Self-assembled nanostructures in ionic liquids facilitate charge storage at electrified interfaces

Abstract

Driven by the potential applications of ionic liquids (ILs) in many emerging electrochemical technologies, recent research efforts have been directed at understanding the complex ion ordering in these systems, to uncover novel energy storage mechanisms at IL–electrode interfaces. Here, we discover that surface-active ILs (SAILs), which contain amphiphilic structures inducing self-assembly, exhibit enhanced charge storage performance at electrified surfaces. Unlike conventional non-amphiphilic ILs, for which ion distribution is dominated by Coulombic interactions, SAILs exhibit significant and competing van der Waals interactions owing to the non-polar surfactant tails, leading to unusual interfacial ion distributions. We reveal that, at an intermediate degree of electrode polarization, SAILs display optimum performance, because the low-charge-density alkyl tails are effectively excluded from the electrode surfaces, whereas the formation of non-polar domains along the surface suppresses undesired overscreening effects. This work represents a crucial step towards understanding the unique interfacial behaviour and electrochemical properties of amphiphilic liquid systems showing long-range ordering, and offers insights into the design principles for high-energy-density electrolytes based on spontaneous self-assembly behaviour.

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Fig. 1: Bulk-phase structural and electrochemical characterization of [C4C1Im][AOT].
Fig. 2: Molecular dynamics simulations reveal unusual EDL structures of [C4C1Im][AOT].
Fig. 3: Elucidation of interfacial molecular layering through AFM force measurements.
Fig. 4: EDL properties probed by impedance measurements.
Fig. 5: Molecular dynamics simulations of other SAILs.
Fig. 6: Electrocapacitive performances of other SAILs.

Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

This work was supported by an MIT Energy Initiative seed grant. X.M. acknowledges financial support from an MIT Skoltech fellowship. C.Č. acknowledges financial support from the Czech Science Foundation (GACR number 19-04150Y). C.Č. and A.A.H.P. thank A. Dequidt of Université Clermont Auvergne for use of the computer program to calculate structure factors. We thank the UK research council STFC for providing beam time at the Institut Laue–Langevin, Grenoble, France. Small-angle neutron scattering experiments were supported under proposal number 9-12-434. X-ray reflectivity measurements were performed at Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, which is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-76SF00515.

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X.M. and P.B. conceived the initial idea. X.M. designed and led the research, carried out the electrochemical experiments, and analysed the experimental and simulation data, under the supervision of T.A.H. P.B. synthesized the SAILs and contributed to the electrochemical experiments, under the supervision of T.A.H. C.Č. carried out the molecular dynamics simulations under the supervision of A.A.H.P. and M.F.C.G. G.H. performed the SANS experiments under the supervision of J.E. and I.G. H.L. performed the AFM force measurements under the supervision of R.A. Y.R. contributed to the synthesis of the SAILs and X-ray reflectivity measurements. D.C. performed the X-ray reflectivity measurements. X.M. wrote the manuscript. All authors revised the manuscript.

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Correspondence to Xianwen Mao or Margarida. F. Costa Gomes or T. Alan Hatton.

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X.M., P.B., M.F.C.G. and T.A.H. have filed a patent application based on this work (US Patent application number 16/323,468). This patent, entitled ‘High-temperature supercapacitors containing surface active ionic liquids’ was filed with the US Patent and Trademark Office on 5 February 2019, and published on 20 June 2019 with publication number US-2019-0189364-A1.

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Supplementary Notes 1–9, Tables 1–6, Figs. 1–15 and references 1–32

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Mao, X., Brown, P., Červinka, C. et al. Self-assembled nanostructures in ionic liquids facilitate charge storage at electrified interfaces. Nat. Mater. 18, 1350–1357 (2019). https://doi.org/10.1038/s41563-019-0449-6

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