). Results showed that, for both PAni and PPy, a smaller size of dopants helps to form a more compact coating with better conductivity in which ions could easily diffuse in and out of the conducting polymers. Thus, better electrochemical performance and higher capacitance could be obtained. However, during charge-discharge process the conducting polymers with smaller dopants were much easier to swell and shrink that makes their degradation faster. On the other hand, bigger size dopants showed better stability at higher scan rates and current densities. This work demonstrated that the size of dopants played a big role on the electrochemical performance and stability of conducting polymers when they were used as supercapacitor electrodes.
Integrating Pseudocapacitive Function into Air-Cathodes with 3D-Wired Nanoscale MnOx: Enabling Pulse-Power for Zn–Air Batteries
Authors: Chervin Christopher N. , Long Jeffrey W. , Sassin Megan B. , Parker Joseph F. , Hopkins Brandon J. , Rolison Debra R.
Published: February 2020

Metal–air batteries have the highest specific (packaged) energy among common battery chemistries (over 400 Whkg–1 for Zn–air) with the added benefits conferred by aqueous electrolytes of safety and affordability. But air-dependent batteries are power-limited by catalytic and mass-transfer restrictions associated with reduction of molecular oxygen at the air cathode—an electrode that must support multiple tasks: O2 transport, ion transport, electron conduction, and electrocatalytic reactivity. To address this power limitation, we have redesigned the air cathode using an architectural and nanoscopic perspective that marries complementary characteristics of batteries and electrochemical capacitors. We demonstrate that air cathodes comprising fiber paper–supported carbon nanofoams modified with conformal nanoscale coatings of Na+-birnessite-type MnOx (10–20 nm thick) exhibit dual functionality: (i) oxygen reduction to sustain long-term energy delivery and (ii) faradaic pseudocapacitance associated with Mn(III/IV) redox to provide intermittent, farads-worth discharge pulses over 10s of seconds [1,2]. Manganese (III) sites in the post-pulsed oxide spontaneously recharge to Mn(IV) in the presence of oxygen from air, making the oxide ready and available for subsequent pulse-power discharge.

[1] Redesigning air cathodes for metal–air batteries using MnOx-functionalized carbon nanofoam architectures. C.N. Chervin, J.W. Long, N.L. Brandell, J.M. Wallace, and D.R. Rolison, J. Power Sources
2012, 207, 191–199.

[2] Dual-function air cathode nanoarchitectures for metal–air batteries with air-independent pulse power capability. J.W. Long, C.N. Chervin, N.W. Kucko, E.S. Nelson, and D.R. Rolison, Adv. Energy Mater.
2013, 3, 584–588.
Edge-Dominated Graphene Cluster Wrapped Metal Nanoparticles on Carbon Fibers for AC-Line Filtering Electrochemical Capacitors
Authors: Li Wenyue , Fan Zhaoyang
Published: February 2020

Ultrafast electrochemical capacitors running at frequencies in the range of hundreds or even kilo hertz have attracted considerable interest for AC-line filtering, pulse power generation and other applications. The electrode structure design, by facilitating electron conduction in the electrode itself and electrolytic ion transportation in the pores of electrode material, as well as providing an easily accessible large surface area, plays key roles in determining the frequency response and achievable capacitance of the electrochemical capacitor. Herein, we present studies of edge-dominated graphene cluster wrapped metal nanoparticles for design of ultrafast electrochemical capacitors. CoNi nanoparticles were loaded on cellulose fiber sheets, which were treated in CH4/H2 plasma for a few minutes. The high temperature plasma pyrolyzed cellulose fiber sheets into carbon fibers as freestanding electrodes, while high-density edge-oriented graphene flakes wrapped around metal nanoparticles to intimately bond with carbon fibers for a minimized electrode resistance. The densely packed and edge-oriented graphene clusters offer a large but easily accessible surface area. Such an electrode showed enhanced areal capacitance of 0.9 mF and ultrafast frequency response with a phase angle of -82.5o at 120 Hz. Electrode fabrication, nanostructure characterization, electrochemical studies and preliminary device applications will be presented.
Fabrication of ZIF-67 Derived Co3S4-WS2 Heterostructures for All-Solid-State Symmetrical Supercapacitors
Authors: Shrivastav Vishal , Sundriyal Shashank , Tiwari Umesh K. , Kim Ki Hyun , Deep Akash
Published: February 2020

Development and modification of the existing material for renewable energy area is gaining its significance however a long way to get commercialization. Transition metal dichalcogenides (TMDs) are gaining interest rapidly in the field of supercapacitors due to their 2D layered morphology, however loses interest for long cyclic stability. Herein we employed hollow Co3S4 microspheres using nitrogen rich ZIF-67 as a precursor. Furthermore, these hollow Co3S4 microspheres are insitu modified by covering with WS2 nanostructures to enhance its energy density. The obtained Co3S4/WS2 composite when employed as an electrode, delivers the specific capacitance of 412.7 F/g at the current density of 1 A/g. Furthermore, the Co3S4/WS2 composite electrodes are used to fabricate an all solid-state symmetrical supercapacitor device, exhibiting a wide potential window of 2 V. The assembled device delivers a very high energy density of 70.7 Wh/kg along with a high-power density of 10.2 kW/kg. The excellent electrochemical performance with 92 % capacity retention proves its potential toward the practical application. The high value of specific capacitance and splendid cycle stability is attributed to the synergy of high surface area ZIF-67 derived Co3S4 microsphere with 2D networked WS2 material.
(Invited) Synthesis of Heavy-Metal-Free Quantum Dots and Their Applications
Authors: Chen Hsueh-Shih
Published: February 2020

Recently, low-Cd-content colloidal quantum dots (QDs) have been developed to replace the conventional core/shell CdSe/ZnS QDs in lighting and displays. Due to concerns of heavy metals including Cd and Pb to the environment, scientists are also developing new synthetic methods for heavy-metal-free QDs. Among them, InP is one of the promising candidates that shows fairly high performance in both optical property and material stability. However, synthesis of InP QDs generally requires an extremely toxic phosphine precursor such as PH3 or tris(trimethylsilyll)phosphine, which spontaneously ignites in air and can cause severe burns for human tissues. In this report, we will compare synthetic processes and material properties of InP QDs synthesized by different phosphine precursors including PH3, tris(trimethylsilyll)phosphine and aminophosine. In particular, a facile synthetic process of InP QDs derived from aminophosine will be presented. Application of the prepared InP QDs in lighting and environmental sensing will be also investigated in this study.
(Invited) Multifunctional Triboelectric Nanogenerators for Self-Powered Electronics
Authors: Kim Sang-Woo
Published: February 2020

Energy harvesting systems based on triboelectric nanomaterials are in great demand, as they can provide routes for the development of self-powered devices which are highly flexible, stretchable, mechanically durable, and can be used in a wide range of applications. Our recent research interest mainly focuses on the fabrication of high-performance triboelectric nanogenerators (TENGs) based on various kinds of nanomaterials. Flexible TENGs exhibit good performances and are easy to integrate which make it the perfect candidate for many applications, and therefore crucial to develop. In this presentation, I firstly introduce the fundamentals and possible device applications of TENGs, including their basic operation modes. Then the different improvement parameters will be discussed. As main topics, I will present a couple of recent achievements regarding highly stretchable transparent flexible TENGs, textile-based wearable TENGs, highly robust and efficient TENGs with multifunctional materials, etc. The recent research and design efforts for enhancing power generation performance of TENGs to realize self powering of wearable and body-implanted electronics will also be discussed in this talk. Finally I am going to introduce a 2D materials-based tribotronics for possible future application toward tactile sensors, robots, security, human-machine interfaces, etc.
(Invited) Scalably-Nanomanufactured Tellurene: An Emerging 2-D Multifunctional Material
Authors: Wu Wenzhuo
Published: February 2020

The reliable production of atomically-thin crystals with tailored properties is essential for exploring new science and implementing novel technologies in the 2-D limit. However, ongoing efforts are limited by the vague potential in scaling-up, restrictions on growth substrates and conditions, small sizes and instability of synthesized materials. In this talk, I will discuss our recent progress in the discovery and production of tellurene (2-D form of elemental tellurium) with an intriguing chiral-chain structure. The solution-synthesized tellurene crystals exhibit process-tunable thickness from a monolayer to tens of nanometers, lateral sizes ~ 100 mm, and can be transferred to designer substrates. I will further discuss our findings in 2-D tellurene’s material properties, as well as our efforts in the prototypical device explorations with the acquired fundamental understandings of tellurene’s properties. Our results show that the air-stable tellurene, as an emerging 2-D material, exhibits a plethora of intriguing physical properties appealing for applications in electronics, optoelectronics, energy, sensors, and quantum devices.
(Invited) Multi-Functional Memristive Transistors Based on Van Der Waals Heterostructures
Authors: Park Hyunik , Kim Jihyun
Published: February 2020

In recent years, the demand for biologically inspired systems for diverse applications, such as object recognition, autonomous vehicles, and humanoid robot, has been continuously increasing. Particularly, neuromorphic computing, imitating the energy-efficient neural network in the human brain, has become powerful candidate that can overcome the limitations of the conventional von Neumann architecture, named ‘von Neumann bottleneck’. To realize neuromorphic computing, a concept of crossbar array of memristors was suggested. A memristor is a two-terminal circuit element that can store binary information within the active layer in a nonvolatile fashion by the electrical stimulation. In addition to its nonvolatility, it has received a remarkable attention for future memory and neuromorphic applications owing to its high endurance, fast switching speed, small form factor, low energy consumption, and multi-state switching capability. Nevertheless, two-terminal memristors cannot fully provide the complex functions of neurons, and thus, they are still not enough to operate complex and massive datasets. In this regard, tremendous efforts have been dedicated to the development of miscellaneous functional circuit elements for neuromorphic and in-memory computing. One way to enable this is combining the memristor with various functional circuit elements such as capacitor, inductor, or transistor.

In our study, we demonstrated a programmable and multi-functional memristive transistor by integrating a hexagonal boron nitride (h-BN) memristor and a molybdenum disulfide (MoS2) transistor. The device was fabricated by sequential stacking semiconducting MoS2, semi-metallic graphene, and insulating h-BN on a SiO2/p++-Si substrate in order, followed by deposition of contact metal. The memristive transistor consisted of a vertical graphene/h-BN/Ti/Au memristor and a lateral graphene/MoS2/Ti/Au transistor as shown in the figure. Mechanically exfoliated h-BN-based memristors showed stable resistive switching behavior as the conducting filament(s) formed and annihilated through the breakdown path within the h-BN layer. The behavior of the memristive transistor can be programmably modulated by switching the resistance state of the memristor, and it showed extremely low standby power consumption of ~16 pW. Notably, multi-state resistance and tunable current on/off ratio can be attained by controlling the synaptic weight of the device. This novel architecture can also perform logical operation by changing the electrical circuitry. The proposed device provides a higher degree of functionality in both logic and memory applications, thereby a step toward the achievement of energy-efficient in-memory computing is expected. The details of the experimental results will be presented.

Figure 1
(Invited) A Leaf-Inspired Photon Management Scheme for Graphene/Silicon Solar Cells
Authors: Roy Tania
Published: February 2020

Graphene/silicon (Gr/Si) Schottky junction solar cells have attracted much attention due to the ease and low cost of fabrication, along with the lucrative properties of high electron mobility, transparency and mechanical flexibility of graphene as a transparent conducting electrode.1 Utilizing its inherent mechanical flexibility, graphene can be integrated with thin flexible crystalline Si substrates opening up a new regime of applications in flexible and wearable electronics. Reducing Si absorber thickness below 50 um offers advantages of reduced material cost, along with mechanical flexibility and light weight.2 But Si at such thicknesses suffers from low photon absorption in the solar spectrum. To compensate for the low light absorption in such thin substrates, light management schemes become essential. Light trapping in Gr/Si solar cells is enabled by engineering the Si surface to form nanopillars, nanowires etc., which decreases the reflection loss and allows more light to couple in to the Si substrate. The structured Si absorber increases surface area and surface recombination, which is detrimental to the solar cell efficiency. Thus, it is imperative to use a light-trapping scheme devoid of Si structuring to enhance the photo-conversion efficiency.

We present an all-dielectric light-trapping scheme on planar Gr/Si Schottky junction solar cells with the use of bottom layer of titania spheres and top layer of silica spheres.3 An optimal Si thickness coupled with an optimized light-trapping scheme leads to efficient electron-photon harvesting. The photo-conversion efficiency of a 20 um thick nanosphere-decorated Gr/Si solar cell improves to 9%, which is 1.3x higher than the pristine cell’s PCE of 7%. FDTD simulations are performed for optimizing the diameter of nanoparticles in each of the layers. The ratio of size of nanoparticles in the top to bottom layer plays a crucial role in advanced light management. An optimized structure of silica spheres, having diameter larger than that of titania spheres, suppresses reflection over wide angles of incidence and increases absorption in active Si layer over AM1.5G solar spectrum. The non-absorbing dielectric spheres can be easily realized by the well-known Stober technique. Additionally, the photovoltaic characteristic of the laminated solar cell shows negligible change after several bending cycles having bend radius ranging from 5 mm to 10 mm. After continuous bending and straightening, the ultra-thin solar cell can retain its performance, revealing the excellent stability and flexibility of the device. Such simple, low-cost light trapping schemes are universal in nature, devoid of recombination losses and are potentially viable for any solar cell technology.

References

1 B Li, et al. Adv. Mater. 22, 2743 (2010).

2 K E Petersen, Proc. IEEE 70, 420 (1982).

3 S. Das, et al. Nano Energy 58, 47 (2019).
Rutile to Cubic Phase Transformation Induced By Electron Beam
Authors: Tian Mengkun , Zhang Lizhi , Mahjouri-Samani Masoud , Sachan Ritesh , Yoon Mina , Puretzky Alexander A. , Geohegan David B. , Eres Gyula , Duscher Gerd
Published: February 2020

Cubic TiO2 was predicted to be a great visible light absorption material partially due to the smallest bandgap among all the TiO2 polymorphs. The cubic TiO2 can be synthesized using the common TiO2 polymorph i.e. anatase as a precursor, but it mixes with other metastable phases as byproducts including baddeleyite, TiO2 (OI) and TiO2 (OII). The traditional syntheses of cubic TiO2 have to be performed at a high-temperature and high-pressure conditions. Such harsh condition limits the potential applications of the cubic TiO2.

In this work, cubic TiO­2 was grown on the surface of a rutile nano-template at room temperature in vacuum. The rutile precursors were made by pulsed laser deposition (PLD). As shown in Figure 1, the growth of cubic TiO2 was induced by the electron beam in an aberration corrected scanning transmission electron microscopy (STEM). Monochromated electron energy loss spectroscopy (EELS) was used to investigate the phase transformation under the electron illumination. The STEM images show that lattice matching relationships are (001)R // (001)C and [100]R // [100]C with only 4.6% lattice mismatch, where R and C refer to rutile and cubic, respectively. Density functional theory (DFT) reveals that rutile-to-cubic TiO2 transformation can be highly favorable if the strain is > 10% at the interface. The driving forces originate from three resources: (1) lattice mismatch at the interface; (2) defective structure of the black rutile; (3) oxygen vacancies created by the electron beam. In addition to knock out oxygen, the electron beam also provides energy for the Ti atomic displacement in the rutile during phase transformation.

Figure 1. Phase transformation from rutile to cubic TiO2 by electron beam exposure.

Figure 1
High-Quality GaN Films Grown on AlN/Sapphires for Power Electronic Device Applications
Authors: Feng Xiajia , Yun Yu
Published: February 2020

High quality GaN films have been successfully grown on multi-AlN/sapphire templates by metal organic chemical vapor deposition system. The Hall mobility and the carrier concentration of 720 cm2/Vs and 6.7x1016 cm-3 at 300K, respectively, along with low dislocation density of 4.1x109 cm-2 have been achieved. The X-ray rocking curve full-width at half-maximum were 160 and 290 arcsec for (0004) and (20-24) reflection planes also obtained, respectively. Besides that, the atomic force microscopy images showed smooth surface morphology and a higher intensity near the band edge was also observed by photoluminescence measurement result.
Growth of AlGaN on GaN/AlN/Sapphire Substrates for Power Electronic Application
Authors: Bun Kheam
Published: February 2020

The effect of a thin AlN layer inserted between AlxGa1-xN/GaN heterostructures grown on sapphire substrates was investigated for possible application in power electronic devices. The heterostructures with two different Al compositions (0.35 and 0.49) were applied to study. After growing a thin AlN (~21nm) on the buffer GaN/sapphire substrate, then a thick AlxGa1-xN/GaN heterostructure were grown and investigated the Al mole fractions. Low rocking curves were also achieved with 0.35 and 0.48 for the 0.3 and 0.49 Al compositions, respectively. The experimental results show that with an increasing of Al composition, the crystallinity is also improved with lower surface roughness. The AlxGa1-xN/GaN heterostructures with 0.35 and 0.49 Al compositions were also investigated the electrical characteristics to confirm that they are suitable for development in optoelectronic and power electronic devices such as HEMTs, LED, etc.
Prediction of Local Water Distributions and Performance of PEMFC using Pore Network Modeling
Authors: Alam Shahriar , Medici Ezequiel , Tajiri Kazuya , Allen Jeffrey S
Published: February 2020

Oxygen transport and water drainage in the cathode side Catalyst Layer (CL) affect the performance of Polymer Electrolyte Fuel Cells (PEFC). A unique mathematical model has been developed to track local fluxes of water, both liquid in pores and lambda in the ionomer, through a fuel cell electrode. The local pressure, temperature, and heat generation is also predicted. The role of pore size distribution, ionomer to carbon (I/C) ratios, RH, and operating conditions on fuel cell performance has been studied. The results and polarization curves provide insight as to how local water balances affect electrode performance.
Understanding the Degradation of Pt Electrocatalysts in Proton Exchange Membrane Fuel Cells By 3D Identical Location STEM
Authors: Yu Kang , Li Chenzhao , Xie Jian , Ferreira Paulo Jorge
Published: February 2020

Proton exchange membrane fuel cells (PEMFCs) are promising energy conversation devices due to their high power/energy density, high efficiency and the absence of carbon dioxide emission. Yet, one of the largest obstacles for the commercial application of PEMFC is the degradation of platinum group metal (PGM)-based electrocatalysts during long term operation. Four degradation mechanisms have been widely reported, namely, 1) Ostwald ripening; 2) particle migration and coalescence; 3) carbon corrosion and 4) Pt dissolution and reprecipitation in the membrane. However, the role played by each degradation mechanism is not fully understood.

In this work, scanning transmission electron microscopy (STEM) tomography coupled with identical location TEM (IL-TEM) are utilized to delineate the roles of each degradation mechanism for Pt catalysts on Vulcan XC72 carbon support. Compared with conventional TEM, 3D STEM tomography is capable of identifying the following: 1) interparticle distances; 2) 3D structure of carbon black, 3) specific positions of Pt catalysts on carbon support; and 4) shapes and morphologies of Pt catalysts[1]. In addition, IL-TEM allows a comparison of Pt catalysts and carbon support of the same region of interest (R.O.I), at the beginning of life and after potential cycling, which provides direct insight into the effects of potential cycling on the degradation of Pt catalysts.

Figure 1(a-b) shows the 3D illustration of Pt catalysts on the Vulcan XC72 carbon support before and after potential cycling, respectively. We observe that volume shrinkage is insignificant in the bulk of the carbon black particles and that there are no Pt catalysts falling off after potential cycling (0.6V-1.0V vs RHE, 6000 cycles). In addition, we note the appearance of newly-formed Pt nanoparticles or ionic clusters (<1nm in diameter). A careful inspection regarding the formation mechanisms of these particles shows that particle migration and coalescence is not involved. Instead, these newly-formed particles are likely due to Pt dissolution and re-precipitation. The 3D IL-TEM experiments also show the coalescence of Pt NPs. However, the coalescence is unlikely due to long migration distances (>0.5nm). Instead, the growth of Pt nanoparticles result from Ostwald ripening and subsequent sintering.

In summary, 3D STEM tomography coupled with IL-TEM (~1nm spatial resolution) provides unique insight into the roles of each degradation mechanisms for Pt electrocatalysts on Vulcan XC72 during potential cycling.

Reference:

[1]Horn, S. Y.; Sheng, W. C.; Chen, S.; Ferreira, P. J.; Holby, E. F.;Morgan, D. Instability of Supported Platinum Nanoparticles in Low-Temperature Fuel Cells. Top. Catal. 2007, 46, 285-305.

Figure 1
Impact of Porosity Gradients within Catalyst Layer and MPL of a PEM Fuel Cell on the Water Management and Performance: A Neutron Radiography Investigation
Authors: Kartouzian Dena , Mohseninia Arezou , Markötter Henning , Langner Philipp , Scholta Joachim , Manke Ingo
Published: February 2020

The influence of porosity modifications of either microporous layer (MPL) or the catalyst layer of a low temperature Polymer Electrolyte Membrane Fuel Cell (PEMFC) on the performance of the cell has been investigated in many studies. However, we have been focusing in our study on the interactive impact of this morphological modification of both cathode MPL and catalyst layer in the water distribution and transport inside the cell and consequently also on the performance of the cell.

In this study, three types of cathode MPLs with different porosity distributions and three type of membrane electrode assemblies (MEA) with different porosity of the catalyst layer are manufactured. Five one-cell fuel cells are investigated using combinations of those in-house made layers. To modify the porosity of the cathode MPL polymer particle in two sizes of 30 µm and 1.5 µm from company Chemisnow® are introduced into the carbon slurry, which are then evaporated out of the MPL during the sintering process. For the catalyst layers, 0.5 µm Polystyrene particles are mixed in the electrode slurry with variable weight fractions and then washed out of the dried MEA. A more detailed description of the production procedure of these layers will be demonstrated in the presentation.

Performance of fuel cells using these five combination of cathode MPL and electrode are measured using a Fuel Cell with 25 cm² active area and 3 serpentine channel flow fields. Neutron Radiography is used to investigate the water distribution dynamics and water content of the fuel cell. For this purpose, a specially designed single cell with 8 cm² active area and 3 serpentine channel flow fields is used. The measurements are obtained at two different humidity values on both cathode and anode inlet gases of RH=70% and RH=120% and different current densities. With an exposure time of 10 s, images with 12.4 µm pixel size are achieved.

The quantitative analysis of the water thickness in the cathode GDL and cathode electrode of 5 cells at RH=120% for an increasing current density is shown at an exemplary radiogram for water distribution inside the cell with our reference cathode MPL and electrode (without porosity modification). Also the Current (ICell) and Voltage (VCell) evolution of the same cell is shown in figure 1.

All cells have reached a water content plateau even before reaching the highest current density of 1A/cm². The cell with a porous catalyst layer and a double layer cathode MPL shows the highest water content on the cathode side of the cell, whereas the cell with a highly porous catalyst layer and a porous MPL shows the lowest water content of all cells, comparable to the water thickness observed for the cell without modified layers. Further results of both water content and performance dependent on porosity changes will be provided in the presentation.

Figure 1
Understanding Carbon Corrosion Damage to Fuel Cell Electrodes: Towards Reversal Invincible
Authors: Hu Leiming , Hong Bo Ki , Oh Jong-Gil , Litster Shawn
Published: February 2020

Polymer electrolyte membrane fuel cells (PEMFCs) have the advantage of low carbon dioxide emission and high efficiency over internal combustion engines in automobile applications1. However, durability of PEMFCs is one of the major obstacles for a wide range of applications. One such durability issue is the cell reversal caused by hydrogen starvation. It results in severe carbon support corrosion in the anode catalyst layer and dramatically decreases the fuel cell performances2. Material-based solutions to increasing the cell’s tolerance to reversal are preferable since it will not increase the system control complexity. Effective material-based solutions include using more corrosion-resistant carbon supports3 and adding water electrolysis catalysts to promote the oxygen evolution reaction (OER) over carbon corrosion4,5. However, these current solutions are still not resilient enough to guarantee long-term durability of PEMFCs under extreme hydrogen starvation conditions.

This present work has examined different anode configurations and their effectiveness in preventing cell performance degradation during hydrogen starvation events. Three different anode structures are studied and compared, including i) a conventional Pt/C gas diffusion electrode (GDE), ii) an anode made of platinum (Pt) black and gas diffusion layer (GDL), and iii) an anode made of Pt black, a thin titanium (Ti) protection layer (10 μm in thickness), and a GDL. Figure 1a shows the change in cell voltage of PEMFCs when hydrogen was cut off while keeping a constant current density of 0.2 A/cm2. During reversals, the Pt/C GDE anode and the anode made of Pt black and a GDL experienced a sudden voltage drop within 5 min which was attributed to severe carbon corrosion damage. In contrast, the cell with Ti protection layer between the Pt black and the GDL was stable for 60 min of reversal without a large voltage drop. As shown in Figire 1b, the cell voltage of the fuel cell with an anode made of Pt black, a Ti protection layer, and a GDL decreased less than 10 % even after the accumulated voltage reversal time of 240 min. Thus the anode with Ti protection layer is much more effective compared with the common reversal tolerant anode approach of adding OER catalyst (i.e., 50 wt% IrO2) to the anode5. Both hydrogen starvation tests and post-reversal electrochemical characterizations demonstrate that using a carbon-free anode with a corrosion-resistant barrier layer is highly effective in preventing cell reversal damage over long periods of time.

Figure 1
Co and Ce Cation Impact on PEM Fuel Cell Performance
Authors: Cai Yun , Gu Wenbin , Kongkanand Anusorn
Published: February 2020

PtCo/C is often used in the cathode in proton exchange membrane fuel cells to enhance performance. However, Co2+ cations can be released from the catalyst into the ionomer during operation. Additionally, Ce3+ cations are often introduced into the membrane as a chemical stabilizer. These cations replace protons of the ionomer acid cites and can redistribute during operation, resulting in local proton deficiencies, especially, in the cathode at high current densities [1-2]. While studies have shown severe performance loss due to Co2+ migration [3-4], the simultaneous impact of both Co2+ and Ce3+ has not been reported. Moreover, state-of-the-art cathodes often use high-surface-area carbon (HSC) with numerous PtCo particles within carbon internal pores. In absence of ionomer within these internal pores, it is unclear how the cations affect the proton activity and hence fuel cell performance. In this study, we performed controlled experiments where known amounts of Co and Ce cations were exchanged into the membranes and investigated the effect of cation concentration at different operating conditions. We also include the comparison of cation impact on electrodes with Vulcan and HSC supported cathode catalysts. Impedance spectroscopy was used to study the effect of cations on membrane and electrode proton resistances. The experimental results provide direct assessment of the cation impact on the fuel cell performance, and will also be used to improve our fuel cell performance model and provide direction for future fuel cell development.

This work was partially supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

References

T. A. Greszler, T. E. Moylan and H. A. Gasteiger, Chapter 49, Handbook of Fuel Cells – Fundamentals, Technology and Applications. Edited by Wolf Vielstich, Harumi Yokokawa, Hubert A. Gasteriger. Volume 6: Advances in Electrocatalysis, Materials, diagnostics and Durability. 2009 John Wiley & Sons, Ltd.

Y. Cai, J. M. Ziegelbauer, A. M. Baker, W. Gu, R. S. Kukreja, A. Kongkanand, M. F. Mathias, R. Mukundan, and R. L. Borup, J. Electrochem. Soc., 165, F3132–F3138 (2018)

S. Chen, H. A. Gasteiger, K. Hayakawa, T. Tada and Y. Shao-Horn, J. Electrochem. Soc., 157, A82 (2010).T. Okada, H. Satou, M. Okuno and M. Yuasa, J. Phys. Chem. B, 106 1267 (2002)

Y. Cai, A. Kongkanand, W. Gu and T. E. Moylan, ECS Trans., 69, 1047 (2015).
(Invited) Porous Materials for PEM Fuel Cells and Electrolyzers: Heterogeneous and Interfacial Considerations
Authors: Bazylak Aimy
Published: February 2020

While in practical application, polymer electrolyte membrane (PEM) fuel cells and electrolyzers exhibit macroscale length scales spanning centimeters to meters, the inner workings of these energy conversion devices fundamentally rely on microscale and nanoscale transport phenomena and porous materials to facilitate their passive operation. In order to maximize the power density and minimize the cost of the PEM fuel cell or electrolyzer, we must optimize our surface area to volume ratios at the reaction sites; thus, state-of-the-art catalysts exhibit diameters on the order of nanometers. At the same time, we must consider the effective delivery of reactants to these small- catalyst sites, and porous materials provide a means to impart homogeneous distributions of reactants, in theory. In addition to laterally distributing reactants across the planar catalyst layer, porous materials also facilitate multiple pathways for the competing reactant and product pathways to co-exist.

The PEM fuel cell and electrolyzer are composed of multiple porous materials, including the catalyst layer, microporous layer, and substrate. Commercial materials, whether by design or not, typically exhibit highly heterogeneous material and chemical properties. For example, in our past work, we extensively examined the heterogeneous microstructure and hydrophobicity of the gas diffusion layer (GDL). In order to reach cost targets for widespread commercial adoption, we must realize materials that enable more effective multiphase flow phenomena than what currently exists. Mass transport losses in PEM fuel cells and electrolyzers are both prohibitively significant. However, designing these materials requires the a priori knowledge of how the heterogeneous properties of the porous materials and their interfacial contacts influence electrochemical performance. These are factors that are currently not fully understood in the literature. In this work, I will discuss these critical design factors (heterogeneous porous materials and nature of interfacial contacts) and how they influence the flow and mass transport behaviour in PEM fuel cells and electrolyzers. I will also discuss the new materials we have designed and fabricated, informed by in-house numerical modelling and tested through a combination of in operando and ex situ X-ray and neutron beam characterization approaches.
Optimization of Open Pore Cellular Foams for PEM Fuel Cells
Authors: Nandjou Fredy , Haussener Sophia
Published: February 2020

Open pore cellular foams (OPCF) have attracted substantial attention for use as flow-field plates in proton exchange membrane fuel cells due to their potential to enhance the performance [1]. Moreover, they could help to mitigate membrane degradation due to the hygrothermal stresses, which are generally observed in fuel cells that use bipolar plates with the channel-rib structure [2].

In order to optimize the structure and operating conditions of OPCF for use in proton exchange membrane fuel cells, we performed coupled numerical and experimental studies. The numerical studies used a 2-D model that solved mass and momentum conservation and species transport, and coupled them to an electrochemical model in the catalyst layers for the source terms. Parametric studies indicated that some OPCF’s properties like thickness, porosity and pore diameter highly impact the global performance of the cell, and that the optimal properties could considerably be influenced by current density, humidity, and cell dimensions.

The results allowed for the design of optimized OPCF in different operating conditions, which were tested in fuel cells. It was observed that the optimized OPCF were very efficient in enhancing the performance of the cells at high currents, where mass transport limitations are generally observed. Promising solutions for heat and water management were explored.

[1] Ahrae Jo and Hyunchul Ju, Numerical study on applicability of metal foam as flow distributor in polymer electrolyte fuel cells. Int. J. Hydrogen Energy 43, 2018.

[2] Fredy Nandjou et al., Impact of heat and water management on proton exchange membrane fuel cells degradation in automotive application, J. Power Sources 326, 2016.