Carbon-carbon supercapacitors: Beyond the average pore size or how electrolyte confinement and inaccessible pores affect the capacitance

TL;DR

This study uses molecular simulations to show that the pore structure and morphology of carbon electrodes significantly influence supercapacitor capacitance, beyond just average pore size, by affecting ion confinement and accessibility.

Contribution

It demonstrates that electrode morphology and pore order critically impact capacitance, providing insights for designing more effective carbon-based supercapacitors.

Findings

Ordered, well-defined pore structures yield higher capacitance.

Disordered electrodes have many inactive carbon atoms not involved in charge storage.

Ion confinement and accessibility are key factors influencing capacitance.

Abstract

Carbon-carbon supercapacitors are high power electrochemical energy storage systems which store energy through reversible ion adsorption at the electrode-electrolyte interface. Due to the complex structure of the porous carbons used as electrodes, extracting structure-property relationships in these systems remains a challenge. In this work, we conduct molecular simulations of two model supercapacitors based on nanoporous electrodes with the same average pore size, a property often used when comparing porous materials, but different morphologies. We show that the carbon with the more ordered structure, and a well defined pore size, has a much higher capacitance than the carbon with the more disordered structure, and a broader pore size distribution. We analyze the structure of the confined electrolyte and show that the ions adsorbed in the ordered carbon are present in larger quantities and are also more confined than for the disordered carbon. Both aspects favor a better charge separation and thus a larger capacitance. In addition, the disordered electrodes contain a significant amount of carbon atoms which are never in contact with the electrolyte, carry a close to zero charge and are thus not involved in the charge storage. The total quantities of adsorbed ions and degrees of confinement do not change much with the applied potential and as such, this work opens the door to computationally tractable screening strategies.