Solid Oxide Fuels Cells: Facts and Figures by John T.S. Irvine & Paul Connor

Author:John T.S. Irvine & Paul Connor
Language: eng
Format: epub
Publisher: Springer London, London

1 Introduction

The solid oxide fuel cell (SOFC) technology has to face many challenges before its large-scale commercialisation. Cost reduction, along with enhanced reliability, durability, fuel flexibility, load following capabilities and compactness are needed. Yet, despite all the research, the exact underlying mechanisms of the electrochemical reactions have not yet been unambiguously identified. The high-temperature environment promotes physicochemical modifications of the materials that alter the electrochemical and mechanical properties after prolonged use. The driving forces of these degradation processes that arise from chemical interactions between the SOFC materials themselves, on the one hand, and the volatile contaminants transported by the fed gases, on the other hand, have not yet been fully clarified.

The structural reliability of SOFCs is a salient issue. The end of operation of a stack is ultimately caused by the loss of structural integrity of one or several of the cells. This is the result of (i) the accumulation during operation of physicochemical alterations, of plastic and creep deformations, and (ii) the modification of the temperature profile, due to the degradation of the electrochemical performance of the cells. Mechanical issues do not, however, exclusively occur after prolonged use. Inappropriate control during load following, harsh conditions and thermal cycles can induce discrete failures. The in-series assembly of the standard repeating units (SRU) in a stack is a striking weakness, the mitigation of which is hindered by the complexity of the failure modes in the ceramic materials. Mechanical failure in a single cell induces a succession of detrimental effects that act in a coupled manner. For instance, delamination or cracking of the electrode layers breaks the ionic and/or electronic conduction paths to the electrochemical reactions sites. The ensuing local loss of performance induces a harmful redistribution of the current density. Once cracked, the electrolyte or sealant does not any longer ensure the separation of the fuel and air compartments to the fullest extent. The local and unsteady combustion provokes a local increase of the temperature, as well as diverse chemical alterations of the cell layers. All these undesirable phenomena promote in turn additional stresses, which reach critical values in an accelerated manner. The SOFC field is, therefore, characterised by strong interactions between the phenomena.

Despite the evidence of mechanical issues in SOFCs, which are experienced even during laboratory button cell tests, this topic is still receiving limited attention. Efforts are seen as stand-alone tasks, owing to the different experimental and modelling techniques needed to gather the essential information, whereas mechanical failures in SOFCs are likely intricately related to physicochemical and electrochemical aspects.

This survey of the current state of models for the prediction of mechanical failures in SOFC stacks comprises five parts. For conciseness, the emphasis is on planar stack design with anode-supported cells. The description can be easily transposed to other situations, differing in terms of type of cell, geometry or operating temperature. In Sect. 2, the diverse origins of stress in the components are briefly presented, along with the most common failure mechanisms. In Sect. 3, a brief overview of the

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