The Reduction Step and Mechanism Analysis of the NiO-8YSZ Anode Affected by the Hydrogen Concentration

Abstract

The reduction of the NiO-YSZ anode has a direct bearing on the performance of the solid oxide fuel cell, and the hydrogen concentration is a significant condition during the reduction process. Hydrogen at different concentrations were used to reduce the NiO-YSZ anode and the initial performances of the solid oxide fuel cells were tested to reveal the influence of the hydrogen concentration on the reduction of the NiO-YSZ anode. The experimental results suggested that four stages of the open circuit voltage exist during the reduction of the NiO-YSZ anode, namely rapid increase period, temporary stasis period, slow increase period and final stable period successively. The rates of the rapid increase period and the slow growth period showed significant positive correlations with the hydrogen concentration of the reduction while there was a negative correlation between the duration of the temporary stasis period and the hydrogen concentration of the reduction. The values of the temporary stasis period were 0.83V approximately, which were irrelevant to the hydrogen concentration of the reduction. During the final stable period, the variation of the open circuit voltage is less than 1mV/h, which were close to the theoretical open circuit voltage. After the reduction of the NiO-YSZ anode, the ohmic resistance of the solid oxide fuel cell was about 0.31Ω /cm2 and did not reveal a clear correlation with the hydrogen concentration of the reduction. The polarization resistance showed significant negative correlation with the hydrogen concentration of the reduction on open circuit. But under the on-load condition, the same phenomenon only occurred in low hydrogen concentration of the reduction and then the polarization resistance remained largely the same with the increasing hydrogen concentration of the reduction. There was a direct correlation between the initial performances of the solid oxide fuel cell and the polarization resistance under the on-load condition that the initial performances can be improved with the increasing hydrogen concentration during the reduction process of the NiO-YSZ anode when the hydrogen concentration was low and then the initial performances remained constant basically with the continuing increasing hydrogen concentration. The abovementioned phenomena demonstrate that the resistance on open circuit represent nothing to the performances of the solid oxide fuel cell under the on-load condition. Previous researches about the NiO reduction by hydrogen, especially the shrinking core model can be used to explain the abovementioned phenomena. The reduction of the NiO is a volume reduction process. The reaction interface of the Ni/NiO moves inwards during the reduction, forming an unreacted NiO core surrounded by a rim of metallic Ni with much porous structures finally. The H2O produced in the reduction is absorbed onto the surface of the unreacted NiO core, improving the apparent activation energy and limiting diffusion of the Ni atoms to appropriate nucleation centers. Thus, the reduction process is retarded, forming the temporary stasis period. The absorbed H2O removes out through the porous structures of the rim of the metallic Ni. Meanwhile the H2 penetrates deeper and continues to reduce the unreacted NiO core, which is the mechanism of the slow increase period. According to the shrinking core model, higher hydrogen concentration can make the unreacted NiO core smaller, lessening the H2O absorbed onto the surface. Thus, the time needed for the absorbed H2O removal is shorten, leading the duration of the temporary stasis period showed significant negative correlation with the hydrogen concentration. The previous researches attribute the activation of the solid oxide fuel cell to the coarsening of the Ni particle or the further reduction of the unreacted NiO core. In this experiment, the performance of the solid oxide fuel cell with the NiO-YSZ anode reduced by 5% hydrogen concentration was lowest, and thus its potential of the activation was highest. In the first 20 hours of the operation, its performance kept improving and achieved that of the solid oxide fuel cells with the NiO-YSZ anodes reduced by higher hydrogen concentration. But the performance of the solid oxide fuel cells with the NiO-YSZ anodes reduced by higher hydrogen concentration did not significantly improve in the first 20 hours of the operation. The comparison showed that the cause of the activation of the solid oxide fuel cell is quite possible the further reduction of the unreacted NiO core.