硝酸鹽電化學還原反應（Electrochemical nitrate reduction reaction, NO3RR）在常溫下是一種具吸引力的氨生成路徑，但此反應涉及九個質子的轉移，並且需要與亞硝酸鹽生成及析氫反應（Hydrogen evolution reaction, HER）這兩種需要兩個質子的副反應進行競爭。因此，我們設計並合成了一種富含磺酸基團的鋯基金屬有機骨架（Metal-organic frameworks, MOFs），即 SO3-MOF-808，作為塗層修飾於具有催化活性的銅基電極表面。該MOF的負電性磺酸基團能夠有效地富集質子於電極表面，顯著提升NO3RR反應中氨生成的選擇性。相比之下，當MOF塗層引入帶正電的三甲基銨基團時，由於排斥質子的效應，會顯著促進硝酸鹽轉化為亞硝酸鹽，且在所有測試電位下，其選擇性均超過90%。
在最佳反應條件下，修飾SO3-MOF-808 的銅電極實現了 87.5%的氨生成法拉第效率、95.6%的硝酸鹽轉化選擇性以及0.383 mmol/h-mg的氨生成速率，均顯著優於未修飾的銅電極以及修飾Nafion塗層的銅電極。本研究結果表明，磺酸基功能化MOF作為富集質子於電極表面的高效材料，具潛力成為NO3RR中商業化Nafion的先進替代品，並且該類電化學惰性MOF塗層在一系列質子耦合電催化反應中展現了廣泛的用用前景。

The electrochemical nitrate reduction reaction (NO3RR) presents a promising method for ammonia synthesis under ambient conditions. However, this nine-proton transformation process competes with side reactions such as the two-proton reduction to nitrite and the hydrogen evolution reaction (HER), making selective ammonia production particularly challenging.
In recent years, considerable efforts have been directed toward developing efficient electrocatalysts to address this issue. In this study, rather than modifying the catalyst itself, a different approach was employed by incorporating a zirconium-based metal-organic framework (MOF), SO3-MOF-808, as a surface coating on copper-based electrodes. This MOF is electrochemically inactive, porous, and chemically stable, and is characterized by abundant sulfonate groups within its framework. While the coating does not significantly influence the electrochemical surface area or the total reaction rate, its negatively charged functional groups enhance the local proton concentration near the catalyst surface, effectively improving the selectivity for ammonia formation during NO3RR. Conversely, applying a MOF coating containing positively charged trimethylammonium groups, which repel protons, leads to a high selectivity for nitrite production, exceeding 90% across all tested potentials.
Under optimal conditions, the copper electrode with SO3-MOF-808 coating achieved a Faradaic efficiency of 87.5%, a nitrate-to-ammonia selectivity of 95.6%, and an ammonia yield rate of 0.097 mmol/cm2·h. These values surpass those obtained with both the unmodified copper electrode (75.0%, 93.9%, 0.087 mmol/cm2·h) and the Nafion-coated counterpart (83.3%, 86.9%, 0.064 mmol/cm2·h). The findings highlight the potential of using electrochemically inert MOF coatings as effective alternatives to traditional proton-conductive materials such as Nafion, opening new avenues for enhancing proton-coupled electrochemical reactions.

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