Aluminum ions speciation and transport in acidic deep eutectic AlCl3 amide electrolytes

Abstract

Aluminum ion speciation in AlCl3:amide deep eutectic solvents (DES) were explored using variable temperature multi-Nuclear (1H and 27Al) Magnetic Resonance and Electrochemical Impedance Spectroscopies. Focus was on the interplay between the local and bulk interactions on species transport, and the effect of AlCl3 concentration on their dynamics. Acetamide, butyramide and propionamide were chosen and molar ratios of 1:1–1.7:1 of AlCl3:amide were studied. Calculated Walden products show that although the DES electrolytes are ‘subionic,’ they are more conductive than ‘poor ILs’. The ionic conductivity data showed VFT temperature dependences, and calculated fragility values were less than 5 for all three amides and concentrations, indicating high levels of conformational mobility for the aluminum species. 1H chemical shift data showed increased shielding with increasing molar ratios for all three amides indicating an enhancement in the local electron densities, possibly due to an increase in the more shielded cis conformation population. 27Al spectra revealed multiple aluminum species that experienced both intermediate and fast chemical exchanges with increasing AlCl3 concentration. The AlCl2(amide)2+ species population decreased with both increasing temperature and AlCl3 concentration, especially for shorter alkyl chains amides. This suggests it plays a less significant role in the aluminum ion transport at higher AlCl3 concentration. 27Al T1 values for the various species revealed a temperature and concentration dependent plateauing in the T1 of the AlCl2(amide)+ species for molar ratios greater than 1:1. Additionally, with increasing AlCl3 concentration, the merging of the AlCl4− and [AlCl2(amide)2]+ species resulted in its T1 being the shortest and increasing with increasing temperatures, suggesting it experiences greater electric field gradients compared to the other aluminum species.