Entropy-Driven Direct Air Electrofixation.

Area of Science:

• Materials Science
• Electrochemistry
• Catalysis
• Chemical Thermodynamics

Background:

• Catalytic activation of small molecules like N2 and CO2 typically shows negative dependence on O2 due to competitive oxygen reduction reaction (ORR).
• Nitrogen electrofixation is a key route for ammonia production under ambient conditions, but its efficiency is often limited by O2 presence.
• Traditional chemical thermodynamics predicts challenges in O2-involved catalytic processes for small molecule activation.

Purpose of the Study:

• To investigate the positive activity dependence on O2 for nitrogen electrofixation using a novel high-entropy catalyst.
• To elucidate the mechanism behind O2-enhanced N2 activation and ammonia synthesis.
• To demonstrate the potential of high-entropy materials in overcoming traditional thermodynamic constraints in catalysis.

Main Methods:

• Synthesis and characterization of the high-entropy (Ni0.20Co0.20Fe0.20Mn0.19Mo0.21)3S4 catalyst.
• Experimental studies on nitrogen electrofixation in N2/O2 mixtures.
• Theoretical calculations to understand the role of high entropy and O2 adsorption in N2 activation and ORR.

Main Results:

• The high-entropy catalyst exhibited a positive activity dependence on O2 for N2 electrofixation, activating N2 by elongating its triple bond.
• High entropy was found to attenuate the competitive ORR, showing a negative exponential entropy-ORR activity relationship.
• An ammonia yield rate of 47.70 μg h⁻¹ cm⁻² was achieved using direct air electrofixation, 1.5 times higher than with pure N2 feedstock.

Conclusions:

• The high-entropy mechanism in (NiCoFeMnMo)3S4 enables O2 to enhance, rather than inhibit, N2 electrofixation.
• This work demonstrates a new dimension of high entropy in catalysis, enabling systems to operate beyond traditional thermodynamic limitations.
• The findings pave the way for efficient direct air electrofixation of nitrogen for sustainable ammonia production.