Symmetrical solid oxide fuel cells based on titanate nanocomposite electrodes

Nanocomposite electrodes of (Sr_0.7Pr_0.3)_0.95TiO_3±δ?Ce_0.9Gd_0.1O_1.95 are directly prepared by spray-pyrolysis deposition on Zr_0.82Y_0.16O_1.92 electrolytes and their properties are compared with those obtained by the traditional screen-printing powder method. The structural, microstructural and electrical characteristics are investigated for their potential use as both cathode and anode in Solid Oxide Fuel Cells. The nanocomposite electrodes with reduced particle size ～30 nm achieved a polarization resistance at 700 ºC of 0.50 and 0.46 Ω cm² in air and pure H₂, respectively, outperforming those obtained for the analogous screen-printed electrodes with particle size of 450 nm, i.e. 4.8 and 3.9 Ω cm², respectively. An electrolyte-supported cell with symmetrical electrodes reached a maximum and stable power density of 354 mW cm^-2 at 800 ºC. These results demonstrate that the performance of electrode materials with modest electrochemical properties but high phase stability, such as doped-SrTiO₃, can be highly improved by preparing nanocomposite electrodes directly on the electrolyte surface.     

Numerical modeling of solid oxide fuel cells

This paper presents a numerical model for a planar solid oxide fuel cell (SOFC) with mixed ionic-electronic conducting electrodes. Transport of positive or negative charges, which takes place in the direction of down- or up-gradient electric potential, respectively, within the composite electrodes and through the electrolyte membrane, is mimicked by making use of an algorithm for Fickian diffusion in the commercial software. The output cell voltage, which is the potential difference between the two current collectors, is fixed at a given value. The coupled equations describing the conservation of mass, momentum and energy and the chemical and electrochemical processes are solved using the commercial package Star-CD, augmented with subroutines developed in-house. Results for the concentration of chemical species and the distributions of temperature and current density in an anode-supported SOFC with direct internal reforming are presented and discussed. The potential for using this model as a general numerical tool to study the impact of the detailed processes taking place in SOFCs is discussed.     

Film percolation for composite electrodes of solid oxide fuel cells

A film percolation model is proposed for composite electrodes of solid oxide fuel cells (SOFCs). The model is developed to predict the percolation properties of 2D-infinite structures which represent the structural characteristics of composite electrodes of electrochemical devices such as SOFCs. The model can be used to estimate electrode properties, such as percolation probability, active three-phase boundary length and interfacial polarization resistance. Compared with the classic percolation theory, which is particularly applicable to 3D-infinite bulks, the model can explicitly capture the effects of thinly layered nature of composite electrodes, and describes a cross-over between 2D-infinite films and 3D-infinite bulks. It also permits the prediction within whole electrode composition range, and can be easily applied in SOFC modeling.     

Rationally structuring proton-conducting solid oxide fuel cell anode with Ni metal catalyst and porous skeleton

In response to the urgent demand for highly active anodes for lower-temperature proton-conducting solid oxide fuel cells (H–SOFCs) and with the aim to explore the function of metal catalysts and porous skeletons, two series anode-supported BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY)-based single cells with varying Ni catalysts and pore formers were assembled and evaluated comparably. In the exploration of Ni catalyst variable, the NBZCY65-35-20SS (65 wt% NiO in NiO-BZCY prepared with adding 20 wt% starch) anode possesses the highest performance, for the 1:1 vol ratio of Ni-BZCY could offer the maximum effective triple-phase boundary (TPB) area in the prerequisite of having abundant pores achieved with the 20 wt% pore former as a constant. In addition, both the NBZCY65-35-15SS and NBZCY65-35-25SS anode demonstrate inferior electrochemical properties separately due to the inadequate reducing gas transmission channels and reaction sites. The BZCY cell assembled with NBZCY65-35-20SS reveals an excellent performance, in which the peak power densities (PPDs) were 660, 539, 413, 272 mW cm⁻² and the polarization resistances (R_P) were 0.061, 0.126, 0.28, 0.652 Ω cm² at 700, 650, 600, 550 °C, respectively. NBZCY65-35-20SS, which has both a superior TPB area and a fine porous anode skeleton, is a preferable option for anode-supported H–SOFCs. On the whole, the scientific regulations governing metal catalysis and pore-forming could be beneficial to the architecture of fine H–SOFC anode structures.     

Synthesis of NiO-YSZ composite particles for an electrode of solid oxide fuel cells by spray pyrolysis

NiO-YSZ (YSZ: Y₂O₃-stabilized ZrO₂) composite particles for a Ni-YSZ cermet anode in solid oxide fuel cells (SOFCs) were synthesized via spray pyrolysis (SP). The formation mechanism of the composite particles by this process was analyzed. The internal microstructure of the particles synthesized during SP processing was observed at each heating temperature of 200, 300, 400 and 1000 °C, and then the formation mechanism of the composite structure was discussed. As a result, it was found that NiO-YSZ composite particles were formed through the following steps. Firstly, during the evaporation stage up to 200 °C, a filled particle with Ni(CH₃COO)₂ and YSZ fine grains were formed from the atomized droplet containing Ni ion and dispersed YSZ sol by volume precipitation. Secondly, during the continuous thermolysis stage up to 400 °C, YSZ grains were formed and moved to the surface of the composite particle by the outgas and the oxidation of Ni(CH₃COO)₂. Finally, the NiO-YSZ composite particle that has NiO grains uniformly covered with fine YSZ grains was formed after the final sintering stage up to 1000 °C.     

Nano-structuring of solid oxide fuel cells cathodes

Different approaches are proposed for increasing the overall performance of solid oxide fuel cells (SOFC) by promoting the nano-structuring of perovskitic cathode layers. The formation of pore systems in the nanometer scale will make available a larger surface area (three-phase-boundary) for the electrocatalytic processes occurring on the air electrode. Two different concepts can be pursued for this purpose: (i) synthesis of thermally-stable mesoporous particles and ulterior deposition of them as macroporous films using coating techniques such as screen printing; and (ii) production of nano-structured films of regularly-arranged nano-sized particles of electrocatalytic materials over the dense electrolyte. The first progresses in the synthesis of nano-structured LSFC cathode layers following the first approach are depicted as well as their electrochemical characterization when using them as SOFC cathodes assembled on complete anode-supported (Ni-YSZ/YSZ) fuel cells.     

Interconnect materials for next-generation solid oxide fuel cells

Highly-efficient solid oxide fuel cell (SOFC) systems are gaining increased attention for future energy conversion applications. Many planar SOFC stack designs utilise ferritic stainless steel (FSS) interconnect components. During operation, surface corrosion of FSS interconnects degrades stack operation by increasing electrical resistance and introducing other deleterious material interactions. To minimise these effects, various surface modifications and coatings are currently under investigation. Two of these methods under development for this application are: metal organic chemical vapour deposition (MO-CVD); and, large area filtered arc deposition (LAFAD). SOFC interconnect-relevant corrosion behaviour of an MO-CVD coating on Crofer 22 APU, AL453, Fe30Cr and Haynes230, and complex, amorphous LAFAD AlCrCoMnTiYO coatings on FSS 430 were investigated. Both of these surface modifications and coatings exhibit significantly improved corrosion protection as compared with uncoated FSS samples.     

Electrochemical properties of porous bismuth electrodes

The properties of Bi surfaces with different roughnesses were characterized by electron microscopy, cyclic voltammetry, and impedance spectroscopy. Two different strategies were used for preparation of porous bismuth layers onto Bi microelectrode surface in aqueous 0.1 M LiClO₄ solution. Firstly, treatment at potential E < −2 V (vs. Ag|AgCl in sat. KCl) has been applied, resulting in bismuth hydride formation and decomposition into Bi nanoparticles which deposit at the electrode surface. Secondly, porous Bi layer was prepared by anodic dissolution (E = 1 V) of bismuth electrode followed by fast electroreduction of formed Bi³⁺ ions at cathodic potentials E = −2 V. The nanostructured porous bismuth electrode, with surface roughness factor up to 220, has negligible frequency dispersion of capacitance and higher hydrogen evolution overvoltage than observed for smooth Bi electrodes.     

Isothermal models for anode-supported tubular solid oxide fuel cells

Modeling of solid oxide fuel cells (SOFCs) has gained considerable significance in recent years. A detailed phenomenological model for SOFC can be used to understand performance limitations, optimization, in situ diagnostics and control. In this paper, we study the transport and various electrochemical phenomena in an anode-supported tubular SOFC using a steady-state model. In particular, we discuss the importance of modeling different phenomena vis-a-vis their impact on the prediction capability of the model. It is observed that even a reasonably simple model can be sufficiently predictive in a particular operating range. As the operating range of the cell is increased, the predictive capability of a model validated in a narrow range cannot be guarantied. It has also been observed that neglecting momentum conservation in the model for a tubular SOFC can affect the predictive capability of the model at higher overpotentials. An extensively validated model is used to study the percentage conversion of oxygen and oxygen concentration profile within a cell at different operating conditions. All of the simulation studies are supported by experimental data that spans a wide range of operation in terms of the DC polarization, reactant flow rates and operating temperatures.     

Exceptionally stable nanostructured air electrodes for reversible solid oxide fuel cells via crystallization-assisted infiltration

Nanostructures present favorable prospects of manufacturing high-performing air electrodes for reversible solid oxide fuel cells (RSOFCs) with the potential to reduce their operating temperature. Here, we present trichloroacetic acid as an original infiltration agent for the facile nanoengineering of RSOFC electrodes. The new process relies on the thermal decomposition of trichloroacetic acid in water at temperatures above 70 °C, which causes intense CO₂ effervescence and crystallizes out the metal ions in the solution as slightly soluble carbonates. Essentially, this allows for the subsequent infiltration step to be performed immediately after drying, as opposed to conventional infiltration, which requires high-temperature calcination after each infiltration step. The anode-supported RSOFC consisting of a nanostructured LaCoO₃ air electrode permitted smooth switching between fuel cell and electrolysis cell modes with no evidence of degradation. In addition, the RSOFC presented exceedingly durable performance during accelerated stability tests.     

Three-dimensional random resistor-network model for solid oxide fuel cell composite electrodes

A three-dimensional reconstruction of solid oxide fuel cell (SOFC) composite electrodes was developed to evaluate the performance and further investigate the effect of microstructure on the performance of SOFC electrodes. Porosity of the electrode is controlled by adding pore former particles (spheres) to the electrode and ignoring them in analysis step. To enhance connectivity between particles and increase the length of triple-phase boundary (TPB), sintering process is mimicked by enlarging particles to certain degree after settling them inside the packing. Geometrical characteristics such as length of TBP and active contact area as well as porosity can easily be calculated using the current model. Electrochemical process is simulated using resistor-network model and complete Butler-Volmer equation is used to deal with charge transfer process on TBP. The model shows that TPBs are not uniformly distributed across the electrode and location of TPBs as well as amount of electrochemical reaction is not uniform. Effects of electrode thickness, particle size ratio, electron and ion conductor conductivities and rate of electrochemical reaction on overall electrochemical performance of electrode are investigated.     

硫化氢固体氧化物燃料电池研究进展

综述了H2 S固体氧化物燃料电池 (SOFC)的发展历史和研制现状 ,包括固体电解质薄膜如质子传导膜和氧离子传导膜的开发、电极催化材料尤其是阳极催化材料的研制、以及整个电池系统的性能研究。指出H2 SSOFC在工业化过程中所面临和必须解决的关键技术问题是 :电解质薄膜材料的研制及其制备 ,尤其是薄膜化的制备技术 ;电极材料的开发及制备 ,特别是阳极催化材料的选择与制备技术 ;膜 -电极三合一制备技术。并对H2 SSOFC的开发及工业应用前景作了展望     

Electrochemical performance of intermediate temperature micro-tubular solid oxide fuel cells using porous ceria barrier layers

We describe the manufacture and electrochemical characterization of micro-tubular anode supported solid oxide fuel cells (mT-SOFC) operating at intermediate temperatures (IT) using porous gadolinium-doped ceria (GDC: Ce_0.9Gd_0.1O_2−δ) barrier layers. Rheological studies were performed to determine the deposition conditions by dip coating of the GDC and cathode layers. Two cell configurations (anode/electrolyte/barrier layer/cathode): single-layer cathode (Ni–YSZ/YSZ/GDC/LSCF) and double-layer cathode (Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF) were fabricated (YSZ: Zr_0.92Y_0.16O_2.08; LSCF: La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ). Effect of sintering conditions and microstructure features for the GDC layer and cathode layer in cell performance was studied. Current density–voltage (j–V) curves and impedance spectroscopy measurements were performed between 650–800 °C, using wet H₂ as fuel and air as oxidant. The double-cathode cells using a GDC layer sintered at 1400 °C with porosity about 50% and pores and grain sizes about 1 μm, showed the best electrochemical response, achieving maximum power densities of up to 160 mW cm⁻² at 650 °C and about 700 mW cm⁻² at 800 °C. In this case GDC electrical bridges between cathode and electrolyte are preserved free of insulating phases. A preliminary test under operation at 800 °C shows no degradation at least during the first 100 h. These results demonstrated that these cells could compete with standard IT-SOFC, and the presented fabrication method is applicable for industrial-scale.     

Nanostructured La0.5Ba0.5CoO3 as cathode for solid oxide fuel cells

A simple method has been used to synthesize nanostructured La_0.5Ba_0.5CoO₃ (LBCO) powders, by confining chemical precursors into the pores of polycarbonate filters. The proposed method allows us to obtain powders formed by crystallites of different sizes, it is scalable and does not involve the use of sophisticated deposition techniques.The area specific polarization resistance of symmetrical cells was studied to analyze the electrochemical behavior of the LBCO nanostructures as cathodes for Solid-Oxide Fuel Cells.We show that the performance is improved by reducing the size of the crystallites, obtaining area specific resistance values of 0.2 Ωcm² at 700 °C, comparable with newly developed cathodes using novel deposition techniques.     

Enhancing thermal-stability of metal electrodes with a sputtered gadolinia-doped ceria over-layer for low-temperature solid oxide fuel cells

Cathodic activation loss is the dominant loss mechanism in the operation of low-temperature solid oxide fuel cells (LT-SOFCs). The thermal degradation of metallic cathodes decreases the performance of LT-SOFCs, causing practical issues in long-term operation. In this paper, we investigate the effect of the sputtered gadolinia-doped ceria (GDC) over-layer on the thermal stability of platinum (Pt) cathodes. The thermal stability of Pt cathodes with 23 nm-thick GDC over-layers significantly increased compared to that of the Pt-only cathodes after 2hrs’ operation at 450 °C. (<4% vs. 17% performance degradation, respectively).     

Shear strength tests of glass ceramic sealant for solid oxide fuel cells applications

Different approaches are used for the integration of ceramic components in solid oxide fuel cells stacks, where dissimilar materials (ceramics and metals) have to be joined and coupled for a reliable long term operation. This work focuses on the mechanical characterisation of a glass ceramic sealant used for the joining of Crofer22APU metallic interconnect samples as well as the interaction with a preoxidised Crofer22APU. Crofer22APU–glass ceramic sealant joined samples are tested by two different mechanical tests. Hourglass samples with different geometries were tested using an in-house developed torsion test machine at room temperature. In addition, their mechanical strength was also evaluated according to the ISO?13124 standard. The comparison of the two different testing methods, with particular focus on the shear strength of the joined samples, are reviewed and discussed.     

Electrochemical performance of low-temperature solid oxide fuel cells running on syngas from pyrolytic urban sludge

Low temperature solid oxide fuel cells (SOFCs) that efficiently utilize widely available hydrocarbon resources are highly desirable for cost reduction and durability purposes. In this work, SOFCs consisting of highly ionic conductive ceria-carbonate composite electrolytes and lithiated transition metal oxide symmetric electrodes are assembled and their electrochemical performances at reduced temperature (≤650 °C) are investigated using syngas fuel (44.65% H₂, 10.19% CH_4, 2.01% CO and the balanced CO₂) derived from pyrolytic urban sludge. The cell gives a peak power output of 127 mW cm^?2 at 600 °C and shows a relatively stable operation for 11 hours under constant voltage operational conditions. Though the composite electrode presents a moderately high polarization resistance toward CH₄ and CO oxidation and the electrochemical performance is highly correlated with the microstructure of ceria-carbonate electrolyte, it is interesting to see that a higher concentration of methane is obtained after the fuel cell reaction, which may suggest an alternative approach to realize the power and chemical co-generation within such a SOFC reactor. Finally, the symmetric electrode shows high resistance toward carbon deposition, possibly due to its high alkaline nature.     

Highly flexible solid oxide fuel cells using phase-controlled electrolyte support

Flexible solid oxide fuel cells (SOFCs) have attracted increasing attention due to their excellent mechanical stability and lightness. An essential electrolyte material for developing highly flexible SOFCs is 3 mol% yttria-stabilized zirconia (3YSZ), but there remain some difficulties in its application to SOFCs. We report a phase-controlled, bendable, ultra-thin 3YSZ electrolyte with a thickness of ~22 µm, high flexural strength, and improved ohmic resistance that surpasses that of 8 mol% YSZ electrolytes. A flexible cell (total thickness < 60 µm) is fabricated through simple and reproducible methods such as tape casting and screen printing. The highest cell performance is achieved among the reported flexible SOFCs, with the maximum power density of 0.86 W cm^?2 at 900 °C using conventional cermet electrodes, Ni-YSZ anode, and LSM-YSZ cathode. Our study provides a well-defined framework for developing flexible SOFCs as next-generation power sources for mobile devices with high volumetric power.