Solid-state NMR studies of supercapacitors

Abstract

Electrochemical double-layer capacitors, or 'supercapacitors' are attracting increasing attention as high-power energy storage devices for a wide range of technological applications. These devices store charge through electrostatic interactions between liquid electrolyte ions and the surfaces of porous carbon electrodes. However, many aspects of the fundamental mechanism of supercapacitance are still not well understood, and there is a lack of experimental techniques which are capable of studying working devices. Recently, solid-state NMR has emerged as a powerful tool for studying the local environments and behaviour of electrolyte ions in supercapacitor electrodes. In this Trends article, we review these recent developments and applications. We first discuss the basic principles underlying the mechanism of supercapacitance, as well as the key NMR observables that are relevant to the study of supercapacitor electrodes.We then review some practical aspects of the study of working devices using ex situ and in situ methodologies and explain the key advances that these techniques have allowed on the study of supercapacitor charging mechanisms. NMR experiments have revealed that the pores of the carbon electrodes contain a significant number of electrolyte ions in the absence of any charging potential. This has important implications for the molecular mechanisms of supercapacitance, as charge can be stored by different ion adsorption/desorption processes. Crucially, we show how in situ NMR experiments can be used to quantitatively study and characterise the charging mechanism, with the experiments providing the most detailed picture of charge storage to date, offering the opportunity to design enhanced devices. Finally, an outlook for future directions for solid-state NMR in supercapacitor research is offered.3

Solid-state NMR Study of Ion Adsorption and Charge Storage in Graphene Film Supercapacitor Electrodes

“…The weakly adsorbed anions were discovered at the electrolyte loading volume of 75 μL (as schematically shown in the “status 2”). However, it is different from the observation in previous NMR studies that the weakly ( i.e ., the ex-pore) adsorption does not occur until the strongly ( i.e ., the in-pore) adsorption has attained saturation17. In the current work, the strongly adsorbed resonances intensity did not reach a maximum value ( i.e ., the strongly adsorption was not saturated), when the weakly adsorbed anions were observed.…”

“…1c). As is well known, the resonances corresponding to adsorbed species on carbon surface shift to low frequency as compared to free species, due to the diamagnetic contribution of the “ring current” in the carbon17. Consequently, the deconvoluted peaks at low frequencies ( e.g ., −10.02 ppm for electrolyte loading volume of 50 μL) arise from the 11 B atoms of strongly bound BF 4 − anions on carbon surface, which is herein referred to as “strongly adsorbed state”.…”

“…In recent years, nuclear magnetic resonance (NMR) has emerged as an element-selective, highly localized and quantitative technique to obtain the atomic-scale information on the local environments with direct observation817. As for EDLCs employing carbonaceous active materials, the species close to the solid surfaces experience a diamagnetic shielding (arising from the “ring current” in carbon electrodes181920), leading to the shift of NMR signals to low frequencies.…”

“…This chemical shift, i.e. , the so-called nucleus-independent chemical shift (NICS), can be used as a fingerprint to distinguish the adsorbed species (ions and molecules) from the free ones in the bulk electrolyte1718.…”

“…Such an in situ measurements were also conducted on carbons soaked in different organic3334 and aqueous electrolytes3536. The above NMR studies provided useful insights into the molecular mechanisms of the charge storage of EDLCs, including the adsorption of counter-ions into the electrode pores, expulsion of co-ions form pores, and ion exchange between anions and cations817373839.…”