Defect Engineering Strategies for Nitrogen Reduction Reactions under Ambient Conditions

Abstract

The production of ammonia from N2 molecules under ambient conditions [electro (photo) chemical reduction] is one of the most attractive topics in the energy‐related field due to its unique advantages and great potentials. Recently, various catalysts have been explored to show certain activities in nitrogen reduction reactions (NRRs) at room temperature and atmospheric pressure. To further improve the catalytic activity and increase the selectivity, the catalysts should be rationally designed to introduce extra active sites for the N2 molecule adsorption and activation. This review summarizes recent progress of the defect engineering strategies to design highly efficient electrochemical or photocatalytic NRR nanocatalysts. The defect sites would serve as the active center for the NRR and further enhance its intrinsic performance. The defect engineering strategies are summarized in different categories, including vacancies (oxygen, nitrogen, and sulfur vacancies), doping (metal doping and nonmetal doping), amorphous phases (amorphous noble metal and amorphous transition metal), size effects, and structure effects. In addition, the different ammonia determination methods are summarized and compared in order to obtain more credible and veritable NRR performance results.

Nitrogen‐Doped NiO Nanosheet Array for Boosted Electrocatalytic N2 Reduction

“…Figure 2c shows the UV-vis absorption spectra of the electrolytes (stained with indophenol indicator) after 2 h electrolysis on N-NiO/CC at potentials ranging from À 0.4 to À 0.8 V versus RHE, and obtained NH 3 yield and FEs are shown in Figure 2d. As clear, the highest NH 3 yield of 22.7 μg h À 1 mg À 1 and FE of 7.3 % are obtained at À 0.5 V, which are comparable or even superior to those of previously reported state-of-the-art electrocatalysts in aqueous electrolytes [48,49] . The detailed comparison is provided in Table S1 (Supporting Information).…”

“…The Mulliken charge analysis (Figure 3c) reveals that the N-NiO donates 0.22 e more electrons to *N 2 H relative to NiO. The increased electron density of *NNH is beneficial for promoting π back-bonding to weaken the NÀ N triple bond and strengthen the NiÀ N bond [49] . As a consequence, a considerably reduced Gibbs free energy of *N 2 H formation is achieved on N-NiO (0.46 eV) compared to 0.95 eV on NiO, demonstrating that the *N 2 H is energetically more favorable to be stabilized on N-NiO than on NiO.…”

Is It Appropriate to Use the Nafion Membrane in Electrocatalytic N2 Reduction?

“…[5] Nevertheless, this process is still in its infancy stage, thus rigorous and reliable protocols to evaluate the activity of the catalysts are necessary and highly concerned. [6][7][8] Also, recent reports have mentioned that the employed Nafion 2 µg mL −1 of ammonia-containing electrolyte for an example, this will lead to a measurement error of ≈30 wt%. Moreover, it was worth noting that the ammonia transferred from cathode cell may be electrooxidized at the anode owing to the high potential of the anode, and this possibility was investigated and revealed in a typical single cell schemed in Figure S7 of the Supporting Information.…”

Reactive Ionic Liquid Enables the Construction of 3D Rh Particles with Nanowire Subunits for Electrocatalytic Nitrogen Reduction

“…Protons are added to proximal N atom successively after the first NH 3 was formed. Considering no by‐product of N 2 H 4 was formed during the electrolysis process, the associative distal pathway could be suitable on 3D Rh particles as N 2 →*N 2 →*NNH→*NNH 2 →*NNH 3 →*N+NH 3 →*NH→*NH 2 →*NH 3 →NH 3 . The interconnected porous nanostructures of 3D Rh particles exposed more electrochemical active sites to absorb the N 2 molecule due to unsaturated coordination, and then an activated H in electrolyte preferentially combined with the distal N atom to break N≡N triple bond, followed by consecutive addition of two other protons to the N atom until one NH 3 was generated.…”

“…*NH 3 !NH 3 . [27] The interconnected porous nanostructures of 3D Rh particles exposed more electrochemical active sites to absorb the N 2 molecule due to unsaturated coordination, and then an activated H in electrolyte preferentially combined with the distal N atom to break N�N triple bond, followed by consecutive addition of two other protons to the N atom until one NH 3 was generated. After that, the remaining Rh nitrido (Rh�N) bond will occur successive hydrogenation process to form another NH 3 molecule.…”