Fabrication Of Solid Oxide Fuel Cells Research Articles

Three-Dimensional-Printed Composite Structures: The Effect of LSCF Slurry Solid Loading, Binder, and Direct-Write Process Parameters.

In this research, a direct-write 3D-printing method was utilized for the fabrication of inter-digitized solid oxide fuel cells (SOFCs) using ceramic materials. The cathode electrode was fabricated using the LSCF (La0.6Sr0.2Fe0.8Co0.2O3-δ) slurry loading and the Polyvinyl butyral (PVB) binder. The rheological parameters of slurries with varying LSCF slurry loading and PVB binder concentration were evaluated to determine their effect on the cathode trace performance in terms of microstructure, size, and resistance. Additionally, the dimensional shrinkage of LSCF lines after sintering was investigated to realize their influence on cathode line width and height. Moreover, the effect of the direct-write process parameters such as pressure, distance between the nozzle and substrate, and speed on the cathode line dimensions and resistance was evaluated. LSCF slurry with 50% solid loading, 12% binder, and 0.2% dispersant concentration was determined to be the optimal value for the fabrication of SOFCs using the direct-write method. The direct-write process parameters, in addition to the binder and LSCF slurry concentration ratios, had a considerable impact on the microstructure of cathode lines. Based on ANOVA findings, pressure and distance had significant effects on the cathode electrode resistance. An increase in the distance between the nozzle and substrate, speed, or extrusion pressure of the direct writing process increased the resistance of the cathode lines. These findings add to the ongoing effort to refine SOFC fabrication techniques, opening the avenues for advanced performance and efficiency of SOFCs in energy applications.

Modeling and characterization of sintered YSZ/NiO porous electrode structural properties using coarse-graining molecular dynamics method

In solid oxide fuel cell (SOFC), sintering of YSZ and NiO powders into porous anode plays a crucial role in determining its overall performance. In this study, a coarse-graining (CG) molecular dynamics (MD) model is developed to capture sintered half-cell composed of porous functional layer with NiO/YSZ particles and dense YSZ ceramic electrolyte layer with applied in anode-supported SOFC. The sintering process is simulated in two stages, i.e., heating process followed by temperature-holding process. This study also explores effects of sintering parameters (e.g., temperature, powder size and material compositions) on structural characteristics, including distribution of sintered triple-phase boundary (TPB) length, porosity, and pore size. It is found that structural properties vary mainly during the heating stage of sintering, while a higher sintering temperature leads to a larger TPB length and a lower porosity, while increasing in diameter ratio of the NiO powders to YSZ powders is advantageous for increasing TPB length, porosity and size pores for promoting electrode electrochemical activity. This work provides a bottom-up approach for potential optimization of SOFC fabrication condition and parameters.

Chemistry and electrochemistry of CeO<sub>2</sub>-based interlayers: Prolonging the lifetime of solid oxide fuel and electrolysis cells

Research and development of solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) are currently of paramount importance in terms of realizing hydrogen energy and carbon emission reduction programs, which many countries have committed to. Although, there are many outstanding results in the fabrication and characterization of SOFCs and SOECs with promising oxygen-ionic and proton-conducting electrolytes, conventional zirconia electrolytes are still widely used not only in a lab-scale setup, but also in the form of enlarged cells and stacks, with the experimental operation of the latter during 10 000–100 000 h. To ensure good performance stability and microstructural integrity of such multilayered cells, a special attention should be paid to the chemical activity of functional materials toward their interaction with each other, especially in long-term focus. The literature analysis has shown that many undesirable processes occur in SOFCs and SOECs with the classical pairs of zirconia electrolytes and strontium-containing electrodes, including element segregation and interdiffusion, insulating phase formation, microscopic defect appearance, and delamination. Some of these processes can be efficiently eliminated by using so-called interlayers designed from doped ceria materials. Due to their numerous beneficial functions, such interlayers have several synonymous names: blocking, barrier, buffer, or protecting layers. Herein, we review the recent progress and achievements in the fundamental and applied researches dealing with the ceria interlayers and their impact on chemistry and electrochemistry of solid oxide cells based on classical zirconia electrolytes as well as promising oxygen-ionic and proton-conducting analogs. <br> The bibliography includes 405 references.

Sintering of Protonic Ceramics for Tubular Solid Oxide Fuel Cells

Protonic ceramics are considered to be the most promising candidates for intermediate-temperature solid oxide fuel cells (SOFCs) owing to their high proton conductivity and competitive electrochemical performance at intermediate temperatures (typically 500–700 °C). However, the fabrication of proton-conducting SOFCs has been challenging mainly due to the poor sinterability of common proton-conducting electrolyte materials. Significantly high sintering temperatures (1500–1600 °C) and long sintering times (up to 24 h) have been reported for obtaining dense, gas-tight proton-conducting electrolytes. These sintering conditions may lead to undesirable interfacial reactions and dense electrode microstructures, thereby limiting the cell performance. The present work focuses on optimizing the co-sintering temperature of tubular SOFCs with a traditional Ni–yttria-stabilized zirconia (YSZ) anode support and proton-conducting functional layers. BaZr0.3Ce0.5Y0.2O3 (BZCY), Ni–BZCY, and BZCY–BaCo0.4Fe0.4Ce0.1Y0.1O3 were used as the electrolyte, anode functional layer, and cathode, respectively. Various approaches to lowering the co-sintering temperature and resulting cell performances will be discussed. We will also examine the thermal cycling behavior and durability of the fabricated protonic SOFCs.

Enhancement of low-temperature solid oxide fuel cell performance and durability via surface chemistry modification

The development of active cathodes is one of the most critical challenges to lowering the operating temperature for solid oxide fuel cells (SOFCs). Here we demonstrated that by modifying the cathode surface chemistry at a relatively low temperature, the cathode activity and durability can be simultaneously enhanced on high-performing, low-temperature cathodes such as (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ (LSCF) and Sr0.5Sm0.5CoO3-δ (SSC). This low-temperature modification using multi-valent cations activates the highly defected surface and maintains the nanoscale electrocatalysts by bypassing the high-temperature sintering procedure for SOFC fabrication. The modified cathode at 600°C shows an order of magnitude reduction in impedance to only 0.05 Ωcm2 with a peak power density of 1.1 W/cm2 and increases stability over 2000 h. The combination of in situ characterization, distribution of relaxation time analysis on impedance spectroscopy, and surface chemistry analysis reveals the importance of surface chemistry control on the gas-solid reaction activity and durability and provides the design principle for numerous future solid oxide cells.

(Invited) A Novel, Ultra-Fast Fabrication of SOFCs By Flash Light Sintering Process

Solid oxide fuel cells (SOFCs) have strong potential for next-generation energy conversion systems because of their high energy conversion efficiency, fuel flexibility and low emission of pollutant. In the last decades, the SOFC technology has rapid progress in terms of performance and durability aspect, expanding its application ranges from stationary system to portable devices. In the fabrication of SOFCs, a simple and mass productive fabrication process such as screen-printing, tape casting and sol-gel method are widely used in the industry because it is easy, scalable and economical process. However, a high temperature post heat-treatment process is required to ensure material properties of the ceramic electrolyte and electrode in these processes. In particular, to achieve a highly dense electrolyte layer, a high sintering temperature above 1300oC is usually employed in the fabrication process, which causes considerable amount of time and significant manufacturing costs. Therefore, reducing the sintering temperature and long process time is one of the major challenges to reduce the fabrication cost and facilitate the commercialization of SOFCs. To overcome this problem, many efforts have been made to demonstrate alternative sintering process to reduce the sintering temperature and process time. For example, several researchers proposed spark plasma sintering process as an alternative sintering process because of the high heating rate within a short time based on Joule heating mechanism [1, 2]. However this process has a limitation of complex vacuum system configuration and high equipment cost for commercialization. In addition, excimer laser ablation annealing and microwave sintering process have been also suggested but there are still limitations such as extremely narrow spot size and complexity of microwave interactions with materials respectively [3, 4]. On the other hand, the flash light sintering process presented here uses a white flash light generated by a xenon arc lamp discharge as a heat treatment sources and has many advantages of an extremely short process time, scalability and simple process condition as standard ambient and temperature. Most of all, because this process conducted within a few seconds, it can significantly improve the productivity and cost-effectiveness of the SOFCs. Our previous works successfully demonstrated that the solution-based YSZ thin films was annealed by flash light sintering and it showed comparable material properties to the conventional thermal sintering process [5]. Nevertheless, due to the low light absorption property and difficulty to sinter the ceramic particles, studies on the flash light sintering process for the powder-based films are still limited. In this work, we demonstrate for the first time rapid fabrication of SOFCs using a novel flash light sintering process. The ceramic electrolyte and electrode was fabricated by using mass productive screen-printing method and flash light sintering process was employed as an alternative sintering process of conventional thermal sintering method. In the sintering process, the effect of various pulse conditions on the sinterbility of the ceramic components were investigated, and the material properties such as microstructure, crystallinity and electrochemical property were analyzed. The optimized ceramic components were employed to fabricate SOFC unit cell and their cell performance and impedance behavior was evaluated to demonstrate the flash light sintering process. Reference [1] Deepash Shekhar Saini et al. “Improved conductivity of spark plasma sintered Ho-Substituted Ba-ZrO3 electrolyte ceramics for IT-SOFCs.” ACS Applied Energy Materials. 1, 7, 3469-3478 (2018)[2] Tatiana L. Simonenko et al. “Spark plasma sintering of nanopowders in the CeO2-Y2O3 system as a promising approach to the creation of nanocrystalline intermediate-temperature solid electrolytes.” Ceramics International, 44, 16, 19879-19884 (2018)[3] Hao, H., Xu, L., Huang, Y., Zhang, X. & Xie, Z. “Kinetics mechanism of microwave sintering in ceramic materials.” Science in China Series E: Technological Sciences 52, 2727–2731 (2009).[4] Bayati, R. et al. “Modifcation of properties of yttria stabilized zirconia epitaxial thin flms by excimer laser annealing.” ACS applied materials & interfaces 6, 22316–22325 (2014).[5] Park, J.S. et al. Rapid, cool sintering of wet processed yttria-stabilized zirconia ceramic electrolyte thin films. Scientific reports 7, 12458 (2017)

Magnetron sputtered LSC-GDC composite cathode interlayer for intermediate-temperature solid oxide fuel cells

The paper investigates the influence of the La0.6Sr0.4CoO3-δ-Gd0.1Ce0.9O1.95 (LSC-GDC) composite cathode interlayer on the operation of solid oxide fuel cells (SOFCs). Thin composite layers with the different GDC content are obtained by the reactive magnetron sputtering. The impact of the high-temperature annealing on the LSC-GDC phase composition is studied by the X-ray diffraction instrument using additionally a synchrotron radiation. The NiO-YSZ anodes with the YSZ electrolyte thin film and GDC barrier layer are used for the SOFC fabrication. The current-voltage curves and impedance spectra of SOFCs are obtained in the temperature range of 700–800°С. It is shown that not annealed composite layers with ∼50 vol% GDC content possess the most efficient electrochemical activity. The maximum power density of the SOFC with the LSC-GDC interlayer is 1322, 1041 and 796 mW/cm2 at 800, 750 and 700 °C, respectively, which is 20–35% higher than that of the cell without cathode interlayer.

Novel La1−xCaxTi1−yTayO3-δ perovskites with enhanced conductivity for solid oxide fuel cell electrodes

The novel La1−xCaxTi1−yTayO3-δ nano perovskites with high oxygen vacancies have been prepared for solid oxide fuel cells (SOFCs) by facile hydrothermal method. To enhance the oxygen vacancies, structural, thermal and chemical stability, the phase composition of the materials was varied with multiple x & y values like (0, 0.2, 0.4, 0.6, 0.8 and 1). All the as prepared compositions of La1−xCaxTi1−yTayO3-δ were undergone heat treatments in autoclave, calcined at 700 °C and sintered at 1000 °C. These modified perovskite oxides can add in hefty point defects, such as oxygen vacancies that compensate dopants or are part of an inherent off-stoichiometry. These defects have high mobilities in La1−xCaxTi1−yTayO3-δ perovskites, making it most suitable for electrochemical energy conversion devices. The appropriate phase purity, structural properties, lattice parameters and morphology of the perovskite were inspected. Electrical conductivity of sintered pellets was analyzed using electrochemical impedance spectroscopy. The electrical behavior of La1−xCaxTi1−yTayO3-δ is enhanced at lower operating temperature and long-term structural stability, phase purity is also improved owing to the inclusion of Calcium and Tantalum (La1-xCaxTi1−yTayO3-δ). The enhancement in electrical conductivity, electro-catalytic activity is credited by means of partial replacement of Ca and Ta in the perovskite structure. Among them, La0.6Ca0.4Ti0.6Ta0.4O3-δ showed the maximum conductivity of 9.18 × 10-2 S cm-1 at 750 °C. These results demonstrate that the La1−xCaxTi1−yTayO3-δ perovskites are capable as electrode material intermediate temperatures SOFCs fabrication.

Synthesis and Characterization of Gadolinium-Doped Zirconia as a Potential Electrolyte for Solid Oxide Fuel Cells

Zirconia-based composites with high thermochemical stability and electrochemical activity are the most promising solid electrolytes for manufacturing solid oxide fuel cells (SOFCs). In the present work, nanocrystalline composite powders of gadolinium-doped zirconia (GDZ: Gd2xZr2(1−x)O4−x) with various doping fractions (0.01 ≤ x ≤ 0.16) were synthesized by the Pechini method and applied for the fabrication of several electrolyte pellets to evaluate their physicochemical properties, sinterability, and conductivity. The X-ray diffraction (XRD) patterns and the thermogravimetry/differential thermal analysis (TGA/DTA) of the synthesized powders confirmed the successful formation of nanocrystalline GDZ in the tetragonal phase with complete substitution of gadolinium phase into the zirconia (ZrO2) lattice. The synthesized gadolinium zirconate powders were then shaped into pellet forms using the tape casting method, followed by sintering at 1300 °C (for 2.5 h). The microstructural analysis of the electrolyte pellets showed suitable grain boundary welding at the surface with an acceptable grain growth at the bulk of the T-phase GDZ samples. The impedance measurements indicated that the T-phase GDZ-8 could provide a comparably higher ionic conductivity (with 7.23 × 10−2 S/cm in the air at 800 °C) than the other dopant fractions. The results of this work can help better understand the characteristics and electrochemical performance of the T-phase gadolinium zirconate as a potential electrolyte for the fabrication of SOFCs.

A dynamic infiltration technique to synthesize nanolayered cathodes for high performance and robust solid oxide fuel cells

Solution infiltration is a popular technique for the surface modification of solid oxide fuel cell (SOFC) cathodes. However, the synthesis of nanostructured SOFC cathodes by infiltration is a tedious process that often requires several infiltration and high temperature (≥500 °C) calcination cycles. Moreover, fabricating large-area nanostructured cathodes via infiltration still requires serious attention. Here, we propose a facile and scalable urea assisted ultrasonic spray infiltration technique for nanofabrication of SOFC cathodes. It is demonstrated that by using urea as a precipitating agent, the calcination after each infiltration cycle can be omitted and the next infiltration can be performed just after a drying step (≤100 °C). Finally, the precipitates can be converted into a desired catalyst phase in single calcination thus, a nanostructured cathode can be fabricated in a much faster manner. It is also shown that the low calcination temperature of the cathode (≤900 °C) can produce highly durable SOFC performance even without employing a Ce0.9Gd0.1O2 (GDC) diffusion barrier layer which provides the ease of SOFC fabrication. While coupling with an ultrasonic spray technique, the urea assisted infiltration can be scaled up for any desired cathode area. La0.6Sr0.4Co0.2Fe0.8O3 nanolayered cathode was fabricated and it was characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. SEM showed the formation of a nanolayer cathode just after 5 cycles of the urea assisted infiltration while the XRD and TEM confirmed the phase and stoichiometric uniformity of the ∼100 nm cathode nanolayer. The effectiveness of the newly developed technique was further verified by the stable operation of a GDC buffer layer free SOFC having an active cathode area of 25 cm2 during a 1200 h durability test. The research outcomes propose urea assisted ultrasonic spray infiltration as a facile, scalable, and commercially viable method for the fabrication of durable nanostructured SOFC cathodes.

Solid Oxide Fuel Cells with a Thin Film Electrolyte: A Review on Manufacturing Technologies and Electrochemical Characteristicses

Solid oxide fuel cells (SOFCs) are electrochemical systems converting the energy released during fuel oxidation into electrical energy. SOFCs are considered as a promising clean energy technology due to the high efficiency of fuel-to-power conversion and environmental friendliness. The potential applications of SOFCs extend from stationary power generation units for industrial and household facilities to auxiliary power units in vehicles and portable power sources. One of the main elements of SOFCs is a solid oxide electrolyte possessing ionic conductivity at high temperatures (above 700 °C). The main challenge in the SOFC commercialization is related to their high operating temperature, which entails materials degradation, short life-time, long start-up and shut-down times, and high cost. One of the most effective ways to reduce the SOFC operating temperature is to minimize the electrolyte thickness. In this regard, fabrication of SOFCs with a thin film electrolyte has been attracting high research activity over the past few decades. Different fabrication techniques were reported to be applicable for manufacturing thin film SOFCs, and the fuel cell performance was found to be highly dependent on the appropriate selection of materials and processing technologies. The present review is focused on state-of-the-art fabrication technologies of the thin film SOFCs. A brief survey of configurations and geometries of the thin film SOFCs and methods of deposition of solid-oxide films is given. Special attention is focused on the electrical generation performance of the thin film SOFCs.

Rapid fabrication of lanthanum strontium cobalt ferrite (LSCF) with suppression of LSCF/YSZ chemical side reaction via flash light sintering for SOFCs

The chemical side reaction between a lanthanum strontium cobalt ferrite (LSCF) cathode and an yttria-stabilized zirconia (YSZ) electrolyte is a key problem for solid oxide fuel cell (SOFC) fabrication. Herein, we demonstrate rapid fabrication of an LSCF cathode using a novel flash light sintering (FLS) technique to suppress the chemical side reaction; notably, no Gd-doped ceria interlayer is required. The FLS-fabricated LSCF cathode shows comparable crystalline development and microstructural evolution to a cathode fabricated using the conventional high-temperature sintering process. A clear LSCF/YSZ interface appears, indicating suppression of detrimental chemical side reactions via rapid and surface-dominant sintering kinetics. Detailed interfacial microstructure analysis reveals the absence of Sr segregation and SrZrO3 formation at the FLS-LSCF cathode/YSZ electrolyte interface. Remarkable performance of 0.9 W cm–2 power density at 750 °C is achieved for a cell incorporating the rapidly fabricated LSCF cathode on the interlayer-free YSZ electrolyte owing to suppression of the insulating secondary phases and significantly reduced polarization resistance. Hence, rapid LSCF cathode fabrication via the novel FLS method effectively suppresses secondary phase formation at the LSCF/YSZ interface and greatly simplifies SOFC fabrication by eliminating the doped-ceria-based interlayer. Moreover, this novel method could help overcome interfacial chemical reaction problems in various solid-state devices, while also providing new application opportunities for high-performance electrolyte/electrode materials.

Fused filament fabrication for anode supported SOFC development: Towards advanced, scalable and cost-competitive energetic systems

Solid Oxide Fuel Cells (SOFC) are electrochemical devices that efficiently convert hydrogen or other fuels into energy and heat and are considered a promising and environmentally friendly alternative to mitigate the effects of climate change. The difficulties found for the SOFC devices implementation into the market have triggered an enhanced need of research on the manufacturing strategy, since the cells and stack fabrication remain a complicated procedure. In this work, the capabilities of an additive manufacturing (AM) process, as it is Fused Filament Fabrication (FFF), applied to anode supported SOFC were explored for the first time, due to the strengths this technology offers in terms of materials versatility and cost-competitive process. Moreover, the ability of tuning the support porosity during the printing process was demonstrated, minimizing the pore former concentration required and avoiding reformulation of the feedstock to achieve higher porosity in the anode. In the cell fabrication, low-cost techniques for thin film deposition were employed. The preliminary geometry investigated represents a novel SOFC device that shows an electrochemical performance comparable to that found in the conventional SOFC fabrication and it opens the possibility of potential single-cell and stack designs in which the use of ceramic sealing could be avoided and the system durability increased.

Solid oxide fuel cells with zirconia/ceria bilayer electrolytes via roll calendering process

Solid oxide fuel cell (SOFC) with yttria-stabilized zirconia (YSZ)/gadolinium-doped ceria (GDC) bilayer electrolytes has been considered one of representative types of high-performance intermediate-temperature SOFCs, because it enables to apply highly active cathode materials. However, the processing of fully dense bilayer electrolytes with good interfacial structure, which are essential for high-performance and secure operation, has not yet been successfully achieved. Here, we present a simple and cost-effective roll calendering process for fabrication of SOFCs with YSZ/GDC bilayer electrolytes. From understanding on the correlation between processing conditions of roll calendering and microstructure of bilayer electrolytes, the roll calendering process is optimized, and in turn, the bilayer electrolytes with overall thickness of 8.0 μm, relative density above 98%, and enhanced interfacial connectivity of 69% is obtained by co-sintering at reduced temperature of 1250 °C. This excellent structural enhancement in comparison with conventional uniaxial pressing process is explained by a shear force applied during roll calendering process, which presumably facilitates particle rearrangement within bilayer electrolytes. The optimized cell yields promising electrochemical performance of 1.45 W/cm2 at current density of 2 A/cm2 and low ohmic resistance of 0.046 Ω cm2 at 800 °C.

Optimization of sintering temperature for SOFCs by a co-firing method

A single-step co-firing process is employed to fabricate solid oxide fuel cells (SOFCs) with an Sm-doped CeO2 (SDC) electrolyte, aiming to simplify the SOFC fabrication procedure. Complete tri layer green cells are co-fired in a single step at different fabrication temperatures, and the co-firing temperature significantly influences the cell microstructure and thus influences its performance. Electrochemical investigations indicate that the cell resistance, including the ohmic and polarization resistance, varies with the co-firing temperatures used and leads to a different cell power output. Although the cell polarization resistance follows the rule that higher firing temperatures lead to higher polarization resistance values, the cell ohmic resistance does not follow the expectation that the cell fired at 1200 °C gives the largest ohmic resistance. The morphology analysis reveals the mechanism behind this phenomenon. The best cell performance is obtained by co-firing the cell at 1000 °C, reaching 401 mW cm−2 at 700 °C.

Influence of Surface State on the Electrochemical Performance of Nickel-Based Cermet Electrodes during Steam Electrolysis

Due to their fuel flexibility and high efficiency, solid oxide cells are a promising technology for sustainable energy production and storage. Nickel in combination with yttria-stabilized zirconia (YSZ) or gadolinium-doped ceria (GDC), forming Ni-YSZ or Ni-GDC cermets respectively, are the most widely adopted electrodes in solid oxide fuel cell fabrication. Currently, there is an increasing interest in cermet electrodes for hydrogen generation through high temperature steam electrolysis using solid oxide electrolysis cells (SOECs). However, durability remains a major issue for reliable operation of SOEC systems. A variety of processes accountable for permanent performance degradation of SOECs has been identified based on post-mortem cell analysis. Besides, transient/reversible degradation processes are typically examined by indirect methods, like impedance spectroscopy. The reason is that the application of material characterization techniques during SOEC operational conditions is challenging. In this wor...

Electrical properties of gadolinia doped ceria electrolytes fabricated by infiltration aided sintering

Common solid oxide fuel cell (SOFC) electrolyte materials (e.g., gadolinia doped ceria – GDC) demand temperatures exceeding 1400 °C for densification by conventional solid state sintering. It is very desirable to reduce the densification of the SOFC electroltytes to i) avoid microstructural coarsening of the composite anode layers, which are co-sintered with the electolyte layer in the anode supported SOFC fabrication scheme and ii) reduce energy consumption during SOFC manufacturing. We have recently demostrated a novel infiltration-aided sintering route to densify GDC ceramics at 1200 °C. In the present work, we present the electrical properties of GDC ceramics fabricated thusly. Comparison of high density (≥95%) samples fabricated by conventional or infiltration-aided sintering reveal that at 700 °C, similar total electrical conductivities are obtained, while at 300 °C, specific grain boundary resistivity is smaller in the latter. Bulk (grain) conductivity is higher in porous GDC ceramics (relative density ≤ 90%) fabricated by infiltration-aided sintering than the conventionally sintered ones with similar porosities. Finally, open circuit voltage of 0.84 V at 700 °C, obtained under dilute hydrogen and stagnant air conditions suggests that GDC ceramics densified by infiltration-aided sintering are suitable for use as SOFC electrolytes.

Mitigating Carbon Formation in SOFCs with Aluminum Titanate Doped NiO-YSZ Anodes

Solid Oxide Fuel Cells (SOFCs) are high temperature energy conversion devices that produce electricity efficiently and sustainably. Because of the need to electrochemically reduce molecular oxygen and because of the relatively high activation energy required for oxide ions to diffuse through the dense, solid-state electrolyte, SOFCs typically operate at temperatures in excess of 650˚C. High operating temperatures endow SOFCs with many advantages including fuel flexibility and high conversion efficiencies, distinguishing them from many other types of fuel cells. In order to benefit from this role SOFCs must be able to operate well with a variety of fuels including H2, CH4, syngas, biogas, and higher molecular weight hydrocarbons. This requirement necessitates keeping the SOFC anodes clear from excessive carbon deposition or “coking” when exposed to hydrocarbon fuels. Standard SOFC anodes composed of a Ni-YSZ cermet are highly susceptible to coking due to Ni’s high catalytic activity towards activating hydrogen-carbon bonds. Deposited carbon can be detrimental to SOFCs as it can block catalytic sites and impede gas transport, limiting overall cell performance. One way to mitigate this effect is to introduce steam into the fuel feed to remove carbon via oxidation. However, as steam also risks oxidizing the catalytically active Ni metal, materials-focused solutions are often preferred. Modifying the Ni based anodes with secondary materials via infiltration is one such solution that yields significant improvements to the anode stability with minimal impacts to overall SOFC fabrication procedures. Aluminum titanate (Al2TiO5 or ALT) has previously been shown to increase electrochemical performance, mechanical strength, and resilience toward redox cycling in cells operated with H2. We have shown that the addition of this same material can also help limit carbon formation on both the anode surface and within the bulk when operating with CH4 as a fuel. The combination of ALT and NiO-YSZ leads to the formation of secondary phases, including a nickel aluminate spinel (NiAl2O4) and a zirconium titanate (Zr5Ti7O24), that via multi-pathway reaction mechanisms improve the Ni-YSZ anode tolerance to hydrocarbon fuels. These studies have used a combination of operando Raman spectroscopy and electrochemical measurements to identify and quantify carbon formation as a function of applied polarization and temperature. Results show that ALT doped anodes form ~25% less carbon on the surface under open circuit conditions. Additionally, an increase of polarization beyond 50% of maximum cell current results in close to no carbon depositing on the ALT doped surface. The effect of long term exposure to CH4 was studied using spectrochronopotentiometry methods. These experiments suggest that carbon build up within the bulk of the ALT doped cells is minimal over longer CH4 exposures which suggest that the secondary phases formed not only help prevent carbon deposits through a barrier layer mechanism but also function as internal reformers. Results provide evidence that ALT is effective at preventing carbon from accumulating to an extent that would hinder overall cell performance during the exposure to CH4 and that small changes to SOFC anode processing can have substantial effects on the performance.

A review of key parameters for effective electrophoretic deposition in the fabrication of solid oxide fuel cells

This paper reviews recent progress in electrophoretic deposition (EPD), particularly in solid oxide fuel cells (SOFCs). EPD is a simple, cost-effective, and geometrical flexible colloidal process. With its excellent control of thickness and other morphological characteristics, it is favored for the fabrication of SOFCs because each component layer of an SOFC has different requirements. However, the effectiveness of EPD is closely related to the suspension stability and EPD processing parameters. Maintaining a stable suspension and optimizing the EPD processing parameters are essential to achieve a dense and uniform deposition layer. Key parameters in maintaining the suspension stability are generally categorized into colloidal related parameters, including particle size and solid loading, and suspension media related parameters, including dielectric constant and conductivity. The effects of these parameters are often reflected by the zeta potential of the suspension, which can be manipulated by using charging agents to maintain a stable state. The deposition time and applied voltage are key parameters in optimizing the EPD process through their effects on the deposition rate. The effects of these parameters on particle surface charges and on the EPD mechanism are discussed.

Development and evaluation of a 3-cell stack of metal-based solid oxide fuel cells fabricated via a sinter-joining method for auxiliary power unit applications

Recently, metal-based solid oxide fuel cells (SOFCs) receive much attention for potential application in auxiliary power units (APUs). In this study, a sinter-joining method with a silver bonding layer is proposed. This method enables the fabrication of metal-based SOFCs by joining metal plates and conventional ceramic cells using a silver bonding layer. This sinter-joining method has the advantage of full-sintering of the cathode at 1100 °C, which facilitates a lower area specific resistance (ASR) of the cathode. Furthermore, the entire manufacturing process is conducted under air atmosphere. A 5 × 5 cm2 metal-based cell is successfully fabricated by the sinter-joining method, and a maximum power density of 433 mW cm−2 and a low polarization resistance of 0.12 Ω cm2 is obtained. Using the metal-based cells, a prototype 3-cell SOFC stack is developed considering mechanical robustness and diesel reformate fuel supply for future APU system applications. The stack exhibits a maximum power density of 100 mW cm−2 and is tested for 120 h. After the test, a post-mortem analysis is conducted, and the causes of the low electrochemical performance and degradation issue are investigated. In the conclusion, the sinter-joining method is considered as one of the methods for metal-based SOFCs.