Direct utilization of methanol on impregnated Ni/YSZ and Ni-Zr0.35Ce0.65O2/YSZ anodes for solid oxide fuel cells

Some of the limits on fuel cell development include the issues of hydrogen availability and storage. Methanol has many advantages as an alternative fuel for fuel cells but depending on the anode composition, the formation of carbon may be a problem. In this paper, the direct utilization of methanol in solid oxide fuel cells with impregnated Ni/YSZ and Ni-Zr_0.35Ce_0.65O_2−δ (ZDC)/YSZ anodes was investigated at 1073 K. Performance and stability of these anodes, as measured by steady-state polarization and electrochemical impedance spectroscopy, were improved by the presence of ZDC; although, the deposition of carbon, as detected by scanning electron microscopy and temperature-programmed oxidation analysis, was not entirely avoided. The impact of the carbon, however, was different depending on the anode. That is, carbon formation caused the delamination of impregnated Ni/YSZ anodes, while the structural integrity of Ni-ZDC/YSZ anodes was maintained and the cell performance was not negatively impacted. Increasing the fuel utilization decreased coking, as predicted by equilibrium calculations.     

Partial conductivities of mixed conducting BaCe0.65Zr0.2Y0.15O3–δ

The electrical properties of BaCe_0.65Zr_0.2Y_0.15O_3–δ (BCZY) were studied as a function of both oxygen partial pressure and water vapor partial pressure in the temperature range of 500–800 °C, and the partial conductivities of protons, holes, and oxygen vacancies were calculated from the defect model. P-type conduction was dominant in an oxidative atmosphere. In a wet atmosphere, BCZY was a mixed conductor of protons, holes, and oxygen ions. A conduction transition from protons to holes and/or oxygen ions was found with increasing temperature. The calculated activation energy of oxygen ion transport was 0.71 eV. The standard solution enthalpy for water dissolution was best fitted with a slope of −120.19 kJ/mol, which is somewhat smaller in absolute terms than that of BaCe_0.9Y_0.1O_3–δ and of BaCe_0.8Y_0.2O_3–δ. This result also agrees well with the literature reports that the Ba, rather than Sr, occupation of A-site and the Ce, rather than Zr, occupation of B-site in perovskite proton conductors induce more negative hydration enthalpies due to the increasing basicity of the corresponding oxides.     

Hydrogen production from fossil and renewable sources using an oxygen transport membrane

Oxygen transport membranes (OTMs) made of mixed ion-electron conductors can be used to increase the production of hydrogen from fossil and renewable sources. This study describes two methods for producing hydrogen with La_0.7Sr_0.3Cu_0.2Fe_0.8O_3−δ (LSCF7328), an OTM material that is easily prepared, exhibits good mechanical properties, and is stable in severe gas conditions. In tests with thin-film (thickness ≈22 μm) LSCF7328 membranes, hydrogen was produced by flowing simulated product streams from CO₂ gasification of coal on one side of the OTM and steam on the other side. In this method, the so-called coal gas on the oxygen-permeate side drives the removal of oxygen from the other side of the OTM, where hydrogen and oxygen are produced by water splitting. With CO (99.5% purity) flowing on the oxygen-permeate side, the hydrogen production rate was measured to be ≈4.7 cm³/min-cm² at 900 °C, indicating that hydrogen can be produced at a significant rate by using product streams from coal gasification. This process also yields a CO₂-rich product stream that is ready for sequestration. In another test, a tubular LSCF7328 was found to increase the hydrogen production from ethanol reforming by supplying high-purity oxygen from air.     

Silver-modified Ba0.5Sr0.5Co0.8Fe0.2O3−δ as cathodes for a proton conducting solid-oxide fuel cell

Electrochemical performance of silver-modified Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF-Ag) as oxygen reduction electrodes for a protonic intermediate-temperature solid-oxide fuel cell (SOFC-H⁺) with BaZr_0.1Ce_0.8Y_0.1O₃ (BZCY) electrolyte was investigated. The BSCF-Ag electrodes were prepared by impregnating the porous BSCF electrode with AgNO₃ solution followed by reducing with hydrazine and then firing at 850 °C for 1 h. The 3 wt.% silver-modified BSCF (BSCF-3Ag) electrode showed an area specific resistance of 0.25 Ω cm² at 650 °C in dry air, compared to around 0.55 Ω cm² for a pure BSCF electrode. The activation energy was also reduced from 119 kJ mol⁻¹ for BSCF to only 84 kJ mol⁻¹ for BSCF-3Ag. Anode-supported SOFC-H⁺ with a BZCY electrolyte and a BSCF-3Ag cathode was fabricated. Peak power density up to 595 mW cm⁻² was achieved at 750 °C for a cell with 35 μm thick electrolyte operating on hydrogen fuel, higher than around 485 mW cm⁻² for a similar cell with BSCF cathode. However, at reduced temperatures, water had a negative effect on the oxygen reduction over BSCF-Ag electrode, as a result, a worse cell performance was observed for the cell with BSCF-3Ag electrode than that with pure BSCF electrode at 600 °C.     

Thermal expansion and electrochemical properties of Ni-doped GdBaCo2O5+δ double-perovskite type oxides

Layered GdBaCo_2 −x Ni_xO_5 + δ (0 ≤ x ≤ 0.3) complex oxides were synthesized and investigated as cathodes for intermediate-temperature solid oxide fuel cell (IT-SOFCs). All compositions formed an orthorhombic double-perovskite structure after calcination at 1000 °C for 5 h. The thermal expansion coefficient (TEC) was effectively decreased due to the partial substitution of Ni for Co, but the cathodic polarization resistance slightly increased with the increasing Ni content. Among the tested oxides, the GdBaCo_1.7Ni_0.3O_5 + δ composition showed a fairly reduced TEC (15.5 × 10⁻⁶ K⁻¹) and reasonably low polarization resistances (e.g., 0.54 Ωcm² at 600 °C), which was considered as a promising candidate for IT-SOFCs.     

SrCe0.7Zr0.2Eu0.1O3-based hydrogen transport water gas shift reactor

Proton-transport-membrane water gas shift (WGS) reactors, based on thin dense SrCe_0.7Zr_0.2Eu_0.1O_3−δ membranes on tubular Ni–SrCe_0.8Zr_0.2O_3−δ supports, were developed to increase H₂ yields relative to thermodynamic limitations. Pure H₂ permeate, total H₂ production, and reactor side CO conversion and H₂/CO effluent ratio were measured as a function of temperature, flow rate, CO concentration and H₂O/CO feed ratios. CO conversion, total H₂ production and yield, and the H₂/CO in the reactor side effluent increased with increasing temperature and H₂O/CO feed ratios. CO conversions of 84% and 90% were achieved at 900 °C with H₂O/CO feed ratios of 1/1 and 2/1, respectively. These respective 77% and 44% increases in CO conversion compared to feed gas condition thermodynamics resulted in 73% and 42% increases in H₂ production. Permeated H₂ and total H₂ production increased with increasing flow rate and CO concentration. Finally, membrane stability under WGS conditions was significantly improved by Zr substitution.     

Principles and limitations of stable isotopes in differentiating organic and conventional foodstuffs: 2. Animal products

In this review, we examine the variation in stable isotope signatures of the lighter elements (δ²H, δ¹³C, δ¹⁵N, δ¹⁸O, and δ³⁴S) of tissues and excreta of domesticated animals, the factors affecting the isotopic composition of animal tissues, and whether stable isotopes may be used to differentiate organic and conventional modes of animal husbandry. The main factors affecting the δ¹³C signatures of livestock are the C3/C4 composition of the diet, the relative digestibility of the diet components, metabolic turnover, tissue and compound specificity, growth rate, and animal age. δ¹⁵N signatures of sheep and cattle products have been related mainly to diet signatures, which are quite variable among farms and between years. Although few data exist, a minor influence in δ¹⁵N signatures of animal products was attributed to N losses at the farm level, whereas stocking rate showed divergent findings. Correlations between mode of production and δ²H and δ¹⁸O have not been established, and only in one case of an animal product was δ³⁴S a satisfactory marker for mode of production. While many data exist on diet–tissue isotopic discrimination values among domesticated animals, there is a paucity of data that allow a direct and statistically verifiable comparison of the differences in the isotopic signatures of organically and conventionally grown animal products. The few comparisons are confined to beef, milk, and egg yolk, with no data for swine or lamb products. δ¹³C appears to be the most promising isotopic marker to differentiate organic and conventional production systems when maize (C4) is present in the conventional animal diet. However, δ¹³C may be unsuitable under tropical conditions, where C4 grasses are abundant, and where grass-based husbandry is predominant in both conventional and organic systems. Presently, there is no universal analytical method that can be applied to differentiate organic and conventional animal products.     

Ionic conductivity of Ln4Zr3O12 solid electrolytes synthesized by mechanochemistry

To explore their possible application as solid electrolytes in Solid Oxide Fuel Cells (SOFC), this contribution presents the synthesis, characterization and electrical properties of Ln₄Zr₃O₁₂ (Ln = Y, Ho, Er and Yb) zirconates. All samples were obtained by mechanical milling and their electrical properties were analyzed as a function of frequency and temperature, by using impedance spectroscopy. Our results show that defective fluorite-type zirconates might be successfully obtained after milling stoichiometric mixtures of the corresponding oxides, for 30–40 h in a planetary mill. Such structural form persists even after firing the as-prepared Y, Ho and Er zirconates, at very high temperatures (1500 °C); whereas, Yb₄Zr₃O₁₂ shows a transition to a rhombohedral δ-phase on firing. Ionic conductivity (σ) values obtained for all compositions at 700 °C (including fluorites and δ-phase), are comparable to those reported for similar ionic conductors, and within the 10^−3.82 to 10^−6.13 S cm⁻¹ range. Higher σ values were obtained for those zirconates preserving the disordered fluorite-type structure after firing.