bluuebunny/crossref_metadata_2025 huggingface dataset:
Understanding the Evolution of Cathode Material in Electrochemical Process at Multi-Scale
Authors: Wang Xuelong , Hu Enyuan , Bak Seong-Min , Liu Yijin , Yu Xiqian , Xiao Ruijuan , Li Hong , Chen Liquan , Yang Xiao-Qing
Published: February 2020

Nowadays it’s well recognized that developing renewable energy resources and energy conservation are key technique for us to meet with a sustainable future. Lithium ion batteries (LIBs) stood out as promising candidates for energy storage devices. Further increasing energy density and reducing cost are the major goals of researchers. Though for now, on increasing energy density more progress seems to be made with anode rather than cathode, there are still several kinds of cathode material with great expectations such as high-voltage LiCoO2, Li-rich material and Nickel-rich layered material. However, they all suffer from surface degradation, transition metal reduction and structure instability accompanied by oxygen release to some extent during the electrochemical process. Therefore, understanding the chemical compositional change and local structure transformation inside the cathode material is essential for us to contain these hazardous deteriorations from the beginning.

Transmission X-ray microscope (TXM) has been proven to be a powerful experimental method for investigation on material systems at particle level especially when energy resolution is feasible and combined with data-mining technique. With this technique, we can monitor chemical compound evolution and their spatial distribution in cathode material during the electrochemical process. More excitingly it shows the possibility to see the emergence of unexpected minor phase which isn’t anticipated in the first place. Furthermore, with the aid of atomic DFT calculation and mesoscale mechanical simulation we could relate the practical performance of material in real cell with the subtle changes inside a particle. In this report by combining these methods at different length scale we offer our own insights on transition metal dissolution and precipitation, variation of chemical composition in different region of a single particle and the behavior of transition metal at different charge voltage. In general, we hope these findings could benefit our understanding on the material evolution in actual electrochemical process.

Acknowledgment

The work done at Brookhaven National Laboratory were supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program, including Battery500 Consortium under contract DE-SC0012704. Research conducted at ORNL's Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
Oxygen Evolution and Phase Transformation in LiCoO2 Cathode: A Phase-Field Modeling Study
Authors: Yurkiv Vitaliy , Sharifi-Asl Soroosh , Ramasubramanian Ajaykrishna , Shahbazian-Yassar Reza , Mashayek Farzad
Published: February 2020

Thermal runaways, triggered by oxygen release from the oxide cathode materials, pose a major safety concern for widespread application of lithium ion batteries. To allow for a deeper understanding of the processes leading to the thermal runaway, a combined modeling and experimental work of phase transformation and the subsequent oxygen release in LiCoO2 electrodes was undertaken. This contribution presents the modeling part and the accompanied abstract in this meeting reports the experimental details, thus only a brief experimental description is given here. Commercially available and chemically de-lithiated LiCoO2 samples were used. Utilizing in-situ aberration-corrected scanning transmission electron microscopy (STEM) and in-situ electron energy loss spectroscopy (EELS) techniques, phase transformation and the oxygen release from LiCoO2 samples were investigated in the temperature range between 25 °C and 450 °C.1

In order to interpret the experimental measurements and to identify factors governing phase transformation and the subsequent oxygen release, a multiphase-field model (PFM) is established to represent the coupled mechanical, chemical and transport behavior occurring in the LiCoO2 electrodes. The PFM was developed using the MOOSE framework.2,3 The model captures both the kinetics of the oxygen release by employing Fick’s diffusion law, as well as the stress build-up. This is achieved by coupling the evolution equation of the phase-field variables (i.e., oxygen concentration, existed phases) to the mechanical equations allowing to study diffusion induced phase transformation and stress generation. The model was validated against the above described experimental measurements by comparing the rate of phases growth and their thicknesses. This comparison allows the PFM to predict the oxygen composition depth and the stress profile in the layered, spinel and rock salt structures of the LiCoO2 during the heating. The simulation results indicate that the generated stress slows down the oxygen release, and changes the local oxygen concentration within the phases. In addition, the temperature ranges of the critical oxygen release were identified. Thus, this combined work provides suggestions for a safe operating window of the LiCoO2 electrodes.

References

1. S. R. Sharifi-Asl, F. Soto, A. Nie, Y. Yuan, H. Asayesh-Ardakani, T. Foroozan, V. Yurkiv, B. Song, F. Mashayek, R. Klie, K. Amine, J. Lu, P. Balbuena, R. Shahbazian-Yassar, Nano Lett., 17, 2165-2171 (2017).

2. M. R. Tonks, D. Gaston, P. C. Millett, D. Andrs, and P. Talbot, Comput. Mater. Sci., 51, 20–29 (2012).

3. D. Schwen, L. K. Aagesen, J. W. Peterson, and M. R. Tonks, Comput. Mater. Sci., 132, 36–45 (2017)
Improvement of Specific Capacitance in Lithium Ion Batteries By Mesoporous Carbon Hybrid Nanostructures
Authors: Dorvilien Valerio , Rojas Carolina Valeria , Santa Cruz Neida , LopezdeVictoria Monica , Rivera Kevin , Limbu Tej B. , Mendoza Frank , Morell Gerardo , Weiner Brad R.
Published: February 2020

We propose a low cost and simple method to prepare carbon hybrid structure-based electrodes in this study. Hybrid-structure Carbon Nanostrures were obtained using a zeolite as a template. The resulting carbon nanostructures are mostly configured by Graphene and multiwall carbon nanotubes (G/MWCNTs). The G/MWCNTs were dispersed onto the shallow surface of alumina foil forming as the electrodes. The electrical conductivity and dispersed morphology of the G/MWCNTs -based conductive electrode were examined by atomic force microscope (AFM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Using the standard method we assembled the Lithium Ion Battery in the Ar-filled glove box, we used LiPF6 with EC:DMC::1:2 ratio as the electrolyte, lithium foil as the counter electrode and G/MWCNTs material. The specific capacity and cycle-life stability of the G/MWCNTs-based lithium ion battery was investigated by cyclic voltammetry analysis.
Ageing Induced Changes in a Lithium-Ion Battery Analysed By Correlating EIS Simulations and Experiments
Authors: Wolff Nicolas , Heinrich Marco , Röder Fridolin , Seitz Steffen , Krewer Ulrike
Published: February 2020

Electrochemical Impedance Spectroscopy (EIS) is a widely-used measurement technique to analyse properties of Lithium-Ion Batteries, since impedance spectra are sensitive to changes of battery parameters like surface layer thicknesses, material conductivities and reaction rate constants. Physico-chemical models of Lithium-ion batteries are a useful tool to correlate changes of measured impedance spectra to changes of Lithium-ion battery parameters. We will present a model based on the work of Doyle and Newman [1] extended with double layers and aging processes to simulate EIS during degradation of a Lithium-ion battery. The model has been used to simulate impedance spectra and to fit them to measured impedance spectra with a linear least square fitting tool.

Within this study, we provide fits of a physico-chemical battery model to cycle aged cell EIS measurements. Achieving reliable results is only possible if measured and simulated impedance spectra meet linearity, causality and stationarity requirements for EIS. Various ageing induced changes in the charge distributions at the anode surface interphases have been assumed and compared under the preconditions mentioned above. These ageing model approaches differ in their passivated specific surface area for reactions and double layers. Each of these ageing models aim to describe the effect of the Solid Electrolyte Interphase (SEI) on the impedance spectrum during cyclical ageing. It is assumed that the aging model providing the best possible agreement between simulation and experiment for all spectra of a cycled Lithium-ion battery is also the best approximation of the real physical behavior. We will show that the SEI influences the reaction and the double layer at the anode/SEI interphase as well as the reaction at the SEI/electrolyte interphase. Figure 1 shows a good agreement between experiment and simulation results for the assumed aging model. Relative deviations between measurements and simulations, e.g. in the second semicircle, can be reduced to 10
-

6.

References:

[1] M. Doyle, T.F. Fuller, J. Newman, J. Electrochem. Soc. 140 (1993) 1526

Figure 1
Morphology Impact By Graphite Variations on Silicon-Alloy Negative Electrodes for Lithium-Ion-Batteries
Authors: Becker Lisa Kathrin , Figgemeier Egbert , Sauer Dirk Uwe
Published: February 2020

Today, lithium-ion batteries represent the most widely used secondary battery. They play an important role in the further development of electric mobility or mobile applications (smartphones, laptops). They are characterized by their high energy and/or power density. In order to meet the needs of the future, higher energy and power densities, longer cycle life and higher safety standards are required. Due to their wide range of applications, the expectations of their lifetime and performance are increasing. The energy density is a critical factor for future energy storage systems in mobile applications. In order to achieve higher energy densities, an anode material must combine high specific capacities with high columbic efficiencies.

Due to its low average voltage, low voltage hysteresis, good rate capability, low irreversible capacity, low volume expansion during lithiation causing excellent cycle life, graphite has been the material of choice as anode material [1].

However, the low theoretical specific capacity limits the application potential of graphite anodes. With its high theoretical specific capacity, silicon is the most promising anode material. However, the use of pure silicon is severely restricted by its mechanical behavior during charge/discharge cycling. Crystallization effects and massive volume changes during cycling seriously limit the cycle life of battery cells with Silicon as main component [2]. Therefore, Silicon/graphite blends seem to be the preferred route towards high energy anode formulations with relevant life times.

In this context it is essential to understand the interplay between the various components of the blend electrodes for optimized performances, which is addressed in the current publication.

Different industrially available graphites (KS6, KS6L. MCMB, LFP2, ECC Graphit) and a Silicon alloy material were used for anode preparation in order to investigate their structural morphologies and electrochemical performance of the resulting electrodes. The electrodes were tested in coin cell assemblies and charged and discharged at various rates. The morphologies of fresh and aged electrodes were investigated by means of optical and electron microscopy. Furthermore the volume changes of cells were monitored in order to relate it with the morphological appearance, porosity and graphite type.

References

[1] M. N. Obrovac and V. L. Chevrier, “Alloy Negative Electrodes for Li-Ion Batteries,” Chemi. Rev., pp. 11444–11502, 2014.

[2] W. J. Zhang, “A review of the electrochemical performance of alloy anodes for lithium-ion batteries,” J. Power Sources, vol. 196, no. 1, pp. 13–24, 2011.
Mechanical Deformation-Assisted Alkaline Etching for High Performance Porous Silicon As a Lithium Storage Anode
Authors: Sohn Myungbeom , Park Hyeong-Il , Lee Dong Geun , Kim Hansu
Published: February 2020

Development of lithium-ion batteries (LIBs) has led to the success of portable electronics and LIBs have gained a great deal of attention as power sources for emerging applications such as wearable devices, electric vehicles and robots. However, current LIBs reached technological limits for increasing their energy densities. To address this issue, much research effort has been devoted to finding high capacity electrode materials to replace currently used materials such as graphite anode in LIBs. Silicon (Si)-based anodes have attracted tremendous interest due to the huge theoretical capacity (3579 mAh g-1 and 2190 mAh cm-3) of Si. However, the huge volume expansion/contraction of Si during cycling lead to drastic capacity degradation. Recently, porous silicon materials have been revealed to alleviate the adverse effects arising from the large volume change of Si, thus showing outstanding electrochemical performances. However, their use in LIBs suffers from the complicated and limited routes for their synthesis. In this presentation, we represent a porous Si anode material prepared by a scalable and nontoxic chemical etching process. The resulting porous Si anode exhibited excellent cycle performance with a high discharge capacity as well as high dimensional stability. This material synthesis concept presented herein represents a means of improving the electrochemical properties of porous Si anode materials for use in commercial LIBs.
Engineering Solid State Batteries: A Structural, Mechanical and Interface Engineering Adventure
Authors: Rupp Jennifer L. M. , Pfenninger Reto , Struzik Michal
Published: February 2020

The next generation of energy storage and sensing devices may largely benefit from fast Li+ ceramic electrolyte conductors to allow for safe and efficient batteries and real-time monitoring anthropogenic CO2. Recently, Li-solid state conductors based on Li-garnet structures received attention due to their fast transfer properties and safe operation over a wide temperature range. Through this presentation basic theory and history of Li-garnets will first be introduced and critically reflected towards new device opportunities. Central to our research is the fundamental investigation of the electro-chemo-mechanic characteristics and design of electrode-Li garnet interfaces to new battery architectures. Here, we firstly present new Li-garnet battery architectures for which we discuss lithium titanate and antimony electrodes in their making, electrochemistry and assembly to full battery architectures1-4. Secondly, new insights on processing of Li-garnet thin films are presented based on first developed field maps for pulsed laser deposition processing. Here, the thermodynamic stability range of maximum Li-conduction, phase and nanostructure is discussed using high resolution TEM studies, near order Raman investigations on the Li-bands and electrochemical transport measurements. The insights provide novel aspects of material structure designs for both the Li-garnet structures (bulk to films) and their interfaces to electrodes, which we either functionalize to store energy for next generation solid state batteries.

1) Interface‐Engineered All‐Solid‐State Li‐Ion Batteries Based on Garnet‐Type Fast Li+ Conductors

J van den Broek, S Afyon, JLM Rupp

Advanced Energy Materials 6 (19), 2016

2) A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries

S Afyon, F Krumeich, JLM Rupp

Journal of Materials Chemistry A 3 (36), 18636-18648, 2015

3) On the chemical stability of post-lithiated garnet Al-stabilized Li 7 La 3 Zr 2 O 12 solid state electrolyte thin films

M Rawlence, I Garbayo, S Buecheler, JLM Rupp

Nanoscale 8 (31), 14746-14753, 2016

4) Investigating the all-solid-state batteries based on lithium garnets and a high potential cathode-LiMn1.5Ni0.5O4.

C Hänsel, S Afyon, JLM Rupp

Nanoscale, in press 2016
All Solid-State Thin Film Microbatteries Using Bismuth or Antimony As Anode Material
Authors: Larfaillou Séverin , Bailly Nicolas , Boissel Jean-Marc , Porthault Hélène
Published: February 2020

Thin film all-solid-state secondary microbatteries are extensively studied since the 1990’s [1-3]. These systems present specific features such as reduced surface, high thermal resistance, excellent cycle life, absence of liquid electrolyte, etc. that make them the most suitable energy sources for some emerging applications (real time clock, medical implants, secured smart cards, etc.). All-solid-state microbatteries are realized by successive deposition of thin films with a total thickness of less than 20 µm. The corresponding manufacturing steps are mainly carried out using physical vapor deposition processes such as thermal evaporation or sputtering, which are commonly used in the microelectronics industry. Then, microbattery systems need to tolerate microelectronics constraints, namely the solder reflow (T°C > 260°C) and short-circuits during manufacturing. Lithium metal anode is then no longer satisfactory.

In this work, we study two original anode materials: antimony Sb and bismuth Bi. In a first step, the study was focused on half cell composed of Sb or Bi versus lithium metal with LiPON as electrolyte. Thin film materials exhibit two large plateaus around 1V (Fig.1) which were previously reported for bulk materials [4-5]. Bi and Sb thin films also exhibit good electrochemical performance (capacity retention upon cycling and high C-rate, etc.). Materials were then integrated in full cell configuration: LiCoO2/LiPON/Sb or Bi to develop 3V-microbatteries. The main difficulty in Li-ion configuration is to obtain the good cathode/anode capacity balance. At first, a theoretical study was done to determine the influence of under- or over-balance. Afterwards, in the first realized systems, a low initial capacity and a large capacity loss are observed, mainly due to the poor LiPON/anode interface (Fig.2). Good cycling results were then obtained with an improvement of this anode/electrolyte interface (Fig.3). This system exhibits specific capacity close to the LiCoO2 vs Li one and the lower 3V potential needed for microelectronics application. This lithium-ion system can also tolerate short-circuit without any degradation of active materials and be heated beyond the solder reflow temperature (260°C) due to the absence of metallic lithium.

The authors thank EnSO project for financial support. EnSO has been accepted for funding within the Electronic Components and Systems For European Leadership Joint Undertaking in collaboration with the European Union's H2020 Framework Program (H2020/2014-2020) and National Authorities, under grant agreement n° 692482.

[1] Bates et al., J. Power Sources, 54 (1995) 58-62.

[2] Bates et al., Solid State Ionics, 135 (2000) 33-45.

[3] Fleutot et al., J. Power Sources, 196 (2011) 10289-10296.

[4] Aldon et al., J. Power Sources, 119-121 (2003) 585-590.

[5] Xianming et al., J. Power Sources, 104 (2002) 90-96.

Figure 1
Fluoride Phosphors with Mn4+ Ions: Relations between Structure and Emission Spectra
Authors: Brik Mikhail G. , Srivastava Alok M
Published: February 2020

The search for new phosphor materials that can be excited by blue LED and emit in the red spectral range is a very important for development of white LED with high color rendering index and suitable color correlated temperature for emission either of cold or warm white light.

Fluoride materials like K2SiF6, Na2GeF6 etc are suitable candidates for this role. High degree of iconicity of chemical bonds in fluorides ensures a weak nephelauxetic effect and results in shifting of the Mn4+ emission to shorter wavelengths. In the present work we consider how the structure of the fluorides affects the manganese emission wavelengths. Such factors as geometry of the fluorine octahedra around the Mn4+ ions, chemical bond lengths, angles between the chemical bonds, mutual arrangements of the octahedra (either they are aligned or tilted with respect to each other), the nature of chemical bonds with the ions from the second coordination sphere, polarizability of the chemical bonds etc are all very important for the formation of spectroscopic properties of the Mn4+ ions. Thus, they all should by analyzed carefully before making a final conclusion about suitability of a particular fluoride phosphor for applications.
Investigation on Structure-Energy Relationship for Ce3+ Coordinated with Six or Eight Oxygen Ions Based on First-Principles Calculations
Authors: Ogasawara Kazuyoshi
Published: February 2020

As a red phosphor to improve the color rendering index of white LEDs, (Sr,Ca)AlSiN3:Eu phosphor is widely used. However, since the production cost of this phosphor is high, cheaper alternative is strongly desired. As an alternative, Mn4+-doped fluoride phosphor has been recently put into practical use [1]. In the case of oxide host, however, the shortest emission wavelength reported for Mn4+ is ca. 650 nm [2] which is still longer than 630 nm required for the red phosphor for white LEDs. Therefore, establishment of a design guideline to decrease the emission wavelength for Mn4+-doped oxide phosphor is an important issue. In order to provide such information, we have recently constructed the energy level diagrams and maps for Mn4+ in oxides based on first-principles calculations [3]. On the other hand, Ce3+ phosphor is also a candidate for the red phosphor for white LEDs. In this case, the emission wavelength is much shorter than 630 nm in the case of oxide host. Therefore, a design guideline to increase the emission wavelength is required. Recently an emission at ca. 600 nm was reported for Ce3+ at a six-coordination site in Sr6Y2Al4O15 [4]. In this crystal, there are several sites which can be occupied by Ce3+. However, only Ce3+ at the six-coordination site shows red emission, while Ce3+ at eight- or nine- coordination sites show blue emission. In this work, in order to clarify the effect of local structures such as coordination number, bond length, and symmetry on the energy levels of Ce3+, first-principles relativistic molecular orbital calculations were performed for model clusters with various local structures, and the energy level diagrams and maps were constructed.

As model clusters, CeO6
9- and CeO8
13- clusters with Oh or D4h symmetries were constructed. The first-principles relativistic molecular orbital calculations were performed using the relativistic SCAT code [5]. The calculations were performed for clusters with gradually changed bond lengths and bond angles. Various energy level diagrams and maps depending on the bond length and the bond angle were constructed by connecting the obtained orbital energies.

The energy level diagram for CeO6
9- cluster with Oh symmetry shows that both the crystal field splitting of 5d level and that of 4f level increase as the bond length decreases. As a result, the 4f level and 5d level begin to overlap at ca. 1.9 Å. The relationship between the 4f-5d transition energy and the local structure was also clarified based on various energy diagrams and maps. Such information would be useful for the theoretical search of the novel Ce3+-doped oxide red phosphor for white LEDs.

References

[1] J. E. Murphy et al., SID 2015 DIGEST, 927 (2015).

[2] M.H. Du, J. Mater. Chem., C2, 2475 (2014).

[3] K. Ogasawara et al., ECS J. Solid State Sci. Technol., 5, R3191 (2016).

[4] Y. Kawano et al., Opt. Mater. Express, 4, 1770 (2014).

[5] A. Rosen et al., J. Chem. Phys., 65, 3629 (1976).
(Invited) Cr3+ and Mn4+: Dopants for Near-Infrared Emitting Persistent Phosphors
Authors: Poelman Dirk , De Clercq Olivier Q , Du Jia Ren , Verfaillie Gwenny Veronique , Korthout Katleen
Published: February 2020

Since it was realized a few years ago that near-infrared emitting persistent or ‘glow-in-the-dark’ phosphors could be used for medical imaging applications, there is an active research interest in this class of compounds [1]. Persistent phosphors having an emission spectrum in the wavelength range between 650 and 950 nm, in the so-called first tissue transparency window, can be injected and applied as diagnostic and therapeutic agents. To this end, the afterglow should be bright and long enough, and the injected particles should be bio-compatible.

While most of the persistent phosphors for visible light are based on Eu2+ doped compounds, most materials for near-IR are based on emission from Cr3+ ions [2]. Depending on the host material and the corresponding dopant-lattice interactions, Cr3+ shows sharp emission features due to the 2E-4A2 transition (usually accompanied by phonon side-bands), a broad emission band attributed to 4T2-4A2 or a combination of both. In this work, we focus on LiGa5O8as a host lattice for the Cr ions, an inverse spinel compound where the Ga-ions occupy both tetrahedrally and octahedrally coordinated lattice sites. Using a combination of x-ray absorption and magnetic resonance measurements, the incorporation of Cr in the lattice was studied. In addition, the effect of co-dopants – increasing the number of trap levels for energy storage – on the afterglow characteristics was investigated.

Next to Cr3+, Mn4+ is a very promising dopant for near-IR persistent luminescence. The ion’s electronic configuration is identical to that of Cr3+, therefore the absorption and emission characteristics are also very similar. Mn4+ is currently a popular dopant for red-emitting LED phosphors, but has hardly been considered as a near-IR dopant [3]. The challenge here is to incorporate the ion in its 4+ valence state in the host, which requires the necessary precautions or charge-compensating co-dopants. In the present work, we selected the perovskite LaAlO3 as the host for Mn4+. The afterglow of this material is limited by the limited incorporation of Mn in its 4+ state in the host. The latter is however strongly improved by co-doping with ions with a lower valence state for charge compensation.

[1] Q. le Masne de Chermont et al., Nanoprobes with near-infrared persistent luminescence for in vivo imaging. Proc. Natl. Acad. Sci. 104 (2007) 9266–9271.

[2] P.F. Smet, K. Van den Eeckhout, O.Q. De Clercq, D. Poelman. “Persistent Phosphors.” In Handbook on the Physics and Chemistry of Rare Earths, Including Actinides, ed. Jean-Claude Bünzli and Vitalij K Pecharsky, 48 5 (2015) 1–108. Amsterdam, The Netherlands: Elsevier.

[3] R. Cao et al. , Synthesis and photoluminescence properties of LaAlO3:Mn4+, Na+ deep red-emitting phosphor, Appl. Phys. A 122 (2016) 299.

Figure 1
(Invited) All-Inorganic YAG Phosphor-in-Glass Light Convertor for Next-Generation Modular High-Brightness White LEDs/Lds
Authors: Wang Jing , Zhang Xuejie , Yu Jinbo , Cho Yujin , Xie Rong-Jun
Published: February 2020

Super-high brightness, reliability, and modularization are three key features of state-of-the-art high-brightness solid state lighting, such as high-power white light-emitting diodes (white LEDs) and white laser diodes (white LDs). However, these features are inevitably limited by the organic resin packing material, as a crucial component of the white lighting device, because of its unstable property at high temperature and low thermal conductivity. Here, we report a robust light convertor that can simultaneously play key roles as a phosphor and an alternative encapsulating material via phosphor-in-glass (PiG) engineering. We employed a combination of powder X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), EDS mapping, confocal laser scanning microscope (CLSM), cathodoluminescence (CL) mapping, in conjunction with micro-PL system with a point-by-point scanning mode to study the detailed structure of PiG samples. This Y3Al5O12:Ce3+-based PiG exhibits a high external quantum efficiency of ~60%, a high thermal conductivity of ~0.59 W/mK, exceptional thermal stability, and excellent moisture resistance. By combining the as-synthesized PiG with high-power blue chip-on-board (COB), a high luminous efficacy (92 lm/W) modular white LEDs with a luminous flux up to 1076 lm and a high color rendering modular warm white LEDs (Ra = 90.3 and CCT = 3585 K) are achieved. Moreover, a modular white LDs with a higher luminous efficacy (110 lm/W) is also achieved through blue LDs pumping. The results demonstrate that this easy-fabrication, low-cost, and long-term reliable high-brightness modular white LEDs or white LDs is expected to be a promising candidate for next-generation illumination.
Transparent Ce,Sm:TAG Luminescent Ceramics for High Power Blue Laser Driven Lightning and Display Technologies
Authors: Balci Mustafa H , Chen Fan , Cunbul Ahmet Burak , Svensen Oyvind , Akram Muhammad Nadeem , Chen Xuyuan
Published: February 2020

Phosphor converted warm white light with high luminous efficacy and color-rendering index is crucial for the future of blue laser driven display technologies. Particularly in projection design where the high power (over 150 W) blue laser source is focused on the phosphor, the ideal phosphor candidate has to provide better conversion efficiency; longer lifetime (20000 hours) and lower thermal overheat. Conventional Ce:YAG suffers from efficient green and particularly red emission. Tb and Sm, known as green and red line emitters, respectively, can improve the color quality drastically. Ce, Sm: TAG ceramic powders were synthesized through spray pyrolysis from nitrate salts. Transparent ceramics has been produced via spark plasma sintering. Their phase compositions and structure parameters were checked by X-ray diffraction. The luminous properties of the transparent ceramics are improved when compared to the commercial phosphor wheels (Ce:YAG). Transparent ceramic exhibited broad emission with improved green and red region of the visible light compared to conventional Ce:YAG phosphor. Balanced Sm3+, Ce 3+admixture into the TAG lattice provided excellent color coordinates and CRI for the phosphor for warm white light illumination. Optical and luminescence characteristics were investigated throughout absorbance, excitation and, emission spectra with temperature dependence. Luminous efficacy over 250 lm/W with correlated color temperature (CCT) ~3800 K, color rendering index (CRI) >80 are achieved. The high-power blue diode laser driven temperature increase versus luminous efficacy and quantum efficiency is discussed. The results indicate that Ce, Sm:TAG transparent ceramics can serve as potential phosphors candidates for high-power blue diode laser driven display technologies.
Investigation of Blue Excitation Dependent Conversion Efficiency in Phosphors for High Luminance Applications: Part 2
Authors: Raukas Madis , Avallon James , Kelso John , Lenef Alan
Published: February 2020

This study is a continuation of work discussed earlier [1]. The efficiency of high luminance semiconductor pumped phosphor sources depends not only on efficient generation of primary radiation from the semiconductor (LEDs or laser diodes), but also strongly on the conversion material which is often pushed to optical and thermal limits. Such conversion materials are typically operating upon high excitation flux and thermal load and nearly always exhibit sublinear behavior in output at high incident fluxes, due to several reasons. We have separated contributions from thermal and non-thermal quenching mechanisms, and in particular investigated phosphor saturation, or excitation quenching, in the converted light output as a function of blue pump intensity. It is important to distinguish between the true loss mechanisms (for example via Auger-like or excited state absorption processes) and the reduction in converted output due to increased transparency for the pump light (e.g. activator ground state depletion). The behavior of measured conversion efficiency as a function of excitation intensity is discussed in the context of parameters like temperature, activator concentration and excited state lifetime or host lattice. An updated model describing the observed dependencies in different luminescent systems (e.g. Ce3+ and Eu2+ based) is reviewed.

[1] M. Raukas, J. Avallon, J. Kelso, A. Lenef, 230th ECS Fall Meeting (PRiME 2016), Honolulu, HI, Oct 2-7, 2016.
Rare Earth Doped Nanocrystalline Anatase Powders: Structural, Morphological and Optical Properties
Authors: Đorđević Vesna , Milićević Bojana , Dramićanin Miroslav
Published: February 2020

Titanium dioxide (TiO2) is a wide band gap semiconductor which is used in many applications for decades. Its technical application highly depends on its crystalline structure and morphology. Because of its high photocatalytic activity, anatase is preferred TiO2 form in many applications such as photocatalyst for air and water splitting and purification. TiO2 is also considered as a good host candidate for doping with optically active rare earth ions (RE) for potential application in phosphor materials since it has wide band gap and good thermal and mechanical properties. Sol-gel method is the most used wet chemical method in preparing nano TiO2. When doped with RE(III) crystallinity is highly affected by the RE(III)-Ti(IV) ion mismatch which impact structural, morphological and optical properties.

Our goal was to investigate how doping with various rare earth (RE) ions influence crystallographic, morphological and optical properties of sol-gel synthesized nanocrystalline TiO2. To draw the conclusions, fixed content of RE was kept trough all samples, where RE= Pr, Nd, Sm, Eu, Dy, Tb, Ho, Er and Tm. X-ray diffraction measurements showed decrease of crystallite size from 150 Å for the undoped sample to 70 Å for Nd doped sample, while BET measurements showed an increase of specific area from 10 to 100 m2/g for Nd doped sample. Transmission electron microscopy showed densely aggregated crystalline nanoparticles of irregular shapes, dimensions of about 10-20 nm in size. Diffuse reflectance measurements showed small difference in optical absorption thresholds due to modifications of materials density of states after incorporation of trivalent rare earth ions. Photoluminescence measurements showed efficient emission in the case of doping with the following RE(III) ions: Nd, Sm, Eu, Dy, Ho, and Er, while luminescence of low intensity is detected for: Pr, Tb, and Tm. Basic assessment on photocatalytic activity based on MO degradation was performed and when compared to the undoped sample, photocatalytic performance of nanopowder improves with addition of RE in small concentrations except for Pr and Tb ions.
Facile Synthesis of Graphene Oxide and Graphene Oxide Quantum Dots for Trace Eu3+ and Tb3+ Detection
Authors: Liu Botong , Huang Ling , Huang Wei
Published: February 2020

Top-down synthesis techniques are the most commonly used methods for synthesizing graphene oxide (GO) and graphene oxide quantum dots (GOQDs). In most cases, multistep procedures (at least two steps) or long reaction times (more than 3 h) are required. Herein, a temperature controlled, facile, one-pot solvothermal method is demonstrated that allows us to selectively synthesize pure GO and pure GOQDs within 1.5 hours, respectively. More importantly, the final product of either GO and GOQDs with different sizes can be easily controlled by tuning reaction temperatures or the precursor reactant ratios. GOQDs approximately 2.5 nm in size show excellent photoluminescence that can be utilized for bioimaging or selective detection of trace amounts of Eu3+ and Tb3+ from their respective mixtures with other rare earth and/or transition metal ions, at sub-ppm concentrations.
White-Light Emitting Carbon Dot Species from Carbonization in Zeolite Template
Authors: Ko Seung Hyeon , Park Hongjun , Lee Taekyoung , Ryoo Ryong
Published: February 2020

Over the last few years, extensive research efforts have focused on development of solid-state white-light-emitting materials because of their potential for energy-efficient illumination. Common approaches to generate white light emission are either by properly mixing red, green, and blue-emitting semiconductor quantum dots or integrating them with traditional organic phosphors. However, semiconductor nanocrystals containing Cd or other heavy metals have serious toxicity as well as high costs, which considerably limits their practical applications. In recent years, compared to the quantum dots, carbon-based fluorescent nanodots including carbon dots and graphene quantum dots have emerged as promising substitutes for the present fluorescent nanomaterials because of their competitive optical performance - high photostability, non-blinking photoluminescence and versatile tunability of optical properties.

Herein we report on a facile synthesis of carbon dot species from acetylene using Ca2+ ion catalyst in zeolite template. The resulting product directly exhibits strong white-fluorescence in N-methyl-2-pyrrolidone (NMP) solution under a single wavelength excitation. The photoluminescence quantum yield of the white emission was determined to be 13.7 % under 340 nm excitation in NMP solution. The current synthesis method is a reliable low-cost synthetic technique, rendering potential various scientific and industrial applications.

Figure 1
Nonlinear Electrochemical Impedance Spectrosocpic Analysis of Ti Dissolution in HF
Authors: Srinivasan Ramanathan , Pachimatla Rajesh
Published: February 2020

Electrochemical impedance spectroscopy (EIS) is a versatile technique that is commonly used to characterize electrochemical systems [1]. In EIS conducted in pseudo-potentiostatic mode, sinusoidal potential perturbations are applied and the magnitude and phase of the resulting current perturbations are used to calculate impedance as a function of frequency. Typically small amplitude perturbations are employed and the response is analyzed assuming that the system behavior is linear and that higher harmonics are not present in the response. Most of the commercial equipment can measure the response at fundamental frequency but higher harmonics are usually discarded. If large amplitude perturbations are employed, the resulting current will have higher harmonic components, which, if recorded, can provide more information about the system examined [2].

In this work, we report measurement of nonlinear electrochemical impedance spectroscopic (NLEIS) response of titanium dissolution in HF. A potentiostat (EC301), Lockin amplifier (SR830) and a FFT analyzer (SR780) from Stanford Research System were combined and a software was written to seamlessly acquire NLEIS data. The applied perturbation frequency was varied from 100 kHz to 125 mHz and the amplitude was varied from 20 mV to 100 mV. The response current (both magnitude and phase) at fundamental and higher harmonics (up to 3rdharmonic) was measured using Lockin amplifier and FFT anlayzer. In order to measure the phase and magnitude accurately, the frequencies and span were chosen such that the fundamental and harmonics fall exactly in the measurement bins. No windowing was performed during FFT signal processing. The results are presented in Fig. 1. Open symbols are the results from Lockin amplifier while the filled symbols are results from FFT analyzer. At 20 mV perturbation amplitude, second harmonic was low but measurable, but the third harmonic was below detection limit. At 100 mV perturbation, all three harmonics could be measured. The results of Lockin amplifier and FFT analyzer matched well.

Ti dissolution in HF has been studied earlier using potentiodynamic polarization and EIS [3,4]. A four step mechanism with two adsorbed intermediates was proposed to explain the observed results. NLEIS measurements were performed on Ti dissolving in HF and the experimental results are presented in Fig. 1 as magnitude and phase of current response as a function of frequency. The experimental results were compared with the reaction mechanism model predictions.

Figure 1
In-Situ Estimation of Near Electrode pH during the Electrochemical Reduction of CO2 Using Attenuated Total Reflectance Surface Enhanced Infrared Absorption Spectroscopy
Authors: Dunwell Marco , Yan Yushan , Xu Bingjun
Published: February 2020

Localized changes in pH within the electrochemical double layer during various electrochemical processes has been a well-established phenomenon for decades; however, experimental quantification of surface pH changes is difficult. In this work, we demonstrate a new method for the in­-situ­ determination of pH near the electrode surface using the CO2 reduction reaction as an example system. Briefly, attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is employed to monitor the ratio of carbonate and bicarbonate peaks as a function of electrode potential. Peak areas are compared with those in obtained from calibration spectra at various pH adjusted using potassium hydroxide. In CO2 saturated 1.0 M sodium bicarbonate solution, it is found that the pH near the electrode changes by over 1.3 units during electrolysis at -0.9 V vs. RHE, leading to an underestimation of the applied overpotential by over 0.08 V. The effect of buffer concentration was investigated by repeating the same procedure in 0.25 and 0.5 M sodium bicarbonate solutions. It is observed that despite a significant decrease in current, larger pH gradients arise during electrolysis with decreasing buffer capacity of the electrolyte. In addition to the effect of buffer capacity, the effect of stirring the electrolyte on near-electrode pH is studied using a novel spectroelectrochemical cell for ATR-SEIRAS designed to allow for stirring of the electrolyte during spectroscopic measurements.
Effect of Silver Particles Electrodeposited on Reticulated Vitreous Carbon for Nitrate Reduction
Authors: Oishi Silvia Sizuka , Couto Andrea Boldarini , Botelho Edson Cocchieri , Ferreira Neidenei Gomes
Published: February 2020

Nitrate is a serious contaminant of ground and surface water, which cause high concern in the field of health and environmental protection. Many processes have been used for nitrate removal and the electrochemical treatment is one of the most efficient due to its advantages regarding low cost effectiveness and ability to treat highly concentrated nitrate effluents. The cathode material is an important variable for the nitrate electroreduction process and the development of new electrodes to improve their catalytic activity is a challenge. Modification of electrodes by electrodeposition of metals has been frequently used for electrocatalysis of various reactions in solution and carbon materials are often used as supports for electrocatalysts. In this work, the reticulated vitreous carbon (RVC) was used as support for silver eletrodeposition and the effect of different parameters (applied potential, AgNO3 concentration and deposition time) on the morphology and electrocatalytic activity for nitrate reduction was studied. RVC was processed using poly(furfuryl alcohol) (PFA) resin synthesized according to the best condition established previously. Polyurethane foams with 70 ppi (pores per inch) were used as matrix for the PFA anchorage and after curing, they were heat treated at 1000 °C. All electrochemical experiments were performed in a conventional three electrode cell, using the RVC with electrodeposited Ag as working electrode, a Pt screen and Ag/AgCl as the counter and reference electrodes, respectively. The deposition potentials were chosen through cyclic voltammetry at 0.1, -0.3 and -0.7 V vs Ag/AgCl that can be related to the following regions, respectively: onset of Ag deposition, massive deposition and diffusion controlled deposition. The AgNO3 concentrations solutions used were 5, 10 and 20 mmol L-1, and the deposition times applied were 10, 60 and 180 s. The morphologies obtained by field emission scanning electron microscopy (FESEM) showed that Ag deposition is not uniform due to the RVC three-dimensional structure. Ag dendrites were formed on the highest RVC stems as shown in Figure 1. Ag aggregates of different sizes and shapes dispersed on the entire electrode surface could be observed depending on the experimental condition employed and sample depth. Difference in electrocatalytic activity of the Ag/RVC electrodes for nitrate reduction was confirmed by cyclic voltammetry in 0.1 mol L-1 K2SO4 solution containing 0.01 mol L-1 KNO3 and varied with the Ag amount on electrode surface in addition to its morphology.

Figure 1 - FESEM micrograph of Ag electrodeposited on RVC at -0.3 V (vs Ag/AgCl), with a duration time of 60 s and AgNO3 concentration solution of 10 mmol L-1.

Figure 1
Electrocatalytic Performance of Three Dimensional Electrode Cu/Reduced Oxide Graphene/ Carbon Fiber for Nitrate Reduction
Authors: Couto Andrea Boldarini , Oishi Silvia Sizuka , Sardinha Andre Ferreira , Ferreira Neidenei Gomes
Published: February 2020

Nitrate ions are well-known water toxic inorganic pollutants and their high concentrations can cause a serious problem for both aquatic systems and human health. In this sense, it is of great importance to monitor nitrate ions in various kind of source [1]. Among several analytical technologies that been used for its detection, electrochemical technique offers a simple, rapid, selective, sensitive, inexpensive, safety method for this purpose, not to mention its considered green technology [2]. To improve the nitrate process performance and selectivity, novel electrode materials have been widely investigated. This work aims to develop a sensitive and reliable three dimensional electrode consisting of Cu particles/reduced oxide graphene (rGO)/carbon fiber (CF) for monitoring nitrate concentration. CF produced at heat treatment temperature of 1000 ºC was used in this study. The rGO electrodeposition on CF (1 cm2) was performed at a fixed potential of -1.25 V vs. Ag/AgCl/KCl(sat) for 15 min in a 20 mL of the graphene oxide (3mg mL-1) + 0.1 mol L-1 LiClO4 aqueous solution. The Cu electrodeposition on rGO/CF was carried out at a fixed potential of -0.7 V for 3, 6, 10, 20 min in a 10 mmol L-1 CuSO4 + 100 mmol L-1 H2SO4 aqueous solution. Cu/rGO/CF composites morphologies and structures were analyzed by field emission gun scanning electron microscopy (FEG-SEM), X-ray diffraction, and Raman spectroscopy. Nitrate electro-reduction was evaluated using cyclic voltammetry at 50 mV s-1 in 0.1 mol L-1 K2SO4 solution (pH = 3.0) with and without the presence of 10-2 mol L-1 KNO3. For rGO/CF binary composite, FEG-SEM images showed the characteristic aggregates with irregular and flakes-like shape with high particle density on CF substrate. The Cu deposits depicted small grain morphologies distributed throughout the rGO/CF surface. As expected, Cu deposit rate increased with increasing the deposition time. From X-ray diffraction analyses of the Cu particles, a mixture of phases containing cuprous oxide and metallic copper was identified. Under optimal experimental conditions, the Cu/rGO/CF electrode showed good catalytic activity for nitrate electro-reduction. This behavior can be associated to the presence of rGO and CF with high specific surface area in addition to Cu particle with high electron transfer capability. These results showed that Cu/rGO/CF composite has a great potential as new promising electrode for nitrate electroreduction application.

Acknowledgements: This work was supported by FAPESP, Process 2016/13393-9, CAPES, and CNPq Brazilian Agencies.

References

[1]R. Gürkan, N. Altunay, J. Food Composition Anal., 2016.

[2] J. Martínez, A. Ortiz, I. Ortiz, Applied Catalysis B: Environmental, 207, 2017.