GUO Guanlun ¹, WANG Shujie ¹, WU Sheng ²
(1. School of Automotive Engineering, Wuhan University of Technology, Wuhan 430070, Hubei, China; 2. College of Intelligent Manufacturing, Jianghan University, Wuhan 430056, Hubei, China)

Extended Abstract: [Background and purpose] Solid oxide fuel cells (SOFCs), as a clean and efficient energy conversion device, have been widely recognized for their numerous advantages, such as low pollution emissions, high energy conversion efficiency and broad application range. They are regarded as one of the most environmentally friendly power generation methods in the 21st century. Under the condition of ignoring cost, hydrogen is considered to be the most ideal fuel for SOFCs, because it does not cause any environmental pollution during the energy conversion process. However, the high costs associated with the production, storage and transportation of hydrogen have significantly restricted the large-scale application of SOFCs. In contrast, ammonia, as a carbon-free hydrogen carrier fuel, has attracted the attention of researchers, due to its relatively low cost and high energy conversion efficiency. Therefore, direct ammonia-fed solid oxide fuel cells (DA-SOFCs) have gained significant popularity. Nevertheless, ammonia has been found to cause irreversible damage to the anode of SOFCs. To understand the poisoning mechanism of ammonia on Ni-based anodes, this study was aimed to explore the detailed process and provide valuable insights for improving the performance and durability of SOFCs.[Methods] In this research, several experimental methods were employed. Firstly, NiO powder was synthesized by using a hydrothermal method. This method allowed for precise control of the reaction conditions, such as temperature, pressure and reactant concentrations, thus enabling the formation of NiO powder with specific crystal structures and particle sizes. The synthesized NiO powder was then used to prepare NiO/YSZ anode materials by mechanical mixing and co-pressing methods. Mechanical mixing ensured the homogeneous distribution of NiO and YSZ, while co-pressing helped to form a compact and stable anode structure. To simulate the actual ammonia exposure environment, a customized ammonia poisoning test bench for the anode materials was constructed. This bench was designed to accurately control various parameters, including ammonia concentration, temperature and reaction time. Before and after the ammonia poisoning experiment, the anode materials were characterized by using various techniques. X-ray diffraction (XRD) was used to analyze the crystal structure and phase changes of the materials, providing information about the lattice parameters and crystal phases. Scanning electron microscopy (SEM) was employed to observe the microstructural variation of the anode materials, allowing for a detailed examination of the surface morphology and particle distribution. Energy-dispersive spectroscopy (EDS) was utilized to determine the elemental composition and variations, with a particular focus on the changes in the content of nitrogen.[Results] Significant changes were observed in the NiO/YSZ anode materials after ammonia poisoning. The NiO, which originally exhibited crystalline granular structures, showed clear signs of agglomeration. The “granularity” and “roundness” of the NiO particles were noticeably enhanced, indicating a transformation in the particle shape. Additionally, the particle size of NiO increased by 32%, which could potentially affect the electrochemical reaction kinetics and mass transfer processes within the anode. The porosity of the anode material also increased from 18.6% to 33.1%, which may modify the gas diffusion pathways and the overall reactivity of the anode. Notably, the nitrogen content in the NiO/YSZ anode material after ammonia poisoning increased by about fourfold, clearly demonstrating the adsorption and incorporation of ammonia into the anode material and hence suggesting that chemical reactions had occurred between ammonia and the anode material.[Conclusions] NiO powder was synthesized and used to prepare NiO/YSZ anode, The ammonia poisoning mechanism of the Ni-based anode was systematically studied. The changes in microstructural morphology, particle size, porosity and nitrogen content provided valuable insights into the poisoning process. The agglomeration of NiO, the increase in particle size and porosity and the significant rise in nitrogen content all indicated that ammonia had a profound impact on the Ni-based anode. These findings could be used as references to mitigate ammonia poisoning and enhance the performance and stability of SOFCs.
Key words: ammonia poisoning; direct ammonia SOFC; anode material; microscopic structure