Indium as an ideal functional dopant for a proton-conducting solid oxide fuel cell

A high In-dopant level BaCeO₃ material was used as an electrolyte for a proton-conducting solid oxide fuel cell (SOFC). Indium behaved as an ideal dopant for BaCeO₃, which improved both the chemical stability and sinterability for BaCeO₃ greatly. The anode supported BaCe_0.7In_0.3O_3−δ (BCI30) membrane reached dense after sintering at 1100 °C, much lower than the sintering temperature for other BaCeO₃-based materials. Additionally, the BCI30 membrane showed adequate chemical stability against CO₂ compared with the traditional rare earth doped BaCeO₃. The BCI30-based fuel cell also showed a reasonable cell performance and a good long-term stability under the operating condition. Besides, the LaSr₃Co_1.5Fe_1.5O_10−δ (LSCF) was also evaluated as a potential cathode candidate for a proton-conducting SOFC.     

Solid Oxide Fuel Cells damage mechanisms due to Ni-YSZ re-oxidation: Case of the Anode Supported Cell

The effects of Ni-YSZ cermet re-oxidation in anode supported Solid Oxide Fuel Cells (SOFCs) have been investigated. Damage mechanisms have been studied in both cases of direct oxidation in air (i.e., fuel shutdown) or by an ionic current (i.e., fuel starvation).Direct oxidation tests show that the electrolyte cracks for a conversion degree of Ni into NiO ranging between ∼58 and ∼71%. This failure mode has been modelled considering both the bulk expansion of the cermet induced by the transformation of the Ni phase and the change of mechanical stresses in the multilayered cell.In the case of fuel starvation, a thin layer of the cermet was electrochemically re-oxidised at 800 °C and then reduced under a hydrogen stream. This ‘redox’ cycle was repeated until the degradation of the cell. The evolution of the impedance diagrams recorded after each cycle suggests that the cermet damages in an area close to anode/electrolyte interface. The mechanical modelling states that a delamination can occur along the interface between the Anode Functional Layer (AFL) and the Anode Current Collector (ACC) substrate. This theoretical result confirms the experimental trends observed by impedance spectroscopy.     

Sintering and conductivity of BaCe0.9Y0.1O2.95 synthesized by the sol-gel method

Ceramic powders of BaCe_0.9Y_0.1O_2.95 (BCY10) have been prepared by the sol-gel method. Barium and yttrium acetate and cerium nitrate were used as ceramic precursors in a water solution. The reaction process studied by DTA-TG and XRD showed that calcination of the precursor powder at T ≥ 1000 °C produces a single perovskite phase. The densification behaviour of green compacts studied by constant heating rate dilatometry revealed that the shrinkage rate was maximal at 1430 °C. Sintered densities higher than 95% of the theoretical one were thus obtained below 1500 °C. The bulk and additional blocking effects were characterized by impedance spectroscopy in wet atmosphere between 150 and 600 °C. A proton conduction behaviour was clearly identified. The blocking effect can be related to a space-charge depletion layer of protons in the vicinity of grain boundaries.     

Evaluation of fuel diversity in Solid Oxide Fuel Cell system

Operability of Solid Oxide Fuel Cell (SOFC) on numerous fuels has been widely counted as a leading advantage in literature. In a designed system, however, switching from a fuel to another is not practically a straightforward task as this causes several system performance issues in both dynamic and steady-state modes. In order to demonstrate the system fuel diversity capabilities, these consequences must be well-evaluated by quantifying the characteristic measures for numerous fuel cases and also potential combinations. From this viewpoint, the numerical predictive models play a critical role. This paper aims to investigate the performance of a SOFC system fed by various fuels using a demonstrated system level model. Process configuration and streams results of a real-life SOFC system rig published in literature are used to validate the model. The presented model is capable not only of capturing the system performance measures but also the SOFC internal variable distributions, allowing the multiscale study of fuel switching scenarios. The fuel change impacts on the system are simulated by considering various fuel sources, i.e., natural gas, biogas, and syngas. Moreover, applications of simulated fuel mixtures are assessed. The modelling results show significant concerns about fuel switching in a system in terms of variation of efficiencies, stack internal temperature and current density homogeneity, and environmental issues. Moreover, the results reveal opportunities for multi-fuel design to address the operation and application requirements such as optimisation of the anode off-gas recycling rate and the thermal-to-electrical ratio as well as the system specific greenhouse gases, i.e., g-CO_x/Wh release.     

Structure, chemical stability and mixed proton-electron conductivity in BaZr0.9−xPrxGd0.1O3−δ

BaZr_0.9−xPr_xGd_0.1O_3−δ (x = 0.3 and 0.6) was prepared by combustion synthesis and characterised with respect to conductivity and stability in an attempt to combine the desirable properties of the end members. The polycrystalline materials exhibit a cubic or pseudo-cubic structure as determined by X-ray synchrotron radiation and transmission electron microscopy. The chemical stability of the compositions is strongly dependent on the praseodymium content, the materials with more Pr present lower stability. Electron holes dominate the conductivity under oxidising atmospheres in BaZr_0.3Pr_0.6Gd_0.1O_3−δ, while BaZr_0.6Pr_0.3Gd_0.1O_3−δ exhibits a mixed electron hole-proton conducting behaviour. Substitution of Zr by Pr in acceptor-doped BaZrO₃ decreases the sintering temperature and increases the grain growth rate.     

Performance of fuel cells with proton-conducting ceria-based composite electrolyte and nickel-based electrodes

A ceria-based composite electrolyte with the composition of Ce_0.8Sm_0.2O_1.9 (SDC)–30 wt.% (2Li₂CO₃:1Na₂CO₃) is developed for intermediate temperature fuel cells (ITFCs). Two kinds of SDC powders are used to prepare the composite electrolytes, which are synthesized by oxalate coprecipitation process and glycine–nitrate process, respectively, and denoted as SDC(OCP) and SDC(GNP). Based on each composite electrolyte, two single cells with the electrolyte thickness of 0.3 and 0.5 mm are fabricated by dry-pressing technique, using nickel oxide as anode and lithiated nickel oxide as cathode, respectively. With H₂ as fuel and air as oxidant, all the four cells exhibit excellent performances at 400–600 °C, which can be attributed to the highly ionic conducting electrolyte and the compatible electrodes. The cell performance is influenced by the SDC morphology and the electrolyte thickness. More interestingly, such composite electrolytes are found to be proton conductors at intermediate temperature range for the first time since almost all water is observed at the cathode side during fuel cell operation for all cases. The unusual transport property, excellent cell performance and potential low cost make this kind of composite material a good candidate electrolyte for future cost-effective ITFCs.     

固体氧化物燃料电池的神经模糊控制策略研究

固体氧化物燃料电池(Solid Oxide Fuel Cell,SOFC)具有多输入多输出、强耦合的特点,为了使其输出电压稳定设计了高效控制器,采用神经模糊控制方法对其输出电压进行控制。通过机理分析和实验数据拟合方法分别建立SOFC的机理模型和神经网络模型,在此基础上采用模糊控制策略对SOFC的输出电压进行控制,并应用神经模糊控制方法进一步提高了控制精度。通过MATLAB/Simulink仿真实验发现,SOFC神经网络模型得到的预测电压与实际电压之间的误差小于0.008 V,较其机理模型更加准确,所提出的控制策略能有效控制SOFC的输出电压。     

Mass transport limitation in inlet periphery of fuel cells: Studied on a planar Solid Oxide Fuel Cell

It was recently clarified on a microtubular Solid Oxide Fuel Cell (SOFC) that the range of mass transport limitation might commence from the inlet periphery (inlet opening and inlet pipe), i.e., the concentration gradient of reactants may extend inward the inlet periphery. For demonstrating that this phenomenon occurs regardless of the form and type of the fuel cell operating at high reactant utilization rate, herein we investigate the mass transport in the anode side of a one-cell stack of a planar SOFC. The investigation leans upon experimental and numerical data analyzed from both conventional (non spatial) and spatial perspectives. The experimental data were spatially obtained in the lateral direction by applying the segmentation method. Regarding analyses let us to confirm that mass transport limitation occurs in the inlet periphery of the planar stack. Besides, the critical ratio of the consumed/supplied mass fluxes of hydrogen is valid for assessing whether the concentration gradient of hydrogen extends inward the inlet periphery. Furthermore, the virtual inlet opening is useful for accurately calculating the mass transport within the active field of the stack via hypothetically preventing the mass transport limitation in the inlet periphery. These findings are expected to help researchers and engineers for accurately designing and characterizing fuel cell systems at varying scales from cells to stacks.     

Proton-conducting solid oxide fuel cells prepared by a single step co-firing process

Proton-conducting solid oxide fuel cells (SOFCs), consisting of BaCe_0.7In_0.3O_3−δ (BCI30)-NiO anode substrates, BCI30 anode functional layers, BCI30 electrolyte membranes and BCI30-LaSr₃Co_1.5Fe_1.5O_10−δ (LSCF) composite cathode layers, were successfully fabricated at 1150 °C, 1250 °C and 1350 °C respectively by a single step co-firing process. The fuel cells were tested with humidified hydrogen (∼3%H₂O) as the fuel and static air as the oxidant. The single cell co-fired at 1250 °C showed the highest cell performance. The impedance studies revealed that the co-firing temperature affected the interfacial polarization resistance of a single cell as well as its overall electrolyte resistance.     

Chemical durability of Solid Oxide Fuel Cells: Influence of impurities on long-term performance

Because of the fuel flexibility of Solid Oxide Fuel Cells (SOFCs), various types of fuels may be applied directly or via a simple reforming process, including hydrocarbons, alcohols, coal gas, biogas, besides hydrogen. However, various types of minor constituents in practical fuels and/or from the system components can cause chemical degradation of SOFCs, such as anode and cathode poisoning phenomena. In this study, we compare the influence of various external impurities, including sulfur, chlorine, phosphorus, boron, and siloxane for anodes, and H₂O and SO₂ for cathodes, on SOFC performance to have a general overview on long-term chemical durability of SOFCs. Chemical compatibility of Ni with foreign species has also been thermochemically considered. Using common model cells, the stability of cell voltage, electrode overpotential, and ohmic loss up to 3000 h has been experimentally examined for H₂-based fuels, for hydrocarbon-based fuels, and for partially pre-reformed CH₄-based fuels. Increase in degradation rate by impurities was verified for various operational parameters. Impurity poisoning mechanisms are discussed for each specific impurity.     

Fe-based perovskites as electrodes for intermediate-temperature solid oxide fuel cells

A solid-oxide fuel cell (SOFC) based upon Fe perovskites, has been designed and tested. Materials with nominal compositions Sr_0.9K_0.1FeO_3−δ (SKFO) and Sr_1.6K_0.4FeMoO_6−δ (SKFMO) with perovskite structure have been prepared and characterized as cathode and anode, respectively. The anode material exhibits high electrical conductivity values of 407-452 S cm⁻¹ at 750-820 °C in pure H₂. In the test cells, the electrodes were supported on a 300-μm-thick pellet of the electrolyte La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM). The single SOFC cells gave a maximum power density at 850 °C of 937 mW cm⁻² with pure H₂ as a fuel. Sizeable power densities were also observed with alternative fuels: 694 and 499 mW cm⁻² with H₂ containing 5 parts per million of H₂S and CH₄, respectively, at 800 °C. Moreover, only a slight degradation of about 3.6% of the power density has been obtained after 65 different cycles of fuel-cell test in H₂ at 750 °C and 14% at 850 °C in 50 cycles using H₂-H₂S. This remarkable behavior has been correlated to the structural features determined in a neutron powder diffraction experiment in the usual working conditions of a SOFC for a cathode (air) and an anode (low pO₂). On the one hand, the cubic Pm-3m Sr_0.9K_0.1FeO_3−δ cathode material is an oxygen deficient perovskite with oxygen contents that vary from 2.45(2) to 2.26(2) from 600 to 900 °C and high oxygen isotropic thermal factors (4.17(8) Å²) suggesting a high ionic mobility. On the other hand, the actual nature of the anode of composition Sr_1.6K_0.4FeMoO_6−δ has been unveiled by neutron powder diffraction to consist of two main perovskite phases with the compositions SrMoO₃ and SrFe_0.6Mo_0.4O_2.7. The association of two perovskites oxides, SrMoO₃ with high electrical conductivity, and SrFe_0.6Mo_0.4O_2.7 with mixed ionic-electronic conductivity has resulted in an extraordinarily performing anode material for SOFCs.     

Simulation and exergy analysis of a hybrid Solid Oxide Fuel Cell (SOFC)–Gas Turbine System

The simulation and exergy analysis of a hybrid Solid Oxide Fuel Cell–Gas Turbine (SOFC–GT) power system are discussed in this paper. In the SOFC reactor model, it is assumed that only hydrogen participates in the electrochemical reaction and that the high temperature of the stack pushes the internal steam reforming reaction to completion; the unreacted gases are assumed to be fully oxidized in the combustor downstream of the SOFC stack. Compressors and GTs are modeled on the basis of their isentropic efficiency. As regards the heat exchangers and the heat recovery steam generator, all characterized by a tube-in-tube counterflow arrangement, the simulation is carried out using the thermal efficiency-NTU approach. Energy and exergy balances are performed not only for the whole plant but also for each component in order to evaluate the distribution of irreversibility and thermodynamic inefficiencies. Simulations are performed for different values of operating pressure, fuel utilization factor, fuel-to-air and steam-to-fuel ratios and current density. Results showed that, for a 1.5 MW system, an electrical efficiency close to 60% can be achieved using appropriate values of the most important design variables; in particular, the operating pressure and cell current density. When heat loss recovery is also taken into account, a global efficiency of about 70% is achieved.     

Relationships between processing,morphology and discharge capacity of the composite electrode

The cycled capacity of Li_1.1V₃O₈ based positive electrodes varies between 100 and 250 mAh g⁻¹ (C/5 rate, 3.3–2 V) depending on the processing parameters. The initial volatile solvent concentration has a strong impact on the distribution of the electrode constituents. For a concentration below the optimal one, the mechanical energy available for mixing is insufficient to overcome viscosity forces and to reach a good dispersion of the constituents in the bulk of the electrode. Above the optimal concentration, settling of the Li_1.1V₃O₈ and carbon black particles in the low viscosity suspensions creates a concentration gradient. In these two cases the electrochemical performance are degraded. The viscosity of the electrode slurry must be systematically adjusted since the grain size and density depend on the active material.     

Exsolution in Ni-doped lanthanum strontium titanate: a perovskite-based material for anode application in ammonia-fed Solid Oxide Fuel Cell

Hydrogen represents the most conventional fuel to feed Solid Oxide Fuel Cells (SOFCs) for green energy production. However, hydrogen has some drawbacks which prevent the large-scale implementation. Research identified ammonia as promising hydrogen vector. Hereby, highly dispersed Ni nanoparticle are deposited on La-doped strontium titanate by exsolution, greatly affecting the electrochemical performance. The exsolved Ni-doped lanthanum strontium titanate (La_0·45Sr_0·45Ti_0·90Ni_0.10-δO₃ – LSTNOH) was largely characterized. XRD analysis detected 10 mol% of Ni doping has been successfully incorporated in to the perovskite structure and then released when exposed in reducing environment. SEM images show Ni nanoparticles highly dispersed on the surface. XPS confirms the presence of Ni on the surface after the exsolution and allows to exclude other detrimental diffusion towards the bulk. A LSTNOH derived composite based anode has been investigated through impedance spectroscopy using ammonia and hydrogen as fuel. It demonstrates best performances compared to the one obtained by Ni infiltration on LSTO (La_0·45Sr_0·45TiO₃) composite scaffold. Polarization resistance, running on ammonia, decreases raising the temperature and the performances approach those in hydrogen.     

Influence of silica morphology in composite Nafion membranes properties

The introduction of inorganic compounds into the Nafion polymeric matrix represents a possible solution to increase the operative temperature (T > 100 °C) without losing the conductivity, water transport properties and above all improving the mechanical characteristics. Two silica materials with different morphologies were synthesized via sol-gel method (SiO₂ and SBA-16) by a tetraethyl orthosilicate (TEOS, 98%, Aldrich).Composite Nafion membranes were prepared using a 3%wt/wt of each powder and a standardized casting method. The influence of morphology on chemical-physical properties of membranes was investigated. It was highlighted that the silica introduction reduces the swelling parameters compared to bare Nafion membrane (NRecast) at room temperature. A similar behaviour was observed also in the proton conductivity measurements, in fact values of 0.144 S/cm, 0.114 S/cm and 0.078 S/cm were recorded at 80 °C for NRecast, NSBA-16 and NSiO₂ respectively. Polarisation curves carried out at 100 °C have revealed a better performance for composite membranes (NSBA-16) than NRecast with a similar trend of proton conductivity.     

Effect of Ba doping on performance of LST as anode in solid oxide fuel cells

The level of barium doping in lanthanum strontium titanate (La_0.4Sr_0.6−xBa_xTiO₃, 0 ≤ x ≤ 0.2; LST, x = 0; LSBT, x > 0), prepared by solid state synthesis, affects its performance as anode in solid oxide fuel cells (SOFCs). Cell structures of LST and all LSBT were similar. The oxidation state of Ti in all compounds was reduced by a comparable amount when LST or LSBT was heated under reducing conditions to form La_0.4Sr_0.6−xBa_xTi_0.59⁴⁺Ti_0.41³⁺O_2.97. All fuel cells using LST or LSBT had high activity for conversion of hydrogen or methane, and the activity increased with the level of substitution by Ba. In addition, performance was enhanced when H₂S was present in either CH₄ or H₂ fuel. There was good contact between YSZ electrolyte and each LSBT or LSB anode.     

Experimentally validated numerical modeling of Eu doped SrCeO3membrane for hydrogen separation

A numerical solution to the defect chemical equations was used to model the defect population in europium-doped strontium cerate (ESC) at vapor partial pressure and oxygen partial pressure range in hydrogen atmosphere. The results of the simulation compared well with the work previously reported in the literature. The numerically simulated defect concentrations were then used to predict the conductivity and hydrogen permeability of ESC membranes as a function of temperature. Uniquely, the model was then validated by comparing the predictions with experimental data for ESC membranes. The results of that exercise showed that the model is in good agreement with the experiment at temperatures high enough that the effects of defect interaction can be ignored; and where the assumption of a dilute solution of defects is valid. The agreement with the experiment further enabled the model to be used to obtain credible predictions for the ambipolar conductivity of ESC and hydrogen flux through ESC as a function of feed side hydrogen partial pressure.     

Transport, sensitivity, and dimensional optimization studies of a tubular Solid Oxide Fuel Cell

In this paper, the impact of the design and operating parameters of a tubular Solid Oxide Fuel Cell (SOFC) is studied using a well-validated steady-state model. The profiles of species concentrations and pressure in the flow channels when the cell terminal voltage is changed are studied. In addition, both the axial and radial profiles of the species concentrations inside the electrodes are presented. The model is also used to study the effects of the parameters that can significantly influence the design criteria of an anode-supported tubular SOFC. The effects of the flowrate of H₂, inlet pressure, and the cell temperature on the power output from the cell are studied. Cell characteristic parameters such as the porosity of the electrodes, effective diffusivity of the species, and the rate of the electrochemical reactions are varied and their impact on the cell performance is observed. The influence of the cell design parameters such as the thickness of the electrodes and the electrolyte on the steady-state polarization curve are also studied. Finally, a dimensional study is presented. In the dimensional study, the radius of the anode flow channel, the length of the cell, and the annulus size are varied keeping the solid volume and the total cell volume constant. This study shows that it is possible to design guidelines for the optimum performance of the cell.     

Photo-electrochemical properties of oxide semiconductors on porous titanium metal electrodes

For the purpose of producing hydrogen using solar energy, we investigated the potential of porous titanium metal sheet (PTMS) with high surface area for use as the basal plates for various types of oxide semiconductor photo-electrodes. The TiO₂ photoelectrodes were prepared by oxidation of PTMS and flat titanium metal sheet (FTMS). The photocurrents of the TiO₂/PTMS electrodes were always higher than TiO₂/FTMS under the same oxidation conditions. The reflectance of PTMS was lower than FTMS over the entire wavelength spectrum, suggesting that the scattered light was absorbed more effectively on the former. A nanocrystalline WO₃ layer-loaded PTMS electrode (WO₃/PTMS) showed a high photocurrent compared to WO₃/FTMS, suggesting that PTMS is highly suitable as basal plates for semiconductor photoelectrodes.     

Local structural arrangements around oxygen and hydrogen-related defects in proton conducting LaP3O9 investigated by first principles calculations

Local structural arrangements and stabilities of oxygen and hydrogen-related defects in proton-conducting LaP₃O₉ were studied using first principles calculations. When an oxygen was removed from LaP₃O₉, the crystal lattice was significantly distorted. Resulting structural arrangements considerably depended on the oxygen deficient site, and phosphate ions tended to condense by sharing a corner oxygen of PO₄ tetrahedra. On the other hand, when a proton was introduced, the proton was located at the interstitial sites positioned approximately 1 Å away from the nearest oxygen forming an O–H bond. The LaP₃O₉ lattice was only slightly distorted even after introduction of an interstitial proton. Based on the calculation results, the stabilities of the defects under moisturized conditions was discussed.