The influence of the reducing conditions on the final microstructure and performance of nickel-yttria stabilized zirconia cermets

Abstract

Ni-YSZ (yttria stabilized zirconia) cermets are the most widespread composite materials to be used as SOFC fuel electrodes. These materials are generally fabricated by the reduction of NiO to Ni in a NiO-YSZ composite, where the reducing conditions have a great effect in the final microstructure of the electrode. In the present work, several reducing conditions were explored in order to find the most suitable microstructure for anode-supported microtubular solid oxide fuel cells (SOFCs). Samples were firstly reduced in either pure or diluted H2 (dry or humidified), at temperatures ranging from 400 to 800 °C while their DC conductivity was monitored. The highest conductivity value was measured for the sample reduced in pure humidified hydrogen at 800°C. However, this sample experienced conductivity degradation in comparison with samples reduced under dry conditions. For the studied temperature range, nucleation of nano-porous nickel particles is firstly formed during reduction. However, from our experiments it was concluded that those nanoparticles are not stable with time, at least at temperatures between 600 ºC and 800 ºC. Electrochemical characterization of complete microtubular cells under real wet conditions was also performed under current load, confirming that the microstructure of the Ni-YSZ cermet is still evolving during operation.

Highly stable microtubular cells for portable solid oxide fuel cell applications

“…In this experiment, as observed in the chronoamperometry of figure 3, there is an initial non-stationary period during the first 20 hours, associated to reaccommodation of nickel particles after NiO reduction. This effect was previously observed for shorter durability studies [35], where a complete EIS and microstructural study was performed. After this period, the system is about constant obtaining current densities of ∼400 mAcm 2 .…”

Boosting the performance and durability of Ni/YSZ cathode for hydrogen production at high current densities via decoration with nano-sized electrocatalysts

“…Cell A degraded fast through the entire testing period of 750 h with the cell voltage degradation rate of 0.699 V/kh, the average R s degradation rate of 0.388 Ω•cm 2 /kh, and the average Rp degradation rate of 0.075 Ω•cm 2 /kh. The degradation of Rs can be ascribed to the Ni-YSZ conductivity changes 49,50 . The Rp degradation profile of Cell A can be split into two segments: the first 60 h and the following 690 h, since the degradation of Cell A had been significantly different in the first 60 h compared to the rest of the test.…”

Reversible operation of microtubular solid oxide cells using La0.6Sr0.4Co0.2Fe0.8O3-δ-Ce0.9Gd0.1O2-δ oxygen electrodes

“…Electrochemical characterization was carried out in both SOFC and SOEC modes using the experimental setup described elsewhere [4,44,45]. The cells were heated up to 800 ºC under nitrogen atmosphere and then switched to humidified hydrogen for a period of 24 hours, in order to assure full accommodation of the nickel particles after NiO reduction [46,47]…”