Comparative Study on Performance Stability of Ni–Oxide Cermet Anodes under Humidified Atmospheres in Solid Oxide Fuel Cells

Abstract

The influence of the oxide component in Ni-oxide based cermet anodes on the performance stability was examined at 1000 • C with a supply of highly humidified hydrogen, 40% H 2 O-60% H 2. Three types of anodes were applied; Ni-samaria-doped ceria (Ni-SDC), Ni-yttria-stabilized zirconia (Ni-YSZ), and Ni-scandia-stabilized zirconia (Ni-ScSZ). Abrupt deterioration in performance was confirmed during constant current operation for the cells with Ni-YSZ and Ni-ScSZ anodes, accompanied with a drastic increase in ohmic resistance between the anode and reference electrode. In contrast, the performance of cell with Ni-SDC anode was pretty stable for 48 h. These differences in performance stability were significantly related to the local partial pressure of steam in the vicinity of the electrochemically active sites because the surface oxidation of nickel particles resulted in the deterioration. The microstructural changes in Ni-YSZ and Ni-SDC anodes upon discharge were also quantified by the dual-beam focused ion beam-scanning electron microscopy (FIB-SEM). The electrochemical behavior observed in each anode had a strong correlation with the anode microstructure such as triple phase boundary length and nickel particle size.

Redox-Induced Self-Modification of Cermet Anodes of Ni–CeO2-Based Oxide for Solid Oxide Fuel Cells

“…• C. 24 This behavior is quite different from that for the conventional SOFC anode of Ni−YSZ cermet, 23 reflecting the difference in affinity for steam; in the case of Ni−YSZ, the interfacial conductivity showed the parabolic distribution against the partial pressure of oxygen (steam) and had a local maximum value in the humidity range of 15-25%. These results indicate that the redox treatment for Ni-GDC has no impact on the mechanism of electrochemical hydrogen oxidation.…”

“…Furthermore, the active triple phase boundary (TPB) length was calculated by subtracting inactive TPBs connecting to the isolated phase; the electronic conduction of GDC in reducing atmospheres was ignored, and GDC was regarded as the pure ionic conductor. 24 After isothermal redox treatment, the volume fraction of pore decreased, concomitant with an increment in those of Ni and GDC. In response to this change, the reduction of the contact area fraction between Ni and Pore (Ni/Pore) was confirmed, whereas those of GDC/Pore and Ni/GDC increased.…”

Microstructural Evolution of Ni–YSZ Cermet Anode under Thermal Cycles with Redox Treatments

“…[4][5][6][7][8][9] For instance, such degradation phenomena can be observed in the downstream part of the fuel gas flow channel accompanied with the microstructural evolution because the anode is exposed to the severe condition due to lean fuel and high partial pressure of steam. 10,11 The electrochemical oxidation of nickel also sometimes proceeds during discharge under the fuel shortage condition. [12][13][14] On the other hand, severe degradation is induced by the frequent start-stop operation and emergency shutdown.…”

Dimensional analysis of Ni‐NiO grains at anode/electrolyte interface for SOFC during redox reaction

“…As shown in Figure (B), the TPBs are represented by the yellow lines, and the value is about 7.8 μm/μm 3 . This value is larger than those reported in the literature (between 1 and 6 μm −2 ) . For example, the TPB value is about 2 μm −2 when a Ni‐YSZ anode was reduced at 950°C .…”