Recent Advances and Challenges of Electrocatalytic N₂Reduction to Ammonia

Abstract

Global ammonia production reached 175 million metric tons in 2016, 90% of which is produced from high purity N2 and H2 gases at high temperatures and pressures via the Haber–Bosch process. Reliance on natural gas for H2 production results in large energy consumption and CO2 emissions. Concerns of human-induced climate change are spurring an international scientific effort to explore new approaches to ammonia production and reduce its carbon footprint. Electrocatalytic N2 reduction to ammonia is an attractive alternative that can potentially enable ammonia synthesis under milder conditions in small-scale, distributed, and on-site electrolysis cells powered by renewable electricity generated from solar or wind sources. This review provides a comprehensive account of theoretical and experimental studies on electrochemical nitrogen fixation with a focus on the low selectivity for reduction of N2 to ammonia versus protons to H2. A detailed introduction to ammonia detection methods and the execution of control experiments is given as they are crucial to the accurate reporting of experimental findings. The main part of this review focuses on theoretical and experimental progress that has been achieved under a range of conditions. Finally, comments on current challenges and potential opportunities in this field are provided.

In Situ Fragmented Bismuth Nanoparticles for Electrocatalytic Nitrogen Reduction

“…Compared to the Bi‐MOF precatalyst, the in situ obtained Bi NPs exhibit a much smaller charge‐transfer resistance ( R ct = 18.58 Ω), which is beneficial for the electrocatalytic NRR process. [ 46–48 ]…”

Metal–Sulfur Linkages Achieved by Organic Tethering of Ruthenium Nanocrystals for Enhanced Electrochemical Nitrogen Reduction

“…Owing to its high gravimetric hydrogen content and energy density, it is also considered as a promising hydrogen carrier. [1,2] The Haber-Bosch (HB) process has been producing NH 3 for more than 100 years, which consumes energy and emits carbon dioxide in vast amounts. Therefore, it is highly desirable to develop a clean, facile, and sustainable alternative to this process.…”

An Overview on Noble Metal (Group VIII)‐based Heterogeneous Electrocatalysts for Nitrogen Reduction Reaction

“…To date, the mechanisms of nitrogen reduction have been mainly divided into three types: dissociative, associative, and enzymatic pathways (Figure 3c–e) [1a,c,8–9,18,25] . The N≡N bond is fully broken prior to any hydrogenation process, leaving a single N adsorption atom to hydrogenate and further change into NH 3 in the dissociation mechanism.…”

“…N atoms in N 2 molecules can offer lone‐pair electrons to unoccupied d orbitals in the noble metals (Group VIII) through the sp hybridization of N atoms. The occupied d orbitals in the metals can donate electrons in reverse to the π‐ molecular orbitals of the dinitrogen [8] . This process reduces the N≡N bond and strengthens the bond of metal‐nitrogen.…”