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BIsmuth BasEd LOw Temperature Sofc – BIBELOTS

BIsmuth BasEd LOw Temperature Sofc

Design of new composite cathodes compatible with bi-layer electrolyte made of bismuth oxide/ceria (ESB/GDC) for the reduction of the temperature of solid oxide fuel cells (SOFC)

Context and objectives

Hydrogen is rising as one of the possible ways to deliver energy with a low carbon dioxide footprint. Fuel cells, devices converting the chemical energy of H2 in electrical energy appears as an important part of the grid of tomorrow. In particular Solid Oxide Fuel Cells (SOFC) benefit from a high heat + power yield as well as the absence of precious metals in the electrode composition. Stabilized zirconia, the standard electrolyte, does not offer sufficient power density below 700 °C, driving the research focus toward high temperature systems. However, high-temperature engineering of SOFC is limited due to the extreme temperature range considered. As was recently demonstrated, it is possible to decrease the operating temperature of SOFC down to 350 °C through the use of bi-layer electrolyte made of stabilized bismuth oxide (ESB) and stabilized ceria (GDC). However, no compatible cathodes have been developed to perform at such low temperatures. The goal of our project is to search for new materials and fabrication technics to design electrodes compatible with these innovating electrolytes. We focus especially on the study of composite cathodes assembled with materials exhibiting promising properties but that usually cannot be used for high temperature applications due to chemical or mechanical stability issues.

The large decrease in the operating temperature enable us to look at oxides with a metallic behavior such as high Tc superconductors or SrFeO3-delta. The ionic conduction is provided by phases with a structure deriving from delta-Bi2O3 such as ESB, DWSB or the BiMeVOX familly, all of which are not usable at high temperature due to their reactivity and low boiling point.
Finally, the oxygen reduction catalytic activity of compounds such as LnBaCo4O7+delta and LnFe2O4+delta, able to reversibly uptake oxygen at temperature as low as 400 °C, are also studied. By looking at the performances of the combination of many pairs of materials, we hope to better understand the factors limiting the performance of cathodes at low temperature.
In order to reach our objectives, we are carrying out an extensive effort to characterize the stability of the different phases under study. Based on the evaluation of their transport properties we are rationally assembling electrodes for low temperature applications. The performance of the electrodes are evaluated using electrochemical impedance spectroscopy on symmetric cells. The knowledge of the limiting reaction steps will help us to optimize the most promising compositions. The ultimate goal being the evaluation of these electrode in full cells to demonstrate the potential of low temperature SOFC based on bi-layered GDC/ESB electrolytes.

A series of composite Bi1.5Er0.5O3/La1-xSrxMnO3+d cathodes was optimized by tuning the strontium content. Contrary to what could be expected, the increased concentration of strontium prevents the surface segregation of SrO, providing a more reactive surface with accessible manganese sites.

During the developpement of the synthesis of Bi14MO24 (M = Mo, W) we discovered that the reported stoichiometry is probably incorrect, Bi12MO21 being a better approximation.

The successful deposition of dense and thin layers of Bi1.5Er0.5O3 on GDC pellets opens the way to the assembly of cells with a bi-layered electrolyte and their electrochemical characterization.

Elaboration of new synthesis protocol (citrate route) for some compositions (in particular bismuth based ionic conductors). The small grain size obtained through this method will be beneficial for lower temperature sintering of the electrodes.

- Better understanding of the reaction mechanism of LSM/ESB composite cathodes

- Identification of original material combination performing well as SOFC cathode at reduced temperature.

- Performance evaluation of full cells

Pajot, M.; Duffort, V.; Capoen, E.; Mamede, A.-S.; Vannier, R.-N. Influence of the Strontium Content on the Performance of La1-xSrxMnO3/Bi1.5Er0.5O3 Composite Electrodes for Low Temperature Solid Oxide Fuel Cells. J. Power Sources 2020, 450, 227649. doi.org/10.1016/j.jpowsour.2019.227649.

As hydrogen slowly becomes an increasingly popular carbon free energy vector; fuel cells, electrochemical devices able to convert H2 in electricity, gradually appear as a key component of the smart electrical grid of tomorrow. In particular, Solid Oxide Fuel Cells (SOFC) based on an oxygen ion conducting electrolyte are of special interest due to their very high yield in heat/electricity co-generation mode and the absence of noble metal on the electrodes. Yttria Stabilized Zirconia (YSZ), the dominant electrolyte used in SOFC does not allow high power output below 700 °C, driving the research efforts towards high temperature systems. However, durable and flexible high temperature SOFCs are extremely difficult to engineer due to the restrictions on materials set by the temperature range. A recently developed bi-layered electrolyte composed of stabilized ceria (GDC) and bismuth oxide (ESB) offers sufficient ionic conductivity to operate at temperatures as low as 350 °C, however there is still the need to find efficient cathodes to realize the full potential of this electrolyte. Hence, this proposal is focused on the search for new cathode materials and assembly procedures compatible with bi-layer GDC/ESB electrolytes.
The goal of this project is to investigate the possibility of assembling solid oxide fuel cells using this new generation of bismuth based bilayer electrolytes, able to operate at temperatures of 400 °C and lower. We will focus on the design of cathode materials to be deposited on the “bismuth layer” of the electrolyte. For this purpose, we propose to look at the assembly of composite air electrodes made with materials with outstanding properties that are usually out of the scope of SOFC research due to chemical/mechanical compatibility. The considerably reduced temperature range will allow us to investigate oxides with metallic behavior such as high-Tc superconductors or SrFeO3-deta. High oxygen ionic conduction will be provided by structure deriving from delta-Bi2O3 such as ESB, DWSB or BiMeVOXs, usually not applicable to higher temperature range due to the reactivity and low melting point. Finally, the oxygen reduction catalytic properties of structures like LnBaCo4O7+delta and LnFe2O4+delta that are able to reversibly insert oxygen into the bulk of their structure at T < 400°C will be tested. By combining two materials with auspicious properties we hope to understand finely the parameters limiting the cathode performance at reduced temperature.
To reach our goal an important work of characterization of the compatibility of the different phases of interest coupled to the measurement of their transport properties will allow the rational design of innovative electrodes specifically design to operate at reduced temperature. The performance of the electrodes will be evaluated on symmetrical cells using electrochemical impedance spectroscopy. Detailed understanding of the mechanisms impeding the low temperature operation of the air electrode will lead to electrode optimization. Ultimately these new compounds will be tested in full cell using GDC/ESB bi-layer electrolytes to demonstrate the full potential of these electrodes.

Project coordination

Victor Duffort (Unité de Catalyse et de Chimie du Solide)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

UCCS Unité de Catalyse et de Chimie du Solide

Help of the ANR 217,728 euros
Beginning and duration of the scientific project: January 2019 - 48 Months

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