The Electrochemical Performance and Structural Evolution of Solid Oxide Fuel Cell Ni-YSZ Fuel Electrodes

LIU, QINYUAN

doi:https://doi.org/10.21985/n2-hjze-fa43

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The Electrochemical Performance and Structural Evolution of Solid Oxide Fuel Cell Ni-YSZ Fuel Electrodes

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Solid Oxide Fuel Cells (SOFCs) are a low-emission, efficient and fuel flexible energy conversion devices, which have been extensively applied in electricity generation systems. Moreover, it can also be operated in electrolysis mode as Solid Oxide Electrolysis Cell (SOEC) or Reversible Solid Oxide Cell (ReSOC) for intermittent renewable energy storage. However, these systems have limited commercialization due to challenges including long-term durability, costs, and performance. This dissertation presents research on the SOFC Nickel-Yttria Stabilized Zirconia (Ni-YSZ) electrode which is commonly used in industrial application. It has been widely reported that long-term degradation is strongly dependent on microstructural changes, including Ni coarsening, and interfacial delamination. However, there are limited systematic studies about how degradation rate varies as a function of operating conditions, e.g., current density, temperature or gas condition. This work focuses on the degradation mechanism of Ni-YSZ fuel electrode in both reversing and direct current operations. Electrochemical impedance spectroscopy was used to characterize the electrochemical performance. Focused Ion Beam – Scanning Electron Microscopy (FIB-SEM) and Transmission X-ray Microscopy (TXM) techniques were used to investigate the microstructural evolution, how it relates to ohmic and polarization resistance change. Life tests with reversing current operations were carried out at different current densities on Ni-YSZ/YSZ/Ni-YSZ electrode supported symmetric cells. Although voltage and total cell resistances were relatively stable, there was a steady increase in the ohmic resistance (at current densities ≥ 0.6 A/cm2) that was almost offset by a decrease in polarization resistance. The increase in ohmic resistance was associated with extensive electrode-electrolyte delamination. The decrease in polarization resistance appears to relate to appearance of Ni-YSZ nanoparticles at electrode/electrolyte interface which increased triple phase boundary density. The further life tests with direct current operations were carried out at different current densities and different gas conditions. The increase in ohmic resistance was observed at current density ≥ 0.6 A/cm2 in 97% H2 / 3% H2O gas condition, while ohmic resistance was stable at 0.6 A/cm2 in 50% H2 / 50% H2O gas condition. The increase in ohmic resistance was caused by intergranular fracture in YSZ electrolyte near SOEC side. Ni accumulation at grain boundaries from SOEC side was responsible for intergranular separation of YSZ grains. Furthermore, accelerated degradation tests in the high temperature and high humidity condition were performed on Ni-YSZ anode supported full cells. Microstructural quantification were correlated with changes in electrochemical performance. Additionally, “pseudo in situ” measurement, where the same electrode was 3D imaged by TXM with intervening aging steps, was conducted on Ni-YSZ electrode. The actual evolution of structure in the same region, including coalescence of Ni, with time were observed and visualized.

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