Suzanne Zamany Andersen

Ammonia is crucial in fertilizer and chemical industries and is seen as a carbon-free fuel¹. The ammonia electrosynthesis from nitrogen at ambient conditions offers an attractive alternative to the Haber-Bosch process^2,3. The lithium-mediated nitrogen reduction (Li-NRR) represents a promising approach for continuous-flow ammonia electrosynthesis, coupling nitrogen reduction with hydrogen oxidation⁴. However, tetrahydrofuran (THF), commonly used as a solvent, impedes long-term ammonia production due to polymerization and volatility issues. Here we show that a chain ether-based electrolyte enables long-term continuous ammonia synthesis. We find that a chain ether-based solvent exhibits non-polymerization properties, a high boiling point (162 °C), and forms a compact solid-electrolyte interphase (SEI) layer on the gas diffusion electrode (GDE), facilitating ammonia release in the gas phase and ensuring electrolyte stability. We demonstrate 300 h continuous operation in a flow electrolyzer with 25 cm² electrode at 1 bar and room temperature, and achieve a current-to-ammonia efficiency of 64 ± 1% with unprecedented gas phase ammonia content of ~ 98%. Our work highlights the crucial role of the solvent in long-term continuous ammonia synthesis.

Ammonia is a crucial component in the production of fertilizers and various nitrogen-based compounds. Now, the lithium-mediated nitrogen reduction reaction (Li-NRR) has emerged as a promising approach for ammonia synthesis at ambient conditions. The proton shuttle plays a critical role in the proton transfer process during Li-NRR. However, the structure-activity relationship and design principles for effective proton shuttles have not yet been established in practical Li-NRR systems. Here, we propose a general procedure for verifying a true proton shuttle and established design principles for effective proton shuttles. We systematically evaluate several classes of proton shuttles in a continuous-flow reactor with hydrogen oxidation at the anode. Among the tested proton shuttles, phenol exhibits the highest Faradaic efficiency of 72 ± 3% towards ammonia, surpassing that of ethanol, which has been commonly used so far. Experimental investigations including operando isotope-labelled mass spectrometry proved the proton-shuttling capability of phenol. Further mass transport modeling sheds light on the mechanism.

Ammonia is used extensively as a fertilizer in agriculture and in the chemical industry. It also has the potential to store carbon-neutral fuel (hydrogen) in the future. To help meet the future demand for ammonia, the electrochemical nitrogen reduction reaction (NRR) has received much attention. It would serve as a decentralized method of synthesizing ammonia at low temperature and pressure. [1] To this day, the most promising NRR results have been shown via the lithium-mediated approach, in which lithium metal is electroplated in an organic electrolyte containing a lithium salt, saturated nitrogen gas, and a proton shuttle. The saturated nitrogen in the electrolyte reacts with electrodeposited lithium and protons are subsequently delivered by the shuttle. The local availabilities of protons and nitrogen are crucial for achieving a high selectivity towards ammonia rather than hydrogen evolution. The selectivity is heavily affected by the solid electrolyte interphase (SEI) which is formed between the electrodeposited lithium and bulk electrolyte. [2,3,4,5] This study investigates the role of ethanol as the proton shuttle in the lithium-mediated nitrogen reduction reaction (Li-NRR). In situ mass spectrometry and electrochemical quartz crystal microbalance are used to elucidate the importance of ethanol. Furthermore, particularly designed chronopotentiometry experiments are designed to provide additional information, allowing for a detailed discussion of ethanol’s multifaceted role in the Li-NRR. It is shown that ethanol is crucial for the formation of the SEI, but not necessary for the actual synthesis of ammonia. The SEI formation in the Li-NRR system is critically influenced by ethanol at the onset of lithium plating. Ethanol is not necessary to shuttle protons from the anode to the cathode. Furthermore, it is shown that lithium is an active species in the Li-NRR system even without electrochemistry. Finally, it is revealed that ethanol plays an important role in the context of anodic electrode potentials and electrolyte stability. References MacFarlane, D. R. et al. Adv. Mater. 32, 1904804 (2020) Du, H-L et al. Nature 609, 722 (2022) Lazaouski, N. et al. ACS Catal. 12, 5197-5208 (2022) Andersen, S. Z. et al. Energy Environ. Sci. 13, 4291-4300 (2020) Andersen, S. Z. et al. Nature 570, 504 (2019)

The lithium‐mediated nitrogen reduction reaction (Li‐NRR) is a promising method for decentralized ammonia synthesis using renewable energy. An organic electrolyte is utilized to combat the competing hydrogen evolution reaction, and lithium is plated to activate the inert N₂ molecule. Ethanol is commonly used as a proton shuttle to provide hydrogen to the activated nitrogen. In this study, we investigate the role of ethanol as a proton shuttle in an electrolyte containing tetrahydrofuran and 0.2 M lithium perchlorate. Particularly designed electrochemical experiments show that ethanol is necessary for a good solid‐electrolyte interphase but not for the synthesis of ammonia. In addition, electrochemical quartz crystal microbalance (EQCM) demonstrates that the SEI formation at the onset of lithium plating is of specific importance. Chemical batch synthesis of ammonia combined with real‐time mass spectrometry confirms that protons can be shuttled from the anode to the cathode by other species even without ethanol. Moreover, it raises questions regarding the electrochemical nature of Li‐NRR. Finally, we discuss electrolyte stability and electrochemical electrode potentials, highlighting the role of ethanol on electrolyte degradation.

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.

The Haber-Bosch process, which industrially produces ammonia, is one of the most important inventions of the 20^th century. It is argued that without the ability to mass produce ammonia and therefore fertilizer, we would not be able to feed half of the current population. However, the Haber-Bosch process is harmful to the environment as it runs at high pressures and temperatures and is therefore very energy intensive. Furthermore, it utilizes H2 from steam reforming which causes large CO₂ emissions. To mitigate the environmental strain of the Haber-Bosch process, electrochemical ammonia synthesis from renewable electricity sources would be an alternative, however the large activation barrier for dinitrogen splitting and the competition of hydrogen evolution reaction (HER) makes the electrochemical nitrogen reduction very difficult. (1) As of now, only the Li-mediated ammonia synthesis has been successfully proven by several labs to consistently produce ammonia at rates and faradaic efficiencies that could be industrially relevant. The exact mechanism is yet to be elucidated but it widely accepted that the first step is electrochemical plating of metallic Li from Li⁺ ions. The freshly plated Li metal is very reactive and is then believed to react with N₂ in the electrolyte which produces an intermediate Li-N species. Lastly, ammonia is being formed by protonation of this Li-N species (2). Recent breakthroughs in the field managed to push the faradaic efficiencies to 78 %, however that was achieved at low current densities of -4 mA/cm² (3). To make the process industrially relevant the current densities and therefore ammonia formation rates need to be increased significantly. The Department of Energy has stated in their REFUEL program a goal of 300 mA/cm² at faradaic efficiencies of 90 % (4). In our latest publication, we have reached current densities of -100 mA/cm² with high surface area Cu electrodes, however at relatively low faradaic efficiencies of 13 %. To synthesize the high surface area Cu electrodes we used the hydrogen bubble templating procedure that deposits metals at high overpotentials where the competing HER makes a templating structure and forms porous metal foams. We have further improved upon the deposition method by changing the substrate, which not only increased the physical stability but also the electrochemical performance. By varying the deposition conditions and optimizing the electrolyte for ammonia synthesis, we achieved a current density of -1 A/cm² and high faradaic efficiencies of 75 %. The increase in faradaic efficiency is speculated to be due to changes in the solid electrolyte interface (SEI) layer, which we probe with X-ray photoelectron spectroscopy with the help of our in-house build transfer system that limits contact to air and moisture. The results are supported by theoretical models that calculate the Li⁺ conductivity of different constituents of the SEI layer. J. Kibsgaard, J. K. Nørskov, I. Chorkendorff, The Difficulty of Proving Electrochemical Ammonia Synthesis. ACS Energy Lett. 4, 2986–2988 (2019). N. Lazouski, Z. J. Schiffer, K. Williams, K. Manthiram, Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction. Joule. 3, 1127–1139 (2019). K. Li, S. Z. Andersen, M. J. Statt, M. Saccoccio, V. J. Bukas, K. Krempl, R. Sažinas, J. B. Pedersen, V. Shadravan, Y. Zhou, D. Chakraborty, J. Kibsgaard, P. C. K. Vesborg, J. K. Nørskov, I. Chorkendorff, Enhancement of lithium-mediated ammonia synthesis by addition of oxygen. Science (6575). 374, 1593–1597 (2021). G. Soloveichik, in 2019 AIChE Annual Meeting: Topical Conference - Ammonia Energy (AIChE, 2019).

Lithium-mediated electrochemical ammonia synthesis (LiMEAS) is a way-to-go to reduce nitrogen at ambient pressure and temperature, thus avoiding harsh conditions of industrial standard Haber-Bosch process.¹ Inspired by Tsuneto,² we showed how the choice of electrolyte solvent has a considerable effect both in terms of ammonia yield and electrolyte degradation, which affects the long term stability.³ Recently, we introduced a strategy to improve LiMEAS by pulsed instead of constant current over time leading to considerably higher Faradaic efficiency.⁴ The pulsed current kept the electrode cleaner from deposit, which allowed the system to run for a longer time. The thinner deposit decreased the ohmic losses, thus increasing the energy efficiency of the electrosynthesis process. Including our discovery that small amounts of oxygen (O₂) added to nitrogen (N₂) enhance Faradaic efficiency of LiMEAS,⁵ the role of the solid-electrolyte interphase (SEI) has to be understood, in order to make LiMEAS stable, reliable, and more efficient. The SEI is responsible for performance and stability of Li-ion batteries (LIB).⁶ It grows on the working electrode (WE) once the deposited lithium contacts the electrolyte resulting in the materials from electrolyte decomposition. Inorganic, organic and polymeric compounds are the main components of this layer, which can be controlled. The same is expected in LiMEAS, since the organic byproducts in electrolyte are formed during electrosynthesis.³ In an ideal case, only Li⁺ and H⁺ should diffuse through this layer, shielding the reactive electrode against further reduction reactions.⁷ However, the composition, behavior and specific role of SEI in LiMEAS is not known, and the field lacks of studies on it. In this work we show what SEI in LiMEAS is and how its formation and properties can be tuned. An operando electrochemical quartz crystal microbalance (EQCM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), gas chromatography-mass spectrometry (GCMS) and nuclear magnetic resonance (NMR) were employed to determine dynamic, mechanical and chemical SEI properties as a function of time, electrolyte, gases, and additives. We focus on the physical behavior and chemical composition changes on the electrodes and in the electrolyte. The role of SEI was elaborated in detail by comparing electrochemical operation conditions, constant and pulsed current, introducing additives to the electrolyte or exchanging electrolyte components. When the current is pulsed, electrolyte decomposition triggers slower and more controllable interphase growth with some mass stripping during relaxation compared to rapid growth in constant current LiMEAS. Due to these (electro-)chemical processes, the interphase formed on cathodes with pulsed current is thinner, less rigid and more viscous than formed with constant current. Furthermore, addition of O₂ to N₂ suppresses the electrolyte degradation and chemical side reactions, leading to a more efficient and chemically stable LiMEAS. High performance of LiMEAS with optimal O₂ was attributed to the formation of a stable and efficient SEI on the surface of the working electrode, and the subsequent suppression of HER from electrolyte. The main phases in SEI were lithium hydroxides (LiOH∙xH₂O), carbonate (LiCO₃), and ethoxide (LiOEt). However, larger masses in EQCM and NMR describe polytetrahydrofuran (polyTHF) network formation. This work additionally elaborates on the effect of LiMEAS and consequent SEI on the electrolyte decomposition. Our results point towards efficient, stable and robust LiMEAS with highly dynamic structure–activity–stability relationship of the electrode interphase for, but not limited to, LiMEAS. C. Li, T. Wang and J. Gong, Transactions of Tianjin University, 2020, 26, 67-91. A. Tsuneto, A. Kudo and T. Sakata, Chem. Lett., 1993, 22, 851-854. R. Sažinas, S. Z. Andersen, K. Li, M. Saccoccio, K. K., J. B. Pedersen, J. Kibsgaard, P. K. K. Vesborg, D. Chakraborty and I. Chorkendorff, Submitted, 2021. S. Z. Andersen, M. J. Statt, V. J. Bukas, S. G. Shapel, J. B. Pedersen, K. Krempl, M. Saccoccio, D. Chakraborty, J. Kibsgaard, P. C. K. Vesborg, J. Nørskov and I. Chorkendorff, Energy & Environ. Sci., 2020, 13, 4291-4300. K. Li, S. Z. Andersen, M. J. Statt, M. Saccoccio, V. J. Bukas, K. Krempl, R. Sažinas, J. B. Pedersen, V. Shadravan, Y. Zhou, D. Chakraborty, J. Kibsgaard, P. C. K. Vesborg, J. K. Nørskov and I. Chorkendorff, Science (New York, N.Y.), 2021, 374, 1593-1597. S. K. Heiskanen, J. Kim and B. L. Lucht, Joule, 2019, 3, 2322–2333. L. Wang, A. Menakath, F. Han, Y. Wang, P. Y. Zavalij, K. J. Gaskell, O. Borodin, D. Iuga, S. P. Brown, C. Wang, K. Xu and B. W. Eichhorn, Nat. Chem., 2019, 11, 789-796.

Ammonia is one of the most produced chemicals worldwide and is currently synthesized by the Haber-Bosch process, which is a thermally catalyzed method that requires high pressures and temperatures. These harsh conditions, in addition to the prerequisite steam reforming process, leads to about 1 % of the annual energy consumption and 1.4 % of the global CO₂ emission. One way to mitigate some of the Haber-Bosch process is to produce ammonia electrochemically, utilizing renewable energy sources. The electrochemical synthesis of ammonia faces several big issues. One being the selectivity, since the more facile hydrogen evolution reaction (HER) will always dominate over the nitrogen reduction reaction (NRR), and another issue being the activity, given that the nitrogen triple bond is very stable and therefore hard to split. Hence, most often the reported ammonia contents are in the low ppm regime, which makes it very susceptible to contaminations both from the gas stream (NH₃ and NO_x impurities) and the system itself (catalyst, cell, chemicals, nitrile gloves, etc.). To avoid misleading results, several protocols have been published on how to correctly perform NRR experiments [1, 2]. One of these confirm that only the Li-mediated ammonia synthesis (LiMeAS) is currently able to produce ammonia electrochemically [3]. The actual mechanism is not fully understood, but it is generally believed that the first step is Li plating from a Li salt containing non-aqueous electrolyte. The very reactive Li will then react with N₂ solvated in the electrolyte to form Li₃N, which is believed to hydrolyze to ammonia when in contact with a proton source. The currently highest archived faradaic efficiency (FE) is at 69 % at 20 bar N₂ pressure when applying an ionic liquid as a proton shuttle [4]. In this work, we achieved up to 79 % FE at 20 bar N₂ by the addition of 0.8 mol. % O₂ in the reaction atmosphere. The positive effect of O₂ is a very counterintuitive observation, since the original work by Tsuneto et al. [5] showed that the use of synthetic air significantly hindered the reaction, as it was postulated that O₂ inhibits the reaction due to LiO₂ formation and/or leading primarily to the oxygen reduction reaction (ORR). We will present experimental results obtained at 10 and 20 bar with varying O₂ contents, which were measured accurately by a mass spectrometer probing the atmosphere just above the electrolyte inside the pressure vessel. By combining experimental observations with theoretical modelling, we conclude that the unexpectedly beneficial role of small O₂ concentrations has a positive influences the solid electrolyte interface (SEI), which is of great importance in our system. Additional ex-situ X-Ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS) measurements were conducted without exposure to air and moisture, to analyze the SEI layer and deposition after electrochemistry. We believe that this study will not only be beneficial for industrializing the LiMeAS, but will also bring us a step further in understanding the complex mechanism behind this process. [1] S. Z. Andersen et al., "A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements," Nature, vol. 570, pp. 504-508, 2019. [2] H. Iriawan et al., "Methods for nitrogen activation by reduction and oxidation," Nature Reviews Methods Primers, vol. 1, no. 1, pp. 1-26, 2021. [3] J. Choi et al., "Identification and elimination of false positives in electrochemical nitrogen reduction studies," Nature communications, vol. 11, no. 1, pp. 1-10, 2020. [4] B. H. Suryanto et al., "Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle," Science, vol. 372, no. 6547, pp. 1187-1191, 2021. [5] A. Tsuneto, A. Kudo, and T. Sakata, "Lithium-mediated electrochemical reduction of high pressure N2 to NH3," Journal of Electroanalytical Chemistry, vol. 367, no. 1-2, pp. 183-188, 1994.

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