Beijing Key Laboratory of Green Chemical Reaction Engineering, and Technology, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China.
Chemistry. 2018 Dec 10;24(69):18494-18501. doi: 10.1002/chem.201800535. Epub 2018 Aug 10.
The catalytic conversion of dinitrogen (N ) into ammonia under ambient conditions represents one of the Holy Grails in sustainable chemistry. As a potential alternative to the Haber-Bosch process, the electrochemical reduction of N to NH is attractive owing to its renewability and flexibility, as well as its sustainability for producing and storing value-added chemicals from the abundant feedstock of water and nitrogen on earth. However, owing to the kinetically complex and energetically challenging N reduction reaction (NRR) process, NRR electrocatalysts with high catalytic activity and high selectivity are rare. In this contribution, as a proof-of-concept, we demonstrate that both the NH yield and faradaic efficiency (FE) under ambient conditions can be improved by modification of the hematite nanostructure surface. Introducing more oxygen vacancies to the hematite surface renders an improved performance in NRR, which leads to an average NH production rate of 0.46 μg h cm and an NH FE of 6.04 % at -0.9 V vs. Ag/AgCl in 0.10 m KOH electrolyte. The durability of the electrochemical system was also investigated. A surprisingly high average NH production rate of 1.45 μg h cm and a NH FE of 8.28 % were achieved after the first 1 h chronoamperometry test. This is among the highest FEs reported so far for non-precious-metal catalysts that use a polymer-electrolyte-membrane cell and is much higher than the FE of precious-metal catalysts (e.g., Ru/C) under comparable reaction conditions. However, the NH yield and the FE dropped to 0.29 μg h cm and 2.74 %, respectively, after 16 h of chronoamperometry tests, which indicates poor durability of the system. Our results demonstrate the important role that the surface states of transition-metal oxides have in promoting electrocatalytic NRR under ambient conditions. This work may spur interest towards the rational design of electrocatalysts as well as electrochemical systems for NRR, with emphasis on the issue of stability.
在环境条件下将氮气（N ）催化转化为氨是可持续化学中的圣杯之一。作为哈伯-博世（Haber-Bosch）工艺的潜在替代方法，电化学还原 N 为 NH 由于其可再生性和灵活性以及其可持续性，从地球上丰富的水和氮原料生产和储存高附加值化学品，因此具有吸引力。然而，由于动力学复杂且能量挑战性的氮还原反应（NRR）过程，具有高催化活性和高选择性的 NRR 电催化剂很少。在本贡献中，作为概念验证，我们证明通过修饰赤铁矿纳米结构表面可以提高环境条件下的 NH 产率和法拉第效率（FE）。在赤铁矿表面引入更多的氧空位可提高 NRR 的性能，导致在 0.10 m KOH 电解质中-0.9 V 对 Ag/AgCl 的平均 NH 生成速率为 0.46 μg h -1 cm -1 和 NH FE 为 6.04%。还研究了电化学系统的耐用性。令人惊讶的是，在首次 1 h 计时电流测试后，实现了 1.45 μg h -1 cm -1的平均 NH 生成速率和 8.28%的 NH FE。这是迄今为止在使用聚合物电解质膜电池的非贵金属催化剂中报道的最高 FE 之一，并且远高于可比反应条件下贵金属催化剂（例如 Ru/C）的 FE。然而，在 16 h 的计时电流测试后，NH 产率和 FE 分别降至 0.29 μg h -1 cm -1和 2.74%，表明系统的耐用性差。我们的结果表明，过渡金属氧化物的表面状态在促进环境条件下的电催化 NRR 方面起着重要作用。这项工作可能会激发人们对电催化剂以及 NRR 电化学系统的合理设计的兴趣，重点是稳定性问题。
