Cu/MnOx–CeO2 and Ni/MnOx–CeO2 catalysts for the water–gas shift reaction: Metal–support interaction

Monometallic copper and nickel catalysts supported on cerium-manganese mixed oxides are prepared, characterized and evaluated for the Water–Gas Shift (WGS) reaction. Active metal loading of 2.5 wt% and 7.5 wt% are used to impregnate MnO_x–CeO₂ supports with 30% and 50% Mn:Ce molar ratio. The structure of the samples strongly depends on both the active metal employed and the manganese content in the mixed support. For both Cu and Ni samples, the best catalytic behavior is found in samples supported on the MnO_x–CeO₂ oxides with 30% Mn:Ce molar ratio, as a result of the presence of Cu_xMn_yO₄ spinel-type phases in the case of copper catalysts and the presence of a NiMnO₃ mixed oxide with defect ilmenite structure in the case of nickel catalysts.     

Effect of catalyst preparation on Au/Ce1−xZrxO2 and Au–Cu/Ce1−xZrxO2 for steam reforming of methanol

We tested 3 wt% gold (Au) catalysts on CeO₂–ZrO₂ mixed oxides, prepared by co-precipitation (CP) and the sol–gel (SG) technique, for steam reforming of methanol (SRM). Uniform Ce_1−xZr_xO₂ solid solution was dependent on the Zr/Ce ratio, where the incorporation of Zr⁴⁺ into the Ce⁴⁺ lattice with a ratio of 0.25 resulted in smaller ceria crystallites and better reducibility, and was found to be efficient for SRM activity. The catalytic activity was suppressed when the ratio was ≥0.5, which led to the segregation of Zr from solid solution and sintering of Au nanoparticles. It was found that the CP technique produced better catalysts than SG in this case. For the bimetallic catalysts, the co-operation of Au–Cu supported on Ce_0.75Zr_0.25O₂ (CP) exhibited superior activities with complete methanol conversion and low CO concentration at 350 °C. Furthermore, the size of the alloy particle was strongly dependent on the pH level during preparation.     

Dopant effect on hydrogen generation in two-step water splitting with CeO2–ZrO2–MOx reactive ceramics

The role played by the dopant in the H₂-generation step of two-step water splitting has been investigated with CeO₂–ZrO₂–MO_x (M = Mg, Ca, Sr, Ba, Sc, Y, Lu, La, Nd, Sm, Eu, Gd, Dy, Tm, Tb and Pr). The relationship between ionic radius, valence of the dopant, and oxidation ratio was investigated for its effect on H₂ yield. The oxidation ratio increases with an increase in the ionic radii. This tendency increases with an increase in the ionic valence (divalent < trivalent < tetravalent). This suggests that the surface process affects the chemical equilibrium of the reaction. The ionic conductivity measured by AC impedance spectroscopy showed that the increase in ionic conductivity speeds the reaction rate of H₂ generation. This indicates that a bulk diffusion process is the rate-determining step of H₂-generation reaction.     

Improved oxygen permeability of Ce0.8Sm0.2O2−δ–PrBaCo2O5+δ dual-phase membrane by surface-modifying porous layer

A porous PrBaCo₂O_5+δ or Ce_0.8Sm_0.2O_2−δ–50 vol.% PrBaCo₂O_5+δ (SDC–PBCO (5/5)) layer was deposited on dense Ce_0.8Sm_0.2O_2−δ–40 vol.% PrBaCo₂O_5+δ (SDC–PBCO (6/4)) membrane (450 μm) to enhance the oxygen permeability by increasing the surface area contacting with air. The oxygen permeation flux was measured in the temperature range of 825–945 °C. The results revealed that the oxygen permeation performance of Ce_0.8Sm_0.2O_2−δ–PrBaCo₂O_5+δ membranes can be significantly enhanced by coating SDC–PBCO (5/5) porous layer alone on the surface of feed side. The thickness of modification layer has obvious effect on the permeability of surface modified membrane. The modification on the feed side has much better effect than that on the permeate side. At 945 °C, the oxygen permeation flux of dense SDC–PBCO (6/4) membrane modified by porous SDC–PBCO (5/5) layer is 3.56 × 10⁻⁷ mol cm⁻² s⁻¹, 26% higher than that of the unmodified one.     

Optimization of the interface polarization of the La2NiO4-based cathode working with the Ce1–xSmxO2–δ electrolyte system

The performance of La₂NiO₄ cathode material and Ce_1–xSm_xO_2–δ (x = 0.1, 0.2, 0.3, 0.4) electrolyte system was analyzed. Ceria-based materials were prepared by the freeze-drying precursor route whereas La₂NiO₄ was prepared by the nitrate–citrate procedure. Electrolyte pellets were obtained after sintering the powders at 1600 °C for 10 h. Also dense ceria-based electrolytes samples were obtained by calcining the powders at 1150 °C after the addition of 2 mol%-Co. Interface polarization measurements were performed by impedance spectroscopy in air at open circuit voltage, using symmetrical cells prepared after the deposition of porous La₂NiO₄-electrodes on the Ce_1–xSm_xO_2–δ system. X-ray diffraction (XRD) of cathode materials after using in symmetrical cells confirmed no significant reaction between La₂NiO₄ and ceria-based electrolytes. The efficiency of the cathode material is highly dependent on the composition of the electrolyte, and low-content Sm-doped ceria samples revealed an important decrease in the performance of the system. Differences in electrochemical behaviour were attributed principally to the oxide ion transference between cathode and electrolyte, and were correlated to the conductivity of the electrolyte. In this way cobalt-doped electrolytes with a Sm-content ≤30% perform better than free-cobalt samples due to the increase in grain boundary conductivity. Finally, composites of the ceria-based materials and La₂NiO₄ to use as cathode were prepared and an important increase of the interface performance was observed compared to La₂NiO₄ pure cathode. Predictions of maximun power density were obtained by the mixed transport properties of the electrolytes and by the interface polarization results. The use of composite materials could allow to increase the performance of the cell from 170 mW cm⁻² for pure La₂NiO₄ cathode, to 370 mW cm⁻² for La₂NiO₄–Ce_0.8Sm_0.2O_2–δ cathode, both working with Ce_0.8Sm_0.2O_2–δ electrolyte 300 μm in thickness and Ni–Ce_0.8Sm_0.2O_2–δ as anode at 800 °C.     

Performance of double-perovskite Sr2−xSmxMgMoO6−δ as solid-oxide fuel-cell anodes

Double-perovskite Sr_2−xSm_xMgMoO_6−δ (SSMM, 0 ≤ x ≤ 0.8) is investigated as a possible anode material for solid-oxide fuel cells on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolytes. Single-phase SSMM samples with 0 ≤ x ≤ 0.4 are prepared. At x ≥ 0.6, a small amount of SrMoO₄ and Sm₂O₃ impurities are observed. The Mg/Mo ordering in SSMM decreases with increasing Sm content. Substitution of Sm for Sr significantly improves the electrical conductivity of SSMM. At x = 0.6, the sample yields the highest conductivity, with values reaching 16 S cm⁻¹ in H₂ at 800 °C. The maximum power densities of single cells achieved with x = 0.0, 0.2, 0.4, 0.6, and 0.8 anodes on a 300 μm-thick LSGM electrolyte are 693, 770, 860, 907, and 672 mW cm⁻², respectively, in H₂ at 850 °C. The SSMM sample with x = 0.4 is considered as the best anode candidate because of the impurity formation seen in x ≥ 0.6 samples. The x = 0.4 sample not only has a thermal-expansion coefficient closer to that of the LSGM electrolyte but also exhibits good electrochemical performance and stability in commercial city gas containing H₂S, where the maximum power density achieved is 726 mW cm⁻² at 850 °C.     

Chemical diffusivity and ionic conductivity of GdBaCo2O5+δ

The transport properties of layered perovskite GdBaCo₂O_5+δ (GBCO), which has recently been proposed as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs), are investigated as a function of oxygen partial pressure (OPP) over the oxygen partial pressure range of 10⁻⁴ ≤ pO₂ (atm) ≤ 0.21 at 1073 ≤ T (K) ≤ 1323. The increase in total conductivity with increasing temperature below the low-temperature, order-disorder transition indicates a semiconductor-type behaviour with an activation energy of 0.42 eV. When OPP is increased to air pressure at a fixed temperature, the total conductivity increases with an apparent slope (∂log σ/∂log pO₂) of 1/10 to 1/22. The maximum oxygen ion conductivity, as extracted from the oxygen permeation measurements, is around 0.01 S cm⁻¹ under the nitrogen condition, which strongly supports the potential for cathode application. The chemical diffusion coefficient () and surface exchange coefficient (κ) are also calculated from the d.c. conductivity relaxation measurement and the values are best fitted by the following two equations:

     

Synthesis and electrochemical properties of (Pr–Nd)1−ySryMnO3−δ and (Pr1−xNdx)0.7Sr0.3MnO3−δ as cathode materials for IT-SOFC

(Pr–Nd)_1−ySr_yMnO_3−δ (P-NSM, y = 0.2, 0.25, 0.3, 0.35) powders made from commercial Pr–Nd mixed oxide, as well as (Pr_1−xNd_x)_0.7Sr_0.3MnO_3−δ (PN3SM, x = 0, 0.5, 0.7, 1) were synthesized by a glycine-nitrate process and characterized as cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC). XRD patterns showed the powders had formed pure perovskite phase after being calcined at 800 °C for 2 h. (Pr–Nd)_0.7Sr_0.3MnO_3−δ (P-N3SM) achieved a high conductivity of 194 S cm⁻¹ at 500 °C and showed a good chemical stability against YSZ at 1150 °C. And the thermal expansion coefficient of P-N3SM/YSZ cathode was 11.1 × 10⁻⁶ K⁻¹, which well matched YSZ electrolyte film. The tubular SOFC with P-N3SM/YSZ cathode exhibited the maximum power densities of 415, 367, 327 and 282 mW cm⁻² at 850, 800, 750 and 700 °C, respectively, which indicated P-N3SM was potentially applied in SOFC for low cost.     

Adiabatic burning velocity of H2–O2 mixtures diluted with CO2/N2/Ar

Global warming due to CO₂ emissions has led to the projection of hydrogen as an important fuel for future. A lot of research has been going on to design combustion appliances for hydrogen as fuel. This has necessitated fundamental research on combustion characteristics of hydrogen fuel. In this work, a combination of experiments and computational simulations was employed to study the effects of diluents (CO₂, N₂, and Ar) on the laminar burning velocity of premixed hydrogen/oxygen flames using the heat flux method. The experiments were conducted to measure laminar burning velocity for a range of equivalence ratios at atmospheric pressure and temperature (300 K) with reactant mixtures containing varying concentrations of CO₂, N₂, and Ar as diluents. Measured burning velocities were compared with computed results obtained from one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics and good agreement was obtained. The effectiveness of diluents in reduction of laminar burning velocity for a given diluent concentration is in the increasing order of argon, nitrogen, carbon dioxide. This may be due to increased capabilities either to quench the reaction zone by increased specific heat or due to reduced transport rates. The lean and stoichiometric H₂/O₂/CO₂ flames with 65% CO₂ dilution exhibited cellular flame structures. Detailed three-dimensional simulation was performed to understand lean H₂/O₂/CO₂ cellular flame structure and cell count from computed flame matched well with the experimental cellular flame.     

O2/CO2和O2/N2氛围下煤粉富氧燃烧数值模拟

在目前煤炭依然作为能源主体的背景下，控制燃煤污染物排放有着重要意义。基于CFD数值模拟，建立伴流燃烧器模型，控制燃料、氧化剂入口流量恒定，设计了O₂/CO₂、O₂/N₂氧化剂氛围中O₂浓度在21%～40%内的多种工况，对煤粉燃烧特性及燃烧产生的污染物进行了研究。分析了不同工况下煤粉燃烧的温度分布、燃烧速率、碳烟、NO_x的生成情况。结果显示，在O₂/CO₂、O₂/N₂两种氧化剂氛围中，随着O₂浓度的上升，煤粉燃烧温度升高、燃烧速率增大，碳烟生成量均增加，同等O2浓度条件下，O₂/CO₂氛围的煤粉燃烧温度和燃烧速率均高于O₂/N₂氛围，碳烟生成量小于O₂/N₂氛围，且O₂/CO₂...     

Sr0.92Y0.08TiO3−δ/Sm0.2Ce0.8O2−δ anode for solid oxide fuel cells running on methane

Yttria-doped strontium titanium oxide (Sr_0.92Y_0.08TiO_3−δ; SYT) was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). The SYT synthesized by the Pechini method exhibits excellent phase stability during the cell fabrication processes and SOFC operation and good electrical conductivity (about 0.85 S/cm, porosity 30%) in reducing atmosphere. The performance of SYT anode is characterized by slow electrochemical reactions except for the gas-phase diffusion reactions. The cell performance with the SYT anode running on methane fuel was improved about 5 times by SDC film coating, which increased the number of reaction sites and also accelerated electrochemical reaction kinetics of the anode. In addition, the SDC-coated SYT anode cell was stably operated for 900 h with methane. These results show that the SDC-coated SYT anode can be a promising anode material for high temperature SOFCs running directly on hydrocarbon fuels.     

Oxidative steam reforming of ethanol over Ni/Al2O3 catalysts promoted by CeO2, ZrO2 and CeO2–ZrO2

Oxidative steam reforming of ethanol at low oxygen to ethanol ratios was investigated over nickel catalysts on Al₂O₃ supports that were either unpromoted or promoted with CeO₂, ZrO₂ and CeO₂–ZrO₂. The promoted catalysts showed greater activity and a higher hydrogen yield than the unpromoted catalyst. The characterization of the Ni-based catalysts promoted with CeO₂ and/or ZrO₂ showed that the variations induced in the Al₂O₃ by the addition of CeO₂ and/or ZrO₂ alter the catalyst's properties by enhancing Ni dispersion and reducing Ni particle size. The promoters, especially CeO₂–ZrO₂, improved catalytic activity by increasing the H₂ yield and the CO₂/CO and the H₂/CO values while decreasing coke formation. This results from the addition of ZrO₂ into CeO₂. This promoter highlights the advantages of oxygen storage capacity and of mobile oxygen vacancies that increase the number of surface oxygen species. The addition of oxygen facilitates the reaction by regenerating the surface oxygenation of the promoters and by oxidizing surface carbon species and carbon-containing products.     

Evaluation of layered perovskites YBa1−xSrxCo2O5+δ as cathodes for intermediate-temperature solid oxide fuel cells

This study is focused on the structural characteristics, oxygen nonstoichiometry, electrical conductivity, electrochemical performance and oxygen reduction mechanism of YBa_1−xSr_xCo₂O_5+δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5). The high oxygen nonstoichiometry, δ = 0.18–0.43 at 700 °C, indicates the large oxygen vacancy concentrations in oxides. The electrical conductivity is improved due to the greater amount of electronic holes originated from the increased interstitial oxygen, and the conductivities of all samples are above 100 S cm⁻¹ at 400–700 °C in air. The results demonstrate the promising performance of YBa_1−xSr_xCo₂O_5+δ cathodes at intermediate temperatures, as evidenced by low area-specific resistances (ASRs) e.g. 0.21–0.59 Ω cm² at 700 °C. The lowest ASR, 0.44 Ω cm², and the cathodic overpotential, −40 mV at a current density of −136 mA cm⁻², are obtained in YBaCo₂O_5+δ cathode at 650 °C. The dependence of polarization resistance on oxygen partial pressure suggests that the charge transfer process is the rate-limiting step for oxygen reduction reaction in YBaCo₂O_5+δ cathode.     

Investigation of Ba1−xSrxCo0.8Fe0.2O3−δ as cathodes for low-temperature solid oxide fuel cells both in the absence and presence of CO2

Ba_1−xSr_xCo_0.8Fe_0.2O_3−δ (BSCF)(0 ≤ x ≤ 1) composite oxides were prepared and tested as cathodes for low-temperature solid oxide fuel cells (SOFCs) both in the absence and presence of CO₂. It is found that the BSCF cathodes in the whole range of strontium doping levels show promising performance at 500–600 °C in the absence of CO₂, among which the SrCo_0.8Fe_0.2O_3−δ (SCF) cathode gives the highest power density while BaCo_0.8Fe_0.2O_3−δ (BCF) cathode shows the lowest performance. The impedance analysis reveals that both the ohmic resistance and polarization resistance of the fuel cell increases when the strontium content decreases. It is believed that the microstructure and electrical conductivity simultaneously affect the process of oxygen reduction. The presence of CO₂ deteriorates the BSCF performance by adsorbing on the cathode surface and thus obstructing the oxygen surface exchange reaction. The CO₂ exerts a more intense influence on BSCF with higher barium content.     

Effect of B2O3–Bi2O3–PbO frit on the performance of LaBaCo2O5+δ cathode for intermediate-temperature solid oxide fuel cells

The effects of B₂O₃–Bi₂O₃–PbO (BBP) frit on the electrochemical performance, electrical conductivity, and thermal expansion of LaBaCo₂O_5+δ (LBCO) cathode were investigated. BBP frit was found to be effective in lowering the sintering temperature of LBCO cathode by about 200 °C and in improving its electrochemical performance within the intermediate-temperature range of 600–800 °C. LBCO with 5 wt.% BBP frit cathode based on Sm_0.2Ce_0.8O_1.9 electrolyte showed the best electrochemical performance, i.e., the lowest area-specific resistance (ASR) and cathodic overpotential. The ASR values were about 64.1%, 66.1%, and 74.5% lower than those of LBCO at 700, 750, and 800 °C, respectively. The cathodic overpotential decreased from 51.0 mV for LBCO to 8.2 mV at a current density of 0.2 A cm⁻² at 700 °C. The electrical conductivity of LBCO with 5 wt.% BBP frit was about 320–330 S cm⁻¹ at 600–800 °C in air.     

Nd1.8Sr0.2NiO4−δ:Ce0.9Gd0.1O2−δ composite cathode for intermediate temperature solid oxide fuel cells

The (100 − x)Nd_1.8Sr_0.2NiO_4−δ:(x)Ce_0.9Gd_0.1O_2−δ (x = 00, 10, 20, 30, 40 and 50 vol%) composites are obtained by ball milling requisite mixture at 200 rotations per minute for 2 h under acetone followed by sintering at 1000 °C for 4 h. The increase in concentration of Ce_0.9Gd_0.1O_2−δ in composite reduces the crystallite size of host Nd_1.8Sr_0.2NiO_4−δ from 378 ± 0.7 to 210 ± 0.8 nm. The dc (electronic) conductivity of composite decreases moderately with an increase in Ce_0.9Gd_0.1O_2−δ content in composite up to 30 vol%, and it decreases abruptly, thereafter at x > 30. A minimum polarization resistance value of 0.24 Ω cm² (at 700 °C) is obtained for a (70)Nd_1.8Sr_0.2NiO_4−δ:(30)Ce_0.9Gd_0.1O_2−δ composite cathode, and this value is attributed to the optimal dispersion of Ce_0.9Gd_0.1O_2−δ into Nd_1.8Sr_0.2CuO_4−δ matrix. The oxygen partial pressure dependent polarization resistance study suggests that the charge transfer and the non-charge transfer oxygen adsorption–desorption along with diffusion are the major rate limiting steps of overall oxygen reduction reaction process.     

A high performance BaZr0.1Ce0.7Y0.2O3-δ-based solid oxide fuel cell with a cobalt-free Ba0.5Sr0.5FeO3-δ–Ce0.8Sm0.2O2-δ composite cathode

A cobalt-free Ba_0.5Sr_0.5FeO_3-δ–Ce_0.8Sm_0.2O_2-δ (BSF–SDC) composite is employed as a cathode for an anode-supported proton-conducting solid oxide fuel cells (H-SOFCs) using BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) as the electrolyte. The chemical compatibility between BSF and SDC is evaluated. The XRD results show that BSF is chemically compatible with SDC after co-fired at 1000 °C for 6 h. A single cell with a 20-μm-thick BZCY electrolyte membrane exhibits excellent power densities as high as 792 and 696 mW cm⁻² at 750 and 700 °C, respectively. To the best of our knowledge, this is the highest performance reported in literature up to now for BZCY-based single cells with cobalt-free cathode materials. Extremely low polarization resistances of 0.030 and 0.044 Ωcm² are achieved at 750 and 700 °C respectively. The excellent performance implies that the cobalt-free BSF–SDC composite is a promising alternative cathode for H-SOFCs. Resistances of the tested cell are investigated under open circuit conditions at different operating temperatures by impedance spectroscopy.     

(La0.74Bi0.10Sr0.16)MnO3−δ–Ce0.8Gd0.2O2−δ cathodes fabricated by ion-impregnating method for intermediate-temperature solid oxide fuel cells

Porous composite cathodes were fabricated by impregnating (La_0.74Bi_0.10Sr_0.16)MnO_3−δ (LBSM) electronic conducting structure with the ionic conducting Ce_0.8Gd_0.2O_2−δ (GDC) phase. The ion impregnation of the GDC phase significantly enhanced the electrocatalytic activity of the LBSM electrodes for the O₂ reduction reactions, and the ion-impregnated LBSM–GDC composite cathodes showed excellent performance. At 700 °C, the value of the cathode polarization resistance (Rc) was only 0.097 Ω cm² for an ion-impregnated LBSM–GDC cathode, and the performance was gradually improved by increasing the loading of the impregnated GDC. For the performance testing of single cells, the maximum power density was 1036 mW cm⁻² at 700 °C for a cell with the LBSM–GDC cathode. The results demonstrated the unique combination of the LBSM electronic conducting structure with high ionic conducting GDC phase was a valid method to improve the electrode performance, and the ion-impregnated LBSM–GDC was a promising composite cathode material for the intermediate-temperature solid oxide fuel cells.     

Scandium-doped PrBaCo2−xScxO6−δ oxides as cathode material for intermediate-temperature solid oxide fuel cells

Scandium-doped PrBaCo_2−xSc_xO_6−δ(PBCS-x, x = 0.00–1.00) oxides have been evaluated as cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs) with respect to phase structure, oxygen content, thermal expansion behavior and electrical and electrochemical properties. The XRD results have demonstrated a phase transition in PBCS-x due to Sc³⁺ doping from tetragonal double-layered perovskite structure at x = 0.00–0.20, bi-phase mixtures at x = 0.30–0.40, to cubic perovskite structure at x = 0.50–0.90. The oxygen contents (6-δ) and average valences of cobalt ions in PBCS-x decrease with the higher Sc³⁺ content and increasing temperatures in air. Sc³⁺ doping has also led to decreased thermal expansion coefficients, lowered electrical conductivities and enhanced electrochemical reaction activities for PBCS-x characterized by decreased area-specific resistances (ASRs) and smaller reaction activation energies. Among the studied samples, the PBCS-0.50 oxide with Sc³⁺-doping content of x = 0.50 exhibits the best electrochemical performance on Ce_0.9Gd_0.1O_1.95 electrolyte. Its ASR values range from 0.123 Ω cm² at 600 °C to 0.022 Ω cm² at 750 °C, which are much lower than the related cathode materials. These results have demonstrated that the PBCS-0.50 oxide is a promising cathode material for IT-SOFCs.     

(La0.74Bi0.10Sr0.16)MnO3−δ–(Bi2O3)0.7(Er2O3)0.3 composite cathodes for intermediate temperature solid oxide fuel cells

(La_0.74Bi_0.10Sr_0.16)MnO_3−δ (LBSM)–(Bi₂O₃)_0.7(Er₂O₃)_0.3(ESB) composite cathodes were fabricated for intermediate-temperature solid oxide fuel cells with Sc-stabilized zirconia as the electrolyte. The performance of these cathodes was investigated at temperatures below 750 °C by AC impedance spectroscopy and the results indicated that LBSM–ESB had a better performance than traditional composite electrodes such as LSM–GDC and LSM–YSZ. At 750 °C, the lowest interfacial polarization resistance was only 0.11 Ω cm² for the LBSM–ESB cathode, 0.49 Ω cm² for the LSM–GDC cathode, and 1.31 Ω cm² for the LSM–YSZ cathode. The performance of the cathode was improved gradually by increasing the ESB content, and the performance was optimal when the amounts of LBSM and ESB were equal in composite cathodes. This study shows that the sintering temperature of the cathode affected performance, and the optimum sintering temperature for LBSM–ESB was 900 °C.