电化学双电层电容器动态模拟：离子尺寸及扩散系数的优化

doi:10.11949/0438-1157.20190421

摘要/Abstract

摘要：

电化学双电层电容器的低能量密度限制了其在储能动力等领域的应用，而有机电解液作为提升器件能量密度的关键因素，研究其导电电解质的特性具有重要的意义。通过构建电化学双电层电容器的动态模型，模拟了电化学双电层电容器的循环伏安曲线，并定量探究和解析了离子溶剂化尺寸和扩散系数对电容性能的影响。模拟结果表明：在低扫描速率情况下，电容性能主要受控于双电层结构特性，比电容随着离子溶剂化尺寸的减小而增大，而离子扩散系数对其电容性能没有影响；在高扫描速率情况下，电容性能会受控于离子传质过程，比电容随着离子溶剂化尺寸和离子扩散系数的增大而增大。并基于模拟计算和分析结果提出了理性设计和优化电化学双电层电容器的策略。

关键词: 电化学双电层电容器, 动态建模, 电解质, 离子尺寸, 扩散系数, 优化设计

Abstract:

The low energy density of electrochemical double layer capacitors limits its application in the field of energy storage power. Organic electrolytes, as a key factor in improving the energy density of devices, are of great significance in studying the properties of conductive electrolytes. Ion size and diffusion coefficient in organic electrolyte are the critical factors in determining the energy density of electrochemical double layer capacitors. A dynamic model of electrochemical double layer capacitors was established to simultaneously account for transport process, Stern and diffuse layers and the saturation phenomenon of dielectric permittivity. The cyclic voltammetry measurement of electrochemical double layer capacitors was numerically simulated for different ionic solvation sizes and diffusion coefficients. The simulation results indicated that the specific capacitances were only affected by ionic solvation sizes at low scan rates, increasing significantly with decreasing ionic solvation size, whereas the ionic diffusion coefficients had no effect on the specific capacitances which were mainly controlled by the Stern layer at low scan rates. However, the specific capacitances, which were mainly controlled by the ionic transport within the diffuse layer at high scan rates, were simultaneously affected by ionic solvation sizes and diffusion coefficients. The specific capacitances increased considerably with increasing ionic solvation size or diffusion coefficient at high scan rates. Finally, the strategies for rational design of electrochemical double layer capacitors is proposed based on the simulation results.

Key words: electrochemical double layer capacitors, dynamic modeling, electrolytes, ionic solvation size, diffusion coefficient, optimal design

中图分类号:

TQ 152

鲁浩天, 陈怡沁, 周静红, 隋志军, 周兴贵. 电化学双电层电容器动态模拟：离子尺寸及扩散系数的优化[J]. 化工学报, 2019, 70(10): 4021-4031.

Haotian LU, Yiqin CHEN, Jinghong ZHOU, Zhijun SUI, Xinggui ZHOU. Simulation and optimization of electrochemical double layer capacitors: effects of ion size and diffusion coefficient[J]. CIESC Journal, 2019, 70(10): 4021-4031.

图/表 10

图1 电化学双电层电容器模型的示意图

Fig.1 Schematic of electric double layer capacitor modeling

表1 电化学双电层电容动态模型参数

Table 1 Parameters for elctric double capacitor dynamic state model simulation

Parameters	Value
minimum value of imposed potential/V	-0.5
maximum value of imposed potential/V	0.5
scan rate of cyclic voltammetry/(V·s^-1)	10,10²,10⁴,10⁵
cycle number	6
local temperature/K	298.5
bulk electrolyte concentration/(mol·L^-1)	1.0
solvated ion size/nm	0.4, 0.6, 0.8
diffuse layer thickness/μm	200
ion valency	1
initial diffusion coefficient of ions in electrolyte/(m²·s^-1)^[22]	3.17×10^-10
dielectric constant of electrolyte within diffuse layer^[27]	61.4
dielectric constant of electrolyte within Stern layer	6.5

图2 不同溶剂化离子尺寸下模拟的循环伏安曲线

Fig.2 Predicted cyclic voltammetry curves by simulation with different solvated ion diameters

图3 不同溶剂化离子尺寸下比表面电容随着扫描速率的变化

Fig.3 Predicted specific capacitance variations at different scan rates by simulation with different solvated ion diameters

图4 不同扫描速率下不同离子扩散系数对循环伏安曲线的影响

Fig.4 Predicted cyclic voltammetry curves by simulation with different ion diffusion coefficients

图5 不同离子扩散系数下比表面电容随着扫描速率的变化

Fig.5 Predicted specific capacitance variations at different scan rates by simulation with different ion diffusion coefficients

图6 不同溶剂化离子尺寸条件下斯特恩层与扩散层界面处阳离子浓度随扫描电势的变化曲线

Fig.6 Cation concentration at interface of Stern and diffusion layer with different solvated ion diameters

图8 不同溶剂化离子尺寸条件下负极附近阳离子浓度和电势梯度随扫描电势的变化曲线

Fig.8 Cation concentration and potential gradient at interface of Stern and diffusion layer with linear sweep potential for different solvated ion diameters

图7 不同离子扩散系数条件下斯特恩层与扩散层界面处阳离子浓度随扫描电势的变化曲线

Fig. 7 Cation concentration at interface of Stern and diffusion layer with different ion diffusion coefficients

图9 不同溶剂化离子尺寸条件下在斯特恩层与扩散层界面处电势梯度随界面阳离子浓度的变化曲线

Fig.9 Relation of potential gradient with cation concentration at interface of Stern and diffusion layer with different solvated ion diameters

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