Anode-supported Solid Oxide Fuel Cells Research Articles

Relationships between mechanical stability of the anode supports and electrochemical performance of intermediate-temperature SOFCs

Mechanical strength and microstructure of the anode supports are among the most critical factors affecting the long-term stability and performance of the intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this work, the anode supports made of Ni/NiO and 10 mol.% Sc2O3 and 1 mol.% Y2O3 co-stabilized zirconia (10Sc1YSZ) were fabricated by tape casting, followed by sintering and screen-printing of the anode functional layer of 10 mol.% Gd2O3 doped ceria (10GDC) and NiO. Then two dense solid-electrolyte layers, 8 mol.% Y2O3 stabilized zirconia (8YSZ) and 10GDC, were deposited by the reactive pulsed dual magnetron sputtering. Mechanical properties of as-sintered and reduced anode supports were estimated employing three-point bending technique. The power density of model anode-supported SOFCs, where mechanical stability of the support was kept after complete reduction in hydrogen, achieved 0.3 W/cm2 at 800°C and 0.6 V under air/hydrogen gradient.

Numerical simulation and thermal stress analysis of direct internal reforming SOFCs

ABSTRACT Hydrocarbon fuels such as CH4 can be converted into H2 and CO by direct internal reforming (DIR) processes within the anode of solid oxide fuel cells (SOFCs). DIR-SOFCs can reduce system complexity and capital cost due to the elimination of external reformers. However, the strong endothermic reforming reactions at the cell entrance may result in large thermal stress, leading to premature failure. In this study, a numerical simulation study was carried out to analyze the effect of operating conditions and cell structures on temperature, composition and thermal stress distributions in DIR-SOFCs. A two-dimensional axisymmetric model was developed by considering the coupling effects of the chemical/electrochemical reactions; transport processes of mass, charge, or heat; and thermal mechanical stress. The failure probability of the cell was estimated by stress distributions. Simulation results show that the increase of the steam-to-carbon ratio and operating voltage leads to higher thermal stresses and higher failure probability. The introduction of a metal supportive layer may release the thermal stress problem at severe DIR-SOFC operation conditions, which leads to a flatter temperature distribution and smaller failure probability compared to anode-supported SOFCs.

Determination of Factors Governing Degradation of Anode-Supported Solid Oxide Fuel Cells as Influenced by Current Density and Humidity

In order to understand the degradation mode of the Ni/YSZ anode in anode-supported cells, durability measurements were performed as a function of current density (OCV, 0.25, 0.5, and 1 A/cm2) and humidity and evaluated using electrochemical impedance spectroscopy. The impedance spectra were analyzed using distribution of relaxation times (DRT) method and the corresponding resistances were determined for varied p(O2), p(H2), and p(H2O). From the DRT analysis it was revealed that the anode resistance significantly increased when 0.60 bar p(H2O) and 1 A/cm2 was applied to the cell. Microstructural analysis of the cross-section of Ni/YSZ anode using SEM revealed that there are significant changes in the density and particle size of Ni particles and porosity in the anode when higher humidity and current density were applied.

Development of Anode-Supported Solid Oxide Fuel Cell by Sequential Tape-Casting and Co-Sintering

Simple and cost-effective technologies to produce solid oxide fuel cells require control of microstructure, thickness, homogeneity, and reproducibility of the functional layers. The manufacturing of a solid oxide fuel cell (SOFC) involves significant ceramic processing challenges to obtain layers with controlled microstructure. Possibly the most common technique for large-scale production of SOFCs is tape casting. In this study, YSZ electrolyte and 60NiO/YSZ anode slurries were studied for the production of half-cells deposited by the sequential tape casting technique. A co-sintering procedure was developed for the half-cells and after cathode deposition the single cells were tested.

Aqueous Thick-Film Ceramic Processing of Planar Solid Oxide Fuel Cells Using La0.20Sr0.25Ca0.45TiO3 Anode Supports

Recent research into the upscaling and implementation of Rh/Ce0.80Gd0.20O1.90 co-impregnated La0.20Sr0.25Ca0.45TiO3 (LSCTA-) anodes in electrolyte-supported SOFC at short-stack industrial scales has resulted in extremely robust performance under realistic operation and tolerance to harsh conditions. Furthermore, evaluation of the mechanical strength of LSCTA- and incorporation of this material into anode-supported SOFC also yielded promising performance at the button cell scale (using Ni and CeO2 catalyst impregnates). The knowledge on ceramic processing obtained during these previous research campaigns may be used to develop anode-supported SOFC with LSCTA- ‘backbones’ that have been optimised for high mechanical strength, high ‘effective’ electronic conductivity and sufficient porosity. Therefore, this manuscript details the preparation of anode-supported SOFC using the thick-film ceramic processing technique of aqueous tape casting, the optimisation of anode microstructure through addition of aqueous solvent-compatible graphitic and methacrylate polymer pore formers and the co-sintering of a LSCTA- support with a typical SOFC electrolyte material.

Evaluation of Three-Dimensional Placement of Built-in Catalytic Partial Oxidation Catalyst in an Anode-Supported Honeycomb SOFC

Honeycomb solid oxide fuel cells (SOFCs) can achieve high volumetric power density with improved thermo-mechanical durability at high temperatures. In this study, a compact SOFC with catalytic partial oxidation reforming catalyst embedded in the flow channels of a porous honeycomb anode support has been fabricated and evaluated. Current-voltage (I-V) measurements and gas chromatography for the outlet gas were carried out while methane/air mixture gas was fed as fuel. I-V measurements were performed to compare the performance of two flow channel configurations: one with fuel fed to the central channel having a built-in reformer and discharged from surrounding four channels, and the other with fuel fed to the four channels with the reformer and discharged from the central channel. As a result, the concentration overpotential was smaller in the latter case. The gas chromatography detected hydrogen in the reformed gas at the outlets of both cases.

Development of High-Performance Anode-Supported Planar SOFC with Large Area by 4-Layer Co-Firing Process

Solid oxide fuel cell (SOFC) technology has attracted great attention because of higher efficiency, fuel flexibility, and eco-friendly stationary power generation system. However, its commercialization is hindered due to the lack of durability and high-cost. Herein, a facile and exceptionally reliable 4-layered anode supported SOFCs with a large area (12cm×12cm) was fabricated in a single step co-firing process at 1400°C, in which tape-casted green films of NiO-YSZ, NiO-1CeScSZ, 1CeScSZ, and GDC were laminated by isostatic press. The fabrication parameter, such as viscosity of the slurries and mechanical strengths of the green films, were optimized. As a result, finely porous anode layer, and highly dense 1CeScSZ electrolyte and GDC buffer layers on a robust porous anode-support, were formed successfully. The possibility of mass production is verified through the fabrication of large area cells with an active area 100 cm2. The single cell showed high-power output of 28 W at total current 50 A at 700°C and exhibits low degradation rate of 0.2%/kh at 25A for long-term test of 1000 hrs. This facile strategy is time-efficient, reproducible, cost-effective, and highly practical to produce SOFC system satisfying both high-performance and durability.

Nanoscale Analysis of LSM/YSZ Interfaces within Composite Cathodes for Commercial Solid Oxide Fuel Cells

Atom probe tomography (APT) was utilized to probe composition at the nanoscale across cathode/electrolyte interfaces for anode-supported commercial SOFCs. These SOFCs contained a porous composite cathode layer, consisting of sintered (La0.8Sr0.2)0.95MnO3 (LSM) and yttria-stabilized zirconia (YSZ). SOFCs were operated up to a 500 h duration at a 0.75 A/cm2 current density and 800°C. The measured cell voltage increased over the first 100 hours of operation, followed by a steady and linear drop in cell voltage that translated to a 5.35% performance loss per 1000 h. Cation migration behavior was extrapolated through comparisons of compositional profiles acquired by APT across LSM/YSZ particle interfaces for as-sintered, operated, and thermally aged conditions. The results show that operation drives La/Sr away from the LSM nanoparticle surface and Y depletion at the YSZ nanoparticle surface. Energy dispersive spectroscopy (EDS) inside a scanning transmission electron microscope (STEM) provided additional insights regarding Mn aggregation behavior during operation.

Performance Evolution of Ni-YSZ Anode-Supported SOFCs During Initial-Stage Operation

Performance evolution of typical Ni-YSZ anode-supported SOFCs during the initial-stage operation (<100 h) under different electrical currents is systematically evaluated. It is found that the performance evolution can be roughly divided into two stages, namely activation stage (about 0-20 h) and aging stage (> 20 h), regardless of the operating current. In the activation stage, the cell performance maintains stable or slightly increases due to the enhanced anode gas-phase diffusion. While in the subsequent aging stage, the cell performance decreases significantly and gradually approaches a plateau in about 100 hours, mainly due to the weakened anode charge transfer reaction and ionic transport processes. Therefore, it is suggested that more efforts should be focused on improving the stability of anode microstructure, especially the stability of anode functional layer.

An Investigation of the Redox Stability of an Anode-Supported SOFC Stack Using Acoustic Emission Monitoring

The mechanical stability of an SOFC stack is compromised when its cells operate in oxidation-reduction (Redox) environments. Previous studies, based mainly on voltage-current or electrochemical impedance measurements, showed that cell performance degraded upon redox cycling above 600°C. Therefore, the purpose of this work is to investigate the Redox process and the stability of a SOFC stack applying the Acoustic Emission (AE) monitoring technique. The investigation includes the interpretation of AE events and signals considering their waveform and frequency content.

Mechanism of improved electrochemical performance of anode-supported solid oxide fuel cells by mesostructural modification of electrode–electrolyte interface

The mechanism whereby structural modification on the mesoscale order (10–100 μm) improves the electrochemical performance of anode-supported solid oxide fuel cells (SOFCs) is elucidated. After preparing two types of anode-supported SOFC having different electrode–electrolyte interfacial areas, we carry out their structural analyses and electrochemical characterization. Next, we develop a two-dimensional (2D) numerical model in which the structures of the cells are implemented and then verify its validity by comparing experimental and simulation results. It is found that the structural features in the mesoscale-modified cell, such as interfacial area enlargement and thickness inhomogeneity, cause nonuniform distributions of physicochemical quantities that contribute to electrochemical reactions. Consequently, the decreases in the ohmic and activation overpotentials of the mesoscale-modified cell relative to the flat cell are respectively larger and smaller than those estimated under the assumptions that the ionic and charge-transfer current densities are uniformly distributed in a cell. Moreover, the ionic current density distribution has a strong nonuniformity at a high current density, leading to a large relative decrease in ohmic loss. Furthermore, the cell overpotential is more reduced at a higher current density; thus, the mesostructural modification of anode-supported SOFCs can lead to a higher cell performance.

Performance enhancement of a beam and slot interconnector for anode-supported SOFC stack

This study proposes a novel interconnector, termed beam and slot interconnector (BSI), for the anode-supported planar solid oxide fuel cell (SOFC). A detailed comparative investigation is conducted on various transport characteristics and electrical performance of the SOFC stacks with conventional straight channel interconnectors (SCIs) or with novel interconnectors. Results show that the peak power density of a SOFC stack with BSIs is 24.8% higher than it with SCIs at 700 °C and a fuel–air flow rate of 16–40 Nml/(min·cm2). Moreover, BSI can reduce the fuel–air feeding and enhance the fuel–air utilization while maintaining high output power density. Compared with SCI, BSI promotes the gas disturbance, significantly increases the gas velocity and vorticity, and leads the gas to flow in the direction perpendicular to the channel. BSI eliminates the limitation of SCIs on gas diffusion in the electrode and transfers sufficient reactant gas into the electrode function layers for electrochemical reactions. BSI shortens the charge transfer path in SOFC and almost avoids the adverse effects of the electrode-interconnector contact resistance. Compared with the conventional SOFC stack, the novel SOFC stack with BSIs significantly reduces various overpotentials. At 700 °C and 0.5 A/cm2, the activation, concentration, and contact overpotentials are reduced by 8.5%, 47.4%, and 96.4% respectively, and the total overpotential finally drops by 20.0%. Overall, the electrical performance of the SOFC stack with novel interconnectors significantly exceeds the one with conventional interconnectors under the same operating conditions.

TEM Study of Oxygen Partial Pressure Effect on Early LSM-YSZ Surface Interactions in Solid Oxide Fuel Cells

Abstract Long-term stability studies indicate that cathode degradation is one of the main failure mechanisms in anode-supported SOFCs. In order to better understand the microstructural degradation mechanisms of the cathode and the influence of oxygen partial pressure at relevant operating temperatures, the authors of this article acquired TEM samples from technological cells to study cation interdiffusion mechanisms and LSM-YSZ interactions, particularly in areas where LSM grains are in contact with YSZ electrolyte. Here they present and interpret the results.

Operando optical studies of solid oxide fuel cells operating on CO and simulated syngas fuels

Solid oxide fuel cells (SOFCs) are attractive devices for combined power and heat generation because they can operate with a variety of fuels. Syngas (CO + H2) is particularly attractive but many questions remain about its behavior on SOFC anodes including the relative oxidation efficiencies of CO and H2 and tendency to form carbon deposits. In this report, operando spectroelectrochemical measurements, including infrared emission spectroscopy, near infrared thermal imaging, and vibrational Raman scattering, combined with spectrochronopotentiometry, are performed using anode-supported SOFCs operating with simulated syngas at 800 °C. Results quantify the fate of syngas over Ni-YSZ anodes in SOFCs related to their performance. SOFCs operating with simulated syngas mixtures are compared with those for H2 and CO used separately. Thermal imaging shows more SOFC anode heating for H2-rich syngas than for operating on CO or H2 alone. We attribute this result to carbon gasification by water that is produced by H2 oxidation. Raman spectra together with spectrochronopotentiometry experiments illustrate that carbon is not observed on the anode surface at 800 °C – although it is at 750 °C and 700 °C – yet carbon is likely present in the spectroscopically inaccessible, electrochemically active region at the electrolyte-electrode interface.

Additive manufacturing of thin electrolyte layers via inkjet printing of highly-stable ceramic inks

Inkjet printing is a promising alternative for the fabrication of thin film components for solid oxide fuel cells (SOFCs) due to its contactless, mask free, and controllable printing process. In order to obtain satisfying electrolyte thin layer structures in anode-supported SOFCs, the preparation of suitable electrolyte ceramic inks is a key. At present, such a kind of 8 mol% Y2O3-stabilized ZrO2 (8YSZ) electrolyte ceramic ink with long-term stability and high solid loading (> 15 wt%) seems rare for precise inkjet printing, and a number of characterization and performance aspects of the inks, such as homogeneity, viscosity, and printability, should be studied. In this study, 8YSZ ceramic inks of varied compositions were developed for inkjet printing of SOFC ceramic electrolyte layers. The dispersing effect of two types of dispersants, i.e., polyacrylic acid ammonium (PAANH4) and polyacrylic acid (PAA), were compared. The results show that ultrasonic dispersion treatment can help effectively disperse the ceramic particles in the inks. PAANH4 has a better dispersion effect for the inks developed in this study. The inks show excellent printable performance in the actual printing process. The stability of the ink can be maintained for a storage period of over 30 days with the help of initial ultrasonic dispersion. Finally, micron-size thin 8YSZ electrolyte films were successfully fabricated through inkjet printing and sintering, based on the as-developed high solid loading 8YSZ inks (20 wt%). The films show fully dense and intact structural morphology and smooth interfacial bonding, offering an improved structural quality of electrolyte for enhanced SOFC performance.

Feldspar-based CaO–K2O–Na2O–BaO silicate glass sealant for solid oxide fuel cells

Sealants have been widely used in solid oxide fuel cells (SOFCs), and glass sealants have shown considerable potentials due to their low leakage rate. We report a type of feldspar-based glass sealant, mainly composed of CaO–K2O–Na2O–BaO, with a coefficient of thermal expansion (CTE) of 10.11 × 10−6 k−1, a glass transition temperature at 528 °C, and a softening temperature at 573 °C. Using this sealant to seal anode-supported SOFCs, the open-circuit voltage (OCV) remains at 1.08 V after five thermal cycles and 50-h aging at 750 °C, close to the theoretical OCV value. The SEM and EDS reveal stable interface between the glass sealant and the anode, indicating excellent performance of the CaO–K2O–Na2O–BaO glass sealant for SOFC sealing applications.

High-performance Gd0.5Sr0.5CoO3−δ and Ce0.8Gd0.2O1.9 nanocomposite cathode for achieving high power density in solid oxide fuel cells

High power density operation is essential for cost reduction in solid oxide fuel cells (SOFCs), and thus the present work proposes a highly active Gd0.5Sr0.5CoO3−δ (GSC) and Ce0.8Gd0.2O1.9 (GDC) nanocomposite cathode for intermediate-temperature SOFCs. Its nanocomposite structure is formed by using GSC-GDC nanocomposite particles synthesized via spray pyrolysis, which produces uniform-sized secondary particles consisting of nano-sized GSC and GDC crystallites. The resulting cathode has an extensive electrochemically active triple-phase boundary and has electrically conductive networks formed with highly dispersed GSC and GDC crystallites. In addition, the GSC-GDC nanocomposite cathode exhibits a performance comparable to a high-performance Sm0.5Sr0.5CoO3−δ and Ce0.8Sm0.2O1.9 nanocomposite cathode. An anode-supported SOFC with the GSC-GDC nanocomposite cathode achieves remarkably high power densities at 0.75 V, i.e., 3.66, 2.60, 1.66, and 0.98 W cm−2 at 800, 750, 700, and 650 °C, respectively. These results suggest the proposed nanocomposite cathode yields a synergistic effect of a high catalytic activity of GSC-GDC and a fine nanocomposite structure, leading to high power density operation of an SOFC.

Efficient and controlled nano-catalyst solid-oxide fuel cell electrode infiltration with poly-norepinephrine surface modification

There is a growing attention to enhance the performance of solid oxide fuel cell (SOFC) electrodes through the incorporation of nano-catalyst materials within the electrodes’ active sites. In this study, we report a technique for increased efficiency and microstructural control of the nano-catalyst infiltration process through polymerized norepinephrine (pNE) treatment. Nano-CeO2 catalysts were incorporated within both electrodes of commercial anode-supported SOFCs using a single salt solution step after a coating of pNE within the porous microstructure. The optimization of catalyst loading was performed by varying the cerium nitrate solution concentrations between 0.4 and 2.0 M. The ceria nanoparticles are distributed at the near-electrolyte region in both electrodes, but with microstructural variance due to the precursor molarity and solid loading. The time-dependent polarization resistance (Rp) variation was categorized into three zones by the nano-catalyst loading. Zone I referred to the baseline performance for the low-catalyst loading and fluctuating Rp. However, the cells in Zone II and III showed a continuous time-dependent activation as low as 0.275 Ω cm2 at 750 °C. The results suggest that the nano-CeO2 film reduces coarsening and related degradation of the backbone. In addition, the pNE-assisted dip infiltration enhanced infiltrant deposition efficiency by reducing the number of infiltration steps.

Investigation of the Electrochemical Performance of Anode-Supported SOFCs under Steady-State Conditions

Investigation of the Electrochemical Performance of Anode-Supported SOFCs under Steady-State Conditions

Operando Optical Studies of Sulfur Contamination in Syngas Operation of Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are attractive devices for power generation because they can operate under a variety of fuels, including H2, hydrocarbon fuels, biogas, and syngas (CO + H2). Syngas is an important fuel since it is a major product of reforming hydrocarbon logistics fuels (i.e. JP-8, JP10, S8, etc.). However, the mechanism of electrochemical oxidation of CO in SOFCs is still not well understood. This is due, in part, to the difficulty in probing CO consumption and product formation under operating conditions in real time. Herein, the operando methods Fourier transform infrared emission spectra (FTIRES) of the SOFC headspace together with thermal imaging data of the Ni-YSZ anode surface to provide data relevant to understanding these reaction mechanisms. The studies are carried out on anode-supported SOFCs (Ni-YSZ/GDC-LSC button cells) operating on simulated syngas mixtures. The contamination and subsequent degradation of SOFC operation at operational temperatures of 600° C to 800° C, in the presence of sulfur concentrations below 30 ppm, is followed using potentiostatic spectroelectrochemistry, chronocoulometry, electrochemical impedance spectroscopy, and operando optical methodologies to provide a comprehensive picture of the processes occurring in the fuel oxidation. Together, the data obtained indicate that sulfur contaminants 1) competitively inhibit formation of carbon over CO catalytic and electrocatalytic sites, and 2) induce changes of the Ni-YSZ anode permanently perturbing their catalytic and electrocatalytic function.