Yttria-stabilized Zirconia Cathode Research Articles

Effect of lanthanide (Ln=La, Nd, and Pr) doping on electrochemical performance of Ln2NiO4+δ−YSZ composite cathodes for solid oxide fuel cells

In this study, we prepared three different Ruddlesden−Popper (RP)-phase nickelate catalysts, i.e., La2NiO4+δ (LNO), Nd2NiO4+δ (NNO), and Pr2NiO4+δ (PNO) via solid-state route. These materials were used to fabricate composite cathodes through their combination with yttria stabilized zirconia (YSZ), which is the most widely used electrolyte material for solid oxide fuel cells. X-ray diffraction (XRD) analysis revealed that after the initial sintering all three RP-based materials had good chemical compatibility with YSZ without any reactive impurities. The electrochemical impedance spectroscopy study showed that, among the three samples, the PNO−YSZ cathode exhibited the best oxygen reduction reaction (ORR) activity, with an area specific resistance (ASR) of 0.64 Ω cm2 at 800 °C, which was 16% and 41% lower compared to that of NNO-YSZ (0.76 Ω cm2), and LNO-YSZ (1.09 Ω cm2), respectively. During the initial long-term operation for 60 h at 800 °C, the measured ASR degradation rate of LNO−YSZ was 3.56 %h−1, while NNO−YSZ and PNO−YSZ showed better stability in comparison (2.23 and 1.87 %h−1, respectively). The post XRD analysis revealed the presence of a pyrochlore secondary phases (Ln2Zr2O7) in all three samples, which was considered a major degradation mechanism of Ln2NiO4+δ-YSZ composite cathodes. However, the amount of the impurity phase exhibited the same tendency as the degradation rate of different cathodes (LNO > NNO > PNO), demonstrating that Pr doping at the A-site of the RP-phase effectively suppressed the formation of a the pyrochlore impurity phase against YSZ. Thus, our results suggest that the ORR activity and durability of Ln2NiO4+δ−YSZ composite cathodes strongly depends on the A-site lanthanides.

Freeze Casting of Tubular Anode Supports for High-Performance Solid Oxide Fuel Cells

Freeze casting method has received significant attention in recent years for the fabrication of porous materials with unique pore geometries. A typical freeze casting process involves freezing of a liquid suspension forming solvent crystals that grow in the direction of solidification. When these solvent crystals are removed by sublimation under a reduced pressure, a porous structure with unidirectional and gradient pores is achieved. Such a porous structure is considered ideal for solid oxide fuel cell (SOFC) electrodes because it allows for rapid gaseous diffusion through the bulk of the electrode and a large number of active sites toward the electrolyte–electrode interface for electrochemical reactions. In the present work, we developed a freeze casting and freeze drying process to fabricate tubular SOFC anode supports using an aqueous NiO–yttria-stabilized zirconia (YSZ) slurry. Freeze casting was performed in an aluminum mold that contained peripheral coils for flowing a coolant. After complete freezing of the slurry, the mold was connected to a vacuum chamber for freeze drying. With a proper control of the experimental setup and adjustments in the freeze drying profile, we were able to reduce the total drying time from several days to a few hours. The fabricated anode supports were coated with the thin layers of YSZ electrolyte and strontium-doped lanthanum manganite (LSM)-YSZ cathode to obtain the single cells. The electrochemical performance of the single cells was evaluated by the I–V polarization curves and electrochemical impedance spectra. Owing to the favorable gas transport properties of the freeze-cast anode, the concentration polarization was found to be greatly reduced and thus a high power density was achieved. Figure 1

Electrochemical performance and stability of nano-structured Co/PdO-co-impregnated Y2O3 stabilized ZrO2 cathode for intermediate temperature solid oxide fuel cells

Palladium (Pd) is an attractive cathode catalyst component for solid oxide fuel cells (SOFCs) that has high tendency to agglomerate during operation at around 800 °C. This work shows that such agglomeration can be inhibited by alloying Co into Pd. PdO, Pd0.95Co0.05O, Pd0.90Co0.10O, and Pd0.80Co0.20O were synthesized and characterized. Powder X-ray diffraction patterns at 750 and 900 °C confirmed that PdO decomposition to Pd which normally occurred at 840 °C was suppressed for Co containing Pd alloys while thermal gravimetric analyses indicated improved redox reversibility of PdO ↔ Pd conversion for alloys during the thermal cycling between 600 and 900 °C. Scanning electron microscopy images supported these arguments. Pd0.90Co0.10+yttria stabilized zirconia (YSZ) electrode (i.e., 10 mol % Co containing PdO-impregnated YSZ electrode) displayed the highest oxygen reduction reaction (ORR) performance and stability. The polarization resistance for ORR on Pd0.90Co0.10+YSZ cathode is only 0.088 Ω cm2 at 750 °C. During polarization test at 750 °C, Pd0.90Co0.10+YSZ cathode showed stable performance for 30 h while the performance of Pd+YSZ cathode degraded after 10 h.

Nanostructured Cathodes for Solid Oxide Fuel Cells by a Solution Spray-Coating Process

Composite nano-cathodes containing Sr-doped LaMnO3 (LSM) and Gd2O3-doped CeO2 (GDC) were deposited on yttria-stabilized zirconia (YSZ) discs and YSZ/NiO + YSZ bi-layer discs using an aqueous solution. The samples were heated over a range of temperatures from 700°C to 1200°C to form LSM + GDC porous films. The crystallite size was estimated using X-ray diffraction (XRD) Scherrer formula. The crystallite size was in the ∼3 nm to ∼80 nm range. Polarization resistance was measured by electrochemical impedance spectroscopy (EIS) from 400°C to 700°C. Nano-cathodes of ∼70 nm thickness fired at 700°C exhibited polarization resistance of ∼0.23 Ωcm2 at 700°C. By contrast, LSM + YSZ cathodes made by a conventional process exhibited polarization resistance of ∼2 Ωcm2. Fuel cell tests were conducted at 700°C on cells with nano-cathodes and conventional LSM + YSZ cathode. The maximum power density was ∼0.51 Wcm−2 with nano-cathodes. By contrast, the corresponding power density with conventional LSM + YSZ cathodes was ∼0.11 Wcm−2. The present work shows that very thin cathode functional layers can be deposited on sintered YSZ/NiO + YSZ bi-layer discs by using an aqueous solution. This approach in principle could be extended to large area cells.

P25] Assessment of temperature and pressure dependence of molar volume and phase diagrams of Al, Cu, Si, and Zn

The performance of a novel micro-tubular solid oxide fuel cell (SOFC) with an inert support and an integrated current collector for the inner electrode was improved by controlling its microstructural features. Multi-step dip coating and co-sintering methods were used to fabricate the cell containing porous yttria-stabilized zirconia (YSZ), Ni, Ni–YSZ, YSZ, strontium-doped lanthanum manganite (LSM)–YSZ, and LSM as the inert support, anode current collector, anode, electrolyte, cathode, and cathode current collector, respectively. To enhance gas diffusion through the YSZ support by properly tailoring its porosity, a combination of micro-crystalline cellulose and polymethyl methacrylate pore formers was used. Additionally, the porosity of the Ni current collector was improved and the LSM–YSZ cathode was sufficiently thick for high oxygen reduction activity. Owing to its optimized microstructure, the micro-tubular SOFC delivered excellent power output with maximum power densities of 710, 591, 445, and 316 mW cm−2 at 850, 800, 750, and 700 °C, respectively. The effect of redox cycling on cell performance was investigated by alternately exposing the anode to fuel and air atmospheres. The cell had good tolerance toward the redox phenomenon with no apparent degradation in its performance up to 10 redox cycles.

Performance improvement and redox cycling of a micro-tubular solid oxide fuel cell with a porous zirconia support

The performance of a novel micro-tubular solid oxide fuel cell (SOFC) with an inert support and an integrated current collector for the inner electrode was improved by controlling its microstructural features. Multi-step dip coating and co-sintering methods were used to fabricate the cell containing porous yttria-stabilized zirconia (YSZ), Ni, Ni–YSZ, YSZ, strontium-doped lanthanum manganite (LSM)–YSZ, and LSM as the inert support, anode current collector, anode, electrolyte, cathode, and cathode current collector, respectively. To enhance gas diffusion through the YSZ support by properly tailoring its porosity, a combination of micro-crystalline cellulose and polymethyl methacrylate pore formers was used. Additionally, the porosity of the Ni current collector was improved and the LSM–YSZ cathode was sufficiently thick for high oxygen reduction activity. Owing to its optimized microstructure, the micro-tubular SOFC delivered excellent power output with maximum power densities of 710, 591, 445, and 316 mW cm−2 at 850, 800, 750, and 700 °C, respectively. The effect of redox cycling on cell performance was investigated by alternately exposing the anode to fuel and air atmospheres. The cell had good tolerance toward the redox phenomenon with no apparent degradation in its performance up to 10 redox cycles.

Decomposition of carbon monoxide and carbon dioxide using cathode-supported dense yttria-stabilized zirconia electrolyte cell

CO, CO2 and CO–CO2 mixed gas were electrochemically decomposed at 800°C using a cell of Ni–YSZ (8mol% yttria-stabilized zirconia) cathode (5mm thick), dense YSZ electrolyte (40μm thick) and Ru–YSZ anode (30μm thick). The decomposition rate of CO gas (2CO→2C+O2) was in the range of 9–15% at an applied voltage of 1.0V and increased to 67–100% at 2.0–8.0V. Carbon was deposited significantly near the electrolyte interface in the cathode. In the case of CO2 gas decomposition (2CO2→2CO+O2, CO2→C+O2), CO2 gas of 6–8vol% was converted to CO (6–8vol%), O2 (2–9vol%) and C. Deposition of a small amount of C was observed in the cathode. The decomposition of CO–CO2 mixed gas produced O2 gas of 2–10vol% in the anode but carbon deposition hardly occurred. Carbon deposited in the cathode from CO decomposition was easily oxidized to CO gas again by CO2 mixed (C+CO2→2CO).

Mn1.5Co1.5O4−δ infiltrated yttria stabilized zirconia composite cathodes for intermediate-temperature solid oxide fuel cells

The composite cathodes of yttria stabilized zirconia (YSZ) and Mn1.5Co1.5O4 (MCO) are prepared by infiltration of the MCO oxides into porous YSZ backbones using aqueous solutions of the corresponding nitrate salts. Calcinations at 850 °C promote the formation of the MCO spinel oxide and yield nano-scale catalyst coatings on the YSZ pore walls. Impedance measurements on the symmetric MCO–YSZ cathode fuel cells show that the lowest polarization resistance in air at 800 °C is 0.43 Ω cm2 for the MCO impregnated YSZ composite at the MCO volume loading of 13.5%. Analysis of the impedance spectra suggest that the oxygen reduction kinetics is probably limited by double ionization of the adsorbed oxygen atoms or charge transfer at the triple-phase boundaries. Furthermore, introducing the oxide ion conductor of samarium-doped ceria as a second component in the coated catalysts yields much lower polarization resistances, e.g., 0.15 Ω cm2 at 800 °C.

Fabrication of structured anode-supported solid oxide fuel cell by powder injection molding

Powder Injection Molding (PIM) gives the possibility to produce, at an industrial rate, ceramic parts with fine details. It is thus a possible approach to reduce the fabrication costs of Solid Oxide Fuel Cells (SOFCs). This work presents fabrication and electrochemical characterization results of injection-molded structured anode-supported SOFCs.Planar anode-supported SOFCs with fine details have been produced by injection molding of a mixture of nickel oxide (NiO) and yttria-stabilized zirconia (YSZ). The channeling structure and support porosity ensure gas transport on the fuel side. Initial anode support characterizations like sintering shrinkage, porosity and electrical conductivity have been done on simple Ni–YSZ test bars. Thin YSZ electrolyte has been deposited by spin-coating on the PIM green body before half-cell co-sintering.Electrochemical testing was carried out with a lanthanum–strontium manganite (LSM)–YSZ cathode. The performance is comparable to tape-cast anode-supported cells, with 0.45 W cm−2 obtained at 0.6 V and 810 °C. Medium-term galvanostatic testing shows a degradation rate of about 1.1% kh−1. Post-test analysis with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) attribute this mainly to cathode degradation due to Cr and S poisoning. These results are therefore promising for using a PIM fabrication process in the SOFC field.

Impact of Cofiring Ceria in Ni/YSZ SOFC Anodes for Operation With Syngas and n-Butane

This study explores cofiring ceria (CeO2) with NiO and 8 mol% yttria-stabilized zirconia (YSZ) to form Ni-based cermet anodes for high-temperature solid oxide fuel cells (SOFCs) operating on syngas and n-butane/steam fuel feeds. Particular attention is paid to the suppression of carbon deposit growth in Ni-based anodes with carbonaceous fuel feeds. CeO2 was cofired with NiO and YSZ to form a porous Ni cermet anode support layer after reduction in H2 at 800°C. The porous anode support layer (1 mm thick) was combined with a Ni/YSZ functional layer (∼25 μm thick), a dense YSZ electrolyte (10–20 μm thick), and porous La0.8Sr0.2MnO3−x (LSM)/YSZ cathodes (∼50 μm thick) to form anode-supported button cells for electrochemical characterization. The button cells were tested from 700 °C to 800 °C on various fuels including syngas and n-butane/H2O mixtures at steam-to-carbon (S/C) ratios of 1.0 and 1.5. Electrochemical testing revealed that CeO2 addition provided stable performance at 800 °C without compromising power densities—up to 0.6 W/cm2 on syngas and 0.35 W/cm2 on direct butane feeds. Furthermore, the addition of CeO2 suppressed significant carbon deposition as observed for Ni/YSZ anode support layers without CeO2. Testing with syngas at different H2 and CO partial pressures indicated that high power densities can be maintained along an anode channel for up to 50% fuel conversion. The results indicate that cofiring CeO2 in Ni/YSZ anode support layers presents a viable option for stable SOFC operation on either prereformed or internally reformed light-hydrocarbon fuel feeds.

Electrochemical decomposition of CO2 and CO gases using porous yttria-stabilized zirconia cell

Electrochemical decomposition of CO2 and CO gases using a porous cell of Ru-8mol% yttria-stabilized zirconia (YSZ) anode/porous YSZ electrolyte/Ni–YSZ cathode system at 400–800°C was studied by analyzing the flow rate and composition of outlet gas, current density, and phases and elementary distribution of the electrodes and electrolyte. A part of CO2 gas supplied at 50ml/min was decomposed to solid carbon and O2 gas through the cell at the electric field strengths of 0.9–1.0V/cm. The outlet gas at a flow rate of 3ml/min included 61–63% CO2 and 37–39% O2 at 700–800°C and the outlet gas at a flow rate of 50ml/min included 73–96% (average 85%) CO2 and 4–27% (average 15%) O2 at 800°C. On the other hand, the supplied CO gas was also decomposed to solid carbon, O2 and CO2 gases at 800°C. The fraction of outlet gas at a flow rate of 50ml/min during the CO decomposition at 800°C for 5h was 11–36% CO, 59–81% O2 and 2–9% CO2. The detailed decomposition mechanisms of CO2 and CO gases are discussed. Both Ni metal in the cathode and porous YSZ grains under the DC electric field have the ability to decompose CO gas into solid carbon and O2− ions or O2 gas.

The Effect of Humidity and Oxygen Partial Pressure on LSM–YSZ Cathode

AbstractTwo series of anode supported solid oxide fuel cells (SOFC) were prepared, one with a composite cathode layer of lanthanum strontium manganite (LSM) and yttria stabilized zirconia (YSZ) on top and the other further has a LSM current collector layer on top. The fuel cells were heat treated at 1,000 °C in air or nitrogen choosing dry or humid (70% steam) conditions. XRD, SEM, and XPS investigations were performed on the various samples. The most severe modifications were observed in humid nitrogen atmosphere at low oxygen partial pressure. LSM surface change goes along with a decrease of manganese concentration and strontium enrichment on the surface of the materials. Formation of monoclinic zirconia and zirconate phases was also observed. These results give a closer insight into possible degradation mechanisms of SOFC composite cathode materials in dependence of humidity and oxygen partial pressure.

Electrochemical reforming of CH4-CO2 mixed gas using porous yttria-stabilized zirconia cell

Flow rate and compositions of outlet gas and carbon deposition in the cell were studied upon the electrochemical reforming of 50% CH4–50% CO2 mixed gas using a cell of Ru–yttria-stabilized zirconia (YSZ) anode/porous 8 mol %-YSZ electrolyte/Ni–YSZ cathode system at 400–800°C under 1.0 V/cm of electric field strength. The supplied mixed gas of 95–97% changed to H2 and CO fuels at 800°C (CH4 + CO2 → 2H2 + 2CO). As a parallel reaction, pyrolysis of CH4 occurred over a Ni cathode to produce carbon powder in the cell (CH4 → C + 2H2), which decreased the flow rate of outlet gas at a high temperature. The above results were compared with the previously measured results with a porous Gd-doped ceria cell (mixed conductor of O2− ions and electrons). In the GDC cell, a part of deposited carbon was removed as CO gas by the reaction with O2− ions formed from CO2 gas in the cathode (CO2 + 2e− → CO + O2−, C + O2− → CO + 2e−) and the electrons produced were transported to the anode through the mixed conductor electrolyte film.

Microstructural Degradation of ( La , Sr ) MnO3 ∕ YSZ Cathodes in Solid Oxide Fuel Cells with Uncoated E-Brite Interconnects

Transmission electron microscopy was used to investigate the local chemistry and microstructure of the active cathodes in solid oxide fuel cells operated for (at 700 and ) under different electrochemical conditions while exposed to a chromia-forming stainless steel. Several distinct microstructural changes were observed owing to chromium interactions with the (LSM)–yttria stabilized zirconia (YSZ) cathode; the nature and magnitude of which depended on the temperature, electrochemical load, and physical location. Nanosized particles of and were observed on the surface of the YSZ, regardless of the overall extent of degradation; the quantity increased with decreasing temperature and increasing current. The results indicate that Mn species facilitate the formation of a stable Cr–Mn–O nuclei on the YSZ, on which further growth occurs, including growth of . In cases of severe performance degradation, LSM decomposes completely, which does not appear to be strongly correlated with the nanoparticles on the YSZ, but results from a more destructive mechanism. Following this decomposition, severe pore filling of Cr-containing species occurs. The amount of microstructural degradation was largest near the cathode/electrolyte interface directly beneath the interconnect–cathode contact channels. These results indicate that two distinct mechanisms of degradation occur, with the electrochemical decomposition of as the primary cause for severe performance degradation.

A solid oxide membrane electrolyzer for production of hydrogen and syn-gas from steam and hydrocarbon waste in a single step

This paper describes a novel solid oxide membrane (SOM) electrolyzer that can utilize the energy value in any hydrocarbon waste or reductant to lower the energy requirement for hydrogen production from steam. The SOM electrolyzer consists of a one-end closed oxygen-ion-conducting yttria stabilized zirconia (YSZ) tube with Ni–YSZ cathode coated on the outside and liquid metal anode inside the tube. The SOM electrolyzer is operated by feeding steam on the cathodic side and hydrocarbon reductant in the liquid metal anode. By feeding hydrocarbon waste it is possible to lower the chemical potential of oxygen in the liquid metal such that the SOM electrolyzer can spontaneously dissociate steam and generate hydrogen on the cathodic side and oxidize the hydrocarbon waste on the anodic side to produce syn-gas. The rate of these reactions can be increased by applying a small potential across the electrodes.

A symmetrical solid oxide fuel cell prepared by dry-pressing and impregnating methods

In this study, a simple and cost-effective dry-pressing method has been used to fabricate a symmetrical solid oxide fuel cell (SOFC) where the dense yttria-stabilized zirconia (YSZ) electrolyte film is sandwiched between two symmetrical porous YSZ layers in which La 0.75Sr 0.25Cr 0.5Mn 0.5O 3− δ (LSCM) based anode and cathode are incorporated using wet impregnation techniques. The maximum power densities ( P max) of a single cell with 32 wt.% LSCM impregnated YSZ anode and cathode reach 333 and 265 mW cm −2 at 900 °C in dry H 2 and CH 4, respectively. The cell performance is further improved with additional impregnation of a small amount of Sm-doped CeO 2 (SDC) or Ni. When 6 wt.% Ni as catalyst is added to both the anode and cathode, P max values of 559 and 547 mW cm −2 can be achieved, which are better than with SDC. The effect of Ni on the cathode performance is also investigated by impedance spectra analysis.

Effect of Humidity in Air on Performance and Long-Term Durability of SOFCs

Anode-supported solid oxide fuel cells (SOFCs) based on Ni–yttria-stabilized zirconia (YSZ) anodes, YSZ electrolytes, and lanthanum strontium manganite (LSM)–YSZ cathodes were studied with respect to durability in humid air typically over 1500 h. Operating temperature and current density were varied between 750 and and , respectively. The introduction of humidity affected the cell voltage under polarization of the cell, and this effect was (at least partly) reversible upon switching off the humidity. Generally, the studied cells were operated in humid air under technologically relevant conditions over more than 1500 h. Improvements at the cathode/electrolyte interface made it possible to obtain highly stable cells, which can be operated under high current density and at in humid air, conditions that cause significant cell voltage degradation in dry air on cells with LSM/YSZ-based cathodes.

Oxygen Reduction at LSM–YSZ Cathodes Deposited on Anode-Supported Microtubular Solid Oxide Fuel Cells

Three-electrode methods were employed to evaluate the electrochemical activity of the oxygen reduction reaction (ORR) at lanthanum strontium manganite (LSM)–yttria stabilized zirconia (YSZ) composite cathodes deposited on the outer surface of microtubular, anode-supported, solid oxide fuel cells . The performance of these cathodes, deposited on the highly curved, electrophoretically deposited, thin YSZ electrolyte layers in the microtubular configuration, was compared against LSM-YSZ cathodes, deposited similarly and having the same microstructure, on thick YSZ layers in standard SOFC planar cells. While the EIS response of the high and low frequency arcs for LSM-YSZ in the two-cell geometries differed somewhat, the total polarization resistance and the ORR activation energy were similar. It was also demonstrated that LSM-YSZ cathode performance in the system can be further enhanced when the cell is sintered at 1150 vs .

Redox Stability of SrNbxTi1 − xO3 – YSZ for Use in SOFC Anodes

The conductive properties of bulk (SNT) and porous –yttria-stabilized zirconia (YSZ) composites for , 0.05, and 0.20 have been examined under relevant solid oxide fuel cell (SOFC) operating temperatures and redox cycling conditions. The porous composite conductivities were approximately 1 order of magnitude lower than the corresponding bulk material. In order to obtain reasonable conductivity levels for SOFCs, samples were prereduced at in . The conductivity of prereduced samples increased with increasing Nb content when directly measured in humidified ; however, when considering the conductivities measured after redox cycling, there appears to be no benefit to using higher dopant levels. After redox cycling at , a composite conductivity of at in humidified (3% ) was achieved for and 0.05 and a composite conductivity of was achieved for . Fuel cell power densities of at and at were achieved in humidified (3% ) with a porous SNT–YSZ anode infiltrated with Pd and , a thick YSZ electrolyte, and an (LSF), LSF–YSZ cathode. The implications of these results for the development of redox-stable anodes are discussed.

Fabrication and Electrochemical Performance of Anode‐Supported Solid Oxide Fuel Cells by a Single‐Step Cosintering Process

Anode‐supported cone‐shaped tubular solid oxide fuel cells (SOFCs) are successfully fabricated by a single‐step cosintering process. The cone‐shaped tubular single cell, NiO+YSZ/YSZ/LSM+YSZ/LSM, is comprised of a Ni+yttria‐stabilized zirconia (YSZ) anode, YSZ electrolyte, a La0.7Sr0.3MnO3 (LSM)+YSZ cathode layer, and an LSM cathode current collector layer. The fabrication process involves slip casting of the anode, colloidal spray coating of the electrolyte, slurry coating of the cathode, and single‐step cosintering of the cone‐shaped tubular cells in the temperature range of 1200°–1400°C. The cells are tested with humidified hydrogen (75 mL/min) as fuel and ambient air as oxidant. Cell electrochemical test results and scanning electron microscope graphs show that the optimal temperature of the single‐step cosintering process is 1300°C.