CHEN Meilong ^{1, 2}, LI Caifeng ², WU Xueliang ², LU Xinyang ², Dustin Banham ², DU Li ¹, CHEN Min ²

(1. Guangdong Provincial Key Laboratory of Fuel Cell Technology, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510006, Guangdong, China; 2. School of Materials Science and Energy, Guangdong Key Laboratory for Hydrogen Energy Technologies, Foshan University, Foshan 528000, Guangdong, China)

Extended Abstract:[Background and purpose] Solid Oxide Fuel Cell (SOFC for short) is an all-solid-state power generation device that converts chemical energy stored in fuel into electrical energy. It has the advantages of high energy conversion efficiency, low pollution emissions and a wide range of fuel sources. It shows development prospects in distributed power sources and other aspects. Compared with oxygen ions, protons have lower activation energy and have more advantages at medium-low temperatures. Barium cerate-based proton conductors have excellent sintering activity and high electrical conductivity, reaching 0.46 W·cm⁻² at 500 ℃. However, barium cerate-based proton conductors are prone to chemical reactions with CO₂ and H₂O, leading to structural damage and performance degradation. Another commonly used proton conductor, BZY, has received extensive attention, due to its excellent stability in CO₂ and H₂O. However, BZY has the drawback of poor sintering activity, requiring high densification temperatures. During the co-firing process of the anode and the electrolyte, it is likely to cause a low porosity of the anode and small electrolyte grains, resulting in an increase in electrode polarization and electrolyte ohmic impedance and hence further reducing the output performance of the cell. It is found that the sintering activity of the electrolyte can be improved and the cell sintering temperature can be reduced by partially substituting Zr⁴⁺ at the B-site in BZY with transition metals or lanthanide metals. However, the introduction of metal elements into the electrolyte increases the electronic conductivity in the electrolyte. Another method to improve the electrochemical performance of the cell is to use thin-film electrolyte made with the PLD technology to reduce the ohmic impedance of the cell, but this requires high-level equipment. In this work, the electrochemical performance of the cell is optimized by using different types of pore-forming agents to regulate the pore structure of the anode support and reducing the thickness of the electrolyte. The effects of cornstarch and graphite pore-forming agents, as well as different contents of graphite pore-forming agents, on the electrochemical performance of BZY-based H-SOFC are explored. At the same time, by reducing the thickness of the electrolyte, the ohmic impedance of the cell is reduced and the electrochemical performance of the cell is improved.[Methods] Synthesis of BZY electrolyte powder: BaZr_0.8Y_0.2O_3−δ (BZY20) powder was synthesized by using the solid-state method. Barium carbonate (BaCO₃, 99.95%), zirconium dioxide (ZrO₂, 99.95%) and yttrium oxide (Y₂O₃, 99.95%) were weighed according to the stoichiometric ratio, ball-milled and mixed evenly. The mixtures were then calcined at 1150 ℃ for 10 h. Preparation of anode green body: BZY20, NiO and the pore-forming agent were mixed through ball milling according to a mass ratio of 2:3:1. Subsequently, the mixture was pressed into pellets of anode green body with a diameter of 25 mm at 200 MPa for 1 min. Finally, the green body was calcined at 800 ℃ for 2 h. Preparation of anode- supported half-cell: One side of the anode green body was impregnated with BZY20 electrolyte slurry, while the thickness of the electrolyte membrane was regulated by the number of impregnations. Finally, the impregnated half-cell green body was sintered in an air at 1500 ℃ for 8 h to obtain a 19-mm-diameter anode-supported half-cell. Preparation of full-cell: The cathode LSCF-BZCYYb was applied to the electrolyte by using the brushing method and drying at 140 ℃ for 30 min. This process was repeated for 4 times and then the cell was sintered at 1000 ℃ for 2 h to obtain the NiO-BZY20/BZY20/LSCF-BZCYYb7111 cell. The effective area of the cathode was 0.785 cm². Cell testing: The cell was sealed to a self-made test fixture using a ceramic sealant (Aremco Ceramabond^TM 552) to form a double-chamber anode-cathode configuration. Subsequently, the fixture was placed in a high-temperature furnace. When the temperature rose to 700 ℃, humidified (2.7% H₂O) hydrogen (50 sccm) and air (60 sccm) were introduced into the anode and cathode sides, respectively. Electrochemical performance of the cell was tested at 550–700 ℃ using an electrochemical workstation.[Results] From the XRD pattern of BZY20 powder calcined at 1150 ℃ for 10 h, it can be concluded that BZY exhibits diffraction peaks of the perovskite phase. From the SEM images, graphite is in block form with an average particle size of 20.35 μm, while cornstarch particles are approximately elliptical spherical with an average particle size of 11.03 μm. The shrinkage rates of anodes with different pore-forming agents are similar. In the entire test temperature range (room temperature to 1450 ℃), the shrinkage rates of the electrolyte and anode green bodies are 6.10% and 8.3% (cornstarch), 9.5% (graphite), respectively. These results indicate that sintering activity of the NiO-BZY20 anode green body is significantly higher than that of the BZY20 electrolyte. The shrinkage rates of the anodes with different pore-forming agents are different. The OCV values at different test temperatures are slightly lower than the theoretical values calculated with the Nernst equation. This is mainly attributed to the generation of electron-holes in the BZY-based electrolyte in an oxidizing atmosphere, causing internal short-circuiting. In addition, from the current-power density curves in the figure, it can be observed that at a higher test temperature (700 ℃), the peak power density of the cell prepared with graphite as the pore-forming agent (199 mW·cm⁻²) is significantly higher than that of the cell prepared with cornstarch as the pore-forming agent (175 mW·cm⁻²). The Ro and Rp values of the cell prepared with graphite as the pore-forming agent are slightly lower than those of the cell prepared with cornstarch as the pore-forming agent. Among them, the difference in Rp can be mainly attributed to the pore-forming ability of graphite as the pore-forming agent in the anode preparation process. From the high-magnification SEM images of the BZY20 electrolyte film layer, it can be found that the grain size of the BZY20 electrolyte of the cell prepared with graphite as the pore-forming agent is relatively large. This phenomenon can be ascribed to the relatively large shrinkage rate of the anode support containing flaky graphite during the co-sintering process, which promotes the sintering densification and grain growth process of the electrolyte. An effective strategy to further improve the electrochemical performance of the Ni-BZY20 anode-supported configuration cell is to reduce the thickness of the BZY20 dense electrolyte membrane to 12 μm. From the I-V and I-P curves, the power density of the cell at 600 ℃ reaches 210 mW·cm⁻², which benefits from the significant reduction of Ro (at 700 ℃, from 0.62 Ω·cm² to 0.29 Ω·cm²). This performance parameter is better than that of most of the cells reported in the literature with the same electrode and electrolyte materials.[Conclusions] Compared with cornstarch pore-forming agent, graphite pore-forming agent resulted in significantly high porosity (21%) and electrochemical performance. At the same time, the use of graphite pore-forming agent also has a relatively large impact on the sintering shrinkage behavior of the anode support. During co-sintering with the electrolyte, it promotes its densification and grain growth, resulting in a decrease in the Ro of the cell and an increase in power density. In addition, under the condition that the amount of graphite pore-forming agent is limited (due to the mechanical properties of the anode support), reducing the thickness of the BZY20 electrolyte membrane from 39 μm to 12 μm can halve the Ro of the cell, whereas the corresponding power density is increased by two times. This is considered to be the most effective means to improve the electrochemical performance of the cell.

Key words: proton conductor solid oxide fuel cell; pore-forming agent; anode; BaZr_0.8Y_0.2O_3−δ; sintering shrinkage