Lifetime limiting effects in pre-commercial solid oxide cell devices

Abstract

The solidoxide electrochemical cell technology is promising for efficient energy storage,especially when the share of intermittent renewable electricity production ishigh. The technology is being commercialized in niche markets, but large-scale employmentis still hindered by limited durability of the devices. The lifetime limitingmechanisms are addressed in this work. A generalintroduction into mechanisms limiting the durability is presented. A databaseof more than 50 parameters from 150 publications and 1 000 000 hours ofaccumulated testing was established, and a quantitative analysis of degradationand lifetime was conducted. It is shown that the technology is approaching the officialtargets required for commercialization, but that work remains to be done.  It is further recognized that targeting nicheapplications initially will allow for employment of economies of scale, whichwill bring down costs and facilitate entry into larger markets. Here, weexamine electrochemical reduction of CO2 to CO and one of the main failuremechanisms related to it. Carbon formation on the nickel electrocatalyst can bedetrimental to the microstructural integrity of the cell. It is found that thepossible operating window is severely limited due to gradients of temperature, gas concentration and overpotential across the electrode. These affects alsoapply to stack- and system-level, and the results obtained are combined withmodeling and stack testing experiences. Thus, on account of this mechanism thepossible outlet CO concentration is limited by up to 50% below thethermodynamic carbon deposition threshold based on the inlet temperature,depending on design and operating strategy. Replacement of the Ni electrocatalyst would increase the stabilitytowards this issue and may improve the robustness in other ways as well. Ceriahas been reported as a potential candidate in such endeavors. Thin filmelectrodes of nickel and ceria are therefore studied as model systems usingnear-ambient pressure x-ray photoelectron spectroscopy to further thefundamental understanding of the carbon formation mechanism. The reactionoccurs further from the thermodynamic threshold on ceria, and fundamentalmechanisms for electrochemically driven carbon growth are suggested based onobserved adsorbate species.  By infiltrating ceria after degradation has already occurred, the robustness and lifetimeof the cells are increased. Complete reactivation of the fuel electrode isachieved after otherwise detrimental failure mechanisms have occurred, such asreactant starvation and carbon formation. Moreover, the degradation of the electrodeover the course of nearly 2500 hours is essentially eliminated by infiltrating aftermicrostructural stabilization had occurred. Lastly, the method is scaled up byreplicating the positive effects of post-degradation infiltration on an 8-cellstack.