Physical Interpretations of Nyquist Plots for EDLC Electrodes and Devices

Electrochemical impedance spectroscopy (EIS) consists of plotting so-called Nyquist plots representing negative of the imaginary versus the real parts of the complex impedance of individual electrodes or electrochemical cells. To date, interpretations of Nyquist plots have been based on physical intuition and/or on the use of equivalent RC circuits. However, the resulting interpretations are not unique and have often been inconsistent in the literature. This study aims to provide unequivocal physical interpretations of electrochemical impedance spectroscopy (EIS) results for electric double layer capacitor (EDLC) electrodes and devices. To do so, a physicochemical transport model was used for numerically reproducing Nyquist plots accounting for (i) electric double layer (EDL) formation at the electrode/electrolyte interface, (ii) charge transport in the electrode, and (iii) ion electrodiffusion in binary and symmetric electrolytes. Typical Nyquist plots of EDLC electrodes were reproduced numerically for different electrode conductivity and thickness, electrolyte domain thickness, as well as ion diameter, diffusion coefficient, and concentrations. The electrode resistance, electrolyte resistance, and the equilibrium differential capacitance were identified from Nyquist plots without relying on equivalent RC circuits. The internal resistance retrieved from the numerically generated Nyquist plots was comparable to that retrieved from the “IR drop” in numerically simulated galvanostatic cycling. Furthermore, EIS simulations were performed for EDLC devices, and similar interpretations of Nyquist plots were obtained. Finally, these results and interpretations were confirmed experimentally using EDLC devices consisting of two identical activated-carbon electrodes in both aqueous and nonaqueous electrolytes.

电化学阻抗谱（Electrochemical Impedance Spectroscopy, EIS）通过绘制奈奎斯特图（Nyquist plots）进行表征，该图以单个电极或电化学电池的复阻抗实部为横坐标、虚部负值为纵坐标。迄今为止，奈奎斯特图的解读均基于物理直觉和/或等效RC电路的应用，但此类解读并不唯一，且在学术文献中常存在不一致性。本研究旨在为双电层电容器（Electric Double Layer Capacitor, EDLC）电极与器件的电化学阻抗谱测试结果提供明确的物理解读。为此，本研究采用物理化学输运模型，通过数值模拟复现奈奎斯特图，该模型考量了以下三方面内容：(i) 电极/电解质界面处双电层（Electric Double Layer, EDL）的形成；(ii) 电极内部的电荷输运；(iii) 二元对称电解质中的离子电扩散过程。本研究针对不同的电极电导率与厚度、电解质区域厚度，以及离子直径、扩散系数与浓度，通过数值模拟复现了双电层电容器电极的典型奈奎斯特图。无需依赖等效RC电路，即可从奈奎斯特图中提取电极电阻、电解质电阻与平衡微分电容。从数值模拟得到的奈奎斯特图中提取的内阻，与数值模拟恒电流循环过程中得到的“IR降”所得内阻具有可比性。此外，本研究还对双电层电容器器件开展了电化学阻抗谱模拟，得到了与前述一致的奈奎斯特图解读结果。最后，本研究采用由两支相同活性炭电极构成的双电层电容器器件，分别在水系与非水系电解质中开展实验，验证了上述研究结果与解读结论。