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

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 time scales. 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 agrees qualitatively with experimental time scales measured with porous electrodes.

How to speed up ion transport in nanopores

“…S14). The slow timescale τ 2 ≈ 45 min is in agreement with model predictions saying that supercapacitors charge at late times with the timescale τ = α(L/2 + H) 2 /D ≈ 17 min 58 , where L ≈ 150 μm is the electrode separation (which in the experiments is the thickness of the electrode-electrode separator), H ≈ 109 μm is the electrode thickness, D ≈ 10 −11 m 2 / s the bulk diffusion constant of EMIM-BF 4 . The salt concentration prefactor α = 0.59 was determined for the case of a model RTIL with ionic radii the same as in our simulations (0.5 nm) (private communication with Cheng Lian).…”

“…Conversely, the model of ref. 58 does not describe the dynamics of finite size ions in nanopores in as much detail as our simulations do, and hence, ref. 58 could not account for the pore clogging effects central to our article.…”

“…6a). One likely has to account for ionic currents in the quasi-neutral macropores of the porous electrodes 58 , which are ignored in our MD simulations.…”

Charging Dynamics of Overlapping Double Layers in a Cylindrical Nanopore

“…Over the past three decades, numerous studies have been conducted that exploit some form of the RC transmission line model to understand the selfdischarge mechanism [8][9][10], the effect of pore size and shape [11][12][13][14][15][16][17][18], and the effect of electrolyte concentration [19,20], among others [21]. More recently, several studies have argued for more detailed analysis of ion transport inside the DL [4,5,[22][23][24][25][26].…”

“…The ratio of DL thickness to the pore radius impacts the ion transport inside a cylindrical pore [4,5,7,26]. The standard RC transmission line model assumes that the DL thickness is significantly smaller than the pore radius [7].…”