Electrochemical performance of mixed ionic–electronic conducting oxides as anodes for solid oxide fuel cell

Mixed ionic–electronic conducting (MIEC) oxides, SrFeCo_0.5O_x, SrCo_0.8Fe_0.2O_3−δ and La_0.6Sr_0.4Fe_0.8Co_0.2O_3−δ have been synthesized and prepared on yttria-stabilized zirconia as anodes for solid oxide fuel cells. Power output measurements show that the anodes composed of such kinds of oxides exhibit modest electrochemical activities to both H₂ and CH₄ fuels, giving maximum power densities of around 0.1 W/cm² at 950°C. Polarization and AC impedance measurements found that large activation overpotentials and ohmic resistance drops were the main causes for the relative inferior performance to the Ni-YSZ anode. While interlayered with an Ni-YSZ anode, a significant improvement in the electrochemical performance was observed. In particular, for the SrFeCo_0.5O_x oxide interlayered Ni-YSZ anode, the maximum power output reaches 0.25 W/cm² on CH₄, exceeding those of both SrFeCo_0.5O_x and the Ni-YSZ, as anodes alone. A synergetic effect of SrFeCo_0.5O_x and the Ni-YSZ has been observed. Future work is needed to examine the long-term stability of MIEC oxide electrodes under a very reducing environment.     

Characterization of solid oxide fuel cells based on solid electrolytes or mixed ionic electronic conductors

The relation between cell voltage (V_cell), applied chemical potential difference (Δμ(O₂)) and cell current (I_t) for solid oxide fuel cells (SOFC) based on mixed ionic electronic conductors is derived by considering also the effect of electrode impedance. Four-probe measurements, combined with current interruption analysis, are considered to yield the relation between ionic current (I_i) and overpotential (η). The theoretical relations are used to analyze experiments on fuel cells with Ce_0.8Sm_0.2O_1.9 and Ce_0.8Gd_0.2O_1.9 electrolytes with La_0.8Sr_0.16CoO₃ or Pt as the cathode and Ni/Ce_0.9Ca_0.1O_1.9−xor Pt as the anode. The electrode overpotentials of these cells, determined by current interruption measurements, are discussed assuming different models including impeded mass transport in the gas phase for molecular and monoatomic oxygen and Butler-Volmer type charge transfer overpotential.     

Stable and high conductivity ceria/bismuth oxide bilayer electrolytes for lower temperature solid oxide fuel cells

A highly conductive bismuth oxide/ceria bilayer electrolyte was developed to reduce solid oxide fuel cell (SOFC) operating temperatures. Bilayer electrolytes were fabricated by depositing a layer of Er_0.2Bi_0.8O_1.5 (ESB) of varying thickness via pulsed laser deposition and dip-coating on a Sm_0.2Ce_0.8O_1.9 (SDC) substrate. The open-circuit potential (OCP) and ionic transference number (t _i) of ESB/SDC electrolytes were tested in a fuel cell arrangement as a function of relative thickness, temperature, and with H₂/H₂O and CO/CO₂ on the anode side and air on the cathode side. These EMF measurements showed a significant increase in OCP and t _i with the bilayer structure, as compared to the cells with a single SDC electrolyte layer. Furthermore, improvement in the OCP and t _i of bilayer SOFCs was observed with increasing relative thickness of the ESB layers. Hence, the bilayer structure overcomes the limited thermodynamic stability of bismuth oxides and prevents electronic conductivity of ceria-based oxides in reducing atmosphere.     

Electrodeposited cobalt coating on Crofer22APU steels for interconnect applications in solid oxide fuel cells

A gas-tight cobalt-based protective coating was successfully applied by electroplating and subsequent oxidation. The coating covered all the surfaces of the machined gas channels of the metallic interconnect and adhered well to this substrate. Such a gas-tight coating offers effective blocking of Cr diffusion or evaporation from the interconnect and hence a reliable protection against Cr poisoning of the SOFC cathode. Chemical interactions were observed between the cobalt coating and the LSM contact layer, resulting in layer spallation during oxidation under pressureless conditions. Applying a pressure to the layers, spallation was effectively prevented. Although the area specific resistance (ASR) of the coated interconnect was higher than the uncoated one, it decreased steadily with time within the measurement period. The ASR was 28 mΩ cm² after an exposure of 1170 h. It seems thus that electroplating followed by oxidation is a promising method for the fabrication of spinel protective coatings for SOFC interconnects or other balance-of-plant components with complicated gas flow paths.     

Direct ammonia proton-conducting solid oxide fuel cells prepared by a modified suspension spray

A dense (BCSO) membrane was fabricated by a modified suspension spraying on porous NiO–BCSO anode support. In the process, the suspension was directly prepared by ball-milling the BaCO₃, CeO₂, and Sm₂O₃ powders in ethanol. A dense and uniform electrolyte layer in the thickness of 10 μm was successfully prepared on porous anode support by suspension spray process after co-sintering at 1,400 °C for 5 h. With (NSMO) cathode, a single cell was assembled and tested with hydrogen and ammonia as fuels, respectively. The hydrogen-fueled cell exhibits 1.01 V for open circuit voltage (OCV) and 560 mW/cm² for peak power density at 700 °C. In comparison, the cell in ammonia displays a similar performance (1.02 V for OCV and 530 mW/cm² for output), which indicates the liquid ammonia is a promising substitute for hydrogen. Moreover, the fuel cell displays good interface contacts. To sum up, ammonia-fueled solid oxide fuel cells prepared by this simple suspension spray is an alternative way to promote the commercialization.     

High temperature oxidation behavior of interconnect coated with LSCF and LSM for solid oxide fuel cell by screen printing

The current study examined the effect of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and La_0.7Sr_0.3MnO₃ (LSM) coatings on the electrical properties and oxidation resistance of Crofer22 APU at 800 °C hot air. LSCF and LSM were coated on Crofer22 APU by screen printing and sintered over temperatures ranging from 1000 to 1100 °C in N₂. The coated alloy was first checked for compositions, morphology and interface conditions and then treated in a simulated oxidizing environment at 800 °C for 200 h. After measuring the long-term electrical resistance, the area specific resistance (ASR) at 800 °C for the alloy coated with LSCF was less than its counterpart coated with LSM. This work used LSCF coating as a metallic interconnect to reduce working temperature for the solid oxide fuel cell.     

Electrical properties and redox stability of tantalum-doped strontium titanate for SOFC anodes

SrTa_xTi_1−xO₃ (STT) with x = 0.01, 0.05, and 0.10, has been investigated as a potential electron conductor for solid oxide fuel cell (SOFC) anodes. STT was found to be chemically stable under oxidizing and reducing conditions and chemically compatible with yttria-stabilized zirconia (YSZ). The coefficient of thermal expansion (CTE) was near that of YSZ, ranging from 11.3 to 11.8 × 10⁻⁶ K⁻¹. The conductive properties of bulk STT and porous STT-YSZ composites were studied under relevant SOFC operating temperatures and redox cycling conditions. In order to achieve reasonable conductivities, samples were initially reduced at 1673 K. Conductivity after redox cycling was higher for lower dopant concentrations. The redox stable conductivity of a porous composite with x = 0.01 was 1.1 S/cm at 1073 K in humidified H₂ (3% H₂O). Fuel cell tests indicated an anode impedance of 0.4 Ω cm² at 973 K in humidified H₂ for STT-YSZ anodes infiltrated with 3 wt.% CeO₂ and 1 wt.% Pd.     

A tubular segmented-in-series solid oxide fuel cell with metallic interconnect films: A performance study through mathematical simulations

In a segmented-in-series solid oxide fuel cell (SIS-SOFC), an interconnect (IC) provides electrical contact and sealing between the anode of one cell and the cathode of the next. A metallic silver-glass composite (SGC) is considered a promising alternative to ceramic IC materials in SIS-SOFCs. In this work, a simulation study is performed on a tubular SIS-SOFC to assess the effectiveness of the SGC-IC design and to predict the SOFC performance characteristics for various IC geometries and conductivities. The developed model provides detailed information on cell behavior, such as the internal resistance, the potential/current distribution, and the local gas species concentration. The results demonstrate that the SGC material greatly reduces a potential drop across the IC film. Thus, it provides the following substantial advantages over conventional ceramic IC materials: (i) increased power density and (ii) a larger degree of flexibility in the cell design. Moreover, the validation test, i.e., comparison of the simulated results with the experimental data, indicates that the model could serve as a valuable tool for design optimization to achieve the required SOFC performance.     

Fabrication of solid oxide fuel cell based on doped ceria electrolyte by one-step sintering at 800 °C

Ce_0.8Gd_0.05Y_0.15O_1.9 (GYDC) electrolyte was prepared by a carbonate co-precipitation method. Lithium nitrate at 1, 1.5, 2 and 3 mol% was added to GYDC as sintering additive. 96% relative density was achieved for GYDC at sintering temperature of 800 °C with addition of 1.5 mol% LiNO₃. The conductivities of GYDC with sintering aids LiNO₃ were measured by a.c. impedance spectroscopy and showed comparable values to that of pure GYDC sample sintered at 1400 °C. A single cell with 1.5 mol% LiNO₃ infiltrated GYDC electrolyte was fabricated by sintering at 800 °C for only 2 h. Lithiated NiO was synthesized by the glycine-nitrate combustion method and employed as cathode material. The cell was tested at temperatures from 500 to 575 °C and a maximum power density of 73 mW cm^− 2 was obtained at 575 °C. These preliminary results indicate that LiNO₃ is a very effective sintering additive for intermediate temperature solid oxide fuel cell fabrication.     

Ln1−xSrxCoO3(Ln = Sm, Dy) for the electrode of solid oxide fuel cells

The perovskite-type oxides were synthesized in the series of Ln_1−xSr_xCoO₃(Ln = Sm, Dy). The formation of solid solutions in Dy_{1 − x}Sr_xCoO₃ was limited, compared with that in Sm_{1 − x}Sr_xCoO₃. The electrical conductivities of the sintered samples were measured as a function of x in the temperature range 30 to 1000 °C. The highest conductivity of around 500 S/cm at 1000 °C was found in Sm_0.7Sr_0.3CoO₃. The reactivity of all the samples with YSZ was examined at 800–1000 °C for 96 h. The Sr-doped perovskite oxides were more reactive with YSZ and produced SrZrO₃ at 900 °C after 96 h. However, no reaction product between SmCoO₃ and YSZ was observed at 1000 °C for 96 h. The cathodic polarization of the oxide electrodes, sputtered on yttria stabilized zirconia (YSZ), was studied at 800–1000 °C in air. SmCoO₃ shows no degradation of the electrode performance at higher temperatures. The thermal expansion measurements on the sintered samples were carried out from room temperature to 1000 °C. Large thermal expansion coefficients were found in these samples.     

Preparation of honeycomb porous La0.6Sr0.4Co0.2Fe0.8O3−δ-Gd0.2Ce0.8O2−δ composite cathodes by breath figures method for solid oxide fuel cells

Honeycomb porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ-Gd_0.2Ce_0.8O_2−δ (LSCF-GDC) composite cathodes are prepared using the breath figures (BFs) method with nontoxic and easily available water droplets as templates. The fabrication of honeycomb porous membranes is realized in a relatively humid environment, using a volatile solvent. The microstructure and morphology of the membranes produced are investigated by scanning electron microscopy (SEM). The SEM micrographs suggest that experimental conditions, such as ambient temperature, relative humidity, and concentration of polymer and LSCF-GDC powder, which have direct influence on the solvent evaporation affects the pore structure of the porous membranes. Electrochemical impedance spectroscopy (EIS) is used to evaluate the polarization resistance of LSCF-GDC composite cathodes prepared at different experimental conditions. The honeycomb porous LSCF-GDC composite cathode showing average pore diameter of 10 μm illustrates the lowest polarization resistance.     

Influence of colloidal templates on the impedance spectroscopic behaviour of Pr0.7Sr0.3Fe0.8Ni0.2O3 for solid oxide fuel cell applications

The creation of porous materials with three-dimensional periodicity has been identified as being of potential interest for increasing the overall performance of solid oxide fuel cells (SOFC). In this work, we have investigated the formation of pore systems in the nanometer scale by replicating colloidal templates. Templating methods have been used to prepare iron-nickel-based perovskite Pr_0.7Sr_0.3Fe_0.8Ni_0.2O₃ material with nanoporous microstructure. Polymethyl methacrylate (PMMA), polystyrene (PS) and polycarboxylate (PC) microspheres with different diameters were used as pore formers. These samples were synthesized and characterized by thermogravimetric analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction, transmission electron microscopy and field emission scanning electron microscopy. The polarization resistance of the materials was studied by Electrochemical Impedance Spectroscopy. The study demonstrated that templated porosity is maintained and highly influences on the impedance spectroscopic behaviour, being the material synthesized with policarboxylate microspheres the most interesting of the three used templates for SOFC applications.     

Auto-combustion synthesis and properties of Ce0.85Gd0.15O1.925 for intermediate temperature solid oxide fuel cells electrolyte

A typical composition of the system Ce_1 − xGd_xO_2 − δ with x = 0.15 (CGO15) has been synthesized by auto-combustion method. DTA/TGA of the precursor compound indicated the completion of reaction at about 270 °C. Greater than 95% of the theoretical density has been achieved by sintering at 1300 °C for 10 h. Single phase formation in as-burnt stage has been confirmed by its powder X-ray diffraction (XRD) pattern. The structural morphology was studied employing bright field transmission electron micrograph (BFTEM) and high resolution transmission electron micrograph (HRTEM). BFTEM image indicates that particles are highly agglomerated and appear to be dispersed in amorphous matrix. Also BFTEM image reveals that the average particle size is 26 ± 5 nm. The presence of amorphous phase in as-prepared ash was also confirmed by HRTEM and selected area diffraction (SAD). The scanning electron micrograph (SEM) of the thermally etched system shows grains having an average size of 400 nm. Impedance measurements have been made in the frequency range 1 Hz to 1.3 MHz between 200 and 500 °C and the total conductivity was measured. An enhanced conductivity value is observed which may make this system suitable for application as a solid electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs).     

Ceria co-doped with calcium (Ca) and strontium (Sr): a potential candidate as a solid electrolyte for intermediate temperature solid oxide fuel cells

Co-doped samples of Ce_0.95?x Ca_0.05Sr _x O_1.95?x , where (x?=?0.00, 0.01, 0.02, and 0.03), have been prepared by auto-combustion method and characterized to explore their use as a solid electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). Crystal structure, microstructure, and ionic conductivity have been characterized by X-ray diffraction, scanning electron microscopy, and impedance spectroscopy, respectively. All the compositions have been found to be single phase. Results show that the samples co-doped with Ca and Sr exhibit higher ionic conductivity than the samples singly doped with Ca in the intermediate temperature range. Ce_0.93Ca_0.05Sr_0.02O_2?δ exhibits maximum conductivity among all the compositions. This may be a potential candidate as a solid electrolyte for IT-SOFCs.     

Fractional distribution of graphene oxide and its potential as an efficient and reusable solid catalyst for esterification reactions

Graphene oxide (GrO) prepared by the Hummers method was separated into three different fractions (GrO₅₀₀₀, GrO₂₀₀₀, and GrO_res) on the basis of their dispersion stability in the water. Infrared, nuclear magnetic resonance, X‐ray photoelectron spectroscopy, and elemental analyses revealed that GrO₅₀₀₀ possesses a high degree of oxygen functionalities including phenolic, carboxylic, and ?OSO₂H groups, compared with the other fractions. The GrO₅₀₀₀ was found to be a highly efficient and reusable solid catalyst for the esterification of various carboxylic acids with a variety of alcohols to furnish corresponding esters in high to excellent yields. The catalytic activity of the GrO₅₀₀₀ was attributed to the ability of highly polar GrO₅₀₀₀ scaffold to adsorb/attract reactants, where the acid functionalities of GrO₅₀₀₀ facilitated the esterification process efficiently. The chemical and structural features of GrO₅₀₀₀ were discussed to understand the improved catalytic activity compared with GrO₂₀₀₀ and conventional solid acid catalysts. Copyright © 2014 John Wiley & Sons, Ltd.     

Correlation between phase structure and electrical conduction in BISNVOX system for intermediate temperature solid oxide fuel cells (IT-SOFC)

Samples of Sn⁴⁺-substituted bismuth vanadate, formulated as Bi₄Sn _x V_{2? x} O_{11?( x /2)? δ} in the composition range 0.07 ≤ x ≤ 0.30, were prepared by standard solid-state reactions. Sample characterization and the principal phase transitions (α ? β, β ? γ and γ′ ? γ) were investigated by FT-IR spectroscopy, X-ray powder diffraction, differential thermal analysis (DTA) and AC impedance spectroscopy. For composition x = 0.07, the α ? β and β ? γ phase transitions were observed at temperatures of 451 and 536°C, respectively. DTA thermograms and Arrhenius plots of conductivities revealed the γ′ ? γ phase transition at 411 and 423°C for x = 0.20 and 0.30, respectively. AC impedance plots showed that conductivity is mainly due to the grain contribution, which is evident in the enhanced short-range diffusion of oxide ion vacancy in the grains with increasing temperature. The highest ionic conductivity (5.03 × 10^?5 S cm^?1 at 300°C) was observed for the x = 0.17 solid solution with less pronounced thermal hysteresis.     

Co-electrolysis of CO2 and H2O in solid oxide cells: Performance and durability

This study examines the initial performance and durability of a solid oxide cell applied for co-electrolysis of CO₂ and H₂O. Such a cell, when powered by renewable/nuclear energy, could be used to recycle CO₂ into sustainable hydrocarbon fuels. Polarization curves and electrochemical impedance spectroscopy were employed to characterize the initial performance and to break down the cell resistance into the resistance for the specific processes occurring during operation. Transformation of the impedance data to the distribution of relaxation times (DRT) and comparison of measurements taken under systematically varied test conditions enabled clear visual identification of five electrode processes that contribute to the cell resistance. The processes could be assigned to each electrode and to gas concentration effects by examining their dependence on gas composition changes and temperature.This study also introduces the use of the DRT to study cell degradation without relying on a model. The durability was tested at consecutively higher current densities (and corresponding overpotentials). By analyzing the impedance spectra before and after each segment, it was found that at low current density operation (− 0.25 A/cm² segment) degradation at the Ni/YSZ electrode was dominant, whereas at higher current densities (− 0.5 A/cm² and − 1.0 A/cm²), the Ni/YSZ electrode continued to degrade but the serial resistance and degradation at the LSM/YSZ electrode began to also play a major role in the total loss in cell performance. This suggests different degradation mechanisms for high and low current density operation.     

Electrical and electrochemical properties of SrBiMTiO<Subscript>6</Subscript> (M = Fe,Mn, Cr) as potential cathodes for solid oxide fuel cells

A series of perovskite oxides SrBiMTiO₆ (M = Fe, Mn, Cr) have been synthesized and characterized towards application as cathode materials for solid oxide fuel cells (SOFCs). X-ray diffraction (XRD) patterns reveal that all samples are stabilized in \( \mathrm{Pm}\ \overline{3}\mathrm{m} \) space group. Electrical conductivity, AC impedance characteristics, and thermal and chemical stability have been studied in order to assess their possible use as SOFC cathode materials. In comparison with other low electrical conductivity cathodes of SOFC, our results suggest that SrBiMnTiO₆, which has the highest electrical conductivity (4.02 S cm^?1) and moderate polarization resistance (0.104 Ω cm²) at 850 °C, is the most promising candidate among the three perovskite oxides for further study and optimization as a SOFC cathode material.     

Structure, phase formation, and wetting behavior of BaO–SiO2–B2O3 based glass–ceramics as sealants for solid oxide fuel cells

In the present study, glasses from the three different compositional triangles in the BaO–B₂O₃–SiO₂ system with fixed B₂O₃/SiO₂ ratio and different BaO/SiO₂ molar ratios (designated as Ba32, Ba37, and Ba42) were prepared, and suitability of them as sealant in solid oxide fuel cells were investigated. Structure of the glasses was characterized with Raman spectroscopy. According to the results, the structure of the glass with 32 % molar BaO (Ba32) predominantly consisted of Q² structural species. In glasses with 37 and 42 % molar BaO (Ba37 and Ba42), with the substitution of SiO₂ by BaO, distribution of Qⁿ units widened, silicate glass network depolymerized, and concentration of Q¹ structural units increased at the expense of Q² units. X-ray diffraction analyses revealed that in samples Ba32 and Ba37, initially, Ba₃Si₅O₁₃ and Ba₅Si₈O₂₁ phases were crystallized, respectively, and it seemed they acted as the sites for the subsequent growth of BaSi₂O₅ phase. In contrast, the dominant phase in sample Ba42 was Ba₂Si₃O₈. Sintering, wetting, and crystallization behavior of the glasses were studied using hot-stage microscopy and differential thermal analysis, respectively. Delay in the crystallization accompanied by depolymerization of the structure led into deformation at lower temperatures and greater wettability on the steel for Ba37 glass. All the glasses wetted AISI430 alloy at temperatures higher than 1,000 °C.     

Development and characterization of a quasi-ternary diagram based on La0.8Sr0.2(Co,Cu,Fe)O3 oxides in view of application as a cathode contact material for solid oxide fuel cells

Oxides resulting from discrete changes in composition within the quasi-ternary system La_0.8Sr_0.2CuO_2.4 + δ–La_0.8Sr_0.2CoO_3 ? δ–La_0.8Sr_0.2FeO_3 ? δ were investigated under similar experimental conditions with the objective of obtaining an overview of the variation of the relevant properties for possible applications as cathode contact layer in SOFCs. Twenty-two oxide compositions within this system were systematically selected and synthesized under identical conditions by the Pechini method. The distribution of the different crystallographic phases at 1050 °C within this quasi-ternary phase diagram, the DC electrical conductivity at 800 °C and the thermal expansion coefficients are presented. Perovskites of different compositions issued from this ternary diagram were tested as cathode contact material between an La_0.8Sr_0.2FeO₃ cathode and a Crofer22APU interconnect by resistance measurements at 800 °C. The application of a MnCo_1.9Fe_0.1O₄ spinel protection reduced the interfacial reaction between the Crofer22APU and the cathode contact material. Electrical resistance measurements at 800 °C in air up to 1000 h and the analysis by scanning electron microscopy/energy-dispersive X-ray spectroscopy of the sample cross-sections were carried out to verify the surface stability and the electrical performance.