Abstract:
The medium/high-entropy oxides have been intensively investigated due to their entropy-stabilized properties and extraordinary electrochemical activities. The stabilization and solubility of Zn and Cu in rocksalt Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O is the critical characteristic of this oxide. ZnO and CuO, as parent binary oxides, exist as wurtzite and tenorite in nature, respectively, Here, we show the short-range disorder effect in stabilizing Zn beyond the configurational entropy, taking the medium-entropy rocksalt Mg0.50TM0.25Zn0.25O (TM = Co, Ni, Cu) as examples.(Shengnan Sun et al., 2024) We found that the solubility of Zn increases after introducing Co, Ni and Cu, compared with the composition Mg0.75Zn0.25O. Among them, Cu is the best, contrary to the intuition that Co and Ni could be more effective due to the rocksalt structure of CoO and NiO. Calculation results reveal that Cu introduction creates a wide distribution of metal-oxygen bond length, causes short-range disorder, and thus enhances Zn solubility. Besides, Mg0.50Cu0.25Zn0.25O has the lowest formation energy of Zn substitution for Mg among these three oxides. From the orbital perspective, the square-planar coordination of Cu-O is more effective in increasing Zn solubility in MgO. Moreover, we also noticed that the medium entropy 1.04 R is enough for stabilizing Cu (25 at%) and Zn (25 at%) simultaneously in Mg0.50Cu0.25Zn0.25O. Additionally, Mg0.50Ni0.25Zn0.25O and Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O containing Ni show a stronger resistance to alkaline and have strong structural stability, compared with Mg0.50Co0.25Zn0.25O and Mg0.50Cu0.25Zn0.25O.
We further applied the high-entropy Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O to the electrochemical ammonia generation from nitrate reduction based on the Cu-Co synergistic effect and the assumption that dispersed homo-cations in a multi-component oxide could suppress possible N-N formation. We obtained an excellent ammonia generation performance with a yield rate 26.6 mg mgcat−1 h−1 and Faradaic efficiency 99.3%. Surprisingly, Li doping Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O weakens the ammonia generation performance. After ruling out the possible factors that influence the performance, we found that the Co spin state is crucial for ammonia generation that a high spin Co in Cu-Co pair reduces the barrier from NH2 to NH3 and thus facilitate ammonia generation.(S. Sun et al., 2024) This result also agrees with the theoretical findings by Chorkendorff, Nørskov and Wang(Cao et al., 2022; Xu et al., 2022) on spin promoted ammonia synthesis in Haber-Bosch process, and indicates the spin effect generality in electrochemical and thermal ammonia synthesis. After nitrate reduction, we found the valence states of Co, Ni and Cu decrease slightly in Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O and the structure is stable, and the surface change can only be observed within a few nanometer-thick at some localized areas. These findings provide significant insights in understanding the stabilization and solubility of metal elements in solid solution from the perspective from short-range disorder and the complementation of orbital distribution and employing the spin states and synergistic effect for selective electrochemical reaction.
References
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Sun, S., Dai, C., Zhao, P., Xi, S., Ren, Y., Tan, H. R., Lim, P. C., Lin, M., Diao, C., Zhang, D., Wu, C., Yu, A., Koh, J. C. J., Lieu, W. Y., Seng, D. H. L., Sun, L., Li, Y., Tan, T. L., Zhang, J., . . . Seh, Z. W. (2024). Spin-related Cu-Co pair to increase electrochemical ammonia generation on high-entropy oxides. Nat Commun, 15(1), 260. https://doi.org/10.1038/s41467-023-44587-z
Sun, S., Zhou, J., Xi, S., Tan, H. R., Wei, F., Seng, D. H. L., Lieu, W. Y., Ren, Y., Wang, S., & Seh, Z. W. (2024). Short-range disorder mediated stability of Zn in rock-salt MgO beyond configurational entropy. Journal of Materials Chemistry A, 12(31), 20064-20076. https://doi.org/10.1039/d4ta02175d
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