The development of new recyclable and inexpensive electrochemically active species for water oxidation catalysis is the most crucial step for future utilization of renewables. Particularly, transition metal complexes containing internal multiple, cooperative metal centers to couple with redox catalysts in the inorganic Keggin-type polyoxometalate (POM) framework at high potential or under extreme pH conditions would be promising candidates. However, most reported Ni-containing POMs have been highly unstable towards hydrolytic decomposition, which precludes them from application as water oxidation catalysts (WOCs). Here, we have prepared new tri-Ni-containing POMs with variable oxidation states by charge tailored synthetic strategies for the first time and developed them as recyclable POMs for water oxidation catalysts. In addition, by implanting corresponding POM anions into the positively charged MIL-101(Cr) metal–organic framework (MOF), the entrapped Ni²⁺/Ni³⁺ species can show complete recyclability for water oxidation catalysis without encountering uncontrolled hydrolysis of the POM framework. As a result, a low onset potential of approximately 1.46 V vs. NHE for water oxidation with stable WOC performance is recorded. Based on this study, rational design and stabilization of other POM-electrocatalysts containing different multiple transition metal centres could be made possible.

中文翻译：

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具有可变氧化态的含镍 Keggin 型多金属氧酸盐作为电化学水氧化的新型催化剂的稳定性


开发用于水氧化催化的新型可回收且廉价的电化学活性物质是未来利用可再生能源最关键的一步。特别是，含有内部多个协同金属中心的过渡金属配合物在高电位或极端 pH 条件下与无机 Keggin 型多金属氧酸盐 (POM) 框架中的氧化还原催化剂偶联，将是有前途的候选者。然而，大多数报道的含镍聚甲醛对水解分解高度不稳定，这妨碍了它们作为水氧化催化剂（WOC）的应用。在这里，我们首次通过电荷定制合成策略制备了具有可变氧化态的新型含三镍聚甲醛，并将其开发为用于水氧化催化剂的可回收聚甲醛。此外，通过将相应的POM阴离子注入带正电的MIL-101（Cr）金属有机框架（MOF）中，捕获的Ni ²⁺ /Ni ³⁺ 物种可以表现出完全的可回收性用于水氧化催化，而不会遇到 POM 骨架的不受控制的水解。结果，记录了水氧化的大约 1.46 V 的低起始电位（相对于 NHE）和稳定的 WOC 性能。基于这项研究，包含不同多个过渡金属中心的其他 POM 电催化剂的合理设计和稳定化成为可能。