以模擬引導提升二氧化錳@網狀玻璃碳及活性碳全固態混合型電容器之功率及能量密度

Abstract

超級電容器是一種電化學儲能裝置，目前已經被應用在許多攜帶式電子產品和電動車的煞車回充系統，然而現今面臨的挑戰為相對低的能量密度，雖然目前有許多材料已被開發並做為超級電容器的電極，實驗方面仍需要進行多次重覆測試才能找出最佳化的超級電容器，此過程耗時費力。因此若能夠運用模擬軟體先進行超級電容器性能的預測，將能有效引導實驗的進行，並節省實驗上的時間及成本。本研究使用COMSOL模擬軟體預測不同參數對於超級電容器性能的影響，藉由模擬預測的結果引導實驗進行電極改質，改變的參數包含正極孔洞大小、厚度，以及負極活性物質的含量，以利得到最佳化的超級電容器。為了能得到較精準的預測結果，參數皆以實驗量測而非文獻的估計值。 電極材料方面選用水熱法合成的二氧化錳@網狀玻璃碳複合電極。二氧化錳為具有高比電容值和高電化學可逆性的擬電容材料；而網狀玻璃碳是一種具有高導電性、低銹蝕性、低密度和低熱膨脹率的孔洞材料；負極採用具有高比表面積和高操作區間的活性碳；搭配膠體聚合電解質形成全固態混合型超級電容器。過往水溶液電解質在1.6 V附近開始水解，會限制了超級電容器的能量密度，且流體易溢漏的問題也會造成安全上的疑慮。而膠體聚合物電解質擁有較高的操作電位，其固態的特性也可以防止電解質溢漏。 藉由模擬引導的最佳化超級電容器，以正極厚度2 mm、孔徑200 μm，負極活性碳含量0.8 g有最好的性能，實驗以電化學阻抗分析和充放電實驗檢測性能，充放電實驗的最高操作電位為3 V，也對系統的電化學可逆性做長時間10000圈充放電實驗，庫倫效率高達98%且比電容值仍維持第一圈的70%，最大能量密度和功率密度分別為21 Wh kg-1和4200 W kg-1。比較模擬和實驗的充放電圖可以得到相同的趨勢，證明可以利用模型來引導實驗最佳化的過程。

Supercapacitors have found increasing applications in portable electronics and the regenerative braking system of electric vehicles. The major challenge facing the state-of-the-art supercapacitors is their relatively low energy density. Though many materials have been developed and used as the electrodes of supercapacitors, repetitive experiments are needed to optimize the performance of supercapacitors. This process is not only time-consuming but it also wastes significant experimental resources. If simulation prediction can be used to guide experimental trials in advance, a lot of time and cost can be saved for device development and optimization. In this study, COMSOL software is employed to conduct simulations to guide the experiment for optimization of hybrid capacitors. Parameters including pore sizes, thicknesses of positive electrode and active material ratios of negative electrode are considered. Furthermore, all of the relevant parameters are measured through experiments so that the simulation prediction is more accurate. MnO2/RVC composite electrode synthesized via hydrothermal technique is used as the positive electrode. MnO2 is shown to be a pseudocapacitive material in the aqueous system, which has high capacitance and high electrochemical reversibility. Reticulated vitreous carbon (RVC), meanwhile, is a porous, glassy carbon material advantageous in its high electrical conductivity, low density, high corrosion resistance, and low thermal expansion. And the capacitive negative electrode is comprised of activated carbon (AC) because of its high specific surface area and wide operable potentials. As for electrolyte, a PVDF based gel polymer electrolyte (GPE) is selected. The conventional aqueous electrolyte has its limit because of the decomposition of water. Moreover, the electrolyte leakage leads to safety concerns. On the other hand, GPE has wider operable voltage window and the solid-state characteristics preventing the electrolyte from leakage. The electrochemical performance of the simulation guided all-solid-state supercapacitor with MnO2@RVC//AC electrodes and LiClO4 GPE is investigated by impedance spectroscopy (EIS), cyclic voltammetry (CV) and galvanostatic charge/discharge experiments (GCD), where GCD is conducted with an upper voltage limit of 3 V. MnO2@RVC//AC has Coulombic efficiency 98% and capacitance retention 70% over 10000 cycles, and energy density and power density are 21 Wh kg-1 and 4200 W kg-1, respectively. The GCD curves of simulation and experiment show the same trend, proving that the model can be used to guide the optimization of hybrid capacitor.