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Augmented Lifetime of Infiltrated Solid Oxide Fuel Cells

Periodic Reporting for period 1 - ALifeInSOFCs (Augmented Lifetime of Infiltrated Solid Oxide Fuel Cells)

Reporting period: 2015-09-01 to 2017-08-31

The main focus of the research project was the fundamental understanding and quantification of the microstructural degradation mechanisms affecting the decrease in electrochemical performance of Ni-infiltrated anodes for solid oxide fuel cells (SOFCs). This was achieved by successfully coupling a variety of experimental, characterisation and modelling techniques, such as: i) fabrication and electrochemical testing (via real-time impedance spectroscopy) of infiltrated anodes, ii) ex-situ tomographic reconstruction and advanced characterisation of electrode microstructure before and after degradation, iii) physically-based modelling to quantitatively link the microstructural evolution to the electrochemical response. Complementary studies were carried out in order to develop and apply tools for the electrochemical and microstructural characterisation of electrodes, analysis of reaction and gas transport phenomena, establishment of design guidelines.
The following main achievements were obtained:
• establishment of a validated modelling framework to decouple the microstructural contribution from the electrochemical performance via impedance spectroscopy;
• fundamental understanding of the nickel microstructural evolution at different length scales (micron and nanometres);
• indications of material modifications to reduce the electrode degradation, as well as the identification of useless practices currently proposed in the scientific literature;
• design and application of new and simplified characterisation techniques to quantify the microstructural evolution of metals on different substrates.
These activities demonstrated how the synergic integration of experimental activities, microstructural characterization and model simulations can efficiently identify and quantify the underlying mechanisms of electrode degradation, thus enabling to suggest strategies to enhance their durability and to discard ineffective common practices. This has already stimulated other European research groups, working mainly on battery and energy storage research, to apply the same methodology to analyse other types of degradation mechanisms and consequently to engineer advanced electrode microstructures.
The following main activities have been performed during the project:
- validation of a physically-based continuum electrochemical model with tomographic and electrochemical impedance spectroscopy (EIS) data of Ni:ScSZ (scandia stabilized zirconia) anodes (both conventional and scaffold electrodes) prepared and tested in previous projects;
- fabrication of more than 10 symmetric SOFC anodes: scaffolds of ScSZ, prepared with custom ink, sintered on YSZ (yttria stabilized zirconia) electrolyte disk, infiltrated up to 19 times with Ni(NO3)2 (nickel nitrate) solution, fired and reduced;
- degradation experiments by annealing selected samples at constant temperature for up to 200h while monitoring electrochemical impedance in real time;
- 3D tomographic reconstruction (focused ion beam-SEM) of selected fresh and degraded anodes, image segmentation and advanced quantification of the microstructural properties;
- interpretation of the electrochemical degradation data according to the validated physically-based model and the 3D tomographic results, with quantification of the fractal roughness and the microstructural degradation mechanisms occurring at different length scales.
Additional activities complemented the research, such as:
- application of the same methodology described above to redox-cycled electrodes;
- development of open-source software (TauFactor) to quantify relevant microstructural properties from tomographic datasets;
- guidelines for the rational design of 3D manufactured and conventional electrodes by using a model-guided approach;
- multi-length scale X-ray tomography and characterisation of diffusion in porous electrodes.

The main result of the project is the development of an integrated experimental/modelling framework which allowed for the fundamental understanding of the electrochemical phenomena in SOFC electrodes and, more specifically, of the degradation processes affecting nickel nanoparticles. In particular, the project proved that the rapid electrochemical degradation of infiltrated SOFC anodes stems from the evolution of interfaces at the nanoscale, beyond the resolution currently achieved by tomographic techniques, thus solving a long-standing dilemma regarding nanostructured and redox-cycled electrodes. As an immediate consequence, the research results suggest that nickel wetting properties are key to stop the degradation while practices currently adopted by the community, such as sintering inhibitors, are ineffective. Additional guidelines for structured SOFC electrodes were also published.

The list of papers authored/co-authored by the Fellow during the project is reported in the ""Publication"" page. They include 9 papers in peer-reviewer international scientific journals (plus 2 currently under review and 4 in preparation), 1 contribution in a book chapter, 4 contributions in conference proceedings, the participation to 7 national and international conferences delivering oral presentations, the dissemination of results in 2 seminars and in 2 workshops."
The achievements of the project have a significant impact mainly on the scientific community working on solid oxide fuel cells and batteries, going beyond the state-of-the-art regarding the following points:
- a long-standing dilemma concerning the rapid degradation of nanostructured Ni-based anodes has been resolved, demonstrating that the degradation stems from the microstructural evolution of the fractal roughness of Ni nanoparticles at a length scale not detectable with 3D tomography. This suggests that the degradation can be reduced by modifying the interfacial energy between nickel and the ceramic scaffold, while the adoption of sintering inhibitors as suggested in the scientific literature is ineffective;
- it was shown that integrating physically-based models with electrochemical impedance spectroscopy and 3D tomography allows for a comprehensive understanding of the fundamental processes governing the efficiency and degradation of electrodes, going much beyond what these techniques can do on their own. This model-guided approach can speed up the optimisation of both fuel cells and batteries, thus saving time and resources if compared with the traditional trial-and-error empirical approach.
These points have already been positively received by the scientific community working on electrochemical energy research, triggering research groups to adopt the same methodology and to work on the implications of the research outcomes.
In the long term, the knowledge gained during the project will help in the rational design of low-temperature solid oxide fuel cells, which represent one of the pillars for the clean and sustainable energy production for residential applications in the near future. Should this technology meet the economical and lifetime targets, which have been addressed in this project, the whole society will benefit from the reduction of carbon emission as well as the boost of the hydrogen economy, which is expected to create new job opportunities and promote the advancement of education programmes centred on the new low-carbon technologies.
Methodology to investigate the degradation in nanostructured solid oxide fuel cell anodes