Catalyst-free, highly selective synthesis of ammonia from nitrogen and water by a plasma electrolytic system

Abstract

There is a growing need for scalable ammonia synthesis at ambient conditions that relies on renewable sources of energy and feedstocks to replace the Haber-Bosch process. Electrically driven approaches are an ideal strategy for the reduction of nitrogen to ammonia but, to date, have suffered from low selectivity associated with the catalyst. Here, we present a hybrid electrolytic system characterized by a gaseous plasma electrode that facilitates the study of ammonia formation in the absence of any material surface. We find record-high faradaic efficiency (up to 100%) for ammonia from nitrogen and water at atmospheric pressure and temperature with this system. Ammonia measurements under varying reaction conditions in combination with scavengers reveal that the unprecedented selectivity is achieved by solvated electrons produced at the plasma-water interface, which react favorably with protons to produce the key hydrogen radical intermediate. Our results demonstrate that limitations in selectivity can be circumvented by using catalyst-free solvated electron chemistry. In the absence of adsorption steps, the importance of controlling proton concentration and transport is also revealed.

Fig. 1. Catalyst-free, electrolytic NH

₃ production from N₂ and water using a plasma electrolytic system. (A) Schematic of the plasma electrolytic system operated by a dc power supply and galvanostatically controlled using a resistor (R) in series. The direction of electron flow (e⁻) is indicated. (B) Total NH₃ produced after 45 min at 6 mA and pH 3.5 for various gas configurations and controls. (C) Potentially important species contained in the plasma, such as vibrationally excited N₂ [N₂(v)], and in the water, such as solvated electrons [e⁻_(aq)], and their involvement in reactions, such as the generation of hydrogen radicals (H·), that lead to NH₃ formation. The overall reactions for N₂ reduction to NH₃ and H₂ evolution (under acidic conditions) at the cathode are shown.

Fig. 2. NH

₃ yield and efficiency in the plasma electrolytic system. (A) Total NH₃ produced and corresponding faradaic efficiency after different processing times at 6 mA and pH 3.5. No NH₃ is produced at 0 min based on the untreated electrolyte solution. (B) Total NH₃ produced and corresponding faradaic efficiency as a function of current after 45 min at pH 3.5. No NH₃ is produced at 0 mA based on the control experiment (see table S1A).

Fig. 3. Stability and trapping of NH

₃ in the plasma electrolytic system. (A) Comparison of total NH₃ produced in split-compartment versus single-compartment cells at 6 mA and pH 3.5 after different processing times. (B) Total NH₃ captured in the main reaction cell and a secondary trap vessel as a function of pH in the main reaction cell. The secondary trap was a strongly acidic H₂SO₄ bath (pH 2).

Fig. 4. Influence of NO

_x on NH₃ formation and in situ NO_x production in the plasma electrolytic system. (A) Potential scavenging reaction pathways of NO₃⁻ (green) and NO₂⁻ (purple) before and after solvation of electrons in water. (B) Comparison of total NH₃ produced after 45 min at 6 mA and pH 3.5 in the presence of NO₃⁻ and NO₂⁻ at 10 mM and 1 M concentrations. The total NH₃ produced for the same conditions in the absence of any scavenger is included for reference. (C) NO_x concentration measured after different processing times at 6 mA and pH 3.5. (D) NO_x concentration measured as a function of current after 45 min and pH 3.5.