Modulating Confined Interlayer Environment By Hard Lewis Acid Cation for Enhancing Oxygen Evolution Reaction

Abstract With the increasing demand for hydrogen energy, the non-noble metal catalysts in water splitting for hydrogen production are significant for addressing global environmental issues. [1] Among them, layered transition metal oxide/hydroxide (LTMO/LTMH) materials have drawn more attention for their high tunability of surface or bulk structural transformations after introducing different cations inside the interlayer, which could manifest OER activity. [2] Therefore, birnessite-layered manganese dioxide (MnO 2 ) is considered a potential material for its ability to create a container for cation intercalation, which could significantly enhance its behavior in the oxygen evolution reaction (OER). [3] However, most of the investigations of its application in OER considered MnO 2 layers as active sites, which may underestimate the function of intercalated cations and hence blur its real reaction active sites. Using the hard and soft acids and bases (HSAB) theory recently sheds new light and provides a new road for improving OER performance; hard Lewis acid captures hydroxyl and thus speeds up the cleavage of water molecules, generating a demanding local alkaline environment. [4-6] Herein, we grounded the perspective of the intercalated cations based on the HSAB theory, using layered MnO 2 as the “container” for accommodating hard Lewis acid cations, achieving using Lewis acid cations for active sites, to enhance OER activity. We use the electrodeposition method to synthesize the precursor layered catalyst, TBA + /MnO 2 . With the ion exchange process, cations with different Lewis acidities (K + , Fe 3+ , Co 2+ , Ni 2+ , Cu 2+ , and Zn 2+ ) were intercalated inside this “nano-cell” (interlayer of layered MnO 2 ) to discuss how they influenced the confined surrounding electrolyte environment and electron/mass transport. X-ray diffraction, Raman, and inductively coupled plasma verified the successful synthesis of layered MnO 2 with different cations intercalated. The OER results of cation-intercalated MnO 2 catalysts exhibited that intercalating the harder Lewis acid significantly influences the interlayer electrolyte environment, markedly accelerating and enhancing the reaction kinetics and OER activity. Furthermore, comparing the OER behavior of intercalated cations, free cations, and bulk metal foils, the intercalated hard Lewis acid cations showed superior reaction kinetics. This work thus provides a clear picture of the role of cations when intercalated into the MnO 2 layers during OER and highlights a novel utilization of Lewis acid cations as active sites for enhancing OER activity. Reference [1] Wang, S.; Lu, A.; Zhong, C.-J. Hydrogen Production from Water Electrolysis: Role of Catalysts. Nano Converg. 2021 , 8 (1), 4. [2] Kim, Y.; Choi, E.; Kim, S.; Byon, H. R. Layered Transition Metal Oxides (LTMO) for Oxygen Evolution Reactions and Aqueous Li-Ion Batteries. Chem. Sci. 2023 , 14 (39), 10644–10663. [3] Ju, M.; Chen, Z.; Zhu, H.; Cai, R.; Lin, Z.; Chen, Y.; Wang, Y.; Gao, J.; Long, X.; Yang, S. Fe(III) Docking-Activated Sites in Layered Birnessite for Efficient Water Oxidation. J. Am. Chem. Soc. 2023 , 145 (20), 11215–11226. [4] Pearson, R. G. Hard and Soft Acids and Bases. J. Am. Chem. Soc. 1963 , 85 (22), 3533–3539. [5] Zhao, S.; Wang, Y.; Hao, Y.; Yin, L.; Kuo, C.-H.; Chen, H.-Y.; Li, L.; Peng, S. Lewis Acid Driving Asymmetric Interfacial Electron Distribution to Stabilize Active Species for Efficient Neutral Water Oxidation. Adv. Mater. 2024 , 36 (7), 2308925. [6] Guo, J.; Zheng, Y.; Hu, Z.; Zheng, C.; Mao, J.; Du, K.; Jaroniec, M.; Qiao, S.-Z.; Ling, T. Direct Seawater Electrolysis by Adjusting the Local Reaction Environment of a Catalyst. Nat. Energy 2023 , 8 (3), 264–272. Figure 1