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In recent decades, the world has faced significant environmental challenges. As a result, humanity has increasingly turned toward green solutions, focusing on sustainable energy sources, cost-effective energy production, and more affordable technologies. Solid oxide electrolysis cells (SOECs) represent a highly efficient technology for producing hydrogen (H2), a sustainable fuel and eco-friendly energy carrier with broad applications.

An SOEC air electrode, also known as the oxygen electrode or anode, is the essential component where oxygen ions (O2-) are oxidised to produce oxygen gas (O2) during the steam electrolysis process. To survive the high operating temperatures and oxidising environment of the system, this component must possess high electronic and ionic conductivity, as well as excellent chemical stability.

State-of-the-art SOC air electrode materials are usually mixed ionic-electronic conducting perovskites, such as La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). Over the last few decades, these materials have been heavily optimised to provide high electronic conductivity, significant ionic conductivity, and fast oxygen exchange kinetics. However, many of these modern materials rely on critical elements like cobalt or rare earth elements. While essential for technological advancements, these resources are predominantly sourced from a limited number of countries, raising critical cost and sustainability concerns.

Consequently, replacing these standard materials with cheaper, more abundant, and sustainable alternatives has been considered in recent years. Among these, eco-friendly calcium and iron oxides (Ca-Fe-O) have emerged as promising candidates. Unlike other common air electrodes, this system is not thermodynamically stable in the perovskite form; instead, it exhibits a brownmillerite crystal structure (Ca2Fe2O5+δ).

The primary disadvantage of this brownmillerite system is its very low electronic conductivity —a crucial parameter for air electrodes—which subsequently leads to high polarisation and ohmic resistance. To overcome this drawback, several strategies are currently being explored:

i) Substitution of the Fe-site with Ni, Mn, or Cu,

ii) Composite formation with a highly conductive Co-free perovskite,

iii) Use of ceramic current collector layers with tailored properties.

By applying these strategies either individually or in combination, significant progress has been made in improving the electrochemical properties of the Ca-Fe-O system for air electrode applications. Preliminary results obtained at the material level—including crystal structure, electronic conductivity, and oxygen exchange kinetics—as well as electrochemical testing on symmetrical cells, are highly promising. These initial findings demonstrate that through careful investigation of fundamental mass and charge transport properties, alongside tailored adaptations in cell design, it is entirely feasible to develop high-performance, long-term stable air electrodes based on sustainable raw materials.

Mehmet Aksoy

Technical University of Leoben / Chair of Physical Chemistry, Leoben, Styria, Austria

mehmet.aksoy@unileoben.ac.at

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