Letteratura scientifica selezionata sul tema "Triple phases région (TPB)"

To resolve the existing issues of solid oxide cells such as degradation and efficiency improvement, it is essential to understand reaction mechanisms on the surface/interface such as triple phase boundary (TPB) as a highly active site that consists of catalysts, electrolytes, and gas phases. However, the reliable TPB model has not been still uniquely defined to discuss the property. In this study, we have focused on the TPB model comprising Ni catalysts, yttria-stabilized zirconia (YSZ) electrolytes, and gas phases and aimed to theoretically identify a reliable TPB model. In concrete, we identified firstly the stable structure of YSZ surface models by using density functional theory calculations considering different oxygen vacancy positions, yttrium atom arrangements, yttria concentration, and YSZ surfaces. Thereafter, we discussed a reliable Ni/YSZ interface model based on our proposed YSZ model by evaluating different Ni structure types, Ni interfaces in contact with the YSZ surface, and interface positions.

This study presents a novel model for investigating the microstructural evolution of nickel (Ni), yttria-stabilized zirconia (YSZ), and gas phases in a solid oxide cell (SOC), and its effects on cell performance. The triple-phase-boundary (TPB), which is the interface between the three phases, plays a crucial role in the electrochemical reaction of the SOC. However, during operation, nickel particles coarsen or migrate, leading to the redistribution of the TPB. To study this phenomenon, a phase field method was utilized to simulate the fuel electrode's detailed structure, and an approach was developed to track the TPB lines (TPBl) and voxels (TPBv). The study then employed the open-source computational fluid dynamics library, OpenFOAM, to simulate the half-cell performance. The results provide a detailed understanding of the dynamics of the TPB and its impact on multiphysical transport phenomena.

AbstractA method is described for quantitatively analyzing the level of interconnectivity of solid-oxide fuel cell electrode phases. The method was applied to the three-dimensional microstructure of a Ni–Y2O3-stabilized ZrO2 (Ni-YSZ) anode active layer measured by focused ion beam scanning electron microscopy. Each individual contiguous network of Ni, YSZ, and porosity was identified and labeled according to whether it was contiguous with the rest of the electrode. It was determined that the YSZ phase was 100% connected, whereas at least 86% of the Ni and 96% of the pores were connected. Triple-phase boundary (TPB) segments were identified and evaluated with respect to the contiguity of each of the three phases at their locations. It was found that 11.6% of the TPB length was on one or more isolated phases and hence was not electrochemically active.

Two-dimensional (2D) electron back scattered diffraction (EBSD) is a powerful tool for microstructural characterization of crystalline materials. EBSD enables visualization and quantification of the effect of synthesis methods on the microstructure of individual grains, thus correlating the microstructure to mechanical and electrical efficiency. Therefore, this work was designed to investigate the microstructural changes that take place in the Ni-SDC cermet anode under different synthesis methods, such as the glycine–nitrate process (GNP) and ball-milling. EBSD results revealed that different grain size and distribution of Ni and SDC phases considerably influenced the performance of the Ni–SDC cermet anodes. The performance of the Ni–SDC cermet anode from GNP was considerably higher than that of Ni-SDC from ball-milling, which is attributed to the triple-phase boundary (TPB) density and phase connectivity. Due to the poor connectivity between the Ni and SDC phases and the development of large Ni and SDC clusters, the Ni-SDC cermet anode formed by ball milling had a lower mechanical and electrical conductivity. Moreover, the Ni–SDC cermet anode sample obtained via GNP possessed sufficient porosity and did not require a pore former. The length and distribution of the active TPB associated with phase connectivity are crucial factors in optimizing the performance of Ni-SDC cermet anode materials. The single cell based on the Ni–SDC composite anode prepared through GNP exhibited a maximum power density of 227 mW/cm2 and 121 mW/cm2 at 800 °C in H2 and CH4, respectively.

Non-Faradaic electrochemical modification of catalytic activity (NEMCA) with electric field applications in solid oxide cells (SOCs) is thought to be induced by spillover effects of lattice oxygen from the bulk, although the detailed mechanism has not still been clear. In SOCs, important phenomena such as fuel decomposition, charge transfer, etc. occur at the triple phase boundary (TPB) as a highly active site that consists of catalyst, electrolyte, and gas phases. NEMCA is expected to be also induced strongly by the surface mechanism on TPB, and understanding surface reactions on TPB is essential for improving catalyst and cell performances. However, the reliable TPB model has not been still uniquely defined to discuss the property of TPB although various studies have been reported. Therefore, in this study, we have focused on the TPB model comprising Ni catalyst cluster; YSZ electrolyte; and gas phase, and aimed to identify a reliable TPB model for theoretical studies by using first-principles calculations as an initial step. In concrete, we identified firstly the stable structures of YSZ surface models by using DFT calculations taking into account oxygen vacancy positions, yttrium atom arrangements, yttria concentration, and other factors. Thereafter, we discussed a reliable Ni/YSZ interface model based on the most stable YSZ model proposed above results by evaluating the Ni structure, interface stability, and so on. In this study, DFT calculations with a plane-wave basis set were implemented using CASTEP, and GGA-PBE exchange-correlation functional was used. The plane-wave cutoff energy was set as 489.8 eV, the OTFG-ultrasoft was used as the pseudopotentials, and the spin-polarization is considered because YSZ is a ferromagnetic substance. In YSZ surface models, yttria concentrations are set to 4.35 mol% and 9.1 mol% which shows the maximum ion conductivity of ZrO2. The three-layer YSZ (111) slabs with 15 Å vacuum layer with 2×2 and 2×4 unit cells were used for repeated slab models. The DFT+U method is used to obtain the correct electric structure of metal oxides with partially filled d or f-orbital shells, and k-points were set to 4×2×1. In Ni/YSZ interface models, we considered Ni cluster and Ni belt type models based on the most stable 2×4 YSZ surface model (9.1 mol%) proposed in this study. We also considered both cases of (111) and (100) facets for the contact interface. In the case of Ni/YSZ interface models, the DFT+U method was not considered to improve the calculation convergence and k-points were set to 4×2×1. A schematic diagram of the Ni/YSZ interface model is shown in the attached Figure. In the case of the 2×2 YSZ model with the yttria concentration of 9.1 mol%, the YSZ model is stabilized when oxygen vacancy is on the second O atom layer and the second neighbor to Y atoms, indicating that improvement in the geometry instability of ZrO2 for 8-coordination is more important than keeping local electron neutrality. We have also found that oxygen vacancy positions are more sensitive to the YSZ surface stability than Y atom arrangements. In the case of the 2×4 YSZ model with the yttria concentration of 4.35 mol%, the YSZ model where there are Y atoms on the first layer is stabilized. The crystal structure achieves a more stable structure by varying bond lengths when Y3+ with the larger ionic radius is replaced with Zr4+. Therefore, the structure is easier to stabilize when the Y atom exists on the surface than in the bulk due to the higher degree of freedom of the Y atom. The most stable structure of the 2×4 YSZ model with the yttria concentration of 9.1 mol% given based on the above results is 0.18 eV more stable than the previously reported structure [1]. This is because the number of Y atoms on the first layer with the second neighbor from oxygen vacancy is larger than the previously reported structure. We evaluated then the structural stability of the Ni/YSZ interface model based on the above YSZ model. As a result, we have found that adhesion energy between Ni and YSZ is independent of the relative position of Ni atoms. In addition, the larger the number of Ni atoms is, the more stable the structure is. This is because the electronic property of Ni atoms approaches the metal as increasing the number of Ni atoms. Other results and a detailed discussion will be reported in our meeting publications (ECS Transactions) and the presentation. [1] M. Shishkin and T. Ziegler, Phys. Chem. Chem. Phys., 16, 1798-1808 (2014). Figure 1

Solid oxide electrolysis cell (SOEC) cathodes require a good porosity and a fine microstructure in order to maximize the triple phase boundary (TPB) between electronic conductor, ionic conductor and the gas phase involved in the reaction. Nickel oxide and yttria stabilized zirconia (NiO/YSZ) composite, one of the most desired candidates for SOEC cathode material, is synthesized via the glycine-nitrate combustion process and mixed with corn starch and carbon black pore formers in order to observe how they modify its microstructure and porosity. XRD spectra indicate a distinct cubic phases of both NiO and YSZ. SEM micrographs were able to confirm that the addition of selected pore formers lead to an increase in porosities. Apparent and relative density measurements show that corn starch pore former produces the lowest density among the sintered pellets. EIS measurements revealed that samples with lower density also showed lower total conductivity.

Ni-YSZ is a key electrode material for solid oxide electrochemical cells (SOC) applications, such as fuel or electrolysis cell applications. The number of active sites, specifically the triple-phase boundaries (TPB), strongly affects the electrode performance. Thus, in order to achieve good electrode performance, a desirable microstructure of the electrode is essential. This study investigated the effect of precursor particle size without using pore former in developing porous Ni-YSZ electrode materials. Precursors were prepared with different particle sizes using a planetary ball mill. In comparison, Ni-YSZ with carbon black as a pore former was also prepared. From the XRD patterns, major peaks can be attributed to the cubic phases of NiO and YSZ. SEM images revealed that a highly dense as-sintered NiO-YSZ electrode was achieved; however, a desirable porous microstructure was obtained after reduction to Ni-YSZ, even without a pore former. In comparison, the prepared electrode with carbon black pore former has a larger pore size than the sample prepared without pore former. The obtained total bulk conductivities for the reduced Ni-YSZ without pore former at 700 oC is 3.94x10-1 S/cm with Ea of 0.02 eV at 500-700 oC. Thus, a desirable porous Ni-YSZ electrode with more TPBs and high total conductivity can be achieved even without a pore former. Figure 1

Abstract Power generation with renewable energy using solid oxide cells (SOCs) has been widely researched. To solve the existing problems of SOCs, such as degradation and efficiency improvement, it is essential to understand reaction mechanisms on the surface/interface such as triple phase boundary (TPB) composed of catalysts, electrolytes, and gas phases. However, a reliable TPB model has not been uniquely defined to discuss the property. This study focused on the TPB model comprising Ni catalysts, yttria-stabilized zirconia (YSZ) electrolytes, and gas phases, and aimed to theoretically identify a reliable TPB model by using density functional theory calculations. The stable structure of YSZ surface models was first identified considering various oxygen vacancy positions, yttrium atom arrangements, yttria concentration, and YSZ surfaces. Thereafter, a reliable Ni/YSZ interface model was discussed by evaluating various Ni structure types, Ni interfaces in contact with the YSZ surface, and interface positions. As a result, we have proposed a more reliable YSZ surface structure than previous reports and reasonable Ni/YSZ interface models considering the computational cost to discuss the properties of TPB. These findings will contribute to the improved design of SOCs as high-performance energy conversion systems for sustainable energy storage.

Les piles à combustible à membrane échangeuse d'anions (AEMFC) ont récemment attiré l'attention en tant que piles à combustible alternatives à faible coût aux piles à combustible à membrane échangeuse de protons traditionnelles en raison de l'utilisation possible d'électrocatalyseurs non-nobles. Bien que l'AEMFC ressemble à la PEMFC, les problèmes de gestion de l'eau sont plus prégnants dans une AEMFC car l'ORR en milieu alcalin nécessite de l'eau, tandis qu'en même temps, de l'eau est produite en grande quantité du côté de l'anode. Pour mieux comprendre la gestion de l'eau dans ce type de pile à combustible, il faut d'abord développer et acquérir de l'expérience avec ce type de pile à combustible à l'échelle du laboratoire. Puisque les matériaux prêts à l'emploi n'existaient pas au commencement de la thèse, nous avons dû fabriquer nos propres assemblages électrode-membranes (AMEs) à partir des matériaux disponibles dans le commerce. Etant donné que la thématique de fabrication des AMEs est nouvelle pour les chercheurs du LEMTA, cette thèse est articulée en deux parties, une dédiée à la formulation, la préparation et l'optimisation des AMEs pour PEMFC ; et une autre dédiée au développement d'AEMFC. Les résultats ont indiqué que la composition et préparation de l'encre, ainsi que la manière de déposer l'encre modifient systématiquement la structure de l'électrode, de même que ses performances en piles à combustible. En outre, l'étude fournit des informations sur les procédures et les méthodes pour les tests en AEMFC. Ici, nous souhaiterions partager notre savoir-faire avec les nouveaux venus dans le domaine de la préparation des AMEs pour piles à combustibles à membranes échangeuses d'ions
Anion exchange membrane fuel cells (AEMFCs) have recently attracted significant attention as low-cost alternative fuel cells to traditional proton exchange membrane fuel cells as a result of the possible use of platinum-group metal-free electrocatalysts. Although AEMFC is a mimic of PEMFC but working in an alkaline medium, water management issues are more severe in AEMFC because ORR in alkaline media requires water, while at the same time water is produced at the anode side. To better understand water management in this type of fuel cell, it is necessary first to develop and gain experience with this kind of fuel cell on the laboratory scale. Since no ready-to-use materials are available at the beginning of the project, the necessity of fabricating homemade MEAs from commercially available materials becomes a reality that we must face. As MEA fabrication is a new topic to LEMTA's researchers, this is why this thesis was divided into two parts: one part dedicated to the formulation, preparation, and optimization of MEAs for PEMFC through physico-chemical and electrochemical characterizations; another part dedicated to the development of AEMFC. The results indicated that ink deposition, composition, and preparation systematically change the electrode structure and thus affect fuel cells performance. Furthermore, the study provides information on the AEMFC procedures and methods. Here, we would like to share our know-how with newcomers in the field of preparation of MEA in ion exchange membrane fuel cells

The catalyst layer (CL) in polymer electrolyte membrane (PEM) fuel cells is one of the key components regulating the overall performance of the cell. In PEM fuel cells, there are two CLs having identical composition for hydrogen oxidation (HO) and oxygen reduction (OR) reactions. There are four phases inside the CL, namely: carbon, Pt particles, ionomer and voids. In this work, a micro-model of the cathode CL has been developed mathematically using finite volume (FV) technique to investigate the transport phenomena of reactants and product species, ions and electrons by incorporating the above stated phases at the cathode side only, due to the fact that the OR reactions are the rate limiting as compared to HO reaction. The 3D CL has been reconstructed based on a regularly distributed sphere’s method with dimensions 4.1 × 4.1 × 4.1 μm3. Platinum particles combined with carbon spheres (C/Pt) are regularly placed in the domain, an ionomer layer of a given thickness is extruded from the sphere surfaces. The C/Pt, ionomer and void distribution, as well as the triple phase boundary (TPB) are analysed and discussed. A microscopic model has been developed for water generation and species transport via Knudsen diffusion through the voids and the proton transport in the ionomer has been included here to aim for the rigorousness of the work. In addition, the electrochemical reactions have been simulated on the surface of Pt particles fulfilling the TBP conditions.

Abstract Manufacturing of MnCo2O4 spinel coatings by solution precursor plasma spraying (SPPS) was studied in order to produce thin ceramic coating on a ferritic stainless steel interconnect for SOFC’s. The main purpose to use MnCo2O4 coating in SOFC devices is to prevent the migration of harmful CrO3 and Cr2(OH)2 compounds to the triple phase barriers (TPB) of the cathode. In this study Mn(NO3)2•4H2O and Co(NO3)2•6H2O were diluted to deionized water and mixture of deionized water and ethanol at 3 M mixture rate. The solutions were sprayed on 0.5 mm thick Crofer 22 APU substrate by Sulzer Metco F4-MB plasma gun with a modified solution feeder. Microstructural characterizations for the as-sprayed coatings were done by using a field-emission scanning electron microscopy (FESEM) with SE-mode. Elemental analyses were done with energy dispersive spectroscopy (EDS) and an X-ray diffraction (XRD) was used for crystallographic studies. The coating with full equivalence of the crystallographic structure of MnCo2O4 spinel was sprayed using argon-helium plasma and water based solution. Plasma gas with hydrogen as a secondary or ternary gas and ethanol based solutions caused the formation of the mixed phases of CoO and MnCo2O4. Although the microstructures of sprayed coatings were still quite porous, the influence of relevant gun and solution parameters were found in order to improve coating denseness in further studies.