The current critical socio-economic situation in Europe exposes the need for a self-sustainable energy sector, with emphasis on green-energy development. In this context, hydrogen is a viable alternative to fossil fuels, converting chemical energy to electrical energy in fuel cells with high efficiency and zero emissions. Nevertheless, green-hydrogen production requires significant development of zero-emission technologies such as electrolysis cells. Electrochemical cells based on ceramic components are good candidates to produce green hydrogen from renewable electric power in electrolysis mode (solid oxide electrolysis cell, SOEC), and convert hydrogen to electrical power in fuel-cell mode (solid oxide fuel cell, SOFC). The high operation temperature (800-1000 ºC) of SOFCs and SOECs leads to high efficiency, but also to stability problems and increased costs. As a solution, innovative materials should be developed to reduce the working temperature to 800 ºC or lower to improve device feasibility. However, lowering the polarisation resistance of the air electrode and obtaining suitable electrode performance at intermediate temperature is a challenge. Cobalt-based materials, such as Sr(Co,Sb)O_3-δ, show good electrochemical response as air electrodes, but the compatibility with the electrolyte should be improved, due to mismatch in thermal expansion.  Moreover, cobalt is in high demand and expensive, in addition to presenting socio-economic issues associated to its extraction in highly volatile regions.

This work is focused on the effect of substitution of cobalt for iron in Sr(Co,Fe,Sb)O_3-δ-based air-electrodes. The system was firstly prepared by the Pechini route, which required higher synthesis temperature as the Fe content increased.  Microstructural analysis confirmed greater grain size in samples prepared at higher temperature. Although the electrochemical response of the Fe-based material is promising, the higher synthesis temperature caused stability issues, which may result, at least partially, from strontium segregation. Spray pyrolysis synthesis-and-deposition methodology was then employed as a solution to reduce the processing temperature and improve the electrochemical performance. The Sr(Co,Fe,Sb)O_3-δ system was synthesised in-situ and deposited on Gd-doped ceria electrolyte by spray pyrolysis, showing improved electrochemical response and stability compared to the Pechini-synthesised analogues, most likely due to the reduction of processing temperature from the range 1100-1300 ºC to 800-850 ºC. The temperature reduction promotes smaller grain size, improving the electrode-electrolyte interaction. This improvement is especially remarkable in SrFe_0.9Sb_0.1O_3-δ, which shows comparable air-electrode performance with Co-based compounds under both cathodic and anodic polarisation bias (SOFC and SOEC configurations, respectively).