On Electrochemically Driven Phase Change and Accelerated Test Protocols in Solid Oxide Cells

Abstract

The aim of this talk is to address three fundamental questions in solid oxide fuel cells and solid oxide electrolysis cells:(1) Whether or not the oxygen reduction reaction is indeed the limiting step in solid oxide cells and how to quantify it definitely?(2) What is the origin for the so-called “electrochemically driven phase transformation and how to translate the understanding to achieve a high-performance and highly stable electrode?(3) How to develop reliable accelerated test protocols and what’s the physical process that was accelerated?Quantifying electrode polarization in fuel cells and electrolyzers is of significant importance. It, however, remains difficult because of the substantial overlap of polarizations from both negative and positives electrodes either in a dc or an ac measurement. This challenge leads to a compelling gap in our scientific understanding of electrode phenomena and designing high-performance and durable electrochemical devices. In this talk, we will describe a reproducible method to quantify the polarization of an individual electrode of a solid oxide cell. This method is generic and can be applied readily to other electrochemical cells.We will then address the greater phase transformation of an oxygen electrode during operation when compared to thermally annealed powders. Theoretical analysis suggests that the presence of a reduced oxygen partial pressure at the interface between the oxygen electrode and the electrolyte is the origin for the phase change in an oxygen electrode. Guided by the theory, an addition of the electronic conduction in the interface layer leads to the significant suppression of phase change, while improving cell performance and performance stability.Reliable accelerated test protocols are needed for solid oxide fuel cell research to facilitate rapid learning on key durability issues, identify potential modes of failure expeditiously, and eventually predict the calendar lifetime of an electrochemical cell. In this work, solid oxide fuel cells operated at a constant current density were compared to cells undergoing accelerated measurements, which are composed of intermittent current injection to the cell. A general accelerated test profile was developed by cycling a solid oxide fuel cell from open circuit to a predetermined operating current density that is the same as the current density during a steady-state operation, to accelerate the local redox environment. Up to 1,320,000 cycles were generated in this work. The cell degradation was accelerated by nearly 10×, suggesting the feasibility of using this protocol for acceleration test to predict life performance of solid oxide fuel cells.Finally, we will present a theoretical model that underlies those three questions and can be used to address stability and durability issues in solid oxide fuel cells and solid oxide electrolysis cells.