Ni YSZ Research Articles

Relation Between Ni Particle Shape Change and Ni Migration in Ni–YSZ Electrodes – a Hypothesis

AbstractThis paper deals with degradation mechanisms of Ni–YSZ electrodes for solid oxide cells, mainly solid oxide electrolysis cells (SOECs), but also to some extent solid oxide fuel cells (SOFCs). Analysis of literature data reveals that several apparently different and even in one case apparently contradicting degradation phenomena are a consequence of interplay between loss of contact between the Ni–YSZ (and Ni–Ni particles) in the active fine‐structured composite fuel electrode layer and migration of Ni via weakly oxidized Ni hydroxide species. A hypothesis that unravels the apparent contradiction and explains qualitatively the phenomena is presented, and as a side effect, light has been shed on a degradation phenomenon in solid oxide fuel cells (SOFCs) that has been observed during a decade.

구형 Ni과 나노 YSZ Powder를 이용하여 제조한 Ni/YSZ Core-shell의 SOFC 연료극 특성

We reviewed the electrical properties of SOFC anode manufactured using spherical Ni and nano YSZ powder. When core-shell is fabricated by using submicron Ni as core and nano-sized YSZ as shell for SOFC anode, the electrical conductivity of the <TEX>$0.2{\mu}m$</TEX> Ni-YSZ core-shell was 3 times higher than that of <TEX>$1.0{\mu}m$</TEX> NiO or <TEX>$1.0{\mu}m$</TEX> Ni-YSZ. Hydrogen selectivity was similar at <TEX>$800^{\circ}C$</TEX>, but hydrogen selectivity and methane conversion rate under <TEX>$750^{\circ}C$</TEX> was 10~25% higher, Power density was more than 2 times, ASR was about 1/3, when exposed to <TEX>$H_2$</TEX> atmosphere at <TEX>$750^{\circ}C$</TEX> for a long time, Ni particles did not have any growth or cut off conduction path.

CFD-simulation of effective carbon gasification strategies from high temperature SOFC Ni–YSZ cermet anodes

Carbon formation during internal reforming and irreversible nickel re-oxidation are main performance degradation sources of high temperature nickel–yttrium stabilized zirconia (Ni–YSZ) cermet anodes. In order to prolong the lifetime of electrodes and of solid oxide fuel cells (SOFCs), it is of key importance to find strategies to effectively and permanently gasify carbon deposits, that exclude the re-oxidation risk. Thus, this computational fluid dynamics (CFD) study scrutinizes four fundamentally diverse carbon gasification strategies, including carbon removal of a polarized cell, in the temperature range of 400–800 °C for their implementation in SOFC applications. Surface species that are precursors to before-mentioned degradation phenomena are temporally and spatially identified and resolved. Pure CO2, and a mixture of CO2 and O2 showed high carbon gasification rates. Nevertheless, a high nickel re-oxidation potential was identified. It is highlighted that humidified hydrogen and our novel approach of a polarized cell provided a safe gasification process by creating a reducing surface condition preventing from re-oxidation. These two methods can be used to extend a cell's lifetime and bring SOFCs closer to commercialization.

Incorporation of a Pd catalyst at the fuel electrode of a thin-film-based solid oxide cell by multi-layer deposition and its impact on low-temperature co-electrolysis

To reveal the reaction mechanism of low-temperature co-electrolysis (LT-Co-EC), insertion of Pd into the Ni–YSZ fuel electrode functional layer by multi-layer thin film deposition was studied.

Dependence of power density on anode functional layer thickness in anode-supported solid oxide fuel cells

To improve the power densities of anode-supported multilayer solid oxide fuel cells (SOFCs) based on an anode functional layer (AFL) and an anode support of Ni–YSZ (8 mol%Y2O3–ZrO2), we systematically investigated how the AFL thickness influenced power density and impedance spectra, preparing the AFL using 65 wt% NiO–35 wt% YSZ with the anode support using 60 wt% NiO–40 wt% YSZ. Among the prepared SOFCs with AFL thicknesses from 0 to 100 μm, the ∼10-μm-AFL SOFC (AFL10) showed the highest power density. Impedance spectroscopy of the prepared SOFCs was composed of an ohmic resistance as well as three constant phase elements (CPEs) resulting from activation- and concentration-polarizations. The impedance spectroscopy showed that AFL10 effectively decreased both the contact- and activation polarization-resistances. In addition, a part of the concentration polarization resistances for AFL10 was lower among the prepared SOFC samples. As such, AFL10 exhibited the highest power density among the prepared SOFC samples.

Degradation Phenomenon of Ni-YSZ Anode Support Cells Under Various Operating Conditions

Solid oxide fuel cells (SOFCs) is an electrochemical device which can directly convert chemical energy to electrical energy. They have many advantages including the high efficiency and multi-fuel flexibility. [1,2] Although high temperature-SOFCs have advantages in terms of performances high production cost and the limited durability are one of issues for making commercialization of SOFCs. Generally, SOFCs are required over 40,000 h life-time to enter into the real markets. [3,4] Furthermore, SOFCs should produce electrical power stably under a variety of severe operating conditions, such as thermal cycle, sudden electrical load change, and frequent electrical load cycle. In this study, various accelerating degradation testing are performed to find the degradation patterns and disclose the main degradation mechanism of SOFCs. The durability of SOFCs are tested on a variety conditions of load trip and load cycle. The load trip is cycled at the current density of between 0 and 200 mA cm-2 at 700oC. Each step is held at 12 h and repeated 30 times. The load cycle is cycled under the current density of between 120 and 200 mA cm-2 at 700oC. Each step is held at 12 h and repeated 12 times. The thermal cycle is tested at 700oC and ramped down at 1℃ min-1 to 400oC. This step is repeated 30 times. As a periodic diagnostic tool of I-V (current-voltage) characterization and electrochemical impedance analysis are performed cells, using an electrochemical impedance spectroscopy (EIS) with a Potentiostat/Galvanostant device using a frequency range from 1 MHz to 1Hz. The physicochemical analysis of tested cell are carried out using field emission-scanning electron microscopy (FE-SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). 1. Steele, Brian CH, and Angelika Heinzel. "Materials for fuel-cell technologies."Nature 414.6861 (2001): 345-352. 2. Yamamoto, Osamu. "Solid oxide fuel cells: fundamental aspects and prospects." Electrochimica Acta 45.15 (2000): 2423-2435. 3. Khan, Muhammad Shirjeel, et al. "Fundamental mechanisms involved in the degradation of nickel–yttria stabilized zirconia (Ni–YSZ) anode during solid oxide fuel cells operation: A review." Ceramics International 42.1 (2016): 35-48. 4. Hagen, Anke, et al. "Durability study of SOFCs under cycling current load conditions." Fuel Cells 9.6 (2009): 814-822. * Corresponding author: jyoung@sejong.ac.kr (J.-Y. Park).

Degradation of solid oxide cells during co-electrolysis of steam and carbon dioxide at high current densities

Abstract In this work, the durability of Ni–YSZ based solid oxide cells was investigated during co-electrolysis of steam and carbon dioxide (45% H 2 O + 45% CO 2 + 10% H 2 ) at current density of −1.5 or −2.0 A cm −2 . The cell consists of ∼300 μm Ni–YSZ support, ∼10 μm Ni–YSZ electrode, ∼10 μm YSZ electrolyte and ∼15 μm LSM–YSZ oxygen electrode. The gas conversion was 45% at −1.5 A cm −2 and 60% at −2.0 A cm −2 , and the operating durations were up to 700 h. The detailed electrochemical analysis revealed significant increase of the ohmic resistance, oxide ion transport resistance in the Ni–YSZ composite electrodes and the electrochemical reaction resistance at the Ni–YSZ triple-phase boundaries. The performance degradation is mainly ascribed to the microstructural change in the Ni–YSZ electrode close to the YSZ electrolyte, including the percolation loss of Ni, the contact loss between Ni and YSZ electrolyte and the decomposition of YSZ close to Ni–YSZ|YSZ interface. The electrochemical performance and the microstructure of the oxygen electrode were found to be relatively stable.

Oxidation of carbon deposits on anode material Ni–YSZ in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrochemical devices that convert the chemical energy of fuels directly into electricity with high conversion efficiency. A major advantage of SOFC systems compared to low-temperature fuel cells is that SOFCs can use humidified hydrocarbons as fuel. However, the use of hydrocarbons may lead to an undesirable carbon deposition as hydrocarbons are oxidized at the anode side in SOFCs. Such carbon deposits on the nickel–yttrium stabilized zirconia (Ni–YSZ) anode material are the most probable mode of deactivation of SOFCs. Therefore, commercially available NiO–YSZ anode material for high-temperature fuel cells was activated by a temperature-programmed reduction in a hydrogen atmosphere. In the next step, carbon was deliberately deposited on the reduced samples by isothermal deposition in a methane–argon atmosphere. The carbon deposits were then burned off with a temperature-programmed oxidation (TPO) in an oxygen–argon atmosphere at different heating rates using thermo-analytical equipment. The TPO was followed by TG, DTG, DTA and QMS measurements. The easiest way to distinguish among various types of deposited carbon was to follow the QMS curves. The obtained QMS curves were processed with Netzsch Peak Separation software in order to extract several peaks corresponding to various forms of carbon deposits. It was found that high amorphous and low amorphous carbon are burned off at relatively low temperatures (up to 650 °C), fibrous carbon is oxidized up to 750 °C, graphite is burned out up to 830 °C and finally, carbon diluted in nickel leaves the system in the 900–1000 °C range, depending on the oxidizing conditions. The isoconversional method was used to calculate activation energies of various carbon oxidation processes. The absolute values of the activation energies increase from amorphous carbon via fibrous carbon to graphite.

Modeling of gas transport with electrochemical reaction in nickel-yttria-stabilized zirconia anode during thermal cycling by Lattice Boltzmann method

Abstract This work reports an investigation of the impact of microstructure on the performance of solid oxide fuel cells (SOFC) composed of nickel yttria-stabilized zirconia (Ni YSZ). X-ray nano computed tomography (nano-CT) was used to obtain three-dimensional (3D) models of Ni-YSZ composite anode samples subjected to different thermal cycles. Key parameters, such as triple phase boundary (TPB) density, were calculated using 3D reconstructions. The electrochemical reaction occurring at active-TPB was modeled by the Lattice Boltzmann Method for simulation of multi-component mass transfer in porous anodes. The effect of different electrode geometries on the mass transfer and the electrochemical reaction in anodes was studied by TPB distributions measured by nano CT for samples subjected to different thermal cycles. The concentration polarization and the activation polarization were estimated respectively. The results demonstrate that a combined approach involving nano-CT experiments in conjunction with simulations of gas transport and electrochemical reactions using the Lattice Boltzmann method can be used to better understand the relationship between electrode microstructure and performance of nickel yttria-stabilized zirconia anodes.

Microstructure, texture, and crystallography in Ni–GDC and Co–GDC porous cermets obtained from directionally solidified eutectic ceramics

Lamellar NiO–GDC (Gadolinium-doped Ceria) and CoO–GDC directionally solidified eutectic ceramics (DSECs) were produced by the laser floating-zone technique and subjected to reduction in order to obtain porous cermets of Ni–GDC and Co–GDC, which have potential applications as anodes in solid oxide fuel cells (SOFC). The reduction of these DSECs into porous cermets was studied at 650 °C in NiO–GDC and at 500 and 700 °C in CoO–GDC, all of them processed with similar reduction kinetics. In comparison to similar Ni–YSZ and Co–YSZ lamellar cermets previously studied, no sharp reduction front was observed. The interface between the reduced and nonreduced zones is broader, with pores homogenously distributed in wide areas. Afterwards, the microstructure, texture, and crystallography of the samples were studied by electron microscopy as well as by electron and X-ray diffraction when completely reduced. The single crystal NiO and CoO lamellae transformed into porous polycrystalline metallic lamellae. Moreover, microscopy observations revealed a porous nanostructure of Co particles obtained by reduction at low temperatures (500 °C). Many of the Co and Ni particles seemed to have roughly maintained the previous crystallographic orientation with respect to the GDC phase, although the disorder of the crystallographic orientation increased significantly. In addition, a significant amount of the Ni particles reoriented to form an epitaxial interface with the (100)–GDC surface.

Electrochemically Scavenging the Silica Impurities at the Ni–YSZ Triple Phase Boundary of Solid Oxide Cells

Silica impurity originated from the sealing or raw materials of the solid oxide cells (SOCs) accumulating at the Ni-YSZ triple phase boundaries (TPBs) is known as one major reason for electrode passivation. Here we report nanosilica precipitates inside Ni grains instead of blocking the TPBs when operating the SOCs at |i| ≥ 1.5 A cm(-2) for electrolysis of H2O/CO2. An electrochemical scavenging mechanism was proposed to explain this unique behavior: the removal of silica proceeded through the reduction of the silica to Si under strong cathodic polarization, followed by bulk diffusion of Si into Ni and reoxidation of Si in the Ni grain.

Impact of nanostructured anode on low-temperature performance of thin-film-based anode-supported solid oxide fuel cells

The impact of a nanostructured Ni–yttria-stabilized zirconia (Ni–YSZ) anode on low-temperature solid oxide fuel cell (LT-SOFC) performance is investigated. By modifying processing techniques for the anode support, anode-supported SOFCs based on thin-film (∼1 μm) electrolytes (TF-SOFCs) with and without the nanostructured Ni–YSZ (grain size ∼100 nm) anode are fabricated and a direct comparison of the TF-SOFCs to reveal the role of the nanostructured anode at low temperature is made. The cell performance of the nanostructured Ni–YSZ anode significantly increases as compared to that of the cell without it, especially at low temperatures (500 °C). The electrochemical analyses confirm that increasing the triple-phase boundary (TPB) density near the electrolyte and anode interface by the particle-size reduction of the anode increases the number of sites available for charge transfer. Thus, the nanostructured anode not only secures the structural integrity of the thin-film components over it, it is also essential for lowering the operating temperature of the TF-SOFC. Although it is widely considered that the cathode is the main factor that determines the performance of LT-SOFCs, this study directly proves that anode performance also significantly affects the low-temperature performance.

Microstructural finite element modeling of redox behavior of Ni–YSZ based ceramic SOFC anodes

One of the main challenges in solid oxide fuel cell (SOFC) is the redox problem of Ni-based anodes. Therefore, the redox tolerance of the anode should be improved to increase the service life of the system. Since the real SOFC anodes have a heterogeneous structure with different grain sizes, a micro-level modeling is necessary. Especially, the re-oxidation step which creates large amount of stress is critical during the redox cycles. Therefore, in this study, the effects of re-oxidation temperatures and exposure times on the oxidation rate and stresses developed are investigated on a synthetically generated porous anode functional layer microstructure. A mathematical model is developed to characterize the re-oxidation process and solved numerically. The results indicate that the re-oxidation strongly depends on the re-oxidation temperature and exposure time. The time required for the full oxidation is found to be 7458.55, 313.14, 200.8 and 33.78s at 800, 850, 900 and 950°C, respectively for the generated anode microstructure. In addition, the created stresses in the anode structure are well above the critical material limits.

Fabrication and evaluation of NiO/Y2O3-stabilized-ZrO2 hollow fibers for anode-supported micro-tubular solid oxide fuel cells

In this work NiO/3mol% Y2O3–ZrO2 (3YSZ) and NiO/8mol% Y2O3–ZrO2 (8YSZ) hollow fibers were prepared by phase-inversion. The effect of different kinds of YSZ (3YSZ and 8YSZ) on the porosity, electrical conductivity, shrinkage and flexural strength of the hollow fibers were systematically evaluated. When compared with Ni–8YSZ the porosity and shrinkage of Ni–3YSZ hollow fibers increases while the electrical conductivity decreases, while at the same time also exhibiting enhanced flexural strength. Single cells with Ni–3YSZ and Ni–8YSZ hollow fibers as the supported anode were successfully fabricated showing maximum power densities of 0.53 and 0.67Wcm−2 at 800°C, respectively. Furthermore, in order to improve the cell performance, a Ni–8YSZ anode functional layer was added between the electrolyte and Ni–YSZ hollow fiber. Here enhanced peak power densities of 0.79 and 0.73Wcm−2 were achieved at 800°C for single cells with Ni–3YSZ and Ni–8YSZ hollow fibers, respectively.

High-performance Ni–YSZ thin-walled microtubes for anode-supported solid oxide fuel cells obtained by powder extrusion moulding

A fast near-net-shape and cost-effective manufacturing method (PEM) were applied to produce Ni–YSZ thin-walled microtubes for anode-supported SOFCs applications. The effects of NiO particle size on rheology and final microstructure are addressed.

Comparative study on ammonia oxidation over Ni-based cermet anodes for solid oxide fuel cells

In the current work, we investigate the performance of solid oxide fuel cells (SOFCs) with Ni‒yttria-stabilized zirconia (Ni–YSZ) and Ni‒gadolinia-dope ceria (Ni–GDC) cermet anodes fueled with H2 or NH3 in terms of the catalytic activity of ammonia decomposition. The cermet of Ni–GDC shows higher catalytic activity for ammonia decomposition than Ni–YSZ. In response to this, the performance of direct NH3–fueled SOFC improved by using Ni–GDC anode. Moreover, we observe further enhancement in the cell performance and the catalytic activity for ammonia decomposition with applying Ni−GDC anode synthesised by the glycine–nitrate combustion process. These results reveal that the high performance of Ni–GDC anode for the direct NH3–fueled SOFC results from its mixed ionic–electronic conductivity as well as high catalytic activity for ammonia decomposition.

Gas Composition Analysis on Ni-YSZ Anode in Direct Internal Reforming Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) is one of the most promising power generation devices. In Japan, the residential SOFC cogeneration systems have been commercialized since 2011. In this system, the supplied methane-rich fuel is reformed in an external reactor and the pre-reformed gas (hydrogen-rich fuel) is introduced to the anode chamber. In contrast, high operating temperature of SOFCs provides the possibility of a direct internal reforming operation, in which the anode material plays a role in the promotion of fuel reforming and fuel oxidation. Because the supplied fuel is reformed over anode catalyst, the concentration distribution of reactants and products appears along the direction of gas flow. Moreover, under discharge operation, the exothermic reaction of electrochemical oxidation as well as the endothermic reaction in the fuel reforming proceed simultaneously at the anode. In this case, the temperature distribution is more complicated than that in a supply of hydrogen fuel. Up to now, there are few reports on the direct evaluation of gas composition and temperature distributions over the anode although the theoretical calculation have been conducted. This study, therefore, aimed to investigate the methane steam reforming and electrochemical oxidation over Ni–YSZ anode in an anode-supported cell. The distribution of gas composition over the anode surface along the direction of gas flow was examined by gas chromatograph under the open circuit and discharge conditions in a direct internal reforming SOFC. The effect of total flow rate on the reaction process was studied under the open circuit condition. Regardless of total flow rate, the methane conversion remarkably increased at the upstream part. The extent of conversion enhancement was more significant at slower flow rates. In contrast, the correlation between the obtained methane conversion and the gas-catalyst contacting time agreed well in all flow rate conditions. The temperatures of anode surface at all measurement positions were lower than that under supplying only hydrogen due to the endothermic reaction in methane reforming process. The temperature variation against the direction of gas flow corresponded to the methane conversion. The surface temperature decreased at the upstream parts and then gradually rose toward the downstream parts. During discharge, the enhancement of methane conversion and the decrement of CO concentration were accomplished with an increase in current density. This result suggests that the steam reforming reaction and water gas shift reaction were promoted by the consumption of hydrogen and the formation of steam in the electrochemical hydrogen oxidation.

Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface

The catalytic effect of tin (Sn) as a mediator in a direct carbon fuel cell (DCFC) is investigated. Sn is applied onto the Ni–YSZ anode surface with varying in its amount for the Sn quantity optimization. The existence of Sn placed on the anode gives considerably enhanced power density. In addition, the electrochemical behaviors are propense to show the best power density within the region between 15 and 30mg of Sn per geometric surface area (3.14cm2). After the tests, the distribution of Sn and carbon is confirmed by anode morphology images and analyses through SEM and EPMA/WDS. To better understand the observed phenomena, model experiments are also carried out. The model study suggests that oversupplied Sn mediator becomes agglomerated resulting in less reaction sites, and hence a diminution in power generation.

Computation of Effective Steady‐State Creep of Porous Ni–YSZ Composites with Reconstructed Microstructures

This paper investigates the effective steady‐state creep response of porous Ni–YSZ composites used in solid oxide fuel cell applications by numerical homogenization based on three‐dimensional microstructural reconstructions and steady‐state creep properties of the constituent phases. The Ni phase is found to carry insignificant stress in the composite and has a negligible role in the effective creep behavior. Thus, when determining effective creep, porous Ni–YSZ composites can be regarded as porous YSZ in which the Ni phase is counted as additional porosity. The stress exponents of porous YSZ are the same as that of dense YSZ, but the effective creep rate increases by a factor of 8–10 due to porosity. The relationship of creep rate and volume fraction of YSZ computed by numerical homogenization is underestimated by most existing analytical models. The Ramakrishnan–Arunchalam creep model provides the closest approximation among all analytical models.

Novel light-weight, high-performance anode-supported microtubular solid oxide fuel cells with an active anode functional layer

Influence of the air-gap, the distance from the tube-in-orifice spinneret to the upper surface of the external coagulant bath during the extrusion/phase-inversion process, on the microstructure of nickel – yttria-stabilized zirconia (Ni–YSZ) hollow fibers has been systematically studied. When the air-gap is 0 cm, the obtained Ni–YSZ hollow fiber has a sandwich microstructure. However, when the air-gap is increased to 15 cm, a bi-layer Ni–YSZ hollow fiber consisting of a thin layer with small pores and a thick support with highly porous fingerlike macrovoids has been achieved. The output power density of microtubular solid oxide fuel cells (MT-SOFCs) with a cell configuration of Ni–YSZ/YSZ/YSZ–LSM increases from 594 mW cm−2 for the cells with the Ni–YSZ anode of sandwich microstructure to 832 mW cm−2 for the cells with the Ni–YSZ anode of bi-layer microstructure at 750 °C, implying that to achieve the same output power density, the weight of the cells with the bi-layer anode support can be reduced to 41.5% compared with that of the cells with the sandwich anode support. Thermal-cycling test shows no obvious degradation on the open-circuit-voltage (OCV), indicating that the MT-SOFCs have robust resistance to thermal cycling.