Electrochemical Ammonia Synthesis Research Articles

Local Proton Source Enhanced Nitrogen Reduction on a Combined Cobalt‐Molybdenum Catalyst for Electrochemical Ammonia Synthesis

AbstractElectrochemical nitrogen reduction reaction (NRR) under ambient conditions has attracted considerable scientific and engineering interest as a green alternative route for NH3 production. Molybdenum is a promising candidate as an electrocatalyst for NRR as it has a suitable binding strength with N species. However, the design of an efficient Mo‐based catalyst remains elusive. To enhance the selectivity of NRR toward NH3, we have developed a carbon nanofiber catalyst embedded with molybdenum and cobalt (Co−Mo−CNF). Co with a strong ability to dissociate water enhances local proton source near Mo, where the hydrogenation step of the NRR occurs. A NH3 formation rate of 72.72 μg h−1 mg−1 and a Faradaic efficiency of 34.5 % were obtained at −0.5 V vs. RHE. We also attempted to provide a mechanistic understanding of the NRR via in situ attenuated total reflectance surface‐enhanced infrared absorption spectroscopy (ATR‐SEIRAS) and isotopic labeling experiments using 15N2 and D2O.

Local Proton Source Enhanced Nitrogen Reduction on a Combined Cobalt-Molybdenum Catalyst for Electrochemical Ammonia Synthesis.

Electrochemical nitrogen reduction reaction (NRR) under ambient conditions has attracted considerable scientific and engineering interest as a green alternative route for NH3 production. Molybdenum is a promising candidate as an electrocatalyst for NRR as it has a suitable binding strength with N species. However, the design of an efficient Mo-based catalyst remains elusive. To enhance the selectivity of NRR toward NH3 , we have developed a carbon nanofiber catalyst embedded with molybdenum and cobalt (Co-Mo-CNF). Co with a strong ability to dissociate water enhances local proton source near Mo, where the hydrogenation step of the NRR occurs. A NH3 formation rate of 72.72 μg h-1 mg-1 and a Faradaic efficiency of 34.5 % were obtained at -0.5 V vs. RHE. We also attempted to provide a mechanistic understanding of the NRR via in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and isotopic labeling experiments using 15 N2 and D2 O.

A combinatorial descriptor for volcano relationships of electrochemical nitrogen reduction reaction

Though touted as a potential way to realize clean ammonia synthesis, electrochemical ammonia synthesis is currently limited by its catalytic efficiency. Great effort has been made to find catalysts with improved activity toward electrochemical nitrogen reduction reaction (eNRR). Rational screening of catalysts can be facilitated using the volcano relationship between catalytic activity and adsorption energy of an intermediate, namely, the activity descriptor. In this work, we propose ΔG*NH2+ΔG*NNH as a combinatorial descriptor, which shows better predictive power than traditional descriptors using the adsorption free energies of single intermediates. The volcano plots based on the combinatorial descriptor exhibits peak activity fixedly at the descriptor value corresponding to the formation free energy of NH3, regardless of the catalyst types; while the descriptor values correspond to the top activities for eNRR on volcano plots based on single descriptors usually vary with the types of catalysts.

Rigorous Assessment of Cl- -Based Anolytes on Electrochemical Ammonia Synthesis.

Many challenges in the electrochemical synthesis of ammonia have been recognized with most effort focused on delineating false positives resulting from unidentified sources of nitrogen. However, the influence of oxidizing anolytes on the crossover and oxidization of ammonium during the electrolysis reaction remains unexplored. Here it is reported that the use of analytes containing halide ions (Cl- and Br- ) can rapidly convert the ammonium into N2 , which further intensifies the crossover of ammonium. Moreover, the extent of migration and oxidation of ammonium is found to be closely associated with external factors, such as applied potentials and the concentration of Cl- . These findings demonstrate the profound impact of oxidizing anolytes on the electrochemical synthesis of ammonia. Based on these results, many prior reported ammonia yield rates are calibrated. This work emphasizes the significance of avoiding selection of anolytes that can oxidize ammonium, which is believed to promote further progress in electrochemical nitrogen fixation.

(Digital Presentation) Where Are We in Terms of Energy Efficiency to Produce Green Ammonia – Electrochemical Synthesis of Ammonia Compared to Haber-Bosch Process

The electrochemical synthesis of ammonia, in the literature often found under the name Electrochemical Nitrogen Reduction Reaction (ENRR), might arise as an alternative to the green Haber-Bosch process in some market applications. In this work, we compare electrical energy efficiency (power-to-fuel efficiency) of the ENRR and the Haber-Bosch process.Not so long time ago, the ENRR was considered hardly possible to conduct, because there arose a doubt the literature is plagued by false-positive measurements. In ENRR measured ammonia concentrations were often within the range of ambient ammonia contamination. Some ENRR works produced ammonia by (unintentionally) reducing other, more reactive forms of nitrogen (NOx, nitrates, nitrites) or getting ammonia from a decomposition of the catalyst, while reporting reduction of inert dinitrogen to ammonia. Several rigorous experimental protocols were published during the last couple of years (from 2018), 1 , 2 , 3 , 4 , 5 and these made it easier to distinguish reliable results from the false-positive ones. In general, the ENRR reports that follow the rigorous synthesis protocols are considered valid. If they do not follow the protocols, but still produce ammonia concentrations which are far above contamination levels, the works are also considered valid.Taking a starting point in the reasoning above, we evaluated 120 articles and found out about 20% of ENRR works in the last three years are reliable. The result of this evaluation emphasizes that researchers should consider rigorous experimental protocols more seriously if they wish to stimulate progress in the field. We took the reliable works and evaluated them for power-to-fuel efficiency, by relating the low heating value of ammonia and total energy input to produce ammonia using ENRR.Our analysis shows that some of the reliable ENRR reports have power-to-fuel efficiency of a maximum 40% which is comparable to the current Haber-Bosch ammonia plants which are often in the range of 35-60%.6 Although energy efficiency of ENRR (40%) is lower than that of the modern Haber-Bosch plant (60%), such process would be probably relevant for the market where lower capital costs are more important than higher operating costs. About a third of the mostly reliable ENRR reports have energy efficiency between 20-35%. The others have less than 20% energy efficiency and need more improvements to be comparable with the Haber-Bosch process in terms of power-to-fuel efficiency. The results are surprising and show electrochemical synthesis of ammonia is maturing and can compete with the Haber-Bosch process, however, improvements are needed to bring the energy efficiency higher up. References (1) Suryanto, B. H. R.; Du, H.-L.; Wang, D.; Chen, J.; Simonov, A. N.; MacFarlane, D. R. Challenges and Prospects in the Catalysis of Electroreduction of Nitrogen to Ammonia. Nat. Catal. 2019, 2 (4), 290–296. https://doi.org/10.1038/s41929-019-0252-4.(2) Andersen, S. Z.; Čolić, V.; Yang, S.; Schwalbe, J. A.; Nielander, A. C.; McEnaney, J. M.; Enemark-Rasmussen, K.; Baker, J. G.; Singh, A. R.; Rohr, B. A.; Statt, M. J.; Blair, S. J.; Mezzavilla, S.; Kibsgaard, J.; Vesborg, P. C. K.; Cargnello, M.; Bent, S. F.; Jaramillo, T. F.; Stephens, I. E. L.; Nørskov, J. K.; Chorkendorff, I. A Rigorous Electrochemical Ammonia Synthesis Protocol with Quantitative Isotope Measurements. Nature 2019, 570 (7762), 504–508. https://doi.org/10.1038/s41586-019-1260-x.(3) Tang, C.; Qiao, S.-Z. How to Explore Ambient Electrocatalytic Nitrogen Reduction Reliably and Insightfully. Chem. Soc. Rev. 2019, 48 (12), 3166–3180. https://doi.org/10.1039/C9CS00280D.(4) Du, H.-L.; Gengenbach, T. R.; Hodgetts, R.; MacFarlane, D. R.; Simonov, A. N. Critical Assessment of the Electrocatalytic Activity of Vanadium and Niobium Nitrides toward Dinitrogen Reduction to Ammonia. ACS Sustain. Chem. Eng. 2019, 7 (7), 6839–6850. https://doi.org/10.1021/acssuschemeng.8b06163.(5) Greenlee, L. F.; Renner, J. N.; Foster, S. L. The Use of Controls for Consistent and Accurate Measurements of Electrocatalytic Ammonia Synthesis from Dinitrogen. ACS Catal. 2018, 8 (9), 7820–7827. https://doi.org/10.1021/acscatal.8b02120.(6) Smith, C.; K. Hill, A.; Torrente-Murciano, L. Current and Future Role of Haber–Bosch Ammonia in a Carbon-Free Energy Landscape. Energy Environ. Sci. 2020, 13 (2), 331–344. https://doi.org/10.1039/C9EE02873K.

Solvation and Stability in Lithium-Mediated Nitrogen Reduction

The lithium-mediated method of electrochemical nitrogen reduction, pioneered by Tsuneto et al1 then verified by Andersen et al2, is currently the sole paradigm capable of unequivocal electrochemical ammonia synthesis. Such a system could allow the production of green, distributed ammonia for use as fertiliser or a carbon-free fuel. However, despite great improvements in Faradaic efficiency and stability since just 20193, fundamental understanding of the mechanisms governing nitrogen reduction and other parasitic reactions is lacking.Lithium Ion Battery (LIB) research can provide insight; since both lithium-mediated electrochemical ammonia synthesis and LIBs utilise an organic solvent and lithium salt, both form a Solid Electrolyte Interphase (SEI), which is electronically insulating but ionically conducting, at the electrode surface. In LIBs, this is necessary to stabilize and cycle low potential materials4. In lithium-mediated ammonia synthesis, the SEI could also have a critical role in controlling the access of protons and other key reactants to the catalytically active sites and promoting greater selectivity toward nitrogen reduction to ammonia5. While some characterisation of the SEI has been carried out for the lithium-mediated nitrogen reduction system6, the literature still lacks holistic studies which aim to carefully characterise the bulk electrolyte and SEI components and link them to system performance.In this work we use insight from battery science to tackle a significant stability problem in lithium-mediated nitrogen reduction. The traditional electrolyte employed by Tsuneto et al. was 0.2 M LiClO4 in a 99:1 tetrahydrofuran to ethanol mix. While this system can produce ammonia, the working electrode potential becomes more negative over time. Our initial investigations show that this problem stems from an unstable SEI which becomes increasingly organic. Simply by raising the concentration of LiClO4 in the electrolyte, we vastly improve stability, as shown in figure 1(a), and boost Faradaic efficiency. Bulk electrolyte salt solvation properties are investigated through Raman spectroscopy, as shown in figure 1(b). Here we observe the emergence of a shoulder at around 930 cm-1 with increasing LiClO4 concentration, which we assign to the emergence of Contact-Ion-Pairs (CIPs) through comparison to Density Functional Theory calculations. These CIPs mean that perchlorate anion degradation products are more abundant in the formed SEI, as shown in our X-Ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass spectrometry results. This more inorganic SEI protects the electrolyte against further degradation, preventing the working electrode drift to more negative potentials. We then link this behaviour to a peak observed in the Faradaic efficiency of ammonia synthesis at 0.6 M LiClO4 by also considering decreasing N2 solubility and diffusivity, as well as a more ionically conductive SEI, in an increasingly concentrated electrolyte. We also present never-before seen cross-sectional images of the SEI using cryogenic Focussed Ion Beam milling and Scanning Electron Microscopy, further aiding understanding of how salt solvation affects the morphology of the formed SEI and system performance. Our results emphasise the need to consider SEI properties in electrolyte design for lithium-mediated nitrogen reduction, as well as the need to balance desirable SEI properties with desirable bulk electrolyte properties. Tsuneto, A., Kudo, A. & Sakata, T. Efficient Electrochemical Reduction of N 2 to NH 3 Catalyzed by Lithium . Chemistry Letters vol. 22 851–854 (1993).Andersen, S. Z. et al. A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 570, 504–508 (2019).Westhead, O., Jervis, R. & Stephens, I. E. L. Is lithium the key for nitrogen electroreduction? Science. 372, 1149–1150 (2021).Peled, E. & Menkin, S. Review—SEI: Past, Present and Future. J. Electrochem. Soc. 164, A1703–A1719 (2017).Singh, A. R. et al. Electrochemical Ammonia Synthesis—The Selectivity Challenge. ACS Catal. 7, 706–709 (2017).Li, K. et al. Enhancement of lithium-mediated ammonia synthesis by addition of oxygen. Science. 1597, 1593–1597 (2021). Figure 1

Electrochemical Synthesis of Ammonia from Nitrate on Preferentially Oriented Cu2 O

Electrochemistry-based processes to produce ammonia have recently gained widespread interest by the scientific community because they provide a way to reduce the high carbon emissions currently associated with ammonia production. Global NH3 demand is expected to rise since it is included in various energy scenarios as a possible green transportation fuel and storage medium of renewable electricity involving fuel cells.1 Several electrochemical and hybrid electrochemical processes with plasma integration are currently being investigated to produce NH3 via different nitrogen sources, such as N2, NO3 -/NO2 - or NO. The utilization of NO3 - from wastewater sources can be valuable for the environmental and health issues associated with NO3 - contamination. Additionally, NO3 - electroreduction in aqueous media is an attractive approach to investigate the catalysis due to its high solubility and the low dissociation energy needed to break the N=O bond. Cu and Cu-based materials have extensively been studied for NH3 production through NO3 - electroreduction. Earlier studies on Cu single crystals2 in alkaline media showed that Cu (100) performs better than Cu (111) in reducing the NO2 - intermediate to hydroxylamine, which is believed to be the precursor to NH3. On the contrary, recent density functional theory calculations3 showed that a Cu−Cu couple with 2.57 Å distance serves as a catalytic center, leading to the conclusion that Cu (111) is the most active surface termination. In this work, preferentially oriented Cu2O crystals with (111), (100), and (110) terminations were obtained via electrodeposition and evaluated for their performance in NO3 - electroreduction to NH3 (Figure 1). To understand which oxidation state of Cu represents the active phase, the samples were investigated by in-situ Raman spectroscopy. According to a recent study, the active phase was a combination of Cu and Cu+.4 Our results point to the immediate reduction of remaining Cu+ species during application of -0.3V vs. Ag/AgCl and formation of Cu(OH)2 at more negative potentials. The presence of Cu(OH)2 was also apparent from ex-situ XPS and UV-Vis analysis. Moreover, post-reaction XRD analysis showed a retainment of the crystal orientation on the reduced Cu surfaces. All the nanostructured catalysts were active for NO3 - reduction and produced NH3 with faradaic efficiencies above 40%. Cu2O (111) was the most efficient for NH3 synthesis reaching a faradaic efficiency of up to 99%.1 D. Anastasiadou, Electrochem. Sci. Adv.,submitted. 2 E. Pérez-Gallent, Electrochim. Acta, 2017, 227, 77–84.3 T. Hu, ACS Catal., 2021, 11, 14417–14427.4 Y. Wang, Angew. Chemie Int. Ed., 2020, 59, 5350–5354. Figure 1

Electrochemical Synthesis of Ammonia from Nitrogen and Nitric Oxide at Room Temperature over Nanostructured Transition Metal Catalysts

Transition metals have been recognized as excellent and efficient catalysts for transforming gaseous molecules into value-added chemicals. Electrochemical ammonia synthesis through the atmospheric reduction of nitrogen and nitrogen oxide is a promising method for sustainable fertilizer and carbon-free hydrogen energy carriers. However, it is a great challenge to main the activity and stability of the catalyst due to its corrosive nature and vigorous reaction conditions involved in the electrochemical transformations. We report stable core-shell electrocatalysts that utilize transition metal nanoparticles in the core and nitrogen-doped porous carbon architecture in the shell for the efficient electroreduction of N2 and NO to ammonia (NH3). The carbon shell prevents the dissolution of Ni nanoparticles. It ensures the long-term stability of the catalyst, whereas the Ni nanoparticles involve in the catalytic reduction of NO to NH3 during electrolysis. The catalyst images reveal a core-shell structure composed of dark metallic spherical particles at the core and layered carbon as a shell surrounding the nanoparticles (Fig. 1a). A broad particle size distribution of 15 nm to 50 nm was observed, and the core-shell particles were interconnected by a smooth carbon matrix. The heteroatom content and the shell thickness strongly influence the catalytic activity. A high faradaic efficiency (FE) of 72.3% at 0.16 VRHE is achieved (Fig. 1b). Moreover, the full-cell electrolyzer was constructed by coupling developed cathodes and commercial RuO2 as anode for oxygen evolution reaction, which delivered a stable performance over several cycles at 1.5 V. Further, we demonstrate the solar to fuel transformation by integrating with a PV-electrolyzer cell, it presented an appreciable FE of >50%. Fig. 1. (a) TEM image of core-shell catalyst, (b) comparison of FE and NH3 yields on various electrocatalysts for the nitric oxide reduction reaction. Figure 1

Increasing Ammonia Formation Rates of Li-Mediated Ammonia Synthesis with High Surface Area Copper Electrodes

The Haber-Bosch process, which industrially produces ammonia, is one of the most important inventions of the 20th 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).

An in-Situ FTIR Study of Lithium-Mediated Electrochemical Nitrogen Reduction

Electrochemical ammonia synthesis under ambient conditions is a promising sustainable alternative to the highly carbon-intensive Haber-Bosch process. The lithium-mediated system1 is to date the only rigorously verified method of reducing dinitrogen that has been reproduced by several laboratories; nonetheless, its mechanism is not yet understood. It is generally accepted that lithium is key to the success of this system. Metallic lithium binds strongly to dinitrogen and can directly reduce it under ambient conditions. However, it is possible that the apparently unique ability of this system to reduce dinitrogen under ambient conditions lies in its formation of a Solid Electrolyte Interphase (SEI)2 on the working electrode, like those formed in lithium-ion batteries. This likely provides a kinetic barrier to the competing hydrogen evolution reaction by moderating, but not blocking, proton mobility, thus favouring nitrogen reduction.While some post-mortem characterisation of the SEI for this system has been published3, specific SEI forming reactions and the roles of SEI species in the success of the lithium-mediated system are not yet understood. Optimising this system requires optimisation of the properties of the SEI. It is therefore highly desirable to identify key components which govern its properties and develop a mechanistic understanding of their formation. Just as lithium-ion battery electrolytes are tailored to optimise SEI properties, a deeper understanding of the formation of SEI species from bulk electrolyte components in this system would then inform specific tailoring of the electrolyte to optimise selectivity towards nitrogen reduction. Some reports thus far1,4,5 have proposed that electrodeposited metallic lithium reduces dinitrogen1; others that metallic Li or a LiH/Li3N layer provides a catalytic surface for direct protonation5. However, to the best of our knowledge, no proposed mechanism is yet supported by operando spectroscopic evidence, and no operando characterisation of the SEI has been published.In this work, we present a series of studies using operando surface-enhanced infrared spectroscopy to investigate the mechanisms of the two key processes using a bespoke ATR cell. Firstly, we investigate mechanisms of SEI formation reactions in different electrolyte formulations at different applied potentials, thus identifying the key SEI components that may govern the system’s stability and selectivity, and the potentials at which they form. Secondly, we exploit the surface-enhancement effect to directly probe nitrogen reduction reaction intermediates on the working electrode. By detecting potential NH intermediates, we identify the mechanism of nitrogen reduction and the active sites, potentially informing the design of novel electrocatalysts with greater activity and selectivity. A. Tsuneto, A. Kudo and T. Sakata. Chemistry Letters 22, 851-854 (1993).O. Westhead, R. Jervis and I.E.L. Stephens. Science 372, 1149-1150 (2021)K. Li et al. Science 1597, 1593-1597 (2021).A. Bagger, H. Wan, I.E.L. Stephens and J. Rossmeisl. ACS Catalysis 11, 6596-6601 (2021)J. Schwalbe et al. ChemElectroChem 7, 1-9 (2020).

Iron Based Catalysts for Nitrogen Reduction Reaction

Ammonia (NH3) is a fundamental feedstock for the global population not only for its wide use as fertilizer and chemical, but also for its potential as energy storage medium. Its synthesis at the industrial scale is based on the well-known Haber Bosch process, operating at high temperature (400-500°C) and high pressure (150-300atm), thus making it highly pollutant. Considering the global climate emergency, it is mandatory to develop a green ammonia synthesis that relies on milder conditions and may adopt renewable energy sources. The electrochemical synthesis, of NH3 from N2 and H2O at ambient conditions and using renewable energy driven electricity could be a promising approach. An efficient electrochemical ammonia synthesis however, is currently still lacking. The main reasons are the absence of adequate catalysts capable of dissociating the N2 triple bond and the competition with the hydrogen evolution reaction (HER) at the cathode, since the required potential is close to that of nitrogen reduction reaction (NRR), required for the ammonia formation.There are some reaction models in the literature but it is commonly accepted that the dissociative adsorption of N2 is the rate limiting step and extensive research has been done on nitrogen interaction with metal surfaces. Ru, Co, Bi, Au, Fe, Mo, etc based materials have been extensively tested for the green ammonia synthesis, but results are still conflicting also because a complete evaluation of environmental contaminations is still lacking.In this work we adopted as NRR catalyst Fe based nanoparticles on carbon cloth substrate via a simple and fast electroless deposition technique. We prepared a FeCl3 solutions with different concentrations in the range 1-10mM and deposited a drop on (3x1.5) cm2 samples of carbon cloth, on a hot plate, at 80°C. After evaporation of the water, the as deposited substrates were dipped in NaBH4 solution while stirring.The catalyst morphology has been studied by Scanning Electron Microscopy (SEM), as a function of the concentration. Very small particles, with a size ranging from 10 to 70nm, were obtained with the most diluted solution. The cluster size increases with increasing FeCl3 concentration in the solution, giving rise to strong coalescence effects and to the formation of a thicker and continuous layer in the case of the most concentrated solution. Scanning Transmission electron Microscopy (STEM) and Electron Energy Loss Spectroscopy (EELS) have been adopted to determine the composition. EELS spectra showed that the iron L-edge is that typical of Fe3O4 materials.The electrochemical ability to reduce nitrogen, with the formation of ammonia, was evaluated in a standard two compartment cell, with a phosphate buffered solution (PBS 0.1M) adopting a Zirfon membrane as a gas separator. Ar or N2 gas (both with a purity of 99.9999%) flowed in the cathode chamber after going through an acidic trap and a water trap, to be further purified. A rigorous protocol has been adopted to evaluate the ammonia production, including a two steps measurement of the environmental ammonia at the open circuit potential. The ammonia was measured by spectrophotometric analysis using the indophenol blue method. The electrochemical production of ammonia was obtained by chronoamperometry under constant voltage.For the iron-based catalysts, a very efficient activation procedure has been developed, based on cyclic voltammetry under N2 flow. The activation process allows an improvement up to 10 times in the ammonia generation rate. Moreover, a strong correlation has been found between the particle size of the catalyst and its activity for NRR, with the best results achieved with the sample covered with nanoparticles, exhibiting also the highest electrochemical active area. Iron based nanoparticles showed excellent activity for NRR, with a faradaic efficiency of 15% at -0.35 V vs RHE and a maximum ammonia production rate of 85µg mg-1 cat h-1. Acknowledgements:This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No101006941.

Revealing Nanoscale Passivation Films and Their Role in Reactivity in Lithium-Mediated Ammonia Synthesis

Ammonia is an indispensable component of modern fertilizers and a key nitrogen source in chemical synthesis, making it one of the highest-volume commodity chemicals.1–3 However, its synthesis via the Haber-Bosch process contributes significantly to global carbon emissions, motivating the development of alternative pathways for ammonia synthesis.4–6 Of many alternative methods, an electrochemical lithium-mediated process is one of the most promising. Since its initial implementations,7–9 novel cell designs10–12 and electrolytes13–15 have greatly improved the selectivity and productivity of lithium-mediated electrochemical ammonia synthesis (LiMEAS), but questions persist regarding the mechanism of nitrogen fixation and the role of lithium in this process (Figure 1a). In particular, many in the literature have alluded to the possible importance of lithium passivation species, commonly known as the solid electrolyte interphase (SEI), but their small length-scale and reactivity make direct observation challenging.11,13,15 In this work, we leveraged a multiscale approach to reveal connections between device-scale performance and nanoscale passivation behavior in LiMEAS. We used four model systems, varying the presence of proton donor (no proton donor or 0.1 M ethanol), and the feed gas (Ar or N2), which allowed us to examine the influence of the two reactive components in LiMEAS which are not usually present for lithium metal electrodeposition in the literature: proton donor and nitrogen. For each model system, we quantified major reaction products (Figure 1b, f). Ammonia and lithium nitride were measured by the salicylate colorimetric assay,16 hydrogen by online gas chromatography (GC), and residual lithium by galvanostatic stripping. We used scanning electron microscopy to observe microscale morphology, and to gain deeper insights into nanoscale surface morphology, we implemented cryo-electron microscopy (cryo-EM) (Figure 1c-d, g-h). Cryo-EM is a powerful technique that enables high-resolution observation of beam-sensitive metallic lithium and SEI materials by holding them at cryogenic temperatures, which preserve their native state.17 In addition to nanoscale morphological information, cryo-EM provided chemical information in the form of selected-area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS), which were paired with X-ray photoelectron spectroscopy (XPS) to develop an understanding of the chemistry of the interphase.Our results indicate that the proton donor governs reactivity toward nitrogen reduction in LiMEAS via its influence on the SEI. In the absence of proton donor, the lithium deposit forms a passivating, mosaic-structured SEI that preserves the deposited lithium, regardless of whether the feed gas is argon or nitrogen (Figure 1e). With the addition of ethanol, the surface changes significantly. The resultant SEI does not passivate the lithium, allowing it to react with nitrogen and other electrolyte components (Figure 1i). This permeable SEI appears to be mosaic in structure, with organic phases dominated by ethanol breakdown products, and its formation is key to the functionality of lithium-mediated ammonia synthesis. These findings can help guide the development of optimal SEIs for selectivity and stability of LiMEAS, and beyond that, offer insights into the characteristics of the lithium SEI at a reactive rather than passivated interface. Figure 1. (a) Schematic of possible lithium reactions. (b) and (f) Faradaic efficiencies of quantified reaction products, with no proton donor (b) and with ethanol present (f). (c-d) and (g-h) Cryo-TEM images of material deposited with no proton donor (c-d), and with ethanol present (g-h). (e) and (i) schematics of surface reactivity, with no proton donor (e) and with ethanol present (i). References J. W. Erisman, M. A. Sutton, J. Galloway, Z. Klimont, and W. Winiwarter, Nat. Geosci., 1, 636–639 (2008).G. R. Maxwell, Synthetic Nitrogen Products, Kluwer Academic Publishers, New York, (2004).FAO, World fertilizer trends and outlook to 2022, (2019).J. G. Chen et al., Science, 360 (2018).G. Soloveichik, Nat. Catal., 2, 377–380 (2019).Z. J. Schiffer and K. Manthiram, Joule, 1, 10–14 (2017).F. Fichter, P. Girard, and H. Erlenmeyer, Helv. Chim. Acta, 13, 1228–1236 (1930).A. Tsuneto, A. Kudo, and T. Sakata, Chem. Lett., 22, 851–854 (1993).A. Tsuneto, A. Kudo, and T. Sakata, J. Electroanal. Chem., 367, 183–188 (1994).N. Lazouski, M. Chung, K. Williams, M. L. Gala, and K. Manthiram, Nat. Catal., 3, 463–469 (2020).S. Z. Andersen et al., Energy Environ. Sci, 13, 4291–4300 (2020).K. Li et al., ACS Energy Lett., 30, 35 (2021).N. Lazouski et al., ACS Catal., 12, 5197–5208 (2022).B. H. R. Suryanto et al., Science, 372, 1187–1191 (2021).K. Li et al., Science, 374, 1593–1597 (2021).H. Verdouw, J. A. Van Echteld, and E. M. L. Dekkers, Water Res., 12, 399–402 (1977).Y. Li et al., Science, 358, 506–510 (2017). Figure 1

Role and Control of Solid-Electrolyte Interphase in Lithium-Mediated Electrochemical Ammonia Synthesis

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.

Enhancement of Li-Mediated Electrochemical Ammonia Synthesis By Modifying Main Element of Ylide Proton Shuttle

Ammonia is an essential chemical used various fields including agriculture and chemical engineering. It is also considered as one of the candidates for carbon-free hydrogen storage. Electrochemical nitrogen reduction reaction (ENRR) has been studied as an alternative to current energy-intensive and greenhouse gas emitting Habor-Bosch process. Lithium mediated nitrogen reduction reaction (Li-NRR), in which organic compounds replace water as the proton shuttle, is currently considered as the most promising route in terms of ammonia production rate. However, ethanol (EtOH), acommon proton shuttle in Li-NRR, has a limited electrochemical stability because it is prone to oxidation at the anode or can be consumed by forming byproducts at the cathode. Therefore, it is highly desirable to search for alternative proton shuttles. In this study, two different ylides—namely, tetrabutylphosphonium chloride (TBPCl) and tetrabutylammonium chloride (TBNCl)—were selected as the proton shuttle in place of EtOH. Both compounds showed spontaneous reaction with lithium nitride by forming ammonia. Both TBPCl and TBNCl showed a higher ammonia production rate and faradaic efficiency than EtOH. Interestingly, tetrabutylammonium chloride showed higher ammonia production rate than tetrabutylphosphonium chloride which may originate from the difference of the acidity. This work demonstrates the possibility of using ylides as an effective proton shuttle and points out the important role of the center atom of the ylides in Li-NRR.

Recent Advances in Designing Efficient Electrocatalysts for Electrochemical Nitrate Reduction to Ammonia

As a raw material for producing chemical fertilizers, ammonia plays an essential role in human production and life. Due to the severe energy consumption and pollution caused by the industrial Haber–Bosch process of NH3 synthesis, developing the NH3 synthesis reaction under ambient conditions is vital. Electrochemical nitrogen reduction reaction (NRR) has recently emerged as a potential method. However, its limited NH3 yield and selectivity are unsatisfactory. NO3 −, as an oxidized form of nitrogen, universally exists in drinking water (<50 mg L−1) and industrial wastewater (2000‐4000 mg L−1). Electrochemical nitrate reduction reaction (NO3 −RR), with higher production and Faradaic efficiency, is a promising strategy for water treatment and NH3 production. In this review, a detailed overview of the recent progress in NO3 −RR for NH3 production with precious group metal (PGM) electrocatalysts, PGM‐free electrocatalysts, and nonmetal electrocatalysts is summarized. In addition, effective design strategies for efficient electrocatalysts, existing challenges, and research prospects for the next stage are also discussed. This review may provide some directions for improving efficient electrocatalysts in electrocatalytic NO3 −RR and offer inspiration for the electrochemical ammonia synthesis process.

Role of the Membrane Transport Mechanism in Electrochemical Nitrogen Reduction Experiments.

The electrochemical synthesis of ammonia through the nitrogen reduction reaction (NRR) is receiving much attention, since it is considered a promising alternative to the Haber–Bosch process. In NRR experiments, a Nafion membrane is generally adopted as a separator. However, its use is controversial since ammonia can be trapped in the membrane, to some extent, or even pass through it. We systematically investigate the interaction of a Nafion membrane with ammonia and with an electrolyte and compare it with Zirfon as a possible alternative separator. We show that Nafion containing ammonia can easily release it when immersed in a 0.1 M Na2SO4 ammonia-free electrolyte, due to the cation exchange mechanism (Na+-NH4+). Since Na2SO4 is a commonly adopted electrolyte for NRR experiments, this may cause serious measurement errors and non-reproducible results. The same experiments performed using the polysulfone Zirfon separator clearly show that it is immune to interactions with ammonia, because of its different ion conduction mechanism. The findings provide a deeper understanding of the choice of membrane and electrolyte to be adopted for NRR tests, and may allow one to obtain more accurate and reliable results.

Electrosynthesis of ammonia with high selectivity and high rates via engineering of the solid-electrolyte interphase.

SummaryAmmonia is a large-scale commodity essential to fertilizer production, but the Haber-Bosch process leads to massive emissions of carbon dioxide. Electrochemical ammonia synthesis is an attractive alternative pathway, but the process is still limited by low ammonia production rate and faradaic efficiency. Herein, guided by our theoretical model, we present a highly efficient lithium-mediated process enabled by using different lithium salts, leading to the formation of a uniform solid-electrolyte interphase (SEI) layer on a porous copper electrode. The uniform lithium-fluoride-enriched SEI layer provides an ammonia production rate of 2.5 ± 0.1 μmol s−1 cmgeo−2 at a current density of −1 A cmgeo−2 with 71% ± 3% faradaic efficiency under 20 bar nitrogen. Experimental X-ray analysis reveals that the lithium tetrafluoroborate electrolyte induces the formation of a compact and uniform SEI layer, which facilitates homogeneous lithium plating, suppresses the undesired hydrogen evolution as well as electrolyte decomposition, and enhances the nitrogen reduction.

Sustainable ammonia synthesis: Just around the corner?

Li-mediated electrochemical synthesis of ammonia has been put forward as a possible alternative to the Haber-Bosch process, which is harmful to the environment and requires centralized operation. However, it has historically been plagued by low Faradaic efficiency, rate, and current density. In this issue of <i>Joule</i>, Chorkendorff and co-workers present a version of the process that meets certain industrial benchmarks and unveil new insights into the reaction kinetics along the way. Their observations are borne out by a recent study in <i>Nature</i> from MacFarlane and co-workers which also demonstrates extremely high Faradaic efficiency and high rates toward ammonia synthesis.

Tandem electrocatalytic N2 fixation via proton-coupled electron transfer.

New electrochemical ammonia (NH3) synthesis technologies are of interest as a complementary route to the Haber-Bosch process for distributed fertilizer generation, and towards exploiting ammonia as a zero-carbon fuel produced via renewably sourced electricity1. Apropos of these goals is a surge of fundamental research targeting heterogeneous materials as electrocatalysts for the nitrogen reduction reaction (N2RR)2. These systems generally suffer from poor stability and NH3 selectivity; the hydrogen evolution reaction (HER) outcompetes N2RR3. Molecular catalyst systems can be exquisitely tuned and offer an alternative strategy4, but progress has been thwarted by the same selectivity issue; HER dominates. Here we describe a tandem catalysis strategy that offers a solution to this puzzle. A molecular complex that can mediate an N2 reduction cycle is partnered with a co-catalyst that interfaces the electrode and an acid to mediate proton-coupled electron transfer steps, facilitating N-H bond formation at a favourable applied potential (-1.2 V versus Fc+/0) and overall thermodynamic efficiency. Certain intermediates of the N2RR cycle would be otherwise unreactive via uncoupled electron transfer or proton transfer steps. Structurally diverse complexes of several metals (W, Mo, Os, Fe) also mediate N2RR electrocatalysis at the same potential in the presence of the mediator, pointing to the generality of this tandem approach.

Efficient electrocatalytic nitrogen reduction to ammonia with FeNi-Co/carbon mat electrodes

Electrochemical nitrogen reduction reaction has recently received increasing attention due to its ability to synthesize ammonia under ambient conditions. However, the low ammonia yields and Faradaic efficiency hinder its further development, which necessitates the use of high-performance catalysts. Hence, in this study, a self-supporting carbon mat with FeNi-doped Co catalysts (Fe1Ni0.2Co145) fabricated by electrospinning was used as an electrode (FeNi-Co@CM) for electrochemical ammonia synthesis. The electrochemical performance of the FeNi-Co@CM electrode is evaluated in a three-electrode setup with FeNi-Co@CM as working electrode, a platinum mesh as counter electrode, and a Ag/AgCl electrode as reference electrode. The experimental investigations produce an ammonia yield rate of 27.9 μg h−1 mg−1 and a Faradaic efficiency of 1.52 % in 1.0 M KOH, which is higher than most of the reported data where iron-group-based electrocatalysts were employed. The verification results from multiple sets of control experiments indicated that the ammonia detected is generated by the nitrogen reduction reaction rather than the contaminants in the surrounding environment. In addition, the results of the stability test show that the FeNi-Co@CM electrode can maintain a high level of ammonia production for more than ten cycles. The working mechanism of nitrogen reduction reaction based on FeNi-Co@CM electrode is also discussed at various scales, including the electrode scale, the catalyst scale, and the atomic scale. This study therefore provides an in-depth understanding of the nitrogen reduction reaction from various perspectives, such as mass transport, reaction interfaces, and reaction intermediates.