Synthesis and electrochemical properties of layered perovskite substituted with heterogeneous lanthanides for intermediate temperature-operating solid oxide fuel cell

In this study, phase synthesis and electrochemical properties of Sm_1-xNd_xBa_0.5Sr_0.5Co₂O_5+d (x = 0–0.9) oxide systems where neodymium and samarium were replaced at the A-site of SmBa_0.5Sr_0.5Co₂O_5+d layered perovskite are investigated for use as cathode materials in Intermediate Temperature-operating Solid Oxide Fuel Cells (IT-SOFCs).The structure of Sm_1-xNd_xBa_0.5Sr_0.5Co₂O_5+d (x = 0–0.9) oxide systems can exist in either an orthorhombic (x = 0–0.4) or tetragonal (x = 0.5–0.9) form. The maximum electrical conductivities in Sm_1-xNd_xBa_0.5Sr_0.5Co₂O_5+d (x = 0–0.9) oxide systems were obtained from Sm_0.2Nd_0.8Ba_0.5Sr_0.5Co₂O_5+d (SNBSCO8) and their values are 1280 and 280 Scm^?1 at 50 °C and 900 °C, respectively. The area specific resistances (ASRs) of SBSCO are 3.019, 0.611, and 0.092 Ωcm² at 500, 600, and 700 °C, respectively. However, SNBSCO8 single phase gives the lowest ASRs of 1.751, 0.244 and 0.044 Ωcm² at the same temperatures tested. SNBSCO8 is thus a promising candidate cathode material for IT-SOFC applications.     

Structural, thermal and electrochemical properties of layered perovskite SmBaCo2O5+d, a potential cathode material for intermediate-temperature solid oxide fuel cells

The synthesis, conductivity properties, area specific resistance (ASR) and thermal expansion behaviour of the layered perovskite SmBaCo₂O_5+d (SBCO) are investigated for use as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The SBCO is prepared and shows the expected orthorhombic pattern. The electrical conductivity of SBCO exhibits a metal–insulator transition at about 200 °C. The maximum conductivity is 570 S cm⁻¹ at 200 °C and its value is higher than 170 S cm⁻¹ over the whole temperature range investigated. Under variable oxygen partial pressure SBCO is found to be a p-type conductor. The ASR of a composite cathode (50 wt% SBCO and 50 wt% Ce_0.9Gd_0.1O_2−d, SBCO:50) on a Ce_0.9Gd_0.1O_2−d (CGO91) electrolyte is 0.05 Ω cm² at 700 °C. An abrupt increase in thermal expansion is observed in the vicinity of 320 °C and is ascribed to the generation of oxygen vacancies. The coefficients of thermal expansion (CTE) of SBCO is 19.7 and 20.0 × 10⁻⁶ K⁻¹ at 600 and 700 °C, respectively. By contrast, CTE values for SBCO:50 are 12.3, 12.5 and 12.7 × 10⁻⁶ K⁻¹ at 500, 600 and 700 °C, that is, very similar to the value of the CGO91 electrolyte.     

Layered perovskite GdBaCoFeO5+x as cathode for intermediate-temperature solid oxide fuel cells

A layered perovskite oxide, GdBaCoFeO_5+x (GBCF), was investigated as a novel cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). A laboratory-sized Sm_0.2Ce_0.8O_1.9 (SDC)-based tri-layer cell of NiO–SDC/SDC/GBCF was tested under intermediate-temperature conditions of 550–650 °C with humidified H₂ (∼3% H₂O) as a fuel and the static ambient air as oxidant. A maximal power density of 746 mW cm⁻² was achieved at 650 °C. The interfacial polarization resistance was as low as 0.42, 0.18 and 0.11 Ω cm² at 550, 600 and 650 °C, respectively. The experimental results indicate that the layered perovskite GBCF is a promising cathode candidate for IT-SOFCs.     

Characterization of layered perovskite oxides NdBa1−xSrxCo2O5+δ (x = 0 and 0.5) as cathode materials for IT-SOFC

The crystal structure, thermal expansion, and electrochemical properties of the layered perovskite series NdBa_1−xSr_xCo₂O_5+δ (x = 0 and 0.5) were investigated to study the effects of substituting Sr for Ba on a layered perovskite oxide.     

Cobalt and molybdenum co-doped Ca2Fe2O5 cathode for solid oxide fuel cell

Recently, Brownmillerite oxides Ca₂Fe_2-xM_xO₅ (0 ≤ x ≤ 0.2) (M = transition metal such as Co, Mo), have been drawing attention as they possess mixed ionic and electronic conductivity. Fe site of parent Ca₂Fe₂O₅ (CFO) structure is partially substituted by Co and/or Mo as well as CoMo co-doping and tested as cathodes in SOFC. Physical characterizations such as X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), and Brunauer–Emmett–Teller (BET) have been carried out to assess the phase formation, microstructure, presence of constituent elements, particle size, and surface area of the cathode, respectively. The Co doped CFO cathodes have better percolation, large surface area, and extended triple phase boundary. Further, the doped CFO cathodes exhibited chemical compatibility with other cell components during fabrication and cell testing as evident from SEM micrographs. The Ca₂Fe_2-xM_xO₅ (0 ≤ x ≤ 0.2) oxides show a semiconductor behaviour having sufficient electrical conductivity values in the SOFCs operating temperature 600–800 °C range. The best electrical conductivity, 0.47 S/cm at 800 °C and the corresponding activation energy of 0.17 eV is exhibited by Ca₂Fe_1.8Co_0.2O₅ (CFCO), whereas Ca₂Fe_1.8Mo_0.2O₅ (CFMO) and Ca₂Fe_1.8Mo_0.1Co_0.1O₅ (CFMCO) cathode shows electrical conductivity 0.11 S/cm and 0.15 S/cm at 800 °C, respectively. CFMO performed better with SDC than YSZ electrolyte between 600 and 700 °C although the lowest area specific resistance (ASR) of 1.28 Ω cm² at 800 °C is observed for CFMO with YSZ electrolyte. Similarly, CFMCO provided low ASR at lower temperature with SDC than that with YSZ electrolyte but exhibited lowest ASR of 0.41 Ω cm² at 800 °C with YSZ. The CFCO cathode shows lower ASR with YSZ than that with SDC for all the temperature and provided lowest value of ASR 0.21 Ω cm² at 800 °C. CFCO cathode has been tested in 900 μm thick electrolyte (SDC/YSZ) supported solid oxide fuel cell (SOFC) CFCO-SDC/SDC/NiO-SDC and CFCO-YSZ/YSZ/NiO-YSZ provided maximum power densities of 171 and 506 mW/cm² (i-R corrected) at 800 °C, respectively.     

A novel layered perovskite cathode for proton conducting solid oxide fuel cells

BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) exhibits adequate proton conductivity as well as sufficient chemical and thermal stability over a wide range of SOFC operating conditions, while layered SmBa_0.5Sr_0.5Co₂O_5+δ (SBSC) perovskite demonstrates advanced electrochemical properties based on doped ceria electrolyte. This research fully takes advantage of these advanced properties and develops novel protonic ceramic membrane fuel cells (PCMFCs) of Ni-BZCY7|BZCY7|SBSC. The results show that the open-circuit potential of 1.015 V and maximum power density of 533 mW cm⁻² are achieved at 700 °C. With temperature increase, the total cell resistance decreases, among which electrolyte resistance becomes increasingly dominant over polarization resistance. The results also indicate that SBSC perovskite cathode is a good candidate for intermediate temperature PCMFC development, while the developed Ni-BZCY7|BZCY7|SBSC cell is a promising functional material system for next generation SOFCs.     

Electrochemical performances of NdBa0.5Sr0.5Co2O5+x as potential cathode material for intermediate-temperature solid oxide fuel cells

Layered perovskite oxide NdBa_0.5Sr_0.5Co₂O_5+x is investigated as a cathode material for intermediate-temperature solid oxide fuel cells. The NBSC cathode is chemically compatible with the electrolyte La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) at temperatures below 1000 °C. It is metallic in nature and the maximum and minimum conductivities are 1368 S cm⁻¹ at 100 °C and 389 S cm⁻¹ at 850 °C. The area specific resistance (ASR) value for the NBSC cathode is as low as 0.023 Ω cm² at 850 °C. The electrolyte-supported fuel cell generates good performance with the maximum power density of 904, 774 and 556 mW cm⁻² at 850, 800 and 750 °C, respectively. Preliminary results indicate that NBSC is promising as a cathode for IT-SOFCs.     

Alternative perovskite materials as a cathode component for intermediate temperature single-chamber solid oxide fuel cell

This paper exploits the suitability of three perovskite materials Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF), GdBaCo₂O_5+δ (GBC) and Ba_0.5Sr_0.5Mn_0.7Fe_0.3O_3−δ (BSMF) as SOFC cathodes in the single-chamber configuration operating at the intermediate temperature range. TG analysis showed high thermal stability depending on the crystalline phases of the materials. The catalytic activity of these three materials for hydrocarbon conversion was investigated under a realistic feed, i.e. with hydrocarbon, oxygen, water and carbon dioxide. Electrochemical impedance spectroscopy of the various cathodes tested in symmetric cell configuration revealed a B-site dependence of the electrode catalytic activity for oxygen reduction. High temperature (1000 °C) powder reactivity tests over a gadolinium doped-ceria (CGO) and perovskite cathode revealed excellent chemical compatibility of BSMF and CGO. Catalytic tests associated with thermal and structural characterization attest to the suitability of these materials in the single-chamber configuration.     

Performance of double-proveskite YBa0.5Sr0.5Co2O5+δ as cathode material for intermediate-temperature solid oxide fuel cells

Double-proveskite YBa_0.5Sr_0.5Co₂O_5+δ (YBSC) was investigated as potential cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). YBSC material exhibited a good chemical compatibility with the La_0.9Sr_0.1Ga_0.8Mg_0.115Co_0.085O_2.85 (LSGMC) electrolyte up to 950 °C for 2 h. The substitution of Sr for Ba significantly enhanced the electrical conductivity of the YBSC sample compared to undoped YBaCo₂O_5+δ, and also slightly increased the thermal expansion coefficient. At 325 °C a semiconductor-metal transition was observed and the maximum electrical conductivity of YBSC was 668 S cm⁻¹. The maximum power densities of the electrolyte-supported single cell with YBSC cathode achieved 650 and 468 mW cm⁻² at 850 and 800 °C, respectively. Preliminary results suggested that YBSC could be considered as a candidate cathode material for application in IT-SOFCs.     

Electrochemical characteristic of non-stoichiometric SmBa0.45Sr0.5Co2O5+d layered perovskite oxide system for IT-SOFC cathode

In this study, the composition of the layered perovskite SmBa_0.5Sr_0.5Co₂O_5+d was changed to different non-stoichiometric compositions by changing the substitution amount of Ba and Sr.SmBa_0.45Sr_0.5Co₂O_5+d (SBSCO-0.45/0.5) showed the lowest area specific resistance (ASR) because the area % caused by high binding energy (HBE) of O_1s in the X-ray photoelectron spectroscopy (XPS) analysis was the largest compared to all other tested compositions and also the unit cell volume was smaller than that of other samples.The dense SmBa_0.5Sr_0.48Co₂O_5+d (SBSCO-0.5/0.48) showed the highest electrical conductivity. This is because SBSCO-0.5/0.48 has the smallest decrease in oxygen contents compared to other samples when subject to temperature increase. Also, through XPS analysis, it was found that the area % of the Co³⁺ and Co⁴⁺ coexistence regions of Co_2p of SBSCO-0.5/0.48 was the largest. The electrical conductivity values and behaviors of the porous SBSCO-0.45/0.5 and SBSCO-0.5/0.48 were not significantly different.Comparing the single-cell performance with composite cathodes comprised of SBSCO-0.45/0.5 and SBSCO-0.5/0.48 with CGO91, the results show that the single-cell with the SBSCO-0.45/0.5 and CGO91 cathode showed higher maximum power density.     

High performance of proton-conducting solid oxide fuel cell with a layered PrBaCo2O5+δ cathode

A dense BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte is fabricated on a porous anode by in situ drop-coating method which can lead to extremely thin electrolyte membrane (10 μm in thickness). The layered perovskite structure oxide PrBaCo₂O_5+δ (PBCO) is synthesized by auto ignition process and initially examined as a cathode for proton-conducting IT-SOFCs. The electrical conductivity of PrBaCo₂O_5+δ (PBCO) reaches the general required value for the electrical conductivity of cathode absolutely. The single cell, consisting of PrBaCo₂O_5+δ (PBCO)/BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY)/NiO-BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) structure, is assembled and tested from 600 to 700 °C with humidified hydrogen (3% H₂O) as the fuel and air as the oxidant. An open-circuit potential of 1.01 V and a maximum power density of 545 mW cm⁻² at 700 °C are obtained for the single cell, and a low polarization resistance of the electrodes of 0.15 Ω cm² is achieved at 700 °C.     

Preparation and electrochemical properties of Sr-doped Nd2NiO4 cathode materials for intermediate-temperature solid oxide fuel cells

Cathode materials Nd_2 − xSr_xNiO₄ were prepared by the glycine-nitrate process and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), AC impendence spectroscopy and DC polarization method, respectively. The results show that no reaction occurred between the electrode and the CGO electrolyte at 1100 °C and the electrode formed good contact with the electrolyte after being sintered at 1000 °C for 4 h. The rate-limiting step for oxygen reduction reaction on Nd_1.6Sr_0.4NiO₄ electrode changed with oxygen partial pressure and measurement temperature. The Nd_1.6Sr_0.4NiO₄ electrode gave a polarization resistance of 0.93 Ω cm² at 700 °C in air, which indicates that Nd_2 − xSr_xNiO₄ electrode is a promising cathode material for intermediate-temperature solid oxide fuel cell (IT-SOFC).     

Layered GdBa0.5Sr0.5Co2O5+δ as a cathode for intermediate-temperature solid oxide fuel cells

The layered GdBa_0.5Sr_0.5Co₂O_5+δ (GBSC) perovskite oxides are synthesized by Pechini method and investigated as a novel cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The single cell of NiO–SDC (Sm_0.2Ce_0.8O_1.9)/SDC (20 μm)/GBSC (10 μm) is operated from 550 to 700 °C fed with humidified H₂ as fuel and the static air as oxidant. An open circuit voltage of 0.8 V and a maximum power density of 725 mW cm⁻² are achieved at 700 °C. The interfacial polarization resistance is as low as 0.88, 0.29, 0.13 and 0.05 Ω cm² at 550, 600, 650 and 700 °C, respectively. The ratio of polarization resistance to total cell resistance decreases with the increase in the operating temperature, from 60% at 550 °C to 21% at 700 °C, respectively. The experimental results indicate that GBSC is a promising cathode material for IT-SOFCs.     

High performance praseodymium nickelate oxide cathode for low temperature solid oxide fuel cell

The praseodymium nickelate oxide Pr₂NiO_4+δ, a mixed conducting oxide with the K₂NiF₄-type structure, was evaluated as cathode for low temperature solid oxide fuel cells (T = 873 K). The electrochemical performance of the cathode has been improved by optimization of the microstructure of the porous cathode combined with the use of a ceria barrier layer in between the cathode and zirconia electrolyte. Both low polarization and ohmic resistances were obtained using Pr₂NiO_4+δ-powders with a median particle size of 0.4 μm, and sintering the screen printed layer at a sintering temperature of about 1353 K for 1 h. These manufacturing conditions resulted in a cathode microstructure with well established connections between the cathode particles and good adhesion of the cathode on the electrolyte. Full-sized anode supported cells have been manufactured using the same process conditions for the Pr₂NiO_4+δ cathode and tested. The best results were obtained when using a dense Ce_0.8Gd_0.2O_1.9 (20CGO) barrier layer. While a complete optimization of the cell preparation has not yet been achieved, the electrochemical performances of anode supported cells with Pr₂NiO_4+δ are higher than those with the well known state-of-the-art La_0.6Sr_0.4Fe_0.8Co_0.2O_3−δ (LSFC) material.     

Structural,electrical, and electrochemical properties of cobalt-doped NiFe2O4 as a potential cathode material for solid oxide fuel cells

In this work, Co-doped NiFe₂O₄ spinels (NFCO-x) are successfully fabricated and characterized as possible cathode materials for the intermediate-temperature solid oxide fuel cells (SOFC). Results of the binding energy calculations using the density functional theory suggest that the reverse spinel structure is stable when Co³⁺ occupies the octahedral interstitial sites. Total and ionic-only conductivities indicate that NFCO-x are a kind of mixed electronic-ionic conductors. Ionic transferring numbers are approximately 0.049 and 0.006 for NFCO-0.1 and NFCO-0.5, respectively, measured at 700 °C in air. Co dopant in the NFCO-x improves the electronic conductivity at the expense of the ionic conductivity. For NFCO-0.5, electronic and ionic conductivities are approximately 0.24 and 9.6 × 10⁻⁴ S cm⁻¹, respectively, measured also at 700 °C in air. Unlike behaviour of the conductivities, the polarization resistance of symmetric cells with NFCO-x electrodes decreases when increasing the Co content (x) to a certain level, and then increases. The cell containing the NFCO-0.5 electrode exhibits the lowest polarization resistance (R_p), which is approximately 1.51 Ω cm² at 650 °C. For single cells, the maximum power density is 320 mW cm⁻² measured at 650 °C using a 38-μm-thick SDC electrolyte and an NFCO-0.5 cathode.     

Degradation behavior of anode-supported solid oxide fuel cell using LNF cathode as function of current load

We investigated the effect of current loading on the degradation behavior of an anode-supported solid oxide fuel cell (SOFC). The cell consisted of LaNi_0.6Fe_0.4O₃ (LNF), alumina-doped scandia stabilized zirconia (SASZ), and a Ni-SASZ cermet as the cathode, electrolyte, and anode, respectively. The test was carried out at 1073 K with constant loads of 0.3, 1.0, 1.5, and 2.3 A cm⁻². The degradation rate, defined by the voltage loss during a fixed period (about 1000 h), was faster at higher current densities. From an impedance analysis, the degradation depended mainly on increases in the cathodic resistance, while the anodic and ohmic resistances contributed very little. The cathode microstructures were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).     

GdBaCo2O5+x layered perovskite as an intermediate temperature solid oxide fuel cell cathode

GdBaCo₂O_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 H₂ 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.     

High performance layered SmBa0.5Sr0.5Co2O5+δ cathode for intermediate-temperature solid oxide fuel cells

The layered SmBa_0.5Sr_0.5Co₂O_5+δ (SBSC) perovskite oxide is synthesized by the Pechini method and investigated as a novel cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). A laboratory-sized Sm_0.2Ce_0.8O_1.9 (SDC)-based tri-layer cell of NiO–SDC/SDC/SBSC is operated from 500 to 700 °C fed with humidified H₂ (3% H₂O) as a fuel and the static ambient air as oxidant. A maximum power density of 1147 mW cm⁻² is achieved at 700 °C. The interfacial polarization resistance is as low as 1.01, 0.38, 0.16, 0.06 and 0.03 Ω cm² at 500, 550, 600, 650 and 700 °C, respectively. The experimental results indicate that SBSC is a very promising cathode material for IT-SOFCs.     

The role of electrical conductivity of substrate on the YSZ film formed by EPD for solid oxide fuel cell applications

Thick YSZ (8 mol% Y₂O₃–ZrO₂) films were coated by EPD, which was performed on non-conducting and graphite-coated NiO–YSZ composites. Weights of depositions had depended on the applied voltage linearly for both types of NiO–YSZ composites. In addition, weights of depositions were significantly higher when graphite-coated substrates were used. Uniform and dense depositions were successfully obtained by both methods. Adhesion strength was evaluated by micro hardness testing. However, it was observed that the interfacial adhesion was improved when graphite-coated substrates were used. The qualities of the deposited YSZ films were different after the subsequent sintering and cracks were developed on the surface of sintered YSZ films when non-conducting substrates were used. SEM observations showed that there were thicker YSZ films on the graphite-coated substrates than on the non-conducting ones. After the reduction process, NiO–YSZ composites were altered to Ni–YSZ cermet, which showed a good electrical conductivity all over it.     

A short review of cathode poisoning and corrosion in solid oxide fuel cell

Solid oxide fuel cell (SOFC) has been recognized as a promising energy conversion device that is expected to play a critical role in solving the global energy and environmental challenges, however, the durability of SOFC under practical working conditions has limited its wide spread deployment and commercialization. Specifically, SOFC cathode often suffers from various contaminations such as Cr and Si arising from the interconnect and sealing materials, respectively, as well as humidity and CO₂ which are inherent in ambient air, resulting in serious issues in long-term performance degradation. In this review, the impacts of certain poisoning and corrosions on SOFC cathode are introduced, and the latest results of durability research on the corrosion resistant properties of cathode under CO₂, humidity, Cr and Si-containing conditions are reviewed. The poisoning and corrosion mechanism and durability of these aspects are systematically assessed and discussed.