Mechanical characterization of oxide coating–interconnect interfaces for solid oxide fuel cells

Interfacial strength and fracture energy of individual carbon nanofibers in epoxy matrix as a function of surface conditions

Chromium(Cr) poisoning of the traditional LSM cathode is one of the most critical issues that accounts for instability of solid oxide fuel cells (SOFCs). The poisoning course will introduce alien species in the cathode active regions and it causes phase change and structure deformation, reducing the sites for electrochemical reactions. A 3D model is thus developed by coupling the computational fluid dynamics (CFD) approach with the finite element method to unravel the involved electrochemical processes in chromium poisoning of SOFCs. A function is proposed based on the experimental results to describe the distribution of Cr-related compounds in cathode. The results indicate that chromium poisoning can induce a dramatic decrease in the electric current density, which can also lead to increase of activation polarizations and lower the temperature. Three kinds of thermal stresses are strongly affected by the invasion of chromium into cathodes, which are all significantly reduced with the poisoning extent. The resulting conclusions are beneficial to deeply understand the Cr poisoning of SOFCs and also to material design to prevent cathodes from Cr-based interconnect attack.

Theoretical consideration on composite oxide scales and coatings

The short beam shear test and single fiber fragmentation test with an energy-based model were conducted to characterize the interfacial properties of two types of carbon fiber–reinforced epoxy resin composites. The temperature dependences of interlaminar shear strength and interfacial fracture energy ( Γi) from room temperature to 130°C were investigated. The correlation between interlaminar shear strength and the micro-interfacial property at elevated temperature was discussed. The scanning electron microscope images of fractographic cross-sections of the composite laminates were used to analyze the failure mode transition as the temperature rising. The results show that the interfacial properties of the two carbon fiber composites with the same matrix are obviously different, especially at high temperature. The changes of interlaminar shear strength and interfacial fracture energy are different at elevated temperature, mainly ascribed to the different treating ways of thermal residual stress and matrix property in the two methods. The similar temperature sensitivities in the changes of Γi and the amount of matrix adhering on carbon fibers in the short beam shear failure images suggest the variations of the micro-interfacial properties in the short beam test as increasing temperature.

Effect of CF 4 plasma treatment on the interfacial fracture energy between inkjet-printed Ag and flexible polyimide films

Bond properties between carbon fibre-reinforced polymer plate and fire-damaged concrete

This paper focuses on the long-term tests for 1000 hours of different Membrane-Electrode-Interconnect Assemblies (MEIAs), consisting of a steel interconnect and an asymmetrical solid electrochemical cell based on the Ce0.8Sm0.2O2−δ electrolyte, the double-layer working electrode with the La2NiO4+δ – Ce0.8Sm0.2O2−δ composite functional layer and the LaNi0.6Fe0.4O3−δ current collector layer, and the platinum reference electrode (O2, LaNi0.6Fe0.4O3−δ|La2NiO4+δ – Ce0.8Sm0.2O2−δ|Ce0.8Sm0.2O2−δ|Pt, O2). These tests were carried out on MEIAs with two types of Cr-based stainless steels, Crofer 22 APU and 08X17T, with and without protective coatings at 850°C in air. The electrochemical performance of MEIAs was studied without polarization as well as under cathodic and anodic polarization (current density was 0.5 A cm−2) by means of electrochemical impedance spectroscopy (EIS) and the distribution of relaxation times method (DRT). It was found that the proposed protective coatings for the stainless steels inhibit chromium poisoning without polarization and under anodic polarization: for Crofer 22 APU the degradation of the polarization resistance after ~1100 hours of exposure was 0.35 ± 0.05 Ω cm2, and for 08X17T it was from 0.40 ± 0.05 Ω cm2. In the case of cathodic polarization, the degradation of the polarization resistance remained at a very high level, comparable to uncoated samples: for Crofer 22 APU and 08X17T with coatings, it was about 0.6 to 0.8 Ω cm2 after ~1100 hours. Post-test analysis of the microstructure and chemical composition of the interconnect - double-layer electrode interface was carried out using scanning electron microscopy (SEM), wavelength dispersive X-ray spectrometry (WDS), and respective image analysis of the obtained data. The influence of chromium poisoning on the degradation was considered for different stages of the electrode performance. It was demonstrated that chromium predominantly accumulates in the current collector layer in the case of anodic polarization and in the functional layer in the cases of open circuit and cathodic polarization. For the first time, a combined electrochemical-chemical degradation mechanism of the La2NiO4+δ – Ce0.8Sm0.2O2−δ functional layer was discussed. Highlights Long-term tests of Membrane-Electrode-Inteconnect Assembles (MEIAs) were done. Chromium poisoning was detected on MEIAs with the interconnects without coating. Cathodic polarization terminates the protective properties of the coatings. Chromium poisoning decreases electrode activity to oxygen exchange and diffusivity. A combined electrochemical-chemical degradation mechanism of the functional layer was revealed.

Numerous studies have shown that protective surface coatings are effective in reducing the well-documented problem of Mn ion dissolution from the surface of the spinel LiMn2O4 (LMO) Li-ion battery cathode.[1] Although recent computational work has identified many promising coating chemistries based on bulk thermochemical properties,[2,3] there remains much to be understood regarding the interface formed between protective coatings and the LMO electrode and how such interfacial properties may influence electrochemical performance. In this work, first principles density functional theory calculations are performed on models for both oxide and metallic coatings on the LMO surface. First, we elucidate the mechanism for the onset of Al2O3 film growth by atomic layer deposition (ALD) and show that precursor decomposition in early ALD pulses leads to adsorption site blocking and sub-monolayer film growth.[4] During ALD, the trimethylaluminum [Al(CH3)3] precursor loses its -CH3 groups to the LMO surface, where the Al(CH3)x* and CH3* surface adsorbates each behave as Lewis bases (electron donors), where near-surface Mn ions are the Lewis acidic adduct (electron acceptor). These studies are extended to various thermodynamically stable low- and high-index LMO surface models,[5] demonstrating that ALD on LMO is structure-sensitive toward stepped sites, which are more electron accepting as suggested by calculated oxygen vacancy formation energies. Moreover, we find that the thermodynamics of decomposed intermediates are linearly related to the corresponding oxygen vacancy formation energy, or the relative Lewis acidity of near-surface Mn ions, of each given surface facet and termination. We propose that selective ALD leads to partial coatings formed at defect sites which may be particularly vulnerable to Mn dissolution. Next, we model the process of delithiation near the surface of an LMO coated with metallic Au, which effectively mitigates Mn dissolution. Moreover, Au coatings have been shown to improve kinetics and do not strip under the potentials where LMO electrochemistry occurs.[6] DFT calculations show that a Li+-deficient near-surface is thermodynamically favored for Au-coated LMO. We demonstrate through local density of states and chemical bonding analyses that Au hybridizes with LMO upon delithiation, leading to partial oxidation on Au electronic states rather than full localization of holes on Mn ions. The Au coating has a lower work function than LMO, driving electron transfer from Au to LMO. This is also associated with downward bending of the LMO conduction band in response to Fermi level pinning at the LMO/Au interface. Operando X-ray diffraction experiments show that these interfacial effects may translate to a reduction in the amount of Li+ that can intercalate into the bulk of Au-coated LMO. These studies help elucidate the structure-property relationships of protective coatings at the LMO surface. In particular, this work highlights the driving forces for nucleation and growth of protective coatings by ALD, as well as the influence of interfacial electronic structure on intercalation chemistry of coated electrode materials. This research is supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Solid oxide fuel cells (SOFCs) address critical needs in the energy infrastructure due to their high efficiency, low emisssions and noiseless operation. The intermediate temperature SOFCs(IT-SOFCs) enable the use of metallic interconnect instead of traditional ceramic interconnect. However, chromium poisoning due to transport from metallic interconnect and BOP components is deleterious to cell performance. To prevent the chromium poisoning, either a new cathode material with a high tolerance of chromium poisoning needs to be deployed, or a protective coating with a high electrical conductivity needs to applied to the interconnect. In this study, electrolyte-supported baseline half-cell with 8mol% YSZ as the electrolyte and (La0.8Sr0.2)0.95MnO3-δ as the working and counter electrode was fabricated. Its electrochemical characteristics were tested in dry air and 50 mA/cm2 cathodic current in the present of bare chromia-forming interconnect, and an interconnect with a protective coating. Further, to investigate the effect of alternate electrode compositions, new electrodes, e.g. Cr- doped LSM, were employed as the working electrode instead of LSM in the baseline cell, and tested under same conditions.

This paper presents a nonlinear local bond-slip model for fiber reinforced polymer (FRP) laminates externally bonded to concrete at elevated temperature for future use in the theoretical modeling of fire resistance of FRP-strengthened concrete structures. The model is an extension of an existing two-parameter bond-slip model for FRP-to-concrete interfaces at ambient temperature. The two key parameters employed in the proposed bond-slip model, the interfacial fracture energy, Gf, and the interfacial brittleness index, B, were determined using existing shear test data of FRP-to-concrete bonded joints at elevated temperature. In the interpretation of test data, the influences of temperature-induced thermal stress and temperature-induced bond degradation are properly accounted for. As may be expected, the interfacial fracture energy, Gf, is found to be almost constant initially and then starts to decrease when the temperature approaches the glass transition temperature of the bonding adhesive; the interfacial brittleness index, B, exhibits a similar trend. The proposed temperature-dependent bond-slip model is shown to closely represent the test data upon which it is based, despite the large scatter of the test data.

Full-range behavior of FRP-to-concrete bonded joints subjected to combined effects of loading and temperature variation

Due to the rapid increase in energy demand, the equipment used in the transmission and distribution network is subjected to thermal overloading. Additionally, electrical stress due to high voltage and contaminations from environmental/structural factors lead to the life of insulating material to decrease rapidly. Among the equipment used in power systems, power transformers are one of the most affected ones from these undesired and distorted conditions. Particularly for oil type transformers, it is quite difficult to provide energy continuity due to failures caused by insulation problems, serious economic losses occur and more importantly, there might be cases where life loss is experienced. In order to avoid from such this cases, the electrical and mechanical properties of the insulation material should be analyzed correctly and the behaviors against different stresses should also be examined. In this context, it is crucial to investigate the problem of insulation between windings, which encountered quite often and has an important place among transformer failure reasons. Along with creeping of spacer part between the windings, the windings get closer to each other and by breaking down of the insulation material around the conductor due to overheating cause failure. In this study, a test system is designed and realized to determine the creep characteristic of spacer component, which is used to provide oil flow between the windings and for easier cooling, under different thermal and mechanical stresses. In this designed test set up, the spacer is placed in transformer oil and exposed to both mechanical and thermal stresses whose amplitudes can be controlled. By this way, it is aimed to investigate the effects of various thermal and mechanical stresses on the creeping of the spacer part. The results of the tests show that the creep curves will help to make the power transformers’ aging calculations and failure prediction algorithms more accurate.

Cu- and Ni-doped Mn1.5Co1.5O4 spinel coatings on metallic interconnects for solid oxide fuel cells

Dynamic interfacial bond behaviour between basalt fiber reinforced polymer sheets and concrete

For the stabilization of insulation performance in resin-molded insulators, strong adhesion between the resin and ceramic is required. In this paper, a new technique for the control of the interfacial strength in resin-molded structures has been proposed. The adhesive strength of the internal interface in a resin-molded structure was estimated as the interfacial fracture energy by using the molecular dynamics method. The interfacial fracture energy was qualitatively in agreement with the adhesive strength index obtained by a shear experiment. Based on the simulation results, the control of the interfacial strength was performed, and it became clear from the result that the interfacial strength between resin and ceramic can be improved up to the strength level of the glaze adhesion.