A study of lanthanum nickelate cathodes for employment in solid oxide fuel cells

Harrison, Christopher ORCID: 0000-0003-0214-7869 (2023). A study of lanthanum nickelate cathodes for employment in solid oxide fuel cells. University of Birmingham. Ph.D.

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Abstract

The current state-of-the-art cathode materials used in Solid Oxide Fuel Cells (SOFC) are doped lanthanum cobaltite perovskites (e.g. La0.6Sr0.4Co0.2Fe0.8O3 (LSCF6428)). These ceramics, with the ability to transport both electrons and oxygen ions through their crystal lattice are employed in many commercial cells available today. However, there are several degradation-related concerns relating to these materials and there have been numerous attempts to explore alternatives. One family of materials that has been suggested to offer promise in overcoming these issues is the lanthanum nickelates (e.g. doped LaNiO3, La2NiO4). In particular, it has been suggested by a number of researchers that these materials can alleviate the degradation associated with so-called chromium poisoning. However, despite some apparent associated advantages, a move away from the use of lanthanum cobaltite cathodes has failed to materialise. One possible reason for this is that, simply, these alternatives offer an inferior standard of performance in comparison with the conventional materials. However, given the sheer volume of research work in this area and the very many attempts to characterise and optimise electrode performance, it can be difficult to ‘benchmark’ materials and understand the performance range that is achievable. Further, many authors fail to offer direct comparisons with the state-of-the-art, often making results difficult to interpret due to a lack of a relative comparison. The work detailed in this thesis attempts to address this by offering a more direct understanding of how lanthanum nickelate electrodes perform in relation to the state-of-the art. Importantly, this can help to inform on future research directions in this area.

In this work, the lanthanum nickelates are shown to offer relatively inferior performance in comparison with the state-of-the-art LSCF. This is apparent both by considering data in the literature (Chapter 2) and by conducting direct comparisons under the same experimental conditions (Chapters 4 and 5). This would appear to contradict statements around the suitability of the lanthanum nickelates in replacing state-of-the-art electrodes. However, it is also apparent that the performance of materials can be optimised via a number of routes. Notably, an increase in electrode sintering temperatures is shown to promote electrode performance. This improvement is thought likely to be associated with enhanced electrode morphology and particle connectivity. A further approach to boosting lanthanum nickelate electrode performance is to combine them with other phases to form composite electrodes. In particular, the combination of LaNi0.6Fe0.4O3 (an excellent electronic conductor) with an oxide conducting phase (e.g. GDC) has been shown to offer real promise in this regard.

In addition to observations on the performance of the studied materials, the extent of the experimental work conducted in this thesis (and the breadth of the materials studied) allows for some comment on certain SOFC testing approaches. Firstly, it has been shown that the use of noble metal pastes (often applied to electrodes in experimental works to provide electrical contact) can disproportionately impact on the reported performance of electrode materials. For those electrode materials with relatively low electrical conductivity, for example, the addition of gold paste can significantly enhance electrode performance. This effect is lessened for materials with more favourable conductivity properties. This creates an interesting dilemma with respect to comparing results across different studies (where pastes may or may not be applied).

As is evident in the literature, the so-called symmetrical cell testing approach is a common means by which the performance of electrodes can be quantified. These tests have several advantages (including the ability to remove associated contributions from the anode side of the cell). However, as is shown in Chapter 5, the conclusions that may be drawn from such tests can be misleading; the results from the symmetrical cell testing methodology (performed at a ‘zero current’ condition) do not always correspond well with those recorded in more conventional ‘single cell’ testing approaches (i.e. performance under polarisation). This is a notable finding given the prevalence of this testing approach in the literature, with some authors using this as a sole means of identifying promising materials. The results from this thesis suggest that such a singular approach should be avoided.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):