Modification of Misfit-Layered Cathode Materials for Improving Electrocatalyic Activity in Reversible Protonic Ceramic Cells

The demand for sustainable regeneration energies such as solar and wind has been continuously increasing by the rise of environmental issues. However, several major technical issues such as intermittent electricity production and high system construction cost are major obstacles to large-scale commercialization for regeneration energies [1]. Reversible protonic ceramic cells (RPCCs) can be an alternative choice because of their high efficiency, long-term durability at intermediate temperatures, and bifunctionality for producing both hydrogen and electricity in an electrochemical device [2]. Until now, perovskite-structured cathode (air/steam electrode) materials have been in spotlight due to their fast oxygen reaction kinetics in RPCCs [3]. However, in spite of their high electrocatalytic activity for oxygen electrode reactions (oxygen evolution and reduction), low phase stability in humid atmospheres, thermal expansion mismatch to common proton electrolytes, and relatively low proton conductivity of RPCCs are still major obstacles to enter the era of hydrogen economy [4, 5]. Hence, we demonstrate novel misfit-layered structure Ca 3 Co 4 O 9+δ (CCO)-based materials to overcome these technical issues with superior catalytic activity for high performance RPCCs. The thermal expansion coefficient value of CCO is 10-12×10 −6 K −1 , which is relatively similar to that of the BaCeO 3 -based proton electrolytes, implying that the CCO-based materials are thermo-mechanically stable during cell operations [6]. In addition, the needle-like morphology of the CCO-based materials is quite valuable in the supply of steam (or air) to the catalytic active sites and elimination the water produced by the electrochemical reaction. Furthermore, with the doping of alkali metal ion (Li, Na, and K) into Ca-site, CCO generates extra charge carrier species with the modified Co oxidation state. This triggers the proton uptake and diffusion properties of CCO-based materials with the improved electrical conductivity. As a result, 5 mol% Na-doped CCO-cell shows exceptionally high performance in both protonic ceramic fuel cell and electrolysis cell modes for RPCCs. References [1] Z. Y. Luo, Y. C. Zhou, X. Y. Hu, N. Kane, W. L. Zhang, T. T. Li, Y. Ding, Y. Liu, M. L. Liu, ACS Energy Letters 7 , 2970-2978 (2022) [2] C. C. Duan, R. Kee, H. Y. Zhu, N. Sullivan, L. Z. Zhu, L. Z. Bian, D. Jennings, R. O’Hayre, Nature Energy 4 , 230-240 (2019) [3] R. Zohourian, R. Merkle, G. Raimondi, J. Maier, Advanced Functional Materials 28 , 1801241 (2018) [4] Y. S. Song, Y. B. Chen, W. Wang, C. Zhou, Y. J. Zhong, G. M. Yang, W. Zhou, M. L. Liu, Z. P. Shao, Joule 3 , 2842-2853 (2019) [5] R. Z. Ren, Z. H. Wang, C. M. Xu, W. Sun, J. S. Qiao, D. W. Rooney, K. N. Sun, Journal of Chemistry A 7 , 18365-18372 (2019). [6] M. Saqib, I.-G. Choi, H. Bae, K. Park, J.-S. Shin, Y.-D. Kim, J.-I. Lee, M. J, Y.-C. Kim, K.-S. Lee, S.-J. Song, E. D. Wachsman, J.-Y. Park, Energy & Enviromental Science 14 , 2772-2484 (2021).