Ni Yttria-stabilized Zirconia Cermet Research Articles

Characterization and Testing of Exsolution-Based Solid Oxide Cells for Reversible Operations in CO2 Electrolysis

Reversible solid oxide cells (rSOCs) are a promising electrochemical technology able to work both as energy storage devices in solid oxide electrolysis (SOEC) mode, and as power generators in solid oxide fuel cell (SOFC) mode. rSOCs implemented for CO2 electroreduction into valuable CO-rich steams through electrolysis would provide opportunities for CO2 utilization, and hence become useful tools for the reduction of greenhouse gases emissions. High-temperature co-electrolysis of CO2/H2O mixtures has recently become of interest since high conversion and energy efficiency are thermodynamically favored in addition to the reduction in cell area-specific resistance when compared to pure CO2 electrolysis [1]. However, reversible operation with CO2-containing feeds requires developing flexible, high-performing, and long-lasting materials for the rSOCsManaging issues such as coking and low redox tolerance in the state-of-the-art (SoA) Ni-Yttria-stabilized-zirconia (Ni-YSZ) cermet fuel electrode in the rSOCs is crucial. Ni-YSZ cermet exhibits high electronic conductivity and electrochemical activity in hydrogen. Nonetheless, Ni is prone to oxidation due to operation under high steam concentrations, and rapid variations in the fuel supply. Additionally, Ni is well-known for its coke-formation tendency which becomes detrimental in CO2-rich mixtures for electrolysis [2]. Thus, ceramics with mixed ionic and electronic conductivity (MIEC) have emerged as alternative fuel electrodes thanks to the larger electrochemically active area compared to standard cermets. Among the MIEC electrodes, the SrTi0.3Fe0.7O3 (STF) perovskite has proven to give low polarization resistances especially when modification techniques such as exsolution of metal nanoparticles are employed [3]. This work focuses on the investigation of the electrocatalytic properties of exsolution-based STF electrodes operating with CO/CO2 and H2O/CO2 mixtures. The mixtures used were 50% CO2/50% CO, 25% H2O/25% CO2/25%H2/25%CO, and 45% H2O/45% CO2/10% H2. A comparison between the performance and microstructure of the studied electrode formulation is provided.Electrolyte-supported cells were manufactured via screen-printing of the electrode inks on scandia-stabilized zirconia (ScSZ) electrolytes. STF perovskite-based electrodes were used, namely: SrTi0.3Fe0.7O3 (STF), Sr0.95(Ti0.3Fe0.63Ni0.07)O3 (STF-Ni), and Sr0.95(Ti0.3Fe0.63Ru0.07)O3 (STF-Ru). The latter two formulations allowed the enhancement of the perovskite structure via exsolution of catalytic nanoparticles (Ni, Co alloyed with Fe) on the oxide surface during cell operation. Each cell configuration included a gadolinium-doped ceria (GDC) buffer layer coupled with an LSCF-GDC (La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O1.95) oxygen electrode. Baseline performances of cell mounting the Ni-YSZ cermet electrode were also evaluated for comparison with the perovskite alternatives, both in the short and long term. Electrochemical characterization was done via impedance spectroscopy, polarization experiments, and durability tests in potentiostatic mode. Chemical characterization was performed via X-ray diffraction, scanning electron microscopy, and temperature-programmed reduction analyses.Compared to Ni-YSZ, significant improvement in terms of reversibility and maximum current densities of the current-voltage (I/V) curves in both SOFC and SOEC modes is found on the STF-based cells at 750°C under CO/CO2 mixtures (Figure 1B). The initial difference in the performance of the STF cells with respect to the Ni-YSZ cermet in humidified H2 (SOFC mode, Figure 1A) was overcome by using the exsolved electrode formulations. Aging experiments up to 100 h performed in alternating operation modes from SOFC (6 h) to SOEC (6 h) indicated an initial performance degradation within the first 24 h for both the cermet and perovskite-based fuel electrodes which later stabilized in time. Compared to the undoped STF formulation, improvement in performance stability was observed for the exsolved STF-Ni and STF-Ru fuel electrodes while working under the target CO/CO2 mixture. This was attributed to the combination of the STF structure benefits (low polarization resistance towards CO/CO2 and H2O/CO2 mixtures) with the boost that metallic nanoparticles provide to the electrode’s heterogeneous catalytic reactivity and overall stability over time.

Cellular Nickel-Yttria/Zirconia (Ni-YSZ) Cermet Foams: Manufacturing, Microstructure and Properties.

Open-celled ceramic composite foams were prepared from NiO and yttria-stabilized zirconia (YSZ) powders by the polymer sponge replication (Schwartzwalder) technique using the respective aqueous dispersions. Mechanically stable NiO–YSZ foams with an average porosity of 93 vol.% were obtained. After chemical reduction of the NiO phase with hydrogen, cellular Ni–YSZ cermet structures were obtained. They are characterized by an electric conductivity up to 19∙103 S∙m−1 which can be adjusted by both, the Ni volume fraction, and the sintering/reduction procedure. The NiO–YSZ ceramic foams, as well as the cellular Ni–YSZ cermets prepared therefrom, were characterized with respect to their microstructure by scanning electron microscopy, confocal Raman microscopy and X-ray diffraction with Rietveld analysis. In addition, the compressive strength, the electric conductivity and the thermal conductivity were determined. The collected data were then correlated to the sample microstructure and porosity and were also applied for modelling of the mechanical and electric properties of the bulk Ni–YSZ strut material.

Preparation of YSZ electrolyte coating on conducting porous Ni–YSZ cermet by DC and pulsed constant voltage electrophoretic deposition process for SOFCs applications

In this article, electrolyte coating of solid oxide fuel cells (SOFCs) based on yttria stabilized zirconia (YSZ) on Ni-YSZ cermet fuel cells anode in ethanol was investigated using pulsed constant voltage and DC electrophoretic deposition (EPD). The NiO-YSZ substrate was prepared by hydraulic pressing of mixed YSZ, NiO and starch powders following pre-sintering at 1200 °C for 3 h. The NiO-YSZ composite was reduced in hydrogen atmosphere in order to produce Ni-YSZ cermet. The suspension properties and EPD process parameters were investigated. The YSZ electrolyte was sintered at 1350 °C for 4 h. The morphology of sintered YSZ electrolyte coating was investigated by scanning electron microscope (SEM) and optical microscopy (OM). Results showed that dense and uniform coatings were deposited in ethanol with solid concentration of 10 g/L of YSZ, 0.5 g/L of iodine at distance of anode to cathode being 5 mm. The thickness of YSZ electrolyte after 6 min of deposition with pulsed constant voltage and DC electrophoretic deposition were 20 and 88 ± 1 μm, respectively.

Improving intermediate temperature performance of Ni-YSZ cermet anodes for solid oxide fuel cells by liquid infiltration of nickel nanoparticles

Liquid infiltration of NiO followed by reduction to form Ni nanoparticle catalysts in solid oxide fuel cell (SOFC) can produce a high density of electrochemical reaction sites. In recent years, electrode architectures utilizing porous oxide substrates with ionic conductivity or mixed ionic-electronic conductivity and connected networks of nickel produced by liquid infiltration have become a popular approach to improve SOFC anode catalytic performance, especially for operating temperatures less than 800 °C. However, infiltrated nickel structures suffer from poor durability, demonstrating significant loss in performance during the first 100 h of use. In contrast, traditional Ni-yttria stabilized zirconia (Ni-YSZ) cermet SOFC anodes exhibit long-term performance stability. However, Ni-YSZ cermet anodes have micron sized structures, and consequently have a significantly lower density of electrochemical reaction site density than infiltrated nickel structures, which have dimensions of around 100 nm. In this study, the performance impact of liquid phase infiltration of nickel nanoparticles into Ni-YSZ cermet anode supported SOFCs is studied by measuring the electrochemical behavior of infiltrated cells at 800 °C, 700 °C, and 600 °C, and comparing them to the performance of an uninfiltrated cell. Durability of the nanoparticles after electrochemical testing is also assessed using a method for quantifying particle statistics from fracture cross sections.

Joint strength of Ag–9Pd–9Ga brazed interconnect and anode-supported electrolyte for solid-oxide fuel cell applications

A newly developed Ag–9Pd–9Ga active filler was vacuum brazed, and the mechanical properties between the metallic interconnects (SS430, Crofer22 APU, Crofer22 H) and a Ni–yttria-stabilized zirconia cermet anode were systematically investigated. The results indicate that the bonding between metal and cermet is well established and that the interface is smooth. The joint strength evaluated at both 25 ℃ and 800 ℃ under shear and tensile loading conditions confirmed that the brazed Ag–9Pd–9Ga sealant compared favorably with its commercially available glass-ceramic GC-9 counterpart.

Cracking Reaction of Octane Fuel and Electrochemical Reaction on Ni-YSZ Cermet Anode of Internal Steam Reforming Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) can, in principle, use not only hydrogen but also hydrocarbon fuels which have high energy density. Then, SOFC is a promising power source for many potential mobile robot applications. Octane is a good candidate as fuel due to high boiling point enough to be in the liquid phase even at high temperatures in summer. Performance of the internal reforming SOFC is governed by anode electrode material, internal reforming method, and operating conditions. Ni-yttria stabilized zirconia (Ni-YSZ) cermet is the most promising anode material for SOFC because of its low cost, chemical stability, and closely matched thermal expansion coefficients with the dense YSZ electrolyte. Steam reforming is the first choice for the internal reforming method because it has been frequently-studied for the direct use of methane fuel. Then, operation of SOFC by internal steam reforming of octane has been studied. However, cracking reactions on the YSZ anode, which is affected by the operating conditions, have not been elucidated sufficiently. The purpose of this study is to clarify the cracking reaction of octane fuel and the electrochemical reaction of the Ni-YSZ anode electrode under various operating conditions. In this study, the fuel-cracking reaction on the Ni-YSZ anode electrode is estimated by gas chromatography with thermal conductivity detector. Anode electrode performance is studied by electrochemical measurement. Two types of octane (n-octane; linear alkane, iso-octane; branched chain alkane) are used in order to elucidate the effect of difference in carbon-chain structure. Octane and steam of equilibrium vapor pressure were supplied to the SOFC cell by bubbling carrier gas (He) at a constant rate. Temperature of the octane bubbler and the carrier gas flow rate was 30 oC and 10 ml/min, respectively. The carrier gas flow rate of the water bubbler was 83 ml/min. Steam/carbon ratio (S/C) was controlled over an S/C ratio from 0.5 to 5.5 by changing the temperature of the water bubbler from 30 to 80 oC. The cell temperature range was 700-950 oC. The octane internal steam reforming operation of SOFC was demonstrated by I-V and I-P characteristics and stable power generation property by constant current. The effects of operating conditions (S/C ratio and cell temperature) on the open circuit voltage (OCV) and power generation property were investigated. The chemicals generated by the cracking reaction of octane at various operating conditions were analyzed by gas chromatography and compared. The difference in outlet gas compositions from the cell of open circuit condition and power generation condition revealed the main species consumed for the power generation reaction on the anode electrode. The electrochemical reaction resistance of the Ni-YSZ anode was measured by ac impedance method and the rate-determining process was identified. In this study the following was revealed. When the S/C and/or the operating temperature are high, the reforming reaction rate is large. By the reforming reaction of n-octane, H2, CO, CO2, and CH4 is mainly produced. When the iso-octane is used as the fuel, small amount of C2H6 is also generated in addition to them. Only H2 and CO contribute to the power generation directly. Rate-determining process of the electrode reaction is a cracking reaction to H2 and CO.

Novel high-performance solid oxide fuel cells with bulk ionic conductance dominated thin-film electrolytes

The overall performance of ionic conducting electrolyte layers is a key factor for determining the power density of solid oxide fuel cells (SOFCs). The aim of this work is to investigate high performance SOFC electrolyte layers developed in our lab via a low cost wet-chemical processing method. In this paper, SOFCs with bulk ionic conductivity dominated thin-film electrolyte demonstrate superior electrochemical performances. Conventional materials for SOFCs are applied in this work: Ni–YSZ cermet as the anode, yttria-stabilized zirconia (YSZ) as the electrolyte, gadolinia-doped ceria (CGO) as the Sr-diffusion barrier layer, and LSCF or LSC as the cathode. At 0.7 V and 600 °C, single cells with an active LSCF and LSC cathode area of 4 × 4 cm2 obtain a power density of 0.7 and 1.4 W cm−2, respectively. According to electrochemical impedance spectroscopy (EIS), the ohmic resistance of the single cells is almost one order of magnitude lower than the conventionally fabricated SOFCs. Due to the improved performance of the electrolyte, SOFCs are able to deliver high power output at reduced operating temperature and increased cell voltage.

New fabrication technique for a Ni–YSZ composite anode from a core–shell structured particle

In this study, we propose a new fabrication technique for a Ni–YSZ (yttria-stabilized zirconia) composite anode widely used in solid oxide fuel cells (SOFCs). A spherical core–shell structure consisting of a micron-sized Ni (core) and nano-sized YSZ (shell) was prepared by a high-speed mixing technique from metallic a Ni and YSZ powder mixture. The core–shell Ni–YSZ powder was then heat-treated at 1450 °C in an air atmosphere. During the early stage of the heat-treatment between 300 and 550 °C, it seemed that the surface of the Ni core was oxidized and the Ni particles were coated with a relatively dense NiO thin layer. As the heat-treatment temperature increased, the Ni core particles erupted, which led to uniformly distributed Ni particles in the YSZ framework. The resulting Ni–YSZ composite exhibited better electrical conductivity and ASR for anode polarization than a commercially available Ni–YSZ cermet anode.

Hydrogen Production Using Solid Oxide Membrane Electrolyzer with Solid Carbon Reductant in Liquid Metal Anode

A laboratory-scale solid oxide membrane (SOM) steam electrolyzer that can potentially utilize the energy value of coal or any hydrocarbon reductant to produce high purity hydrogen has been fabricated and evaluated. The SOM electrolyzer consists of an oxygen-ion-conducting yttria-stabilized zirconia (YSZ) electrolyte with a Ni–YSZ cermet cathode coated on one side and liquid tin anode on the other side. Hydrogen production using the SOM electrolyzer was successfully demonstrated between 900 and by feeding a steam-rich gas to the Ni–YSZ cermet cathode and solid carbon reductant into the liquid tin anode. It was confirmed that the energy required for hydrogen production can be effectively lowered by feeding a solid carbon reductant in the liquid tin anode. A polarization model for the SOM electrolyzer was developed. The experimental data obtained under different operating conditions were curve fitted into the model to identify the various polarization losses. Based on the results of this study, work needed toward increasing the electrochemical performance of the SOM electrolyzer is discussed.

Electrochemical and microstructural analysis of nickel–yttria-stabilized zirconia electrode operated in phosphorus-containing syngas

Trace impurities in coal-derived syngas pose a threat on the performance and durability of solid oxide fuel cells (SOFCs) operated in syngas. A Ni–YSZ/YSZ/Ni–YSZ (YSZ stands for yttria-stabilized zirconia) half-cell is constructed in order to evaluate the effects of a phosphorus impurity on the performance of the Ni–YSZ cermet anode in the simulated coal-derived syngas containing 30.6% H 2, 30% CO, 11.8% CO 2, 27.6% H 2O and 20 ppm PH 3. Electrochemical impedance analysis shows that both the charge transfer resistance and the diffusion resistance increase with time during exposure to the P-containing syngas. The diffusion resistance increases faster than the charge transfer resistance. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies show that phosphorus is incorporated into the Ni–YSZ electrode, leading to the formation of secondary phases of nickel phosphate and zirconium phosphate. The incorporation of phosphorus into the anode is accelerated by an applied electric field.

GdBaCo 2O 5+ x layered perovskite as an intermediate temperature solid oxide fuel cell cathode

GdBaCo 2O 5+ x (GBCO) was evaluated as a cathode for intermediate-temperature solid oxide fuel cells. A porous layer of GBCO was deposited on an anode-supported fuel cell consisting of a 15 μm thick electrolyte of yttria-stabilized zirconia (YSZ) prepared by dense screen-printing and a Ni–YSZ cermet as an anode (Ni–YSZ/YSZ/GBCO). Values of power density of 150 mW cm −2 at 700 °C and ca. 250 mW cm −2 at 800 °C are reported for this standard configuration using 5% of H 2 in nitrogen as fuel. An intermediate porous layer of YSZ was introduced between the electrolyte and the cathode improving the performance of the cell. Values for power density of 300 mW cm −2 at 700 °C and ca. 500 mW cm −2 at 800 °C in this configuration were achieved.

Characterization of sulfur poisoning of Ni–YSZ anodes for solid oxide fuel cells using in situ Raman microspectroscopy

The changes in the surface phase and the morphology of nickel–yttria stabilized zirconia (YSZ) cermet anodes for solid oxide fuel cells (SOFCs) upon exposure to a fuel containing 100 parts per million (ppm)-level hydrogen sulfide (H 2S) at elevated temperatures were investigated using in situ Raman microspectroscopy as well as ex situ techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The in situ Raman experiment revealed that nickel surfaces underwent significant sulfidation when the Ni–YSZ cermet was cooled slowly (at ∼ 2–5 °C/min) from the testing temperature (∼ 500–800 °C) to room temperature in a fuel with pH 2S/ pH 2 = 100 ppm, although no sulfides were detected at elevated temperatures (> 500 °C) in the same fuel. In comparison, the ex situ measurements at room temperature may not reflect what actually happens to samples during exposure to H 2S at elevated temperatures: results may be complicated by inevitable changes induced during the cooling of samples to room temperature in order to perform the ex situ measurements. Thus, in situ measurements are vital to the confirmation of the validity or even the relevance of the results from ex situ measurements in the study of the sulfur poisoning mechanism for Ni-based anodes of SOFCs.

Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilalyer electrolyte prepared by wet ceramic co-sintering process

To protect the ceria electrolyte from reduction at the anode side, a thin film of yttria-stabilized zirconia (YSZ) is introduced as an electronic blocking layer to anode-supported gadolinia-doped ceria (GDC) electrolyte solid oxide fuel cells (SOFCs). Thin films of YSZ/GDC bilayer electrolyte are deposited onto anode substrates using a simple and cost-effective wet ceramic co-sintering process. A single cell, consisting of a YSZ (∼3 μm)/GDC (∼7 μm) bilayer electrolyte, a La 0.8Sr 0.2Co 0.2Fe 0.8O 3–GDC composite cathode and a Ni–YSZ cermet anode is tested in humidified hydrogen and air. The cell exhibited an open-circuit voltage (OCV) of 1.05 V at 800 °C, compared with 0.59 V for a single cell with a 10-μm GDC film but without a YSZ film. This indicates that the electronic conduction through the GDC electrolyte is successfully blocked by the deposited YSZ film. In spite of the desirable OCVs, the present YSZ/GDC bilayer electrolyte cell achieved a relatively low peak power density of 678 mW cm −2 at 800 °C. This is attributed to severe mass transport limitations in the thick and low-porosity anode substrate at high current densities.

Effects of pre-calcined YSZ powders at different temperatures on Ni–YSZ anodes for SOFC

The particle size distribution and the specific surface area of yttria stabilized zirconia (YSZ) powders could be changed by pre-calcining. Accordingly, the performances of Ni–YSZ cermet might be changed too. In this paper, the effects of pre-calcined YSZ powders on Ni–YSZ cermet were investigated. The performances of Ni–YSZ cermet, including the basic physical characterization, sintering shrinkage and electrochemical properties, were studied in connection with the quality of YSZ powders pre-calcined at 900 °C, 1100 °C and 1200 °C, respectively. The results indicated that the crystal structure was not changed, but the porosity of open holes in NiO–YSZ pellets was increased with the pre-calcining temperature increasing. In addition, the total shrinkage of different anode materials was changed by using different YSZ powders, the value of Ni–YSZ cermet doping with YSZ pre-calcined at 900 °C and 1100 °C was closer to that of YSZ electrolyte. The cermet with YSZ powders pre-calcined at 900 °C showed the highest electric conductivity, and its polarization performance was closed to that of cermet with raw YSZ powders.

Electrochemical Characterisation of Supporting SOFC Anodes

In order to get a detailed insight into the electrochemical performance of the single anode in an anode supported SOFC cell, symmetrical cells consisting of two nominally identically prepared electrode and support layers have been prepared. Different electrode layers were investigated: Ni-yttria stabilized zirconia (Ni-YSZ) with different ratios of the fine and coarse YSZ, Ni-gadolinium modified ceria (Ni-CGO), and Ni-yttria-titania modified zirconia (Ni-YZT). High quality impedance data are obtained in this set-up and the influence of the Ni-YSZ cermet micro structure corresponds well to results reported earlier. The impedance indicates that the electrode performance is limited rather by electrochemical reaction steps in the Ni-cermet electrode than by gas transport phenomena.

5 mol% TiO 2-doped Ni–YSZ anode cermets for solid oxide fuel cells

The influence of doping YSZ (8 mol% yttria stabilized zirconia) with 5 mol% TiO 2 on the thermal expansion coefficient, electrical conductivity and microstructure was examined for different cermets as a function of Ni-content. Thermal expansion measurements showed that TiO 2 contributes to the reduction of the thermal expansion coefficient values of the cermet material. The incorporation of TiO 2 into the YSZ lattice was found to reduce slightly the ionic conductivity of the pure ceramic, while under reducing conditions the electronic contribution, due to the reduction of Ti +4 to Ti +3, results in a decrease of the activation energy for conduction in the order of 13%. For the TiO 2-doped cermets the electrical conductivity was always better than that of the undoped, exhibiting a lower degradation rate at 1000°C. Based on microstructure examination of the samples, this degradation was attributed to the lower agglomeration trend of the Ni-particles of the doped cermets compared to the Ni–YSZ cermet.

Portable solid oxide fuel cells using butane gas as fuel

A portable solid oxide fuel cell using butane gas as fuel was demonstrated. A yttria stabilized zirconia (YSZ) tube (7 mm in diameter and 90 mm in length) was used as the electrolyte with a porous Pt cathode and a Ni–YSZ cermet anode. The cell showed an output power of 0.14 W using a commercial butane gas bomb.

High-performance electrodes for reduced temperature solid oxide fuel cells with doped lanthanum gallate electrolyte: II. La(Sr)CoO 3 cathode

The reduced temperature solid oxide fuel cell (SOFC) with 0.5 mm thick La 0.9Sr 0.1Ga 0.8Mg 0.2O 3− α (LSGM) electrolyte, La 0.6Sr 0.4CoO 3− δ (LSCo) cathode, and Ni-(CeO 2) 0.8(SmO 1.5) 0.2 (SDC) cermet anode showed an excellent initial performance, and high maximum power density, 0.47 W/cm 2, at 800°C. The results were comparable to those for the conventional SOFC with yttria-stabilized zirconia (YSZ) electrolyte, La(Sr)MnO 3-YSZ cathode and Ni–YSZ cermet anode at 1000°C. Using an LSCo powder prepared by spray pyrolysis, and selecting appropriate sintering temperatures, the lowest cathodic polarization of about 25 mV at 300 mA/cm 2 was measured for a cathode prepared by sintering at 1000°C. Life time cell test results, however, showed that the polarization of the LSCo cathode increased with operating time. From EPMA results, this behavior was considered to be related to the interdiffusion of the elements at the cathode/electrolyte interface. Calcination of LSCo powder could be a possible way to suppress this interdiffusion at the interface.