Water increases Faradaic selectivity of Li-mediated nitrogen reduction.

Abstract

The lithium-mediated system catalyses nitrogen to ammonia under ambient conditions. Herein we discover that trace amount of water - in contrast to prior reports from the literature – can effect a dramatic improvement in the Faradaic selectivity of N2 reduction to NH3. We report an optimal water concentration of 35.5 mM and LiClO4 salt concentration of 0.8 M allows a Faradaic efficiency up to 27.7% at ambient pressure. We attribute the increase in Faradaic efficiency to the incorporation of Li2O in the solid electrolyte interphase, as suggested by our X-ray photoelectron spectroscopy measurements. Our results highlight the extreme sensitivity of lithium mediated N2 reduction to small changes in the experimental conditions.

Nonaqueous Li-Mediated Nitrogen Reduction: Taking Control of Potentials

“… 26 (iv) These Ag salts are also hygroscopic: this accelerates degradation and may strongly affect N 2 reduction experiments because of its sensitivity to water content in the electrolyte. 32 , 33 Going back to LiFePO 4 , its reported tedious preparation—carried out using an electrolyte that is essentially the same as the one where the electrode will then be used—is not as challenging as it seems and has been swiftly adapted from commonly used Li-ion battery electrolytes (e.g., LiPF 6 in cyclic/linear carbonate). 20 , 21 In an Ar glovebox, a LiFePO 4 disc (Ø 18 mm) was assembled in a coin cell ( Figure S1a ) at the positive side, against a Li metal negative electrode, separated by a glass fiber separator wetted with 1 M LiNTf 2 (i.e., LiN(SO 2 CF 3 ) 2 ) in THF (omitting ethanol due to incompatibility of lithium with proton sources).…”

“…(ii) Ag + ions tend to leak through that same frit into the bulk electrolyte and can affect electrochemistry by coplating with lithium for instance. ,, (iii) Ag/Ag + references need to be freshly prepared for every experiment since the Ag salts used to make their electrolyte are light-sensitive and degrade rather quickly . (iv) These Ag salts are also hygroscopic: this accelerates degradation and may strongly affect N 2 reduction experiments because of its sensitivity to water content in the electrolyte. , Going back to LiFePO 4 , its reported tedious preparationcarried out using an electrolyte that is essentially the same as the one where the electrode will then be usedis not as challenging as it seems and has been swiftly adapted from commonly used Li-ion battery electrolytes (e.g., LiPF 6 in cyclic/linear carbonate). , In an Ar glovebox, a LiFePO 4 disc (Ø 18 mm) was assembled in a coin cell (Figure S1a) at the positive side, against a Li metal negative electrode, separated by a glass fiber separator wetted with 1 M LiNTf 2 (i.e., LiN­(SO 2 CF 3 ) 2 ) in THF (omitting ethanol due to incompatibility of lithium with proton sources). Discharging the LiFePO 4 electrode at a rate of 1.56 mA.g –1 LiFePO 4 (0.01C rate) until a cutoff voltage of 4 V vs Li yielded a stable phase, reproducibly equilibrating to a potential of +3.428 ± 0.003 V vs Li (10 repeats) after relaxation, which remained stable for at least 7 days in this configuration (Figure S1b,c).…”