Abstract
Supercapacitors store charge through the electrosorption of ions on microporous electrodes. Despite major efforts to understand this phenomenon, a molecular-level picture of the electrical double layer in working devices is still lacking as few techniques can selectively observe the ionic species at the electrode/electrolyte interface. Here, we use in situ NMR to directly quantify the populations of anionic and cationic species within a working microporous carbon supercapacitor electrode. Our results show that charge storage mechanisms are different for positively and negatively polarized electrodes for the electrolyte tetraethylphosphonium tetrafluoroborate in acetonitrile; for positive polarization charging proceeds by exchange of the cations for anions, whereas for negative polarization, cation adsorption dominates. In situ electrochemical quartz crystal microbalance measurements support the NMR results and indicate that adsorbed ions are only partially solvated. These results provide new molecular-level insight, with the methodology offering exciting possibilities for the study of pore/ion size, desolvation and other effects on charge storage in supercapacitors.
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Acknowledgements
A.C.F., J.M.G. and C.P.G. acknowledge the Sims Scholarship (A.C.F.), EPSRC (through the Supergen consortium for J.M.G.) and the EU ERC (through an Advanced Fellowship to C.P.G.) for financial support. P.S. and W.-Y.T. acknowledge support from the European Research Council (ERC, Advanced Grant, ERC-2011-AdG, Project 291543–IONACES). P.S. also acknowledges financial support from the Chair ‘Embedded Multi-Functional Nanomaterials’ from the Airbus Group Foundation. A.C.F. and J.M.G. thank the NanoDTC Cambridge for travel funding. We also thank A. Kornyshev, S. Kondrat and C. Merlet for useful discussions.
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J.M.G., A.C.F. and C.P.G. designed the research. J.M.G. made supercapacitor cells and electrolytes, performed NMR experiments and analysed the NMR data. W.-Y.T., P.-L.T. and P.S. designed the EQCM work. W.-Y.T. carried out the EQCM experiments and W.-Y.T., P.-L.T. and P.S. analysed the data. All authors contributed to discussion of the data and writing the paper.
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Griffin, J., Forse, A., Tsai, WY. et al. In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors. Nature Mater 14, 812–819 (2015). https://doi.org/10.1038/nmat4318
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DOI: https://doi.org/10.1038/nmat4318
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