Development of CO2 tolerant cathode materials for low-temperature solid oxide fuel cells

Abstract

Solid oxide fuel cells (SOFCs) are gaining interest as a radical electrochemical power generation device for commercial and industrial applications. This way of electricity generation is much more efficient, environmentally compatible, and economically viable as compared to conventional methods of power generation from fossil fuels.However, the main obstacle for the practical application of SOFC technology is its high operating temperature (≥ 800 °C). These high operating temperatures lead to the long-term stability of cells, poor sealing problems, slow start-up/shut-down procedures and require expensive interconnect materials to tolerate high temperatures. Lowering the operating temperature to 500-750 °C is considered a practical approach to mitigate these issues and has attracted extensive research interests in recent years. To date, cobalt-based perovskites are regarded as benchmark cathodes for intermediate-temperature SOFCs (IT-SOFCs) due to their high oxygen reduction reaction (ORR) activity. However, the much higher cost of cobalt; chemical incompatibility; poor thermal expansion coefficient (TEC) mismatch with conventionally used electrolytes and high CO2 susceptibility impedes their use for practical IT-SOFCs. The development of cobalt-free cathodes such as SrFeO3-δ (SF) is paramount to the practical application of the IT-SOFC technologies. However, the sluggish catalytic performance of the iron-based electrode towards ORR activity at IT is a persistent concern. Therefore, this thesis aims to circumvent the significant challenges of cobalt-free perovskite cathode, including sluggish ORR activity and poor CO2 tolerance for SOFC operating at 700-500 °C, by incorporating alkali metal ions into the Sr(Fe, Nb, Ta)O3-δ lattice. This project also investigates the doping effects of the alkali metal dopants and their roles in ORR efficacy in the presence of CO2. The knowledge and insights generated from this thesis project contributed to the understanding of doping effects in perovskite electrode materials. A new avenue is presented to produce IT-SOFC cathode materials by in-situ dopant diffusion and formation of molten carbonate at the surface.In the first part of the experimental section, we investigated the doping effect of monovalent alkali metals on ORR activity. We synthesized the cobalt-free Sr0.95A0.05Fe0.8Nb0.1Ta0.1O3-δ (A= Li, Na, and K) cathodes for the application of LT-SOFCs. The optimized 5 mol% substitution of Li+ for Sr2+ host stabilizes the cubic perovskite structure. It leads to a cathode performance that is significantly higher than Na- and K- doped cathodes, with an ASR as low as ~ 0.12 Ω. cm2 and a maximum power density of ~1.02 W.cm-2 using an SDC electrolyte at 600 °C. Additionally, the CO2 tolerance of the alkali metal-doped cathode materials is also boosted by more than four times than the pristine SFNT. This arises due to the higher O2 partial pressure during in-situ alkali carbonation at the surface of the cathodes. This part of the experimental work demonstrates the decisive role of alkali carbonate and directs our experimental interest towards the molten alkali carbonate.In the second part of the experimental section, we focused on the co-substitution of alkali metals in pristine SFNT and in-situ formation of molten carbonate at the surface of cathode materials. We developed the binary alkali metals doped Sr0.95(x,y)0.05Fe0.8Nb0.1Ta0.1O3-δ (x, y = Li, Na, K), and proposed surface tailoring mechanism to enhance the ORR activity and CO2 tolerance of synthesized cathodes at 600 °C. Our novel perovskite cathode Sr0.95Li0.026K0.024Fe0.8Nb0.1Ta0.1O3-δ (S(LK)FNT) exhibited a superior ORR activity, with the lowest ASR ever reported for cobalt-free iron-based cathodes. This new cathode satisfies the targeted ASR requirement of £ 0.1 Ω.cm2 at 600 °C for practical SOFC cathodes. The outstanding cathode performance of the S(LK)FNT both in air and in 10 vol.% CO2 air mixture and outweigh performance recovery upon CO2 removal is attributed to the improved O2- ions conductivity through the percolated carbonate network around the grain boundaries of cathodes.The objective of the third part was to fabricate the composite cathode by infiltrating the La2NiO4+d (LNO) onto the S(LK)FNT cathode backbone. The ORR performance of the core-shell composite cathode is evaluated at intermediate temperature. A 5 wt.% infiltration of LNO oxide not only improves the CO2 tolerance of S(LK)FNT backbone cathode but also enhances the ORR activity of overall cathode. The infiltrated LNO leads to a 40% decrease in ASR and a 30% improvement of CO2 tolerance at 600 °C. This remarkable improvement is attributed to the synergistic effects of an improved oxide ionic conductivity and surface charge transfer kinetics by extending the TPBs.Overall, this work has proposed a rational strategy in developing a robust and highly efficient cobalt-free cathode material for low-temperature SOFC applications. The outcomes of this project could also benefit other key industrial applications such as high-temperature oxygen separation and high-temperature electrolysis.