Lithium-mediated nitrogen reduction is a proven method to electrochemically synthesize ammonia; yet the process has so far been unstable, and the continuous deposition of lithium limits its practical applicability. One...

Boosting ammonia with a little oxygen Ammonia synthesis from nitrogen for fertilizer production is highly energy intensive. Chemists are therefore exploring electrochemical approaches that could draw power from renewable sources while generating less waste. One promising cycle involves the reduction of lithium ions at an electrode, with the resultant metal in turn reducing nitrogen and regenerating the ions. Li et al . report the counterintuitive result that small quantities of oxygen could enhance the efficiency of this process, which they attribute to diffusional effects that limit excessive lithium reduction. —JSY

Ammonia is a critical component in fertilizers, pharmaceuticals, and fine chemicals and is an ideal, carbon-free fuel. Recently, lithium-mediated nitrogen reduction has proven to be a promising route for electrochemical ammonia synthesis at ambient conditions. In this work, we report a continuous-flow electrolyzer equipped with 25–square centimeter–effective area gas diffusion electrodes wherein nitrogen reduction is coupled with hydrogen oxidation. We show that the classical catalyst platinum is not stable for hydrogen oxidation in the organic electrolyte, but a platinum-gold alloy lowers the anode potential and avoids the decremental decomposition of the organic electrolyte. At optimal operating conditions, we achieve, at 1 bar, a faradaic efficiency for ammonia production of up to 61 ± 1% and an energy efficiency of 13 ± 1% at a current density of −6 milliamperes per square centimeter.

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The lithium-mediated ammonia synthesis is so far the only proven electrochemical way to produce ammonia with promising faradaic efficiencies (FEs). However, to make this process commercially competitive, the ammonia formation rates per geometric surface area need to be increased significantly. In this study, we increased the current density by synthesizing high surface area Cu electrodes through hydrogen bubbling templating (HBT) on Ni foam substrates. With these electrodes, we achieved high ammonia formation rates of 46.0 ± 6.8 nmol s −1 cm geo −2 , at a current density of −100 mA/cm geo −2 at 20 bar nitrogen atmosphere and comparable cell potentials to flat foil electrodes. The FE and energy efficiency (EE) under these conditions were 13.3 ± 2.0% and 2.3 ± 0.3%, respectively. Additionally, we found that increasing the electrolyte salt concentration improves the stability of the system, which is attributed to a change of Li deposition and/or solid electrolyte interphase.

Lithium-mediated non-aqueous electrochemical ammonia synthesis (LiMEAS) as an efficient and green ammonia production way was studied by GCMS in different organic electrolytes to evaluate the stability of electrochemical systems.

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Although oxygen added to nonaqueous lithium-mediated electrochemical ammonia synthesis (LiMEAS) enhances Faradaic efficiency, its effect on chemical stability and byproducts requires understanding. Therefore, standardized high-resolution gas chromatography–mass spectrometry and nuclear magnetic resonance were employed. Different volatile degradation products have been qualitatively analyzed and quantified in tetrahydrofuran electrolyte by adding some oxygen to LiMEAS. Electrodeposited lithium and reduction/oxidation of the solvent on the electrodes produced organic byproducts to different extents, depending on the oxygen concentration, and resulted in less decomposition products after LiMEAS with oxygen. The main organic component in solid-electrolyte interphase was polytetrahydrofuran, which disappeared by adding an excess of oxygen (3 mol %) to LiMEAS. The total number of byproducts detected was 14, 9, and 8 with oxygen concentrations of 0, 0.8, and 3 mol %, respectively. The Faradaic efficiency and chemical stability of the LiMEAS have been greatly improved with addition of optimal 0.8 mol % oxygen at 20 bar total pressure.