Electrochemically reducing N₂ at mild temperatures is an appealing approach to lower the carbon footprint of ammonia synthesis. In particular, synthesis of ammonia via a Li-mediated process, which operates at close-to-ambient temperatures, has been gaining traction in recent years. However, a challenge associated with this method is the relatively negative electrochemical potential required to produce the Li metal mediator from Li⁺ during the reaction, which limits the energy efficiency. Finding other metals that can mediate ammonia synthesis at more positive potentials could address this issue. Now, Melinda Krebsz, Douglas MacFarlane, Alexandr Simonov and colleagues at Monash University report that Mg^0/2+ — which has a more positive redox potential than Li^0/+ — is capable of reducing N₂ and the resulting material can be converted to ammonium ions.

The researchers employ a two-step method. In the first step, Mg⁰ is electrodeposited from a Mg²⁺-containing electrolyte and this freshly deposited Mg⁰ reacts with N₂ to produce nitride. A key issue to navigate here is the disadvantageous tendency of Mg⁰ to passivate, a process in which a thin, unreactive oxide/hydroxide layer forms on the surface. To address this, the team use an electrolyte that, in addition to Mg²⁺, contains Li⁺, BH₄⁻ and bis(trifluoromethylsulfonyl)-imide ions; this mixture of species prevents the Mg⁰ surface from passivation, maintaining its reactivity. In the second step, the nitride-containing deposits are reacted with HCl, resulting in the formation of ammonium ions. The researchers report a Faradaic efficiency of 7% and a yield rate of 66 nmol s⁻¹ cm⁻². While the current-to-product efficiency is lower than that reported for Li-mediated ammonia synthesis, the work acts as a proof-of-concept that opens the door to further exploration of Mg as a mediator.

在温和温度下电化学还原 N ₂是降低氨合成碳足迹的一种有吸引力的方法。特别是，通过锂介导的工艺合成氨，该工艺在接近环境温度下运行，近年来一直受到关注。然而，与该方法相关的一个挑战是在反应过程中从Li ⁺产生Li金属介体所需的相对负的电化学势，这限制了能量效率。寻找其他能够以更积极的潜力介导氨合成的金属可以解决这个问题。现在，Melinda Krebsz、Douglas MacFarlane、Alexandr Simonov 及其莫纳什大学的同事报告说，Mg ^0/2+ （比 Li ^0/+具有更正的氧化还原电位）能够还原 N ₂并且所得材料可以转化为铵离子。

研究人员采用了两步法。在第一步中，从含Mg ²⁺的电解质电沉积Mg ⁰ ，并且该新沉积的Mg ⁰与N ₂反应产生氮化物。这里要解决的一个关键问题是 Mg ⁰钝化的不利趋势，在该过程中，表面上会形成一层薄薄的、不反应的氧化物/氢氧化物层。为了解决这个问题，该团队使用了一种电解质，除了 Mg ²⁺之外，还含有 Li ⁺ 、BH ₄ ^-和双（三氟甲基磺酰基）酰亚胺离子。这种物质的混合物可防止 Mg ⁰表面钝化，从而保持其反应性。在第二步中，含氮化物沉积物与 HCl 反应，形成铵离子。研究人员报告法拉第效率为 7%，产率为 66 nmol s ^-1 cm ^-2 。虽然电流到产品的效率低于锂介导的氨合成的报告，但这项工作作为概念验证，为进一步探索镁作为介体打开了大门。