Highly Selective Electrochemical Reduction of Dinitrogen to Ammonia at Ambient Temperature and Pressure over Iron Oxide Catalysts

Abstract

The catalytic conversion of dinitrogen (N ) into ammonia under ambient conditions represents one of the Holy Grails in sustainable chemistry. As a potential alternative to the Haber-Bosch process, the electrochemical reduction of N to NH is attractive owing to its renewability and flexibility, as well as its sustainability for producing and storing value-added chemicals from the abundant feedstock of water and nitrogen on earth. However, owing to the kinetically complex and energetically challenging N reduction reaction (NRR) process, NRR electrocatalysts with high catalytic activity and high selectivity are rare. In this contribution, as a proof-of-concept, we demonstrate that both the NH yield and faradaic efficiency (FE) under ambient conditions can be improved by modification of the hematite nanostructure surface. Introducing more oxygen vacancies to the hematite surface renders an improved performance in NRR, which leads to an average NH production rate of 0.46 μg h cm and an NH FE of 6.04 % at -0.9 V vs. Ag/AgCl in 0.10 m KOH electrolyte. The durability of the electrochemical system was also investigated. A surprisingly high average NH production rate of 1.45 μg h cm and a NH FE of 8.28 % were achieved after the first 1 h chronoamperometry test. This is among the highest FEs reported so far for non-precious-metal catalysts that use a polymer-electrolyte-membrane cell and is much higher than the FE of precious-metal catalysts (e.g., Ru/C) under comparable reaction conditions. However, the NH yield and the FE dropped to 0.29 μg h cm and 2.74 %, respectively, after 16 h of chronoamperometry tests, which indicates poor durability of the system. Our results demonstrate the important role that the surface states of transition-metal oxides have in promoting electrocatalytic NRR under ambient conditions. This work may spur interest towards the rational design of electrocatalysts as well as electrochemical systems for NRR, with emphasis on the issue of stability.

Oxygen vacancy enhancing mechanism of nitrogen reduction reaction property in Ru/TiO2

“…To further clarify the difference, the O 1 s spectrum of Ru/TiO 2 -Vo was fitted to three individual peaks representing different oxygen environment. Three different peaks located at around 530.1, 531.4 and 532.3 eV were assigned to the oxygen lattice (O L ), oxygen species in the vicinity of the oxygen vacancy (Vo) and the chemisorbed oxygen species (O C ), respectively [47,48] . Unfortunately, the O 1 s spectra of Ru/TiO 2 was only fitted to two peaks located at around 530.1 and 532.3 eV representing O L and O C , which was similar with the sample of Ru/P25 (Fig.…”

Current Progress of Electrocatalysts for Ammonia Synthesis Through Electrochemical Nitrogen Reduction Under Ambient Conditions

“…The Fe 2 O 3 -CNTs had ag ood electrochemical stability and could be used for at least 60 h. The data also indicated that the electrocatalytic syn-thesis of active sites in NH 3 mayb ea ssociated with specific carbon sites formed att he interface between Fe 2 O 3 and CNTs. Cui et al [118] proved that the a-Fe 2 O 3 /CNT catalyst with abundant oxygen vacancies could improvet he NH 3 yield andF Eo f N 2 reduction under ambient conditions. Electrochemical tests in 0.1 m KOH as the electrolyte as ap otential of À0.9 Vv s. Ag/ AgCl revealed an improved NRR performance.…”

Amorphous/Crystalline Hetero‐Phase TiO2‐Coated α‐Fe2O3 Core–Shell Nanospindles: A High‐Performance Artificial Nitrogen Fixation Electrocatalyst

“…The FEs of α‐Fe 2 O 3 @mTiO 2 ‐400 for NH 3 formation are shown in Figure e. The FEs are 9.9 %, 13.3 %, 9.5 %, 2.1 %, and 1.1 % at −0.4, −0.5, −0.6, −0.7, and −0.8 V, respectively. In addition, it is noteworthy that the FE (13.3 %) at −0.5 V is better than the catalytic activity of that of most reported aqueous‐ and Fe‐ based NRR electrocatalysts under ambient temperature and pressure conditions, including γ‐Fe 2 O 3 ‐NC/CF (12.28 %), Fe/Fe 3 O 4 (8.29 %), Fe 2 O 3 /CNT (0.164 %), Fe 2 O 3 ‐Ar/CNT (8.28 %) . Notably, the competition between the NRR and the HER lead to the yield rate and FE of NH 3 value are decreased with potential being more negative (<‐0.5 V) .…”