Electrolyte acidification from anode reactions during lithium mediated ammonia synthesis

Cu(N2)-Li Battery for Ammonia Synthesis

The cathodic mechanism of Li-N2 batteries is similar to Li-mediated N2 reduction (LiNR). Herein, the Li-N2, LiNR, and Cu-Li battery were amalgamated to a milliliter-scale Cu(N2)-Li system. The utilization of a lithium anode with lithium oxidation reaction (LiOR), ensures an uninterrupted supply of lithium ions to active N2. LiOR not only enhances electrolyte stability but also reduces voltage by stripping Li ions, in contrast to the inert platinum anode, commonly employed in LiNR. Notably, an unusual observation of ammonia accumulation within the anode chamber elucidates the presence and role of reaction intermediates. The charging process aimed at lithium regeneration faces high polarization, and a cycling procedure involving low-current charging was proposed to improve cycling. This study integrates insights from three distinct research directions to leverage their respective advantages and scientific insights. The Li-N2 battery emerges as a highly advantageous strategy for ammonia synthesis due to the progressiveness of lithium anode.

Phenol as proton shuttle and buffer for lithium-mediated ammonia electrosynthesis

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.

Li-Mediated Electrochemical Nitrogen Fixation: Key Advances and Future Perspectives

The electrochemical nitrogen reduction reaction holds great potential for ammonia production using electricity generated from renewable energy sources and is sustainable. The low solubility of nitrogen in aqueous media, poor kinetics, and intrinsic competition by the hydrogen evolution reaction result in meager ammonia production rates. Attributing measured ammonia as a valid product, not an impurity, is challenging despite rigorous analytical experimentation. In this regard, Li-mediated electrochemical nitrogen reduction is a proven method providing significant ammonia yields. Herein, fundamental advances and insights into the Li-mediated strategy are summarized, emphasizing the role of lithium, reaction parameters, cell designs, and mechanistic evaluation. Challenges and perspectives are presented to highlight the prospects of this strategy as a continuous, stable, and modular approach toward sustainable ammonia production.

Is Ethanol Essential for the Lithium-Mediated Nitrogen Reduction Reaction?

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.

Searching for the Rules of Electrochemical Nitrogen Fixation

Li-mediated ammonia synthesis is, thus far, the only electrochemical method for heterogeneous decentralized ammonia production. The unique selectivity of the solid electrode provides an alternative to one of the largest heterogeneous thermal catalytic processes. However, it is burdened with intrinsic energy losses, operating at a Li plating potential. In this work, we survey the periodic table to understand the fundamental features that make Li stand out. Through density functional theory calculations and experimentation on chemistries analogous to lithium (e.g., Na, Mg, Ca), we find that lithium is unique in several ways. It combines a stable nitride that readily decomposes to ammonia with an ideal solid electrolyte interphase, balancing reagents at the reactive interface. We propose descriptors based on simulated formation and binding energies of key intermediates and further on hard and soft acids and bases (HSAB principle) to generalize such features. The survey will help the community toward electrochemical systems beyond Li for nitrogen fixation.

Electrochemical Nitrogen Fixation for Green Ammonia: Recent Progress and Challenges

Ammonia, a key feedstock used in various industries, has been considered a sustainable fuel and energy storage option. However, NH3 production via the conventional Haber-Bosch process is costly, energy-intensive, and significantly contributing to a massive carbon footprint. An electrochemical synthetic pathway for nitrogen fixation has recently gained considerable attention as NH3 can be produced through a green process without generating harmful pollutants. This review discusses the recent progress and challenges associated with the two relevant electrochemical pathways: direct and indirect nitrogen reduction reactions. The detailed mechanisms of these reactions and highlight the recent efforts to improve the catalytic performances are discussed. Finally, various promising research strategies and remaining tasks are presented to highlight future opportunities in the electrochemical nitrogen reduction reaction.

The role of ion solvation in lithium mediated nitrogen reduction

Since its verification in 2019, there have been numerous high-profile papers reporting improved efficiency of lithium-mediated electrochemical nitrogen reduction to make ammonia. However, the literature lacks any coherent investigation systematically linking bulk electrolyte properties to electrochemical performance and Solid Electrolyte Interphase (SEI) properties. In this study, we discover that the salt concentration has a remarkable effect on electrolyte stability: at concentrations of 0.6 M LiClO₄ and above the electrode potential is stable for at least 12 hours at an applied current density of -2 mA cm^-2 at ambient temperature and pressure. Conversely, at the lower concentrations explored in prior studies, the potential required to maintain a given N₂ reduction current increased by 8 V within a period of 1 hour under the same conditions. The behaviour is linked more coordination of the salt anion and cation with increasing salt concentration in the electrolyte observed via Raman spectroscopy. Time of flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy reveal a more inorganic, and therefore more stable, SEI layer is formed with increasing salt concentration. A drop in faradaic efficiency for nitrogen reduction is seen at concentrations higher than 0.6 M LiClO₄, which is attributed to a combination of a decrease in nitrogen solubility and diffusivity as well as increased SEI conductivity as measured by electrochemical impedance spectroscopy.

Li-N2 Battery for Ammonia Synthesis and Computational Insight

Electrochemical synthesis of ammonia is deemed as an alternative to the fossil-fuel-driven Haber-Bosch (HB) process, in which Li-mediated nitrogen reduction (LiNR) is the most promising scheme. Continuous lithium-mediated nitrogen reduction for ammonia synthesis (C-LiNR) has recently been reported in high-level journals with many foggy internal reactions. Synthesizing ammonia in a separate way may be profitable for understanding the mechanism of LiNR. Herein, an intermittent lithium-mediated nitrogen reduction for ammonia synthesis (I-LiNR) was proposed, three steps required for I-LiNR were achieved in the cathode chamber of a Li-N₂ battery. Discharge, stand, and charge in the Li-N₂ battery correspond to N₂ lithification, protonation, and lithium regeneration, respectively. It can also realize the quasi-continuous process with practical significance because it could be carried out through identical batteries. Products such as Li₃N, LiOH, and NH₃ are detected experimentally, which demonstrate a definite reaction pathway. The mechanism of the Li-N₂ battery, the Li-mediated synthesis of ammonia, and LiOH decomposition are explored through density functional theory calculations. The role of Li in dinitrogen activation is highlighted. It expands the range of LiOH-based Li-air batteries and may guide the study from Li-air to Li-N₂; attention has been given to the reaction mechanism of Li-mediated nitrogen reduction. The challenges and opportunities of the procedure are discussed in the end.

Near ambient N2 fixation on solid electrodes versus enzymes and homogeneous catalysts

The Mo/Fe nitrogenase enzyme is unique in its ability to efficiently reduce dinitrogen to ammonia at atmospheric pressures and room temperature. Should an artificial electrolytic device achieve the same feat, it would revolutionize fertilizer production and even provide an energy-dense, truly carbon-free fuel. This Review provides a coherent comparison of recent progress made in dinitrogen fixation on solid electrodes, homogeneous catalysts and nitrogenases. Specific emphasis is placed on systems for which there is unequivocal evidence that dinitrogen reduction has taken place. By establishing the cross-cutting themes and synergies between these systems, we identify viable avenues for future research.

Oxygen Functionalization-Induced Charging Effect on Boron Active Sites for High-Yield Electrocatalytic NH3 Production

Ammonia has been recognized as the future renewable energy fuel because of its wide-ranging applications in H₂ storage and transportation sector. In order to avoid the environmentally hazardous Haber-Bosch process, recently, the third-generation ambient ammonia synthesis has drawn phenomenal attention and thus tremendous efforts are devoted to developing efficient electrocatalysts that would circumvent the bottlenecks of the electrochemical nitrogen reduction reaction (NRR) like competitive hydrogen evolution reaction, poor selectivity of N₂ on catalyst surface. Herein, we report the synthesis of an oxygen-functionalized boron carbonitride matrix via a two-step pyrolysis technique. The conductive BNCO₍₁₀₀₀₎ architecture, the compatibility of B-2p_z orbital with the N-2p_z orbital and the charging effect over B due to the C and O edge-atoms in a pentagon altogether facilitate N₂ adsorption on the B edge-active sites. The optimum electrolyte acidity with 0.1 M HCl and the lowered anion crowding effect aid the protonation steps of NRR via an associative alternating pathway, which gives a sufficiently high yield of ammonia (211.5 μg h^-1 mg_cat^-1) on the optimized BNCO₍₁₀₀₀₎ catalyst with a Faradaic efficiency of 34.7% at - 0.1 V vs RHE. This work thus offers a cost-effective electrode material and provides a contemporary idea about reinforcing the charging effect over the secured active sites for NRR by selectively choosing the electrolyte anions and functionalizing the active edges of the BNCO₍₁₀₀₀₎ catalyst.