Publication date: 10th April 2024
Most of Solid Oxide Fuel/Electrolyser Cells (SOFC/SOEC) air electrode materials, such as
La0.8Sr0.2MnO3 (LSM) or La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), display the perovskite structure or related
structures. They are made of Rare Earth Elements (REE) on the A site whose partial substitution allows
to tune their transport properties.[1] However, REE have been listed as critical resource by the European
Union, mostly due to the concentration of the mining industries in China. In this context, the aim of the
NOUVEAU project, funded by the EU, is the development of novel electrode coatings and interconnect
for sustainable and reusable SOEC with the objective to reduce by at least 30 % the amount of REE
used in a SOEC stack. This implies to find new REE free materials, mainly for the air electrode.
Interestingly, recent papers showed good mixed ionic-electronic conductivity, promising oxygen
permeability, thermal expansion coefficients (TECs) compatible with typical solid electrolytes and low
area specific resistance (ASR) values for Ca2Fe2O5 derivatives. [2-7] ASR of 0.22 Ω.cm2 at 750 °C with
a cerium gadolinium oxide (CGO) electrolyte and 0.23 Ω.cm2 at 700 °C with Sm0.2Ce0.8O1.9 (SDC)
electrolyte were reported for Ca2Fe1.3Mn0.7O5+δ and Ca2Fe1.8Co0.2O5+δ, respectively, the target for the
application being an ASR lower than 0.15 Ω cm2 at 700°C.[6-7]
These materials display the brownmillerite structural type which can be viewed as an oxygen deficient
perovskite with the nominal formula A2B2O5+δ or ABO2.5+δ. The oxygen vacancies ordering in the
brownmillerite structure results in alternating layers of corner-sharing BO6 octahedra and BO4
tetrahedra. The latter being oxygen deficient, consequently, oxygen diffusion along these chains is
expected while a good electronic conductivity can be obtained using transition metal ions with flexible
red-ox properties such as manganese, iron and cobalt on the B-site.
Based on the promising results reported in literature, di calcium iron oxides-based materials with
manganese or cobalt in the B site were synthesized via a citrate-nitrate route. First ASR measurements
were carried out by electrochemical impedance spectroscopy. An ASR of only 1.99 Ω cm2 at 800 °C
was measured for the most promising Ca2Fe0.5Mn1.5O6-δ material mainly because of a lack of ionic and
electrical conductivity. The ASR was considerably improved when the material was studied in
composite with CGO, reaching 0.60 Ω.cm2 at 800 °C. Moreover, the electronic conductivity was
improved by doping with Bi, Y or La on the Ca site. This led to an ASR of only 0.74 Ω.cm2 at 800 °C
for the pure Ca1.9Bi0.1Fe0.5Mn1.5O6-δ. Study of composite cathode is in progress.
A careful study of the electrochemical response of these systems will be presented.
CNRS, Ministère de l’Enseignement Supérieur et de la Recherche and European Union’s Horizon Europe research
and innovation program under grant agreement N°101058784 are acknowledged for funding.
The present publication reflects only the author’s views. The Commission is not responsible for any use that may
be made of the information contained therein.
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