Title

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

Document Type

Article

Publication Date

2017

Keywords

Porous materials, Solid-state NMR, Supercapacitors

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 with 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 and so on), while supercapacitors are more promising for second or subsecond 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 electrodes1,2, and by using ionic liquid electrolytes that are stable at higher voltages3,4. Both approaches have afforded some increases in energy density, though not without sacrificing power density. The delicate balance between energy and power must be understood if supercapacitors are to be used in a wide range of applications.

Fundamental studies based on spectroscopic5,6,7,8,9,10,11,12,13,14, and theoretical15,16,17,18, methods have recently revealed the complex nature of charging in supercapacitors. Before charging, the electrode pores contain a large number of electrolyte ions15,19,20, 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 depending on the choice of electrode and electrolyte, and the electrode polarization21,22. For example, charging often proceeds via the exchange of counter-ions and co-ions (co-ions are defined as having charge of the same sign as the electrode in which they are located), while charging by co-ion desorption alone is also a possibility. We introduced the charging mechanism parameter, X, to quantify these different processes, with X taking values of +1, 0 and −1, for the extreme cases of charging by counter-ion adsorption, counter-ion–co-ion exchange, and co-ion desorption, respectively, while intermediate X values indicate contributions from more than one mechanism21.

An understanding and control of the charge storage mechanism (X value) may hold the key to optimizing the energy and power performance of supercapacitors for different applications. However, a crucial missing part of our understanding is how the electrolyte ions diffuse and migrate in supercapacitor electrodes. Theoretical studies based on molecular dynamics and mean-field theories have shown that ion–ion interactions, ion–carbon interactions and the electrode pore size all influence the rates of ionic diffusion in supercapacitor electrodes16,18,23,24, though there is not yet a clear consensus on which factors are most important, nor the order of magnitude by which diffusion is influenced by confinement and by charging. Experimental methods to directly probe in-pore motion in an ion-selective and electrode-selective way have until now been lacking.

Here we show how an in situ pulsed field gradient (PFG) NMR approach can be used to measure ionic diffusion in the nanopores of supercapacitor electrodes. Confinement results in reductions in self-diffusion coefficients by over two orders of magnitude compared with neat electrolyte solutions, while in situ measurements show that changes of in-pore ion populations during charging modulate in-pore ionic diffusion. We also show that the electrolyte concentration and nanopore size have significant effects on in-pore diffusion, and on the exchange of ions between bulk and in-pore sites. Our findings offer detailed insights into diffusion and exchange processes in porous electrodes and bring new opportunities for understanding and controlling the charging dynamics of supercapacitors.

Comments

Open Access; Authors retain copyright

Publisher Attribution

The full article is available from Nature Energy via doi: 10.1038/nenergy.2016.216