Salt concentration and charging velocity determine ion charge storage mechanism in nanoporous supercapacitors

C Prehal,H Amenitsch,O Paris,V Presser,C Koczwara

doi:10.1038/s41467-018-06612-4

Abstract

A fundamental understanding of ion charge storage in nanoporous electrodes is essential to improve the performance of supercapacitors or devices for capacitive desalination. Here, we employ in situ X-ray transmission measurements on activated carbon supercapacitors to study ion concentration changes during electrochemical operation. Whereas counter-ion adsorption was found to dominate at small electrolyte salt concentrations and slow cycling speed, ion replacement prevails for high molar concentrations and/or fast cycling. Chronoamperometry measurements reveal two distinct time regimes of ion concentration changes. In the first regime the supercapacitor is charged, and counter- and co-ion concentration changes align with ion replacement and partially co-ion expulsion. In the second regime, the electrode charge remains constant, but the total ion concentration increases. We conclude that the initial fast charge neutralization in nanoporous supercapacitor electrodes leads to a non-equilibrium ion configuration. The subsequent, charge-neutral equilibration slowly increases the total ion concentration towards counter-ion adsorption.

Highlights

When carbon-based supercapacitors are charged, the electrode charge is counter-balanced by the ionic charge within the pore space
Changes of cation and anion concentrations on time scales much larger than the time of the actual charging were detected during chronoamperometry (CA) measurements, suggesting that the first fast time regime does not lead to the final equilibrium configuration of the system
cyclic voltammetry (CV) curves of cells with the lowest salt concentration tend to show a distinct minimum around the potential of zero charge (PZC) at low scan rates

Summary

Introduction

When carbon-based supercapacitors are charged, the (nonFaradaic) electrode charge is counter-balanced by the ionic charge within the pore space. Ions rearrange to optimally screen repulsive interactions between counter-ions by preferentially occupying sites with highest possible degree of confinement[12] This mechanism naturally explains the often reported increase of surface-normalized capacitance with decreasing micropore size[13,14]. Experimental conditions and keyparameters determining the dominating ion charge storage mechanism still remain to be identified Both atomistic/molecular parameters, such as carbon/ion interactions, ion mobilities or hydration enthalpies, and macroscopic properties of the entire system, like cell design or cycling rates, might influence the charge storage mechanism in a yet unknown way. In situ XRT and small-angle X-ray scattering experiments during charging and discharging in a custom-built supercapacitor cell[16] reveal distinct dependencies of the ion charge storage mechanism on the electrolyte salt concentration, the charging and discharging rates, the specific cell design and partially the nature of the used ions. Changes of cation and anion concentrations on time scales much larger than the time of the actual charging were detected during chronoamperometry (CA) measurements, suggesting that the first fast time regime does not lead to the final equilibrium configuration of the system

Methods

Results

Conclusion