Study of cobalt-free composite oxygen electrodes for solid oxide cells

Abstract

Solid oxide cells are energy conversion devices that can operate in two modes: as fuel cells to produce electricity from hydrogen, and as electrolysis cells to generate hydrogen from electricity. In these devices, the oxygen electrode often faces challenges, such as slow reaction kinetics or interfacial degradation, which can significantly impact overall cell performance and thus require careful optimization. Traditionally, materials containing cobalt are widely used as oxygen electrodes due to their excellent kinetics for reactions with oxygen. However, one of the current challenges is to replace this critical element in energy applications due to concerns related to health, the environment, and limited geographic availability. In this thesis, the compounds Sr2Fe1.5Mo0.5O6-δ (SFM) and Lan+1NinO3n+1 (n = 1 and 3, LNO) are explored as cobalt-free oxygen electrodes, demonstrating excellent mixed ionicelectronic conduction properties. The electrodes were initially optimized through successive deposition of material layers onto the electrolyte substrate. This process not only optimized the electrode thickness but also improved the distribution of ionic current from the electrolyte into the electrode, resulting in a reduction in polarization resistance (Rpol). We suggest that the formation of a higher solid fraction of electrode material at the electrolyte interface may contribute to an improved ionic current extension into the bulk of the electrode. Consequently, the optimized electrodes achieved Rpol values of ~0.6 Ω cm2 for SFM, ~4.9 Ω cm2 for La2NiO4+δ (L2N1), and~12.9 Ω cm2 for La4Ni3O10-δ (L4N3) at 700 °C. Additionally, a new composite electrode of SFM with approximately 34 vol% of praseodymia-doped ceria (Ce0.8Pr0.2O2-δ) was developed, leading to improvements in incorporation kinetics. However, the overall performance of this composite electrode was compromised by insufficient electronic conductivity (Rpol ~7.3 Ω cm2 at 700 °C). For the LNO electrodes, impregnation with praseodymium oxide (~10 wt%) resulted in a reduction in polarization resistance by ~7 times for L2N1 (Rpol ~0.7 Ω cm2) and ~17 times (Rpol ~0.8 Ω cm2) for L4N3 at 700 °C. This significant improvement is attributed to the catalytically active PrOx sites, which enhance oxygen dissociation and charge transfer processes. Overall, this work provides critical insights into the microstructural and compositional criteria essential for the future development of high-performance cobalt-free oxygen electrodes, contributing to the advancement of more sustainable and efficient solid oxide cells. By facilitating the transition to cleaner energy sources and reducing reliance on cobalt, these developments can play a vital role in decreasing greenhouse gas emissions and mitigating the impacts of climate change, while also benefiting the environment and the health of individuals exposed to cobalt, ultimately supporting a more sustainable future for society.