Search

The anode-supported solid oxide fuel cell (SOFC) comprises of NiO-8YSZ | 8YSZ | LSM-GDC | LSCF and the performance durability is executed for over 1000 hours. It shows low degradation phenomena under constant current operation during the complete testing period. The cell performance decreases with the decreasing of the temperature, and the maximum power densities are 408, 265, and 163 mW cm-2at 800, 750, and 700 °C, respectively. According to the EIS analysis with the equivalent circuit model of five serial components, all resistances decrease with the testing time except the non-charge transfer resistance of the cathode. However all resistances increase with the decreasing of the temperature on the contrary. The ohmic resistance of the cell (RO) dominates the cell performance under the whole durability test period as well as the operation temperature. In this study, the ROis determined by the interfacial contact resistances, which occurred between the cell and the connecting components. The LSM-GDC | LSCF interfaces formed the discontinuous gap due to the weak attachment and external loading. The result of the activation energy analysis shows that the rate-determination step of the cell is existed in the anode side between 700 and 800 °C. However, the cell performance is controlled from the domination of the ROat 800 °C shift to the joint contributions of the RO, anodic polarization (RAP), and cathodic polarization (RCP) at 700 °C.

Yttria-stabilized zirconia (YSZ) is the most commonly used electrolyte material for solid oxide fuel cells. The use of relatively inexpensive metal components would require a trade-off in the lowering of the operating temperature below 800 °C. However, the reduced temperature results in an increased resistivity of YSZ. This can be overcome by reducing the electrolyte thickness using thin film techniques. In this study, the preparation of a thin film of YSZ via the sputter deposition technique was reported. YSZ electrolyte films were deposited on anode substrates at 650/750 °C. The gas permeability test indicated a gas-tight electrolyte with a gas leak rate − 7 lcm − 2 s − 1 . The measured open circuit voltage was 1.1 V, while the maximum power density was 515 mW cm − 2 .

The anode-supported solid oxide fuel cell (SOFC) with low-porosity anode structure is fabricated and the electrochemical characteristics are investigated. The electrochemical characterization of the cell shows a periodic oscillation phenomenon of the cell voltage under the constant current density operation. The low-porosity anode structure results in the decrease in the effective diffusion coefficient and the accumulation of water vapor. The cell voltage oscillation is mainly caused by the concentration polarization as well as the boundary migration of the reaction zone. The profound influence on the concentration polarization can be observed when the cell test is executed with operation condition of higher current density, lower hydrogen concentration, and lower hydrogen flow rate in the anode side.

Direct evidences of cerium valence state transformation from Ce4 + to Ce3 + in an anode-supported solid oxide fuel cell after long-term operation have been identified. Single cell with samarium-doped ceria (Sm0.2Ce0.8O2 − δ, SDC) thin film electrolyte is prepared with maximum power density of 608 mW cm− 2 at 650 °C when the fuel/oxidant flow rates are 335/1005 sccm, respectively. The long-term durability tests are executed by fixed-current operation for 950 h. The power density and voltage degradation are observed and critically attributed to the chemical state transformation of cerium in the electrolyte. From X-ray photoelectron spectroscopy (XPS), the Ce3 + ratio increases from 30.3 to 52.9% in the electrolyte before and after operation. The focused ion beam (FIB)/transmission electron microscopy (TEM) techniques are utilized to investigate the structure variation and the selected area diffraction patterns of the chosen grains towards electrodes identify the phase transformation from CeO2 to Ce2O3, suggesting that the Ce3 + species increase at the near-anode side. The direct evidence of Ce3 + presence shows one of the key factors for the degradation of a low temperature solid oxide fuel cell (LT-SOFC).

Dense electrolyte films ∼15 μm thick made of samarium-doped ceria (SDC) are fabricated by spin-coating. The SDC powders are synthesized by the glycine nitrate combustion process. It is found that nanoscale SDC powders can be obtained at 1000 °C. Cells constructed with an SDC electrolyte, a NiO + SDC composite anode, and an SSC–SDC/SSC bi-layer cathode are fabricated and tested at temperatures from 400 to 650 °C. SEM micrographs show that the SDC electrolyte layer adheres well to the porous anode and the cathode. The maximum power densities of the cell are 38, 84, 185, 303, 438, and 549 mW cm −2 at 400, 450, 500, 550, 600, and 650 °C, respectively. Analysis of the impedance spectra indicates that the electrode polarization dominates the total cell resistance at temperatures below 550 °C, and the ohmic resistance dominates the total cell resistance above 550 °C. The activation energies of the resistances show that the cell performance is significantly controlled by the electrode polarization resistance. Durability tests are performed over 950 h and indicate that the power density and the voltage gradually degrade with time at a rate of ∼0.03 mW cm −2 h −1 and ∼0.07 mV h −1 , respectively. Hence, a low-temperature solid oxide fuel cell has been developed.

The various composite cathode layers on the solid oxide fuel cells (SOFCs) with a dense electrolyte of yttria-stabilized zirconia (YSZ) are investigated in this study. The ceramic powders of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF), Ba0.5Sr0.5Co0.2Fe0.8O3−δ (BSCF), La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM), and Gd0.2Ce0.8O3−δ (GDC) are prepared by the glycine-nitrate combustion process. The different formations of composite cathode layers are coated onto the NiO–YSZ/YSZ half-cells via a screen printing method. Scanning electron microscopy (SEM) micrographs show that all of the samples can be well adherent to the YSZ electrolyte. From the energy dispersive X-ray (EDX) result, the undesired compound of SrZrO3 is formed between the YSZ electrolyte and the composite cathode layers with LSCF and BSCF at the sintering temperature of 1000 °C. According to the results of the electrochemical test and electrochemical impedance spectroscopy (EIS), the composite cathode layer with YSZ–La0.8Sr0.2MnO3−δ (LSM) interlayer exhibits the best electrochemical performance. The maximum power density can achieve the value above 400 mW/cm2 for the samples with YSZ–LSM interlayer.

The Sm 0.5 Sr 0.5 CoO 3− δ (SSC) with perovskite structure is synthesized by the glycine nitrate process (GNP). The phase evolution of SSC powder with different calcination temperatures is investigated by X-ray diffraction and thermogravimetric analyses. The XRD results show that the single perovskite phase of the SSC is completely formed above 1100 °C. The anode-supported single cell is constructed with a porous Ni–yttria-stabilized zirconia (YSZ) anode substrate, an airtight YSZ electrolyte, a Sm 0.2 Ce 0.8 O 2− δ (SDC) barrier layer, and a screen-printed SSC–SDC composite cathode. The SEM results show that the dense YSZ electrolyte layer exhibits the good interfacial contact with both the Ni–YSZ and the SDC barrier layer. The porous SSC–SDC cathode shows an excellent adhesion with the SDC barrier layer. For the performance test, the maximum power densities are 464, 351 and 243 mW cm −2 at 800, 750 and 700 °C, respectively. According to the results of the electrochemical impedance spectroscopy (EIS), the charge-transfer resistances of the electrodes are 0.49 and 1.24 Ω cm 2 , and the non charge-transfer resistances are 0.48 and 0.51 Ω cm 2 at 800 and 700 °C, respectively. The cathode material of SSC is compatible with the YSZ electrolyte via a delicate scheme employed in the fabrication process of unit cell.

The anode-supported single cell was constructed with porous Ni–Yittria-stabilized zirconia (YSZ) as the anode substrate, an airtight YSZ as the electrolyte, and a screen-printed La 0.8 Sr 0.2 MnO 3−δ (LSM)–Gadolinia-doped ceria (GDC)/LSM double-layer cathode. The SEM results show that the YSZ thin film is highly integrated, fully dense with a thickness of 13 μm, and exhibits excellent compatibility between cathode and electrolyte layers. The effects of feed rates of the reactants, temperature, and contact pressure between the current collector and the unit cell were systematically investigated. The results are based on the assumption that the anode contribution to the polarization resistance is negligible. Our analysis showed that the electrochemical reaction is limited by mass transfer control when the airflow rate is decreased to 500 ml min −1 . The maximum power density is 204.6 mW cm −2 at 800 °C with H 2 and air at flow rates of 800 and 2000 ml min −1 , respectively. According to the AC-impedance data, the resistances of charge transfer at the electrode/electrolyte interface are 3.79 and 1.90 Ω cm 2 . The resistances of oxygen-reduction processes are 3.63 and 1.01 Ω cm 2 at 700 and 800 °C, respectively. The results from the sensitivity analysis of the variation of contact pressure between current collectors and membrane electrode assembly (MEA) show that the influence is enhanced at the regions of the high current density.

Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3− δ (BSCF) and gadolinia-doped ceria (GDC) were synthesized via a glycine-nitrate process (GNP). A cubic perovskite of BSCF was observed by X-ray diffraction (XRD) at a calcination temperature above 950 °C. An anode-supported solid-oxide fuel cell was constructed from the porous NiO + YSZ as the anode substrate, the yittria-stabilized zirconia (YSZ) as the electrolyte, and the porous BSCF-GDC layer as the cathode with a GDC barrier layer. For the performance test, the maximum power density was 191.3 mW cm −2 at a temperature of 750 °C with H 2 fuel and air at flow rates of 335 and 670 sccm, respectively. According to the AC-impedance data, the charge-transfer resistances of the electrodes were 0.10 and 1.59 Ω cm 2 , and the oxygen-reduction and oxygen-ion diffusion resistances were 0.69 and 0.98 Ω cm 2 at 750 and 600 °C, respectively. SEM microstructural characterization indicated that the fuel cell as fabricated exhibited good compatibility between cathode and electrolyte layers.

Yttria-stabilized zirconia (YSZ) is the key material for an electrolyte of a solid oxide fuel cell (SOFC). In order to prepare a nanoscale/SOFC-grade 8YSZ material, the hydrothermal coprecipitation process is intensively investigated for process improvement and product identification. From the characterization results of the 8YSZ product, the operation conditions were selected for process optimization. The criteria used in optimization condition determination were the reproducibility of the 8YSZ product and the particle size, the crystallinity, as well as the operation simplicity. Experimental results showed that a nanoscale/SOFC-grade 8YSZ powder was directly prepared. The quasi-optimum conditions of the process are proposed.

The first energy recovery step in the ammonium hydrogen sulfate (AHS) cycle is the formation of H2SO4(l) from H2O(g) and SO3(g). It has been determined that the optimum way to accomplish this is by the use of a double pipe tubular reactor. In this paper, a mathematical model for the reactor is presented, applied to the three reaction zones, and a method of numerical solution discussed. Three horizontal pilot-scale configurations, 0.234 × 106 to 5.863 × 106 kJ/h energy release, are discussed and sizing presented. Also, the results for a vertical configuration are presented. The need for additional work on two-phase gas-liquid flow in condensing systems and in annuli has been identified. The most important conclusion is that a high temperature can be achieved in the reactor by the use of a front end adiabatic section followed by nonadiabatic sections to recover the heat released.

not Available.

The anode-supported solid oxide fuel cells (SOFCs) with an yttria-stabilized zirconia (YSZ) electrolyte have been successfully prepared by the sequential steps of fabricating technique including tape casting, spin coating, and screen-printing. Two types of the SOFCs are manufactured and their cell performances are characterized via the detail analysis of the electrochemical impedance spectroscopy (EIS). The maximum power densities are 411 and 289 mW/cm 2 for the high and low performance cells at 800°C, respectively. According to the EIS analysis, the electrochemical performance of the poor cell is dominated by the anodic non-charge-transfer resistance, which is mainly resulted by the gas diffusion polarization due to the low porosity of the anode. The high performance cell is controlled by the simultaneous contributions of the ohmic resistance (R O ), the anodic polarization (R AP ), and the cathodic polarization (R CP ) at the operation temperature of 800°C. However, the R CP will dominate the performance of the cell as the temperature is lowered down to 700°C.

The endothermic decomposition of metal pyrosulfate in molten phase to produce metal sulfate and sulfur trioxide gas is an important chemical energy storage reaction. A chemical reaction engineering analysis of certain literature data was carried out to obtain a model for the decomposition and the rate constant as a function of temperature. The analysis indicates that the rate controlling reaction event is the cation-anion interaction of K+ and S2O7−2 ions to form KS2O7−1, simultaneously polarizing the latter to weaken the SOS bridge and release SO3. Based on this view transition state theory was applied and yielded an expression for the rate constant in essential agreement with experimental data. A preliminary model for a pilot-scale reactor is presented.

An important energy recovery step in the ammonium hydrogen sulfate (AHS) cycle is the recombination reaction producing NH4HSO4. It has been determined that the optimum way to accomplish this, and prevent side reactions, is by the heterogeneous gas-liquid reaction of H2SO4 and NH3. A mathematical model is presented and applied to the two reaction zones and a method of numerical solution is discussed. Three horizontal pilot-scale configurations, 0.17 × 106 to 4.2 × 106 kJ/h energy release, are discussed and sizing is presented. The most important conclusion from the work is that the energy can be released most effectively by carrying out the reaction in a double pipe tubular reactor operating in the annular flow regime with both gas and liquid in turbulent flow.