LaCoO3-δ-coated Ba0.5Sr0.5Co0.8Fe0.2O3-δ: A promising cathode material with remarkable performance and CO2 resistance for intermediate temperature solid oxide fuel cells

LaCoO_3-δ (LC)-coated Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) cathode is fabricated by the solution impregnation method and promoted electrochemical performance is obtained. After being coated by LC shell, the polarization resistance can be as low as 0.197 Ω cm² and the peak power density is 0.243 W cm^?2 at the operating temperature of 600 °C. The excellent CO₂ resistance of LC-coated BSCF cathode is verified by the CO₂-poisoning test. Even in the operating atmosphere with high CO₂ concentration, the polarization resistance change of LC-coated BSCF cathode is much smaller than that of the blank BSCF cathode. By long-term test of single cells, the remarkable electrochemical performance stability of LC-coated BSCF cathode is shown. The promoted electrochemical performance, excellent CO₂ resistance and remarkable long-term stability make LC-coated BSCF cathode promising for intermediate temperature solid oxide fuel cells.     

Nano Ni layered anode for enhanced MCFC performance at reduced operating temperature

Nanoparticles of Ni and Ni–Al₂O₃ were coated on a molten carbonate fuel cell (MCFC) anode by spray method to enlarge the electrochemical reaction sites at triple phase boundaries (TPBs). Both nano Ni coated anode and nano Ni–Al₂O₃ anode exhibited significant reduction of anode polarization, thanks to smaller charge transfer resistance. The maximum power density of nano Ni coated anode was 159 mW cm⁻² at current density of 300 mA cm⁻² operating at 600 °C. This is about 7% increase from the standard cell performance tested and compared in the study. Although low performance of nano coated Ni–Al₂O₃ cell is observed due to electrolyte consumption, the stability of cell performance during operation time is more favorable in MCFCs operation.     

Remarkable ionic conductivity and catalytic activity in ceramic nanocomposite fuel cells

Although ceramic nanocomposite fuel cells (CNFCs) have attracted the attention of the fuel cell community due to their low operating temperature (<600 °C), often the performance of the cells is limited due to the low ionic conductivity of the electrolyte and the sluggish reaction kinetics at the electrodes. This results in high ohmic and charge transfer losses in the cell performance. Here we report nanocomposite electrolyte (GDC-NLC) and electrodes (NiO-GDC-NLC and LSCF-GDC-NLC as anode and cathode respectively) with enhanced ionic conductivity and catalytic activity respectively, which significantly improve the ionic transport in the electrolyte layer (ohmic losses ≈ 0.23 Ω cm²) and the reaction kinetics at the electrodes (polarization losses ≈ 0.63 Ω cm²). Microstructural and phase changes in the materials were characterized with X-ray diffraction, scanning electron microscopy, and differential scanning calorimetry to understand the mechanisms in the cells. Our button fuel cell produced an outstanding performance of 1.02 W/cm² at 550 °C.     

Electrochemical and thermal properties of SmBa0.5Sr0.5Co2O5+δ cathode impregnated with Ce0.8Sm0.2O1.9 nanoparticles for intermediate-temperature solid oxide fuel cells

SmBa_0.5Sr_0.5Co₂O_5+δ (SBSC55) impregnated with nano-sized Ce_0.8Sm_0.2O_1.9 (SDC) powder has been investigated as a candidate cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The cathode chemical compatibility with electrolyte, thermal expansion behavior, and electrochemical performance are investigated. For compatibility, a good chemical compatibility between SBSC55 and SDC electrolyte is still kept at 1100 °C in air. For thermal dilation curve, it could be divided into two regions, one is the low temperature region (100–265 °C); the other is the high temperature region (265–850 °C). In the low temperature region (100–265 °C), a TEC value is about 17.0 × 10^?6 K^?1 and an increase in slope in the higher temperatures region (265–800 °C), in which a TEC value is around 21.1 × 10^?6 K^?1. There is an inflection region ranged from 225 to 330 °C in the curve of d(δL/L)/dT vs. temperature. The peak inflection point located about 265 °C is associated to the initial temperature for the loss of lattice oxygen and the formation of oxygen vacancies. For electrochemical properties, the polarization resistances (R_p) significantly reduced from 4.17 Ω cm² of pure SBSC55 to 1.28 Ω cm² of 0.65 mg cm^?2 of SDC-impregnated SBSC55 at 600 °C. The single cell performance of SBSC55∣SDC∣Ni-SDC loaded with 0.65 mg cm^?2 SDC exhibited the optimum power density of 823 mW cm^?2 at operating temperature of 800 °C. Based on above-mentioned properties, SBSC55 impregnated with an appropriate SDC is a potential cathode for IT-SOFCs.     

Electrochemical investigations of cobalt-free perovskite cathode material for intermediate temperature solid oxide fuel cell

A novel cobalt-free perovskite zinc-doped lanthanum strontium iron oxide (La_0.8Sr_0.2Zn_xFe_1?xO_3?δ, LSZF, x = 0.1–0.3) is synthesized and evaluated as cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC) with samarium doped ceria (SDC) electrolyte. LSZF cathode at x = 0.2 composition demonstrates the remarkable electrochemical activity at intermediate temperature (550 °C): such as, high electrical conductivity (13.63 S cm^?1), excellent thermal stability with SDC electrolyte (12.10 μK^?1), high surface area (4.52 m² g^?1), extremely reduced area specific resistance (0.69 Ω cm^?2) and low activation energy (0.117 eV). Furthermore, single fuel cells are fabricated using LSZF as a cathode, which exhibits the excellent performance by achieving the high power density of 409 mW cm^?2 under natural gas as a fuel and ambient air as an oxidant at 550 °C with good stability over 10 h. All experimental results indicate that the LSZF is a promising cathode material for natural gas based intermediate temperature fuel cell applications.     

A novel La2NiO4+δ-La3Ni2O7-δ-Ce0.55La0.45O2-δ ternary composite cathode prepared by the co-synthesis method for IT-SOFCs

A novel La₂NiO_4+δ-La₃Ni₂O_7?δ-Ce_0.55La_0.45O_2?δ (L2N1-L3N2-LDC) ternary composite with a weight ratio of 0.3:2.5:2.2 was prepared by a one-step co-synthesis method and employed as cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). X-ray diffraction (XRD) profiles confirmed the successful synthesis of the composite consisted of L2N1, L3N2 and LDC phases, without any other impurity. Compared with the cathode prepared by the physical mixing method, the co-synthesized composite cathode possessed a porous microstructure with the smaller particle size and more uniform distribution of various elements. The ternary composite cathode on Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte revealed improved electrochemical performance, achieving the polarization resistance value of 0.06 Ω cm² at 800° C in stationary air. Electrochemical impedance spectra under various oxygen partial pressures indicated the charge transfer process was the rate limiting step for oxygen reduction reaction. Furthermore, a SDC electrolyte (about 350 μm) supported single cell with L2N1-L3N2-LDC as cathode and Ni-SDC as anode demonstrated a maximum power density of 253 mW cm^?2 at 800° C. These results confirmed that L2N1-L3N2-LDC ternary composite prepared by co-synthesized method is a very promising cathode material for IT-SOFCs.     

Investigation of electrospun Ba0.5Sr0.5Co0.8Fe0.2O3−δ-Gd0.1Ce0.9O1.95 cathodes for enhanced interfacial adhesion

Fibrous Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ-Gd_0.1Ce_0.9O_1.95 (BSCF-GDC) composite cathodes are fabricated by a facile electrospinning method. However, the electropun BSCF-GDC cathode shows poor adhesion to a GDC electrolyte because of the high shrinkage rate of the electrospun BSCF-GDC cathode during sintering. To solve this adhesion issue, mixed BSCF fiber-GDC powder cathode is investigated. As a result, mixed BSCF fiber-GDC powder cathode with an enhanced adhesion is successfully fabricated. This improvement can be attributed to the modified microstructure with the GDC powder that joins the BSCF fibers to the GDC electrolyte at the cathode and electrolyte interface. The polarization resistance of the mixed BSCF fiber-GDC powder cathode is 0.10 Ω cm², which is lower than 0.13 Ω cm² of conventional BSCF-GDC powder cathode at 700 °C. It is attributable to the improved oxygen gas and lattice oxygen diffusion, and the surface exchange of the mixed BSCF fiber-GDC powder cathode. The single cell with a mixed BSCF fiber-GDC powder cathode show 500 mW cm^?2 at 700 °C, which is 25% higher than conventional BSCF-GDC powder cathode.     

PdO/ZrO2 engineered (La0.8Sr0.2)0.95MnO3-δ-(Y2O3)0.08(ZrO2)0.92 cathode for use in intermediate temperature solid oxide fuel cells

To enhance the electrochemical performance of (La_0.8Sr_0.2)_0.95MnO_3-δ-8 mol. % Y₂O₃ stabilized ZrO₂ (LSM-YSZ) cathode at reduced temperatures, PdO and ZrO₂ (Pd/Zr = 0.8/0.2) are co-infiltrated into the LSM-YSZ scaffold. Such prepared composite cathode is investigated at temperatures between 600 and 750 °C and cathodic current densities of 400 and 800 mA cm^?2. It is observed that PdO particles are uniformly deposited on the surface of the LSM-YSZ and surrounded by nano-sized ZrO₂ particles. This distinctive microstructure possesses improved thermal stability under current at 750 °C due to the hindering effect of ZrO₂ on the agglomeration and growth of PdO particles. As a result, the electrocatalytic activity of the cathode for oxygen reduction reaction (ORR) is greatly enhanced due to presence of the self-limited PdO particles. At open circuit voltage, the initial polarization resistance decreases from 1.68 to 0.40 Ω cm² as temperature increases from 600 to 750 °C; and the polarization resistance is fully stabilized at the level of 0.36 and 0.34 Ω cm², respectively, after current polarization at 750 °C under 400 and 800 mA cm^?2 for less than 200 h.     

A liquid-feed solid polymer electrolyte direct methanol fuel cell operating at near-ambient conditions

Results on the performance of a 25 cm² liquid-feed solid polymer electrolyte direct methanol fuel cell (SPE-DMFC), operating under near-ambient conditions, are reported. The fuel cell can sustain a load current density of 100 mA cm⁻² with an output voltage of c. 450 mV at 90°C with 2 M aqueous methanol and air-fed cathode at near-ambient pressures with a catalyst loading of 5 mg cm⁻² of Pt. Preliminary data on the performance of a liquid-feed SPE-DMFC stack comprising two 25 cm² cells are also reported. These data are sufficient to suggest that further developmental work on liquid-feed SPE-DMFCs operating under near-ambient conditions (0 barg O₂ at 90°C) is well worthwhile.     

Lifetime Expectancy of molten carbonate fuel cells: Part I. Effect of temperature on the voltage and electrolyte reduction rates

Electrolyte depletion is a significant setback in the operation of molten carbonate fuel cells (MCFCs). The electrolyte loss mostly occurs as a result of the high operating temperatures of over 873 K. The effect of temperature on MCFCs was studied using several 7 cm² coin-type MCFCs operated at 873, 973 and 1073 K. Lithium-potassium carbonate (Li/K) was used as an electrolyte in this study. A decrease in cell performance with time was observed at all temperatures. The performance degradation was found to be more severe at 1073 K than at 973 K and 873 K. The electrolyte loss rate was observed by chemical means to have increased with increasing temperature. A more severe electrolyte loss rate was observed in cells operated at 1073 K, such that the electrolyte amount reduced by half after 250 h of cell operation. In this research work, a factor, F_WV, which correlates the electrolyte loss rate, voltage reduction rate, and cell life, is introduced. Its dependence on the cell electrode area and operating temperature make it a suitable parameter for simulating MCFC's lifetime.     

Electrode performance of a new La0.6Sr0.4Co0.2Fe0.8O3 coated cathode for molten carbonate fuel cells

To improve the cathode performance in molten carbonate fuel cells (MCFCs), Lanthanum Strontium Cobalt Ferrite (La_0.6Sr_0.4Co_0.2Fe_0.8O₃, LSCF) of perovskite structure was coated on a porous Ni plate by a vacuum suction method. The electrochemical performance of modified cathode was examined and compared with that of uncoated conventional cathode via single cell operation and electrochemical impedance analysis (EIS). The cell voltage of the single cell using the LSCF coated cathode, measured at 650 °C with current density of 150 mA/cm² is 0.837 V and it is higher than that of the cell with uncoated conventional cathode, 0.805 V. The higher performance and the lower charge transfer resistance were obtained at 600–700 °C after LSCF coating. The lower activation energy of oxygen reduction reaction was also obtained. The lower activation energy of oxygen reduction reaction after LSCF coating shows that LSCF on lithiated NiO cathode plays a role of catalyst on the oxygen reduction reaction in cathode.     

Performance and durability of an anode-supported solid oxide fuel cell with a PdO/ZrO2 engineered (La0.8Sr0.2)0.95MnO3-δ-(Y2O3)0.08(ZrO2)0.92 composite cathode

PdO/ZrO₂ co-infiltrated (La_0.8Sr_0.2)_0.95MnO_3-δ-(Y₂O₃)_0.08(ZrO₂)_0.92 (LSM-YSZ) composite cathode (PdO/ZrO₂+LSM-YSZ), which adsorbs more oxygen than equal amount of PdO/ZrO₂ and LSM-YSZ, is developed and used in Ni-YSZ anode-supported cells with YSZ electrolyte. The cells are investigated firstly at temperatures between 650 and 750 °C with H₂ as the fuel and air as the oxidant and then polarized at 750 °C under 400 mA cm^?2 for up to 235 h. The initial peak power density of the cell is in the range of 438–1207 mW cm^?2 at temperatures from 650 to 750 °C, corresponding to polarization resistance from 1.04 to 0.35 Ω cm². This result demonstrates a significant performance improvement over the cells with other kinds of LSM based cathode. The cell voltage at 750 °C under 400 mA cm^?2 decreases from initial 0.951 to 0.89 V after 170 h of current polarization and remains essentially stable to the end of current polarization. It is identified that the self-limited growth of PdO particles is responsible for the cell voltage decrease by reducing the length of triple phase boundary affecting the high frequency steps involved in oxygen reduction reaction in the cathode.     

Monolithic flat tubular types of solid oxide fuel cells with integrated electrode and gas channels

A new monolithic solid oxide fuel cell (SOFC) design stacked with flatten tubes of unit cells without using metallic interconnector plate is introduced and evaluated in this study. The anode support is manufactured in a flat tubular shape with fuel channel inside and air gas channel on the cathode surface. This design allows all-ceramic stack to provide flow channels and electrical connection between unit cells without needing metal plates. This structure not only greatly reduces the production cost of SOFC stack, but also fundamentally avoids chromium poisoning originated from a metal plate, thereby improving stack stability. The fuel channel was created in the extrusion process by using the outlet shape of mold. The air channel was created by grinding the surface of pre-sintered support. The anode functional layer and electrolyte were dip-coated on the support. The cathode layer and ceramic interconnector were then spray coated. The maximum power density and total resistance of unit cell with an active area of 30 cm² at 800 °C were 498 mW/cm² and 0.67 Ωcm², respectively. A 5-cell stack was assembled with ceramic components only without metal plates. Its maximum power output at 750 °C was 46 W with degradation rate of 0.69%/kh during severe operation condition for more than 1000 h, proving that such all-ceramic stack is a strong candidate as novel SOFC stack design.     

High performance ceramic nanocomposite fuel cells utilizing LiNiCuZn-oxide anode based on slurry method

A multi-oxide material LiNiCuZn-oxide was prepared through a slurry method as an anode for ceramic nanocomposite fuel cell (CNFC). The CNFCs using this anode material, LSCF as cathode material and a composite electrolyte consisting of CaSm co-doped CeO₂ and (NaLiK)₂CO₃ produced ～1.03 W/cm² at 550 °C due to efficient reaction kinetics at the electrodes and high ionic transport in the nanocomposite electrolyte. The electrochemical impedance spectroscopy revealed low ionic transport losses (0.238 Ω cm²) and low polarization losses (0.124 Ω cm²) at the electrodes. The SEM measurements revealed the porous microstructures of the composite materials at electrode and the dense mixture of CaSm co-doped CeO₂ and (NaLiK)₂CO₃. The Brunauer-Emmett-Teller (BET) analysis revealed high surface areas, 4.1 m²/g and 3.8 m²/g, of the anode and cathode respectively. This study provides a promising material for high performance CNFCs.     

Porous YFe0.5Co0.5O3 thin sheets as cathode for intermediate-temperature solid oxide fuel cells

In this work, porous YFe_0.5Co_0.5O₃ (YFC) thin sheets were synthesized by citric acid method. The crystal structure, morphology, thermal expansion, electrical conductivity, and electrochemical properties of YFC were investigated to evaluate it as a possible cathode on BaZr_0.1Ce_0.7Y_0.2O₃ (BZCY) electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs). An orthorhombic perovskite structure was observed in YFC. The conductivity of YFC is 183 S cm ^?1 at 750 °C in air. The coefficient of thermal expansion of composite cathode YFC-BZCY is closer to BZCY electrolyte than YFC. The composite cathode represents a relatively low polarization resistance (R_p) of 0.07 Ω cm² at 750 °C in air due to the porous thin sheet-like cathode. The oxygen reduction reaction process and the reaction activation energy of cathode were also analyzed. An anode-supported cell of NiO-BZCY∣BZCY∣YFC-BZCY is fabricated by a simple method of co-pressing. The power density of the cell is 303 mW cm^?2 at 750 °C as the thickness of electrolyte is 400 μm. The results suggest that YFC is a promising cathode candidate for IT-SOFC.     

The electrolyte-layer free fuel cell using a semiconductor-ionic Sr2Fe1.5Mo0.5O6-δ – Ce0.8Sm0.2O2-δ composite functional membrane

Commercial double Perovskite Sr₂Fe_1.5Mo_0.5O_6-δ (SFM), a high performance and redox stable electrode material for solid oxide fuel cell (SOFC), has been used for the electrolyte (layer) -free fuel cell (EFFC) and also as the cathode for the electrolyte based SOFC in a comprehensive study. The EFFC with a homogeneous mixture of Ce_0.8Sm_0.2O_2-δ (SDC) and SFM achieved a higher power density (841 mW cm^?2) at 550 °C, while the SDC electrolyte based SOFC, using the SDC-SFM composite as cathode, just reached 326 mW cm^?2 at the same temperature. The crystal structure and the morphology of the SFM-SDC composite were characterized by X-ray diffraction analysis (XRD), and scanning electron microscope (SEM), respectively. The electrochemical impedance spectroscopy (EIS) results showed that the charge transfer resistance of EFFCs were much lower than that of the electrolyte-based SOFC. To illustrate the operating principle of EFFC, we also conducted the rectification characteristics test, which confirms the existence of a Schottky junction structure to avoid the internal electron short circuiting. This work demonstrated advantages of the semiconductor-ionic SDC-SFM material for advanced EFFCs.     

Electrochemical behavior and performances of Ni-BaZr0·1Ce0·7Y0.1Yb0.1O3−δ cermet anodes for protonic ceramic fuel cell

High performance Ni-BCZYYb cermet anode were prepared at 1300 °C using electrolyte powders prepared by combustion and commercial NiO. The cermets are porous (39 vol% of porosity), show a high electronic conductivity (1097 S cm^?1) and sufficient mechanical properties. The electrochemical behavior of the Ni-BCZYYb/BZCYYb-ZnO/Ni-BCZYYb symmetrical cell elaborated by co-pressing and co-sintering was investigated using electrochemical impedance spectroscopy. The impedance spectroscopy study show that the electrode reaction involves three steps. The total polarization Area Specific Resistance decreases by about one order of magnitude when increasing the temperature from 450 to 600 °C or the H₂ concentration from 5 to 100 vol% to reach 0.049 Ω cm² at 600 °C under pure hydrogen.     

Study on LiI and KI with low melting temperature for electrolyte replenishment in molten carbonate fuel cells

Molten carbonate fuel cells (MCFCs) are regarded as the closest fuel cell to commercialization due to their high capacity and energy efficiency. However, they are operated at a high temperature (620 °C or higher), where liquid electrolyte loss occurs during operation; hence, their lifetime is limited. For the long-term operation of MCFCs, it is essential to develop a novel method to replenish the electrolyte during operation. However, it is very difficult to directly inject the electrolyte, (Li_0.62K_0.38)₂CO₃, into each unit cell of the stack unless it is supplemented through liquid or gas phase at low temperature. It was verified whether LiI and KI, which have low melting points and high vapor pressures, could replenish the lost electrolyte in MCFCs. In this study, the LiI and KI injected into the unit cell in liquid phase showed a similar tendency to the Li/K carbonate electrolyte. This is because LiI and KI react with the CO₂/O₂ gases supplied to the cathode during MCFC operation to form Li/K carbonate electrolytes.     

Effects of Gd0.8Ce0.2O1.9−δ coating with different thickness on electrochemical performance and long-term stability of La0.8Sr0.2Co0.2Fe0.8O3-δ cathode in SOFCs

The commercialization of Solid oxide fuel cells (SOFCs) has always been limited by the poor catalytic activity and the severe degradation of cathode in the intermediate and low operating temperature. Here we report a Gd_0.8Ce_0.2O_1.9?δ (GDC) coated La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ (LSCF) composite cathode material, which can significantly improve the electrochemical performance and durability of LSCF cathode. The effects of different GDC coating thickness on the electrochemical performance and long-term working stability of LSCF cathode are investigated, and the optimal coating thickness is established. The polarization impedance of GDC coated LSCF (LSCF@GDC) cathode with 9 nm of GDC coating is 0.08 Ω cm² at 800 °C, which is only one quarter of that of the raw LSCF cathode, and the degradation rate of constant current polarization with 100 mA cm^?2 is only 0.42%/100 h at 700 °C, which is far less than that of the raw LSCF cathode. The X-ray photoelectron spectroscopy (XPS) results show that the degree of Sr segregation decreases with the increase of the thickness of the coated GDC layer. The potential LSCF@GDC composite material is expected to increase the operability of SOFCs and accelerate its commercialization.     

High performance of protonic solid oxide fuel cell with BaCo0.7Fe0.22Sc0.08O3−δ electrode

Solid oxide fuel cells (SOFCs) have attracted tremendous attention for their combination of environmental power generation and fuel flexibility. Proton conducting SOFCs (P-SOFCs) demonstrate advantages over oxygen-ion conducting SOFCs, such as less activation energies on ionic transport and higher fuel utilization efficiency. Central to the devices is a suitable cathode with high catalytic activity. Herein, a cubic perovskite BaCo_0.7Fe_0.22Sc_0.08O_3?δ (BCFSc) has been applied as the cathode in proton-conducting solid state fuel cell (SOFC) with BaZr_0.1Ce_0.7Y_0.2O_3?δ (BZCY) electrolyte. Peak power densities of 760, 591, 452 and 318 mW cm^?2 are obtained at 650, 600, 550 and 500 °C with humidified hydrogen as the fuel and air as the oxidant. A low polarization resistance of 0.05 Ω cm² under open circuit at 650 °C is observed.