Thermal field in SOFC fed by hydrogen: Inlet gases temperature effect

In the present work, the effect of the hydrogen and the air temperature values on the temperature distribution in a Planar Solid Oxide Fuel Cell is studied by the aid of a two-dimensional mathematical model. Two different configurations of the Solid Oxide Fuel Cells are examined: i) the Anode Supported Planar Solid Oxide Fuel Cell (ASP_SOFC) and ii) the Electrolyte Supported Planar Solid Oxide Fuel Cell (ESP_SOFC). In order to describe the temperature distribution within the SOFC, the coupling of the mass and energy transport phenomena along with the electrochemistry is required. The studied parameters are: a) the hydrogen and the air temperature values and b) the geometry configurations. The complex system of the governing equations is numerically solved with the finite differences method and the calculation of the temperature distribution within each domain of the SOFCs is calculated via the 2-D mathematical model processed by FORTRAN language. Finally, the mathematical model predictions for the temperature distribution under the influence of the studied parameters are thoroughly discussed.     

Metal-Ci oxygen-evolving catalysts generated in situ in a mild H2O/CO2 environment

Under mild conditions (near-neutral aqueous solutions, room temperature, and atmospheric pressure), an attractive electrolyte was expanded for splitting water and three novel oxygen-evolution catalysts (Co-C_i, Ni-C_i, Ag-C_i) were generated. The Co-C_i catalyst showed a high oxygen-evolution rate (24.42 μmol h⁻¹), a low oxygen-evolution overpotential (395 mV) and excellent stability. In addition, the non-amorphous Ag-C_i catalyst, different from reported catalyst, was generated. The catalyst has been further characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and X-ray photoelectron spectroscopy (XPS).     

Two-dimensional numerical study of temperature field in an anode supported planar SOFC: Effect of the chemical reaction

In the present work the effect of the chemical reaction on the temperature field in an anode supported planar SOFC is numerically studied by the aid of a two-dimensional mathematical model. For the model development the mass transport phenomena, the energy conservation, the species flow governed by Darcy’s law and the electrochemistry are coupled. The finite difference method is used to solve numerically the system of the equations.The temperature field within each component of the SOFC (interconnection, cathode, anode and electrolyte) is calculated via the mathematical model which is implemented in FORTRAN language. The model results demonstrate the effect of different expressions of the chemical heat source, expressed in terms of enthalpy and entropy, on the temperature field and the location of the higher temperatures that occur within the SOFC during its operation.     

H2O chemisorption and H2 oxidation on yttria-stabilized zirconia: Density functional theory and temperature-programmed desorption studies

The mechanism of H₂O dissociation as well as the adsorption and oxidation reaction of H₂ on yttria-stabilized zirconia (YSZ), commonly used as part of solid oxide fuel cell (SOFC) anodes, was investigated employing temperature-programmed desorption (TPD) spectroscopy and density functional theory (DFT). In agreement with theory the experimental results show that interaction of gaseous H₂O with YSZ results in dissociative adsorption leading to strongly bound OH surface species. In the interaction of gaseous H₂ with an oxygen-enriched YSZ surface (YSZ + O) similar OH surface species are formed as reaction intermediates in the H₂ oxidation. Our experiments showed that in both the H₂O/YSZ and the H₂/YSZ + O heterogeneous reaction systems noticeable amounts of H₂O are “dissolved” in the bulk as interstitial hydrogen and hydroxyl species. The experimental H₂O desorption data is used to access the accuracy of the H₂/H₂O/YSZ adsorption/desorption and surface reaction kinetics data, employed in previous modeling studies of the electrochemical H₂ oxidation on Ni-pattern/YSZ model anodes by Vogler et al. [J. Electrochem. Soc., 156 (2009) B663] and Goodwin et al. [J. Electrochem. Soc., 156 (2009) B1004]. Finally a refined experimentally validated H₂/H₂O/YSZ adsorption/desorption and surface reaction kinetics data set is presented.     

Analysis and performance assessment of NH3 and H2 fed SOFC with proton-conducting electrolyte

The present paper investigates the performance of a solid oxide fuel cell based on proton-conducting electrolyte (SOFC-H⁺) using one-dimensional steady-state model. The analysis covers a detailed electro-chemical model for H₂ and NH₃ fuels. The direct internal reforming of NH₃ is examined, and the effects of some operating parameters (e.g. temperature, pressure, fuel utilization and oxidant utilization) on the reversible cell potential are investigated. In addition, the overpotentials (including activation, ohmic and concentration) are calculated to study the irreversible behavior of the SOFC-H⁺ with some actual data operating conditions and material properties taken from the literature. In addition, effects of some operation and structural parameters on cell performance were examined. The present results indicate that the activation and the ohmic losses are considerable. The concentration overpotential at the anode side is negligible due to the fact that H₂O is produced at the cathode side. The maximum power density is calculated as 3212 and 3113 W/m² at 1073 K and 1 atm for the fuels of H₂ and NH₃. The results further show that H₂ provides better performance than NH₃ at the same partial pressure. Moreover, NH₃ is an excellent hydrogen carrier which is a potential candidate for SOFC-H⁺ due to its high hydrogen content and considerable cell performance.     

Temperature gradients measurements within a segmented H2/air PEM fuel cell

A commercial Nafion 112 membrane loaded with the catalysts and catalysts’ support in a segmented way was used for a H₂/air PEM fuel cell. On this membrane, the anodic and cathodic catalysts with their support were loaded on five consecutive places in a back to back style forming five catalyst islands of same dimensions on each side of the membrane. So, five sub-fuel cells were existed in one fuel cell compartment. These subcells are connected ionically but not electronically. The polarization behavior for these subcells was measured separately when the other subcells were at a zero load and simultaneously when they were polarized at the same load. Also, the temperature gradient within this segmented PEM fuel cell was measured in front of cathode side of the sub-fuel cells when hydrogen and air gases were flown in a parallel or opposite direction to each other. Temperature gradients were correlated with the observed performance of the fuel cell.     

Cycle analysis of low and high H2 utilization SOFC/gas turbine combined cycle for CO2 recovery

A major factor in global warming is CO₂ emission from thermal power plants, which burn fossil fuels. One technology proposed to prevent global warming is CO₂ recovery from combustion flue gas and the sequestration of CO₂ underground or near the ocean bed. Solid oxide fuel cell (SOFC) can produce highly concentrated CO₂, because the reformed fuel gas reacts with oxygen electrochemically without being mixed with air in the SOFC. We therefore propose to operate multi-staged SOFCs with high utilization of reformed fuel to obtain highly concentrated CO₂. In this study, we estimated the performance of multi-staged SOFCs considering H₂ diffusion and the combined cycle efficiency of a multi-staged SOFC/gas turbine/CO₂ recovery power plant. The power generation efficiency of our CO₂ recovery combined cycle is 68.5%, whereas the efficiency of a conventional SOFC/GT cycle with the CO₂ recovery amine process is 57.8%.     

Hydrogen permeation through a Pd-based membrane and RWGS conversion in H2/CO2, H2/N2/CO2 and H2/H2O/CO2 mixtures

This study investigated the effect of gases such as CO₂, N₂, H₂O on hydrogen permeation through a Pd-based membrane −0.012 m² – in a bench-scale reactor. Different mixtures were chosen of H₂/CO₂, H₂/N₂/CO₂ and H₂/H₂O/CO₂ at temperatures of 593–723 K and a hydrogen partial pressure of 150 kPa. Operating conditions were determined to minimize H₂ loss due to the reverse water gas shift (RWGS) reaction. It was found that the feed flow rate had an important effect on hydrogen recovery (HR). Furthermore, an identification of the inhibition factors to permeability was determined. Additionally, under the selected conditions, the maximum hydrogen permeation was determined in pure H₂ and the H₂/CO₂ mixtures. The best operating conditions to separate hydrogen from the mixtures were identified.     

The study of Au/MoS2 anode catalyst for solid oxide fuel cell (SOFC) using H2S-containing syngas fuel

Au/MoS₂ is a promising anode catalyst for conversion of all components of H₂S-containing syngas in solid oxide fuel cell (SOFC). MoS₂-supported nano-Au particles have catalytic activity for conversion of CO when syngas is used as fuel in SOFC systems, thus preventing poisoning of MoS₂ active sites by CO. In contrast to use of MoS₂ as anode catalyst, performance of Au/MoS₂ anode catalyst improves when CO is present in the feed. Current density over 600 mA cm⁻² and maximum power density over 70 mW cm⁻² were obtained at 900 °C, showing that Au/MoS₂ could be potentially used as sulfur-tolerant catalyst in fuel cell applications.     

2D thermal modeling of a solid oxide electrolyzer cell (SOEC) for syngas production by H2O/CO2 co-electrolysis

Solid oxide fuel cells (SOFCs) can be operated in a reversed mode as electrolyzer cells for electrolysis of H₂O and CO₂. In this paper, a 2D thermal model is developed to study the heat/mass transfer and chemical/electrochemical reactions in a solid oxide electrolyzer cell (SOEC) for H₂O/CO₂ co-electrolysis. The model is based on 3 sub-models: a computational fluid dynamics (CFD) model describing the fluid flow and heat/mass transfer; an electrochemical model relating the current density and operating potential; and a chemical model describing the reversible water gas shift reaction (WGSR) and reversible methanation reaction. It is found that reversible methanation and reforming reactions are not favored in H₂O/CO₂ co-electrolysis. For comparison, the reversible WGSR can significantly influence the co-electrolysis behavior. The effects of inlet temperature and inlet gas composition on H₂O/CO₂ co-electrolysis are simulated and discussed.     

Photoreduction of carbon dioxide with H2 and H2O over TiO2 and ZrO2 in a circulated photocatalytic reactor

The photocatalytic reduction of carbon dioxide (CO₂) was studied in a self-designed circulated photocatalytic reaction system under titanium dioxide (TiO₂, Degussa P-25) and zirconium oxide (ZrO₂) photocatalysts and reductants at room temperature and constant pressure. The wavelengths of incident ultraviolet (UV) light for the photocatalysis of TiO₂ and ZrO₂ were 365 and 254 nm, respectively. Experimental results indicated that the highest yield of the photoreduction of CO₂ were obtained using TiO₂ with H₂+H₂O and ZrO₂ with H₂. Photoreduction of CO₂ over TiO₂ with H₂+H₂O formed CH₄, CO, and C₂H₆ with the yield of 8.21, 0.28, and 0.20 μmol/g, respectively, while the photoreduction of CO₂ over ZrO₂ with H₂ formed CO at a yield of 1.24 μmol/g. The detected reaction products supported the proposition of two reaction pathways for the photoreduction of CO₂ over TiO₂ and ZrO₂ with H₂ and H₂O, respectively. Additionally, a one-site Langmuir-Hinshewood (L-H) kinetic model was successfully applied to simulate the photoreduction rate of CO₂.     

Effect of process modification and presence of H2O2 in the synthesis of samaria-doped ceria powders for fuel cell applications

A fundamental step for a sustainable industrial development based on “H₂ Economy” is the implementation of fuel cell technology, in terms of new devices, materials and convenient processes for their production. Rare earth doped ceria oxides are suitable materials for the new generations of cells and their cost effective production becomes fundamental as the price of rare earths is increasing. In this view, our study investigates a modified method of co-precipitation of Ce_0.8Sm_0.2O_1.9−x (SDC) evaluating the effects of adding of H₂O₂ in the process. The parameters controlled were the molar ratio [H₂O₂]/[M³⁺], (M³⁺ = Ce³⁺, Sm³⁺ present in starting nitrate salts solutions) and the pH of precipitation; in some cases the precipitates were also treated under reflux at 373 K overnight. The powder catalysts, both as fresh precipitates and calcined oxides were analyzed via N₂ adsorption (BET), X-Ray diffraction (XRD) and temperature programmed reduction (TPR) techniques and their morphological, structural and redox properties were correlated with the synthesis parameters used. The electrical conductivity properties of these materials have also been investigated via electrochemical impedance spectroscopy (EIS) and the results compared with those of a commercial oxide. The synthesis approach was shown to be very versatile in the development of materials with properties exploitable for applications in catalysis and in intermediate temperature Solid Oxide Fuel Cell (IT-SOFC) systems.     

Synthesis and electrochemical behaviour of ceria-substitution LSCM as a possible symmetric solid oxide fuel cell electrode material exposed to H2 fuel containing H2S

12.5 wt.% ceria-substituted on the A-sites of La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) for La_0.75Sr_0.125 Ce_0.125Cr_0.5Mn_0.5O_3−δ (LSCCM) has been synthesized by the sol–gel process and evaluated as the electrode materials of symmetric solid oxide cells. The orthorhombic perovskite-type structure was demonstrated using X-ray diffraction (XRD) and part of cerium has been successfully doped to LSCM and presents two valence states (+3 and +4). In addition, the surface adsorption oxygen content increases due to ceria-doping using X-ray photoelectron spectroscopy (XPS). The measured electrical conductivity shows that the addition of ceria yields increase in total conductivity in air and humidified H₂. Electrochemical performance test of yttria-stabilized zirconia (YSZ) electrolyte-supported symmetric solid oxide fuel cell with the configuration of LSCCM|YSZ|LSCCM was performed, and shows peak power density of 33.12 mW cm⁻² at 1173 K when operating in wet 3% H₂–1% H₂S, far greater than the one of LSCM in the same test conditions.     

Kinetics of the NH reaction with H2 and reassessment of HNO formation from NH + CO2, H2O

The reaction of ground-state NH with H₂ has been studied in a high-temperature photochemistry (HTP) reactor. The NH(X³Σ) radicals were generated by the 2-photon 193 nm photolysis of NH₃, following the decay of the originally produced NH(A³Π) radicals. Laser-induced fluorescence on the transition at 336 nm was used to monitor the progress of the reaction. We obtained , with ±2σ precision limits varying from 12 to 33% and corresponding accuracy levels from 23 to 39%. This result is in excellent agreement with that of Rohrig and Wagner [Proc. Combust. Inst. 25 (1994) 975] and the data sets can be combined to yield . Starting with this agreement, it is argued that their rate coefficients for NH + CO₂ could not be significantly in error [Proc. Combust. Inst. 25 (1994) 975]. This, combined with models of several combustion systems, indicates that HNO + CO cannot be the products, contrary to their suggestion [Proc. Combust. Inst. 25 (1994) 975]. Ab initio calculations have been performed which confirm this conclusion by showing the barriers leading to these products to be too high compared to the measured activation energies. The calculations indicate the likelihood of formation of adducts, of low stability. These then may undergo further reactions. The NH + H₂O reaction is briefly discussed and it is similarly argued that HNO + H₂ cannot be the products, as had been previously suggested.     

Reactivity between La(Sr)FeO3 cathode,doped CeO2 interlayer and yttria-stabilized zirconia electrolyte for solid oxide fuel cell applications

Detailed X-ray diffraction (XRD) analysis of two different Sr-doped LaFeO₃ cathodes, YSZ electrolyte and two Sm/Gd-doped CeO₂ interlayer and their mixtures were used to evaluate the formation of undesired secondary reaction compounds. The analysis of room temperature X-ray diffraction data of the mixtures indicates the crystallization of strontium and/or lanthanum zirconates between the cathode and the electrolyte materials and no detected reaction between the cathode and the interlayer materials.     

Influence of pyrazole derivatives in I/I3 redox electrolyte solution on Ru(II)-dye-sensitized TiO2 solar cell performance

The influence of pyrazole additives in an I⁻/I₃⁻ redox electrolyte solution on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized TiO₂ solar cell was studied. The current–voltage characteristics of the cell were measured using 18 different pyrazole derivatives. All of the pyrazole additives enhanced the open-circuit photovoltage (V_oc) and the solar energy conversion efficiency (η), but reduced the short-circuit photocurrent density (J_sc). Most of the pyrazoles improved fill factor (ff). The physical and chemical properties of the pyrazoles were computationally calculated in order to elucidate the reasons for the additive effects on cell performance. The greater the partial charge of the nitrogen atom at position 2 in the pyrazole group, the larger the V_oc, but the smaller the J_sc values. As the dipole moment of the pyrazole derivatives increased, the V_oc value increased, but the J_sc value decreased. The V_oc of the cell also increased as the ionization energy of the pyrazoles decreased. These results suggest that the electron donicity of the pyrazole additives affected the interaction with the nanocrystalline TiO₂ photoelectrode, the I⁻/I₃⁻ electrolyte, and the acetonitrile solvent, which changed the Ru(II)-dye-sensitized solar cell performance.     

Balancing the electron conduction and mass transfer: Effect of nickel foam thickness on the performance of an alkaline direct ethanol fuel cell (ADEFC) with 3D porous anode

Nickel foam has been applying as the electrode support material for alkaline direct ethanol fuel cells, since its unique three-dimensional network structure helps efficiently use the catalyst to improve the cell performance. In this work, the effect of the thickness of nickel foam electrodes on cell performance is investigated. The experimental results show that the nickel foam thickness influences both the electron conduction and mass transfer, and the optimal thickness is a trade-off between them. Through XRD, SEM image, polarization curve test, EIS test and CV test, it is found that the nickel foam electrode with the thickness of 0.6 mm has better performance than that of 0.3 mm and 1.0 mm. The thinner the nickel foam, the better the conductivity. However, the corresponding three-dimensional space becomes narrower, which leads to partial agglomeration of the catalyst and hindrance of mass transfer. In addition, the influence of catalyst loading on the performance of 0.6 mm nickel foam electrode is explored. The maximum power density of 1.0 mg cm⁻² Pd loading reaches 56.3 mW cm⁻² at 60 °C, which is higher than that of 2.0 mg cm⁻² loading, indicating that the three-dimensional network structure of nickel foam can efficiently utilize the catalyst, and fully exert the catalytic function of the catalyst even at a lower catalyst loading. Moreover, the effects of operating temperature and ethanol concentration on cell performance are also studied. The cell performance increases with the increase of temperature, and it reaches the highest with 3 M ethanol.     

Influence of aminothiazole additives in I/I3 redox electrolyte solution on Ru(II)-dye-sensitized nanocrystalline TiO2 solar cell performance

The influence of aminothiazole additives in acetonitrile solution of an I⁻/I₃⁻ redox electrolyte on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′- bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized TiO₂ solar cell was studied. The current–voltage characteristics were investigated under AM 1.5 (100 mW/cm²) for nine different aminothiazole compounds. The aminothiazole additives tested had varying influences on the solar cell performance. Most of the additives enhanced the open-circuit photovoltage (V_oc), but reduced the short circuit photocurrent density (J_sc) of the solar cell. Both the physical and chemical properties of the aminothiazoles were computationally calculated in order to determine the reasons that the additive influenced solar cell performance. The larger the calculated partial charge of the nitrogen atom in the thiazole, the higher the V_oc value. The V_oc value increased as the dipole moment of aminothiazoles in acetonitrile increased. Moreover, the V_oc of the solar cell also increased as the size of the aminothiazole molecules decreased. These results suggest that the electron donicity of the aminothiazole additives influenced the interaction with the TiO₂ photoelectrode, which altered the dye-sensitized solar cell performance.     

The effects of hydrogen sulfide on the polymer electrolyte membrane fuel cell anode catalyst: H2S-Pt/C interaction products

The performance of a polymer electrolyte membrane fuel cell (PEMFC) operating on a simulated hydrocarbon reformate is described. The anode feed stream consisted of 80% H₂, ∼20% N₂, and 8 ppm hydrogen sulfide (H₂S). Cell performance losses are calculated by evaluating cell potential reduction due to H₂S contamination through lifetime tests. It is found that potential, or power, loss under this condition is a result of platinum surface contamination with elemental sulfur. Electrochemical mass spectroscopy (EMS) and electrochemical techniques are employed, in order to show that elemental sulfur is adsorbed onto platinum, and that sulfur dioxide is one of the oxidation products. Moreover, it is demonstrated that a possible approach for mitigating H₂S poisoning on the PEMFC anode catalyst is to inject low levels of air into the H₂S-contaminated anode feeding stream.     

Experimental and numerical investigation of the effect of H2 enrichment on laminar methane-air flame thickness

The Rayleigh scattering technique has been applied to a V-shaped methane-air flame in order to investigate the effect of H₂ enrichment on the laminar flame thickness for a wide range of equivalence ratios (0.4 to 0.9) and a wide range of enrichment rates (from 10% to 40% in volume). From the Rayleigh scattering signal, the temperature is estimated using an original noniterative experimental-numerical coupled method where the variation of the cross section across the flame front is taken into account. The determination of temperature gradient is carried out using a statistical criterion and all data post-processing are validated by comparing experimental and numerical results. As expected, a decrease in flame thickness is observed when H₂ is added to methane-air flames. There is very good agreement between numerical and experimental results for equivalence ratios higher than 0.55. Below this value, a great discrepancy is observed and a large scatter between various chemical mechanisms can be observed. In the validated domain of GRImech3.0, the numerical results are used to give explanation elements concerning the decrease in the flame thickness when methane is H₂-enriched. From the energy equation analysis, two different mechanisms emerge. The first takes place at high temperature and leads to an increase of the global heat release rate magnitude. It mainly involved the chain branching reaction H + O₂ ? O + OH. The second takes place at low temperature and affect the preheat zone. It involves the reaction H + O₂(+ M) ? HO₂(+ M) and allows the methane to be oxidized at lower temperature and thus an earlier heat release.