Comparison of Cobalt-Based Vs. Cobalt-Free Infiltrated Nano-Catalyst Cathodes for Low Temperature Solid Oxide Cells

Abstract

Tailoring inexpensive cathode materials which demonstrate high catalytic activity at lower temperature is crucial for the deployment of solid oxide cells as a next generation premier clean energy source.1 Cobalt is a crucial component and widely used in the most state-of-the-art cathodes of many types of batteries, fuel cells, and electrolysis cells.2 However, due to supply chain issues, replacement of cobalt (which is the most frequently used electrocatalyst to facilitate the oxygen reduction reaction) with other transition metals that are far more available, cheaper, and have less geopolitical, environmental, and social concerns have been explored in this work.In order to overcome the lethargic oxygen reduction reaction (ORR) kinetics, various transition metal oxide electrocatalysts (Ni, Fe, and Zn) and Co (for baseline comparison) were infiltrated on porous scaffold backbone support along with SrO and PrOx. This scalable spray coated porous scaffold of mixed ionic electronic conductor (MIEC) Gd0.1Ce0.9O2- δ (GDC) electrolyte material increases triple phase boundary for ORR reaction. SEM images shown in Figure 1(a) and 1(b) confirms nano-catalyst formation inside scaffold pores.At first, multiphase Co oxide-based nano-catalysts were tested on symmetric cells for baseline data, which demonstrated an area specific resistance (ASR) of 0.08 Ωcm2 at 650°C. Ni, Fe and Zn oxides were investigated as potential replacement electrocatalysts for CoOx because they exhibited similarly low ASR on symmetric cells in the range of 0.08 to 0.16 Ωcm2 at 650°C. To further assess their ability to perform as electrocatalysts for ORR, solutions of these transition metal nitrates were infiltrated, and heat treated to form oxides into SOFC cathode scaffolds and their performance in solid oxide fuel cell operating conditions was characterized. At optimal infiltration loading, Fe and Zn exhibited peak power density of 1.0 W/cm2 and 0.5 W/cm2 with polarization ASR of ~0.03 Ωcm2 and ~0.06 Ωcm2 respectively at 650°C as shown in Figure 1(c). Fe and Zn are considered relatively non-toxic, and are very cheap, however, further optimization will be required for enhanced performance.Conversely, Ni and Co as infiltrated nano-catalyst cathodes both showed extremely low polarization ASR of ~0.01 Ωcm2 with peak power density of 1.7 and 1.9 W/cm2 respectively at 650°C, as shown in Figure 1(c). Cobalt-free Ni based cathode demonstrated remarkably high peak power density of 1 W/cm2 and 1.5 W/cm2 at 550°C and 600°C respectively. Further exploration of cobalt-free, electrocatalytically active materials may open doors to new opportunities for systems like regenerative fuel cells that can operate both as a fuel cell and electrolysis cell with integrated system for energy conversion, and storage.3 Wachsman, E. D., & Lee, K. T. (2011). Lowering the temperature of solid oxide fuel cells. Science, 334(6058), 935– https://doi.org/10.1126/science.1204090.Olivetti, E., Ceder, G., Gaustad, G., & Fu, X. (2017). Lithium-Ion battery supply chain considerations: Analysis of potential bottlenecks in critical metals. Joule, 1(2), 229–243. https://doi.org/10.1016/j.joule.2017.08.019.Thangarasu, S., Dhanabalan, K., Roh, S., Kim, S. W., Park, K., Jung, S., Kurkuri, M. D., & Jung, H. (2017). A comprehensive review on unitized regenerative fuel cells: Crucial challenges and developments. International Journal of Hydrogen Energy, 42(7), 4415–4433. https://doi.org/10.1016/j.ijhydene.2016.10.140. Figure 1