Double-perovskites YBaCo2−xFexO5+δ cathodes for intermediate-temperature solid oxide fuel cells

Double-perovskites YBaCo_2−xFe_xO_5+δ (YBCF, x = 0.0, 0.2, 0.4 and 0.6) are synthesized with a solid-state reaction and are assessed as potential cathode materials for utilization in intermediate-temperature solid oxide fuel cells (IT-SOFCs) on the La_0.9Sr_0.1Ga_0.8Mg_0.115Co_0.085O_2.85 (LSGMC) electrolyte. The YBCF materials exhibit chemical compatibility with the LSGMC electrolyte up to a temperature of 950 °C. The conductivity of the YBCF samples decreases with increasing Fe content, and the maximum conductivity of YBCF is 315 S cm⁻¹ at 325 °C for the x = 0.0 sample. A semiconductor-metal transition is observed at about 300-400 °C. The thermal expansion coefficient of the YBCF samples increases from 16.3 to 18.0 × 10⁻⁶ K⁻¹ in air at temperatures between 30 and 900 °C with increase in Fe content. The area-specific resistances of YBCF cathodes at x = 0.0, 0.2 and 0.4 on the LSGMC electrolyte are 0.11, 0.13 and 0.15 Ω cm² at a temperature of 700 °C, respectively. The maximum power densities of the single cells fabricated with the LSGMC electrolyte, Ce_0.8Sm_0.2O_1.9 (SDC) interlayer, NiO/SDC anode and YBCF cathodes at x = 0.0, 0.2 and 0.4 reach 873, 768 and 706 mW cm⁻², respectively. This study suggests that the double-perovskites YBCF (0 ≤ x ≤ 0.4) can be potential candidates for utilization as IT-SOFC cathodes.     

Assessment of perovskite-type La0.8Sr0.2ScxMn1−xO3-δ oxides as anodes for intermediate-temperature solid oxide fuel cells using hydrocarbon fuels

Composites formed by the infiltration of 40 wt% La_0.8Sr_0.2Sc_xMn_1−xO_3-δ (LSSM) oxides (x = 0.1, 0.2, 0.3) into 65% porous yttria-stabilized zirconia (YSZ) are investigated as anode materials for intermediate-temperature solid oxide fuel cells for hydrocarbon oxidation. The oxygen non-stoichiometry and electrical conductivity of each LSSM-YSZ composite are determined by coulometric titration. As the concentration of Sc increases, the composites show higher phase stability and the electrical conductivity of LSSM is significantly affected by the Sc doping, the non-stoichiometric oxygen content, and oxygen partial pressure (p(O₂)). To achieve better electrochemical performance, it is necessary to add ceria-supported palladium catalyst for operation with humidified CH₄. Anode polarization resistance increases with Sc doping due to a decrease in electrical conductivity. An electrolyte-supported cell with a LSSM-YSZ composite anode delivers peak power densities of 395 and 340 mW cm⁻² at 923 K in humidified (3% H₂O) H₂ and CH₄, respectively, at a flow rate of 20 mL min⁻¹.     

Effect of Sm0.2Ce0.8O1.9 on the carbon coking in Ni-based anodes for solid oxide fuel cells running on methane fuel

To directly use hydrocarbon fuel without a reforming process, a new microstructure for Ni/Sm_0.2Ce_0.8O_2−δ (Ni/SDC) anodes, in which the Ni surface of the anode is covered with a porous Sm_0.2Ce_0.8O_2−δ thin film, was investigated as an alternative to conventional Ni/YSZ anodes. The porous SDC thin layer was coated on the pores of the anode using the sol–gel coating method. The cell performance was improved by 20%–25% with the Ni/SDC anode relative to the cell performance with the Ni/YSZ anode due to the high ionic conductivity of the Ni/SDC anode and the increase of electrochemical reaction sites. For the SDC-coated Ni/SDC anode operating with methane fuel, no significant degradation of the cell performance was observed after 180 h due to the surface modification with the SDC film on the Ni surface, which opposes the severe degradation of the cell performance that was observed for the Ni/YSZ anode, which results from carbon deposition by methane cracking. Carbon was hardly detected in the SDC-coated Ni/SDC anode due to the catalytic oxidation of the deposited carbon on the SDC film as well as the electrochemical oxidation of methane in the triple-phase-boundary.     

Electrochemical performance of (Ba0.5Sr0.5)0.9Sm0.1Co0.8Fe0.2O3−δ as an intermediate temperature solid oxide fuel cell cathode

This study presents the electrochemical performance of (Ba_0.5Sr_0.5)_0.9Sm_0.1Co_0.8Fe_0.2O_3−δ (BSSCF) as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFC). AC-impedance analyses were carried on an electrolyte supported BSSCF/Sm_0.2Ce_0.8O_1.9 (SDC)/Ag half-cell and a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF)/SDC/Ag half-cell. In contrast to the BSCF cathode half-cell, the total resistance of the BSSCF cathode half-cell was lower, e.g., at 550 °C; the values for the BSSCF and BSCF were 1.54 and 2.33 Ω cm², respectively. The cell performance measurements were conducted on a Ni-SDC anode supported single cell using a SDC thin film as electrolyte, and BSSCF layer as cathode. The maximum power densities were 681 mW cm⁻² at 600 °C and 820 mW cm⁻² at 650 °C.     

Cu1−xPdx/CeO2-impregnated cermet anodes for direct oxidation of methane in LaGaO3-electrolyte solid oxide fuel cells

A series of ceramic-metal composite anodes containing 1.0 wt.% Cu_1−xPd_x alloys (where x = 0, 0.15, 0.25, 0.4, 0.5, 0.75 and 1.0) were prepared by impregnation of the respective metal salts and 5.0 wt.% CeO₂ into a porous La_0.4Ce_0.6O_2−σ anode skeleton. The performance of these anodes was evaluated in both dry H₂ and CH₄ in the temperature range of 700-800 °C using the 300-μm thick La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−σ (LSGM) electrolyte-supported solid oxide fuel cells (SOFCs). The addition of Pd to Cu significantly increased the performance of the single cells in dry CH₄, with the cell maximum power density changed from 66 mW cm⁻² for Cu_1.0Pd_0.0 to 345 mW cm⁻² for Cu_0.0Pd_1.0 at 800 °C. In H₂, however, the performance improvement was not as significant compared to that in CH₄. In addition, carbon formation was greatly suppressed in the Cu-Pd alloy-impregnated anodes compared to that with pure Pd after exposure to dry CH₄ at 800 °C, which led to different performance stability behaviors for these cells operating with dry CH₄.     

Coking-resistant NbOx-Ni-Ce0.8Sm0.2O1.9 anode material for methanol-fueled solid oxide fuel cells

NbO_x is added in Ni-Ce_0.8Sm_0.2O_1.9 by impregnation as an anode material for solid oxide fuel cells fed with methanol. Nb (IV) and Nb (V) exist in the reduced anode. The addition of Nb reduces the binding energy of Ni. The catalytic activity of the anode and the performance of the single cell both increase with the increase of Nb. At 700 °C, the cell with 5NbO_x-Ni-Ce_0.8Sm_0.2O_1.9 anode and Ce_0.8Sm_0.2O_1.9-carbonate electrolyte shows a output power density of 687 mW cm^?2. Meanwhile, water produced in the anode is absorbed by NbO_x and forms surface hydroxyl groups, which facilitates the removal of carbon. The addition of NbO_x decreases the amount of deposited carbon in the humidified methanol atmosphere significantly, and an improved stability of the single cell is achieved.     

Fabrication and characterization of anode-supported micro-tubular solid oxide fuel cell based on BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte

Anode-supported micro-tubular solid oxide fuel cells (SOFCs) based on a proton and oxide ion mixed conductor electrolyte, BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb), have been fabricated using phase inversion and dip-coating techniques with a co-firing process. The single cell is composed of NiO-BZCYYb anode, BZCYYb electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)-BZCYYb cathode. Maximum power densities of 0.08, 0.15, and 0.26 W cm⁻² have been obtained at 500, 550 and 600 °C, respectively, using H₂ as fuel and ambient air as oxidant.     

SmBaCo2O5+x double-perovskite structure cathode material for intermediate-temperature solid-oxide fuel cells

SmBaCo₂O_5+x (SBCO), an oxide with double-perovskite structure, has been developed as a novel cathode material for intermediate-temperature solid-oxide fuel cells (IT-SOFCs). The electrical conductivity of an SBCO sample reaches 815–434 S cm⁻¹ in the temperature range 500–800 °C. XRD results show that an SBCO cathode is chemically compatible with the intermediate-temperature electrolyte materials Sm_0.2Ce_0.8O_1.9 (SDC) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM). The polarization resistances of an SBCO cathode on SDC and LSGM electrolytes are 0.098 and 0.054 Ω cm² at 750 °C, respectively. The maximum power densities of a single cell with an SBCO cathode on SDC and LSGM electrolytes reach 641 and 777 mW cm⁻² at 800 °C, respectively. The results of this study demonstrate that the double-perovskite structure oxide SBCO is a very promising cathode material for use in IT-SOFCs.     

Performances of LnBaCo2O5+x-Ce0.8Sm0.2O1.9 composite cathodes for intermediate-temperature solid oxide fuel cells

Double-perovskite oxides, LnBaCo₂O_5+x (LnBCO) (Ln = Pr, Nd, Sm, and Gd), are prepared using a solid-state reaction as cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The performances of LnBCO-Ce_0.8Sm_0.2O_1.9 (SDC) composite cathodes were investigated for IT-SOFCs on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte. The thermal expansion coefficient can be effectively reduced in the case of the composite cathodes. No chemical reactions between LnBCO cathodes and SDC electrolyte, and LnBCO and LSGM are found. The electrochemical performances of LnBCO cathodes and LnBCO-SDC composite cathodes decrease with decreasing Ln³⁺ ionic radii, which is closely related to the decrease of the electrical conductivity and fast oxygen diffusion property. The area specific resistances of the LnBCO cathodes and LnBCO-SDC composite cathodes on LSGM electrolyte are all lower than 0.13 Ω cm² and 0.15 Ω cm² at 700 °C, respectively. The maximum power densities of single-cell consisted of LnBCO-SDC composite cathodes, LSGM electrolyte, and Ni-SDC anode achieve 758-608 mW cm⁻² at 800 °C with the change from Ln = Pr to Gd, respectively. These results indicate that LnBCO-SDC composite oxides are candidates as a promising cathode material for IT-SOFCs.     

Effect of Co doping on the properties of Sr0.8Ce0.2MnO3−δ cathode for intermediate-temperature solid-oxide fuel cells

The effect of the Co doping on the structure, electrical conductivity and electrochemical properties of Sr_0.8Ce_0.2MnO_3−δ was investigated. The Co doping decreased the sintering temperature by about 100 °C and cubic structure was synthesized for Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ. The electrical conductivity of Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ reached 102 S cm⁻¹ at 700 °C, which was sufficient to provide low ohmic losses at the cathode. In comparison with Sr_0.8Ce_0.2MnO_3−δ, the area-specific resistance of Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ was 0.10 Ω cm² at 750 °C, which was about 20 times lower than that of Sr_0.8Ce_0.2MnO_3−δ. While the exchange current density i₀ of Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ was 0.49 A cm⁻² at 800 °C, that for Sr_0.8Ce_0.2MnO_3−δ was 0.11 A cm⁻². The results show that the Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ cathode had high catalytic activity for oxygen reduction reaction in the temperature range of 700–800 °C.     

Direct CH4 fuel cell using Sr2FeMoO6 as an anode material

A double-perovskite Sr₂FeMoO₆ (SFMO) has been synthesized with a combined citrate-EDTA complexing method. The material shows a double-perovskite structure after reduction in 5% H₂/Ar at 1100 °C for 20 h. A single fuel cell using this material as anode is constructed with the configuration of SFMO?La_0.8Sr_0.2Ga_0.83Mg_0.17O₃?Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃. The cell exhibits a remarkable electrochemical activity in both H₂ and dry CH₄, respectively. With Oxygen as oxidant, the maximum power density is 863.7 mW cm⁻² with H₂ as the fuel and 604.8 mW cm⁻² with dry CH₄ as the fuel at 850 °C, respectively. SFMO has an almost linear thermal expansion coefficient from 30 to 900 °C and is very close to that of La_0.8Sr_0.2Ga_0.83Mg_0.17O₃. A durability test of the single cell indicates that SFMO is stable in dry CH₄ operation. Therefore SFMO can be recommended as a promising anode material for LaGaO₃-based solid oxide fuel cells operating with both H₂ and dry CH₄.     

A highly coking-resistant solid oxide fuel cell with a nickel doped ceria: Ce1−xNixO2−y reformation layer

A single phase mixed oxide ion-electron conducting electrochemical catalyst of Ce_1−xNi_xO_2−y is employed as an anode functional reformation layer for a coking-resistant solid oxide fuel cell (SOFC) based on oxide ion conducting electrolyte operated in methane and ethanol. The high catalytic activity of Ce_1−xNi_xO_2−y oxide for fuel reformation is demonstrated by the excellent cell performances in various fuels at relatively low temperatures (550–650 °C). The fast oxygen ions flux and formed steam at anode side are also found to be favorable for hydrocarbon reformation to promote the cell performance and long term stability. At 650 °C, maximum power densities of 415 and 271 mW cm⁻² are achieved in methane and ethanol respectively. The resistance against carbon deposition is significantly improved with stable voltage output in a long-term durability operation.     

Initialization of a methane-fueled single-chamber solid-oxide fuel cell with NiO + SDC anode and BSCF + SDC cathode

The initialization of an anode-supported single-chamber solid-oxide fuel cell, with NiO + Sm_0.2Ce_0.8O_1.9 anode and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ + Sm_0.2Ce_0.8O_1.9 cathode, was investigated. The initialization process had significant impact on the observed performance of the fuel cell. The in situ reduction of the anode by a methane–air mixture failed. Although pure methane did reduce the nickel oxide, it also resulted in severe carbon coking over the anode and serious distortion of the fuel cell. In situ initialization by hydrogen led to simultaneous reduction of both the anode and cathode; however, the cell still delivered a maximum power density of ∼350 mW cm⁻², attributed to the re-formation of the BSCF phase under the methane–air atmosphere at high temperatures. The ex situ reduction method appeared to be the most promising. The activated fuel cell showed a peak power density of ∼570 mW cm⁻² at a furnace temperature of 600 °C, with the main polarization resistance contributed from the electrolyte.     

Preparation and electrochemical performance of Pr2Ni0.6Cu0.4O4 cathode materials for intermediate-temperature solid oxide fuel cells

Cathode material Pr₂Ni_0.6Cu_0.4O₄ (PNCO) for intermediate-temperature solid oxide fuel cells (IT-SOFCs) is synthesized by a glycine-nitrate process using Pr₆O₁₁, NiO, and CuO powders as raw materials. X-ray diffraction analysis reveals that nanosized Pr₂Ni_0.6Cu_0.4O₄ powders with K₂NiF₄-type structure can be obtained from calcining the precursors at 1000 °C for 3 h. Scanning electron microscopy shows that the sintered PNCO samples have porous microstructure with a porosity of more than 30% and grain size smaller than 2 μm. A maximum conductivity of 130 S cm⁻¹ is obtained from the PNCO samples sintered at 1050 °C. A single fuel cell based on the PNCO cathode with 30 μm Sm_0.2Ce_0.8O_1.9 (SCO) electrolyte film and a 1 mm NiO-SCO anode support is constructed. The ohmic resistance of the single Ni-SCO/SCO/PNCO cell is 0.08 Ω cm² and the area specific resistance (ASR) value is 0.19 Ω cm² at 800 °C. Cell performance was also tested using humidified hydrogen (3% H₂O) as fuel and air as oxidant. The single cell shows an open circuit voltage of 0.82 V and 0.75 V at 700 °C and 800 °C, respectively. Maximum power density is 238 mW cm⁻² and 308 mW cm⁻² at 700 °C and 800 °C, respectively. The preliminary tests have shown that Pr₂Ni_1−xCu_xO₄materials can be a good candidate for cathode materials of IT-SOFCs.     

Combustion synthesis and properties of highly phase-pure perovskite electrolyte Co-doped La0.9Sr0.1Ga0.8Mg0.2O2.85 for IT-SOFCs

The highly phase-pure perovskite electrolyte, La_0.9Sr_0.1Ga_0.8Mg_0.115Co_0.085O_2.85 (LSGMCO), was prepared by means of glycine–nitrate process (GNP) for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The perovskite phase evolution, sintering, electrical conductivity and electrochemical performance of LSGMCO were investigated. The results show that the highly phase-pure perovskite electrolyte LSGMCO can be obtained after calcining at 1150 °C. The sample sintered at 1450 °C for 20 h in air exhibited a better sinterability, and the relative density of LSGMCO was higher than 95%. The stoichiometric indexes of the elements in the sintered sample LSGMCO determined experimentally by EDS were in good agreement with the nominal composition. The electrical conductivities of the sample were 0.094 and 0.124 S· cm⁻¹ at 800 °C and 850 °C in air, respectively. The ionic conduction of the sample was dominant at high temperature with the higher activation energies. While at lower temperature the electron hole conduction was predominated with the lower activation energies. The maximum power densities of the single cell fabricated with LSGMCO electrolyte with Ce_0.8Sm_0.2O_1.9 (SDC) interlayer, SmBaCo₂O_5+x cathode and NiO/SDC anode achieved 643 and 802 mW cm⁻² at 800 °C and 850 °C, respectively.     

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.     

Preparation and electrochemical properties of a Sm2−xSrxNiO4 cathode for an IT-SOFC

Cathodic materials Sm_2−xSr_xNiO₄ (0.5 ≤ x ≤ 1.0) for an IT-SOFC (intermediate temperature solid oxide fuel cell) were prepared by the glycine-nitrate process and characterized by XRD, SEM, ac impedance spectroscopy and dc polarization measurements. The results showed that no reaction occurred between the Sm_2−xSr_xNiO₄ electrode and the Ce_0.9Gd_0.1O_1.9 (CGO) electrolyte at 1100 °C, and the electrode formed good contact with the electrolyte after sintering at 1000 °C for 2 h. The electrochemical properties of these cathode materials were studied using impedance spectroscopy at various temperatures and oxygen partial pressures. Sm_1.0Sr_1.0NiO₄ exhibited the lowest cathodic overpotential. The area specific resistance (ASR) was 3.06 Ω cm² at 700 °C in air.     

Prontonic ceramic membrane fuel cells with layered GdBaCo2O5+x cathode prepared by gel-casting and suspension spray

In order to develop a simple and cost-effective route to fabricate protonic ceramic membrane fuel cells (PCMFCs) with layered GdBaCo₂O_5+x (GBCO) cathode, a dense BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) electrolyte was fabricated on a porous anode by gel-casting and suspension spray. The porous NiO–BaZr_0.1Ce_0.7Y_0.2O_3−δ (NiO–BZCY7) anode was directly prepared from metal oxide (NiO, BaCO₃, ZrO₂, CeO₂ and Y₂O₃) by a simple gel-casting process. A suspension of BaZr_0.1Ce_0.7Y_0.2O_3−δ powders synthesized by gel-casting was then employed to deposit BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) thin layer by pressurized spray process on NiO–BZCY7 anode. The bi-layer with 10 μm dense BZCY7 electrolyte was obtained by co-sintering at 1400 °C for 5 h. With layered GBCO cathode synthesized by gel-casting on the bi-layer, single cells were assembled and tested with H₂ as fuel and the static air as oxidant. An open-circuit potential of 0.98 V, a maximum power density of 266 mW cm⁻², and a low polarization resistance of the electrodes of 0.16 Ω cm² was achieved at 700 °C.     

Cobalt-free layered perovskite GdBaFe2O5+x as a novel cathode for intermediate temperature solid oxide fuel cells

A cobalt-free layered perovskite oxide, GdBaFe₂O_5+x (GBF), was investigated as a novel cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). Area-specific resistance (ASR) of GBF was measured by impedance spectroscopy in a symmetrical cell. The observed ASR was as low as 0.15 Ω cm² at 700 °C and 0.39 Ω cm² at 650 °C, respectively. A laboratory sized Sm_0.2Ce_0.8O_1.9 (SDC)-based tri-layer cell of NiO-SDC/SDC/GBF was tested under intermediate temperature conditions of 550-700 °C with humidified H₂ (∼3% H₂O) as a fuel and the static ambient air as an oxidant. A maximal power density of 861 mW cm⁻² was achieved at 700 °C. The electrode polarization resistance was as low as 0.57, 0.22, 0.13 and 0.08 Ω cm² at 550, 600, 650 and 700 °C, respectively. The experimental results indicate that the layered perovskite GBF is a promising cathode candidate for IT-SOFCs.     

High performance composite interconnect La0.7Ca0.3CrO3/20 mol% ReO1.5 doped CeO2 (Re = Sm,Gd, Y) for solid oxide fuel cells

This paper reports and discusses composite interconnect materials that were modified from La_0.7Ca_0.3CrO_3−δ (LCC) by addition of Re doped CeO₂ (Re = Sm, Gd, Y) for improved conductivity at relative low temperatures. It is found that the addition of small amounts of RDC (ReO_1.5 doped CeO₂) into LCC dramatically increased the electrical conductivity. For the best system studied, LCC + 5 wt% SDC (Sm_0.2Ce_0.8O_1.9), LCC + 3 wt% GDC (Gd_0.2Ce_0.8O_1.9) and LCC + 3 wt% YDC (Y_0.2Ce_0.8O_1.9), the electrical conductivities reached 687.8, 124.6 and 104.8 S cm⁻¹ at 800 °C in air, respectively. The electrical conductivities of the specimens, LCC + 3 wt% SDC, LCC + 1 wt% GDC and LCC + 2 wt% YDC in H₂ at 800 °C were 7.1, 3.8 and 5.9 S cm⁻¹, respectively. With the increase of RDC content, the relative density increased, indicating that RDC served as an effective sintering aid in enhancing the sinterability of the powders. The average coefficient of thermal expansion (CTE) at 30–1000 °C in air increased with the increase of the RDC content. The oxygen permeation measurements indicated a negligible oxygen ionic conduction, indicating that the efficiency loss of a solid oxide fuel cell by permeation is negligible for the general cell design using LCC + RDC as interconnect. Therefore, the composite materials La_0.7Ca_0.3CrO₃/20 mol% ReO_1.5 doped CeO₂ are very promising interconnecting ceramics for solid oxide fuel cells (SOFCs).