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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.
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Managing the Nitrogen Cycle via Plasmonic (Photo)Electrocatalysis: Toward Circular Economy
As renewable energy sources are either intermittent in nature or remote in location, developingng cost-effective, sustainable, modular systems and technologies to store and transport renewables at an industrial scale is imperative. Storing cheap renewable electricity into chemical bonds (i.e., chemical energy storage) could be a transformative opportunity for reliable and resilient grid energy storage. This approach enables renewables to be stored and shipped similarly to fossil fuels. Currently, the chemical industry primarily consumes fossil feedstock as an energy source, which has been the standard for over a century. A paradigm shift is required to move toward a more sustainable route for chemical synthesis by electrifying and decarbonizing the modern chemical industry. As renewable electricity costs decrease, (photo)electrosynthesis is gaining interest for synthesizing high-value and high-energy fuels and molecules in a clean, sustainable, and decentralized manner. The nitrogen cycle is one of the Earth’s most critical biogeochemical cycles since nitrogen is a vital element for all living organisms. Artificial nitrogen fixation via a (photo)electrochemical system powered by renewables provides an alternative route to resource- and carbon-intensive thermochemical processes.(Photo)electrochemical nitrogen fixation at a large scale necessitates the discovery of active, selective, and stable heterogeneous (photo)electrocatalysts. In addition, the use of advanced in situ and operando spectroscopic techniques is needed to pinpoint the underlying reaction mechanisms. The selectivity of nitrogen (N2) molecules on the catalyst surface and suppressing thermodynamically favorable side reactions (e.g., hydrogen evolution reaction) are the main bottlenecks in improving the rate of (photo)electrochemical nitrogen fixation in aqueous solutions. The rational design of electrode, electrolyte, and reactors is required to weaken the strong nitrogen−nitrogen triple bond (NN) at or near ambient conditions. This Account covers our group’s recent advances in synthesizing shape-controlled hybrid plasmonic nanoparticles, including plasmonic−semiconductor and plasmonic−transition metal nanostructures with increased surface areas. The nanocatalysts’ selectivity and activity toward nitrogen conversion are benchmarked in liquid- and gas-phase electrochemical systems. We leverage operando vibrational-type spectroscopy (i.e., surfaceenhanced Raman spectroscopy (SERS)) to identify intermediate species relevant to nitrogen fixation at the electrode−electrolyte interface to gain mechanistic insights into reaction mechanisms, leading to the discovery of more efficient catalysts. Operando SERS revealed that the nitrogen reduction reaction (NRR) to ammonia on hybrid plasmonic−transition metal nanoparticle surfaces (e.g.,Pd−Ag) occurs through an associative mechanism. In the NRR process, hydrazine (N2H4) is consumed as an intermediate species. A femtosecond pulsed laser is used to synthesize hybrid plasmonic photocatalysts with homogeneously distributed Pd atoms on a Au nanorod surface, resulting in enhanced optoelectronic and catalytic properties. The overarching goal is to develop modular photoelectrochemical systems for long-duration renewable energy storage. In the context of nitrogen fixation, we aim to propose strategies to manage the nitrogen cycle through the interconversion of N2 and active nitrogen-containing compounds (e.g., NH3,NOx), enabling a circular nitrogen economy with sustainable and positive social and economic outcomes. The versatile approaches presented in this Account can inform future opportunities in (photo)electrochemical energy conversion systems and solar fuel-based applications.
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- Award ID(s):
- 1904351
- PAR ID:
- 10343560
- Editor(s):
- Burrows, Cynthia J.
- Date Published:
- Journal Name:
- Accounts of chemical research
- Volume:
- 54
- Issue:
- 23
- ISSN:
- 0001-4842
- Page Range / eLocation ID:
- 4294-4306
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/acs.accounts.1c00446
