Simulation Technology on SOFC Durability With an Emphasis on Conductivity Degradation of ZrO2-Base Electrolyte

Abstract

Attempts have been made to simulate numerically the conductivity degradation of solid oxide fuel cell (SOFC) YSZ electrolyte; physicochemical model has been constructed on the basis of experimental conductivities of Pt/1%NiO-doped YSZ/Pt cells under OCV condition. The temperature effect was extracted from the time constant for degradation caused by one thermal activation process (namely Y-diffusion), whereas the oxygen potential effect was determined by those Raman peak ratios between the tetragonal and the cubic phases which linearly change in relation to the conductivity. The electrical properties of the YSZ electrolyte before and after the transformation are taken into account. The time constant is directly correlated with Y-diffusion with proper critical diffusion length (∼10 nm), while the Y-diffusion can be enhanced on the reduction of NiO; this gives rise to the oxygen potential dependence. The most important objective of simulating the conductivity degradation is to reproduce the oxygen potential profile shift on transformation. Detailed comparison between experimental and simulation results reveal that the shift of oxygen potential profile, therefore, the conductivity profile change inside the YSZ electrolyte can well account for the Raman spectra profile. This also reveals that with decreasing temperature, there appear other kinetic factors of weakening or diminishing enhancing effects by NiO reduction. This may be important in interpreting the ohmic losses in real stacks, because there are differences in time constant or in magnitude of degradation between the pellets and those industrial stacks in which transformation was confirmed by Raman spectroscopy.

Recent Achievements of NEDO Durability Project with an Emphasis on Correlation Between Cathode Overpotential and Ohmic Loss

“…These oxygen potentials are quite important in understanding chemical behaviors of electrode materials. This is also important to evaluate the oxygen potential distribution inside the electrolyte, which becomes key in understanding/predicting conductivity degradations due to the tetragonal transformation to be affected significantly by oxygen potential as well as temperature , , .…”

“…This approach will be extended to treat more complicated phenomena and to reproduce the actual features of tetragonal transformation and resulting conductivity degradations. These features are summarized in our previous paper , .…”

Achievements of NEDO Durability Projects on SOFC Stacks in the Light of Physicochemical Mechanisms

“…The main features of the performance degradations determined by CRIEPI were already reported and discussed in details elsewhere ; here, summaries at the final stage are given in Table . Separation of total voltage losses into several contributions (cathode and anode overpotentials, ohmic loss etc.)…”

“…In our previous investigation , the conductivity degradation has been discussed in terms of three regions in the oxygen potential vs . temperature diagram.…”

“…Figure is reproduced from Fig. 19 in by considering other effects to be discussed in the present investigation. The first is the oxidative region where the NiO phase is stable against the reduction; here, the 1% NiO content in 8YSZ is selected as in the anode‐support cells.…”