Insight into the Electrical Double Layer of Ionic Liquids Revealed through Its Temporal Evolution

Abstract

Ionic liquids (ILs) are proposed as potentially ideal electrolytes for use in electrical double layer capacitors. However, recent discoveries of long‐range electrostatic screening in ILs have revealed that this understanding of the electrical double layer in highly concentrated solutions is still incomplete. Through precise time‐dependent measurements of wide‐angle X‐ray scattering and surface forces, novel molecular insight into their electrical double layer is provided. An ultraslow evolution of the nanostructure of three imidazolium ILs is observed, which reflects the reorganization of the ions in confined and unconfined (bulk) states. The observed phase transformation in the bulk consists of the ILs ordering over at least 20 h, reflected in an expansion or contraction of the spacing between the ions organized in domains of ≈10 nm. This transformation justifies the evolution of the electrical double layer measured in force measurements. Subtle differences between the ILs arise from the intricate balance between electrostatic and non‐electrostatic interactions. This work reveals a new time scale of the evolution of the IL structure and offers a new perspective for understanding the electrical double layer in ILs, with implications on diverse areas of inquiry, such as energy storage, lubrication, as well as micro‐ and nanoelectronics devices.

Tuning Ionic Screening to Accelerate Electrochemical CO2 Reduction in Ionic Liquid Electrolytes

“…Such an interpretation is consistent with discussions from the surface force community, where it is proposed that large electrostatic screening lengths present in ionic liquids cause them to behave analogously to dilute electrolytes, where very few "free" ions are available to redistribute and screen charged surfaces. 25,26,30,33 Most importantly, we observed a remarkable increase in CO 2 reduction rates, in the form of enhanced current densities, as we increased the concentration of EMIm BF 4 to exceed dilute limits (ca. 150 mM).…”

Local and Long-Range Organization in Room Temperature Ionic Liquids

“…10 nm Debye length, structural order has been reported in a host of RTILs on length scales ranging from 1 to 60 nm. − Much of this structural order is seen within the first several nm of (charged) surfaces, where “herringbone” order and charged layers have been reported. RTIL organization on this length scale has also been seen to evolve over time . While the details of this organization differ substantially from traditional Gouy–Chapman-Stern or Helmholtz behavior, what is observed is in much closer proximity to the interface than have been reported by several other groups.…”

Gelation, Clustering and Crowding in the Electrical Double Layer of Ionic Liquids

“…Gebbie et al [35,36] interpreted these results as ILs behaving as "dilute electrolytes", with 1% of ions free and the remaining ions being bound in neutral ion pairs, which do not participate in electrode screening. Whereas, Han et al [39] have proposed that these long-ranged screening lengths arise because of the ∼ 10 nm domain formation of nano-aggregates in ILs, which were independently verified from X-Ray scattering.…”

“…This manifests through decaying oscillations in charge density around an ion in the bulk or as a function of distance from an interface [25][26][27], referred to as overscreening [28], which can give long-ranged electrostatic screening lengths [29][30][31][32][33]. Surprisingly, surface force measurements performed in ILs have reported extremely long-ranged monotonic decay lengths [34][35][36][37][38][39][40][41], referred to as underscreening [42,43]. Gebbie et al [35,36] interpreted these results as ILs behaving as "dilute electrolytes", with 1% of ions free and the remaining ions being bound in neutral ion pairs, which do not participate in electrode screening.…”