LaCo0.6Ni0.4O3−δ as cathode contact material for intermediate temperature solid oxide fuel cells

LaCo_0.6Ni_0.4O_3−δ (LCN64) was prepared through the polymeric steric entrapment precursor route with Polyvinyl alcohol (PVA) as the entrapment agent and was evaluated as a contact material between the metallic interconnect and the cathode in planar intermediate temperature solid oxide fuel cell stacks (IT-SOFC). The ratio of PVA to metal nitrates and the calcination temperature of the precursor were optimized for the process. The electrical conductivity and thermal expansion coefficient (TEC) of the synthesized LCN64 and its chemical compatibility with SUS 430 were also characterized. The results indicate that 1:4 is a proper ratio of PVA to metal nitrates for process control and safety management; and calcination of the precursor at temperatures above 650 °C leads to formation of single perovskite phase LCN64. The conductivity of fully sintered LCN64 is above 1150 S cm⁻¹ in the temperature range between 100 °C and 800 °C, which is higher than those of conventional contact materials La_1−xSr_xMnO₃ (LSM) and LaNi_yFe_1−yO₃ (LNF). The average TEC is 17.22 × 10⁻⁶ K⁻¹ at temperatures below 900 °C, which is higher than those of the metallic interconnect and cell components. Mn and Cr elements contained in SUS 430 migrated into the porous LCN64 layer at 800 °C without chemically forming resistive phases.     

In situ sinterable cathode with nanocrystalline La0.6Sr0.4Co0.2Fe0.8O3−δ for solid oxide fuel cells

A nanocrystalline powder with a lanthanum based iron- and cobalt-containing perovskite, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF), is investigated for solid oxide fuel cell (SOFC) applications at a relatively low operating temperature (600-800 °C). A LSCF powder with a high surface area of 88 m² g⁻¹, which is synthesized via a complex method with using inorganic nano dispersants, is printed onto an anode supported cell as a cathode electrode. A LSCF cathode without a sintering process (in situ sintered cathode) is characterized and compared with that of a sintering process at 780 °C (ex situ sintered cathode). The in situ sintered SOFC shows 0.51 A cm⁻² at 0.9 V and 730 °C, which is comparable with that of the ex situ sintered SOFC. The conventional process for SOFCs, the ex situ sintered SOFC, including a heat treatment process after printing the cathodes, is time consuming and costly. The in situ sinterable nanocrystalline LSCF cathode may be effective for making the process simple and cost effective.     

Enhanced electrochemical performance of solution impregnated La0.8Sr0.2Co0.8Ni0.2O3−δ cathode for intermediate temperature solid oxide fuel cells

Solution impregnated La_0.8Sr_0.2Co_0.8Ni_0.2O₃ + Gd-doped CeO₂ (LSCN + GDC) cathodes for intermediate temperature solid oxide fuel cells (IT-SOFC) are prepared and their electrochemical properties are evaluated and compared with the conventional LSCN cathodes. The results indicate that the cathode performance can be enhanced by the presence of the nanosized microstructure produced with the solution impregnation method. It is determined that the amount of LSCN loading in the LSCN + GDC composite cathode needs to be higher than 35 wt% in order to achieve a performance superior to that of the conventional LSCN cathode. The optimum amount of LSCN loading is in the range of 45-55 wt% with an activation energy near 1.32 eV for oxygen reduction. At temperatures between 600 and 750 °C, the polarization resistance of the 55 wt% LSCN loaded LSCN + GDC cathode is in the range of 1.07 and 0.08 Ω cm², which is only about one half of that for the conventional cathode.     

Nano-structured cathodes based on La0.6Sr0.4Co0.2Fe0.8O3−δ for solid oxide fuel cells

Lanthanum-based iron- and cobalt-containing perovskite is a promising cathode material because of its electrocatalytic activity at a relatively low operating temperature in solid oxide fuel cells (SOFCs), i.e., 700-800 °C. To enhance the electrocatalytic reduction of oxidants on La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF), nanocrystalline LSCF materials are successfully fabricated using a complexing method with chelants and inorganic nano dispersants. When inorganic dispersants are added to the synthesis process, the surface area of the LSCF powder increases from 18 to 88 m² g⁻¹, which results in higher electrocatalytic activity of the cathode. The performance of a unit cell of a SOFC with nanocrystalline LSCF powders synthesized with nano dispersants is increased by 60%, from 0.7 to 1.2 W cm⁻².     

La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes infiltrated with samarium-doped cerium oxide for solid oxide fuel cells

Porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathodes are coated with a thin film of Sm_0.2Ce_0.8O_1.95−δ (SDC) using a one-step infiltration process. Examination of the microstructures reveals that small SDC particles are formed on the surface of LSCF grains with a relatively narrow size distribution. Impedance analysis indicates that the SDC infiltration has dramatically reduced the polarization of LSCF cathode, reaching interfacial resistances of 0.074 and 0.44 Ω cm² at 750 °C and 650 °C, respectively, which are about half of those for LSCF cathode without infiltration of SDC. The activation energies of the SDC infiltrated LSCF cathodes are in the range of 1.42-1.55 eV, slightly lower than those for a blank LSCF cathode. The SDC infiltrated LSCF cathodes have also shown improved stability under typical SOFC operating conditions, suggesting that SDC infiltration improves not only power output but also performance stability and operational life.     

Low-temperature solid oxide fuel cells with novel La0.6Sr0.4Co0.8Cu0.2O3−δ perovskite cathode and functional graded anode

The perovskite La_0.6Sr_0.4Co_0.8Cu_0.2O_3−δ (LSCCu) oxide is synthesized by a modified Pechini method and examined as a novel cathode material for low-temperature solid oxide fuel cells (LT-SOFCs) based upon functional graded anode. The perovskite LSCCu exhibits excellent ionic and electronic conductivities in the intermediate-to-low-temperature range (400-800 °C). Thin Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte and NiO-SDC anode functional layer are prepared over macroporous anode substrates composed of NiO-SDC by a one-step dry-pressing/co-firing process. A single cell with 20 μm thick SDC electrolyte on a porous anode support and LSCCu-SDC cathode shows peak power densities of only 583.2 mW cm⁻² at 650 °C and 309.4 mW cm⁻² for 550 °C. While a cell with 20 μm thick SDC electrolyte and an anode functional layer on the macroporous anode substrate shows peak power densities of 867.3 and 490.3 mW cm⁻² at 650 and 550 °C, respectively. The dramatic improvement of cell performance is attributed to the much improved anode microstructure that is confirmed by both SEM observation and impedance spectroscopy. The results indicate that LSCCu is a very promising cathode material for LT-SOFCs and the one-step dry-pressing/co-firing process is a suitable technique to fabricate high performance SOFCs.     

High performance proton-conducting solid oxide fuel cells with a stable Sm0.5Sr0.5Co3−δ-Ce0.8Sm0.2O2−δ composite cathode

A Sm_0.5Sr_0.5CoO_3−δ-Ce_0.8Sm_0.2O_2−δ (SSC-SDC) composite is employed as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs). BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) is used as the electrolyte, and the system exhibits a relatively high performance. An extremely low electrode polarization resistance of 0.066 Ω cm² is achieved at 700 °C. The maximum power densities are: 665, 504, 344, 214, and 118 mW cm⁻² at 700, 650, 600, 550, and 500 °C, respectively. Moreover, the SSC-SDC cathode shows an essentially stable performance for 25 h at 600 °C with a constant output voltage of 0.5 V. This excellent performance implies that SSC-SDC, which is a typical cathode material for SOFCs based on oxide ionic conductor, is also a promising alternative cathode for H-SOFCs.     

Electrochemical performance of La0.9Sr0.1Co0.8Ni0.2O3−δ-Ce0.8Sm0.2O1.9 composite cathode for solid oxide fuel cells

The mixed ionic and electronic conductors (MIEC) of La_0.9Sr_0.1Co_0.8Ni_0.2O_3−δ (LSCN)-Ce_0.8Sm_0.2O_1.9 (SDC) were investigated for potential application as a cathode material for solid oxide fuel cells (SOFCs) based on a SDC electrolyte. Electrochemical impedance spectroscopy (EIS) technique was performed over the temperature range of 600-850 °C to determine the cathode polarization resistance, which is represented by area specific resistance (ASR). This study systematically investigated the exchange current densities (i₀) for oxygen reduction reaction (ORR), determined from the EIS data and high-field cyclic voltammetry. The 70LSCN-30SDC composite cathode revealed a high exchange current density (i₀) value of 297.6 mA/cm² at 800 °C determined by high-field technique. This suggested that the triple phase boundary (TPB) may spread over more surface of this composite cathode and revealing a high catalytically active surface area. The activation energies (E_a) of ORR determined from the slope of Arrhenius plots for EIS and high-field techniques are 96.9 kJ mol⁻¹ and 90.4 kJ mol⁻¹, respectively.     

Thermochemical compatibility and polarization behaviors of La0.8Sr0.2Co0.8Ni0.2O3−δ as a cathode material for solid oxide fuel cell

Thermochemical compatibilities with Ce_0.8Gd_0.2O_2−δ (GDC) electrolyte and electrochemical behaviors under the condition of anodic or cathodic current treatment are investigated for La_0.8Sr_0.2Co_0.8Ni_0.2O_3−δ (LSCN) cathode of solid oxide fuel cell (SOFC). X-ray diffractometer (XRD) shows that cation exchange at 1150 °C leads to the formation of solid state solution between the cathode and electrolyte. Considering thermal expansion coefficient (TEC) and conductivity, La_1−xSr_xCo_1−yNi_yO_3−δ with the composition of La_0.8Sr_0.2Co_0.8Ni_0.2O_3−δ is indicated as a promising cathode for intermediate temperature SOFC. Electrochemical measurement reveals that the performance of LSCN cathode shows reversibility under anodic with subsequent cathodic current treatment. Further, the low frequency electrode process is strongly affected by anodic current. While the high frequency arc shows independent relationship with current polarization.     

A cobalt-free Sm0.5Sr0.5Fe0.8Cu0.2O3−δ-Ce0.8Sm0.2O2−δ composite cathode for proton-conducting solid oxide fuel cells

A cobalt-free composite Sm_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ-Ce_0.8Sm_0.2O_2−δ (SSFCu-SDC) is investigated as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs) in intermediate temperature range, with BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) as the electrolyte. The XRD results show that SSFCu is chemically compatible with SDC at temperatures up to 1100 °C. The quad-layer single cells of NiO-BZCYYb/NiO-BZCYYb/BZCYYb/SSFCu-SDC are operated from 500 to 700 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. It shows an excellent power density of 505 mW cm⁻² at 700 °C. Moreover, a low electrode polarization resistance of 0.138 Ω cm² is achieved at 700 °C. Preliminary results demonstrate that the cobalt-free SSFCu-SDC composite is a promising cathode material for H-SOFCs.     

Electrochemical stability of La0.6Sr0.4Co0.2Fe0.8O3−δ-infiltrated YSZ oxygen electrode for reversible solid oxide fuel cells

La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)-YSZ (yttria stabilized zirconia) oxygen electrodes were developed by an infiltration process for reversible solid oxide fuel cells (RSOFCs). Electrochemical performance of the LSCF-YSZ composite oxygen electrode was investigated in both fuel cell and steam electrolysis modes. Galvanostatic polarization operated at ±600 mA cm⁻² and 750 °C showed that the cell has a voltage degradation rate of 3.4% and 4.9% for fuel cell mode and steam electrolysis mode, respectively. Post-test SEM (scanning electronic microscopy) analysis of the electrodes indicates that the agglomeration of infiltrated LSCF particles is possibly responsible for the performance degradation of the cell.     

Fabrication and characterization of anode-supported single chamber solid oxide fuel cell based on La0.6Sr0.4Co0.2Fe0.8O3−δ–Ce0.9Gd0.1O1.95 composite cathode

Anode-supported solid oxide fuel cells consisting of nickel–gadolinium doped ceria (NiO–CGO, 60:40 wt%) anode, gadolinium doped ceria (CGO) electrolyte and lanthanum strontium cobaltite ferrite–gadolinium doped ceria (LSCF–CGO) cathode are developed and operated under single-chamber conditions, utilizing methane/air mixture. The cell performance is optimized regarding the electrolyte microstructure, cathode composition and testing conditions. The performance of the cell improves with the decrease of the thickness of the electrolyte and the increase of the ratio of methane to oxygen. The test cell with LSCF–CGO cathode (70:30 wt%) that was sintered at 1100 °C for 2 h and 150 μm dense electrolyte exhibits the maximum power output of ∼260 mW cm⁻² at 600 °C in CH₄/O₂ = 2 atmosphere.     

La0.6Sr0.4Fe0.8Cu0.2O3−δ perovskite oxide as cathode for IT-SOFC

A La_0.6Sr_0.4Fe_0.8Cu_0.2O_3−δ (LSFCu) perovskite was investigated as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFC). The LSFCu material exhibited chemical compatibility with the Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte up to a temperature of 1100 °C. The electrical conductivity of the sintered sample was measured as a function of temperature from 100 to 800 °C. The highest conductivity of about 238 S cm⁻¹ was observed for LSFCu. The average thermal-expansion coefficient (TEC) of LSFCu was 14.6 × 10⁻⁶ K⁻¹, close to that of typical CeO₂ electrolyte material. The investigation of electrical properties indicated that the LSFCu cathode had lower interfacial polarization resistance of 0.070 Ω cm² at 800 °C and 0.138 Ω cm² at 750 °C in air. An electrolyte-supported single cell with 300 μm thick SDC electrolyte and LSFCu as cathode shows peak power densities of 530 mW cm⁻² at 800 °C.     

Effects of bi-layer La0.6Sr0.4Co0.2Fe0.8O3−δ-based cathodes on characteristics of intermediate temperature solid oxide fuel cells

In this study, anode-supported planar IT-SOFCs, with a thin Sm_0.2Ce_0.8O_2−δ (SDC) electrolyte film and a bi-layer cathode, are fabricated using tape-casting and screen-printing processes. The bi-layer cathode consists of a current collector La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) layer and a functional LSCF-SDC composite layer in various thicknesses. Microstructure studies reveal that the interfaces among various layers show good adhesion, except for Cell A equipped with a cathode of pure LSCF. Cell A reports the lowest ohmic (R₀) and polarization (R_P) resistances. R_P, which increases with the thickness of the LSCF-SDC composite layer in the cathode, rises rapidly as the temperature drops, particularly at temperatures ≤550 °C. This indicates the high electrical conductivity of the cathode as a major contribution to the decrease of R_P at 500 °C. The best cell performances are observed at 650 °C for all cases, in which Cell A shows a maximum power density of 1.51 W cm⁻² and an open circuit voltage of 0.80 V. Considering both of the electrical and the mechanical integrity of the single cell, insertion of the composite layer is required to guarantee a good adhesion of cathode layer to electrolyte layer. However, the thickness of the composite layer should be retained as thin as possible to minimize the R₀ and R_P and maximize the cell performance.     

Performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes under solid oxide fuel cells operation conditions

The performance stability and degradation mechanism of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathodes and LSCF impregnated Gd_0.1Ce_0.9O_2−δ (LSCF-GDC) cathodes are investigated under solid oxide fuel cell operation conditions. LSCF and LSCF-GDC cathodes show initially performance improvement but degrade under cathodic polarization treatment at 750 °C for 120 h. The results confirm the grain growth and agglomeration of LSCF and in particular GDC-LSCF cathodes as well as the formation of SrCoO_x particles on the surface of LSCF under cathodic polarization conditions. The direct observation of SrCoO_x formation has been made possible on the surface of dense LSCF electrode plate on GDC electrolyte. The formation of SrCoO_x is most likely due to the interaction between the segregated Sr and Co from LSCF lattice under polarization conditions. The formation of SrCoO_x would contribute to the deterioration of the electrocatalytic activity of the LSCF-based electrodes for the O₂ reduction in addition to the agglomeration and microstructure coarsening.     

Performance evaluation of La0.4Sr0.6Co0.2Fe0.7Nb0.1O3−δ as both anode and cathode material in solid oxide fuel cells

Solid oxide fuel cell (SOFC) has experienced a growing interest in the last few decades because of generating energy more efficiently than the conventional combustion of fossil fuels. By using the same material as anode and cathode of SOFC (symmetric fuel cell), the production of reliable and repeatable cells would be simpler. In this work, La_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3−δ (LSCFN) perovskite has been prepared and evaluated as both cathode and anode material of symmetric fuel cell. The results of symmetric fuel cell show that a maximum peak power density of 500 mW cm⁻² has been achieved and the total electrode polarization resistances of the cell is only 0.22 Ω cm² at 850 °C which is much lower than that of typical symmetric fuel cell with La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ as electrode material. All of these results indicate that LSCFN can potentially be a promising candidate for the electrode material of symmetric fuel cell.     

La0.8Sr0.2Mn1−xRuxO3−δ as a cathode material for solid oxide fuel cells

Oxides of La_0.8Sr_0.2Mn_1−xRu_xO_3−δ (LSMR) (x = 0, 0.25, 0.50, 0.75, or 1.0) were prepared to fabricate cathodes in solid oxide fuel cells. The crystal structure changed from trigonal (x = 0 or 0.25) to a mixture of trigonal and orthorhombic (x = 0.5) and to orthorhombic (x = 0.75 or 1.0). X-ray photoelectron spectroscopy analysis after electrochemical testing indicated that the relative concentrations of Ru⁴⁺ to Ru⁶⁺ and Mn⁴⁺ to Mn²⁺ influence the performance of a single cell. The transformation from Ru⁴⁺ to Ru⁶⁺ releases two electrons but that from Mn⁴⁺ to Mn²⁺ creates two electron holes (an oxygen vacancy). The relative concentrations in LSMR were determined using the stoichiometric ratio (x) of Ru, and then, the concentrations of electrons and electron holes for influencing the cathode electrochemical catalytic reactivity were estimated. x = 0.25 represented the better cell performance, and Ru may stabilize the LSMR grain size during electrochemical testing.     

Characteristics of nano La0.6Sr0.4Co0.2Fe0.8O3−δ-infiltrated La0.8Sr0.2Ga0.8Mg0.2O3−δ scaffold cathode for enhanced oxygen reduction

The chemical compatibility and electrochemical properties of nanoLa_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)-infiltrated La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM) scaffold were manufactured and assessed for the application as a solid oxide fuel cell cathode with an LSGM electrolyte. When the LSCF and LSGM powder mixture was fired above 950 °C, the characteristic peaks of the two materials merged and an insulation peak (derived from LaSrGaO₄) was observed. To prevent reactions between LSCF and LSGM, an infiltration technique was utilized with the LSGM as a scaffold. Using this infiltration technique, nano LSCF particles (approximately 100 nm) can be uniformly coated on the LSGM scaffold surface. Good nano particle adhesion was observed at the LSGM/LSCF interface, even at relatively low firing temperatures (850 °C). The cathode polarization resistance (R_p) of the nano LSCF infiltrated LSGM scaffold cathode was lower than that of a conventional LSCF cathode. The improvement in performance of the nano LSCF-infiltrated cathode was attributed to an increase in the number of triple phase boundaries (TPB) as a result of the nano LSCF coating. In addition, the oxygen reduction reaction (ORR) paths were extended from the TPBs to the LSCF surface because LSCF particles are considerably smaller than the LSCF oxygen ion penetration depth (3–4 μm) over the temperature range of 700 °C–800 °C.     

Optimization of BaZr0.1Ce0.7Y0.2O3−δ-based proton-conducting solid oxide fuel cells with a cobalt-free proton-blocking La0.7Sr0.3FeO3−δ-Ce0.8Sm0.2O2−δ composite cathode

BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY)-based proton-conducting solid oxide fuel cells (H-SOFC) with a cobalt-free proton-blocking La_0.7Sr_0.3FeO_3−δ-Ce_0.8Sm_0.2O_2-δ (LSF-SDC) composite cathode were fabricated and evaluated. The effect of firing temperature of the cathode layer on the chemical compatibility, microstructure of the cathode and cathode-electrolyte interface, as well as electrochemical performance of single cells was investigated in detail. The results indicated that the cell exhibited the most desirable performance when the cathode was fired at 1000 °C; moreover, at the same firing temperature, the power performance had the least temperature dependence. With humidified hydrogen (∼2% H₂O) as the fuel and ambient air as the oxidant, the polarization resistance of the cell with LSF-SDC cathode fired at 1000 °C for 3 h was as low as 0.074 Ω cm² at 650 °C after optimizing microstructures of the anode and anode-electrolyte interface, and correspondingly the maximum power density achieved as high as 542 mW cm⁻², which was the highest power output ever reported for BZCY-based H-SOFC with a cobalt-free cathode at 650 °C.