An official website of the United States government
Abstract Recently, lithium‐mediated nitrogen reduction reaction (Li‐NRR) in nonaqueous electrolytes has proven to be an environmentally friendly and feasible route for ammonia electrosynthesis, revealing tremendous economic and social advantages over the industrial Haber‐Bosch process which consumes enormous fossil fuels and generates massive carbon dioxide emissions, and direct electrocatalytic nitrogen reduction reaction (NRR) which suffers from sluggish kinetics and poor faradaic efficiencies. However, reaction mechanisms of Li‐NRR and the role of solid electrolyte interface (SEI) layer in activating N2remain unclear, impeding its further development. Here, using electronic structure theory, we discover a nitridation‐coupled reduction mechanism and a nitrogen cycling reduction mechanism on lithium and lithium nitride surfaces, respectively, which are major components of SEI in experimental characterization. Our work reveals divergent pathways in Li‐NRR from conventional direct electrocatalytic NRR, highlights the role of surface reconstruction in improving reactivity, and sheds light on further enhancing efficiency of ammonia electrosynthesis.
more »
« less
Alternative ammonia production processes and the use of renewables
The Haber–Bosch synthesis of ammonia is an energy-intensive process that uses coal or natural gas as a fuel and feed. Direct electrochemical nitrogen reduction represents a potential alternative to the Haber–Bosch process that can be less polluting. This alternative route to ammonia from dinitrogen is not likely to require the same large capital investments as does the Haber–Bosch process, thus suggesting a distributive production structure of ammonia relative to the existing ammonia industry. In addition, the flexibility borne from the use of electrochemistry yields technologies that are better fit for the use of renewable energy sources that supply intermittent electricity. We show that under certain scenarios, at levels of efficiency (as determined by the required overpotential and the Faradaic efficiency) that might reasonably be achieved, direct electrochemical nitrogen reduction would be a sustainable and economically viable alternative to the Haber–Bosch process.
more »
« less
- Award ID(s):
- 1955014
- PAR ID:
- 10347839
- Editor(s):
- Murthy, G. S.; Gnansounou, E.; Khanal, S. K.; Pandey, A.
- Date Published:
- Journal Name:
- Biomass, Biofuels, Biochemicals
- Page Range / eLocation ID:
- 243-258
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Electro- and photocatalytic reduction of N 2 to NH 3 —the nitrogen reduction reaction (NRR)—is an environmentally- and energy-friendly alternative to the Haber-Bosch process for ammonia production. There is a great demand for the development of novel semiconductor-based electrocatalysts with high efficiency and stability for the direct conversion of inert substrates—including N 2 to ammonia—using visible light irradiation under ambient conditions. Herein we report electro-, and photocatalytic NRR with transition metal dichalcogenides (TMDCs), viz MoS 2 and WS 2 . Improved acid treatment of bulk TMDCs yields exfoliated TMDCs (exTMDCs) only a few layers thick with ∼10% S vacancies. Linear scan voltammograms on exMoS 2 and exWS 2 electrodes reveal significant NRR activity for exTMDC-modified electrodes, which is greatly enhanced by visible light illumination. Spectral measurements confirm ammonia as the main reaction product of electrocatalytic and photocatalytic NRR, and the absence of hydrazine byproduct. Femtosecond-resolved transient absorption studies provide direct evidence of interaction between photo-generated excitons/trions with N 2 adsorbed at S vacancies. DFT calculations corroborate N 2 binding to exMoS 2 at S-vacancies, with substantial π -backbonding to activate dinitrogen. Our findings suggest that chemically functionalized exTMDC materials could fulfill the need for highly-desired, inexpensive catalysts for the sustainable production of NH 3 using Sunlight under neutral pH conditions without appreciable competing production of H 2 .more » « less
-
Electrochemical nitrogen reduction reaction (NRR) for ammonia synthesis might offer an alternative means to the capital- and carbon-intensive thermochemical process (Haber-Bosch) in a clean, sustainable, and decentralized way if the process is coupled to renewable electricity sources. One of the challenges in electrochemical ammonia synthesis is finding catalysts with a suitable activity for breaking N2 triple bonds at or near ambient conditions. Improving the design of electrocatalysts, electrolytes, and electrochemical cells is required to overcome the selectivity and activity barrier in electrochemical NRR. In-situ and operando surface-enhanced Raman spectroscopy (SERS) is a well-suited technique to probe electrochemical reactions at the solid-liquid (electrode/electrolyte) interface. Operando SERS allows for the detection of intermediate species even in low abundance and is used to provide insights into NRR mechanisms using hybrid plasmonic nanostructures (e.g., Au-Pd) by combining spectroscopy and electrochemistry. A potentiostat is used to apply potential on a SERS active substrate that is then monitored by changes in a spectrum. The spectroelectrochemical cell is developed to operando probe the trace of NH3 and possible intermediate species produced at the electrode/electrolyte interface. This work would aid in understanding the reaction mechanism and ultimately designing more efficient catalysts for electrochemical energy conversion systems. This material is based upon work supported by the National Science Foundation under grant no. 1904351.more » « less
-
Artificial ammonia synthesis is vital to modern life; however, the Haber-Bosch process, by which most ammonia is synthesized, is capital and carbon intensive. Zero-valent-metal-mediated ammonia synthesis is a promising alternative but requires a metal that is both a strong reductant and forms a stable nitride. Only a small number of metals, like lithium, can satisfy these constraints. Therefore, we developed an electrochemical paradigm enabling the use of different reductants by orthogonalizing the roles of the zero-valent metal between sodium metal and a Ti active site. These components are cheaper than lithium by two orders of magnitude. Using a sodium-naphthalene-titanium cascade, we achieved a rate of 475 nmol cm-2 s-1 and a Faradaic efficiency of 24% and found that the reaction rate depends primarily on current density.more » « less
-
In light of the enormous energy footprint of the Haber–Bosch process (1–2% of global energy consumption), alternative green routes of generating ammonia (NH 3 ) are needed. The electrochemical reduction of NO 3 − from waste streams is a promising method to produce NH 3 using renewably-sourced electricity. However, catalyst selectivity is a grand challenge that hinders NO 3 − to NH 3 conversion technologies. In this manuscript, we fabricate Nafion-modified metal catalysts for NO 3 − reduction. Although Nafion composites are commonly used to facilitate proton transfer, this work investigates electrodes covered by Nafion overlayers, which possess unique reactivity. We find that Cu versions of these catalysts reduce NO 3 − to NH 3 with a faradaic efficiency of up to (91 ± 2)%, making them among the most selective catalysts reported. Voltammetry studies, surface-enhanced Raman spectroscopy, and density functional theory calculations indicate that the Nafion overlayer activates the N–O bond of a key Cu–NO intermediate, thus facilitating NH 3 production. Lastly, we demonstrate that these catalysts are effective at denitrifying polluted groundwater samples in the field.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1016/B978-0-12-819242-9.00007-5
