Abstract
A detailed understanding of ion adsorption within porous carbon is key to the design and improvement of electric double-layer capacitors, more commonly known as supercapacitors. In this work nuclear magnetic resonance (NMR) spectroscopy is used to study ion adsorption in porous carbide-derived carbons. These predominantly microporous materials have a tuneable pore size which enables a systematic study of the effect of pore size on ion adsorption. Multinuclear NMR experiments performed on the electrolyte anions and cations reveal two main environments inside the carbon. In-pore ions (observed at low frequencies) are adsorbed inside the pores, whilst ex-pore ions (observed at higher frequencies) are not adsorbed and are in large reservoirs of electrolyte between carbon particles. All our experiments were carried out in the absence of an applied electrical potential in order to assess the mechanisms related to ion adsorption without the contribution of electrosorption. Our results indicate similar adsorption behaviour for anions and cations. Furthermore, we probe the effect of sample orientation, which is shown to have a marked effect on the NMR spectra. Finally, we show that a (13)C →(1)H cross polarisation experiment enables magnetisation transfer from the carbon architecture to the adsorbed species, allowing selective observation of the adsorbed ions and confirming our spectral assignments.
Highlights
Institute, Drexel University, Philadelphia, PA 19104, USA e Universite Paul Sabatier, CIRIMAT UMR CNRS 5085 Toulouse, 31062, France and Reseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, France † Electronic supplementary information (ESI) available: SEM images and porosity analysis of carbons, spectral fits, derivation of formulae used to calculate area per anion, supporting nuclear magnetic resonance (NMR) spectra and pulse sequences
We probe the effect of sample orientation, which is shown to have a marked effect on the NMR spectra
To study the poresize effect, here we focus our attention on carbide-derived carbons (CDCs) with controlled pore sizes and narrow pore size distributions
Summary
Drexel University, Philadelphia, PA 19104, USA e Universite Paul Sabatier, CIRIMAT UMR CNRS 5085 Toulouse, 31062, France and Reseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, France † Electronic supplementary information (ESI) available: SEM images and porosity analysis of carbons, spectral fits, derivation of formulae used to calculate area per anion, supporting NMR spectra and pulse sequences. NMR has been used to study the adsorption of molecules on activated carbons,[25,26,27,28] and the same techniques have been used to study adsorption inside carbon nanotubes (CNTs).[29,30,31,32] It has been well established that resonances corresponding to molecules adsorbed on carbon surfaces show a shift to low frequencies (relative to the corresponding free species), making NMR a very useful tool to study ion adsorption This approach has been extended to study the effect of an applied potential during the operation of supercapacitors with activated carbon electrodes. A through-space magnetisation transfer experiment (from 13C enriched carbon to the 1H spins of the electrolyte cations) enables identification of species in close proximity to the carbon surface, providing conclusive evidence for our peak assignments
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