Selective observation of charge storing ions in supercapacitor electrode materials

Alexander C Forse,John M Griffin,Clare P Grey

doi:10.1016/j.ssnmr.2017.10.003

Abstract

Nuclear magnetic resonance (NMR) spectroscopy has emerged as a useful technique for probing the structure and dynamics of the electrode-electrolyte interface in supercapacitors, as ions inside the pores of the carbon electrodes can be studied separately from bulk electrolyte. However, in some cases spectral resolution can limit the information that can be obtained. In this study we address this issue by showing how cross polarisation (CP) NMR experiments can be used to selectively observe the in-pore ions in supercapacitor electrode materials. We do this by transferring magnetisation from 13C nuclei in porous carbons to nearby nuclei in the cations (1H) or anions (19F) of an ionic liquid. Two-dimensional NMR experiments and CP kinetics measurements confirm that in-pore ions are located within Ångströms of sp2-hybridised carbon surfaces. Multinuclear NMR experiments hold promise for future NMR studies of supercapacitor systems where spectral resolution is limited.

Highlights

Supercapacitors are high power energy storage devices that store charge at the interface between porous carbon electrodes and an electrolyte solution
We have demonstrated the application of cross polarisation (CP) Nuclear magnetic resonance (NMR) as a method to selectively observe in-pore ions in supercapacitor electrode materials wetted with ionic liquid
Together with two-dimensional NMR and CP kinetics, our results confirm that in-pore ions are located within Ångstro€m distances of the carbon surfaces, while ex-pore ions are more remote

Summary

Introduction

Supercapacitors are high power energy storage devices that store charge at the interface between porous carbon electrodes and an electrolyte solution. Observation of resolved in-pore resonances in the NMR spectra of supercapacitor electrodes offers a large amount of valuable information; (i) the intensities of the in-pore resonances allow the number of in-pore anions and cations to be determined, and the composition of the carbon pores and supercapacitor charging mechanisms can be studied [8,9,25,26,27], (ii) the chemical shift difference between in- and ex-pore resonances gives information about the carbon structure, with both carbon ordering and pore size distribution affecting the shift [33,34,35], and (iii) the linewidth of the in-pore resonance can offer information about the dynamics of the in-pore ions [25]. Ex-pore ions are not detected in the CP experiments due to the much longer distances between these ions and the carbon surfaces and the faster dynamics of these free ions, making the CP technique a useful spectral editing method for studying adsorbed species in supercapacitor electrodes

Carbon materials

NMR sample preparation

NMR experiments

Results and discussion

Conclusions