Electrophoretically deposited thin film electrolyte for solid oxide fuel cell

Abstract

Thin films of 8 mol% yttria stabilised zirconia (YSZ) electrolyte have been deposited on non-conducting porous NiO–YSZ anode substrates using electrophoretic deposition (EPD) technique. Deposition of such oxide particulates on non-conducting substrates is made possible by placing a conducting steel plate on the reverse side of the presintered porous substrates. Thickness of the substrates, onto which the deposition has been carried out, varied in the range 0·5–2·0 mm. Dense and uniform YSZ thin films (thickness: 5–20 μm) are obtained after being cofired at 1400°C for 6 h. The thickness of the deposited films is seemed to be increased with increasing porous substrate thickness. Solid oxide fuel cell (SOFC) performance is measured at 800°C using coupon cells with various anode thicknesses. While a peak power density of 1·41 W cm^?2 for the cells with minimum anode thickness of 0·5 mm is achieved, the cell performance decreases with anode thickness.     

Robust and reliable solid oxide fuel cells with modified thin film YSZ electrolyte

Reduce electrolyte thickness can improve solid oxide fuel cell (SOFC) performance. However, thinner electrolyte often contains prominent defects and flaws, which may decrease the yield and increase operation risk. This work proposes a method to modify the thin film YSZ electrolyte, to improve cell reliability and durability. The as-sintered anode supported half-cell with screen printed YSZ electrolyte was immersed in precursor solution of Y(NO₃)₃·6H₂O and Zr(NO₃)₄·5H₂O, and being treated under hydrothermal condition of 150°C for 12 h. As a result, the modified cells show slight increase in the OCV values. Furthermore, the hydrothermal modification effectively promotes interface sintering between YSZ electrolyte and GDC barrier layer, yielding a smaller ohmic resistance of .142 Ω·cm² (a decrease of ∼11%) and a higher peak power density of .964 W/cm² (an increase of ∼18%) at 750°C, than pristine cell. Moreover, the modified cell operates stably over 300 h, while the pristine cell presents large and irregular voltage fluctuations. This work suggests that the hydrothermal modification is an effective and promisingly industrial applicable method for thin film electrolyte recovery in SOFCs.     

Enhanced sintering behavior of LSGM electrolyte and its performance for solid oxide fuel cells deposited by vacuum cold spray

How to obtain dense La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) electrolyte at low sintering temperature (<1300 °C) is a challenge to improve solid oxide fuel cell (SOFC) performance at intermediate operation temperature. In this study, a double-layer design method for vacuum cold spray (VCS) prepared-LSGM electrolyte assisted with two-step sintering at a low temperature was proposed. The sintering behavior of VCS deposited LSGM layers at 1200 °C was investigated. The LSGM layers became denser in most regions except the appearance of some cracks. Subsequently, the effect of a second LSGM layer on the sintered top layer was studied to block cracks. Results showed that the co-sintered layer with a thickness of approximately 5 μm presented a maximum open circuit voltage of ∼0.956 V at 650 °C and a maximum power density of 592 mW/cm² at 750 °C. Result indicates that the sintering assisted VCS is a promising method to prepare the LSGM electrolyte applied in intermediate temperature SOFCs.     

Effect of yttrium-stabilized bismuth bilayer electrolyte thickness on the electrochemical performance of anode-supported solid oxide fuel cells

Lowering operating temperature and optimizing electrolyte thickness, while maintaining the same high efficiencies are the main considerations in fabricating solid oxide fuel cells (SOFCs). In this study, the effect of yttrium-stabilized bismuth bilayer electrolyte thickness on the electrical performance was investigated. The yttrium-stabilized bismuth bilayer electrolyte was coated on the nickel–samarium-doped composite anode/samarium-doped ceria electrolyte substrate with varying bilayer electrolyte thicknesses (1.5, 3.5, 5.5, and 7.5 μm) via dip-coating technique. Electrochemical performance analysis revealed that the bilayer electrolyte with 5.5 μm thickness exhibited high open circuit voltage, current and power densities of 1.068 V, 259.5 mA/cm² and 86 mW/cm², respectively at 600 °C. Moreover, electrochemical impedance spectroscopy analysis also exhibited low total polarization resistance (4.64 Ωcm²) at 600 °C for the single SOFC with 5.5 μm thick yttrium-stabilized bismuth bilayer electrolyte. These findings confirm that the yttrium-stabilized bismuth bilayer electrolyte contributes to oxygen reduction reaction and successfully blocks electronic conduction in Sm_0.2Ce_0.8O_1.9 electrolyte materials. This study has successfully produced an Y_0.25Bi_0.75O_1.5/Sm_0.2Ce_0.8O_1.9 bilayer system with an extremely low total polarization resistance for low-temperature SOFCs.     

Low-temperature constrained sintering of YSZ electrolyte with Bi2O3 sintering sacrificial layer for anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFCs) have strong potential for next-generation energy conversion systems. However, their high processing temperature due to multi-layer ceramic components has been a major challenge for commercialization. In particular, the constrained sintering effect due to the rigid substrate in the fabrication process is a main reason to increase the sintering temperature of ceramic electrolyte. Herein, we develop a bi-layer sintering method composed of a Bi₂O₃ sintering sacrificial layer and YSZ main electrolyte layer to effectively lower the sintering temperature of the YSZ electrolyte even under the constrained sintering conditions. The Bi₂O₃ sintering functional layer applied on the YSZ electrolyte is designed to facilitate the densification of YSZ electrolyte at the significantly lowered sintering temperature and is removed after the sintering process to prevent the detrimental effects of residual sintering aids. Subsequent sublimation of Bi₂O₃ was confirmed after the sintering process and a dense YSZ monolayer was formed as a result of the sintering functional layer-assisted sintering process. The sintering behavior of the Bi₂O₃/YSZ bi-layer system was systematically analyzed, and material properties including the microstructure, crystallinity, and ionic conductivity were analyzed. The developed bi-layer sintered YSZ electrolyte was employed to fabricate anode-supported SOFCs, and a cell performance comparable to a conventional high temperature sintered (1400 °C) YSZ electrolyte was successfully demonstrated with significantly reduced sintering temperature (<1200 °C).     

Bi-layer glass-ceramic sealant for solid oxide fuel cells

A bi-layered concept of glass-ceramic (GC) sealant is proposed to overcome the challenges being faced by solid oxide fuel cells’ (SOFCs). Two separated layers composed of glasses (Gd-0.3 and Sr-0.3) were prepared and deposited onto interconnect materials using a tape casting approach. After heat treating the bi-layered structure at 850 °C for 1–100 h, smooth and void free interfaces over the entire cross-section of joint were obtained. Micro-Raman analysis confirmed the presence of a higher amount of residual glassy phase in Gd-0.3 in comparison to Sr-0.3. The bi-layered GC showed good wetting and bonding ability to the Crofer22APU metallic plate. Slight increase of electrical conductivity with increasing annealing time was observed due to partial crystallization of the glass, but the overall conductivity levels of GC bi-layers were low enough to grant good electrical insulation. This set of relevant properties makes the investigated bi-layered sealants suitable for SOFC applications.     

A highly conductive Ce0.8Nd0.2O1.9 electrolyte with double sintering aids for intermediate-temperature solid oxide fuel cells

In this paper, the influence of Bi/Zn mass ratio on the phase composition, microstructure, sintering properties, and electrical properties of Bi/Zn co-added Nd_0.2Ce_0.8O_1.9 (NDC) used for intermediate-temperature solid oxide fuel cells (SOFCs) was investigated. At 700 °C, the total conductivity of the NDC-based electrolyte (3Bi/1Zn-NDC) with the mass ratio 3:1 for Bi₂O₃ and ZnO was as high as 5.89 × 10^?2 S cm^?1, 4.60 and 4.51 times higher than the single addition of 4 wt% Bi₂O₃ and 4 wt% ZnO, respectively. In addition, the 3Bi/1Zn-NDC electrolyte exhibited a good physical and chemical compatibility with the electrode materials. The open circuit voltage (OCV) of the cell supported by the 3Bi/1Zn-NDC electrolyte was 0.67 V, and the output power density could reach 402.25 mW cm^?2 at 700 °C. It showed stable power output and OCV in the long-term stability test within 50 h. Overall, the combination of 3 wt% Bi₂O₃ and 1 wt% ZnO was a very effective dual sintering aid for NDC electrolyte.     

Solid oxide fuel cells in combination with biomass gasification for electric power generation

Biomass, a source of renewable energy, represents an effective substitute to fossil fuels. Gasification is a process that organics are thermochemically converted into valuable gaseous products(e.g. biogas). In this work, the catalytic test demonstrated that the biogas produced from biomass gasification mainly consists of H_2,CH_4, CO,and CO_2, which were then be used as the fuel for solid oxide fuel cell(SOFC). Planar SOFCs were fabricated and adopted. The steam reforming of biogas was carried out at the anode of a SOFC to obtain a hydrogen-rich fuel.The performance of the SOFCs operating on generated biogas was investigated by I–V polarization and electrochemical impedance spectra characterizations. An excellent cell performance was obtained, for example,the peak power density of the SOFC reached 1391 mW·cm~(-2) at 750℃ when the generated biogas was used as the fuel. Furthermore, the SOFC fuelled by simulated biogas delivered a very stable operation.     

Optimization of a ScCeSZ/GDC bi-layer electrolyte fabrication process for intermediate temperature solid oxide fuel cells

Cost-effective wet ceramic coating techniques for fabricating ScCeSZ/GDC bi-layer electrolyte anode-supported button cells were investigated in this study. Aqueous ceramic slurries were prepared by ball milling and then used for Ni/ScCeSZ half cell fabrication by tape casting and spin coating. Prepared cells were tested at operating temperature between 700 and 800°C with a fuel composition of hydrogen:nitrogen 3:1 and air at the cathode. The cell with a spin coated GDC film showed the maximum power density of 1.142, 1.012, 0.813 W?cm^?2 at 800, 750, and 700°C, respectively. It was also able to produce power output around 0.7 W?cm^?2 for 500 h at 750°C, which confirms the cell operational stability. More importantly, the GDC film prepared by spin coating effectively avoided the formation of the (Zr,Ce)O₂^?based solid solution at the ceria/zirconia interface compared with the other cells with the co-casted and sintered GDC film.     

Development of solid oxide fuel cells based on a Ce(Gd)O2−x electrolyte film for intermediate temperature operation

Initial tests have been carried out with the fuel cell arrangement La_0.6Sr_0.4Co_0.2Fe_0.8O₃Ce_0.9Gd_0.1O_1.95Ni/YSZ, incorporating dense film (5–10 μm) Ce_0.9Gd_0.1O_1.95 electrolyte tape cast onto the supporting anode, to investigate the feasibility of intermediate temperature operation (500–700°C). A good open circuit voltage of approx. 0.8 V was obtained at 550°C using moist hydrogen as the fuel. Slightly lower open circuit voltages were found at higher temperatures, which may have been caused by minor gas leakage and the electronic conductivity of the electrolyte. Power outputs in excess of 100 mW/cm² were obtained at 650°C, and the cell resistance was 0.8Ω cm² at this temperature. This resistance, and the greater resistance at lower temperature, was predominantly due to the cathode according to AC impedance measurements. Experiments were also carried out at 600°C using direct methanol fuels at the anode; the maximum power output was approximately half of that obtained with hydrogen.     

Fabrication of electrolyte materials for solid oxide fuel cells by tape-casting

In this research, solid oxide fuel cell electrolytes were fabricated by aqueous tape-casting technique. The basic compositions for SOFC electrolyte systems were focused on yttria-stabilized zirconia (YSZ) system. The powders used in this study were from different sources. ZrO₂-based system doped with 3, 8, and 10 mol% of Y₂O₃, and 8YSZ electrolyte tape illustrated the desirable properties. The grain size of the sintered electrolyte tapes was in the range of 0.5–1 μm with 98–99% of theoretical density. Phase and crystal structure showed the pure cubic fluorite structure for 8–10 mol% YSZ and tetragonal phase for 3 mol% doped. The electrolyte tapes sintered at 1450 °C for 4 h had the highest ionic conductivity of 30.11 × 10⁻³ S/cm which was measured at 600 °C. The flexural strengths were in the range of 100–180 MPa for 8–10 mol% YSZ, and 400–680 MPa for 3 mol% YSZ.     

Polarization effects in electrolyte/electrode-supported solid oxide fuel cells

The effects of activation, ohmic and concentration polarization on the overall polarization in solid oxide fuel cells are presented. A complete analysis was conducted based on thermodynamic principles for the calculation of cell voltage. Treating the fuel cell as a control volume, the irreversibility term in a steady flow thermodynamic system was related to the overall polarization. The entropy production was calculated and related to the lost work of the fuel cell, while the heat loss from the cell was determined from the entropy balance. To generalize the cell voltage–current density expression, the Butler–Volmer model was used in the calculation of activation polarization and both ordinary and Knudsen diffusions were considered in the calculation of concentration polarization. The overall cell resistance was deduced from the generalized cell voltage–current density expression. The concentration resistance at the anode can be minimized by humidifying the hydrogen with an appropriate amount of water, depending on the thickness of the anode used. Comparison of polarization effects on the cell performance between the electrolyte-supported and anode-supported cells showed that the latter would give a better cell performance.     

Synthesis and densification of lanthanum silicate apatite electrolyte for intermediate temperature solid oxide fuel cell via co-precipitation method

Lanthanum silicate apatite (LSA, La_9.33+xSi₆O_26+1.5x, x = 0–0.67) has been widely investigated as a promising electrolyte material for intermediate temperature solid oxide fuel cell (SOFC). In this work, a facile and low-cost co-precipitation method is used to synthesize LSA precursor powders. The well dispersed nanopowders (ca. 70 nm) with pure hexagonal LSA phase are obtained by calcining the precursor at 900 °C. Impurity of La₂SiO₅, caused by the different precipitation productivities of La(NO₃)₃ and TEOS, can be eliminated through lowering the La/Si ratio in the starting mixtures. The dispersant (PEG200) plays a crucial role in co-precipitation processes, which can effectively mitigate the agglomeration and therefore significantly improve the sinterability of the nanoparticles. Dense LSA ceramic with relative density of 98% is obtained after sintering at 1550 °C, which exhibits a conductivity of 0.13 mS cm⁻¹ at 500 °C.     

A high-performance solid oxide fuel cell with a layered electrolyte for reduced temperatures

The application of conventional zirconia-based electrolytes is limited to relatively high temperatures (ie > 750°C) due to their poor conductivities at low temperatures. Doped ceria has much higher conductivities; however, when exposed to fuel, electronic current develops within the material, which impairs cell performance and efficiency. Herein, we report a novel layered electrolyte structure consisting of a 10 µm samaria-doped ceria primary layer and a 2 µm scandia-ceria-stabilized zirconia protection layer on the fuel side. The cell had five layers and was fabricated using a tape casting and ultrasonic spraying technique. By carefully selecting the raw materials, the bilyer electrolyte was sintered to full density at a low temperature of 1250°C. The adverse interdiffusion and undesirable reactions between the two layers were largely avoided. A fuel cell with the layered electrolyte structure, operated on hydrogen fuel, produced a high open circuit voltage 1.07 V and a power density of 321 mW/cm² at 0.8 V and 600°C, 76% improvement compared to the fuel cell with a scandia-stabilized zirconia/samaria-doped ceria bilayer electrolyte reported in literature.     

硫化氢固体氧化物燃料电池研究进展

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

BaZr0.1Co0.4Fe0.4Y0.1O3-SDC composite as quasi-symmetrical electrode for proton conducting solid oxide fuel cells

Symmetric solid oxide fuel cells (SSOFCs) with the identical anode and cathode electrocatalysts show promise to reduce material and system cost while increasing the cell lifespan. In this work, BaZr_0.1Co_0.4Fe_0.4Y_0.1O₃ (BZCFY) oxide perovskite is proposed as a symmetric electrode for SSOFCs based on proton conducting electrolyte, with targets of reducing temperature and high-performance application. Active oxygen ionic conductor and catalyst, SDC, is composited to improve the cell performance and electrode durability. Those materials show good chemical compatibility while BZCFY is decomposed to alloy and mixed oxide composite, which significantly affects electrode activity. SDC-BZCFY composite gives an electrode polarization resistance of 1.35–13.7 Ω cm² and 0.32–1.59 Ω cm² for hydrogen oxidation reaction and oxygen reduction reaction on the proton conducting electrolyte, BZCY, at the temperature range of 700–550 °C, respectively. Moreover, it displays an excellent oxygen reduction kinetics with an impressive activation energy of 0.91 eV. The polarization resistances are significantly reduced in the fuel cell condition owning to the electrochemical promotion effect under open-circuit condition. Quasi-SSOFCs with BZCY electrolyte in a thickness of 480 μm and electrode thickness of 25 μm give a peak power density of 114.8 and 74.3 mW cm⁻² at 650 and 600 °C, respectively. In addition, SSOFC also displays acceptable durability under constant voltage operational condition for 25 h. This work highlights alternative active electrode material for symmetric solid oxide fuel cells for low temperature operation.     

Mechanical and electrochemical behavior of novel electrolytes based on partially stabilized zirconia for solid oxide fuel cells

In this study, 3 mol% yttria stabilized zirconia (3YSZ) is investigated as a SOFC electrolyte alternative to 8 mol% yttria stabilized zirconia (8YSZ). The mechanical and electrochemical properties of both materials are compared. The mechanical tests indicate that the thickness of 3YSZ can be reduced to half without sacrificing the strength compared to 8YSZ. By reducing the thickness of 3YSZ from 150 µm to 75 µm, the peak power density is shown to increase by around 80%. The performance is further enhanced by around 22% by designing of novel electrode structure with regular cut-off patterns previously optimized. However, the cell with novel designed 3YSZ electrolyte exhibits 30% lower maximum power density than that of the cell with 150 µm-thick standard 8YSZ electrolyte. Nevertheless, the loss in the performance may be tolerated by decreasing the fabrication cost revealing that 3YSZ electrolyte with cut-off patterns can be employed as SOFC electrolyte alternative to 8YSZ.     

Simulation of membrane microreactor for fuel cell with methane feed

Microreactors are composed of channels with a dimension of 1–1000 μm in width and depth. Three types of membrane microreactors for production of electric power by fuel cells with methane feed, i.e. (1) Pd membrane microreactor (Pd-MMR) followed by polymer electrolyte fuel cell (PEFC), (2) oxide-ion conducting solid oxide fuel cell (SOFC) and (3) proton conducting solid oxide fuel cell (H⁺-SOFC) were simulated and compared.

The Pd-MMR followed by PEFC was the most effective system for electric power generation compared with the other two types of SOFC although the system was rather complex. However, the performances were dependent on the permeation properties of membranes (Pd, YSZ, perovskite) as well as kinetics of catalysts.     

Oxygen vacancy levels on gadolinia-doped ceria interlayer deposited by atmospheric pressure plasma jet for solid oxide fuel cells

The oxygen vacancy levels as a factor on different gadolinia-doped ceria interlayer (GDCi) films deposited on yttria stabilized zirconia (YSZ) electrolyte substrates by an atmospheric pressure plasma jet (APPJ) via precursor solution of nitrate salts are investigated. Focusing on the effect of carrier gases, scanning electron microscopy (SEM), Raman, and X-ray diffraction (XRD) are implemented for the materials characterization of the as-deposited GDCi films and sintered-GDCi films at various temperatures. The higher level of oxygen vacancies in GDCi films adhered on 8YSZ electrolyte are evidently analyzed using Ar as the carrier gas during the deposition, of which the interdiffusion resulted in the formation of (GDC + YSZ) solid solution for sintering over 1300?°C degraded the total conductivity. The deposition of GDCi films on 8YSZ by APPJ method using O₂ carrier gas significantly improved the total conductivities of the whole electrolyte layers. Moreover, this study provides the useful insight into the oxygen vacancy levels on GDC films as interlayer (GDCi) to improve the values of open circuit voltage in LSM/GDCi/YSZ/Pt full-cell, as well as offering the efficiency of APPJ as one step deposition process.     

Synthesis and characterization of Ga-doped Ba3MoNbO8.5 electrolytes for intermediate temperature-solid oxide fuel cells

Ba₃MoNb_1-xGa_xO_8.5-δ (BMNG, 0 ≤ x ≤ 0.2) powders are successfully prepared by sol-gel autoignition method. The effects of acceptor-type Ga³⁺ doping on Ba₃MoNbO_8.5 (BMN) are characterized by thermogravimetric analysis, X-ray diffraction, scanning electron microscope, Raman spectroscopy, and electrochemical impedance spectroscopy. All BMNG samples crystallize as a single phase in the R-3m space group and show great phase stability at various environments. Doping a small amount of gallium can effectively improve bulk conductivity and sintering density of BMN. The ionic conductivity of Ba₃MoNb_0.9Ga_0.1O_8.5-δ (BMNG10) is the highest, which can reach 2.05 × 10^?2 S cm^?1 at 800 °C. The enhanced ionic conductivity is primarily related to the increase of oxygen vacancy concentration and the number of tetrahedral units within the structure. In addition, through successfully assembling and evaluating a single cell supported by the BMNG10 electrolyte, it is proved that the practical utilization of BMNG10 in intermediate temperature-solid oxide fuel cells (IT-SOFCs) is feasible. In short, hexagonal perovskite derivative BMNG10 is a promising oxide ion conductor for IT-SOFCs.