Fundamental Degradation Mechanisms Research Articles

Enhanced steel machining performance using texture-controlled CVD alpha-alumina coatings: Fundamental degradation mechanisms

Cemented carbide inserts coated with CVD α-alumina, particularly those exhibiting a (0001) texture, have proven highly effective in steel turning. Despite the established superior performance of (0001) textured alumina coatings, the underlying reasons remain unclear. This study explores the influence of the crystallographic texture of alumina on wear mechanisms in various chip-tool contact zones on the insert rake face. The objective is to establish a fundamental understanding of the active degradation mechanisms and machining performance by relating coating texture to the orientation and deformation of individual Al2O3 grains. Two multilayered coatings, Al2O3 on Ti(C,N), featuring (0001)- and (112‾0)-textured CVD α-alumina, were assessed in dry turning of a bearing steel. The wear rate of the (112‾0) coating was double that of the (0001) coating. Worn coatings exhibit nano-terrace formation at the insert edge, likely due to chemical etching. In the sticking zone, plastic deformation leads to larger facets for grains oriented with the chip flow direction, while rounded surfaces result if this condition is not met. In the transition zone, both (0001) and (112‾0) textured coatings undergo increased plastic deformation accompanied by sub-surface dislocations. (0001) texture deforms more by basal slip creating a wavy coating pattern with steps present at larger misalignments of the lattice planes in neighboring grains while (112‾0) texture deforms by several slip systems creating elongated ridges and ruptured-like areas resulting in rougher surface. This difference in surface morphology is then inherited by the abrasion of submicron coating fragments embedded in the chip (more in (112‾0) texture) in the sliding zone resulting in an even rougher surface. Chemical reaction with the hot chip may also contribute to wear acting as an additional mechanism. This fundamental understanding contributes to the potential enhancement of steel machining using texture-controlled CVD alumina coatings, ultimately improving coated cutting tool performance.

A review on computational modeling of instability and degradation issues of halide perovskite photovoltaic materials

AbstractHybrid halide perovskite solar cells have been recognized as one of the most promising future photovoltaic technologies due to their demonstrated high‐power conversion efficiency, versatile stoichiometry and low cost. However, degradation caused by environmental exposure and structural instability due to ionic defect migration hinders the commercialization of this technology. While the experimental studies try to understand the phenomenology of the degradation mechanisms and devise practical measures to improve the stability of these materials, theoretical studies have attempted to bridge the gaps in our understanding of the fundamental degradation mechanisms at different time and length scales. A deeper understanding of the physical and chemical phenomena at an atomic level through multiscale materials modeling is going to be crucial for the knowledge‐based prognosis and design of future halide perovskites. There have been increased efforts in this direction in the last few years. However, the instability fundamentals explored through atomistic modeling and simulation methods have not been reviewed comprehensively in the literature yet. Therefore, this paper is an attempt to present a critical review, while identifying the existing gaps and opportunities in the investigation of the degradation and instability issues of the halide perovskites using computational methods. The review will primarily focus on the instability caused due to the intrinsic ionic defect migration and degradation due to thermal, moisture and light exposure. The findings from the simulation studies conducted primarily using density functional theory, ab initio molecular dynamics, classical molecular dynamics and machine learning methods will be presented.This article is categorized under: Software > Molecular Modeling Structure and Mechanism > Computational Materials Science Data Science > Artificial Intelligence/Machine Learning

(Battery Division Early Career Award Sponsored by Neware Technology Limited) Design, Synthesis, and Characterization of Cathode Microstructures in Lithium Batteries

The propagation of redox reactions governs the electrochemical properties of battery materials and their critical performance metrics in battery cells. The recent research progress, especially aided by advanced analytical techniques, has revealed that incomplete and heterogeneous redox reactions prevail in many electrode materials. Advanced high-capacity cathode materials are mostly polycrystalline materials that exhibit complex charge distribution (the valence state distribution of the redox-active cations) due to the presence of numerous constituting grains and grain boundaries. The redox reactions in individual grains typically do not proceed concurrently due to their distinct geometric locations in polycrystalline particles. As a result, these unsynchronized local redox events collectively induce heterogeneous and anisotropic charge distribution, building up intergranular and intragranular stress. Therefore, these polycrystalline materials may exhibit weak mechanical stability, leading to undesired chemomechanical breakdown during battery operation. Grain engineering in polycrystalline materials provides a large playground to modulate the materials properties beyond controlling the chemical composition, and electronic and crystal structures. In particular, the anisotropic ion-conducting pathways in layered oxides make the grain crystallographic orientation a critical factor in determining the modality of the redox reactions in these materials.This presentation will discuss our recent progress in the design, synthesis, and characterization of cathode microstructures in lithium batteries.First, we will discuss how the charge distribution is guided by grain crystallographic orientations in polycrystalline battery materials. We elucidate the spatially resolved charge distribution in lithium layered oxides with different grain crystallographic arrangements and establish a model to quantify their charge distributions. While the holistic “surface-to-bulk” charge distribution prevails in polycrystalline particles, the crystallographic orientation-guided redox reaction governs the charge distribution in the local charged nanodomains. Compared to the randomly oriented grains, the radially aligned grains exhibit a lower cell polarization and higher capacity retention upon battery cycling. The radially aligned grains create less tortuous lithium-ion pathways, thus improving the charge homogeneity as statistically quantified from over 20 million nanodomains in polycrystalline particles. This study provides an improved understanding of the charge distribution and chemomechanical properties of polycrystalline battery materials.Second, we will discuss how the grain arrangement affects the thermal stability of polycrystalline cathode materials in rechargeable batteries. We performed a systematic in situ study on the Ni-rich polycrystalline cathode materials to investigate the fundamental degradation mechanism of charged cathodes at elevated temperatures, which is essential for tailoring material properties and improving performance. Using multiple microscopy, scattering, thermal, and electrochemical probes, we decoupled the major contributors to the thermal instability from intertwined factors. Based on our findings, the cathode grain microstructure has a forgotten yet important role in the thermal stability of polycrystalline rechargeable batteries. Oxygen release, as an important process during the thermal runaway, can be regulated through engineering grain arrangements. The grain arrangement can modulate the macroscopic crystallographic transformation pattern and oxygen diffusion length in layered cathodes to offer more possibilities for cathode material design and synthesis.Third, we will discuss our new understanding of particle behaviors in composite cathodes. We capture and quantify the particle motion during the solidification of battery electrodes and reveal the statistics of the dynamically evolving motion in the drying process, which has been challenging to resolve. We discover that the particle motion exhibits a strong dependence on its geometric location within a drying electrode. Our results also imply that the final electrode quality can be controlled by balancing the solvent evaporation rate and the particle mobility in the region close to the drying surface. We formulate a network evolution model to interpret the regulation and equilibration between electrochemical activity and mechanical damage of these particles. Through statistical analysis of thousands of particles using x-ray phase-contrast holotomography in a Ni-rich cathode, we found that the local network heterogeneity results in asynchronous activities in the early cycles, and subsequently the particle assemblies move toward a synchronous behavior. Our study pinpoints the chemomechanical behavior of individual particles and enables better designs of the conductive network to optimize the utility of all the particles during operation.

The Role of Gas Evolution in Particle Surface Cracking in Nickel-Rich Lithium-Ion Cathode Materials

Nickel-rich layered oxide cathode materials (LiNixTM(1-x)O2 where x > 0.8) are of great interest because they offer increased capacity compared to current commercial materials while maintaining compatibility with supply chain and production processes already in place. However, recent studies have shown that these materials in their current form are unsuitable for commercial applications due to the accelerated degradation caused by replacing more stable transition metals with the relatively unstable nickel. While various strategies have already been proposed to mitigate this issue, the fundamental degradation mechanism still needs to be better understood to inform the design of the next generation of nickel-rich cathode materials. In this work, differential electrochemical mass spectrometry (DEMS) is combined with titration mass spectrometry (TiMS) to measure gases evolved in a lithium half-cell during cycling as well as surface species which evolve gas upon addition of strong acid to an extracted cathode. Along with qualitative observations of particle cracking by scanning electron microscopy (SEM), these results reveal correlations between particle cracking, electrolyte reactivity, and carbonate oxidation and deposition on the changing cathode surface in nickel-rich cathode materials.

Effect of porous structure and morphology of cathode on the degradation of lithium-ion batteries

This study investigates the effect of the micromorphology of cathodes with 3D porous structures and geometries on the degradation and electrochemical performance of lithium-ion batteries. Active particle aggregates with different particle diameters and volume fractions are randomly positioned to generate a 3D structure of the cathode in simulation. A physics-based electrochemical model that describes the degradation of the separator, anode, and cathode is developed. Simulation results show that aggregates with a smaller particle radius have a larger surface area, which leads to a higher capacity with a lower concentration gradient but also severe overall degradation. However, with a larger surface area, the volume fraction of the cathode decreases faster, whereas that of the anode decreases slower because of the difference in the fundamental degradation mechanisms of both electrodes. These results indicate that a cathode with a smaller surface area and an anode with a larger surface area are preferred to minimize battery degradation. This morphological information also impacts the diffusivity, which must be carefully optimized for maximizing the capacity and minimizing the degradation. The model developed in this study is expected to be useful to understand degradation mechanisms and to optimize battery design and manufacturing.

Valve Regulated Lead Acid Battery Evaluation under Peak Shaving and Frequency Regulation Duty Cycles

This work highlights the performance metrics and the fundamental degradation mechanisms of lead-acid battery technology and maps these mechanisms to generic duty cycles for peak shaving and frequency regulation grid services. Four valve regulated lead acid batteries have been tested for two peak shaving cycles at different discharge rates and two frequency regulation duty cycles at different SOC ranges. Reference performance and pulse resistance tests are done periodically to evaluate battery degradation over time. The results of the studies are expected to provide a valuable understanding of lead acid battery technology suitability for grid energy storage applications.

Characterization of Polycrystalline Cathode Materials in Five Dimensions (5D)

Thermal stability is one of the most desired features for large-scale energy applications, such as batteries used in electronics and electric vehicles. Thermal runaway, heat generation, and gas release are likely to happen for batteries under thermal abuse conditions, leading to severe performance degradation and safety risks. Although the decomposition of electrolytes at high temperatures has been widely studied in the community, a systematic and in situ study of the intrinsic instability of the cathodes in Li ion batteries is lacking. Polycrystalline nickel-rich Li layered oxides (LiNi x Mn y Co1-x-y O2, NMC) were chosen as the platform materials considering the recent improvements in specific capacity but also the rising safety concerns in Ni-rich cathodes.Here we used one of state-of-the-art techniques in microscopy: the full-field transmission X-ray microscope (TXM) to capture the 3D tomography of charged NMC particles, together with the X-ray absorption near edge structure (XANES) spectroscopic capability for Ni oxidation state. Moreover, we performed the XANES-3DTXM measurements during the whole process of heating the NMC cathodes from room temperature up to 250°C, which enables us to monitor the full evolution of NMC secondary particles in real time in 5 dimensions (3D spatial + tunable X-ray energy + temperature).This in situ XANES-3DTXM approach allows us to observe the real-time morphological and chemical evolution at elevated temperatures. The large field of view and reasonably high spatial resolution offer the capability to analyze NMC cathodes at different length scales, ranging from voids of tens of nanometers, to cracks of hundreds of nanometers, to secondary particles of tens of micrometers.With quantification of morphological (e.g., relative volume change, porosity, crack propagation) and chemical (e.g., the oxidation state of Ni, local charge distribution) characteristics as a function of temperature and time, the fundamental degradation mechanisms in polycrystalline nickel-rich Li layered oxides under thermally abuse conditions are thereupon unveiled. Based on these analyses and discoveries, we proposed to alter the primary particle arrangement as an effective solution to alleviate the thermal issues in nickel-rich Li layered oxides.Figure: Schematic of the in situ heating setup, 3D rendering of a NMC secondary particle, and selected morphology section and Ni oxidation state mapping at different temperatures. Figure 1

(Invited) Vertical Gallium Nitride PN Diodes for Grid Resiliency and Medium-Voltage Power Electronics

Due to their fast avalanche breakdown response, gallium nitride (GaN) diodes with vertical device geometry hold promise for protection of the electric grid against fast (<1 us) voltage transients induced by electromagnetic pulses (EMPs). Additionally, comparable critical electric field and superior mobility relative to silicon carbide (SiC) suggest that vertical GaN devices compare favorably to SiC for medium-voltage (defined here as 1.2 to 20 kV) power electronics, with the advantage becoming more pronounced as the voltage rating exceeds 10 kV. However, vertical GaN power devices are significantly less mature than their SiC counterparts, and further work on epitaxial growth, characterization of fundamental material properties, device processing, manufacturability, and reliability / failure analysis is needed. This talk will describe our team’s efforts to advance the state-of-the-art for vertical GaN PN diodes. One key challenge is the growth of thick (tens of um), controllably low-doped (<1016 cm-3) drift layers on native GaN substrates by metal-organic chemical vapor deposition (MOCVD), and our team has achieved >50 um thick layers with net doping in the low-1015 cm-3 range suitable for medium-voltage devices. Additionally, significant challenges exist in the processing of such devices, notably in the edge termination used to prevent premature breakdown. Using the aforementioned drift layers coupled with a multi-step-etched junction termination extension (JTE), our team has demonstrated PN diodes with breakdown voltage >4 kV and specific on-resistance <4 mOhm cm2. Future work aims to extend the growth and fabrication such that devices up to 20 kV breakdown are achieved. Additionally, a parallel foundry effort aiming to establish the manufacturability of vertical GaN technology, focused thus far on 1.2-kV-class devices, has performed extensive metrology on incoming epitaxial GaN wafers prior to fabrication of PN diodes with areas ranging from ~0.1 up to 1 mm2. Using various combinations of implanted guard rings and JTEs as the edge termination, devices with breakdown voltages >1.2 kV have been achieved, and the yield has been evaluated based on a combination of both breakdown voltage and the forward current characteristic. The foundry effort will be extended to 3.3 and later 6.5 kV devices, possibly using alternative edge termination approaches such as bevels that are currently under development. Further, in order to understand the fundamental degradation mechanisms in vertical GaN devices and ultimately to mitigate them, extensive reliability testing of 1.2-kV-class vertical GaN PN diodes has been underway, including tests such as high-temperature reverse bias and high-temperature operating life. Such testing has revealed several distinct degradation behaviors, and failure analysis has been performed on select devices to determine the root cause(s) of degradation and/or failure. Finally, a transmission-line-based setup has been used to quantify the time response of the avalanche breakdown of 1.2-kV-class GaN PN diodes to determine whether it is fast enough for the diodes to be used for protection of the grid against EMPs, and the measured breakdown time of ~1 ns is indeed sufficient for such an application. This work was supported by the ARPA-E OPEN+ Kilovolt Devices Cohort directed by Isik Kizilyalli. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Development Progress at Hexis SOC Activities

HEXIS is developing and manufacturing SOFC-based micro-CHP systems for houses and / or small companies. While being a subsidiary of the Viessmann Group (until 06/2020) the company has been mostly focusing on the new generations of highly efficient (>95 % LHV) fuel cell modules – these modules are running in small quantities at customer sites - meanwhile, Hexis belongs to the h2e group and has expanded its technical activities.This paper will present the long-term experiences on long-term and cyclic testing of fuel cell modules in lab tests as well as results on robustness and performance in the field. It was observed that degradation rates as low as approx. 0.3 % / kh, i.e. 13 mΩ cm² / kh on the short-stack level for over 50,000 h can be reached. These stacks have been analyzed with electrochemical impedance spectroscopy (EIS) at regular intervals, which can provide valuable insights in understanding fundamental degradation mechanisms for long term operation of SOFC stacks.Moreover, the paper will include highlights of new developments since the new era for Hexis as an important part in the h2e group has started. These new developments are improvements on cell and stack level as well as new applications for the Hexis technology in general. Finally, the paper will outline some of the future challenges in Hexis’ development activities.

Development Progress at Hexis SOC Activities

HEXIS is developing and manufacturing SOFC-based micro-CHP systems for houses and/or small companies. This paper will present the long-term experiences on long-term and cyclic testing of fuel cell modules in lab tests as well as results on robustness and performance in the field. It was observed that degradation rates as low as approx. 0.3 %/kh, i.e. 13 mΩ cm²/kh on the short-stack level for over 50,000 h can be reached. These stacks have been analyzed with electrochemical impedance spectroscopy (EIS) at regular intervals, which can provide valuable insights in understanding fundamental degradation mechanisms for long term operation of SOFC stacks. Moreover, the paper will include highlights of new developments such as improvements on cell and stack level as well as new applications for the Hexis technology in general. Finally, the paper will outline some of the future challenges in Hexis’ development activities.

Structural and Electrochemical Properties of "Disordered, Spinel-like" Structures Derived from NCM111 Materials By Chemical Delithiation

Li-ion batteries (LIBs) are being widely used in various consumer electronics applications and battery electric vehicles. In current and next-generation LIBs, particularly Ni-rich NCMs (e.g. Li1+x[Ni0.6Co0.2Mn0.2]1-xO2) are considered as a promising class of cathode active materials (CAMs) due to their high specific capacities, their high power and energy densities, and their good structural stability.[1] In order to meet cycle-life requirements and to maintain a safe state of operation, LIBs are only cycled within a specific potential range, significantly limiting the usage of the specific capacity achievable with these materials. A major problem for NCMs are over-charge and/or harsh cycling conditions, which lead to highly aggravated capacity fading and ultimately to cell-failure. One fundamental degradation mechanism for NCM materials is the deleterious oxygen evolution from the NCM surface at high degrees of delithiation (>80% state of charge, SOC),[1,2] which results in the formation of an oxygen-depleted spinel and/or rock salt phase on the NCM particle surface as well as in a concomitant oxidation of the electrolyte.[3,4] A thorough understanding of the exact structures of the O-depleted phases produced by lattice oxygen release from the NCM materials and of their effects on the cycling performance of LIBs are still missing, which hampers the development of countermeasures. In the literature, oxygen release from partially delithiated NCMs has been frequently studied by thermal treatment, showing that with increasing Ni content the temperature at which oxygen is released decreases while the amount of released oxygen increases.[3,5–7] A detailed study of chemically delithiated NCM111 (Li1+x[Ni1/3Co1/3Mn1/3]1-xO2) using electrochemical testing methods, thermogravimetric analysis coupled with mass spectrometry (TGA-MS), X-ray diffraction (XRD), scanning electron microscopy, and gas physisorption measurements is the main objective in this work. Chemical delithiation to various degrees was executed with the oxidation agent NO2BF4 in anhydrous acetonitrile. Electrochemical testing suggests minor modification of the NCM material upon chemical delithiation resulting in less specific reversible capacity and that an electrochemically inactive phase is formed after oxygen depletion, which increases polarization and significantly decreases the reversible specific capacity of the NCM material. TGA-MS analysis is used to characterize the oxygen release behavior of the chemically delithiated samples, while XRD analysis by Rietveld refinement leads to the observation and allows the quantification of a spinel phase after thermal treatment of the chemically delithiated materials. Disintegration of the secondary structure of the NCM material was observed during chemical and electrochemical delithiation, resulting in a surface area increase by a factor of »10 and »5, respectively. The obtained results help for a better understanding of the processes happening during the oxygen release of NCM materials at high SOC, while providing reference data of phases found electrochemically in over-charge and/or under harsh cycling conditions.

Origin of relaxation frequency shift in blue organic light-emitting diodes

Recent technological innovations in organic light-emitting diodes (OLEDs) have enabled their applicability to be expanded to not only displays but also the lighting industry. In addition, the high scalability and flexibility of OLEDs render them promising candidates for next-generation displays. However, their insufficient lifetime and low uniformity/stability are challenging issues, mainly because of inadequate understanding regarding the fundamental degradation mechanism of OLEDs owing to their complex device structure. Herein, based on impedance spectroscopy (IS), we report the origin of the relaxation frequency (fR) shift in blue OLEDs as a function of electrical stress time for up to 1000 h with a constant current stress condition of 1 mA. These experimental conditions allow us to separately study the degradation mechanisms of the emission and transport layer (TPL) interfaces. On the basis of thorough understanding of the theoretical expression of fR, we mainly attribute the variation in fR at the charge-balancing voltage to the degradation of the TPL interface; this is further rationalized by the negligible difference between fR values obtained from IS and the equivalent circuit theory. This strong correlation consequently renders fR a powerful degradation indicator for the TPL interface. Our results provide a basis for the realization of highly reliable blue OLEDs with a sufficient lifetime.

Perovskites: Interface Engineering Driven Stabilization of Halide Perovskites against Moisture, Heat, and Light for Optoelectronic Applications (Adv. Energy Mater. 30/2020)

In article number 2000768, Atanu Jana, Jin Young Kim, Kwang S. Kim and co-workers review the current and existing challenges facing perovskite stability. This Review focuses on materials, power conversion efficiencies, fundamental degradation mechanisms, and solar cell design for enhancing their environmental stability. Finally, existing challenges and a road map for future directions of perovskite stability are provided.

A diagnostic tool for assessing the conservation condition of cellulose nitrate and acetate in heritage collections: quantifying the degree of substitution by infrared spectroscopy

Cellulose nitrate and acetate are materials at risk in heritage collections because it is not possible to predict the evolution of their conservation state over time. Knowing that the degree of substitution (DS) of these materials correlates with their state of conservation because the fundamental degradation mechanism is hydrolysis, in this work, DS was measured in historical objects and artworks. Infrared spectra were used to develop and optimize calibration curves for cellulose nitrate and acetate references that were next applied to calculate DS values of heritage objects. The extent of hydrolysis measured, with this tool, correlated well with the physical deterioration assessed through the sample hardness (Shore A) which was measured with a Durometer. Calibration curves were optimized in reference materials by Attenuated Total Reflectance (ATR-FTIR) and Micro Fourier Transform Infrared Spectroscopy (μFTIR). The DS values of the AC reference materials was previously calculated by nuclear magnetic resonance spectroscopy. The calibration curves were obtained plotting DS as a function of the ratio between a reference peak (which does not suffer relevant changes during degradation) and selected peaks that monitor the degradation for cellulose acetate and nitrate polymers (avoiding the interference of plasticizers). The reference peak for both was the COC stretching mode (νCOC). The probe peak was, for cellulose nitrate, the NO2 asymmetric stretching (νaNO2) and, for cellulose acetate, the OH stretching mode (νOH). This ratio was then applied to calculate DS values of historical materials, in good and poor conservation condition; in situ by ATR, and in micro-samples collected from artworks by μFTIR. This selection comprises cinematographic and photographic films dated from the 1890s to the 1960s, and contemporary works of art made with cellulose acetate sheets by Portuguese artist José Escada dated from the 1960s. Finally, by comparison with the original estimated DS values, we show how this tool permits to define the state of degradation of these complex polymer matrixes. Thus, establishing the quantification of the DS as a novel tool to monitor the degradation of cellulose ester plastics, contributing in this way for a sustainable preservation of an irreplaceable heritage.

On the relation between the Mason number and the durability of MR fluids

The durability of magnetorheological (MR) fluid is a fundamental aspect of any MR device, as MR fluid is submitted to shear stresses that cause its degradation. While various studies have identified MR fluid degradation behaviors and proposed improvements, there exists no generalized failure theory to help understand the in-use degradation of MR fluids. In this regard, this study suggests that a relation exists between the Mason number and the maximum achievable lifetime dissipated energy (LDE) of MR fluids. To validate this hypothesis, custom (perfluoropolyether) and commercial (hydrocarbon) MR fluids are aged in various Mason number conditions using a clutch-type durability device. Experimental results show that under a critical Mason number of ∼1, LDE of the MR fluids can be related to the Mason Number using an analogy to metal fatigue failure. Above this critical Mason number, the fundamental degradation mechanism appears to change from a dominant mechanical wear of the particles (i.e. particle-limited) to another mechanism (i.e. fluid-limited) which is not yet fully understood. Since MR generally operate well below the critical Mason number of ∼1, the relation between the Mason number and the particle-limited regime durability of MR fluids can be very useful to design novel MR fluids or MR devices, as it provides a way to estimate the impact of various design strategies on the total expected life of the device.

Progress in HEXIS’ SOFC Development

HEXIS is developing and manufacturing SOFC-based micro-CHP systems for houses and / or small companies. In the last years, we have been developing the new generation (Leonardo) of the Viessmann/HEXIS SOFC system and have them running now in field and lab tests. Additionally, the development of the next system (da Vinci) generation was kicked off and first results on prototype level achieved.This paper will report the newest results of short-stack tests and of system tests based on the new system combined with historical data of the earlier Galileo systems. These include tests running for more than 40’000 h. Moreover, results of the next system generation, reaching electrical efficiencies by steam-reforming with 50 % AC, net and total efficiencies of 95 % (LHV) will be shown.Finally, long-term and cyclic testing of systems in lab tests as well as results on robustness and performance in the field will be discussed:Degradation rates as low as approx. 0.3 % / kh, i.e. 12 mΩ cm² / kh could be shown on short-stack level successfully over 40’000 h. These stacks have been analysed with electrochemical impedance spectroscopy (EIS) in regular intervals, which will enable valuable insights in understanding fundamental degradation mechanisms in long term operation of SOFC stacks. The paper will link these results with corresponding observations on full scale stack and button cells level.

Iron(tris pyridyl-imine) Complexes As Redox Couples for Non-Aqueous Redox Flow Battery Applications

Grid-scale energy storage technologies are much needed for efficient integration of intermittent energy sources, such as wind and solar, into the electrical grid. Redox flow batteries (RFBs) have decoupled power and stored energy qualities, in contrast to traditional batteries, storing energy chemically in scalable solutions, and thus have the potential to solve the large-scale energy storage issue. Conventional redox flow chemistries are aqueous-based, such as Iron-chrome and all vanadium, where operating cell voltages are limited by water splitting reactions. Therefore, non-aqueous RFBs, which can operate at much higher voltages, are gaining attention and are typically only limited by the potential window of the non-aqueous solvent. Despite having enlarged potential windows that could provide higher energy density, most of the non-aqueous RFBs still operate at very low concentration, current densities and suffer from poor cycle life. In addition, the high cost of metals, such as vanadium, ruthenium, cobalt, etc., are an obstacle to achieving the DOE’s long term target of 150 USD per kWh for RFBs. In this presentation, therefore, we will discuss the synthesis and characterization of iron pyridine-imine complexes for redox couples. The performance of these redox couples in a bulk electrolysis cell and flow battery set-up will also be discussed. A mechanistic study to understand the fundamental degradation mechanisms of the redox couples under continuous charge/discharge operations will also be discussed. Our results indicate that iron complexes possess good reversibility and have the potential to compete, and potentially replace, other redox systems used in non-aqueous RFBs. Acknowledgement This work is supported by the Laboratory Directed Research & Development Program at Los Alamos National Laboratory.

Durability Test of Anode-Supported Solid Oxide Fuel Cell Under Diverse Load Conditions

Solid oxide fuel cells (SOFCs) provide an appealing electrochemical devices to convert chemical energy into electrical power with high efficiency and negligible carbon emissions. For utilizing in a wide spectrum of applications, SOFCs need to prove the operational stability over 40,000 h with less than 10% of power decay [1]. The performance deteriorations of SOFCs are induced by complex factors such as redox cycle [2], carbon deposition [3], S poisoning [4], and Cr poisoning. Therefore, understanding of fundamental degradation mechanisms under various operating conditions are important to improve the lifetime of SOFCs. For past few decades, the durability study of SOFCs has carried out under prolonged periods of time in mainly ‘laboratory conditions’, i.e. constant current operations, well controlled fuel composition, and low level of impurities [5,6]. However, the real operating conditions normally include harsh conditions such as temperature fluctuations, current fluctuations, and fuel and air stop. Especially, the different load conditions can result in accelerating the degradation of cell components because of the local potential and temperature changing. In this research, systematic durability investigations of anode-supported SOFCs are performed under a wide range of operating conditions including load cycle and load trip. In order to understand degradation mechanisms of the cell, various electrochemical investigations are performed such as the current-voltage polarization and electrochemical impedance spectroscopy. The morphology and microstructure of anode-supported cell are examined using field emission scanning electron microscoy and electron probe X-ray microanalyzer. References Y.-S. Chou, J.W. Stevenson, J.-P. Choi, J. Power Sources 250 (2014) 166–173.T. Klemensø, C. Chung, P.H. Larsen, M.Mogensen, J. Electrochem. Soc. 152 (2005) A2186–A2192.Mai, M. Becker, W. Assenmacher, F. Tietz, D. Hathiramani, E. Ivers-Tiffee, D. Stöver, W. Mader, Solid State Ionics 177 (2006) 1965–196H. Yokokawa, T. Horita, K. Yamaji, H. Kishimoto, T. Yamamoto, M. Yoshikawa, Y. Mugikura, K. Tomida, Fuel Cells 13 (2013) 526–535.Hagen, J.V.T. Høgh, R. Barfod, J. Power Sources 300 (2015) 223–228.H. Yokokawa, ECS Trans. 35 (2011) 207–216.T.-H. Lee, K.-Y. Park, J.-T. Kim, Y. Seo, K.B. Kim, S.-J. Song, B. Park, J.-Y. Park, J. Power Sources 276 (2015) 120–132.

ADOPTING TRANSMISSION KIKUCHI DIFFRACTION TO CHARACTERIZE GRAIN STRUCTURE AND TEXTURE OF ZR-2.5NB CANDU PRESSURE TUBES

The microstructure and texture of Zr-2.5Nb pressure tubes is greatly influenced by the manufacturing route. Although the general nature of the microstructure remains consistent between different manufacturing routes, subtle differences in the relative size and aspect ratios of the elongated α-Zr grains differ with pressure tubes of different pedigree. These differences have been shown to correlate well with in-reactor deformation; however, the ability to consistently and efficiently characterize the microstructures hinders an understanding of the fundamental degradation mechanisms. This paper outlines a new approach to characterize Zr-2.5Nb pressure tubes using thin foils characterized with both diffraction contrast in a conventional transmission electron microscope (TEM) and transmission Kikuchi diffraction (TKD) in a scanning electron microscope (SEM). The combined approaches enable a characterization of the same region of material with both techniques and capitalize on the advantages of each approach. ...

Evolution of the degradation mechanism of pure zinc stent in the one-year study of rabbit abdominal aorta model.

In the present study, pure zinc stents were implanted into the abdominal aorta of rabbits for 12 months. Multiscale analysis including micro-CT, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and histological stainings was performed to reveal the fundamental degradation mechanism of the pure zinc stent and its biocompatibility. The pure zinc stent was able to maintain mechanical integrity for 6 months and degraded 41.75±29.72% of stent volume after 12 months implantation. No severe inflammation, platelet aggregation, thrombosis formation or obvious intimal hyperplasia was observed at all time points after implantation. The degradation of the zinc stent played a beneficial role in the artery remodeling and healing process. The evolution of the degradation mechanism of pure zinc stents with time was revealed as follows: Before endothelialization, dynamic blood flow dominated the degradation of pure zinc stent, creating a uniform corrosion mode; After endothelialization, the degradation of pure zinc stent depended on the diffusion of water molecules, hydrophilic solutes and ions which led to localized corrosion. Zinc phosphate generated in blood flow transformed into zinc oxide and small amounts of calcium phosphate during the conversion of degradation microenvironment. The favorable physiological degradation behavior makes zinc a promising candidate for future stent applications.