Salting-out effect promoting highly efficient ambient ammonia synthesis

Abstract

The electroreduction of nitrogen to ammonia offers a promising alternative to the energy-intensive Haber–Bosch process. Unfortunately, the reaction suffers from low activity and selectivity, owing to competing hydrogen evolution and the poor accessibility of nitrogen to the electrocatalyst. Here, we report that deliberately triggering a salting-out effect in a highly concentrated electrolyte can simultaneously tackle the above challenges and achieve highly efficient ammonia synthesis. The solute ions exhibit strong affinity for the surrounding H2O molecules, forming a hydration shell and limiting their efficacy as both proton sources and solvents. This not only effectively suppresses hydrogen evolution but also ensures considerable nitrogen flux at the reaction interface via heterogeneous nucleation of the precipitate, thus facilitating the subsequent reduction process in terms of both selectivity and activity. As expected, even when assembled with a metal-free electrocatalyst, a high Faradaic efficiency of 71 ± 1.9% is achieved with this proof-of-concept system.

Recent advances in material design and reactor engineering for electrocatalytic ambient nitrogen fixation

“…Our group emphasized the importance of the salting-out effect to achieve an efficient NRR process and proposed a successful strategy to promote the NRR selectivity and activity under ambient conditions. 248 We verified this phenomenon from a more macroscopic point of view-finite element simulation. Pure N 2 was continuously injected into the system at a stable flow rate of 30 sccm, and the 2D contour map of N 2 volume fraction after 100 s was extracted to evaluate its accumulation on the surface of the electrocatalyst.…”

Unveiling the Synergy of O‐Vacancy and Heterostructure over MoO3‐x/MXene for N2 Electroreduction to NH3

“…Before the NRR tests, all feeding gases are purified through an acid trap (0.05 M H 2 SO 4 ) and an alkaline trap (0.1 M KOH)) to remove any possible NH 3 /NO x contaminants (Figure S12, Supporting Information). [ 50 ] The linear sweep voltammetry (LSV, Figure S13, Supporting Information) curve of MoO 3‐ x /MXene in N 2 ‐saturated electrolyte presents a noticeably high current density relative to that in Ar‐saturated solution, confirming the feasibility of MoO 3‐ x /MXene for N 2 electroreduction. Prior to quantitative NRR evaluation, 1 H isotopic labeling nuclear magnetic resonance (NMR) experiments are first conducted to track the N‐source of yielded NH 3 by the NRR.…”

“…As shown in Figure 5f,g (Figure S34, S35, Supporting Information), it is clear that OV‐rich MoO 3‐ x and MoO 3‐ x /MXene show much higher g ( r ) values than their corresponding OV‐poor MoO 3 and MoO 3 /MXene, implying a stronger N 2 affinity to OV‐containing catalyst systems. [ 50 ] Consequently, a high N 2 accumulation around catalyst can greatly facilitate the N 2 diffusion and chemisorption toward the active sites, which is favorable for the expedited NRR activity.…”

Interfacial Microextraction Boosting Nitrogen Feed for Efficient Ambient Ammonia Synthesis in Aqueous Electrolyte

“…Then we performed isotope-labeled quantification experiments, the detailed methods are presented in Experimental Section. The calibration curve for integral area ratio of the 1 H NMR peak with 14 S24 and S25, Supporting Information). The results demonstrate that the obtained ammonia was produced from the NRR exclusively and both of yield rate and FE is comparable with that detected by the indophenol blue method (Figure 5f).…”

“…[10][11][12] Instead, most electrons and protons tend to go toward hydrogen evolution, and the enrichment and overflow of H 2 can further limit the N 2 available in aqueous solutions for catalysts, which would severely lower the efficiency of NRR. [13][14][15][16] Hence, the extremely low solubility of N 2 in aqueous electrolytes, especially under ambient conditions, is an essential bottleneck to further enhance the ammonia production rate and NRR selectivity. [17][18][19] In order to address the above obstacles, choosing non-aqueous solutions with better N 2 dissolution capacity seems a good breakthrough to improve the efficiency of NRR.…”