bluuebunny/crossref_metadata_2025 huggingface dataset:
Reliability Characteristics of Low Dielectric Constant Materials Under Mechanical-Electrical Stress
Authors: Cheng Yi-Lung , Huang Yao-Liang , Lee Chih-Yen
Published: February 2020

Electrical characteristics and reliability of dielectric materials are commonly studied under electric fields without considering mechanical stress effect. Electrical and reliability characteristics of silicon dioxide (SiO2) and porous low-k dielectric (p-low-k) films under mechanical-electrical compounded field effect using metal-insulator-semiconductor (MIS) structures were compared in this study. Under mechanical stress in both tension and compression, the increased capacitance and leakage current, and the reduced breakdown field were observed for both SiO2 and p-low-k films. P-low-k films displayed a larger degradation under mechanical stress. After mechanical unloading, electrical properties and reliability cannot be recovered. Additionally, the tensile and compressive stresses caused a similar degrading effect on a p-low-k film. In case of SiO2 films, a larger degradation was detected under compressive stress. This investigation demonstrates mechanical stress can influence the electrical characteristics and reliability of dielectric and are needed to pay more attention in this topic.
(Invited) Passivation of High K/GaN Interfaces for GaN Tunnel FETs
Authors: Kummel Andrew , Li Wenjun , Kerr Amanda , Chagarov Evgueni , Gu Siyuan , Kaufman-Osborn Tobin , Madisetti Shailesh , Wu Jason , Asbeck Peter , Oktyabrsky Serge , Fay Patrick
Published: February 2020

A combined wet and dry cleaning process for GaN(0001) has been investigated with XPS, capacitance voltage measurement. Capacitance–voltage characterization showed that combined ex-situ wet sulfide passivation and in-situ cyclic trimethylaluminum (TMA)/hydrogen plasma exposure led to reductions in the densities of both interface traps and border traps. In situ XPS studies show that after the wet sulfur treatment on GaN(0001), sulfur desorbs in vacuum at 25C prior to gate oxide deposition. Ex-situ depth profiling ARXPS post-ALD deposition shows that the a-Al2O3 gate oxide bonds directly to the GaN substrate leaving both the gallium surface atoms and the oxide interfacial atoms with bulk-like charge. DFT calculations predict that the oxide/GaN(0001) interface will have bulk-like charges and a low density of band gap states. This passivation is consistent with the oxide restoring the surface gallium atoms to tetrahedral bonding by eliminating the gallium empty dangling bonds on bulk terminated GaN(0001). A key application of gate oxide on GaN is GaN tunnel FETs. Tunneling FETs (TFETs) are one promising option for reducing power consumption per logic function due to their potential for subthreshold slopes steeper than the thermionic limit of 60 mV/decade. While TFETs based on narrow-gap semiconductors have been demonstrated, the narrow band gaps of these materials also lead to limited on-off current ratios, compromising their power efficiency. GaN-based TFETs offer a potential solution to this problem, since the large band gap effectively reduces off-state current, and the large polarization fields present in III-N heterostructures (which arise from the lack of inversion symmetry in the wurtzite crystal structure of the III-N system)—and GaN/InGaN/GaN heterojunctions in particular—can be used to dramatically increase tunneling currents (and thus TFET on-state currents). However, implementation of a GaN-based TFET required a low defect gate oxide/GaN interface. This has been achieved using the two-step wet-dry surface passivation.
(Invited) Homoepitaxial GaN Growth on Free-Standing Substrates
Authors: Hite Jennifer K , Anderson Travis J , Mastro Michael A , Luna Lunet E , Gallagher James C , Eddy Charles R.
Published: February 2020

The availability of high quality, free-standing GaN substrates opens windows for new device applications in III-nitrides, especially in vertical structures. With the introduction of these native substrates, the properties of nitrides are no longer dominated by defects introduced by heteroepitaxial growth. However, additional materials challenges are coming to the forefront that need to be understood and surmounted in order to allow homoepitaxial devices to achieve their full potential.

In order to enable device-quality epitaxial layers, a deeper understanding of substrate preparation and the effects of the substrate and growth initiation on the characteristics of the epitaxial layers is required for metal organic chemical vapor deposition (MOCVD) growth of homoepitaxial films. We investigate these effects on epi morphology, uniformity, and impurity incorporation at the interface and in the films. Although the initial substrate factors influencing the epi can be subtle, they can have far reaching impact on device performance. By studying these effects using wafers from several different vendors, with substrates from both hydride vapor phase epitaxy (HVPE) and ammonothermal techniques, an understanding of the requirements for device quality MOCVD homoepitaxy can be determined.
Characterization of Homoepitaxial GaN Films on Commercial GaN Substrates
Authors: Luna Lunet E , Anderson Travis J , Hite Jennifer K , Hobart Karl D. , Kub Francis J
Published: February 2020

The realization of next-generation medium to high power vertical GaN devices relies heavily on the epitaxial growth of GaN on commercially available, high-quality GaN substrates. Recent advances in commercially available GaN substrates have enabled the design and fabrication of vertical GaN devices that demonstrate promise and feasibility, but are limited by materials challenges of homoepitaxial GaN growth. Although homoepitaxial GaN is expected to provide more control over growth characteristics (e.g. crystallinity and doping), its reliability and reproducibility suffer at the hand of the native GaN substrates available to date. The variations in commercial GaN substrates include defect density, surface roughness, wafer bow, and photoluminescence properties. Thus, elucidating the role of GaN substrate properties on the growth and characteristics of resulting homoepitaxial GaN films has emerged as a new challenge towards next-generation GaN power devices. In this presentation, a systematic study of the morphological, electrical, and optical properties of GaN films on GaN substrates will be presented to identify substrate and epitaxial film metrics that administer high mobility, low leakage current, low on-resistance, and high breakdown voltage.
Interface State Density of Atomic Layer Deposited Al2O3 on Beta-Ga2O3
Authors: Su Chen Yi , Hoshii Takuya , Muneta Iriya , Wakabayashi Hitoshi , Tsutsui Kazuo , Iwai Hiroshi , Kakushima Kuniyuki
Published: February 2020

Abstract

In this study, an atomic layer deposited (ALD) Al2O3 with a thickness of 40 nm was used as a dielectric layer for gallium oxide (Ga2O3) metal-oxide-semiconductor (MOS) capacitors. Interface state (D
it) of 1.6×1012 cm-2/eV was extracted at 0.7 eV from conduction band (E
C).

Introduction

β-Ga2O3 is a promising candidate for power device applications due to its wide bandgap of 4.7–4.9 eV with a breakdown electric field 8 MV/cm, which is about three times larger than those of SiC and GaN power device materials. Along with the reports on high voltage Schottky diodes, MOS transistors have now been reported. In this research, ALD-Al2O3 with a thickness of 40 nm has been used as a gate dielectric and its interface properties are investigated.

Experiment

A 40-nm-thick-Al2O3 gate dielectric film was deposited by ALD on the surface of a Ga2O3 substrate with n-type epitaxial layer. The doping density (N
d) of the epitaxial layer was 2.3×1016 cm-3. The samples were transferred to a sputter chamber and 50-nm-thick W film followed by 50-nm-thick TiN was deposited by RF magnetron sputtering as a gate electrode. The capacitor was patterned by reactive ion etching (RIE) to form gate electrodes. Finally, a 10-nm-thick Ti followed by a 50-nm-thick TiN was deposited on the backside of the substrate as an Ohmic contact. Figure 1 shows the schematic illustration of the fabricated MOS capacitor.

Results

Capacitance-voltage (C-V) curves measured from 1 kHz to 100 kHz are shows in fig.2. The equivalent oxide thickness (EOT) of the dielectric layer was 18.3 nm. The D
it of the capacitor was evaluated by conductance method. Figure 3 shows the extracted D
it distribution in the bandgap of Ga2O3, where a 1.6×1012 cm-2/eV was obtained at 0.7 eV from E
C.

Conclusion

Interface property of Al2O3/β-Ga2O3 was evaluated by conductance method. A 1.6×1012 cm-2/eV was obtained at 0.7 eV from E
C.

References

[1] M. Higashiwaki et al., Appl. Phys. Lett, 103, 123511 (2013)

[2] K. Zeng, Y. Jia, and U. Singisetti, IEEE Electron Device Lett. 37, 906 (2016)

Figure 1
Performance Enhancement of GaN-Based Light-Emitting Diodes with Magnesium Nitride Inter-Layers
Authors: KIM Sang-Mook
Published: February 2020

High quality GaN epilayers were obtained by using a magnesium nitride (MgxNy) inter-layer. X-ray photoelectron spectroscopy (XPS) reveals Mg 2p core-level spectra from the MgxNy inter-layers. The roughness of the MgxNy layers increased with the growth time, though a prolonged processing time resulted in a decrease in the roughness. A high-resolution X-ray diffraction ω-scan rocking curve was used to reveal that the screw dislocation density (TDD) of GaN with an MgxNy inter-layer was reduced and the crystalline quality of the GaN epitaxial layer was enhanced. Furthermore, the luminous efficiency of an LED with the MgxNy layers was increased by 20 % relative to a reference LED.

Figure 1
Designing Coupled Quantum Dot with ZnS-CdSe Hybrid Structure for Enhancing Exciton Lifetime
Authors: Pala Raj Ganesh
Published: February 2020

Fabrication of coupled quantum dot (CQD) may provide an excellent platform for the realization of high end opto-electronic applications as well as quantum information processing. CQD can be synthesized by well-known cation exchange method and less explored ‘nanoparticle fusion’ method, in which the latter involves the coupling between constituting facets of two different semiconductors. Herein, we elucidate the mechanistic formation pathway of different heterostructures with ZnS and CdSe quantum dots (QD) with an emphasis on the formation of CQD comprised of bicompartmental Janus structure (i.e. Janus structure consisting of two compartments with the two QDs coupled) via ‘nanoparticle fusion’. With increase in the ratio of Cd/Zn from 0.9:1 to 1.3:1 to 2.5:1 to 4:1 to 12:1, we observe the evolution of structure from CdZnSeS alloy to Acorn Janus to Bicompartmental JanusA with ZnS Zinc Blende (ZB) – CdSe Wurtzite (Wz) to Bicompartmental JanusB (ZnS/CdSe-both ZB) and eventually to CdZnSeS alloy core - CdSe thick shell. Interestingly, the CQD possess two distinct emission bands (570/630 nm) in which 570 nm emission arises from the formation of new electronic states due the strong coupling between the two QDs whereas 630 nm is the characteristic CdSe emission. Further, the coupling enhances the exciton lifetimes of 570/630 nm emission (Bicompartmental JanusA - 36/31 ns and Bicompartmental JanusB : 41/94.8 ns) which can be exploited in various applications. Further, the DFT simulations provide the heuristics behind the formation of certain heterostructure with strain and interfacial energies of particular facets dictating the morphology during coupling of QDs.
A Reconfigurable Field-Effect Sensor By Single-Layer Graphene for Opto-Electro-Chemical Sensing Applications
Authors: Hsu Wei-En , Ku Ting-An , Lee Chao-Yu , Wu Chih-I , Lin Chih-Ting
Published: February 2020

In the era of internet-of-things (IoT), sensors play important roles in every situation. It is of interest to reduce power consumption and chip size of sensing devices for mobile applications. In this work, we design and fabricate a solid-state device with chemical sensing and photodetecting, e.g. a dual-functional and reconfigurable sensor on single graphene field-effect transistor (GFET). Based on the single-layer graphene, this device couples these two solid-state sensing mechanisms. This graphene field-effect sensing device has a double-gate configuration, in which the electrolyte solution gate controls the top-gate GFET as the chemical sensor. At the same time, the bottom gate voltage is biased for noise reduction. In the developed GFET, a photodetector can also be created by taking advantage of high carrier mobility of graphene. As a consequence, applying bottom gate voltage is able to regulate charge induction on the graphene surface created by photo-excitation carriers inside the silicon substrate. Both of the two sensing mechanisms could be modulated under each gate independently. In other words, when one sensing mechanism is in operation mode, the signal current from the other sensing mechanism could be minimized by field-effect control, and vice versa. This phenomenon demonstrated the concept of electro-reconfigurable sensing devices. In this work, we demonstrate the photocurrent could be reduced by bottom gate voltage modulation when the device operating under chemical sensing mode. And the proposed electro-reconfiguration mechanism is investigated. This can be attributed to the ambipolar field-effect characteristic and surface electron traps of silicon dioxide layer as the bottom gate dielectric. In our experiments, the photodetector could be modulated by a +/- 7 V bottom gate voltage in the bottom-gate configuration. With the same device, the chemical sensor could be modulated by a +/- 500 mV solution gate voltage in the top-gate configuration. The newly developed electro-reconfigurable solid-state sensor has the advantages of small size, low cost and low operation voltage by utilizing the same device and sensing region. In addition, this device offers an alternative solution to photocurrent reduction in solid-state sensing devices by regulating photocurrent through field effect control. It would pave the way for real practice of IoT sensors and point-of-care testing devices.

Figure 1
Multiplexed CVD Biomarker Detection in Human Serum Using Aptamer Immobilized High Electron Mobility Transistor
Authors: Tai Tse-Yu , Sinha Anirban , Lee Gwo-Bin , Wang Yu-Lin
Published: February 2020

Cardiovascular diseases (CVDs) are the major causes of death in the world today. Identifying and managing CVD risk is of primary importance in efficient patient care. Proper risk assessment strategies can provide information to healthy as well as high risk individuals on how to better manage their health. This can greatly reduce the mortality rate of cardiovascular disease. Using protein based cardiac biomarkers for risk assessment can improve early diagnosis and aid in prognosis of CVDs. Semiconductor based electronic sensors are very attractive for rapid and sensitive protein detection. Highly Electron Mobility Transistor (HEMT) based sensors have been used in a variety of applications such as ion, gas and biomolecule sensing and possess desirable features such as high sensitivity and resistance to harsh chemical environment. In this research, A unique sensing methodology using AlGaN/GaN HEMT has been developed to directly detect target protein in high salt concentration environment. The target protein was captured on the surface of the gate electrode, which is separated from the transistor channel by a small gap, and functionalized with target specific aptamer. By forming an electrical double layer (EDL) structure, the solution capacitance modulates the FET drain current. In this way, highly sensitive determination of cardiac biomarkers in high ionic strength media such as 1X PBS and human serum can be realized.

HEMT is fabricated using 4 mask processes: mesa isolation and defining active device area, ohmic contact deposition and annealing, metal interconnect deposition and device passivation. For electrical measurement, a pulsed gate electrode voltage is supplied to the gate electrode which creates a solution capacitance in the gap between the gate electrode and transistor dielectric. This solution capacitance modulates the potential drop across the transistor dielectric and hence the drain current. The solution capacitance changes when the device is functionalized with the target specific aptamer and when the target protein binds to the receptor. The aptamer based AlGaN/GaN HEMT biosensor can detect target proteins with high sensitivity, selectivity and a wide dynamic range of detection in 1X PBS (with 4% BSA) and clinical human serum samples.
Solution-Processed High-k Dielectric Films for Wearable Neuroelectronics
Authors: Yu Byoung-soo , Ha Tae-Jun
Published: February 2020

Recently, neuroelectronics based on neurotechnology have been extensively investigated for sensitive detection of neural signals and emulation of human brains. However, there have been some difficulties and challenges with respect to realization of high performance wearable neuroelectronics. Solution-processed high-k dielectric films have attracted considerable attention due to their outstanding electrical characteristics and technological merits. For this reason, big efforts to employ such dielectric films into the wearable neuroelectronic devices have been intensively investigated in the field of bio-electronics. In this presentation, we will demonstrate high-quality solution-processed high-k dielectric films composed of zirconium oxide (ZrO2) and hafnium oxide (HfO2) for wearable neuroelectronic devices. With an optimized process of fabrication and a careful choice of materials, high-quality dielectric films can be achieved. The characterization of dielectric films by atomic force microscopy, scanning electron microscopy, transmittance and x-ray photoelectron spectroscopy measurements can explain the origin of suppressed leakage currents and intrinsic defect states in the films. We will also discuss the demonstration of neuroelectronics consisting of solution-processed high-k dielectric films, which describes the short-term and long-term memories. This work was supported by the National Research Foundation of Korea grant funded by the Korea government (MSIP) (NRF-2017R1A2B2003808).
Copper Oxide-Based Photocathodes and Electrocatalysts for the (Photo)Electrochemical Reduction of Protons and Carbon Dioxide
Authors: Mayer Matthew T. , Schreier Marcel , Luo Jingshan , Graetzel Michael
Published: February 2020

The use of renewable electricity to drive chemical reactions is an approach to storing the energy from intermittent sources (solar, wind) in the form of chemical bonds. Combining photoabsorber and electrocatalytic components into integrated devices, we developed copper oxide-based photocathodes for proton reduction to hydrogen (1,2), as well as carbon dioxide reduction to carbon monoxide (3), in demonstrations of efficient solar-to-fuels conversion.

Recently, oxide-derived copper emerged as an intriguing electrocatalyst for the conversion of carbon dioxide into hydrocarbon products. In an effort to modify the product selectivity of copper oxide nanowire arrays, we used atomic layer deposition to apply conformal layers of tin oxide to the electrode surface. This copper-tin-oxide hybrid exhibited nearly unity selectivity for carbon monoxide as CO2 reduction product, deviating significantly from the behavior expected for copper or tin alone, neither of which typically produce CO at high selectivity. This result suggests sub-monolayer composition modification of electrode surfaces can be a powerful strategy for tuning electrocatalyst selectivity.

J. Luo, L. Steier, M.-K. Son, M. Schreier, M. T. Mayer and M. Grätzel, Nano Letters, 2016, 16, 1848–1857.

P. Dias, M. Schreier, S. D. Tilley, J. Luo, J. Azevedo, L. Andrade, D. Bi, A. Hagfeldt, A. Mendes, M. Grätzel and M. T. Mayer, Adv. Energy Mater., 2015, 5, 1501537.

M. Schreier, J. Luo, P. Gao, T. Moehl, M. T. Mayer and M. Grätzel, J. Am. Chem. Soc., 2016, 138, 1938–1946.

M. Schreier, F. Héroguel, L. Steier, S. Ahmad, J. S. Luterbacher, M. T. Mayer, J. Luo and M. Grätzel, Nature Energy, 2017, 2, 17087.
A Regular Dimpled Surface Morphology for the Oxygen Evolution Reaction
Authors: Taylor Audrey K. , Andreu Irene , Gates Byron D.
Published: February 2020

Optimizing electrochemical reactions is essential to improving the energy efficiency of many renewable energy technologies, such as for increasing their competitive advantage across many sectors of the market. The oxygen evolution reaction (OER) is of particular importance for its applicability to chemical generation and energy storage, and its reliance on low cost materials, such as nickel based catalysts. Nickel electrodes with surface oxides (NiOx) are frequently used as anodes in these systems because they also offer reduced activation energies, exhibit sufficient catalytic activity, and durability under alkaline conditions. Persistent bubble accumulation on highly active electrodes can, however, result in blocked active sites and reduced system performance for these and other OER catalysts. The identification and development of surface morphologies that can effectively evolve and remove oxygen bubbles from the electrode surfaces could be a highly beneficial technology to further enhance the efficiency of the OER.

In this work, regular dimpled Ni surfaces were prepared using self-assembled poly(styrene) templates with a distinct diameter (e.g., 1 μm). The electrodeposition of Ni into these assembled templates was tuned to produce four types of dimpled surface textures. The electrochemical activity of these regular features were evaluated for the OER to investigate their influence on the mass transport properties. Enhancements to the OER efficiency were demonstrated for these systems when compared to flat Ni electrodes. The wettability of the dimpled Ni electrodes was characterized by contact angle measurements acquired both before and after electrochemical cycling. Theoretical wetting models were applied to the surface morphologies to correlate the results with adhesion of the oxygen bubbles and partial wetting with the electrolyte. The regular design of these micro- and nanostructured surfaces enables further correlation of structural morphologies in Ni based electrodes to their electrochemical performance.
(Invited) Techno-Economic Perspectives of Catalysts Development for Low Temperature Water Electrolysis
Authors: Xu Hui , Zhao Shuai , Mittelsteadt Cortney K
Published: February 2020

Catalyst development for low temperature water electrolysis has been pursued in the electrochemistry community. Booming research and development in this area are due in part to near-commercialization of fuel cell vehicles and potential adoption of renewable electricity for water electrolysis. Two types of reactions, oxygen evolution reaction (OER) and hydrogen evolution reactions (HER) have been widely studied. Studies on the OER prevail in both acidic and basic media, while the HER studies are predominant in basic media. The increasing research interests have been reflected in this recurring symposium. At Giner Inc., we have developed a variety of highly active and durable catalysts (e.g, Ir nanotubes, Ir supported on titanium oxide, cobalt oxide on nanocarbon) for both proton and anion exchange membrane electrolysis.

This work aims to provide insightful guidance on the catalyst development from an industrial point of view. Compared to fuel cells, the catalyst cost in a water electrolyzer is less significant due to prohibitive cost of feedstock (electricity). Therefore, when regular (expensive) electricity is used for water electrolysis, the catalyst studies should be centered on the intrinsic activity improvement to lower the over-potential so as to increase the energy efficiency. On the other hand, when the renewable (low-price) electricity becomes available, the catalyst studies should be focused on the mass activity to lower the platinum-group metal (PGM) loading. Provided that the global supply of industry standard OER catalyst Ir is very limited, the development of non-PGM catalysts for both proton and anion exchange membrane electrolysis is very meaningful.

The second consideration is the catalyst performance in a real electrolyzer cell. The vast majority of research work is on the rotating disk electrodes (RDE) using liquid electrolyte. However, the RDE data may not be necessarily reproduced in real electrodes that use ionomer as an ion conductor. The interaction of the catalyst with other components (e.g., membrane, ionomer and gas diffusion media) determines electrode morphology and structure thus tremendously impacting the electrode performance.

The last consideration is the long-term durability of the catalysts, which is equal and even more important than the initial performance, from an industrial point of view. However, unlike the well-established durability benchmarks for fuel cells, the catalyst durability of water electrolyzer has not been thoroughly studied. Therefore, accelerated stress tests (AST) need to be implemented to evaluate the long-term durability of the catalysts.

Acknowledgement: The financial support is from the Department of Energy under the Contract Grant DE-SC0007471.
IrO2 Decorated Self-Doped TiO2 Nanotube Arrays: A Binder-Free and More Stable Electrode for Oxygen Evolution Reaction in Acid Condition
Authors: Shi Yan , Lu Zhuoxin , Wang Zhida , Guo Lili , Tan Hongyi , Guo Changqing , Yan Changfeng
Published: February 2020

The effective, stable and low cost anodic catalyst or electrode for OER is still a big challenge for water electrolysis in acid condition. Many efforts were devoted to develop suitable supports for depositing Ir and Ru metal or oxides to obtain an enhanced activity or stability for OER.

In this work, self-doped TiO2 nanotube arrays (sTNTA) which has a higher electrical conductivity than TNTA was chosen as the support, it was fabricated by a simple electrochemical reduction of TNTA in neutral electrolyte. Then IrO2 nano particles were deposited on it by electrochemical pulse deposition method to form an OER electrode. The cyclic voltammetric behavior, electrical conductivity and micro structure of sTNTA prepared under different reduction potential were characterized to obtain an optimal performance for depositing IrO2, the OER activity and stability of new electrode were also determined. The obtained sTNTA shows an increased current in CV test than TNTA, which is due to the increased carrier density. As the reduction potential move to more negative level (from -1.5V to -1.9V vs. Ag/AgCl), the carrier density was found increased for 3 order of magnitudes, and XPS characterization confirm the existence of Ti3+ which formed in electrochemical reduction process, and sTNTA prepared under -1.9V shows the best electrochemical performance. After depositing IrO2, IrO2/sTNTA exhibited higher OER activity than IrO2/TNTA, according to the CV and EIS test, this can be ascribed to the enhanced electrical conductivity of support, which results in more active sites and lower charge transfer resistance of the electrode. The OER stability of IrO2/sTNTA was tested under constant current of 5mA/cm2, IrO2/TNTA and IrO2/Ti which prepared by electrochemical deposition on Ti foil were also tested for comparison. A higher stability of IrO2/sTNTA electrode was observed than the other two electrodes, and this may be due to the interaction between the IrO2 and sTNTA. XPS studies show a clear peak shift of Ir4f in IrO2/sTNTA, indicating a possible lower valence of Iridium, and such lower oxidation state of Iridium may be response for the enhanced OER stability.

In summary, our studies have demonstrated the possibility of self-doped TNTA as a potential anodic support for OER. Comparing with the TNTA, the high electrical conductivity of sTNTA facilitate the charge transfer rate and thus improve the OER activity. Additionally, a higher OER stability was observed for IrO2/sTNTA electrode, and this may be due to possible catalyst-support interactions between IrO2 and self-doped TNTA.

Figure 1
Cobalt Platinum Bronze for an Active and Durable Oer Electrocatalyst of PEM Electrolysis without Ir or Ru
Authors: Kamitaka Yuji , Morimoto Yu
Published: February 2020

Water electrolysis is an essential technology for realizing the renewable energy society. PEM electrolysis, in particular, is promising because of its high power density despite its high cost per unit active area. Its OER catalyst often contains Ir or Ru (or both) because of their high OER activities and durabilities. Although the current prices of these two metals are not extremely high compared with other precious metals because of their relatively low demand, Ir and Ru are, however, ones of the scarcest elements on earth and this extreme scarcity could be a major obstacle for wide distribution of PEM electrolysis technology.

Recently we have started fundamental studies on mixed oxides of platinum so called "platinum bronze" (Bronze in this context means mixed oxides with metallic sheen) consisting of platinum, oxygen and other metal ions (such as Li, Na, Mg, Ca, Zn, Cd, Co, Ni [1]) as a versatile electrocatalyst for ORR, OER and HOR.

Co-Pt bronze was prepared according to the literature [1] with some modifications. Platinum oxide and Co(NO3)2·6H2O powders were mixed and heat-treated under air conditions at 650 ºC for 5 hours. The obtained powder was washed with heated (80 °C) aqua regia for 3 times and the remaining powder was then thoroughly rinsed with pure water.

The electronic conductivity of the bronze measured with a compression cell was compared in Fig. 1 with those of Vulcan XC-72 (a typical conductive carbon black), IrO2 (a typical OER catalyst) and PtO2. The Co-Pt bronze showed a comparable conductivity with XC-72 and IrO2, which is much higher than that of PtO2, an insulator.

OER activities were measured by potential sweep and potential hold experiments. The electrodes were prepared by depositing the catalysts on a glassy carbon disk electrode. Fig. 2 a) and b) show that the Co-Pt bronze has a similarly active and stable OER activity with IrO2 not like Pt black and Pt/Vulcan, which show rapid decay of OER current due to formation of PtO2.

This study is financially support by the New Energy and Industrial Technology Development Organization.

References

[1] R.D. Shannon, et al., Inorg. Chem., 21, 3372 (1982)

Figure 1
(Invited) Engineering Advanced Transition-Metal Based Electrocatalysts for Oxygen Evolution Reaction
Authors: Song Junhua , Fu Shaofang , Shi Qiurong , Zhu Chengzhou , Du Dan , Lin Yuehe
Published: February 2020

Increasing energy demands and environment awareness have promoted extensive research on the development of alternative energy conversion and storage technologies with high efficiency and environmental friendliness. Among them, electrochemical water splitting is very appealing, and is receiving more and more attention. Essential to the water splitting is the kinetically sluggish oxygen evolution reaction (OER), which demands efficient electrocatalysts. To make the water splitting process more energy-efficient and economical, developing high-performance noble metal-free catalysts, i.e., transition-metal-based nanomaterials, has aroused great attention due to their distinct advantages, such as relatively low cost, low toxicity, great flexibility in terms of structure and morphology, and satisfying electrochemical performance. We focus on the rational design of porous nanostructures with tunable composition and other favorable synergistic factors, aiming at increasing the number of the active sites and enhancing the reactivity of the active sites. Taking advantage of these favorable structural and compositional parameters, the resultant nanostructures exhibited excellent OER activity and stability, holding great promise in water splitting.

References:

Song, C. Zhu, B. Xu, S. Fu, M. Engelhard, R. Ye, D. Du, S. Beckman, Y. Lin. Bimetallic Cobalt-Based Phosphide Zeolitic Imidazolate Framework: CoPx Phase-Dependent Electrical Conductivity and Hydrogen Atom Adsorption Energy for Efficient Overall Water Splitting. Advanced Energy Materials 7, 2017, 1601555

S Fu, C Zhu, J Song, M Engelhard, X Li, D Du, Y Lin. Highly Ordered Mesoporous Bimetallic Phosphides as Efficient Oxygen Evolution Electrocatalysts. 2016, 1, 792-796

Zhu, S. Fu, D. Du, Y. Lin. Facilely tuning porous NiCo2O4 nanosheets with metal valence states alteration and abundant oxygen vacancies as robust electrocatalysts towards water splitting. Chemistry European J. 2016, 22, 4000-4007

Fu, J. Song, C. Zhu, G. Xu, K. Amine, C. Sun, X. Li; M. H Engelhard, D. Du, Y. Lin. Ultrafine and highly disordered Ni2Fe1 Nanofoams Enabled Highly Efficient Oxygen Evolution Reaction in Alkaline Electrolyte. Nano Energy. Submitted
Ion Intercalation Induced Amorphization of High Surface Area Cobalt (II, III) Oxide (Co3O4) for Enhanced Water Oxidation Activity
Authors: Gupta Prashant Kumar , Saha Sulay , Kishor Koshal , Sharma Ashutosh , Pala Raj Ganesh
Published: February 2020

Amorphous material can result in highly roughened electrode coupled with enhanced specific activity due to higher degree of under-saturation in comparison to the crystalline material. The synthesis of amorphous materials is generally done by high temperature heating followed by rapid quenching which is undesirable in two ways-they are highly energy intensive and generally result in lower surface area. To address these issues, we propose a generalized room temperature approach wherein Lithium insertion/de-insertion into the crystal lattice induces high stresses resulting in the collapse of the crystal structure giving rise to high surface area amorphous materials. Also, this method provides an utility for materials recovered from lithium ion battery. The crystalline->amorphous transformation can be induced by other electrochemical processes such as multiple electrochemical cycles of hydrogenation-dehydrogenation over the electrocatalyst. We have compared the suitability of above mentioned processes for preparation of better oxygen evolution reaction (OER) electrocatalyst of amorphous Co3O4. The specific activity of OER, ion intercalation induced amorphization of Co3O4 is superior than crystalline Co3O4 whereas hydrogenation-dehydrogenation induced amorphization of Co3O4 has activity between crystalline Co3O4 and ion intercalation induced amorphization of Co3O4. Further, higher bulk-oxygen vacancies are created through ion intercalation process leading to higher conductivity and reducing overall charge-transport resistance through electrocatalyst. On the other hand, hydrogenation-dehydrogenation induced amorphization is restricted to surface only. The above study demonstrates the utility of multicycle ion intercalation over hydrogenation-dehydrogenation technique for obtaining amorphous OER electrocatalysts.

Keywords:

Co3O4, oxygen evolution reaction, Lithiation/delithiation, hydrogenation-dehydrogenation, Ion intercalation, oxygen vacancies.
Morphology Control of Carbon-Free Spinel NiCo2O4 Catalysts for Enhanced Bifunctional Oxygen Reduction and Evolution in Alkaline Media
Authors: Devaguptapu Surya , Zhao Shuai , Gupta Shiva , Xu Hui , Wu Gang
Published: February 2020

Spinel NiCo2O4 is considered a promising precious metal-free catalyst that is also carbon-free for oxygen electrocatalysis applications. Current efforts mainly focus on optimal chemical doping and substituent to tune its electronic structures for enhanced activity. Here, we study its morphology control and elucidate the morphology-dependent catalyst performance for bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Three types of NiCo2O4 catalysts with significantly different morphologies were prepared using temple-free, Pluronic-123 (P-123) soft, and SiO2 hard templates, respectively, via hydrothermal methods following by a calcination. While the hard-template yields sphere-like dense structures, soft-template assists the formation of a unique nano-needle cluster and flower-like assembly containing abundant meso- and macro pores. Then, the effect of morphology of NiCo2O4 on their corresponding bifunctional catalytic performance was systematically investigated. The flower-like nano-needle assembly NiCo2O4 catalyst via the soft template method exhibited the highest catalytic activity and stability for both ORR and OER. In particular, it exhibited an onset and half-wave potentials of 0.94 and 0.82 V vs. RHE, respectively, for the ORR in alkaline media. Although it is still inferior to Pt, the NiCo2O4 represents one of the best ORR catalyst compared to other reported carbon-free oxides. Meanwhile, remarkable OER activity and stability were achieved with an onset potential of 1.48 V and a current density of 15 mA/cm2 at 1.6 V, showing no activity loss after 20,000 potential cycles (0 to 1.9 V). The demonstrated stability is even superior to Ir for the OER. The morphology-controlled approach provides an effective solution to create a robust 3D architecture with increased surface areas and enhanced mass transfer. Importantly, the soft template can yield high degree of spinel crystallinity with ideal stoichiometric ratios between Ni and Co, thus promoting structural integrity with enhanced electrical conductivity and catalytic properties. The high performance bifunctional oxide catalyst is carbon free and can eventually overcome the stability issue resulting from carbon corrosion in reversible fuel cells and metal-air batteries.
In Situ Scanning Probe Microscopy of Li-Ion Battery Cathode Materials
Authors: Baghgar Bostan Abad Mina , Ignatova Tetyana , Turvey Mackenzie W. , Collins Philip G.
Published: February 2020

Designing the next generation of high performance energy storage devices requires a deeper understanding of structural degradation. To this end, a wide variety of scanning probe microscopy techniques can help characterize charge transport and aging mechanisms in Li-ion battery materials and their interfaces. For example, we use in-situ probe microscopy and Kelvin probe force microscopy (KPFM) to investigate the mechanical and potentiostatic changes in MnO2 cathodes as they undergo charge cycling. Topography monitors mechanical expansion during charging, and the dependence of expansion on scan rate reveals the separate roles of Mn3+/4+ redox pseudocapacitance and faster, double-layer surface capacitance. On the same electrodes, KPFM observes the evolution of surface potentials with lateral resolution as low as 40 nm. At this length scale, inhomogeneous lithiation is clearly observed as a highly nonuniform, fractal growth or shrinkage of Mn3+ and Mn4+ phases. Via surface potential, KPFM also reveals "dead" zones that do not participate in charging, "hot" zones that charge or discharge most readily, and the evolution of each type as an electrode is repeatedly cycled.

This work was supported as part of the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.
Analyzing Multi-Modal Degradation Interplay in Li-Ion Battery Electrodes
Authors: Verma Ankit , Mukherjee Partha P.
Published: February 2020

Lithium-ion batteries (LIBs) have found widespread usage in small scale electronics and are slowly inching towards large scale applications like electric vehicles and grid storage. The acceptance of LIBs for these applications is predicated on mitigation of performance and safety issues intrinsic with application at varying operating conditions. Performance and safety degradation of LIBs with cycling is attributed to electrode pulverization coupled with solid electrolyte interphase (SEI) formation and Li plating on the anode surface. Extreme operating conditions (e.g. low temperature and high current rates) can exacerbate Li plating phenomena at the anode. Consequently, it is essential to delineate the effects of coupled mechanical (fracture) and electrochemical (SEI, plating) degradation modes in lithium ion batteries. In this work, we focus on investigating these coupled mechano-electrochemical interactions, based on stochastic analysis and modeling to elucidate the interplay between the trifecta of degradation modes, with the representative particle-electrolyte domain under investigation shown below.

Figure 1
The Role of Cathode Architecture in Conversion Reaction Chemistries
Authors: Tozier Dylan , Giordani Vincent , Uddin Jasim , Tan Hongjin , Chase Gregory V , Addison Dan , Greer Julia R
Published: February 2020

State of the art commercial lithium ion batteries use cathodes such as lithium cobalt oxide which rely on insertion and removal of lithium ions from a host material. However, insertion cathode materials are limited in their capacity, and replacing them with a cathode that employs growth and dissolution of new phases could significantly increase a battery’s energy density. For example, oxygen and sulfur cathodes have been widely researched to this end, with both cases involving the growth of a lithium-rich compound on a current collector/catalyst support.

Consider a similar phase-forming conversion chemistry[1], whereby a molten nitrate salt serves as both an active material and the electrolyte. Molten nitrate salts have been previously studied as an active material in a primary lithium battery where lithium oxide irreversibly forms as nitrate reduces to nitrite. We will describe how the use of a nanoparticle heterogeneous catalyst allows the reversible growth and dissolution of large (several micron) lithium oxide crystals in this system, as substantiated by SEM, XRD, and TEM.

After introducing this molten salt lithium battery, we address the effect of cathode geometry on the electrochemical performance of this model system. In particular, we note that the growth of such large, solid phase species on the surface of the catalyst support imposes new design restrictions when optimizing a cathode for energy density. For instance, it is not just the surface area of the catalyst support that determines the discharge capacity, but also the amount of usable pore volume which can accommodate this solid phase discharge product. As a proof of concept, we design and implement an architected electrode with large pore volume and relatively small surface area, comparing it with the more typical geometries of thin films and nanoparticles. These electrode design principles can be extended to other phase-forming conversion chemistries.

[1] Addison, D.; Bryantsev, V.; Chase, G. V.; Giordani, V.; Uddin, J.; Walker, W Rechargeable Batteries Employing Catalyzed Molten Nitrate Positive Electrodes. US Patent 2016/0204418, August 8, 2013.
Abnormal Self-Discharge in Lithium-Ion Batteries
Authors: Kang Kisuk , Seong Won Mo , Park Kyu-Young , Lee Myeong Hwan , Park Hyeokjun , Oh Kyungbae , Lee Sechan
Published: February 2020

Lithium-ion batteries are expected to serve as a key technology for large-scale energy storage systems (ESSs), which will help satisfy recent increasing demands for renewable energy utilization. Besides their promising electrochemical performance, the low self-discharge rate (<5% of the stored capacity over 1 month) of lithium-ion batteries is one of their most significant advantages for ESSs. Herein, contrary to conventional belief, we report that the self-discharge of LIBs can be abnormally accelerated when the battery had been exposed even to a routine short-term thermal exposure. We demonstrate that, this thermal ‘history’ in addition to the temperature itself is memorized in the battery and accelerates the self-discharge rate. The series of characterizations performed in our work reveal that the electrolyte salt acts as a strong oxidizing agent by vigorously damaging the surface of the cathode, producing an internal ‘parasitic’ lithium source that continuously supplies lithium for the self-discharge. Although it is widely known that battery operation at elevated temperature generally induces faster degradation of capacity over cycles, the key finding here is that not only the operation temperature but also the ‘thermal history’ of the battery should be carefully considered because this history remains and continues to affect the self-discharge rate afterwards. The self-discharge of LIBs has remained largely neglected; however, our findings suggest that close attention must be paid to the self-discharge of LIBs applied to large-scale ESSs, which, unlike mobile electronic devices, would be exposed to various outdoor temperature conditions.
Systematic Study of Efficient Adsorbents and Mediators for Advanced Lithium Sulfur Battery
Authors: Liu Fang , Lu Yunfeng
Published: February 2020

Lithium-sulfur battery is one of the most promising systems for electric vehicles. Unfortunately, its application has been impeded by its slow charging/discharging rates, fast capacity degradation, and short cycling life. Although several materials have been identified as efficient adsorbents or mediators for lithium polysulfides, their working mechanism remains ambiguous. Here we report a systematic study of the interactions between transition metal oxides/sulfides and lithium polysulfides based on their electron structures. This study provides not only mechanistic insights into the fundamental issues associated with sulfur cathodes, but also offers quantified design framework for advanced lithium-sulfur batteries. Besides, we developed a class of regenerative polysulfide-scavenging layer and a series of polysulfide-mediators. The resulted cells exhibit high gravimetric energy densities, high power output and extended cycling lifetime, showing great promise for practical applications.
Characterization and Chemical Mapping of High Capacity Intermetallic Li-Ion Anodes Using X-Ray Microscopy Techniques
Authors: Gonzalez-Malabet Hernando J. , Ausderau Logan , Juarez-Robles Daniel , Xiao Xianghui , De Andrade Vincent , Liu Yijin , Mukherjee Partha P. , Nelson George J.
Published: February 2020

A combined electrochemical and x-ray imaging study has been performed for a high capacity Cu6Sn5 alloy anode material for Li-ion batteries. The electrodes investigated include Cu6Sn5 pellets and composite electrodes containing alloy, carbon additive, and a polymer binder. X-ray microtomography (µCT), segmentation, and analysis was performed on anodes subject to lithiation and cycling. The observation of affected and unaffected samples from lithiated, cycled, and pristine electrodes were compared. Significant structural changes were observed in pellet electrodes subjected to full lithiation (0 V vs. Li/Li+). Cycled composite electrodes show microstructural changes that correspond with capacity fade. These changes include decreases in both particle size, indicating pulverization of the active material, and reduction in pore/carbon/binder regions, suggesting impacts on effective transport properties. The µCT results show distinct absorption patterns indicating varied bulk chemical composition, but detailed chemical composition is difficult to discern. To address this drawback x-ray absorption near edge structure (XANES) imaging was performed, obtaining absorption spectra for Cu, Cu6Sn5, and Li2CuSn. Using these spectra, an analysis was performed on 2D images from samples of cycled electrodes to map the chemical composition in the phases that contained Cu. The capabilities to distinguish the different materials on mixed samples shows that microstructure and composition changes resulting from lithiation and delithiation in Cu6Sn5, can be observed with x-ray imaging methods. In operando XANES imaging of Cu6Sn5 lithiation in composite electrodes has been performed. The in operando testing methodology will be shown and explained; results show particle motion and disintegration during cycling, the analyzed particles will be characterized using absorption analysis, in order to accurately discern the evolution of the composite during cycling. These features appear mostly on delithiation and show that, in addition to microstructural change at the particle scale, the networks supporting mass and charge transport may evolve during cycling. This multiscale microstructural evolution is expected to have significant impact on cycling performance and capacity fade.