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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - DOLORES (Degradation of Lifetime of fuel cell Resistance by Electrochemical impedance Spectroscopy)

Teaser

Summary

An increasingly important technology in the generation of clean electricity is based on solid oxide cells (SOCs) which can be operated in either fuel cell (SOFC) or electrolyser (SOEC) mode. SOCs are remarkably flexible in their: mode of operation, fuel characteristics, operating temperature and in their ability to switch mode of operation. They have high electrical efficiencies which, combined with their high temperature of operation, lead to overall system efficiencies as high as 80% for power generation. SOCs, therefore, are a key component of power supplies ranging from small scale domestic applications and electric or hybrid vehicles, to larger scale, combined heat and power systems for buildings and industrial complexes. They can play a key role in the hydrogen economy and chemical industries with potential for not only large scale production of hydrogen from water but also for conversion of carbon dioxide to syngas, which is a precursor for many synthetic fuels and chemicals. Degradation rates for commercial SOFC stacks are typically 1.5-2%/1000 hours which hinders their wider adoption. Understanding the origins and mechanisms of degradation is difficult, in part because several factors are involved and it is not clear which one is dominant.
The ceramic electrolyte of choice in SOCs is YSZ due to its high oxide ion conductivity and stability in reducing atmospheres. Recent work at Sheffield showed that YSZ pellets with platinum electrodes, show the onset of electronic conductivity under the action of a small applied voltage. Following the previous results obtained in Sheffield, the main objective of the proposal is to better understand the electronic conductivity observed in YSZ ceramics. At present, SOCs have not reached full-scale commercial production; providing a solution to a previously-unrecognised degradation mechanism brings that a step closer to fruition.
The main technique used to fully characterise the electrical performance of YSZ electrolytes was Impedance Spectroscopy. A new methodology the take full advantage of the technique has been developed for this project and it can be used not only to characterise ionic conductors, but also materials with other electrical properties such as electronic or mixed conductors or dielectrics. The use of the new methodology implemented for data analysis has resulted on new and unexpected results. The research has been focused on understanding the obtained results.

Work performed

The project consisted of 4 different work packages, all of them intimately related with the Impedance Spectroscopy technique and YSZ. The first work package consisted in a detailed study of YSZ samples as ceramics or as single crystals, by Impedance Spectroscopy technique and required a total incorporation of Dr Vendrell in the host group. The research activities and results can be summarized as follows:
It is standard practice in the analysis of impedance data of YSZ to present data in the form of impedance complex plane plots, Z\'\'â„Z\' and to obtain bulk conductivity data; sometimes, the data are fitted to a semi-circle whose centre is depressed below the Z\' axis. However, Z* plots on linear scales give undue weighting to the largest resistances in a sample and effectively, exclude from view any low resistance components such as those associated with inhomogeneous ceramics that may have conductive grain cores but resistive grain boundaries. Therefore, our first step was to develop a methodology to fully characterise the prepared materials. By using the new methodology on the analysis of YSZ single crystals, new and unexpected results were obtained. The appropriate fitting to equivalent circuits required the presence of a dielectric element in the equivalent circuit in addition to the usual element that represents the bulk conductivity. As far as we are aware, the contribution of short-range dielectric processes in parallel with long-range ionic conduction has not been well recognised previously in the analysis of impedance data of YSZ. These results on a single crystal sample show that the bulk response contains two components, representing dielectric and conduction processes.
The results obtained were recently published in:
a) X. Vendrell & A.R.West, J.Electrochem. Soc. 165(11), 2018, F966.
In parallel to the above results on YSZ single crystals, YSZ ceramics were also characterised by Impedance Spectroscopy under different conditions of pO2, bias voltage, temperature and time. Impedance data were strongly sensitive to a small dc bias, in the range 1-16 V. Application of a dc bias had two effects on the impedance data: (i) the total impedance shifted to lower resistances, maintaining the overall features of the complex plane plots and (ii) the low frequency spike collapsed drastically to give a depressed arc. Both effects were fully reversible on removal of the dc bias. The decrease in total resistance was voltage-dependent. The effect of 12 V dc bias on total sample resistance in three atmospheres N2, air and O2 was also tested. These effects on application of a voltage to the sample appears to be precursors to the recently discovered, flash sintering technique, in concrete are intimately related with the Stage I or incubation stage of the flash process. A new paper is in preparation where the application of a dc bias together with the effect of the atmosphere is discussed:
b) X. Vendrell & A.R. West. In preparation, J. Eur. Ceram. Soc.
A new collaboration with Prof Rishi Raj from the University of Colorado in the United States of America was initiated. In 2010, Prof. R Raj presented the first publication describing a process where a ceramic can be sintered by a simultaneous application of an electric field and temperature. Prof R Raj visited Sheffield three times during November 2016 - May 2017, some flash sintered samples prepared in Boulder were characterised in Sheffield by Impedance Spectroscopy following the methodology developed previously and explained above. Collaboration between the University of Sheffield and the University of Colorado Boulder, spurred by the appointment of Prof R Raj as a Distinguished Visiting Scholar of the Royal Academy of Engineering and a secondment of Dr X Vendrell in the University of Colorado Boulder. The results of this work were recently sent for publication and are under review:
c) X. Vendrell, D. Yadav, R. Raj & A.R. West, J. Eur. Ceram Soc., under review.

Final results

By the end of the fellowship Dr Vendrell has been established as a highly networked researcher with in depth knowledge of Impedance Spectroscopy technique and has become expert in data analysis of different materials, form ionic conductors to insulators. Moreover, after a secondment in Profâ€™s R. Raj laboratory, he has become one of the few researchers involved in a new sintering technique which is currently a â€˜hot topicâ€™, which will give him a high degree of competence in executing experimental trials and research projects in different areas. The MSCA fellowship is having a high impact on Dr Vendrellâ€™s career; he currently has a Beatriu de PinÃ³s fellowship in the Universitat PolitÃ¨cnica de Catalunya, a prestigious and very competitive fellowship from the Catalan government. He has developed a high degree of competence in executing and preparing research projects.