A Blessing and a Curse: How a Supercapacitor's Large Capacitance Causes its Slow Charging

TL;DR

This paper introduces a model explaining why supercapacitors with large capacitance charge slowly, highlighting the impact of electrode structure and potential on charging dynamics, and aligning with experimental observations.

Contribution

The paper presents a novel electrode stack model that elucidates the slow charging behavior of supercapacitors, bridging the gap between experimental and theoretical timescales.

Findings

Charging slows at high potentials with two relaxation time scales.

Model aligns qualitatively with experimental measurements.

Porous electrodes exhibit similar relaxation dynamics.

Abstract

The development of novel electrolytes and electrodes for supercapacitors is hindered by a gap of several orders of magnitude between experimentally measured and theoretically predicted charging timescales. Here, we propose an electrode model, containing many parallel stacked electrodes, that explains the slow charging dynamics of supercapacitors. At low applied potentials, the charging behavior of this model is described well by an equivalent circuit model. Conversely, at high potentials, charging dynamics slow down and evolve on two relaxation time scales: a generalized $RC$ time and a diffusion time, which, interestingly, become similar for porous electrodes. The charging behavior of the stack-electrode model presented here helps to understand the charging dynamics of porous electrodes and qualitatively agrees with experimental time scales measured with porous electrodes.