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

Gadolinium-doped ceria (GDC) is widely recognized as an effective diffusion barrier layer in solid oxide fuel cells (SOFCs) due to its ability to prevent undesirable chemical reactions. However, the high sintering temperatures required for GDC densification often lead to the formation of a Ce-Zr solid solution phase, which compromises the long-term stability and performance of SOFCs. To address these issues, this study investigates a new ternary-doped ceria composition, Ce(Gd, Bi, Yb)O2-δ(GYBC), as an alternative diffusion barrier layer material. The GYBC layer was fabricated using ultrasonic spray coating, producing a thin, dense layer with optimal microstructural properties. When applied in a SOFC unit cell (Ni-YSZ/YSZ/GYBC/La0.4Sr0.6Co0.2Fe0.8O3-δ (LSCF)-GDC/LSCF), the GYBC layer showed a significant improvement in electrochemical performance. The SOFC unit cell achieved a power density of 2.32 W cm-2 at an operating temperature of 800 °C, which is higher than cells that use a conventional GDC diffusion barrier layer. This improvement is attributed to the high oxygen storage capacity (OSC) of the GYBC layer, which enhances oxide ion supply at the cathode/electrolyte interface and substantially boosts performance under the high current conditions typical of practical SOFC applications. These results suggest that GYBC has the potential as a diffusion barrier material for SOFCs, improving their performance and durability.

Exceptional cathodic performance of nanostructured La–Sr–Co–Fe–O highly dispersed in porous yttria-stabilized zirconia frameworks for intermediate-temperature solid oxide fuel cells

Solid oxide fuel cells (SOFCs) attract attentions as high-efficiency energy-conversion devices. In order to establish high-performance SOFCs, the slow electrode kinetics, which is often a main cause of the energy loss, should be improved. In present SOFCs, mixed ionic-electronic conducting (MIEC) oxides, such as (La,Sr)CoO3 -δ and (La,Sr)(Co,Fe)O3 -δ , are typically used as the cathode materials. In such an MIEC cathode, it is generally believed that electrochemical oxygen reduction reaction takes place mainly at the oxide surfaces (double phase boundary, DPB) and the contribution of the reaction at the electrode/electrolyte/gas interface (triple phase boundary, TPB) is insignificant. Therefore, researches on the kinetics of SOFC cathodic reaction have mostly concerned to investigate the surface reaction on the electrode oxides. In our previous works, the reaction distributions in MIEC SOFC electrodes were experimentally evaluated by using operando micro X-ray absorption spectroscopy (operando μ-XAS) [1, 2]. For more quantitative and precise investigation of the reaction distribution, we proposed a patterned thin film electrode, which is a kind of the columnar electrode simplifying the microstructures of a porous electrode [2]. However, the patterned thin film electrode in our previous work did not have triple phase boundaries, and it was thus impossible to discuss the contribution of TPB reaction to the total reaction. In this work, a patterned thin film electrode having triple phase boundaries was fabricated. (La,Sr)CoO3- d (LSC) was chosen as the model MIEC oxide for SOFC cathodes. The reaction distribution in this new type of the patterned thin film electrode was experimentally determined by using operando μ-XAS, and the results were compared with ones obtained with the old type of the patterned thin film electrode without TPB. It was found the effective reaction area tended to shrink by the introduction of TPB to the patterned thin film electrode. This fact might suggest that the reaction at TPBs in SOFC cathodes is not negligible even when MIEC oxides are used as electrode materials.

Review: 469 refs.

Solid Oxide Fuel Cell (SOFC) cathode materials rely on the catalytic processes at their surfaces to reduce and incorporate oxygen. For many common cathode materials surface passivation occurs through 'A site' segregation at common operating temperatures [1]. These processes have a negative effect upon the oxygen reduction rates and therefore overall cell performance. Quantifying the true extent these segregation effects have on the exchange properties has not been fully explored but it is clear that the properties of the surface are essential. Many techniques have been employed to study cell properties in-situ, however, surface studies are typically restricted to post mortem or ex-situ analysis due to the necessity of a high vacuum. In this study High Temperature Environmental Scanning Electron Microscopy (HT-ESEM) has been employed to analyse the morphology changes of common SOFC cathode materials in-situ. Samples of La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and La0.6Sr0.4CoO3-x (LSC) were annealed from room temperature to 1000oC at pressure ranges between 3 – 6mbar in atmospheres of Oxygen and Water. Using secondary electron imaging during the thermal annealing cycle morphological and compositional changes were able to be documented at temperature for the first time (Figure 1). This technique has enabled detailed analysis of the surface degradation that occurs during SOFC fuel cell operation. Secondary phase precipitation rate and formation behaviour has been quantified over a range of temperatures and time scales furthering the understanding of SOFC cathode surfaces significantly. [1] M. Kubicek, A. Limbeck, T. Fromling, H. Hutter, and J. Fleig, "Relationship between cation segregation and the electrochemical oxygen reduction kinetics of La0.6Sr0.4Co0.2Fe0.8O3 thin film electrodes" Journal of The Electrochemical Society, vol. 158, no. 6, pp. B727–B734, 2011. Figure 1 - Showing in-situ growth of secondary phase particulates on a polished surface of LSCF Figure 1

Solid oxide fuel cells (SOFCs) offer direct conversion of hydrogen or hydrocarbon fuels to electricity with high energy conversion efficiency (>50%). Impurities that block active reaction sites on the electrode surface can markedly degrade the oxygen exchange rate, and lead to a reduction in long term durability of such devices. In this work, we propose to develop electrodes that are tolerant to Si which is one of the most prevalent cathode impurities. This work draws on our recent method to fully recover oxygen exchange kinetics following Si induced aging of ceria containing cathodes. We take advantage of elements, which over time during SOFC operation, are released by exsolution to actively trap (i.e. getter) Si impurities in a self-regulating chemical fashion. The objective is to evaluate impurity scavenging by added reactive elements that are intentionally exsolved from the SOFC cathode electrode during operation. In this study, we chose to focus our effort on one cathode material: Pr doped ceria (PCO). This material has the advantage of not containing strontium, an element known to segregate to the surface of the well-known perovkite based cathodes materials, thereby degrading their performance. As for the Si getter, we choose to focus on lanthanum, which is added in PCO in solid solution.

SrCo0.4Fe0.4Zr0.1Y0.1O3-δ, A new CO2 tolerant cathode for proton-conducting solid oxide fuel cells

Solid oxide fuel cell (SOFC) technology requires further maturations to become commercialized broadly. Many existing problems are expected to be solved by intermediate temperature SOFC (IT-SOFC). On the other hand, electrochemical resistances of cathode materials increase rapidly at lower temperature. So it is necessary to understand the underlying mechanisms of oxygen reduction reaction and develop new materials. Although many experimental and theoretical researches for this purpose are in progress, traditional trial-and-error materials search must be inefficient and take too long time. Considering the above situation, we explore critical factors that can explain the oxygen exchange rate of perovskite oxide cathode in SOFCs. It has been said that the bulk ionic conductivity is strongly related to the oxygen exchange rate, while the electronic conductivity is not as far as it is high enough. However, we found that the combination of ionic and electronic conductivities shows much stronger correlation than ionic conductivity alone while analyzing reported experimental data on 18 materials in figure 1. Materials which have high oxygen exchange rate can be explained by the multiplication of bulk ionic conductivity and electronic conductivity while others explained by the ionic conductivity. Fig. 1 Relationship between oxygen exchange rate and bulk electronic conductivity, ionic conductivity, and the model function with both conductivities. Figure 1

Overcoming phase instability of RBaCo2O5+δ (R=Y and Ho) by Sr substitution for application as cathodes in solid oxide fuel cells

Integrated gasification fuel cell (IGFC) systems combining coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally friendly utilization of coal for power production. Most IGFC system analyses performed to-date have used nondimensional thermodynamic SOFC models that do not resolve the intrinsic constraints of SOFC operation. In this work a quasi-two-dimensional (2D) finite volume model for planar SOFC is developed and verified using literature data. Special attention is paid to making the model capable of supporting recent SOFC technology improvements, including the use of anode-supported configurations, metallic interconnects, and reduced polarization losses. Activation polarization parameters previously used for high temperature electrolyte-supported SOFC result in cell performance that is much poorer than that observed for modern intermediate temperature anode-supported configurations; thus, a sensitivity analysis was conducted to identify appropriate parameters for modern SOFC modeling. Model results are shown for SOFC operation on humidified H2 and CH4 containing syngas, under coflow and counterflow configurations; detailed internal profiles of species mole fractions, temperature, current density, and electrochemical performance are obtained. The effects of performance, fuel composition, and flow configuration of SOFC performance and thermal profiles are evaluated, and the implications of these results for system design and analysis are discussed. The model can be implemented not only as a stand-alone SOFC analysis tool, but also a subroutine that can communicate and cooperate with chemical flow sheet software seamlessly for convenient IGFC system analysis.

Integrated gasification fuel cell (IGFC) systems combining coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally friendly utilization of coal for energy production. Most IGFC system analyses performed to date have used non-dimensional thermodynamic SOFC models that do not resolve the intrinsic constraints of SOFC operation. In this work, a one-dimensional finite volume model for planar SOFC is developed and verified using literature data. Special attention is paid to making the model capable of supporting recent SOFC technology improvements, including the use of anode-supported configurations, metallic interconnects, and reduced polarization losses. Results are presented for SOFC operation on humidified hydrogen and methane-containing syngas, under co-flow and counter-flow configurations; detailed internal profiles of species mole fractions, temperature, current density and electrochemical performance are obtained. The effects of performance, fuel composition and flow configuration on SOFC performance and thermal profiles are evaluated, and the implications of these results for system design and analysis are discussed.

Solid Oxide Fuel Cell (SOFC) is an energy conversion device consisting of porous electrodes and a dense electrolyte in-between which converts a gas fuel into electrical power with high efficiency (figure 1). A few micron-thick (1-2 μm) dense yttria-stabilized zirconia (YSZ) electrolyte has been recently fabricated by inkjet printing on a 9x9 cm2 green NiO/YSZ substrate in our facilities using a modified photo inkjet printer (figure 2). The resulting cells were tested and compared to those obtained on fuel cells fabricated by tape cast electrolyte, showing superior performances. With an objective of continuing improving SOFC performances, development of a cathode layer by inkjet printing is being studied in this project. Inkjet printing inks developed within this project are water-based, and hence exhibit a high surface tension. When printing onto surfaces such as sintered half-cell (electrolyte + anode), covering issue might happen if the ink surface tension is not optimized, leading to poor performances of the cell. In this project, influence of surface tension of Lanthanum Strontium Cobalt ink (LSC) on the homogeneity of the printed film has been studied. Several solvents mixtures and surfactants have been investigated in order to reduce the ink surface tension hence optimizing the covering of the substrate surface.

Solid oxide fuel cells (SOFCs) which can directly covert the chemical energy of fuel to electric energy have received as one of alternative energy sources because of their high energy conversion efficiency, fuel flexibility and low emission of pollutant to the environments. However, there are still several technical issues for making commercialization of SOFCs such as limited durability and long start-up time. Hence, recent studies of the SOFCs have focused on improving performance of SOFCs in the intermediate temperature (IT) to achieve technical goals such as high stability, low-cost component materials, easier sealing and quicker start-up time. In particular, stable long-term performances of SOFCs are one of the essential factors for commercialization of fuel cell systems. However, the sudden unstable situations such as frequent electrical load on-off and high load can often occur during the SOFCs operations, and thus, these unexpected situations can affect the durability of SOFCs systems. Therefore, we need to investigate performance change of SOFCs under various electrical load conditions including the constant current, current on-off cycles and irregular current change. In addition, post-mortem analysis of SOFCs are necessary to find the main degradation mechanisms after various durability tests. In this work, Ni-cermet anode supported cells are fabricated by the tape-casting method with neodymium doped ceria and double-perovskite structure based cathode materials (NdBa0.5Sr0.5Co1.5Fe0.5O5+δ). The performance of the cells are investigated by I-V measurement and the durability test also conducted by using load trip (0~200 mAcm-2) and load cycle (120~200 mAcm-2) techniques with controlled current condition at 650 oC. For electrochemical analysis, electrochemical polarization and characterizations are conducted using an electrochemical impedance spectroscopy (EIS) with a Potentiostat/Galvanostat instrument using a frequency range from 1 MHz to 1Hz. The several physicochemical analyses are also investigated using field emission-scanning electron microscopy (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). References E. D. Wachsman and K. T. Lee, Science 334 (2011) 935-939.S. B. Adler, Chem. Rev. 104 (2004) 4791-4843.E. Park, S. Taniguchi, T. Daio, J. T. Chou and K. Sasaki, Solid State Ionics 262 (2014) 421-427.B. C. H. Steele and A. Heinzel, Nature 414 (2001) 345-352.S. Omar, E. D. Wachsman and J. C. Nino, Solid State Ionics 177 (2006) 3199-3203.M. Kahlaoui, S. Chefi, A. Inoubli, A. Madani and C. Chefi, Ceram. Int. 39 (2013) 3873-3879. Keywords : Solid oxide fuel cells; intermediate-temperature; durability; perovskite.

Doubling the diffusivity measurement efficiency in solid oxide fuel cells (SOFCs) via a bi-sensor electrochemical cell

Solid oxide fuel cells (SOFCs) are getting more importance with their promising future, due to their high-energy conversion efficiency, low-emissions and flexibility of usable fuel type. The performance and the lifetime of SOFCs are keenly dependent on electrode microstructure. In order to recognize the microstructural evolution and its degradation kinetics in SOFC cermet cathodes during long exposure time (up to 20 000 h) under realistic operating conditions ( T= 850 °C, J= 190-250 ), investigations on porous LSM/YSZ cathodes were conducted. The 3D-tomography technique (FIB/SEM) offers extensive data about the microstructures of various cathode aged during different operating times (2 500 h, 15 000 h and 20 000 h), which allows an exact quantification of particle size distribution, phase-connectivity, tortuosity factor and Triple Phase Boundary Length (TPBL). With the increasing operating time no significant 3D microstructural changes in the cathode were noticed in the obtained data. However, additional qualitative X-ray fluorescence measurement, indicate a clear presence of chrome contamination on aged cathodes, which may be the main degradation mechanism in the SOFC cathode. Keywords: chrome poisoning, long operating time, particle size distribution, SOFC, triple phase boundary, SEM, XRF