Temperature and Bias Effects on Sputtered Ceria Diffusion Barriers for 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.

A simple, rapid spray method for preparing anode-supported solid oxide fuel cells with GDC electrolyte thin films

Long term durability was demonstrated on high-performance direct ethanol solid oxide fuel cell operating in gradual internal reforming. Anode supported single cells, fabricated at Forschungszentrum Jülich, composed of (La,Sr)MnO3 cathode, 8 mol% yttria-stabilized zirconia (YSZ) electrolyte, and YSZ-Ni anode were used for long term testing on anhydrous (direct) ethanol. Gadolinia-doped ceria (CGO), with 0.1 wt.% of Ir, catalyst was produced by the impregnation of Ir salt in commercial CGO (Praxair) and mixed with organic additives. The CGO-Ir suspension was deposited onto the surface of the YSZ-Ni anode support with an air brush. Such a catalytic layer acts both as a physical barrier to avoid direct contact of the fuel with Ni-based anode and to promote the gradual internal reforming of ethanol. The CGO-Ir catalyst was designed for the reforming of the main compounds resulting from the thermal decomposition of ethanol at high temperature and to inhibit the formation of ethylene, which is a known source of carbon deposits. After deposition the sample was heated at 850ºC in inert atmosphere to ensure good adhesion between the catalytic layer and the anode. Gold grid and paint were used as current collectors in both cathode and anode sides. The single fuel cells were mounted in the test apparatus, sealed with gold rings and alumina cement and heated to the measuring temperature (850°C) under inert atmosphere. After gas tightness was checked by flowing air on the cathode side and argon on the anode side, the anode was reduced under hydrogen. The fuel cell was kept at 0.6 V while monitoring the current drained from the cell at 850 °C. Samples were initially operated in hydrogen for a few hours and fuel was switched to dry ethanol, carried by nitrogen. It is important to mention that no oxidizing agent was added to the fuel and that water produced by the electrochemical reaction of hydrogen at the anode promoted the reforming of the ethanol in the CGO-based layer, in the so-called gradual internal reforming. In such conditions the current output was recorded and observed to not be significantly different form that under hydrogen. Moreover, the direct ethanol fuel cell was continuously operated for 650 hours without major losses due to carbon formation. The experimental results demonstrate that the water released by the electrochemical oxidation of reformate hydrogen at the electrolyte/anode active interfaces is sufficient for the reforming of the primary fuel in the catalytic layer, provided that a minimum fuel utilization is respected. After the durability tests, energy dispersive X-ray spectroscopy and scanning electron microscopy analyses of the samples revealed no carbon deposit formation in the anode. The experimental results provided compelling evidence for the viability of gradual internal reforming of ethanol in high performance solid oxide fuel cells.

A systematic study was initiated of anode-supported solid oxide fuel cells (SOFCs) with Nd2NiO4 cathodes. This type of cathode, a mixed conducting K2NiF4-type material, can be considered as a candidate for SOFC applications. In this study, the influence of (1) the presence of a Ce0.8Gd0.2O1.9 (CGO) interlayer between the electrolyte and the cathode, (2) sintering conditions, and (3) the grain size of the powder on the performance of SOFCs with Nd2NiO4 cathodes was investigated in more detail. Results from current density-voltage characteristics and permeation and gas diffusion measurements showed that the electrochemical performance was promising for SOFCs including a CGO interlayer and a Nd2NiO4 cathode (prepared with a powder with a d50 of 0.5A∕cm2 or 0.8μm) sintered at 1100°C or higher; current density at 800°C and 700mV between 1.2 and 1.3A∕cm2. The microstructure of the cathode was such that no gas diffusion problems occurred.

Microtubular anode-supported solid oxide fuel cells (μt-SOFC) were created with a metallic coil embedded in the anode to act as current collector. The electrochemical performance was experimentally examined by comparing the power density (PD) of μt-SOFC with embedded coils of different turns per unit length and composition (nickel and palladium). It is shown that an increase in the turns per unit length results in a proportional current density increase and in a quadratic increment of PD. Additional performance improvement is found for the cell with palladium current collector due to the higher catalytic activity for hydrogen oxidation.

Development and testing of anode-supported solid oxide fuel cells with slurry-coated electrolyte and cathode

In this chapter, the construction, principle of operation, types and production technologies of SOFCs are presented and characterized. Three main types of SOFC are described according to the type of support layer responsible for the mechanical strength of the cell: electrolyte-supported solid oxide fuel cells (ES-SOFC), anode-supported solid oxide fuel cells (AS-SOFC), and metal-supported solid oxide fuel cells (MS-SOFC). Base and raw materials for particular functional layers of solid oxide fuel cells are characterized, i.e., anode, electrolyte, and cathode layers. Thick- and thin-film technologies for the production of particular elements of SOFC fuel cells are presented. Using the example of technology developed in the Institute of Power Engineering Ceramic Department CEREL, the production technology of anode-supported solid oxide fuel cells and the method of characterizing the microstructure and electrochemical properties of the produced cells are presented.

Gadolinium doped ceria (GDC) gains increasing attention as a promising material for both solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). The GDC-based electrodes demonstrate positive features, e.g. enhanced electrochemical performance in low operation temperatures, resistivity to sulfur poisoning and carbon deposition, etc. As GDC is a mixed electronic-ionic conductor in reducing atmosphere, electrochemical reactions can be supported by the double phase boundary (DPB) at the GDC surface. Several studies demonstrated that the DPB reaction may dominate the electrochemical performance over triple phase boundary (TPB) reaction in Ni-GDC electrodes, where it can account for as much as 95 % of the total reaction depending on the cermet microstructure [1]. Recent studies even show that pure ceria electrodes can achieve substantial power density [2,3]. In this context, the microstructure design principle for GDC-based electrodes should focus on maximizing GDC surface area. Particularly, GDC electrodes with infiltrated GDC nanoparticles or electro-catalytic nanoparticles may achieve competitive performance compared with the conventional Ni-YSZ electrodes. Although many strategies have been proposed to improve performance of GDC-based electrodes, it is difficult to quantitatively compare all the reported results. Furthermore, the correlation between microstructure and performance of pure GDC electrode are still scarce.Several different types of GDC-based electrodes are fabricated and tested in the unified characterization procedure in this study. Four basic designs are considered, i.e. Ni-GDC, GDC-perovskite composites, GDC electrodes with deposited Ni nanoparticles and pure GDC electrodes. Several strategies to modify the GDC surface area are discussed. In addition, the substantial enhancements in the DPB density and electrochemical performance were achieved by precipitating Ni-nanoparticles directly on sub-micron GDC powders at 2wt.% loading. The performance of electrodes was evaluated by electrochemical impedance spectra measurements with various humidity and temperature conditions. The results showed clear dependance between GDC surface area and electrode performance. At the same time, it should be noted that the electrode active thickness depends on the electrode microstructure. In particular, it is in the range of 5 µm for the pure nano-GDC electrode. When the GDC electrode is thicker than the active thickness, it results in large Ohmic resistance due to the electronic conduction in the porous electrode.[1] A. Nenning, M. Holzmann, J. Fleig, A.K. Opitz, Mater. Adv. 2 (2021) 5422–5431.[2] M. Ouyang, A. Bertei, S.J. Cooper, Y. Wu, P. Boldrin, X. Liu, M. Kishimoto, H. Wang, M. Naylor Marlow, J. Chen, X. Chen, Y. Xia, B. Wu, N.P. Brandon, J. Energy Chemistry. 56 (2021) 98–112.[3] W. Jung, K.L. Gu, Y. Choi, S.M. Haile, Energy Env. Sci. 7 (2014) 1685–1692.

Three-dimensional computational fluid dynamics modeling of anode-supported planar SOFC

The anode-supported solid oxide fuel cells (SOFCs) with an yttria-stabilized zirconia (YSZ) electrolyte have been successfully prepared by the sequential steps of fabricating technique including tape casting, spin coating, and screen-printing. Two types of the SOFCs are manufactured and their cell performances are characterized via the detail analysis of the electrochemical impedance spectroscopy (EIS). The maximum power densities are 411 and for the high and low performance cells at , respectively. According to the EIS analysis, the electrochemical performance of the poor cell is dominated by the anodic non-charge-transfer resistance, which is mainly resulted by the gas diffusion polarization due to the low porosity of the anode. The high performance cell is controlled by the simultaneous contributions of the ohmic resistance , the anodic polarization , and the cathodic polarization at the operation temperature of . However, the will dominate the performance of the cell as the temperature is lowered down to .

In order to develop solid oxide fuel cells (SOFCs) running on hydrocarbon fuels, we have focused on a new method of improving electrode performance and reducing carbon deposition by coating thin films of samaria-doped ceria (SDC) within the pores of electrode by a sol-gel coating technique. The SDC coating on the pores of anode made it possible to have a good stability for long-term operation due to low carbon deposition and nickel sintering. In this study, we demonstrated a new method of improving electrode performance and reducing carbon deposition by coating thin films of samaria-doped ceria and applied the modification technique to two different types of fuel cell structures, anode-supported SOFC and comb-shaped SOFC. From our results, the maximum power density of an anode-supported cell (electrolyte; 8 mol% YSZ and thickness of 30μm, and cathode; La0.85Sr0.15MnO3) with the modified anode was ∼300mW∕cm2 at 700°C in the mixture of methane (25%) and air (75%) as the fuel, and air as the oxidant. The cell was operated for 500hr without significant degradation of cell performance. For the comb-shaped SOFCs operated in the mixed-fuels fuel cell conditions, the cell performance was 40mW∕cm2 at 700°C in the CH4∕O2 ratio of 1.

Solid oxide fuel cells for stationary, mobile, and military applications

Fabrication and characterization of low-temperature SOFC stack based on GDC electrolyte membrane

1.Introduction Solid oxide fuel cells (SOFCs) have received much attention as promising electrochemical power generation devices because of their high electrical efficiency and environment adaptability. Development of SOFCs is now focused on lowering the operating temperature from the conventional range of 750-900°C to an intermediate range of 500-700ºC, allowing a wider range of materials for SOFC components and improvement of thermal reliability. Preparing the Yttlia Stabilized Zirconia (YSZ) and Gadolinia-doped ceria (GDC)) bilayer electrolytes are effective for reducing operating temperature [1]. In the present work, NiO-YSZ anode supported cells with YSZ/GDC bilayer electrolytes and (La,Sr) (Co,Fe) O3 (LSCF) thin film cathodes were fabricated by pulsed laser deposition (PLD) and screen printing. However, one thermal cycle test of a cell between 200°C and 600°C, with heating and cooling rates of 400°C /h, resulted in detachment of the LSCF thin film and thus reduced durability. To improve the adhesion between the LSCF and GDC, (La,Sr)CoO3(LSC), nano particles were deposited on the GDC thin films by PLD. This step was expected to increase contact area between electrolyte and cathode. We investigated the influence of the LSC nano particle on thermal reliability and I-V characteristic of SOFCs. 2.Experimental Details NiO-YSZ cermet (NiO:YSZ＝60:40 wt%, 500 μm thick) was used as the anode substrate, YSZ (8 mol% Y2O3) and GDC (10 mol% GdO2) as electrolyte materials, and La0.6Sr0.4Co0.2Fe0.8 (LSCF) and La0.6Sr0.4Co (LSC) as cathode materials. YSZ (8 μm thick) and LSCF (30 μm thick) films were prepared by screen printing and GDC (3 μm thick) was deposited by PLD. Two sets of unit cells were prepared: one set was with LSC nano particles deposited between GDC and LSCF thin films by PLD (NiO-YSZ / YSZ / GDC / LSC particle / LSCF) and the other set was without LSC nano particles (NiO-YSZ / YSZ / GDC / LSCF).Thermal cycle test between 200 and 600°C with heating and cooling rates of 400° C /h were performed. Power densities and I-V characteristics were measured. 3.Results (a) Deposition of LSC nano particles by PLD on the GDC thin film. Figure 1 shows a scanning electron microscopy image of the surface of a GDC thin film. Many LSC nano particles (50-nm diameter) were deposited on the dense GDC thin film by PLD, which increased the area of contact between the GDC and LSCF thin films. (b) Thermal cycle test of NiO-YSZ anode supported cells with or without LSC nano particles. Two types of unit cells, (1) NiO-YSZ / YSZ / GDC / LSC particle / LSCF and (2) NiO-YSZ / YSZ / GDC / LSCF were prepared by screen printing and PLD. Photographs of unit cells after one thermal cycle are shown in Fig. 2. It is evident that adding the LSC nano particles succeeded in suppressing detachment of the LSCF thin film. Spike effect resulted in high adhesion between LSCF and GDC. The unit cell with LSC nano particles survived 50 thermal cycles and yielded higher thermal reliability. (c) I-V characteristics of NiO-YSZ anode supported cells with or without LSC nano particles. Figure 3 shows the I-V characteristics of unit cells with and without LSC nano particles. The maximum power densities of unit cells with or without LSC nano particles were 0.5 and 0.3 W·cm-2, respectively. Cell performances were found to be better when LSC nano particles were deposited, because of increase in reactive sites.[1] T. Mukai et al., Journal of Fuel Cell Science and Technology , 10(2013) 061006-1-6.

Precise porosity control is highly desirable for improving the electrochemical performance of solid oxide fuel cells (SOFCs). Freeze casting is an established method for enabling high bulk porosity in structures and controlling pore orientation. In this study, freeze casting was used to fabricate tubular, anode-supported SOFCs with aligned and varying amounts of porosity by controlling the solids/water ratio in different casting slurries. SOFCs were prepared with a Ni/yttria and scandia stabilized zirconia (ScYSZ) anode support (AS), an anode functional layer (AFL), a ScYSZ electrolyte, a lanthanum strontium manganite (LSM)/ScYSZ cathode interlayer (CIL), and an LSM cathode. The permeability of the anode support was found to increase from 1.4 × 10−2 to 1.8 × 10−2 m2 as porosity was increased from 57 to 64 vol%, while the total cell resistance decreased by 35% from 0.93 to 0.60 Ohm cm2. When evaluated with 30 vol% H2 as the fuel at 800 °C, the decrease of concentration polarization enabled an increase in electrochemical performance by 42% from 0.35 to 0.50 W/cm2 as the porosity in the anode support was increased. Mechanical strength characterization using a three-point method showed there is a practical upper limit of the amount of porosity that can be designed into the anode support. This work paves a way for controlling porosity by freeze casting and understanding the correlation between porosity and concentration polarization losses in SOFCs.