Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy

Abstract

Ionic transport inside porous carbon electrodes underpins the storage of energy in supercapacitors and the rate at which they can charge and discharge, yet few studies have elucidated the materials properties that influence ion dynamics. Here we use in situ pulsed field gradient NMR spectroscopy to measure ionic diffusion in supercapacitors directly. We find that confinement in the nanoporous electrode structures decreases the effective self-diffusion coefficients of ions by over two orders of magnitude compared to neat electrolyte, and in-pore diffusion is modulated by changes in ion populations at the electrode-electrolyte interface during charging.Electrolyte concentration and carbon pore size distributions also affect in-pore diffusion and the movement of ions in and out of the nanopores. In light of our findings we propose that controlling the charging mechanism may allow the tuning of the energy and power performances of supercapacitors for a range of different applications.As renewable energy and green technologies such as electric vehicles become prevalent, we must develop new ways to store and release energy on a range of timescales. Rechargeable batteries are ideal for timescales of minutes or hours (electric cars, portable electronic devices, grid storage etc.), while supercapacitors are more promising for second or sub-second timescales and are increasingly being used for transport applications where rapid charging and discharging are required. The superior power handling and cycle lifetime of supercapacitors comes at the expense of energy density, with recent materials-driven research aiming to address this issue by fine-tuning the nanoporous structure of the carbon electrodes, 1,2 and by using ionic liquid electrolytes that are stable at higher voltages. 3,4 Both approaches have afforded some increases in energy density, though not without sacrificing power density. The delicate balance between energy and power density must be understood if supercapacitors are to be used in a wide range of applications.Fundamental studies based on spectroscopic, 5-14 and theoretical, 15-18 methods have recently revealed the complex nature of charging in supercapacitors. Prior to charging, the electrode pores contain a large number of electrolyte ions, 15,19,20 and as a result charge storage is generally more complex than simple counter-ion adsorption (counter-ions are defined as having charge opposite to the electrode in which they are located). [5][6][7]15 A range of different charging mechanisms can operate

Revising the Concept of Pore Hierarchy for Ionic Transport in Carbon Materials for Supercapacitors

“…However, as it is evident from a recent study, [19] application of a voltage of ±1.5 V changed the in-pore diffusivities by factor ≈3 at most (Figure 3 therein) compared to zero voltage, while in the present study the effect is seen and discussed in terms of orders of magnitude. However, as it is evident from a recent study, [19] application of a voltage of ±1.5 V changed the in-pore diffusivities by factor ≈3 at most (Figure 3 therein) compared to zero voltage, while in the present study the effect is seen and discussed in terms of orders of magnitude.…”

“…[19] In micropores, the strong influence of pore walls on the motion of adsorbed molecular species leads to this observed decrease in the diffusion coefficient by several orders of magnitude compared to bulk diffusivities. A higher diffusivity of anions was observed in all experiments due to their smaller size compared to cations.…”

Nitrogen Codoped Unique Carbon with 0.4 nm Ultra‐Micropores for Ultrahigh Areal Capacitance Supercapacitors

“…Thus, ionic transport in microporous electrodes is an important consideration for energy storage in electrochemical supercapacitors. Studies on the effects of the pore size on capacitance could enable further performance improvements by maximizing the electrode surface area accessible to the electrolyte ions . However, to date, no studies have examined effects of micropores smaller than 0.4 nm on the performance of electrochemical capacitors.…”

Using Polymeric Ionic Liquids as an Active Binder in Supercapacitors

“…In-operando NMR and quartz-crystal microbalance could potentially elucidate the charge balance process during high frequencies sweep and determined the role played by the polymer in such process. 42,43 The high R i associated with poly(IL) based electrodes could suggest a higher self-discharge and leakage current than the co-polymer electrodes, however Figures 4d-4e demonstrate otherwise. Figure 4d shows the self-discharge voltage (E self-discharge ) and Figure 4e the leakage current (i leakage ) determined for each of the studied EDLCs.…”