Effect of Redox Cycling on Mechanical Properties of Ni-YSZ Cermets for SOFC Anodes

Abstract

Effects of redox cycling on the mechanical properties of Ni-YSZ cermets were investigated by using an in-situ small punch (SP) testing method. The Ni contents in the Ni-YSZ cermets tested in this study were 0, 15, 30 and 50 vol. %. Electrical conductivity measurements suggested that the Ni particles formed a network-like structure at the Ni contents greater than 20 vol. %. Experimental results obtained from the in-situ SP tests at 800åC demonstrated that ductile-like deformation was induced under reducing environment (at an oxygen partial pressure of 10-18atm) for the Ni contents of 30 and 50vol. %. The ductile-like behavior was attributable to the formation of Ni network. The first complete re-oxidation of Ni-YSZ cermets then caused critical damage and resulted in drastic strength reduction, probably due to Ni agglomeration. This study revealed the first evidence for the presence of potential plastic deformation capacity in Ni-YSZ cermets under reducing environments.

Fracture properties of porous yttria-stabilized zirconia under micro-compression testing

“…Moreover, the Ni in the hydrogen electrode is liable to re-oxidize into NiO in case of system failures such as the re-introduction of air during the system shutdown. Because of the Ni volume expansion upon re-oxidation, high tensile stresses are generated in the YSZ skeleton leading to the generation of micro-cracks [22][23][24][25]. It has been shown that the mechanical damage in the hydrogen electrode can affect significantly the overall cell performance [22,[25][26][27].…”

“…Because of the Ni volume expansion upon re-oxidation, high tensile stresses are generated in the YSZ skeleton leading to the generation of micro-cracks [22][23][24][25]. It has been shown that the mechanical damage in the hydrogen electrode can affect significantly the overall cell performance [22,[25][26][27]. In this frame, several studies have been already dedicated to assess the mechanical degradation of the hydrogen electrode and to quantify its impact on the global cell performances [23,28,29].…”

Fracture of Porous Ceramics: Application to the Mechanical Degradation of Solid Oxide Cell During Redox Cycling

“…The Ni swelling within the cermet generates high tensile stresses in the YSZ backbone leading to the apparition of micro-cracks (3). For classical anode supported cell, the mechanical damage of the ionic conducting phase within the cermet results in a substantial degradation of the overall cell performances (4)(5)(6)(7)(8)(9). It is therefore required to further enhance the hydrogen electrode robustness for a higher redox tolerance.…”

Enhancing Ni-YSZ Anode Resilience to Environmental Redox Stress with Aluminum Titanate Secondary Phases

“…While Ni-based anodes endow SOFCs with many benefits, one significant limitation to Ni-YSZ cermets is the material’s sensitivity to oxidation. Given that the molar volume of NiO is ∼1.66 times larger than the molar volume of Ni, reduction/oxidation (redox) cycling can lead to severe mechanical degradation including cracking, cell fracture, and loss of connectivity between the anode and the YSZ electrolyte. ,− In addition to any mechanical damage suffered by the SOFC, redox cycling will also adversely impact the anode microstructure through decreased triple phase boundaries and lower overall electrochemical conversion capacity. − Anode oxidation can result from fuel starvation under strong polarizing conditions, referred to as electrochemical oxidation, or as a result of either gas leakage between the anode and the cathode or the presence of an oxidizing agent in the fuel, referred to as atmospheric or environmental oxidation. These two sources of redox stress will impact Ni-YSZ cermet anodes in different ways: electrochemical oxidation from fuel starvation will first affect the electrode–electrolyte interface resulting in delamination and loss of electrocatalytic material. , Environmental redox cycling is expected to cause a more uniform degradation throughout the anode with the majority of damage visible on the surface of the electrode.…”