以分子動態模擬研究奈米水溶液電雙層電容器

Abstract

電化學電雙層電容，俗稱超級電容器，是儲存電能的元件之一，且其儲存的電量是傳統電容的數十至數百倍。 本研究以分子動態模擬研究不同電解質水溶液在平板間的電雙層結構，以及電容性質隨充電量與電解質種類與濃度的關係。 一般來說，水的介電係數隨外加電場增加而下降，因此電容也降低。 純水在極板施加高外加電場下，極板間水分子表現出介電飽和現象，並會呈現一個類似冰晶的結構變化。 但在弱外加電場下，半電極電容與外加電場並非單調的遞減關係。 透過數值微分電雙層電位差對表面電荷的函數，我們可計算電雙層的微分電容，並探討其電容值與離子種類以及溶液濃度的關係。 從不同氯化鈉濃度的微分電容曲線，可以看出濃度的增加的確可以提升電雙層電容，從純水的最大值約8microF/cm2增加到4M氯化鈉溶液約15microF/cm2。不同陰陽離子大小的微分電容會出現半徑越大，電容值越高，和實驗相同的定性趨勢。 透過多種不同分析方法，我們發現溶劑水分子對於微分電容值有相當大的影響力以及面對極板帶正電與負電會呈現不同微觀結構上的變化，並可以由此解釋微分電容曲線上不對稱的現象。

Electrochemical double layer capacitors(ECs), also known as the super-capacitor, is an electrical energy storage device whose capacitance can be tens or hundreds times higher than that of the conventional ones. The electric double layer (EDL) structure and capacitance have been studied using flat electrodes with different aqueous electrolytes via classical molecular dynamics simulation. The change in molecular structure and the electrical properties of the electrolyte solutions are investigated by applying different charges to the electrodes. The dielectric constant of water decrease with applied electric field. At high applied electric fields, the water molecules may even align into highly ordered layer structure. At low applied electric fields, the differential capacitance were calculated by numerical differentiation of the obtained surface charge density vs. double layer potential curve. The value of differential capacitance for water is about 8 microF/cm2 and for 4M NaCl solution is 15 microF/cm2.The maximum of differential capacitance increase with concentration and size of ion, which is in qualitatively good agreement with experimental observations. The structure of water layer near the flat electrode surface is affected by the sign of charge of the electrode. The structure of water has a great influence on its screening capability, leading to the asymmetry behavior in the differential capacitance curve.