Lithium-mediated nitrogen reduction reaction (Li-NRR) is recognised as a high rate and selectivity pathway for green ammonia synthesis.^[1] However, practical implementation requires further advancements in yield rate and stability.^[1-3] Our previous study established a stable phosphonium cation/ylide proton shuttling cycle in a LiBF₄ electrolyte, achieving a faradaic efficiency (FE) of 69 ± 1%.^[4] Recently, the phosphonium-based proton shuttle in our robust high-concentration LiNTf₂ electrolyte outperformed LiBF₄, with doubled yield rate and improved FE of 75 ± 4%.^[5],[6] Despite the performance of the ethanol proton shuttle is still better, which is near 100 % FE and ca. 0.5 μmol s^-1 cm^-2 yield rate,^[5] the phosphonium cation is beneficial over the ethanol for no side products through tetrahydrofuranylation.^[6] This motivated us to deeply investigate the paramteres affecting the phosphonium cation shuttle’s performance in the high-concentration LiNTf₂ electrolyte.

Herein, we explored the relationship between the mass-transport characteristics of the electrolyte solution and the Li-NRR performance with three smaller relatives of the originally studied phosphonium cation, [P_6,6,6,14]⁺, specifically  [P_1,2,2,2]⁺, [P_1,4,4,4]⁺, [P_4,4,4,8]⁺ where the subscripted numbers represent the number of carbons in each of the four alkyl chains. An increase in the phosphonium alkyl chain length results in a noticeable decrease in the ionic conductivity and increase in the viscosity of the electrolyte solution. Furthermore, the diffusion coefficient of phosphonium cations measured by pulsed field gradient NMR techniques decreases as the alkyl chain length increases, with [P_6,6,6,14]⁺ having half the diffusion coefficient (0.93 × 10^-12 m² s^-1) of [P_1,2,2,2]⁺. The Li⁺ diffusion coefficient follows a similar, but less pronounced trend as it is moderately reduced from 0.74 × 10^-12 m² s^-1 in the [P_1,2,2,2]⁺ containing solution to 0.67 × 10^-12 m² s^-1 when using [P_6,6,6,14]⁺ shuttle. Interestingly, the type of phosphonium cations has minimal impact on the NTf₂^- diffusion coefficient. Based on these transport behaviours, one might expect that the longer phosphonium alkyl chain would adversely affect the Li-NRR performance because of the lower lithium diffusion. However, contrary to this expectation, the ammonia yield rate and the FE of the process increases steadly in the order [P_1,2,2,2]⁺ < [P_1,4,4,4]⁺ < [P_4,4,4,8]⁺ < [P_6,6,6,14]⁺. This suggests that the size of phosphonium cation directly affects a delicate balance of transport rates of the Li⁺, the proton shuttle and the N₂ transport at the electrolyte|electrode interface. Moreover, differential capacitance analysis demonstrated that the longer phosphonium alkyl chain, the more stable and compact ionic layer on the electrode surface. This well-structured ionic assembly acts as a protective layer that defines the structure of the solid electrolyte interphase and controls the Li⁺ electroreduction.

Optimising the Li-NRR performance with the best-performing [P_6,6,6,14]⁺ shuttle requires a decrease in the LiNTf₂ concentration from 2 to 1.5 M, which leads to an increase in the Li⁺ diffusion coefficient equal to that of the [P_1,2,2,2]⁺ system. Under this condition, the process is remarkably stable over 68 h with an invariable 80 ± 1% FE and an average yield rate of 300 ± 10 μmol s^-1 cm^-2.

Our study emphasises the complexity of the Li-NRR relationship to the mass-transport processes through correlations with bulk electrolyte properties, and to the ordering of the ionic species at the electrode surface. To achieve a 100 % FE target  with the robust phosphonium cation proton shuttles, future investigations will utilise hydrodynamic and computational modeling techniques.