Electrochemical Generation of Catalytically Active Edge Sites in C₂N‐Type Carbon Materials for Artificial Nitrogen Fixation

Abstract

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH3) is a potentially carbon‐neutral and decentralized supplement to the established Haber–Bosch process. Catalytic activation of the highly stable dinitrogen molecules remains a great challenge. Especially metal‐free nitrogen‐doped carbon catalysts do not often reach the desired selectivity and ammonia production rates due to their low concentration of NRR active sites and possible instability of heteroatoms under electrochemical potential, which can even contribute to false positive results. In this context, the electrochemical activation of nitrogen‐doped carbon electrocatalysts is an attractive, but not yet established method to create NRR catalytic sites. Herein, a metal‐free C2N material (HAT‐700) is electrochemically etched prior to application in NRR to form active edge‐sites originating from the removal of terminal nitrile groups. Resulting activated metal‐free HAT‐700‐A shows remarkable catalytic activity in electrochemical nitrogen fixation with a maximum Faradaic efficiency of 11.4% and NH3 yield of 5.86 µg mg−1cat h−1. Experimental results and theoretical calculations are combined, and it is proposed that carbon radicals formed during activation together with adjacent pyridinic nitrogen atoms play a crucial role in nitrogen adsorption and activation. The results demonstrate the possibility to create catalytically active sites on purpose by etching labile functional groups prior to NRR.

Strategies to achieve effective nitrogen activation

“…Although abundant works reported the effects of electrolyte ions and pH on NRR activity and selectivity, the specific mechanism of the electrolyte effects still needs to be clarified and unified. [157][158][159][160] It is theoretically foreseeable 153 that the catalyst exhibits high NRR performance in acidic solutions. The abundant protons not only enhance HER but also provide sufficient proton sources for NRR.…”

Boosting the Faraday Efficiency of Electrochemical Ammonia Synthesis via the Strain Effect Induced by Interfacial Hybrid Formation between BN and Carbon Nanotubes

“…The carbon-based materials themselves have no active sites, so it is necessary to "create" catalytic active sites according to different needs in order to be applied in various fields. 41 Different from the C atom in carbon-based materials, boron (B) has a promising role in eNRR processes due to its Lewis acid characteristics. When the B atom exists as an sp2 hybrid, it has an empty orbital to accept the lone pair of foreign electrons, so the σ-bond electron density of the N 2 molecule could be well accepted by the B active site.…”

“…It is worth noting that the atoms on each surface of metal-based materials can serve as active centers, which is theoretically significantly different from the case of carbon materials. The carbon-based materials themselves have no active sites, so it is necessary to “create” catalytic active sites according to different needs in order to be applied in various fields …”

Metal-Free Light-Driven 1,4-NADH Regeneration System from Nitrogen and Sulfur Codoped Carbon Dots

“…Nitrogen-doped (N-doped) carbon materials, a typical metal-free catalyst, have been extensively used in catalytic hydrogenation fields such as CO 2 reduction, N 2 fixation, and aldehyde reduction. − Constructing metal-free systems based on N-doped carbon materials to directly regenerate NAD­(P)H has aroused interest, while the resulting products are mostly enzymatically inactive isomers. − As was reported, the N element in N-doped carbon materials usually serves as the active site, whereas the product selectivity is closely related to the N species. , Particularly, pyrrolic N is, in theory, capable of catalyzing proton-coupled-electron transfer reactions, a major process in NAD­(P)H regeneration. − However, pyrrolic N usually occupies a small proportion of N-doped carbon materials. Strategies to synthesize N-doped carbon materials with a high proportion of pyrrolic N for direct 1,4-NAD­(P)H regeneration are still lacking but are well worth exploring.…”