Sr2Fe1.5Mo0.5O6−δ as a regenerative anode for solid oxide fuel cells

Sr₂Fe_1.5Mo_0.5O_6−δ (SFM) was prepared using a microwave-assisted combustion synthesis method. Rietveld refinement of powder X-ray diffraction data reveals that SFM crystallizes in the simple cubic perovskite structure with iron and molybdenum disordered on the B-site. No structure transition was observed by variable temperature powder X-ray diffraction measurements in the temperature range of 25-800 °C. XPS results show that the iron and molybdenum valences change with an increase in temperature, where the mixed oxidation states of both iron and molybdenum are believed to be responsible for the increase in the electrical conductivity with increasing temperature. SFM exhibits excellent redox stability and has been used as both anode and cathode for solid oxide fuel cells. Presence of sulfur species in the fuel or direct utilization of hydrocarbon fuel can result in loss of activity, however, as shown in this paper, the anode performance can be regenerated from sulfur poisoning or coking by treating the anode in an oxidizing atmosphere. Thus, SFM can be used as a regenerating anode for direct oxidation of sulfur-containing hydrocarbon fuels.     

Synthesis and electrical properties of Al-doped Sr2MgMoO6-δ as an anode material for solid oxide fuel cells

Aluminum doped Sr₂MgMoO_6-δ (SMMO) was synthesized via citrate-nitrate route. Dense samples of Sr₂Mg_1-xAl_xMoO_6−δ (0 ≤ x ≤ 0.05) were prepared by sintering the pellets at 1500 °C in air and then reducing at 1300 °C in 5%H₂/Ar. The electrical conductivity strongly depended on the preparing atmosphere, samples reduced in 5%H₂/Ar exhibited higher conductivity than those unreduced. Al-doping increased remarkably the electrical conductivity of Sr₂Mg_1-xAl_xMoO_6−δ. The reduced samples displayed a relatively stable electrical conductivity under oxygen partial pressure (Po₂) from 10⁻¹⁹ to 10⁻¹⁴ atm at 800 °C, and exhibited an excellent recoverability in electrical conductivity when cycled in alternative air and 5%H₂/Ar atmospheres. Sr₂Mg_0.95Al_0.05MoO_6−δ material showed a good chemical compatibility with LSGM and GDC electrolytes below 1000 °C, while there was an obvious reaction with YSZ. Al-doping improves the anode performance of SMMO in half-cell of Pt/Sr₂Mg_1-xAl_xMoO_6−δ∣GDC∣Pt in H₂ fuel. The present results demonstrate that Sr₂Mg_1-xAl_xMoO_6−δ is a potential anode material for intermediate temperature-Solid Oxide Fuel Cells (IT-SOFCs).     

Highly carbon– and sulfur–tolerant Sr2TiMoO6−δ double perovskite anode for solid oxide fuel cells

Ni-based cermets are most commonly used anode materials for solid-oxide fuel cells (SOFCs), but poor stability operating on hydrocarbon fuels seriously hampers their commercialization due to carbon deposition and sulfur poisoning. Here, we report a carbon– and sulfur–tolerant double perovskite anode Sr₂TiMoO_6−δ (STMO) combining the characteristics of two simple perovskites of SrTiO₃ and SrMoO₃. The STMO anode exhibits excellent thermal and chemical compatibility with La_0.9Sr_0.1Ga_0.8Mg_0.2O_3–δ (LSGM) and Ce_0.8Sm_0.2O_1.9 (SDC) electrolytes in 5% H₂/Ar. The single cell with STMO anode demonstrates good stability and excellent coking resistance and sulfur tolerance in H₂S-containing syngas during a 60-h period. The maximum power density (P_max) values of a LSGM-electrolyte-supported single cell with STMO anode are 505 and 275 mW cm⁻²at 850 °C in H₂ and H₂S-containing syngas, respectively. The electrochemical performance is further improved by impregnation of Pd nanoparticles, where the P_max values achieve 1009 and 586 mW cm⁻² at 850 °C under the same conditions, respectively, showing great potential as an anode material for SOFCs operating on H₂S-containing syngas. Our study provides a strategy to develop versatile double perovskite materials by combining the relevant characteristics of two separate perovskites.     

Characterization of cation-ordered perovskite oxide LaBaCo2O5+δ as cathode of intermediate-temperature solid oxide fuel cells

A-site cation-ordered perovskite oxide LaBaCo₂O_5+δ (LBCO) was synthesized and evaluated as a cathode material of intermediate-temperature solid oxide fuel cells (IT-SOFCs). LBCO was structurally stable when calcined at 850 °C in air but transformed into cation-disordered structure at 1050 °C. LBCO showed chemical compatibility with Gd_0.1Ce_0.9O_1.95 (GDC) electrolyte at 850 °C and 1000 °C in air. Conductivity of LBCO firstly increased slightly with higher temperature to a maximum of 470 S cm⁻¹ at ∼250 °C and then decreased gradually with further increase in temperature. Electrochemical impedance spectra of the LBCO/GDC/LBCO symmetric cell were measured, and electrode reaction mechanism for the LBCO cathode was analyzed. The electrode polarization resistance of LBCO was mainly contributed by oxygen ionic transfer across the cathode/electrolyte interface and oxygen atom diffusion-electronic charge transfer process. Low area-specific resistances with values ranging from 0.15 Ω cm² at 650 °C to 0.0086 Ω cm² at 800 °C were obtained. These results have demonstrated that the A-site cation-ordered perovskite oxide LBCO is a promising cathode material for IT-SOFCs.     

Perovskite Sr2Fe1.5Mo0.5O6−δ as electrode materials for symmetrical solid oxide electrolysis cells

Perovskite Sr₂Fe_1.5Mo_0.5O_6−δ (SFM) has been successfully prepared by a microwave-assisted combustion method in air and employed as both anode and cathode in symmetrical solid oxide electrolysis cells (SOECs) for hydrogen production for the first time in this work. Influence of cell operating temperature, absolute humidity (AH) as well as applied direct current (DC) on the impedance of single cells with the configuration of SFM|La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM)|SFM has been evaluated. Under open circuit conditions and 60 vol.% AH, the cell polarization resistance, R_P is as low as 0.26 Ω cm² at 900 °C. An electrolysis current of 0.88 A cm⁻² and a hydrogen production rate as high as 380 mL cm⁻² h have been achieved at 900 °C with an electrolysis voltage of 1.3 V and 60 vol.% AH. Further, the cell has demonstrated good stability in the long-term steam electrolysis test. The results showed that the cell electrolysis performance was even better than that of the reported strontium doped lanthanum manganite (LSM) – yttria stabilized zirconia (YSZ)|YSZ|Ni–YSZ cell, indicating that SFM could be a very promising electrode material for the practical application of SOEC technology.     

Potentiality of cobalt-free perovskite Ba0.5Sr0.5Fe0.9Mo0.1O3−δ as a single-phase cathode for intermediate-to-low-temperature solid oxide fuel cells

Cobalt-free perovskite Ba_0.5Sr_0.5Fe_0.9Mo_0.1O_3−δ (BSFMo) was investigated as a single-phase cathode for intermediate-to-low-temperature solid oxide fuel cells (IL-SOFCs). The X-ray diffraction (XRD) Rietveld refinement, electrical conductivity, thermogravimetric (TG) measurements, the phase reaction were investigated. The doping of high-valence Mo cations into Fe-site obviously enhanced the electrical conductivity of BSFMo sample with the maximum value of 174 S cm⁻¹. XRD results showed that BSFMo cathode was chemically compatible with the BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) electrolyte for temperatures up to 1000 °C. Laboratory-sized tri-layer cells of NiO-BZCYYb/BZCYYb/BSFMo were operated from 550 to 700 °C with humidified hydrogen (～3% H₂O) as fuel and the static air as oxidant, respectively. An open-circuit potential of 1.001 V, the maximum power density of 428 mW cm⁻², and a low electrode polarization resistance of 0.148 Ω cm² were achieved at 700 °C. The experimental results indicated that the single-phase BSFMo is a promising candidate as cathode material for IL-SOFCs.     

Evaluation of Sr0.88Y0.08TiO3–CeO2 as composite anode for solid oxide fuel cells running on CH4 fuel

Sr_0.88Y_0.08TiO₃ (YST) was synthesized and the performance of a YST–CeO₂ composite as an alternative anode for the direct utilization of CH₄ in solid oxide fuel cells (SOFCs) was investigated. X-ray diffraction showed that YST had good chemical compatibility with CeO₂ and YSZ (8 mol% Y-doped ZrO₂). The shrinkage of the YST–CeO₂ composite on sintering was less than that of pure YSZ and CeO₂, and its thermal-expansion behavior was similar to that of YSZ. With YSZ as electrolyte, ScSZ (10 mol% Sc-doped ZrO₂)–LSM (La_0.8Sr_0.2MnO₃) as cathode, and YST–CeO₂ composite as anode, single cells were prepared and tested in both H₂ and CH₄. The maximum power density obtained at 900 °C was 161.7 mW cm⁻² in H₂ atmosphere and 141.3 mW cm⁻² in CH₄. The results demonstrated the potential of using YST–CeO₂ composite as the anode for SOFCs.     

A novel doped CeO2–LaFeO3 composite oxide as both anode and cathode for solid oxide fuel cells

A novel composite oxide Ce(Mn,Fe)O₂-La(Sr)Fe(Mn)O₃ (CFM-LSFM) was synthesized and evaluated as both anode and cathode materials for solid oxide fuel cells. The cell with CFM-LSFM electrodes was fabricated by tape-casting and screen printing technique. The power-generating performance of this cell was comparable to that of the cell with Ni-SSZ anode and LSM-SSZ cathode. During the 120 h long-term test in hydrogen at 800 °C, the performance increased by 8.6% from 256 to 278 mW cm⁻². This was attributed to the decrease of polarization resistance and ohmic resistance during the test. The XRD results showed the presence of Fe, MnO and some unknown second phases after heat-treating the electrode materials in H₂ which may be beneficial to the anode electrochemical process. The gradual decrease of polarization resistance as increasing the current density possibly resulted from the increasing content of water in the anode.     

Layered perovskite PrBa0.5Sr0.5Co2O5+δ as high performance cathode for solid oxide fuel cells using oxide proton-conducting electrolyte

The layered perovskite PrBa_0.5Sr_0.5Co₂O_5+δ (PBSC) was investigated as a cathode material for a solid oxide fuel cell using an oxide proton conductor based on BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY). The sintering conditions for the PBSC-BZCY composite cathode were optimized, resulting in the lowest area-specific resistance and apparent activation energy obtained with the cathode sintered at 1200 °C for 2 h. The maximum power densities of the PBSC-BZCY/BZCY/NiO-BZCY cell were 0.179, 0.274, 0.395, and 0.522 W cm⁻² at 550, 600, 650, and 700 °C, respectively with a 15 μm thick electrolyte. A relatively low cell interfacial polarization resistance of 0.132 Ω cm² at 700 °C indicated that the PBSC-BZCY could be a good cathode candidate for intermediate temperature SOFCs with BZCY electrolyte.     

Ta-doped PrBaFe2O5+δ double perovskite as a high-performance electrode material for symmetrical solid oxide fuel cells

Symmetrical solid oxide fuel cells (S-SOFCs) have received considerable attention due to fewer preparation steps in recent years. The PrBaFe₂O_5+δ (PBF) is a candidate material due to good catalytic activity and electrochemical stability. In this work, Ta-substituted PBF materials (PrBaFe_2-xTa_xO_5+δ, denoted as PBFTx, x = 0, 0.1, 0.2, 0.3) are prepared and evaluated as symmetrical electrodes on GDC(Gd_0.1Ce_0.9O_2-δ)-YSZ(yttria-stabilized zirconia)-GDC three-layer electrolyte. The PrBaFe_1.8Ta_0.2O_5+δ (PBFT0.2) symmetrical cell presents the lowest polarization resistance, and the value is 0.171 Ω cm² and 0.503 Ω cm² in air and hydrogen atmosphere at 800 °C, respectively. In addition, the PBFT0.2 cell shows a peak power density of 234 mW/cm² using humidified hydrogen as fuel gas and air as oxidant at 800 °C, which is enhanced by 68% compared with that of PBF (138 mW/cm²). The results indicate that the strategy of Ta doping can improve the electrochemical performance of PBF and PBFT0.2 is a potential electrode for S-SOFCs.     

Proton conducting solid oxide fuel cells with layered PrBa0.5Sr0.5Co2O5+δ perovskite cathode

BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) exhibits adequate protonic conductivity as well as sufficient chemical and thermal stability over a wide range of SOFC operating conditions, while layered perovskite PrBa_0.5Sr_0.5Co₂O_5+δ (PBSC) has advanced electrochemical properties. This research fully takes advantage of these advanced properties and develops a novel protonic ceramic membrane fuel cell (PCMFC) of Ni–BZCY7|BZCY7|PBSC. Experimental results show that the cell may achieve the open-circuit potential of 1.005 V, the maximal power density of 520 mW cm⁻², and a low electrode polarization resistance of 0.12 Ωcm² at 700 °C. Increasing operating temperature leads to the decrease of total cell resistance, among which electrolyte resistance becomes increasingly dominant over polarization resistance. The results also indicate that PBSC perovskite cathode is a good candidate for intermediate temperature PCMFC development, while the developed Ni–BZCY7|BZCY7|PBSC cell is a promising functional material system for SOFCs.     

Layered perovskite LnBa0.5Sr0.5Cu2O5+δ (Ln = Pr and Nd) as cobalt-free cathode materials for solid oxide fuel cells

Cobalt-free layered perovskite LnBa_0.5Sr_0.5Cu₂O_5+δ (Ln = Pr and Nd, PBSC and NBSC) powders are prepared using combined citrate and EDTA complexing method. The performance of PBSC and NBSC cathode materials are evaluated for solid oxide fuel cells (SOFCs). Two oxidation states (Cu²⁺/Cu⁺) for Cu ions exist in LnBa_0.5Sr_0.5Cu₂O_5+δ oxides. The main valence of Pr ions in PBSC is 3+. The average thermal expansion coefficients (TECs) of PBSC and NBSC are 14.2 and 14.6 × 10^?6 K^?1 between 30 and 950 °C, which are similar to the TECs of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3?δ (LSGM) intermediate-temperature electrolyte. The electrical conductivity of PBSC is slightly higher than that of NBSC. At 800 °C, the polarization resistance (R_p) values of the PBSC and NBSC cathodes on the LSGM electrolyte are 0.043 and 0.057 Ω cm², respectively. The electrolyte-supported single cells were prepared by using PBSC and NBSC as cathode, LSGM as electrolyte (300 μm thickness), Ce_0.9Sm_0.1O_1.95 (SDC) as interlayer and Ni/SDC as anode. At 850 °C, the maximal power densities are obtained as 681 and 651 mW cm^?2 for PBSC and NBSC cathodes.     

Novel layered perovskite SmBaCu2O5+δ as a potential cathode for intermediate temperature solid oxide fuel cells

A novel layered perovskite SmBaCu₂O_5+δ (SBCO) as a potential cathode for intermediate temperature solid oxide fuel cells (IT-SOFC) has been investigated in this paper. The SmBaCu₂O_5+δ oxide was synthesized by EDTA- Citrate complexing sol-gel process. The crystal structure, the thermal expansion, the electrical conductivity and electrochemical properties have been characterized by X-ray diffraction (XRD), dilatometer, four-probe dc method, electrochemical impedance spectroscopy (EIS) and cathodic polarization examinations. The average thermal expansion coefficient (TEC) of SBCO was 14.6 × 10⁻⁶/ °C in the temperature range of 50-800 °C, which matched Sm-doped ceria (SDC) electrolytes. The electrode polarization resistance was 0.469 Ωcm². Considering low thermal expansions and good electrochemical properties, layered perovskite SBCO shows promising performance as cathode material for IT-SOFCs.     

Anode performance of LST–xCeO2 for solid oxide fuel cells

La-doped SrTiO₃ (LST)–xCeO₂ (x = 0, 30, 40, 50) composites were evaluated as anode materials for solid oxide fuel cells in terms of chemical compatibility, electrical conductivity and fuel cell performance in H₂ and CH₄. Although the conductivity of LST–xCeO₂ composite slightly decreased from 4.6 to 3.9 S cm⁻¹ in H₂ at 900 °C as the content of CeO₂ increased, the fuel cell performance improved from 75.8 to 172.3 mW cm⁻² in H₂ and 54.5 to 139.6 mW cm⁻² in CH₄ at 900 °C. Electrochemical impedance spectra (EIS) indicated that the addition of CeO₂ into LST can significantly reduce the fuel cells polarization thus leading to a higher performance. The result demonstrated the potential ability of LST–xCeO₂ to be used as SOFCs anode.     

Characterization and evaluation of NdBaCo2O5+δ cathode for proton-conducting solid oxide fuel cells

The layered perovskite structure oxide NdBaCo₂O_5+δ (NBCO) with rapid oxygen ion diffusion and surface exchange kinetics was synthesized by auto ignition process and initially examined as a cathode for proton-conducting fuel cells (H-SOFCs). The single cell, consisting of NdBaCo₂O_5+δ (NBCO)/BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY)/NiO–BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) structure, was assembled and tested from 650 to 700 °C with humidified hydrogen (∼3% H₂O) as the fuel and air as the oxidant. A maximum power density of 438 mW cm⁻² at 700 °C was obtained for the single cell and electrochemical performance of the cell was studied.     

High performance Ni–Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells

A novel anode consisting of Ni and Sm₂O₃ with negligible oxygen-ion conductivity was developed for intermediate-temperature solid oxide fuel cells (SOFCs). Its triple phase boundary length is pretty small compared with the conventional Ni-samaria doped ceria (SDC) anode, of which SDC is one of the electrolytes having high oxygen-ion conductivity. Even so, single cells with Ni–Sm₂O₃ anodes generated peak power density of 542 mW cm⁻² at 600 °C, comparable to, if not higher than those with the Ni–SDC anodes when the same cathodes and electrolytes were applied. In addition, Ni–Sm₂O₃ exhibited lower interfacial polarization resistance than Ni–SDC. The high electrochemical performance, which might be related to the high catalytic activity of Sm₂O₃ and the unique microstructures of the Ni–Sm₂O₃, suggests a viable alternative to the conventional anodes for SOFCs.     

LaNbO4 toughened NiO–Y2O3 stabilized ZrO2 composite for the anode support of planar solid oxide fuel cells

NiO–Y₂O₃ stabilized ZrO₂ (YSZ) composite is the state-of-the-art material for the anode support of planar solid oxide fuel cells (SOFCs). To improve its fracture toughness (K_C), lanthanum orthoniobate LaNbO₄ is synthesized by the method of solid state reaction and added to the mixture of YSZ and NiO at the weight ratio of 47:53. The content of LaNbO₄ in the composite is in the range between 5 and 30 wt%. The microstructure of the composites is examined by scanning electron microscopy (SEM); and the chemical compatibility among the components is evaluated by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). Vickers hardness test is performed for estimating the K_C of the composites. The results indicate that the K_C increases with the addition of LaNbO₄ in the composites; and the toughening effect is associated with the grain-refinement in the composite and the domain switch in the monoclinic LaNbO₄.     

SrCo0.7Fe0.2Ta0.1O3−δ perovskite as a cathode material for intermediate-temperature solid oxide fuel cells

Perovskite oxide SrCo_0.7Fe_0.2Ta_0.1O_3−δ (SCFT) was synthesized by a solid–state reaction and investigated as a potential cathode material for intermediate-temperature solid oxide fuel cell (IT-SOFC). The single phase SCFT having a cubic perovskite structure was obtained by sintering the sample at 1200 °C for 10 h in air. Introduction of Ta improved the phase stability of SCFT. The SCFT exhibited a good chemical compatibility with the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte at 950 °C for 10 h. The average thermal expansion coefficient was 23.8 × 10⁻⁶ K⁻¹ between 30 and 1000 °C in air. The electrical conductivities of the SCFT sample were 71–119 S cm⁻¹ in the 600−800 °C temperature range in air, and the maximum conductivity reached 247 S cm⁻¹ at 325 °C. The polarization resistance of the SCFT cathode on the LSGM electrolyte was 0.159 Ω cm² at 700 °C. The maximum power density of a single-cell with the SCFT cathode on a 300 μm-thick LSGM electrolyte reached 652.9 mW cm⁻² at 800 °C. The SCFT cathode had shown a good electrochemical stability over a period of 20 h short-term testing. These findings indicated that the SCFT could be a suitable alternative cathode material for IT-SOFCs.     

Chromium deposition and poisoning of cathodes of solid oxide fuel cells – A review

Intermediate temperature solid oxide fuel cells (IT-SOFCs) using chromia-forming alloy interconnect requires the development of cathode not only with high electrochemical activity but also with the high resistance or tolerance towards Cr deposition and poisoning. This is due to the fact that, at SOFC operating temperatures, volatile Cr species are generated over the chromia scale, poisoning the cathodes such as (La,Sr)MnO₃ (LSM) and (La,Sr)(Co,Fe)O₃ (LSCF) and causing a rapid degradation of the cell performance. Thus, a fundamental understanding of the interaction between the Fe–Cr alloys and SOFC cathode is essential for the development of high performance and stable SOFCs. The objective of this paper is to critically review the progress and particularly the work done in the last 10 years in this important area. The mechanism and kinetics of the Cr deposition and Cr poisoning process on the cathodes of SOFCs are discussed. Chromium deposition at SOFC cathodes is most likely dominated by the chemical reduction of high valence Cr species, facilitated by the nucleation agents on the electrode and electrolyte surface and/or at the electrode/electrolyte interface, i.e., the nucleation theory. The driving force behind the nucleation theory is the surface segregation and migration of cationic species on the surface of perovskite oxide cathodes. Overwhelming evidences indicate that the surface segregation plays a critical role in the Cr deposition. The prospect of the development in the Cr-tolerant cathodes for SOFCs is presented.