bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact of Mechanical Operation Conditions on PEM Water Electrolyzer Performance
Authors: Borgardt Elena , Giesenberg Lennard , Müller Martin , Lehnert Werner , Stolten Detlef
Published: February 2020

Hydrogen can be generated by means of PEM water electrolysis that makes use of renewably-produced electricity. To improve and maintain good performance of PEM electrolysis cells and stacks, research has been carried out mostly in the field of operation conditions, catalyst loading reduction and substitution of precious metals, two-phase flow investigation and analysing degradation effects. However, understanding mechanical interactions in the cell can additionally help to improve performance and lower investment costs [1]. In the electrolysis cell, the components are clamped against each other to ensure a good electrical contact. Excessive clamping pressure, however, leads to damage of the components. Studies on PEM fuel cells found that high clamping pressures reduces GDL porosity and leads to mass transport limitation [2]. In this work the influence of clamping pressure on the performance of different PEM electrolysis cell designs has been studied. Therefore, a test cell was designed that allows applying pressure directly on the active cell area. Polarisation curves and ohmic resistances were measured at different clamping pressures. Electrochemical impedance spectroscopy (EIS) was used to show the effect of compression on ohmic losses and mass transfer limitations. Furthermore, the pressure change within the cell over time was investigated. For all cell designs, optimal clamping pressure was found to be approximately 2.3 – 3 MPa. The impedance data show no higher mass transfer losses for higher pressures on the active area. However, the ohmic losses rise above a critical pressure. The results indicate that for PEM electrolysis, not mass transport limitations but protonic ohmic losses are crucial for a worse performance at high clamping pressures. Also it was found that stress relaxation of the membrane lead to pressure loss and thus to a worse performance over time.

Borgardt, E., et al., Mechanical characterization and durability of sintered porous transport layers for polymer electrolyte membrane electrolysis. Journal of Power Sources, 2018. 374: p. 84-91.

Mason, T.J., et al., A study of the effect of compression on the performance of polymer electrolyte fuel cells using electrochemical impedance spectroscopy and dimensional change analysis. International Journal of Hydrogen Energy, 2013. 38(18): p. 7414-7422.
On the Effect of Bipolar Plate Mechanical Properties on the Current Distribution of Proton Exchange Membrane Water Electrolysis
Authors: Al Shakhshir Saher , Zhou Fan , Kær Søren Knudsen
Published: February 2020

The durability and lifetime of the proton exchange membrane water electrolysis (PEMWE) is directly affected by the current distribution (CD). Uniform CD indicates uniform electrochemical reaction and vice versa. CD of PEMWE is affected by many factors, such as flow field patterns, catalyst particle distribution in the catalyst layer, and the contact between different components of PEMWE, especially between the bipolar plates and the current distribution plate. The mechanical properties of the bipolar plate such as the ductility and bristliness might affect the current distribution significantly. In this work, the CD is measured at the cathode side using S++ setup, where the segmented plate is placed between the current distributor and bipolar at the cathode side of the PEMWE cell. The segmented plate consists of 100 segments over the 50 cm2, thus each segment area is 0.5 cm2. Two different materials (graphite and Titanium) for the bipolar plates are used in this work, to investigate the effect of bipolar plate mechanical properties on the CD of the PEMWE.

Initial results show that, the PEMWE with titanium flow field plate has shown a non-uniform CD at higher clamping pressure, and a more uniform CD at lower clamping pressure. Meanwhile, the PEMWE with graphite flow field plate has shown a highly uniform CD at higher torque and a less uniform CD at lower torque. Also, it is concluded that higher PEMWE cell performance not necessary means more uniform CD over PEMWE active area.

Figure 1
Two Phase Flow Modeling and Characterization of Oxygen Bubbles in PEM Water Electrolysis Cells
Authors: Nouri-Khorasani Amin , Kwan Jason Tai Hong , Bonakdarpour Arman , Wilkinson David P.
Published: February 2020

The formation, growth, and stability of oxygen bubbles inside the porous transport layer (PTL) pores of a polymer electrolyte membrane water electrolysis (PEMWE) cell have been modeled. In a PEMWE cell, O2 bubbles form and grow inside the anode catalyst layer (ACL) and PTL pores. The most critical impact of the O2 bubbles on the overall performance is the screening of the ACL surface. Semi-empirical relations for the current density-surface coverage for gas evolving electrodes have previously been developed [1] and were used for our PEMWE modeling [2]. This screening effect has been evaluated as an overpotential term, and the results were used to predict the effects of the catalyst and PTL properties on the electrolysis cell performance. The fluctuations in the cell current density were used to identify different types of bubbles and their detachment frequency from the anode electrode.

In this work, the anode catalyst surface has been described as a smooth, planar and horizontal plane. The PTL pores are described as identical straight hydrophilic cylinders with 1-mm height. Such a structure resembles tunable PTL structures such as those synthesized by Kang et al. [3]. Stable O2 bubble nuclei form mainly at the intersection of the anode catalyst layer and the PTL pore wall [3,4].

Our modeling results show the effects of three different types of bubbles in a PEMWE cell: i) nucleation-driven, ii) drag-driven, and iii) buoyancy-driven bubbles. Figure 1 shows the relative lifetime and overpotential effects of the O2 bubbles in a PEMWE with a PTL consisting of 11 μm dia. pores operated at a fixed temperature of 80°C and a balanced pressure of 1 bar [5]. Comparing the current density fluctuations in chronoamperometry experiments with the bubble detachment frequencies obtained through modeling provides a distribution of different types of bubbles in the electrolysis cell. These frequencies vary according to the PTL structure and the PEMWE cell operating conditions. Understanding of bubble growth and detachment mechanisms are of significant importance for reduction of mass transport-related losses and enhancement of PEMWE performance.

References:

H. Vogt and K. Stephan, Electrochim. Acta, 155, 348–356 (2015).

E. Tabu Ojong et al., Int. J. Hydrogen Energy,
42
, 25831-25847 (2017).

Z. Kang et al., Energy Environ. Sci., 10, 166-175 (2017).

O. F. Selamet et al., Int. J. Hydrogen Energy, 38, 5823–5835 (2013).

A. Nouri-Khorasani, E. Tabu Ojong, T. Smolinka, and D. P. Wilkinson, Int. J. Hydrogen Energy, 42, 28665–28680 (2017).

Figure 1
The Effect of Cathode Catalyst on the Performance of Anion Exchange Membrane Water Electrolyzer
Authors: Liu Zengcai , Sajjad Syed D. , Kaczur Jerry J. , Yang Hongzhou , Masel Rich
Published: February 2020

Water electrolyzers are being developed as a way of storing renewable energy, as a way to produce hydrogen for fuel cell automobiles, and as a route to renewable fuels and chemicals. In this paper the effect of changes in the cathode catalyst on the performance of an anion exchange membrane water electrolyzer with Sustainion® membranes running at 60 °C in 1 M KOH. We choose commercial catalysts for study: Raney nickel from W.R. Grace, NiFeCo and Ni8Fe2 from US Nano, NiMoB, FeCoBSi, Ni80Mo20 from Fraunhofer Institute for Manufacturing Technology (IFAM), and NiMo/C from Pajarito Powder(PP). They are all supported on nickel fiber paper. Surprisingly, Raney nickel showed the best performance. At a fixed cell voltage of 2.0 volts in 1 M KOH at 60 °C, we observe 2.9 A/cm² cell current with the Raney Nickel catalyst, compared to 2.3 and 2.1 A/cm² with the US Nano catalysts, 1.7, 1.65 and 1.1 A/cm² with the IFAM catalysts, 1.5 A/cm² with the Pajarito Powder catalyst, and 0.95 A/cm² with the nickel fiber paper alone. These results demonstrate that catalysts that catalysts that show superior performance in a conventional alkaline water electrolyzer, may not show performance superior to Raney nickel in an anion exchange membrane water electrolyzer.

Figure 1
Insight into Oxygen Evolution Reaction Kinetics from Gas Fed Polymer Electrolyte Water Electrolysis
Authors: Schuler Tobias , Kimura Taro , Schmidt Thomas J. , Büchi Felix N
Published: February 2020

In polymer electrolyte water electrolysis (PEWE) oxygen evolution reaction (OER) kinetics are a major efficiency limitation. At least with thin membranes, the anode kinetic overvoltage is the main voltage loss. The reaction mechanism for IrO2 in the most stable surface (110) configuration in acid environment was investigated by DFT calculation in literature and postulated according to reactions R1 to R4 [1]. The concept of gas feed electrolysis enables the identification of the rate determining step in the OER mechanism by using the additional freedom of water concentration variation. Therefore the activity of water was varied between 0.6 to 0.95 at 80°C. The determined reaction order is compared to proposed reaction mechanisms in literature.

The used materials were validated by determination of the IrO2 activity as function of the loading. CCMs were prepared by automated spray coating process using IrO2/TiO2 catalyst. The loadings were varied from 1mg/cm2 to 3mg/cm2. In analogy to liquid PEWE, the CCMs were tested in cell configuration by using commercial Ti sinter based porous transport layers. The loading trends of exchange current densities estimated from Tafel fitting were compared to specific capacity determined by cyclic voltammetry at 1.0V. The linear dependence of specific capacity with loading exemplifies a constant roughness factor as shown in Figure 1. The catalyst layer utilization doesn’t show any impact on loading/catalyst layer thickness.

Reference

[1] J. Rossmeisl, Z.W. Qu, H. Zhu, G.J. Kroes, J.K. Nørskov, Electrolysis of water on oxide surfaces, J. Electroanal. Chem., 607 (2007) 83-89.

Figure 1
Hydrous Iridium-Metal Oxide Nanoarchitectures for High Activity Oxygen Evolution Electrocatalysts
Authors: Rhodes Christopher , Godinez-Salomon Fernando , Albiter Luis , Ying Yuanfang
Published: February 2020

To further the development of proton-exchange membrane (PEM) electrolyzers, oxygen evolution electrocatalysts with high activity, extended durability, and lower costs are needed. Although iridium oxide (IrOx) electrocatalysts have shown high activity and reasonable stability for the oxygen evolution reaction (OER) in acidic conditions, the high costs and limited supply of iridium have motivated our group and other groups to explore approaches that increase the activity and stability of iridium-based catalysts. We have recently demonstrated bimetallic nickel-platinum two-dimensional (2D) nanoframes that function as oxygen reduction electrocatalysts with high specific activity and improved stability.1 Using this approach, we created self-supported metallic Ni-Ir and Co-Ir alloy nanoarchitectures and evaluated their performance as oxygen evolution electrocatalysts. The Ir-M (M=Ni, Co) alloy 2D nanoframes were synthesized by thermal reduction of iridium-decorated metal hydroxide nanosheets followed by chemical leaching. The interaction of iridium with nickel and cobalt was investigated to tune the surface atomic and electronic structure and increase the OER activity. The nanoframes have a self-supported, carbon-free three-dimensional matrix that allows molecular access to the catalytically active surface sites. After electrochemical conditioning within the OER potential range, the surface is predominantly transformed from metallic to a metal oxide/hydroxide, and the metallic alloy phase is retained below the amorphous surface layer. Oxygen evolution activities were determined using rotating disk electrode configuration. The OER mass activities of hydrous iridium-metal oxide nanoarchitectures were significantly higher than those of commercial IrOx. Different temperature treatments were determined to significantly alter the atomic-level structure and influence the electrochemical activity and stability. Additional efforts are aimed at understanding the effect of altering the non-noble transition metal and treatment conditions on the structure and OER activity and stability. The ability to combine highly catalytically active surfaces within a carbon-free 3D nanoarchitecture provides the opportunity to design OER catalysts with improved activity and stability.

References

Godínez-Salomón, F.; Mendoza-Cruz, R; Arellano-Jimenez, M.J., Jose-Yacaman, M.; Rhodes, C.P; Metallic Two-dimensional Nanoframes: Design of Carbon-free Hierarchical Nickel-Platinum Alloy Electrocatalyst Nanoarchitecture with Enhanced Oxygen Reduction Activity and Stability, ACS Applied Materials & Interfaces, 2017, 9, 18660-18674. DOI: 10.1021/acsami.7b00043
Development and Evaluation of Ir Based Anode Electrocatalysts for Water Electrolysis
Authors: Muto Marika , Nagayama Mayumi , Sasaki Kazunari , Hayashi Akari
Published: February 2020

Introduction

Hydrogen production through PEM type water electrolysis followed by hydrogen storage and use has been focused on as one of the efficient uses of renewable energy. Although PEM type water electrolyzer has an advantage on reduced ohmic resistance between two electrodes, it has also a disadvantage that common carbon supports cannot be used in order to disperse small catalyst particles under the high potential condition over 1.5 V at the anode. That leads to low active surface area of anode electrocatalysts, resulting in high cost. Therefore, the purpose of this study is to develop carbon-free porous anode electrocatalysts with high active surface area. So far, our group has been working on mesoporous carbon materials for PEFC studies1, and we have found that such a porous structure is kept even after being built into the device and can improve the mass transfer within catalyst layers. Then, basic concepts of mesoporous carbon are applied to this study. We propose development of carbon-free porous metal catalysts to enhance the performance of water electrolyzer.

Experimental method

Ir(acac)3 and Pluronic F127 were used as a metal precursor and a template to form a porous structure, respectively. A mixture of the metal precursor, solvent, and surfactant was heated up by step-by-step treatments in N2 or Air atmosphere to make the porous structure. After the fundamental material characterizations of the resulting Ir material, electrochemical measurements were performed using a common half-cell setup in the acidic solution. Oxygen evolution reaction (OER) activity was evaluated as anode electrocatalysts. Also, membrane electrode assembles (MEAs) were developed, and then a single cell performance was evaluated.

Results and discussion

Based on nitrogen sorption measurements and a SEM image as shown in Figure 1, the porous structure consisted by aggregation of particles was confirmed for the obtained Ir material. The resulting pore diameter was mostly smaller than 70 nm. When OER activity of porous Ir material was measured and compared with commercial IrO2 electrocatalyst (Figure 2), initial OER activity was lower than IrO2. This was due to fact that porous Ir material is mainly composed by Ir metal, which was confirmed by XRD. Then, the surface of porous Ir was activated via electrochemical oxidation. As a result, activated porous Ir/IrOx showed equivalent OER activity to commercial IrO2 as seen in Figure 2. Even though OER activity was just equivalent to commercial IrO2 as powder, the difference was seen when electrocatalysts were built into MEAs. The details of IV characteristics will be discussed. Furthermore, we have also worked on durability analyses against potential fluctuation derived by renewal energy, which will be also discussed.

Reference

(1) Hayashi, et al., Electrochim. Acta,
53, 6117-6125 (2008).

Figure 1
Surfactant-Free Preparation of Ir Based Oer Catalysts in Low Boiling Point Solvents and Their Catalytic Evaluation
Authors: Bizzotto Francesco , Quinson Jonathan , Bucher Jan , Zana Alessandro , Kunz Sebastian , Arenz Matthias
Published: February 2020

We introduce a simple and scalable preparation method for colloidal precious metal nanoparticles (NPs) that overcomes the typical restrictions in catalyst synthesis, i.e. the independent control of individual parameters such as size, size-distribution, and particle density (metal loading). Our Co4Cat (Colloids for Catalysts) synthesis concept uses alkaline low-boiling point solvents without the use of additional surfactants. It can be applied to different mono- and multimetallic precious metal NPs as well as different support types. In the presented work, the Co4Cat concept is used to prepare Ir based NPs that are tested as oxygen evolution reaction (OER) catalysts for water electrolysis under acidic conditions. In electrochemical half-cell tests, it is shown that due to the very high surface area, exceptional mass activities can be achieved.

The Co4Cat concept is an advancement of our “toolbox” synthesis approach that was previously applied to Pt based PEM fuel cell catalysts. In the presentation, the general idea as well as the individual synthesis steps are discussed in detail and the electrochemical testing is benchmarked against literature values as well as commercially available OER catalysts.
(Invited) Electrocatalysts for Fuel Cell Cathodes
Authors: Stamenkovic Vojislav
Published: February 2020

The studies of the oxygen reduction reaction on well-defined surfaces have guided the approach to design of the real world nanostructured catalysts. Insights from single crystalline systems and thin film surfaces have yielded atomically precise information about surface structure and alloying with other metals in the quest to improve their intrinsic catalytic activity. These experimental efforts are also producing information necessary for the design of catalysts with enhancing stability. These accomplishments were achieved through electrochemical in situ ICP-MS detection of parts per trillion levels of metal dissolution. Atomic level insight, has guided the chemical synthesis of well-defined nanoscale surfaces and creation of novel architectures that are associated with advanced catalytic performance. The progress in platinum-transition metal alloy nanocatalysts will be in focus of this lecture, with the goal of improving the activity and durability of the oxygen reduction electrocatalyst in a proton exchange membrane fuel cell. Atomic level control of the positioning of elements within alloy nanocrystals enables the design of shapes and surfaces that mimic the key descriptors discovered through research on well-defined surfaces. The meticulous characterization of nanocatalysts using extremely high-purity conditions and cutting-edge techniques provides greater understanding of their performance and degradation pathways. This knowledge allows better implementation of electrocatalysts in membrane electrode assemblies (MEA) for demonstration of expected performance in a fuel cell vehicle. An MEA is still viewed as a complex system with multiple interfaces. The catalyst, its support, water, ionomer, and gaseous reactants create challenging environment to control and engineer at the level that would be capable to address all technical targets that are necessary for wide deployment of this technology. The transition from high-purity liquid electrolytes into MEA still needs to be fully addressed in order to boost fuel cell stack performance. More recent research directions, such as platinum group metal (PGM) free catalysts will also be discussed. The main focus is being placed on replacement of platinum by more abundant cost effective constituents that could significantly lower the total cost of fuel cell. Typical PGM-free catalysts are consisted of transition metal sites coordinated by nitrogen dopants within a carbon matrix. While their intrinsic activity requires improvement, stability is equally critical in order to implement this class of materials in real fuel cells. Once again, well-defined, high-precision characterization and synthesis are proving vital to development of new generation of cathode catalysts. The overall research strategy presented here exhibits the merits of a continuous pipeline from fundamental to applied research in the development of fuel cell technology.
The Effect of Sr Doping on La2NiO4+Δ Doped with 10% Mn
Authors: Sadia Yatir , Skinner Stephen J.
Published: February 2020

Cathodes based on K2NiF4 phases have been shown as high performance cathodes for solid oxide fuel cells (SOFC). The main candidate in this family is La2NiO4+δ (LNO) which is known to have high electronic conductivity, high ionic conductivity and good surface exchange at low temperatures (500-600°C). However, performance of these cathodes needs further improvement to compete with the high temperature cathodes such as LaxSr1-xMnO3 (LSM). An increase in the surface exchange is expected by replacing Ni with a low amount of Mn, however such a replacement is not stable and Sr needs to replace the La in the A site to compensate for the small Mn ion. Samples with varying Sr content have been produced using a citric acid sol gel route and their electrical properties have been studied. Changing the Mn:Sr ratio induced a change in the lattice parameters and as such affected properties of the material. The biggest change was an increase in the electronic conductivity across the solid solution range of 10-50% Sr. However, while the 10% sample behaves very similarly to LNO despite having much lower electronic conductivity, Sr segregation in the high Sr content samples, shown using XPS, has some detrimental effects on the cathode behaviour. The correlation between the structure, electronic conductivity, Sr segregation and impedance results will be presented.
Frequency-Resolved Evaluation of Electrode Oxidation State in La0.6Sr0.4CoO3 Thin Films with Operando XAS
Authors: Gerwe Brian S , Mizuno Keita , Sekizawa Oki , Nitta Kiyofumi , Amezawa Koji , Adler Stuart B.
Published: February 2020

Commercial adoption and implementation of renewable, but intermittent, energy sources relies on a new generation of high-performance energy conversion and storage devices, such as solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC). Performance of the oxygen electrode in SOFC is a limiting factor; however, improvements are hindered by inadequate understanding of electrochemical reaction and ionic diffusion processes. AC impedance and DC polarization techniques are typically used to infer relationships between driving energy and reaction rate, but uniquely understanding reaction mechanisms without direct chemical state measurements of the electrode is challenging.

In recent years, workers have developed in operando X-ray absorption spectroscopy (XAS) to study oxygen chemical potentials in SOFC cathodes under operating temperatures and Po2. Determining the oxygen chemical potential is based on shifts in the X-ray absorption edge of 3d transition metals due to valence changes to compensate for oxygen vacancies. So far, operando XAS has only been measured under DC polarization, and thus lacked the ability to probe processes of different timescales [1]. In this work, we extend operando XAS to AC impedance measurements, allowing direct frequency-resolved measurement of electrode oxidation state. This approach can potentially be applied to nonlinear harmonic responses, and the first step in the spatially-resolved mapping of the local steady-periodic electrode response.

A dense film of La0.6Sr0.4CoO3 (LSC) was deposited on a mirror polished Gd-doped ceria pellet by pulsed laser deposition as the working electrode with porous Pt counter and reference electrodes. Absorption spectra of the Co K-­edge were collected from fluorescent X-rays to determine the detection energy most sensitive to changes in Co valence state. Operando XAS measurements were conducted at 973 K in 10% O2 at the predetermined X-ray energy, typically around 7.718 keV, under electrical perturbations large enough to excite moderate nonlinearity.

The frequency resolved oxidation state response of the electrode is obtained by converting X-ray absorption data to oxygen chemical potentials via sets of calibration XAS spectra at various effective Po2. As a result, the data can be treated by the same analysis methods developed for the voltage response in AC impedance measurements.

References

[1] Y. Orikasa, T. Ina, K. Yamamoto, T. Nakao, et al., Electrochemistry
82(10), 897-900 (2014)
(Invited) Expansion in proton-Conducting Ceramic Based Devices
Authors: Ricote Sandrine , Dubois Alexis , Taghikhani Kasra , Braun Robert J. , Berger John , Kee Robert J.
Published: February 2020

High temperature proton conductors such as barium zirconate/cerate (BaCexZr0.9-xY0.1O3- δ referred to as BCZY) have become increasingly more popular, with a large number of potential applications including protonic ceramic fuel/electrolysis cells (PCFC/PCEC) and membrane reactors for steam methane reforming, methane dehydroaromatization or ammonia synthesis. It was also demonstrated that PCFCs can operate with high efficiency while remaining cost competitive in the temperature range 500-600 °C compared to traditional solid oxide fuel cells at 800 °C [1].

Contrary to traditional oxygen ion conductors (such as yttria stabilized zirconia) that display only thermal expansion, chemical expansion is also present in high temperature proton conductors upon hydration (equation 1) where protonic defects are incorporated into the oxygen sublattice [2-4].

(1) H2O (g) + OO
x + VO°° ↔ 2 OHO°

First, techniques for thermal and chemical expansion measurements will be presented together with the analysis/interpretation challenges. Then, an overview of the literature data on the expansion in high temperature proton conductors will be provided. While all these measurements are performed in a single atmosphere, it is important to be able to predict the expansion with different gas compositions on both sides of the membrane to mimic a real device application. To do so, we developed a computational model based on a Nernst-Planck-Poisson formulation and included a chemo-thermo-mechanical component. Defect concentrations and mobilities were taken from previous conductivity measurement modelling [5], whereas mechanical properties (Young modulus, Poisson ratios) were determined experimentally.

References:

[1] Dubois A., Ricote S., Braun R.J., Benchmarking the expected stack manufacturing cost of next generation, intermediate-temperature protonic ceramic fuel cells with solid oxide fuel cell technology. J. Power Sources 369 (2017) 65-77.

[2] Andersson A.K.E., Selbach S.M., Knee C.S., Grande T., Chemical expansion due to hydration of proton-conducting perovskite oxide ceramics. J. Am. Cream. Soc. 97 (2014) 2654-2661.

[3] Hiraiwa C., Han D., Kuramitsu A., Kuwabara A., Takeuchi H., Majima M., Uda T., Chemical expansion and change in lattice constant of Y-Doped BaZrO3 by hydration/dehydration reaction and final heat-treating temperature. J. Am. Cream. Soc. 96 (2013) 879-884.

[4] Hudish G., Manerbino A., Coors W.G., Ricote S., Chemical expansion in BaZr0.9− xCexY0.1O3−δ ( x = 0 and 0.2) upon hydration determined by high-temperature X-ray diffraction. J. Am. Ceram. Soc. 101 (2018) 1298-1309.

[5] Zhu H., Ricote S., Coors W.G., Kee R.J., Interpreting equilibrium-conductivity and conductivity-relaxation measurements to establish thermodynamic and transport properties for multiple charged defect conducting ceramics. Farad. Discuss. 182 (2015) 49-74.
Modeling of Temperature Distribution in a Proton-Conductive Ceramic Fuel Cell
Authors: Araki Takuto , Okuyama Yuji , Li Kunpeng , Toshiki Kawamura , Yamaguchi Toshiaki , Shimada Hiroyuki
Published: February 2020

Proton-conductive Ceramic Fuel Cells (PCFCs) have potential to achieve higher energy efficiency compared with conventional SOFCs, since protons react with oxygen to generate water at cathode side and the fuel at anode side does not diluted. However, some part of generated electrons reacts with transferred electron holes, and the total numbers of electrons in external circuit decrease. The generation and extinction of holes result in loss of energy and excessive heat generation. Then, the hole current should be carefully examined and controlled.

The conductivity of holes strongly depending on local temperature, thickness of electrolyte, partial pressure of oxygen etc.. Therefore, for optimizing the performances of a given PCFCs, coupled analysis considering local temperature, partial pressures, ionic current density is mandatory. Especially, the temperature is one of the strongest factors of the conductivity. Then, the temperature distribution was discussed with results from coupled numerical model considering local temperature distributions, partial pressures, geometries of a cell, and overpotentials.

Figure 1
Development of a Short Stack with Metal-Based Solid Oxide Fuel Cells Fabricated By Sinter-Joining Method
Authors: Lee Sanghun , Jang Young-hoon , Shin Hoyong , Bae Joongmyeon
Published: February 2020

Solid oxide fuel cells (SOFCs) are attracting attentions because of high energy conversion efficiency and fuel flexibility. Transportation is one of the areas where the application of SOFCs is considered. However, SOFCs are composed of ceramic electrolyte and electrode, which makes SOFCs brittle and unsuitable for application under vibrant condition such as vehicle. To overcome low mechanical robustness of SOFCs, metal-based SOFCs are studied to be applied in transportations. In this study, metal-based cells are fabricated by joining metal plate and ceramic cell using silver bonding materials. This fabrication method has advantages of facile fabrication process, full-sintering of cathode, and sintering process only under air atmosphere. To characterize metal-based SOFCs fabricated by sinter-joining method, microstructure, electrochemical performance, and mechanical strength were studied. To develop a short stack using the metal-based cells fabricated by sinter-joining method, basic configuration of stack was designed. To ensure sealing of the stack, finite element method was applied to achieve uniform stress distribution on compressive gasket. As a result, a short stack was successfully fabricated and tested. After test, post-mortem analysis was conducted.
First-Principles Modelling of the Local Atomic and Electronic Structures of Luminescent Centers in Complicated Materials
Authors: Ma Chong-Geng
Published: February 2020

In the last decades, the local coordination environments and valence states of luminescent centers in optical materials have been investigated mainly by using the combination of the spectroscopy techniques, such as the EPR or optical spectra, and the simple and qualitative theoretical models, such as the crystal-field theory (CFT) [1]. However, the reliability of the obtained local structures and oxidation numbers of doped optical centers is still questionable due to the empirical character of those models. Nowadays, the quick development of quantum chemistry theory for strongly-correlated d- and f-elements [2], together with an increase of the computational resources, has made it possible to quantitatively predict the structural and electronic properties of lanthanide- and transition metal(TM)- doped materials and further provide a deeper insight into the measured spectroscopic data.

In this talk, we will employ three typical examples to demonstrate how the first-principles calculations model the local structures and valence states of impurities in optical materials. First, the loss of the local inversion symmetry of Eu3+ ions doped in disordered compound Ca2La3Sb3O14 will be theoretically confirmed and the loss mechanism obtained here may be used for other systems as well to enhance spectral intensity for an improvement of characteristics of phosphor materials [3]. Second, the first-principles calculations of Cr-doped ZrSiO4 will show us the conversion mechanism from a pair of Cr4+ ions formally appearing in the chemistry concept to the combination of both Cr3+ and Cr5+ centers [4], which indicates the treatment of the valence states of TM ions should be so delicate as to need to be paid more attention. Third, a simple calculation scheme for the structural determination of the 5d lowest excited state of lanthanide ion is proposed and successfully applied to the case of Tb3+-doped Cs2NaYF6
[5]. In addition, a CFT-based methodology for solving the multiple minima problem in the HSE06 calculations of lanthanide-doped materials is suggested and applied to cubic elpasolites doped with Ln
3+ ions (Ln=Ce-Tm), and the calculation results present a good agreement with empirical “zigzag” model of Dorenbos [6].

References:

[1] N.M. Avram, and M.G. Brik, Optical Properties of 3d-Ions in Crystals: Spectroscopy and Crystal Field Analysis, Springer and Tsinghua University Press, 2013.

[2] R. Gillen, S.J. Clark, and J. Robertson, Phys. Rev. B
87 (2013) 125116.

[3] D.X. Liu, C.-G. Ma, P.W. Hu, Z. Li, Y. Tian, P. Su, M.G. Brik, A.M. Srivastava, and S. Tanabe, J. Am. Ceram. Soc. DOI: 10.1111/jace.15406, 2018.

[4] M. Gaft, G. Boulon, G. Panczer, Y. Guyot, R. Reisfeld, S. Votyakov, and G. Bulka, J. Lumin.
87-89 (2000) 1118.

[5] C.K. Duan, P.A. Tanner, A. Meijerink, and V. Makhov, J. Phys. Chem. A
115 (2011) 9188.

[6] P. Dorenbos, Phy
. Rev. B
85 (2012) 165107.
First-Principles Calculations of 4f-5d Transition Energy of Ce3+ in Y(Al1-xGax)5O12
Authors: Ogasawara Kazuyoshi
Published: February 2020

The crystals doped with Ce3+ are utilized as various optical materials such as solid-state lasers, scintillators, and phosphors. Since the 4f-5d transition energy of Ce3+ in crystals strongly depends on the coordination environment, it is important to understand the relationship between the energy levels and the coordination environment for theoretical design of novel optical materials. For this purpose, garnet solid solutions are useful as host crystals since the physical properties can be modified by forming variety of solid-solutions through cation substitution. For example it is known that the 4f-5d transition energy of Ce3+ in Y(Al1-xGax)5O12 increases monotonically as x increases [1].

In this work, in order to investigate the relationship between the 4f-5d transition energy and the coordination environment of Ce3+ in Y(Al1-xGax)5O12, first-principles calculations were performed using the relativistic DV-Xα molecular orbital method [2]. At first YO8 clusters with D2 symmetry were constructed based on the crystal structure of Y(Al1-xGax)5O12 [3,4]. Then Ce was substituted for Y and the Ce-O bond lengths were adjusted based on the Shannon’s crystal radii [5] so as to consider the lattice relaxation effect [6]. In order to produce the effective Madelung potential, point charges were located at the atomic sites outside the cluster. As the exchange-correlation potential, the VWN potential [7] with the relativistic correction [8] was adopted. The 4f-5d transition energy was calculated using the Slater’s transition state method [9].

The calculated results showed that the variations of the average energy and the crystal field splitting of 5d levels of Ce3+ depending on x can be understood by the variations of the bond lengths, they are not monotonic. Although the 4f-5d transition energy is basically determined by these factors, interestingly the resultant theoretical 4f-5d transition energy showed monotonic variation. As a result the experimentally observed increasing tendency of the 4f-5d transition energy for increasing x was well reproduced.

[1] P. Dorenbos, J. Lumin., 134, 310 (2013).

[2] A. Rosén, D.E. Ellis, H. Adachi, and F.W. Averill, J. Chem. Phys., 65, 3629 (1976).

[3] A. Nakatsuka, A. Yoshiasa, and S. Takeno, Acta Crystallogr., B51, 737 (1995).

[4] A. Nakatsuka, A. Yoshiasa, and T. Yamanaka, Acta Crystallogr., B55, 266 (1999).

[5] R.D. Shannon, Acta Crystallogr.,
A32, 751 (1976).

[6] S. Watanabe, T. Ishii, K. Fujimura, and K. Ogasawara, J. Solid State Chem., 179, 2438 (2006).

[7] S. H. Vosko, L. Wilk, and M. Nusair, Can. J. Phys. 58, 1200 (1980).

[8] A. H. MacDonald and S. H. Vosko, J. Phys.
C12, 2977 (1979).

[9] J. C. Slater, Quantum Theory of Molecules and Solids (McGraw-Hill, New York, 1974), Vol. 4.
High-Stable Narrow Red Emitting Homogeneous K2SiF6:Mn4+@K2SiF6 Composite Phosphor for White LEDs: Green Synthesis and Water-Resistant Mechanism
Authors: Wang Jing , Huang Lin
Published: February 2020

Poor water resistance and non-green synthesis remain a great challenge for commercial narrow red emitting A2MF6:Mn4+ (A=alkali metal ion; M=Si, Ge, Ti) for solid state lighting and display. We develop here a simple and green growth route to synthesize homogeneous K2SiF6:Mn4+@K2SiF6 (KSFM@KSF) red emitting composite phosphor with excellent water resistance and high efficiency without the usage of toxic and volatile HF solution. After immersed into water for 6 hours, the as-obtained water-resistant products maintain 76% of the original emission intensity while the non-water-resistant ones steeply drop into 11%. Under 85% humidity and 85 oC for 504 h (21 days), the as-obtained water-resistant products remain 80-90%, much higher than 30-40% for the non-water-resistant products. Surface deactivation enabled growth mechanism was proposed, comprehensively investigated in detail. This composite structure strategy was also successfully used to treat KSFM prepared by other methods. We believe that it will open the opportunity for excellent anti-moisture narrow red emitting A2MF6:Mn4+ to be environment-friendly synthesized in large scale toward white LED industry adoptions.
Luminescent Thermometry Using Mn4+ Doped Li2TiO3 Probe
Authors: Kuzman Sanja , Ristic Zoran , Milićević Bojana , Medic Mina , Djordjevic Vesna , Lukic Petrovic Svetlana R. , Dramicanin Miroslav
Published: February 2020

Luminescent thermometry is extensively investigated because it provides sensitive contactless temperature measurments. This method is proposed for measurements at place which are difficult to access and in a nanoscale environments. Two types of self-referenced luminescent thermometry methods were commonly used: fluorescence intensity ratio method, and thermometry based on an emission decay times.

Titanates activated with transition metals have wide applications. In this paper we show that Mn4+ doped Li2TiO3 powder can be used for the luminescence thermometry. Here, we investigated possibility of using this material for the temperature determination from emission decays over the room temperature to 100 deg temperature range. Emission decay curves were measured at the strongest emission peak (680 nm). Data analysis showed that thermometry with this probe shows 3.8%/K relative sensitivity and superiour temperature resolution of 0.03 K over the complete temperature range.

Figure 1
Synthesis, Luminescence and Crystal Field Calculations of Mn4+- Doped Li2TiO3 Deep Red Phosphor
Authors: Milićević Bojana , Medic Mina , Djordjevic Vesna , Papan Jelena , Kuzman Sanja , Wu Mingmei , Srivastava Alok M , Brik Mikhail G. , Dramicanin Miroslav
Published: February 2020

Red emission of the Mn4+ ions incorporated in different materials is used in various applications, such as lighting, holographic recording, thermoluminescence dosimetry, thermometry, etc. Mn4+ emission can improve color-rendering-index of phosphor-converted white-light-emitting diodes, and it is a promising candidate to replace the well-established rare-earth Eu activator in red phosphors. Here, synthesis, structure, detailed spectroscopic and crystal field analysis of Mn4+ doped Li2TiO3 are presented. We have successfully synthesized Li2Ti0.995Mn0.005O3 powder by a solid state synthesis. All precursors were mixed with stoichiometric ratio Li : Ti = 2 : 1 and calcined at 800⁰C for 5 h to produce pure-phase. Structural analysis was done by X-ray diffraction. It confirmed monoclinic Li2TiO3 structure with space group C2/2, in which the octahedral sites are occupied by Ti4+ and Mn4+. Optical characterization included measurements of photoluminescence excitation and emission spectra, emission decay analyses and diffuse reflection measurements. Optical spectra showed emission and absorption bands originating from d–d electronic transitions (d3 configuration). Kubelka-Munk function of the measured diffuse reflectance spectrum showed characteristic absorption of Mn4+. Deep red emission at 680 nm under λexc= 490 nm is a result of the spin-forbidden 2Eg→4A2 and 4T2→ 4A2 electron transition of the tetravalent manganese ions. The energy levels of the Mn4+ ions in Li2TiO3 were calculated using the crystal field exchange-charge model. Three parameters are needed to describe completely each energy level: the Racah parameters B and C, which determine the positions of the free ion energy levels, and the crystal field strength denoted by Dq, which describes the splitting of the free ion terms. These parameters were calculated and showed good agreement with the experimental photoluminescence excitation and emission spectra.

Figure 1
Discovery of New Solid State Materials for Phosphors and Their Photoluminescence Tuning
Authors: Xia Zhiguo
Published: February 2020

Understanding and exploring the structural design principles of the inorganic hosts are necessary for the discovery of novel phosphors for white LEDs. Our recent work focuses on the discovery of new phosphor materials based on the structural evolution and the crystallographci sites engineering. Firstly, photoluminescence tuning can be realized via the cation substitution on one site, and hereby we have discovered several types of M2SiO4-based and M3(PO4)2-based phosphors with tunable emission for white LEDs.1-2 Secondly, we have recently proposed the concept of “chemical unit cosubstitution” as one potential design scheme, and the chemical unit cosubstitution strategy is applied to the melilite structure class designing the Ca2(Al1−x
Mg
x
)(Al1−x
Si1+x
)O7:Eu2+ solid solution phosphor via the [Mg2+−Si4+] for [Al3+−Al3+] cosubstitution.3 We also design the La5Si3O12N phase from La5Si2BO13 via the [B3+−O2−] by the [Si4+−N3−] cosubstitution.4 Thirdly, we have proposed a new insight into the design of the isostructural phases via the filling of M+ in the void channels of the Mg2Al4Si5O18 phase, so that the charge balance can be kept while the chemical composition varied, and this strategy will be also efficient in other porous inorganic hosts.5 Finally, we have recently studied the NaScSi2O6-based phosphor from CaMgSi2O6-based phosphor via the chemical unit cosubstitution of [Na+–Sc3+] for [Ca2+-Mg2+] unit.6 Further studies show that the isostructural solid-solution of (CaMg)
x
(NaSc)1-x
Si2O6 (0 < x < 1) can be formed, in which cation nanosegregation enables the presence of dilute Eu2+ at two centers and their intensities are linearly proportional to x, so that it represents a new cation nanosegregation tuning of photoluminescence for the exploration of color-tunable phosphor for white LEDs.7-10

References:

[1] H. Ji, Z. Huang*, Z. Xia*, et. al., J. Phys. Chem. C, 2015, 119 2038.

[2] M. Chen, Z. Xia* M. Molokeev, Q. Liu, Inorg. Chem., 2015, 54, 11369.

[3] Z. Xia*, C. Ma*, M. Molokeev, K. R. Poeppelmeier*, et. al. J. Am. Chem. Soc., 2015, 137, 12494.

[4] Z. Xia*, M. Molokeev, W. B. Im*, S. Unithrattil, Q. Liu,
J. Phys. Chem. C, 2015, 119, 9488.

[5] J. Zhou, Z. Xia*, M. Chen, M. Molokeev, Q. Liu, Sci. Rep., 2015, 5, 12149.

[6] Z. Xia*, Y. Zhang, M. Molokeev, V. Atuchin, Y. Luo, Sci. Rep., 2013, 3, 3310.

[7] Z. Xia*, K. R. Poeppelmeier*, Acc. Chem. Res., 2017, 50, 1222.

[8] Z. Xia*, G. Liu*, K. R. Poeppelmeier*, et. al., J. Am. Chem. Soc., 2016, 138, 1158.

[9] Z. Xia*, and Q. L. Liu, Prog. Mater. Sci., 2016, 84, 59-118.

[10] Z. Xia*, and A. Meijerink*, Chem. Soc. Rev., 2017, 46, 275-299.
Layered Li-rich Oxide Cathode Microparticle 0.4Li2MnO3∙0.6Li(Mn0.43Ni0.36Co0.21)O2 Decorated with Nano Grains and its Electroochemical Performance
Authors: Wang Sungeun , Jung Dae Soo
Published: February 2020

The development of alternative energy sources has become the primary strategy to overcome unstable crude-oil prices and the issues associated with climate change. Seasonal variations in alternative energy sources have prompted the need for an energy storage system. Reduction of carbon dioxide is also a key strategy for solving the problem of climate change. Carbon dioxide produced by transportation constitutes almost one-third of total carbon dioxide production. Replacing gasoline-based transportation with electric vehicles represents a promising solution to reduce carbon dioxide emissions.

Among the various types of batteries, lithium secondary batteries are considered the most attractive option because of their light weight and high power density. However, the current technical specifications are not quite satisfactory for full scale commercialization of electric vehicles (EVs) and hybrid electric vehicles (HEVs). Increasing driving range and reducing the cost of batteries remain key issues that need to be addressed.

Nano-composite layered compounds of xLi2MnO3·(1-x) LiMO2 (M = Ni, Co, Mn) were recently shown to exhibit high capacities in the high voltage region of –4.6 V and have lower cost and better safety compared to LiCoO2, which is used in commercial lithium-ion batteries. However, the layered xLi2MnO3·(1-x) LiMO2 shows low rate capability due to low electronic conductivity, which is caused by the insulating Li[Li1/3Mn2/3]O2 component. Therefore, the low rate capability of these batteries needs to be addressed for use in EVs and HEVs, which require fast charge/discharge reactions.

Various methods have been developed to improve the rate capability of layered xLi2MnO3·(1-x) LiMO2 composite particles, such as doping with another transition metal, surface modification, and coating. However, attempts to control the morphology and particles size has not been reported due to limitations in the preparation methods. The cathode particles are normally prepared by a solid-state reaction or liquid phase methods and these methods require a high calcination temperature and long calcination time to obtain desirable composite phase. These calcination conditions result in aggregation and growth of primary particles, which degrades the diffusion of lithium ions between the electrode and electrolyte.

Spray pyrolysis has recently been used for the preparation of multicomponent ceramic particles due to its several advantages, such as being a simple system, ease of controlling particle size and preparing spherical particles of multi-components and formation of a pure well-crystallized phase at low calcination temperature. Thus, in this study, we prepared spherical 0.4Li2MnO3∙0.6Li(Mn0.43Ni0.36Co0.21)O2 microparticles decorated with nano-grains via spray pyolysis and the structural and electrochemical performance of the prepared particles were investigated.