Tsai Yun-Shan, Yang Shang-Cheng, Yang Tzu-Hsien, Wu Chung-Huan, Lin Tzu-Chi, Kung Chung-Wei
Department of Chemical Engineering, National Cheng Kung University, 1 University Road, Tainan City 70101, Taiwan.
Program on Key Materials, Academy of Innovative Semiconductor and Sustainable Manufacturing, National Cheng Kung University, 1 University Road, Tainan City 70101, Taiwan.
ACS Appl Mater Interfaces. 2024 Nov 13;16(45):62185-62194. doi: 10.1021/acsami.4c14786. Epub 2024 Nov 1.
The electrochemical reduction reaction of nitrate (NORR) is an attractive route to produce ammonia at ambient conditions, but the conversion from nitrate to ammonia, which requires nine protons, has to compete with both the two-proton process of nitrite formation and the hydrogen evolution reaction. Extensive research efforts have thus been made in recent studies to develop electrocatalysts for the NORR facilitating the production of ammonia. Rather than designing another better electrocatalyst, herein, we synthesize an electrochemically inactive, porous, and chemically robust zirconium-based metal-organic framework (MOF) with enriched intraframework sulfonate groups, SO-MOF-808, as a coating deposited on top of the catalytically active copper-based electrode. Although both the overall reaction rate and electrochemically active surface area of the electrode are barely affected by the MOF coating, with negatively charged sulfonate groups capable of enriching more protons near the electrode surface, the MOF coating significantly promotes the selectivity of the NORR toward the production of ammonia. In contrast, the use of MOF coating with positively charged trimethylammonium groups to repulse protons strongly facilitates the conversion of nitrate to nitrite, with selectivity of more than 90% at all potentials. Under the optimal operating conditions, the copper electrocatalyst with SO-MOF-808 coating can achieve a Faradaic efficiency of 87.5% for ammonia production, a nitrate-to-ammonia selectivity of 95.6%, and an ammonia production rate of 97 μmol/cm h, outperforming all of those achieved by both the pristine copper (75.0%; 93.9%; 87 μmol/cm h) and copper with optimized Nafion coating (83.3%; 86.9%; 64 μmol/cm h). Findings here suggest the function of MOF as an advanced alternative to the commercially available Nafion to enrich protons near the surface of electrocatalyst for NORR, and shed light on the potential of utilizing such electrochemically inactive MOF coatings in a range of proton-coupled electrocatalytic reactions.
硝酸盐的电化学还原反应（NORR）是在环境条件下生产氨的一条有吸引力的途径，但从硝酸盐转化为氨需要九个质子，这一过程必须与生成亚硝酸盐的双质子过程以及析氢反应相竞争。因此，近期的大量研究致力于开发用于NORR的电催化剂以促进氨的生成。在此，我们并非设计另一种更好的电催化剂，而是合成了一种具有丰富骨架内磺酸根基团的电化学惰性、多孔且化学稳定的锆基金属有机框架（MOF），即SO-MOF-808，并将其作为涂层沉积在具有催化活性的铜基电极顶部。尽管电极的整体反应速率和电化学活性表面积几乎不受MOF涂层的影响，但带负电荷的磺酸根基团能够在电极表面富集更多质子，MOF涂层显著提高了NORR生成氨的选择性。相比之下，使用带有带正电荷的三甲铵基团的MOF涂层来强烈排斥质子，极大地促进了硝酸盐向亚硝酸盐的转化，在所有电位下选择性均超过90%。在最佳操作条件下，涂有SO-MOF-808的铜电催化剂用于氨生产时可实现87.5%的法拉第效率、95.6%的硝酸盐到氨的选择性以及97 μmol/cm² h的氨生成速率，优于原始铜（75.0%；93.9%；87 μmol/cm² h）和优化后的Nafion涂层铜（83.3%；86.9%；64 μmol/cm² h）所达到的所有指标。此处的研究结果表明，MOF作为市售Nafion的先进替代品，可在NORR电催化剂表面富集质子，并为在一系列质子耦合电催化反应中利用这种电化学惰性的MOF涂层的潜力提供了线索。
