Racing to critical milestones is a favorite pastime of historical explorers and scientist. From the race to be the first to the North Pole to the discovery of the Haber–Bosch process, overcoming seemingly impossible goals often comes through persistent teams exploring creative solutions, while at the same time paying close attention to past failures. The race to develop electrochemical routes for ammonia synthesis is like these endeavors, as the path to the pole is long, not clear, and at times treacherous. Over the past decade, researchers have made significant progress in improving electrochemical ammonia synthesis. (1) One critical challenge identified early was inaccurate product analyses. The low levels of ammonia reported were often in the same range as sources of lab-based adventitious ammonia and NOx. (2) This provided researchers several challenges with respect to identifying the true activity of a given catalyst. Thus, mitigating nitrogen contamination emerged as the critical problem to resolve within the field. (3,4) Figure 1. With electrosynthesis of ammonia reaching 100% Faradaic efficiency, will the next milestone be related to energy efficiency and cost? Various research groups have converged on several suggested ammonia measurement methods which, if employed, can aid in ruling out contamination. Suggested controls which employ the use of isotopically labeled nitrogen gas and quantitative nuclear magnetic resonance spectroscopy allow researchers to discern adventitious nitrogen (¹⁴N₂) from isotopically labeled nitrogen (¹⁵N₂). (5) Beyond improving low-level ammonia measurements, movement toward operating conditions which increase ammonia yield well beyond the adventitious ammonia levels is also ideal. Specifically, moving to conditions where the m_product ≫ m_system and C_NH3 > 100 ppm is an important threshold identified which can provide confidence in each measurement. (6) Interestingly, low ammonia levels and false positives are not a new occurrence. In fact, there are many cautionary tales within published scientific literature discussing this issue. For instance, the first observation of electrochemical N₂ fixation, in 1807, was later proven non-reproducible 90 years later, (7,8) and there were several passionate discussions on this subject which were published in the 1990s. (9−12) Despite this setback, with solutions available for low-level ammonia measurements, there has been a significant growth in research centered around both aqueous-phase and non-aqueous-based electrocatalytic and photocatalytic systems. (13−15) Suppressing the hydrogen evolution reaction remains the critical bottleneck for aqueous-based catalytic systems. For these reasons, non-aqueous and lithium-mediated electrocatalytic approaches have emerged as the most likely routes to achieve technology-significant targets in terms of rate of production (current density) and product selectivity (Faradaic efficiency). (16) Lithium-mediated nitrogen reduction (LiNR) most recently showed low Faradaic efficiencies (FE < 20%) and current densities (I < 20 mA cm^–2). (17,18) While significant, these values were well below the U.S. Department of Energy’s feasibility targets (e.g., I = 300 mA cm^–2 and FE = 90%). However, recent research findings by Simonov, MacFarlane, and colleagues and Chorkendorff and colleagues have for the first time demonstrated Faradaic efficiencies which approach 100% (Figure 1, top). (19) MacFarlane and Simonov and colleagues were able to suppress electrolyte decomposition through tuning the local physicochemical properties of the electrode–electrolyte interface with an imide-based lithium-salt electrolyte. Likewise, Chorkendorff and colleagues showed this is also possible through the use of a highly porous Cu electrode, at elevated current density (−1 A cm_geo-2). (20) This is the first demonstration of the lithium-mediated nitrogen reduction reaction at industrially relevant current densities. With these impressive metrics, the field has undoubtedly reached a significant milestone, and in a sense has reached a “North Pole”-like discovery. With the “North Pole” in the rear-view mirror, eyes now are set on the “South Pole” (Figure 1, bottom). For ammonia electrosynthesis to compete with the Haber–Bosch process, significant work is needed to improve the energy efficiency, and several questions remain regarding the technoeconomics. (21) Furthermore, with growing materials criticality issues, will lithium remain a viable substrate, or will other materials emerge? I look forward to the next decade of electrochemical ammonia synthesis. This material is based upon work supported by the National Science Foundation under Grant No. 1846611. This article references 21 other publications. This article has not yet been cited by other publications. Figure 1. With electrosynthesis of ammonia reaching 100% Faradaic efficiency, will the next milestone be related to energy efficiency and cost? This article references 21 other publications.

中文翻译：

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十年电化学合成氨

赛跑到关键的里程碑是历史探险家和科学家最喜欢的消遣。从争先恐后到北极，再到发现 Haber-Bosch 过程，克服看似不可能的目标往往需要坚持不懈的团队探索创造性的解决方案，同时密切关注过去的失败。开发用于合成氨的电化学路线的竞赛就像这些努力一样，因为通往极地的道路很长，不清楚，有时还很危险。在过去的十年中，研究人员在改进电化学氨合成方面取得了重大进展。(1) 早期发现的一个关键挑战是产品分析不准确。报告的低水平氨通常与基于实验室的外来氨和 NOx 的来源处于同一范围内。(2) 这给研究人员在确定给定催化剂的真实活性方面带来了一些挑战。因此，减轻氮污染成为该领域需要解决的关键问题。(3,4)图 1. 随着氨的电合成达到 100% 的法拉第效率，下一个里程碑是否与能源效率和成本相关？各种研究小组已经集中研究了几种建议的氨测量方法，如果采用这些方法，可以帮助排除污染。使用同位素标记的氮气和定量核磁共振波谱的建议控制允许研究人员从同位素标记的氮 ( ¹⁵ N _{2 ) 中区分外来氮 (} ¹⁴ N _{2 )}）。(5) 除了改进低水平氨测量之外，转向使氨产量大大超过偶然氨水平的操作条件也是理想的。具体来说，转移到m_乘积≫ m_系统和C _{NH 3} > 100 ppm 的条件是确定的重要阈值，可以为每次测量提供信心。(6) 有趣的是，低氨水平和误报并非新鲜事。事实上，在已发表的科学文献中有许多关于这个问题的警示故事。例如，第一次观察到电化学 N ₂1807 年的固定，90 年后被证明是不可复制的 (7,8)，并且在 1990 年代发表了关于这个主题的几次激烈讨论。(9-12) 尽管存在这种挫折，但由于可用于低水平氨测量的解决方案，围绕水相和非水基电催化和光催化系统的研究取得了显着增长。(13-15) 抑制析氢反应仍然是水基催化系统的关键瓶颈。由于这些原因，非水和锂介导的电催化方法已成为实现生产率（电流密度）和产品选择性（法拉第效率）方面重要技术目标的最有可能的途径。I < 20 mA cm ^–2 )。(17,18) 这些值虽然意义重大，但远低于美国能源部的可行性目标（例如，I = 300 mA cm ^–2和 FE = 90%）。然而，Simonov、MacFarlane 及其同事和 Chorkendorff 及其同事最近的研究结果首次证明了接近 100% 的法拉第效率（图 1，顶部）。(19) MacFarlane 和 Simonov 及其同事能够通过使用基于酰亚胺的锂盐电解质调整电极-电解质界面的局部物理化学性质来抑制电解质分解。同样，Chorkendorff 及其同事表明，在提高电流密度（-1 A cm_地理2）。(20) 这是在工业相关电流密度下首次展示锂介导的氮还原反应。有了这些令人印象深刻的指标，该领域无疑达到了一个重要的里程碑，在某种意义上已经达到了“北极”般的发现。随着后视镜中的“北极”，现在的视线集中在“南极”上（图 1，底部）。为了使氨电合成与 Haber-Bosch 工艺竞争，需要做大量工作来提高能源效率，并且在技术经济学方面仍然存在一些问题。(21) 此外，随着材料关键性问题的日益严重，锂会继续成为可行的基材，还是会出现其他材料？我期待着电化学氨合成的下一个十年。本材料基于美国国家科学基金会资助的工作，资助号为 1846611。本文引用了 21 种其他出版物。这篇文章尚未被其他出版物引用。图 1. 随着氨的电合成达到 100% 的法拉第效率，下一个里程碑是否与能源效率和成本相关？本文引用了其他 21 种出版物。