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

Abstract

Long-term performance testes by CRIEPI (Central Research Institute for Electric Power Industry) on six industrial stacks have revealed an interesting correlation between cathode polarization loss and ohmic loss. To make clear the physicochemical meaning of this correlation, detailed analyses were made on the conductivity degradation of YSZ electrolyte in button cells and then on the ohmic losses in the industrial cells in terms of time constants which are determined from speed of the tetragonal transformation through the Y diffusion from the cubic phase to the tetragonal phase. In some cases, shorter time constants (faster degradations) were detected than those expected from the two-time-constant (with and without NiO reduction effects) model, suggesting that additional ohmic losses after subtracting the contribution from the tetragonal transformation must be caused from other sources such as cathode-degradation inducing effects. Main cathode degradations can be ascribed to sulfur poisoning due to contamination in air in the CRIEPI test site. An important feature was extracted as this cathode degradations became more severe when the gadolinium-doped ceria (GDC) interlayers were fabricated into dense film. Plausible mechanisms for cathode degradations were proposed based on the Sr/Co depletion on surface of lanthanum strontium cobalt ferrite (LSFC) in the active area. Peculiar cathode degradations found in stacks are interpreted in term of changes in surface concentration by reactions with sulfur oxide, electrochemical side reactions for water vapor emission or Sr volatilization, and diffusion of Sr/Co from inside LSCF.

Mechanism of SrZrO3Formation at GDC/YSZ Interface of SOFC Cathode

“…[1][2][3] Some manufactures have fabricated an interlayer dense enough to suppress Sr diffusion toward the electrolyte side, which is accompanied by no SrZrO 3 formation. 4 In general, however, the interlayer formed on the YSZ electrolyte by printing and high-temperature sintering becomes porous. Therefore, the interlayer cannot completely eliminate SrZrO 3 formation, and the SrZrO 3 is often formed near the ceria/zirconia interface.…”

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

“…The NEDO (New Energy and Industrial Technology Development Organization) projects on durability/reliability of SOFC stacks have been performed for 13 years as three successive projects initiated from 2005FY ; this has been made as cooperated investigations among stack developers, public institutes, namely, AIST (National Institute of Advanced Industrial Science and Technology) and CRIEPI (Central Research Institute for Electric Power Industries) and academic groups in UTokyo (The University of Tokyo), KyotoU (Kyoto University),KyushuU (Kyushu University) and TohokuU (Tohoku University). The stack degradation issues have been extracted from the stack performance tests made by CRIEPI on respective stacks with the same technique of separating measured voltage losses into several different contributions and their degradation rates and also by Tokyo Gas on thermal cycle characteristics .…”

“…This implies that a number of different analytical methods should be well organized in a systematic manner, and also precise enough to distinguish respective processes among complicated competitions. On the basis of such considerations, simulation for predicting life can be set up by including all plausible aspects of complicated features in stack degradations .…”

“…Features of extracted issues for respective stacks have been already reported in details , ; here final results of the 13‐year project including previous projects will be described with an emphasis on the physicochemical reasonings in terms of the thermodynamic (energetic) properties as well as the kinetic properties such as cation diffusivity and surface reactivities. Main purpose is to obtain clues to solving gaps in degradation behaviors among stacks or between stacks and button cells in terms of those physicochemical properties which may be affected by fabrication processes, especially by firing processes and also operation conditions.…”

“…Main purpose is to obtain clues to solving gaps in degradation behaviors among stacks or between stacks and button cells in terms of those physicochemical properties which may be affected by fabrication processes, especially by firing processes and also operation conditions. Since the anode degradations have not been described well previously , one focus will be made on anode degradations in addition to summaries on electrolyte and cathodes.…”