skip to main content


Title: Mechanistic understanding of electrochemical nitrogen reduction reaction on hybrid plasmonic nanostructures using operando surface-enhanced Raman spectroscopy
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
Award ID(s):
1904351
NSF-PAR ID:
10237301
Author(s) / Creator(s):
;
Date Published:
Journal Name:
ACS Spring Meeting 2021
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Clean and efficient energy storage and conversion via sustainable water and nitrogen reactions have attracted substantial attention to address the energy and environmental issues due to the overwhelming use of fossil fuels. These electrochemical reactions are crucial for desirable clean energy technologies, including advanced water electrolyzers, hydrogen fuel cells, and ammonia electrosynthesis and utilization. Their sluggish reaction kinetics lead to inefficient energy conversion. Innovative electrocatalysis, i.e., catalysis at the interface between the electrode and electrolyte to facilitate charge transfer and mass transport, plays a vital role in boosting energy conversion efficiency and providing sufficient performance and durability for these energy technologies. Herein, a comprehensive review on recent progress, achievements, and remaining challenges for these electrocatalysis processes related to water (i.e., oxygen evolution reaction, OER, and oxygen reduction reaction, ORR) and nitrogen (i.e., nitrogen reduction reaction, NRR, for ammonia synthesis and ammonia oxidation reaction, AOR, for energy utilization) is provided. Catalysts, electrolytes, and interfaces between the two within electrodes for these electrocatalysis processes are discussed. The primary emphasis is device performance of OER‐related proton exchange membrane (PEM) electrolyzers, ORR‐related PEM fuel cells, NRR‐driven ammonia electrosynthesis from water and nitrogen, and AOR‐related direct ammonia fuel cells.

     
    more » « less
  2. The production of ammonia for agricultural and energy demands has accelerated research for more environmentally-friendly synthesis options, particularly the electrocatalytic reduction of molecular nitrogen (nitrogen reduction reaction, NRR). Catalyst activity for NRR, and selectivity for NRR over the competitive hydrogen evolution reaction (HER), are critical issues for which fundamental knowledge remains scarce. Herein, we present results regarding the NRR activity and selectivity of sputter-deposited titanium nitride and titanium oxynitride films for NRR and HER. Electrochemical, fluorescence and UV absorption measurements show that titanium oxynitride exhibits NRR activity under acidic conditions (pH 1.6, 3.2) but is inactive at pH 7. Ti oxynitride is HER inactive at all these pH values. In contrast, TiN – with no oxygen content upon deposition – is both NRR and HER inactive at all the above pH values. This difference in oxynitride/nitride reactivity is observed despite the fact that both films exhibit very similar surface chemical compositions – predominantly Ti IV oxide – upon exposure to ambient, as determined by ex situ X-ray photoelectron spectroscopy (XPS). XPS, with in situ transfer between electrochemical and UHV environments, however, demonstrates that this Ti IV oxide top layer is unstable under acidic conditions, but stable at pH 7, explaining the inactivity of titanium oxynitride at this pH. The inactivity of TiN at acidic and neutral pH is explained by DFT-based calculations showing that N 2 adsorption at N-ligated Ti centers is energetically significantly less favorable than at O-ligated centers. These calculations also predict that N 2 will not bind to Ti IV centers due to a lack of π-backbonding. Ex situ XPS measurements and electrochemical probe measurements at pH 3.2 demonstrate that Ti oxynitride films undergo gradual dissolution under NRR conditions. The present results demonstrate that the long-term catalyst stability and maintenance of metal cations in intermediate oxidation states for pi-backbonding are critical issues worthy of further examination. 
    more » « less
  3. null (Ed.)
    Ammonia holds great promise as a carbon-neutral liquid fuel for storing intermittent renewable energy sources and power generation due to its high energy density and hydrogen content. Photo-Electrochemical Ammonia Synthesis: Nanocatalyst Discovery, Reactor Design, and Advanced Spectroscopy covers the synthesis of novel hybrid plasmonic nanomaterials and their application in photo-electrochemical systems to convert low energy molecules to high value-added molecules and looks specifically at photo-electrochemical nitrogen reduction reaction (NRR) for ammonia synthesis as an attractive alternative to the long-lasting thermochemical process. - Provides an integrated scientific framework, combining materials chemistry, photo-electrochemistry, and spectroscopy to overcome the challenges associated with renewable energy storage and transport - Reviews materials chemistry for the synthesis of a range of heterogeneous (photo) electrocatalysts including plasmonic and hybrid plasmonic-semiconductor nanostructures for selective and efficient conversion of N2 to NH3 - Covers novel reactor design to study the redox processes in the photo-electrochemical energy conversion system and to benchmark nanocatalysts’ selectivity and activity toward NRR - Discusses the use of advanced spectroscopic techniques to probe the reaction mechanism for ammonia synthesis - Offers techno-economic analysis and presents performance targets for the scale-up and commercialization of electrochemical ammonia synthesis This book is of value to researchers, advanced students, and industry professionals working in sustainable energy storage and conversion across the disciplines of Chemical Engineering, Mechanical Engineering, Materials Science and Engineering, Environmental Engineering, and related areas. 
    more » « less
  4. Abstract

    Ammonia (NH3) electrosynthesis gains significant attention as NH3is essentially important for fertilizer production and fuel utilization. However, electrochemical nitrogen reduction reaction (NRR) remains a great challenge because of low activity and poor selectivity. Herein, a new class of atomically dispersed Ni site electrocatalyst is reported, which exhibits the optimal NH3yield of 115 µg cm−2h−1at –0.8 V versus reversible hydrogen electrode (RHE) under neutral conditions. High faradic efficiency of 21 ± 1.9% is achieved at ‐0.2 V versus RHE under alkaline conditions, although the ammonia yield is lower. The Ni sites are stabilized with nitrogen, which is verified by advanced X‐ray absorption spectroscopy and electron microscopy. Density functional theory calculations provide insightful understanding on the possible structure of active sites, relevant reaction pathways, and confirm that the Ni‐N3sites are responsible for the experimentally observed activity and selectivity. Extensive controls strongly suggest that the atomically dispersed NiN3site‐rich catalyst provides more intrinsically active sites than those in N‐doped carbon, instead of possible environmental contamination. This work further indicates that single‐metal site catalysts with optimal nitrogen coordination is very promising for NRR and indeed improves the scaling relationship of transition metals.

     
    more » « less
  5. The electrocatalytic nitrogen reduction reaction (NRR) is of significant interest as an environmentally friendly method for NH 3 production for agricultural and clean energy applications. Selectivity of NRR vis-à-vis the hydrogen evolution reaction (HER), however, is thought to adversely impact many potential catalysts, including Earth-abundant transition metal oxynitrides. Relative HER/NRR selectivities are therefore directly compared for two transition metal oxynitrides with different metal oxophilicities—Co and V. Electrocatalytic current–potential measurements, operando fluorescence, absorption, and GC measurements of H 2 and NH 3 production, ex situ X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations are combined to directly compare NRR and HER activities under identical reaction conditions. Results show that cobalt oxynitrides – with Co primarily in the Co( ii ) oxidation state – are NRR active at pH 10, with electrochemical reduction of both lattice nitrogen and dissolved N 2 , the latter occurring without N incorporation into the lattice. Removal of lattice N then yields Co( ii ) oxide, which is still NRR active. These results are complemented by calculations showing that N 2 binding at Co( ii ) sites is energetically favored over binding at Co( iii ) sites. GC analysis demonstrates that H 2 production occurs in concert with ammonia production but at a far greater rate. In contrast, vanadium oxynitride films are HER inactive under the same (pH 10) conditions, as well as at pH 7, but are NRR active at pH 7. DFT calculations indicate that a major difference in the two materials is hindered O–H dissociation of H 2 O adsorbed at O-ligated Co vs. V cation centers. The combined studies indicate significant variation in HER vs. NRR selectivity as a function of employed transition metal oxynitrides, as well as different HER mechanisms in V and Co oxynitrides. 
    more » « less