Pr Doped Barium Cerate as the Cathode Material for Proton‐Conducting SOFCs

BaCe_0.8Pr_0.2O₃ (BCP20) and BaCe_0.6Pr_0.4O₃ (BCP40) powders are successfully synthesized through the Pechini method and used as the cathode materials for proton‐conducting solid state oxide fuel cells (SOFCs). The prepared cells consisting of the structure of a BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY7)‐NiO anode substrate, a BZCY7 anode functional layer, a BZCY7 electrolyte membrane, and a cathode layer, are measured from 600 to 700 °C with humidified hydrogen (∼3% H₂O) as the fuel and static air as the oxidant. The electricity results show that the cell with BCP40 cathode has a higher power density, which could obtain an open‐circuit potential of 0.99 V and a maximum power density of 378 mW cm^–2 at 700 °C. The polarization resistance measured at the open‐circuit condition of BCP40 is only 0.16 Ω cm² at 700 °C, which was less than BCP20.     

Preparation of BaZr0.1Ce0.7Y0.2O3–δ Based Solid Oxide Fuel Cells with Anode Functional Layers by Tape Casting

Solid oxide fuel cells (SOFCs) based on the proton conducting BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY) electrolyte were prepared and tested in 500–700 °C using humidified H₂ as fuel (100 cm³ min^–1 with 3% H₂O) and dry O₂ (50 cm³ min^–1) as oxidant. Thin NiO‐BZCY anode functional layers (AFL) with 0, 5, 10 and 15 wt.% carbon pore former were inserted between the NiO‐BZCY anode and BZCY electrolyte to enhance the cell performance. The anode/AFL/BZCY half cells were prepared by tape casting and co‐sintering (1,300 °C/8 h), while the Sm_0.5Sr_0.5CoO_3–δ (SSC) cathodes were prepared by thermal spray deposition. Well adhered planar SOFCs were obtained and the test results indicated that the SOFC with an AFL containing 10 wt.% pore former content showed the best performance: area specific resistance as low as 0.39 Ω cm² and peak power density as high as 0.863 W cm^–2 were obtained at 700 °C. High open circuit voltages ranging from 1.00 to 1.12 V in 700–500 °C also indicated negligible leakage of fuel gas through the electrolyte.     

An investigation of metal ion diffusion in ceria‐based solid oxide fuel cell with barium‐containing anode

Metal ion diffusion is an effective strategy to suppress the internal electronic short circuit in ceria‐based solid oxide fuel cells (SOFCs). This could be achieved by fabricating an electron‐blocking layer between the barium‐containing anode and ceria‐based electrolyte. In this paper, a 0.6NiO‐0.4BaZr_0.1Ce_0.7Y_0.2O_3‐δ (NiO‐BZCY) anode‐supported cell based on Gd_0.1Ce_0.9O_2‐δ (GDC) electrolyte was employed to evaluate the internal metal ion diffusion behavior. The high open circuit voltages of about 1 V obtained at 550‐700°C can be attributed to in situ formation of an electron‐blocking interlayer between NiO‐BZCY and GDC. Microstructural analyses of the interlayer grains obtained by traditional solid‐state reaction were carried out. Phase identification demonstrated that the electron‐blocking interlayer had a perovskite structure. SEM and TEM analyses indicated formation of a new compound in the interlayer, of which the composition was determined as Zr, Y, and Ni co‐doped BaCe_0.9Gd_0.1O₃ with orthorhombic structure.     

Comparison Between Anode‐Supported and Electrolyte‐Supported Ni‐CGO‐LSCF Micro‐tubular Solid Oxide Fuel Cells

Two types of micro‐tubular hollow fiber SOFCs (MT‐HF‐SOFCs) were prepared using phase inversion and sintering; electrolyte‐supported, based on highly asymmetric Ce_0.9Gd_0.1O_1.95(CGO) HFs and anode‐supported based on co‐extruded NiO‐CGO(CGO)/CGO HFs. Electroless plating was used to deposit Ni onto the inner surfaces of the electrolyte‐supported MT‐HF‐SOFCs to form Ni‐CGO anodes. LSCF‐CGO cathodes were deposited on the outer surface of both these MT‐HF‐SOFCs before their electrochemical performances were compared at similar operating conditions. The performance of the anode‐supported MT‐HF‐SOFCs which delivered ca. 480 mW cm^–2 at 600 °C was superior to the electrolyte‐supported MT‐HF‐SOFCs which delivered ca. six times lower power. The contribution of ohmic and electrode polarization losses of both FCs was investigated using electrochemical impedance spectroscopy. The electrolyte‐supported MT‐HF‐SOFCs had significantly higher ohmic and electrode polarization ASR values; this has been attributed to the thicker electrolyte and the difficulties associated with forming quality anodes inside the small (<1 mm) lumen of the electrolyte tubes. Further development on co‐extruded anode‐supported MT‐HF‐SOFCs led to the fabrication of a thinner electrolyte layer and improved electrode microstructures which delivered a world leading 2,400 mW cm^–2. The newly made cell was investigated at different H₂ flow rates and the effect of fuel utilization on current densities was analyzed.     

A Performance Study of Solid Oxide Fuel Cells With BaZr0.1Ce0.7Y0.2O3–δ Electrolyte Developed by Spray‐Modified Pressing Method

Fuel cells with BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY) proton‐conducting electrolyte is fabricated using spray‐modified pressing method. In the present study the spray‐modified pressing technology is developed to prepare thin electrolyte layers on porous Ni‐BZCY anode supports. SEM data show the BZCY electrolyte film is uniform and dense, well‐bonded with the anode substrate. An anode‐supported fuel cell with BZCY electrolyte and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ (BSCF) cathode is characterized from 600 to 700 °C using hydrogen as fuel and ambient air as oxidant. Maximum power density of 536 mW cm^–2 along with a 1.01 V OCV at 700 °C is obtained. Impedance spectra show that Ohmic resistances contribute minor parts to the total ones, for instance, only ~23% when operating at 600 °C. The results demonstrate that spray‐modified pressing technology offers a simple and effective way to fabricate quality electrolyte film suitable to operate in intermediate temperature.     

The Effect of H2S on the Performance of SOFCs using Methane Containing Fuel

In recent years, the interest for using biogas derived from biomass as fuel in solid oxide fuel cells (SOFCs) has increased. To maximise the biogas to electrical energy output, it is important to study the effects of the main biogas components (CH₄ and CO₂), minor ones and traces (e.g. H₂S) on performance and durability of the SOFC. Single anode‐supported SOFCs with Ni–Yttria‐Stabilised‐Zirconia (YSZ) anodes, YSZ electrolytes and lanthanum‐strontium‐manganite (LSM)–YSZ cathodes have been tested with a CH₄–H₂O–H₂ fuel mixture at open circuit voltage (OCV) and 1 A cm^–2 current load (850 °C). The cell performance was monitored with electric measurements and impedance spectroscopy. At OCV 2–24 ppm H₂S were added to the fuel in 24 h intervals. The reforming activity of the Ni‐containing anode decreased rapidly when H₂S was added to the fuel. This ultimately resulted in a lower production of fuel (H₂ and CO) from CH₄. Applying 1 A cm^–2 current load, a maximum concentration of 7 ppm H₂S was acceptable for a 24 h period.     

Electrochemical performance and microstructure characterization of nickel yttrium‐stabilized zirconia anode

A nickel and yttrium‐stabilized zirconia (Ni‐YSZ) composite is one of the most commonly used anode materials in solid oxide fuel cells (SOFCs). One of the drawbacks of the Ni‐YSZ anode is its susceptibility to deactivation due to the formation of carbonaceous species when hydrocarbons are used as fuel supplies. We therefore initiated an electrochemical study of the influence of methane (CH₄) on the performance of Ni‐YSZ anodes by examining the kinetics of the oxidation of CH₄ and H₂ over operating temperatures of 600–800°C. Anode performance deterioration was then correlated with the degree of carbonization observed on the anode using ex‐situ X‐ray powder diffraction and scanning electron microscopy techniques. Results showed that carbonaceous species led to a significant deactivation of Ni‐YSZ anode toward methane oxidation. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Adsorption of Sulfur‐Containing Species on LaCrO3 (001) Surface: A First‐Principles Study

Density functional theory calculations are employed to investigate the adsorption of sulfur‐containing species on the (001) surface of LaCrO₃ (LCrO). Molecular adsorption is found to be stable with H₂S binding preferentially at O site on the LaO‐terminated surface. The adsorption of H₂S molecule leads to the electrons transferring from the substrate to the molecule and the charges rearrangement within the molecule. In addition, the adsorption of the corresponding S‐containing dissociated species (SH and S) is investigated. SH and S are found to be preferentially bind at the Cr site. We further predict the adsorption energies of sulfur‐containing species increase following the sequence H₂S<SH<S for all the adsorption sites on LCrO (001) surface. Based on the adsorption energy comparison, LCrO is more sulfur‐tolerant than traditional Ni‐based anode materials, which is qualitatively in line with available experimental results. This study provides a scientific basis for rational design of sulfur‐tolerant anode materials for SOFCs.     

Characterization of Ba0.5Sr0.5Co0.7In0.1Fe0.2O3−δ as the Cathode Material for Proton‐conducting SOFCs

Ba_0.5Sr_0.5Co_0.7In_0.1Fe_0.2O_3−δ powders are successfully synthesized as the cathode materials for proton‐conducting solid oxide fuel cells (SOFCs). The prepared cells consisting of the structure of a BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7)‐NiO anode substrate, a BZCY7 electrolyte membrane and a cathode layer, are measured from 600 to 700 °C with humidified hydrogen (ca. 3% H₂O) as the fuel. The electrochemical results show that the cell exhibits a high power density which could obtain an open‐circuit potential of 0.986 V and a maximum power density of 400.84 mW cm⁻² at 700 °C. The polarization resistance measured at the open‐circuit condition is only 0.15 Ω cm² at 700 °C.     

Electrochemical Performance of Cobalt‐free Nd0.5Ba0.5Fe1–xNixO3–δ Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells

A new series of cobalt‐free perovskite‐type oxides, Nd_0.5Ba_0.5Fe_1–xNi_xO_3–δ (0 ≤ x ≤ 0.15), have been prepared by a citric acid‐nitrate process and investigated as cathode materials for proton conducting intermediate temperature solid oxide fuel cells (IT‐SOFCs). The conductivity of the oxides was measured at 300–800 °C in air. It is discovered that partial substitution of Ni for Fe‐sites in Nd_0.5Ba_0.5Fe_1–xNi_xO_3–δ obviously enhances the conductivity of the oxides. Among the series of oxides, the Nd_0.5Ba_0.5Fe_0.9Ni_0.1O_3–δ (NBFNi10) exhibits the highest conductivity of 140 S cm⁻¹ in air at 550 °C. A single H₂/air fuel cell with proton‐conducting BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY) electrolyte membrane (ca. 40 μm thickness) and NBFNi10‐BZCY composite cathode and NiO‐BZCY composite anode was fabricated and tested at 600–700 °C. The peak power density and the interfacial polarization resistance (R_p) of the cell are 490 mW cm⁻² and 0.15 Ω cm² at 700 °C, respectively. The experimental results indicate that NBFNi10 is a promising cathode material for the proton‐conducting IT‐SOFCs.     

Electrochemical Analysis of a System Based on W and Ni Combined with CeO2 as Potential Sulfur‐tolerant SOFC Anode

A formulation of tungsten and nickel combined with CeO₂ (WNi‐Ce) was prepared and evaluated as sulfur‐tolerant anode for SOFC at intermediate temperature. Structural and morphological changes that take place in the system upon interactions with hydrogen sulfide were analyzed. The electrochemical performance was tested in a single cell, WNi‐Ce/LDC/LSGM/LSFC, varying H₂S concentration (0–500 ppm) at 750 °C using I–V curves, impedance spectroscopy and load demands. The highest cell performance was reached in H₂ and decrease with H₂S content increase in the fuel from 226 mW cm⁻² in pure H₂ to 108 mW cm⁻² in 500 ppm H₂S/H₂. Essentially, no decay in the cell performance was observed in the several short‐term load tests studied under several H₂S concentration (0–500 ppm) during 1h, and even in 500 ppm H₂S/H₂ during 70 h, indicating that this material could be a potential sulfur‐tolerant anode.     

An investigation of fuel composition and flow‐rate effects in a H2S fuelled sofc: Experiments and thermodynamic analysis

Hydrogen sulphide (H₂S)‐fuelled solid oxide fuel cells (SOFCs) can potentially generate useful electrical energy while disposing of H₂S, a toxic by‐product of the fossil fuel industry, on site. Experimental results from H₂S fuelled SOFCs exhibit characteristics, for example, an unusual dependence of cell performance on fuel composition and flow‐rate, which are poorly explained in the literature. In this work we: (a) present results for experiments where the composition and flow‐rates were varied for both the fuel and oxidant streams to analyse their effect on fuel cell performance, and (b) develop and use a thermodynamic analysis to help understand these experimental results. Through this work, we shed further light on two basic questions unanswered so far, (1) Why does the flow‐rate of the fuel affect the open circuit potential of the fuel cell? (2) Which of the chemical species present in the fuel is oxidised on the anode? Our experiments and analysis suggest that H₂S, and not H₂ produced from H₂S dissociation, is preferentially electro‐oxidised on the anode in our experiments. © 2011 Canadian Society for Chemical Engineering     

Sr2−XLaXMgMoO6 and Sr2−XLaXMgNbO6 for Use as Sulfur‐Tolerant Anodes Without a Buffer Layer

The objective of this effort is to synthesize and characterize a series of lanthanum‐(La) doped Sr₂MgMoO₆ (SMMO) and La‐doped Sr₂MgNbO₆ (SMNO) anode materials which can be used in combination with lanthanum‐containing electrolytes to mitigate the effects of lanthanum poisoning in solid oxide fuel cells (SOFCs). Currently, an La_0.4Ce_0.6O_1.8 (LDC) buffer layer is used with many perovskite‐based anode materials to prevent La diffusion into the anode from the La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 (LSGM) electrolyte which can create a resistive La species that impedes electrochemical performance. The LDC buffer layer, with diminished electronic conductivity, adds an extra level of complexity in the SOFC manufacturing process. Further, this extraneous layer presents an added experimental challenge when assessing anode material performance. Overall electrochemical performance could be improved if the resistive buffer layer could be removed, thereby allowing the anode material to have direct contact with the electrolyte. To accomplish this, a new class of anode materials was synthesized with the goal of balancing “La” chemical potential between these neighboring materials. La‐doped SMMO and SMNO were prepared and studied. It was hypothesized that by incorporating La into the anode, the gradient of chemical activity between the anode and electrolyte would decrease, which would prevent La diffusion. These anode materials were synthesized via a sol–gel methodology and characterized with X‐ray diffraction to assess phase purity. The conductivity of the materials was analyzed in the presence of both H₂ and 100 ppm H₂S/H₂ to determine the stability and performance of these materials during device operation. The stability experiments demonstrated that 40% La‐doped SMNO is stable in all pertinent environments while not reacting with the LSGM electrolyte.     

La1.7Ca0.3Ni0.75Cu0.25O4‐δ‐Layered Perovskite as Cathode on La0.9Sr0.1Ga0.8Mg0.2O3 or Ce0.8Gd0.2O2 Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells

La₂NiO_4+δ‐based oxides, mixed ionic–electronic conductors with K₂NiF₄‐type structure, have been considerably investigated in recent decades as electrode materials for advanced solid oxide fuel cells (SOFCs) due to their high electrical conductivity and oxidation reduction reaction (ORR) activity. In this study, La_1.7Ca_0.3Ni_0.75Cu_0.25O_4+δ was investigated as a potential cathode on La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ electrolyte support. Furthermore, La_1.7Ca_0.3Ni_0.75Cu_0.25O_4+δ was examined on thin Ce_0.8Gd_0.2O₂ (GDC) electrolyte with Ni‐GDC anode support for intermediate temperature SOFCs (IT‐SOFCs). La_1.7Ca_0.3Ni_0.75Cu_0.25O_4‐δ cathode with gadolinium doped ceria (GDC) electrolyte and NiO‐GDC anode support showed a maximum power density of 0.75 W/cm² in H₂ and lower polarization resistance, Rp (<0.1 Ω cm²), in impedance spectroscopy at 700°C.     

Fabrication of BaZr0.1Ce0.7Y0.2O3 – δ ‐Based Proton‐Conducting Solid Oxide Fuel Cells Co‐Fired at 1,150 °C

Dense proton‐conducting BaZr_0.1Ce_0.7Y_0.2O_{3 – δ} (BZCY) electrolyte membranes were successfully fabricated on NiO–BZCY anode substrates at a low temperature of 1,150 °C via a combined co‐press and co‐firing process. To fabricate full cells, the LaSr₃Co_1.5Fe_1.5O_{10 – δ}–BZCY composite cathode layer was fixed to the electrolyte membrane by two means of one‐step co‐firing and two‐step co‐firing, respectively. The SEM results revealed that the cathode layer bonded more closely to the electrolyte membrane via the one‐step co‐firing process. Correspondingly, determined from the electrochemical impedance spectroscopy measured under open current conditions, the electrode polarisation and Ohmic resistances of the one‐step co‐fired cell were dramatically lower than the other one for its excellent interface adhesion. With humidified hydrogen (2% H₂O) as the fuel and static air as the oxidant, the maximum power density of the one‐step co‐fired single cell achieved 328 mW cm^–2 at 700 °C, showing a much better performance than that of the two‐step co‐fired single cell, which was 264 mW cm^–2 at 700 °C.     

Catalytic Influence of Oxide Component in Ni‐Based Cermet Anodes for Ammonia‐Fueled Solid Oxide Fuel Cells

Recently, the promising prospect of ammonia as a hydrogen carrier for solid oxide fuel cells (SOFCs) has attracted significant interests. In this work, the effects of temperature, fuel content, and total flow rate of anode gas on the performance of Ni/yttria‐stabilized zirconia (Ni/YSZ) anode for ammonia‐fueled SOFCs were investigated. Based on obtained results, the utilization route of ammonia on Ni/YSZ anode was discussed; the results of electrochemical experiments were related with the catalytic decomposition bahavior of ammonia over Ni/YSZ. Moreover, the catalytic activity for ammonia decomposition and anode performance of Ni/samarium‐doped ceria (Ni/SDC) and Ni/yttrium‐doped barium cerate (Ni/BCY) were also investigated. Among these anode materials, Ni/BCY exhibited the highest ammonia decomposition activity and anode performance for ammonia‐fueled SOFCs at intermediate temperatures.     

Biogas Catalytic Reforming Studies on Nickel‐Based Solid Oxide Fuel Cell Anodes

Heterogeneous catalysis studies were conducted on two crushed solid oxide fuel cell (SOFC) anodes in fixed‐bed reactors. The baseline anode was Ni/ScYSZ (Ni/scandia and yttria stabilized zirconia), the other was Ni/ScYSZ modified with Pd/doped ceria (Ni/ScYSZ/Pd‐CGO). Three main types of experiments were performed to study catalytic activity and effect of sulfur poisoning: (i) CH₄ and CO₂ dissociation; (ii) biogas (60% CH₄ and 40% CO₂) temperature‐programmed reactions (TPRxn); and (iii) steady‐state biogas reforming reactions followed by postmortem catalyst characterization by temperature‐programmed oxidation and time‐of‐flight secondary ion mass spectrometry. Results showed that Ni/ScYSZ/Pd‐CGO was more active for catalytic dissociation of CH₄ at 750 °C and subsequent reactivity of deposited carbonaceous species. Sulfur deactivated most catalytic reactions except CO₂ dissociation at 750 °C. The presence of Pd‐CGO helped to mitigate sulfur deactivation effect; e.g. lowering the onset temperature (up to 190 °C) for CH₄ conversion during temperature‐programmed reactions. Both Ni/ScYSZ and Ni/ScYSZ/Pd‐CGO anode catalysts were more active for dry reforming of biogas than they were for steam reforming. Deactivation of reforming activity by sulfur was much more severe under steam reforming conditions than dry reforming; a result of greater sulfur retention on the catalyst surface during steam reforming.     

Fabrication and Characterization of Anode‐Supported Low‐Temperature SOFC Based on Gd‐Doped Ceria Electrolyte

Large‐size, 9.5 cm × 9.5 cm, Ni‐Gd_0.1Ce_0.9O_1.95 (Ni‐GDC) anode‐supported solid oxide fuel cell (SOFC) has been successfully fabricated with NiO‐GDC anode substrate prepared by tape casting method and thin‐film GDC electrolyte fabricated by screen‐printing method. Influence of the sintering shrinkage behavior of NiO‐GDC anode substrate on the densification of thin GDC electrolyte film and on the flatness of the co‐sintered electrolyte/anode bi‐layer was studied. The increase in the pore‐former content in the anode substrate improved the densification of GDC electrolyte film. Pre‐sintering temperature of the anode substrate was optimized to obtain a homogeneous electrolyte film, significantly reducing the mismatch between the electrolyte and anode substrate and improving the electrolyte quality. Dense GDC electrolyte film and flat electrolyte/anode bi‐layer can be fabricated by adding 10 wt.% of pore‐former into the composite anode and pre‐sintering it at 1,100 °C for 2 h. Composite cathode, La_0.6Sr_0.4Fe_0.8Co_0.2O₃, and GDC (LSCF‐GDC), was screen‐printed on the as‐prepared electrolyte surface and sintered to form a complete single cell. The maximum power density of the single cell reached 497 mW cm^–2 at 600 °C and 953 mW cm^–2 at 650 °C with hydrogen as fuel and air as oxidant.     

A novel BaCe0.5Fe0.3Bi0.2O3–δ perovskite-type cathode for proton-conducting solid oxide fuel cells

A perovskite-type BaCe_0.5Fe_0.3Bi_0.2O_3-δ (BCFB) was employed as a novel cathode material for proton-conducting solid oxide fuel cells (SOFCs). The single cells with the structure of NiO-BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY7) anode substrate|NiO-BZCY7 anode functional layer|BZCY7 electrolyte membrane|BCFB cathode layer were fabricated by a dry-pressing method and investigated from 550 to 700 °C with humidified hydrogen (~3% H₂O) as the fuel and the static air as the oxidant. The low interfacial polarization resistance of 0.098 Ω cm² and the maximum power density of 736 mW cm⁻² are achieved at 700 °C. The excellent electrochemical performance indicates that BCFB may be a promising cathode material for proton-conducting SOFCs.     

Modeling of Direct H2S Fueled Solid Oxide Fuel Cells with Reforming Chemical Kinetics

A direct H₂S fueled SOFC model is developed based on Ni‐YSZ/YSZ/YSZ‐LSM button cell test stand. The model considers the detailed reforming chemical processes of H₂S and multi‐physics transport processes in the fuel cell and fuel supply tubes. The model is validated using experimental data. Extensive simulations are performed to study the complicated interactions between multi‐physics transport processes and chemical/electrochemical reactions. The results elucidate the fundamental mechanisms of direct H₂S fueled SOFCs. It is found that suitably increasing the H₂O content in the supplied H₂S fuel can improve SOFC electrochemical performance; high operating temperature may facilitate the reforming of H₂S and improve the electrochemical performance. The sulfur poisoning effect may be mitigated by increasing the H₂O content in the fuel, increasing the operating temperature, decreasing the flow rate, and/or making the cell work at low voltage (or high current) conditions.