Ni Agglomeration Research Articles

A cofuel channel microtubular solid oxide fuel/electrolysis cell

A co-fuel channel microtubular solid oxide fuel/electrolysis cell is successfully fabricated by adding an insulating ceramic connector. The active electrode area of both of the abovementioned cells is approximately 10.73 cm2. Hydrogen-steam mixtures act as either reactants or products in the ceramic tubes. A ring-like anodic current collector is located in the middle of the anode, while silver paste is coated on the cathode as a cathodic current collector. When operating the cells at 750 ℃, 0.6 A cm−2 at 0.6 V and 1.0 A cm−2 at 1.7 V are achieved for the SOFC and SOEC, respectively. By using the co-fuel channel, the power generation of the SOFC is 3.9 W, while the simultaneous power consumption of the SOEC is 18.2 W. After 11 h of operation, no microcracks were observed, and the anode-electrolyte interface structure exhibited no Ni agglomeration or active phase loss.

Remarkable stability of Ni-modified polyoxometalates to H2, CO, and CH4 during propylene oligomerization

Olefin oligomerization is a crucial step in the synthesis of chemicals and fuels via the formation of higher molecular weight intermediate products. Upstream processes that produce olefin building blocks, however, may contain impurities that affect catalyst performance and process design strategies. Supported Ni-modified Wells–Dawson (WD)-type polyoxometalates (POM) have been reported as active and regenerable olefin oligomerization catalysts, but the performance of these catalysts under more extreme conditions has not been reported. In this study, we evaluate the stability of a SBA-15 supported Ni-POM-WD catalyst in the presence of possible feed stream contaminants, such as CO, H2, and CH4, that could be present in alkane dehydrogenation and shale gas valorization plant streams. Propylene dimerization rates over Ni-POM-WD/SBA-15 were unaffected when co-feeding H2 and CH4. However, propane formation was dominant when co-feeding H2 through hydrogenation pathways, while CH4 was inert in the reaction chemistry. Co-feeding CO resulted in an immediate drop in propylene conversion, which was fully restored after the removal of CO and applying thermal regeneration. Evaluation of the catalysts by 31P NMR and energy dispersion x-ray (EDX) mapping after exposure to each of the impurities provided evidence of structural stability with the Ni2+ sites intact after reaction and with no sign of particle or Ni agglomeration.

A new design of metal supported micro-tubular solid oxide fuel cell with sandwich structure

Micro-tubular solid oxide fuel cell (MT-SOFC) is considered as a promising choice for portable applications. In this work, we developed a novel metal supported MT-SOFC with porous 430 stainless steel support| 430 stainless steel-SSZ| SSZ| porous SSZ sandwich structure by dip coating and one-step co-sintering technology. The metal supported MT-SOFC showed good connection between each function layer and exhibited a significant maximum power density of 271 mW cm−2 at 800 °C. Although the power density showed about 19.6% off after 14 thermal cycles between 600 °C and 800 °C, and about 5% per 100 h degradation rate during the 200 h long-term stability test at 700 °C and 0.7 V, the structure of the single cell could be maintained well and no crack and Sr diffusion was observed. As the result, the ohmic resistance of the cell kept unchanged during the thermal cycling and long-term test. The relatively fast degradation was attributed to the Ni and LSM particles coarsening and agglomeration, which will be improved in the further work. This work presented a low-cost and simple way to fabricate the metal supported MT-SOFCs with good electrochemical performance.

Rhenium-based low resistivity and low annealing temperature ohmic contacts to n-GaN

We report an ohmic contact metallization scheme for n-GaN based on a Re–Al–Ni–Au multilayer stack that offers low-resistivity and provides better edge sharpness and lower surface roughness at reduced annealing temperature compared to standard Ti–Al-based metallization. We studied three sets of samples with Re thicknesses of 10, 30, and 60 nm and measured specific contact resistances using the circular transmission line method. We obtained reliable ohmic contacts with specific contact resistance of the order of 10−6 Ω cm2 at 550 °C temperature with the contact stack having a large annealing temperature window of >300 °C for resistivity below 5 × 10−6 Ω cm2. The lowest contact resistivity, below 10−7 Ω cm2, is achieved at annealing temperature around 650–700 °C. A reduction in surface roughness by a factor of 4, with excellent edge definition for Re >30 nm thickness is observed as compared to Ti–Al contacts. Grazing incidence x-ray diffraction and electron dispersive x-ray spectroscopy (EDS) show intermetallic phases of RexNy, Re–Al–Ni, Al–Re, and Al–Au being formed. Atomic force microscope and EDS measurements show the formation of crystalline Re–Al–Ni agglomerates, surrounded by phases of AlAu2 and Al2Au5. The presence of Re seems to suppress the formation of viscous AlAu4 phase, thus minimizing the lateral flow of the metals and providing better edge acuity. The temperature dependence of contact resistivity suggests a field-emission mechanism for current transport across the contact. Our results show that Re-based ohmic contacts, with their lower annealing temperature and excellent edge definition, may offer a promising alternative to Ti–Al contacts.

Reactive sputtered Ni-SDC cermet alloy anode for low-temperature solid oxide fuel cell

The facile deposition process for multi-component films is crucial in the development of thin film low-temperature solid oxide fuel cells (LT-SOFCs). Here, we report on the development and optimization of reactive sputtered Ni-SDC(Ni-Sm:CeO2) films for LT-SOFC anodes. The effects of oxygen partial pressure (PO2/PAr of 0 %–50 % at the sputter pressure of 30 mTorr) and deposition temperature on the physical, chemical and electrochemical properties of Ni-SDC films are studied. While the Ni-SDC anodes deposited at high PO2/PAr (>20 %) exhibited higher initial performance, they suffered from crack formation and massive Ni agglomeration upon operation due to NiO-to-Ni reduction and severe volume change. We further show that increasing the deposition temperature to 500 ℃ was effective in suppressing such Ni agglomeration and preserving the performance during operation.

Metal Supported Thin Film SOFC Fabrication Via RF Sputtering

Thin film solid oxide fuel cell (TF-SOFC) has been studied for past few decades to lower its operating temperature to intermediate temperature range (400℃-700℃). However, it is a challenge to deposit a pinhole free electrolyte thin film on a conventional porous ceramic support. In the case of the Ni-Fe based metal support, even if NiO-Fe2O3 is manufactured as a dense structure, it is reduced to Ni-Fe in a hydrogen atmosphere and forms pores. In this study, Ni-Fe based metal support has been introduced as TF-SOFC support to deposit pinhole free electrolyte. Metal support was fabricated as 290㎛ thick porous bulk(34% porosity) with 10㎛ thick dense layer(<1% porosity) by using tape casting process. Ni-Fe ratio (9:1. 7:3, 5:5, 3:7) and sintering condition (1300℃-1500℃) of metal support were optimized to minimize a mismatch of TEC with thin film and improve microstructure stability of support. After that, NiO-YSZ anode functional layer (AFL) and YSZ electrolyte were continuously deposited on metal support through RF sputtering. By tailoring sputtering condition (substrate temperature, operating pressure, reactive gas flow), 39vol%NiO-YSZ composition AFL and 8.6mol%YSZ pinhole free electrolyte were deposited. Annealing was performed at a temperature of 1100℃ or less to improve the crystallinity of the thin film and suppress Ni agglomeration of Ni-YSZ AFL while minimizing diffusion of Fe in the AFL. In cell operating condition (600℃-H2 atmosphere), the dense layer of metal support and NiO-YSZ AFL formed 28% and 24% of pores, respectively. By using LSM-YSZ cathode through the screen printing process, Metal support (300㎛) / Ni-YSZ (1.3㎛) / YSZ (0.8㎛) / LSM-YSZ (20㎛) button cell was fabricated, and electrochemical analysis was performed. Key wards: Sputtering, Thin film, Metal Support, Ni-Fe, YSZ, Ni-YSZ

Unravelling the significance of catalyst reduction stage for high tar reforming activity in the presence of syngas impurities

Ni catalysts are used widely in tar reforming for syngas purification. This study investigates the aim of the reduction stage toward Ni catalysts during tar reforming in the presence of syngas impurities. A NiAl2O4 catalyst was used for the reforming of a mixture of tar model compounds (naphthalene, toluene, and styrene) in simulated syngas containing 50 ppmv H2S and 500 ppmv HCl. The catalyst exhibited activity in tar reforming even without the pre-reduction treatment, but it was deactivated rapidly due to the favored reaction between H2S and Ni2+ species, causing increased Ni agglomeration and carbon deposition. Pre-reduction of catalysts enhanced the catalyst stability even in the presence of syngas impurities. Higher reforming activity was achieved by increasing the reduction temperature from 800 °C to 900 °C. However, transformation in the support structure at higher reducing temperature could lead to decreased exposure of Ni due to the collapse of pore structures.

Operando observations of active three phase boundary of patterned nickel - Yttria stabilized zirconia electrode in solid oxide cell

In solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs), the substantial performance degradation of nickel (Ni) - yttria-stabilized zirconia (YSZ) fuel electrode has been a growing challenge for commercialization. The microstructure of Ni-YSZ electrode is not stable during long-term operation due to Ni phase redistribution. In the present study, operando observations of patterned Ni film electrodes under SOFC, SOEC and reversible solid oxide cell (RSOC) operation modes are conducted. Confocal laser scanning microscopy is applied to acquire real-time microscopy images of the three phase boundary (TPB) under three operation modes to study the Ni mobility on the YSZ surface. The operando observation showed that the Ni phase dynamically spreads and splits up only under the SOFC mode, while this phenomenon is not observed under the SOEC mode. Ni coarsening occurs regardless of the operation mode. The dynamic motion of active TPB qualitatively correlates with the variation in the current density during the operation. The contact angles of Ni on flat YSZ surface after operation are the smallest for SOFC, followed by RSOC and the largest for SOEC. Competition between the altered Ni wettability and the Ni agglomeration finally determines the complicated Ni morphological changes.

Paths to market for stationary solid oxide fuel cells: Expert elicitation and a cost of electricity model

Solid oxide fuel cells (SOFCs) can efficiently generate continuous electricity for buildings but face technical and economic challenges to achieving mass market acceptance. We conducted a workshop comprising 23 experts to assess near-term markets, system cost and production scale, and system and stack degradation rates. Experts identified commercial- and residential-scale systems as the most favorable entry-level markets in the U.S. and world, respectively. The 2020, 2035, and 2050 median 250 kW electric-only system costs assessed by experts were $2,400/kW, $1,909/kW, and $841/kW (2018 USD) at production volumes of 210, 4000, and 10,000 systems per year, respectively. For small-scale combined heat and power applications, median 1 kW system cost assessments were $11,000/kW, $7,750/kW, and $5,750/kW at 50,000, 80,000 and 175,000 systems per year, respectively. Experts suggested voltage degradation could achieve 0.2%/1000 hrs in 2035–2050 and identified chromium poisoning, microstructural cathode degradation, and Ni agglomeration and coarsening as the most substantial barriers to mitigating degradation. To assess lifecycle performance, we incorporated experts' assessments into a cost of electricity (COE) model. The 250 kW system COE competed with anticipated internal combustion engine, microturbine, and U.S. grid COEs in 2035–2050. The 1 kW system COEs were three times anticipated grid prices in 2035–2050; however, 1 kW systems may be better suited to countries with higher spark spreads. Our results could inform business decisions, funding prioritizations, and technology roadmap development.

Development of Flowerlike Ce1-XZrxO2-δ-Dispersed Paper-Structured Catalyst for Direct-Biogas SOFC

Performance of direct internal reforming (DIR)-solid oxide fuel cell (SOFC) can be improved by the application of the paper-structured catalyst (PSC) on the anode material. In the ceramic fiber network of the PSC, any kinds of functional materials can be easily dispersed during paper-making process.The solid solution based on the equimolar mixture of ceria and zirconia (Ce0.5Zr0.5O2-δ), hereafter called CZ, is a promising oxide support of the Ni metal catalyst due to its high specific surface area and high oxygen storage capacity (OSC). By employing the two-step hydrothermal process developed in this study, we could successfully prepare the CZ with flowerlike geometry, which can prevent Ni from sintering under reducing atmosphere.Ni-loaded PSCs with and without the dispersion of CZ, Ni/CZ-PSC and Ni-PSC, respectively, were prepared. Structures of the PSCs were characterized by X-ray diffractometry (XRD), BET specific surface area measurement, field emission scanning electron microscopy (FESEM), inductively coupled plasma spectrometry (ICP) and thermogravimetric analysis (TG). Catalytic performances of these PSCs for the dry reforming of methane (DRM) in the feed of the equimolar mixture of CH4 and CO2 were evaluated by an automatic gas chromatograph (GC) with the posttest analysis by FESEM observation, temperature-programmed oxidation (TPO) and TG to estimate carbon deposition.In the durability (160 h) tests of the prepared-PSCs for DRM at 750oC under gas hourly space velocity (GHSV) of 2300 h-1 revealed that, for both cases with and without CZ dispersion, methane conversions were stable with the degradation rates (estimated after 10 h) of 13.7 and 21.8% 1000 h-1, respectively. TG measurements after the 160 h tests under oxidizing atmosphere in the range from 300 to 750oC revealed that the weights of the deposited carbon were 0.2 and 3.4 folds of those of PSCs for Ni/CZ-PSC and Ni-PSC, respectively. In the case without CZ dispersion (Ni-PSC), after the TG measurement to burn out the deposited carbon, Ni agglomeration and its detachment from the ceramic fiber could be clearly observed. On the other hand, for the Ni/CZ-PSC, these phenomena were considerably suppressed thanks to the flowerlike morphology for better dispersion of Ni metal particles and higher OSC of CZ solid solution to consume deposited carbon by the lattice oxygen.The developed PSCs will be applied on the anode of electrolyte-supported cell to demonstrate the stable operation of DIR-SOFC fueled by biogas (direct-biogas SOFC).

A Dynamic Multi-Scale Model for Solid Oxide Cells Validated on Local Current Measurements: Impact of Global Cell Operation on the Electrodes Reaction Mechanisms

A dynamic physically based model has been developed to unravel the complex relationships between the overall Solid Oxide Cells (SOCs) response and the reaction mechanisms taking place in the electrodes. This numerical tool combines modules for different length-scales from the electrode microstructure up to the single repeat unit. First, models have been developed to emulate digital twins of electrode microstructures made of the classical SOCs materials [1]. The microstructural properties of the active layers and the current collectors are then computed on the synthetic volumes and used as inputs in the electrochemical models. The microscale models for the active layers take into account the mass and the charge transport phenomena for the solid and gas phases. For each electrode, the proposed reaction mechanism is split in a sequence of elementary steps with different pathways that can be activated depending on the polarization [2, 3]. The electrode models are finally coupled with a macroscopic description including the cell geometry and the design of the gas channels [4]. This model allows computing the cell polarization curves, the overpotentials and the EIS together with the distribution of the local current density and gas composition in the single repeat unit. This multiscale model has been used to study a typical cell made of the classical SOCs materials: the electrolyte was in Yttria Stabilized Zirconia (YSZ) whereas the hydrogen and oxygen electrodes were respectively composed by a cermet of Nickel and YSZ (Ni-YSZ) and a composite of Lanthanum doped Strontium Cobaltite Ferrite and Ceria doped Gadolinium Oxide (LSCF-CGO). For this cell, the representativeness of the electrodes synthetic microstructures have been checked on reconstructions obtained by tomographic techniques. It has been shown that the synthetic electrodes reproduce accurately the real microstructures. Moreover, the models at the electrode level have been carefully validated on polarizations curves and Electrochemical Impedance Spectra (EIS) measured on symmetrical cells using a three-electrode setup. It is worth noting that the electrodes of the tested symmetrical cells were manufactured using the same materials and composition than the complete cell. Finally, a specific attention has been paid in this work to validate the approach with an original experimental setup allowing the measurement of the local current densities on the full cell without electrode segmentation. This setup is composed of nine symmetrical-spaced pins that perform the measurement of local polarization curves from the inlet to the cell outlet. A campaign of tests has been carried out in fuel cell and electrolysis mode with different temperatures and flow rates. A good agreement has been found between the experimental and simulated data. It has been shown that the model is able to reproduce the evolution of the local polarization curves with a higher current density at the cell inlet. Furthermore, the simulated EIS for the cell have been found to be consistent with the experimental ones. The fully validated model has been used to analyze the cell operation in electrolysis and fuel cell mode. The reaction pathways associated to the elementary steps in the active layers have been discussed depending on the position within the electrodes and the polarizations. This model will then be used to implement the degradation laws such as the Ni agglomeration and compute the loss of cell performances and the remaining useful lifetime [5]. Moreover, the model could be applied to interpret the evolution of the EIS upon operation.[1] H. Moussaoui, J. Laurencin, Y. Gavet, G. Delette, M. Hubert, P. Cloetens, T. Le Bihan, J. Debayle, Stochastic geometrical modeling of solid oxide cells electrodes validated on 3D reconstructions, Computational Materials Science. 143 (2018) 262-276.[2] F. Monaco, E. Effori, M. Hubert, E. Siebert, G. Geneste, B. Morel, E. Djurado, D. Montinaro, J. Laurencin, Electrode Kinetics of Porous Ni-3YSZ Cermet Operated in Fuel Cell and Electrolysis Modes for Solid Oxide Cell Application. Under review. [3] E. Effori, J. Laurencin, E. Da Rosa Silva, M. Hubert, T. David, M. Petitjean, L. Dessemond, E. Siebert, An elementary kinetic model for the LSCF and LSCF-CGO electrodes for solid oxide cells: impact of oxygen partial pressure and degradation on the electrode response. Under review. [4] J. Laurencin, D. Kane, G. Delette, J. Deseure, F. Lefebvre-Joud, Modelling of solid oxide steam electrolyser: Impact of the operating conditions on hydrogen production. Journal of Power Sources. 196 (2011) 2080-2093.[5] M. Hubert, J. Laurencin, P. Cloetens, B. Morel, D. Montinaro, F. Lefebvre-Joud, Impact of Nickel agglomeration on Solid Oxide Cell operated in fuel cell and electrolysis modes. Journal of Power Sources. 397 (2018) 240-251. Figure 1

Determination of Factors Governing Degradation of Anode-Supported Solid Oxide Fuel Cells as Influenced by Current Density and Humidity

Anode-supported solid oxide fuel cells (SOFCs) offer the advantage of thinner electrolyte membranes which greatly reduce the ohmic loss and enables lower operating temperature and higher performance. However, a significant contribution to the polarization resistance is attributed to the anode, as the thickest component in the cell, which can influence the distribution of hydrogen partial pressure, p(H2) and water vapor pressure, p(H2O) especially when combined with current density to achieve high fuel utilization. To understand the degradation mechanism in long-term operation of a commonly used anode material, Ni/YSZ, commercial anode-supported cells were evaluated under varied current density (up to 1 A/cm2) and humidity (1-60%) using electrochemical impedance spectroscopy. The impedance spectra were analyzed using distribution of relaxation times (DRT) method to separate the resistance contribution from each component of the cell. The Ni/YSZ anode resistance was observed to increase with higher current density and humidity, suggesting that the degradation may be correlated to microstructural factors such as Ni agglomeration leading to reduced Ni interparticle contact, causing the increase in anode resistance. In addition, analysis using the 1-D transmission line model to determine the effective reaction length (ERL) of Ni/YSZ anode, revealed that the ERL moves further from the TPB with increasing degradation. Details of the post-test microstructural characterization of the cells will be discussed during the presentation.

Modeling Nickel Microstructural Evolution in Ni-YSZ Electrodes Using a Mathematical Morphology Approach

Microstructural evolution of Nickel in Ni-YSZ fuel electrodes is one of the main limiting degradation mechanisms in solid oxide cells [1]. The redistribution of Nickel is ascribed to its high mobility and low wettability with YSZ, related to interfacial surface tensions. Practically, the degradation manifests as the agglomeration of Ni, potentially accompanied by its migration, mainly reported in electrolysis mode.Extensive efforts have been made to understand Ni agglomeration and relocation. In this frame, advanced imaging techniques have been employed to characterize the microstructure of electrodes after aging in a wide range of conditions. In order to support these observations and gain fundamental insights, a number of computer simulations methods such as: multistate kinetic Potts-Monte Carlo models, cellular automata, phase field, and topological boundary dynamics have been successfully employed in modelling grain growth and recrystallization phenomena. Nevertheless, questions still remain on the exact underlying mechanisms of Ni migration.In the shadow of this lack of clear understanding, a morphological modelling approach aiming at mimicking Ni depletion based on the most fundamental definition of chemical potential and surface curvature minimization would be very helpful to guide researchers into gaining clearer insights of this phenomenon. Moreover, the morphological simulations are very fast compared to the physically based models. Therefore, they can offer the possibility to emulate large database of numerical microstructures in order to study the impact of Ni evolution on the microstructural properties [2].The current work falls within this framework and aims at modelling the two main Ni degradation phenomena, namely coarsening and migration. For this purpose, a novel model based on morphological operations from mathematical morphology [3] has been developed and adapted to the Ni-YSZ specificities. The considered driving force for Ni evolution is the minimization of its surface curvature, and implicitly its chemical potential, which strongly depends on the local operating conditions. After calibration, the model showed a capability to predict, starting from an initial 3-D microstructure as input data, Ni relocation that was in agreement with that observed in a large dataset of 3-D reconstructions from pristine and aged cells. This helps understanding the associated microstructural evolutions which are linked to the cell Area Specific Resistance (ASR) through an adapted in-house model. This coupling of microstructural and electrochemical models allows giving clearer insights and practical recommendations for the design of fuel electrodes with improved stability and electro-catalytic activity.[1] M. Hubert, J. Laurencin, P. Cloetens, B. Morel, D. Montinaro, F. Lefebvre-Joud, Journal of Power Sources, 397 (2018) 240-251.[2] H. Moussaoui, R.K. Sharma, J. Debayle, Y. Gavet, G. Delette, J. Laurencin, Journal of Power Sources, 412 (2019) 736-748.[3] J. Serra, Image Analysis and Mathematical Morphology: Theoretical Advances, Academic Press, Cornell University (1988). Figure 1

Atomically dispersed Ni induced by ultrahigh N-doped carbon enables stable sodium storage

<h2>Summary</h2> Building phase interface with enough solid-phase contact is of great importance for improving chemical reaction kinetics and depth. High dispersion of electrode materials, especially at the atomic-level, are known for high interface contact, yet their potential application in batteries is restricted due to low loading. Herein, the atomically dispersed metal Ni (Ni in Ni–N–C is 54.9 wt %) with high loading was achieved by ultrahigh N-doping carbon (N/N–C:29.5 wt %) during the discharging process of nickel sulfide, leading to good reversibility and high-capacity maintenance owing to ultrahigh phase contact during long cycling for sodium-ion batteries. It delivers a stable cycling life (0.061% capacity decay per cycle) compared with the poor cyclability (0.418%) for the Ni agglomeration electrode with lower N-doping. The assembled pouch cells achieve robust stability (92.1% after 50 cycles). DFT calculations reveal that ultrahigh N-doping and electrochemically formed Na₂S can provide thermally stable Na₂S/Ni/NC structures, inhibiting Ni agglomeration during cycling.

Redox Durability of Ni-Co Alloy Cermet Anodes for SOFCs

SOFCs are expected to be commonly used as a clean energy conversion technology. Ni-zirconia cermet, which is commonly used for SOFC anodes as an electronic conductor, has a problem that Ni agglomerates due to redox reactions disrupting the conductive path and leading to irreversible performance degradation. In this study, we fabricated anodes using Ni-Co alloy instead of Ni. By alloying Ni with Co, it was found that the same power generation performance and high redox cycle durability could be obtained.

Nickel-based nanocatalysts promoted over MgO-modified SBA-16 for dry reforming of methane for syngas production: Impact of support and promoters

A series of Ni/SBA-16-MgO catalysts with varying promoters (3 wt% of Y, Ce, and La) were provided through a double-solvent way and tested in reforming of CH4 with CO2 process. The characterization was done using XRD, BET, TPR, and HRTEM for unused catalysts and via TPO, HRTEM, and FESEM equipped with EDX techniques for used catalysts. Among unpromoted and promoted catalysts, the Y-promoted catalyst resulted in the smallest Ni particles (11.5 nm) within the MgO-modified SBA-16, highest Ni dispersion, and oxygen vacancies. The catalytic results illustrated that adding Y and Ce promoters had a positive effect on the catalytic activity and stability (especially for Y-promoted catalyst) while adding La indicated a negative influence because of larger Ni crystallite size and agglomeration. The TPO, HRTEM, and FESEM observations illustrated the deposition of tip type nanotubes carbon on the catalysts surface that was greater on the unpromoted catalyst surface.

Enhanced thermal stability of Ni/GeSn system using pre-amorphization by implantation

Improving the thermal stability of Ni/GeSn intermetallics is of great importance to avoid surface degradation and Sn segregation. For this purpose, we studied the effects of pre-amorphization by ion implantation (PAI) of GeSn layers prior to metallization. The impact of Si, Ge, C, or Ge + C PAI was evaluated in terms of phase sequence, morpohological, and electrical evolution during the solid-state reaction. The overall phase sequence, followed by in situ x-ray diffraction, was comparable with or without PAI and went as follows: the Ni5(GeSn)3 hexagonal phase was obtained first, followed by the mono-stanogermanide phase: Ni(GeSn). Nevertheless, the threshold temperature for phase formation varied. These variations, depending on the nature of the implanted ions, can be related to kinetic and/or thermodynamic factors as supported by the analysis of bibliography for silicides and germanides. Additionally, it was reported that the use of Si or Ge implantation did not significantly impacted the surface morphology of the layers. On the other hand, the implantation of C positively impacts the surface morphology evolution by delaying Sn long-range diffusion and Ni(GeSn) agglomeration. This trend was then highly beneficial for preserving electrical stability in an enhanced process window.

Development of mathematical transfer functions correlating Solid Oxide Fuel Cell degradation to operating conditions for Accelerated Stress Test protocols design

This work presents an innovative model-based approach for the development of mathematical transfer functions capable of correlating Solid Oxide Fuel Cells (SOFCs) degradation rate to the applied operating conditions. Such functions can be used for fuel cell lifetime prediction and Accelerated Stress Test (AST) protocols design. The proposed approach relies on multiscale modelling methodology and links local degradation to high level performance models to evaluate the key operating variables accelerating voltage decay over time. A thorough simulation analysis is performed to convey the correlation among operating variables and degradation rate into mathematical transfer functions. To better illustrate the overall design and application process of such functions, a case study accounting for Ni agglomeration is addressed. The multiscale modelling framework is applied to correlate microscale (i.e., Ni particles size change) and macroscale (i.e., SOFC voltage reduction) levels through the most affected mesoscale parameters. The model is then used to simulate voltage decay over time and link degradation rates to the applied operating conditions. Afterwards, a parametric analysis is performed to investigate the influence exerted by each operating variable on the degradation rate and derive the transfer functions. An example of application for Accelerated Stress Test (AST) protocols design is then given.

The Detection of Monoclinic Zirconia and Non-Uniform 3D Crystallographic Strain in a Re-Oxidized Ni-YSZ Solid Oxide Fuel Cell Anode

The solid oxide fuel cell (SOFC) anode is often composed of nickel (Ni) and yttria-stabilized zirconia (YSZ). The yttria is added in small quantities (e.g., 8 mol %) to maintain the crystallographic structure throughout the operating temperatures (e.g., room-temperature to &gt;800 °C). The YSZ skeleton provides a constraining structural support that inhibits degradation mechanisms such as Ni agglomeration and thermal expansion miss-match between the anode and electrolyte layers. Within this structure, the Ni is deposited in the oxide form and then reduced during start-up; however, exposure to oxygen (e.g., during gasket failure) readily re-oxidizes the Ni back to NiO, impeding electrochemical performance and introducing complex structural stresses. In this work, we correlate lab-based X-ray computed tomography using zone plate focusing optics, with X-ray synchrotron diffraction computed tomography to explore the crystal structure of a partially re-oxidized Ni/NiO-YSZ electrode. These state-of-the-art techniques expose several novel findings: non-isotropic YSZ lattice distributions; the presence of monoclinic zirconia around the oxidation boundary; and metallic strain complications in the presence of variable yttria content. This work provides evidence that the reduction–oxidation processes may destabilize the YSZ structure, producing monoclinic zirconia and microscopic YSZ strain, which has implications upon the electrode’s mechanical integrity and thus lifetime of the SOFC.

Hydrogen Production via Model Diesel Steam Reforming over a High-Performance Ni/Ce0.75La0.25O2−δ-γ-Al2O3 Catalyst with Oxygen Vacancies

Hydrogen-rich syngas production via diesel steam reforming (DSR) is of great interest because of the high H2/CO ratio and available external heat sources. In this study, catalysts (particle size (dp) = 3–5 mm) containing 10 wt % Ni were prepared through wet impregnation, and their performance was evaluated in a semipilot fixed-bed reactor (Φ = 42 mm). The catalyst was doped with La, which resulted in excellent catalytic performance under different operating conditions. The apparent activation energies of the catalysts for the model diesel (n-dodecane) steam reforming increased in the order Ni/Ce0.75La0.25O2−δ-γ-Al2O3 (17 kJ/mol) < Ni/CeO2-γ-Al2O3 (18 kJ/mol) < Ni/γ-Al2O3 (27 kJ/mol). The model DSR over Ni/Ce0.75La0.25O2−δ-γ-Al2O3 showed product stability and produced a high H2 content (32 vol %). Fresh and spent catalysts were characterized by X-ray diffraction, N2 adsorption–desorption isotherms, H2-temperature programmed reduction, inductively coupled plasma–mass spectrometry, X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, and thermogravimetric analysis. The origin of the oxygen vacancies in the La-doped catalyst was elucidated using density functional theory calculations. The improvements in the activity and stability of the Ni/Ce0.75La0.25O2−δ-γ-Al2O3 catalyst are attributed to (1) the enhancement in the oxygen release/storage capacity, (2) the stabilizing interaction between Ni and the surface La2O3 phase, and (3) the small size (11 nm) and uniform distribution of Ni particles. Out of the tested catalysts, the spent Ni/Ce0.75La0.25O2−δ-γ-Al2O3 catalyst showed the lowest coke deposition (5.8 mg/g-cat.) and the least severe Ni agglomeration after 10 h of testing because of its superior ionic mobility and oxygen storage capacity.