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Partial breaking of the Coulombic ordering of ionic liquids confined in carbon nanopores

Nature Materials volume 16, pages 1225–1232 (2017)Cite this article

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Abstract

Ionic liquids are composed of equal quantities of positive and negative ions. In the bulk, electrical neutrality occurs in these liquids due to Coulombic ordering, in which ion shells of alternating charge form around a central ion. Their structure under confinement is far less well understood. This hinders the widespread application of ionic liquids in technological applications. Here we use scattering experiments to resolve the structure of a widely used ionic liquid (EMI–TFSI) when it is confined inside nanoporous carbons. We show that Coulombic ordering reduces when the pores can accommodate only a single layer of ions. Instead, equally charged ion pairs are formed due to the induction of an electric potential of opposite sign in the carbon pore walls. This non-Coulombic ordering is further enhanced in the presence of an applied external electric potential. This finding opens the door for the design of better materials for electrochemical applications.

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Figure 1: Structural anomalies of EMI–TFSI confined in unpolarized carbon pores.
Figure 2: Pore size-dependent anion–anion structure in unpolarized carbon nanopores.
Figure 3: Enhanced co-ion pair formation in carbon nanopores of decreasing pore size.
Figure 4: The distributions of IL molecules and induced charges on carbon pore walls in monolayer and bilayer confinement.
Figure 5: Pore size-sensitive oriented structures of EMI cations and TFSI anions in carbon nanopores of 0.7 nm.
Figure 6: The effect of electrode potential on the structural ordering of ionic liquid inside ultra-narrow pores.

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Acknowledgements

R.F. is supported by TAKAGI Co., Ltd. This research was supported by Grant-in-Aid for Young Scientists (B) (No. 26870240), Young Scientists (A) (No. 17H04953), Scientific Research (A) (No. 24241038), Scientific Research (B) (No. 17H03039), CREST (JPMJCR1324) and Center of Innovation Program from Japan Science and Technology Agency. The synchrotron radiation experiments were performed at the BL02B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2012B1438, No. 2013B1243, No.2014A1167 and No. 2014B1196) and at the BL5S2 of Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan (Approval No. 2016D4005 and No. 201606124). P.S. acknowledges support from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n.102539 (Advanced Grant, Ionaces project). M.J.B. acknowledges the support of the Australian Research Council Discovery Program (DP110101293). Y.G. work on CDC was supported by the Fluid Interface Reactions, Structures & Transport, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences. We are also grateful to K. Van Aken (Drexel University) for providing additional CDC samples.

Author information

Author notes

  1. Julie Ségalini

    Present address: Marion Technologies, Parc Technologique Delta Sud, 09340, Verniolle, France

Authors and Affiliations

  1. Center for Energy and Environmental Science, Shinshu University, 4-17-1, Wakasato, Nagano-City, 380-8553, Japan

    Ryusuke Futamura, Taku Iiyama, Yury Gogotsi, Patrice Simon & Katsumi Kaneko

  2. Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1, Asahi, Matsumoto-City, 390-8621, Japan

    Taku Iiyama & Yuma Takasaki

  3. Department of Material Science and Engineering, and A.J. Drexel Nanomaterials Institute, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania, 19104, USA

    Yury Gogotsi

  4. School of Science, Loughborough University, Leicestershire LE11 3TU, UK

    Mark J. Biggs

  5. School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia, 5005, Australia

    Mark J. Biggs

  6. Sorbonne Universités, UPMC Univ. Paris 06, CNRS, Laboratoire PHENIX, F-75005 Paris, France

    Mathieu Salanne

  7. Réseau sur le Stockage Electrochimique de l’Energie, RS2E FR CNRS 3459, France

    Mathieu Salanne & Patrice Simon

  8. Université Paul Sabatier, CIRIMAT UMR, CNRS 5085, 5085, 118 route de Narbonne, 31062 Toulouse Cedex 4, France

    Julie Ségalini & Patrice Simon

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Contributions

R.F. carried out the experiments, being in main charge of this research. R.F., P.S. and K.K. prepared the manuscript. T.I. developed the HRMC simulation program and supported the structure analysis. M.J.B. supported development of the HRMC program. M.S. computed image charge distributions. R.F. and Y.T. performed HRMC calculations. P.S. and Y.G. provided the CDC samples. J.S. contributed development of in situ X-ray diffraction measurements. R.F., M.S., T.I., P.S., Y.G., M.J.B. and K.K. discussed the results and edited the paper.

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Correspondence to Patrice Simon or Katsumi Kaneko.

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Futamura, R., Iiyama, T., Takasaki, Y. et al. Partial breaking of the Coulombic ordering of ionic liquids confined in carbon nanopores. Nature Mater 16, 1225–1232 (2017). https://doi.org/10.1038/nmat4974

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