全釩液流電池之電極改質與充電狀態感測及系統模擬研究

Abstract

全釩液流電池(vanadium redox flow battery, VRFB)為一種具發展潛力之電能存儲裝置，特別適用於搭配間歇式再生能源，強化綠能應用性。本論文之研究內容分成三部分，施以創新改質技術，納菲離子聚合物溶液(Nafion® ionomer solution)含浸改質石墨氈(graphite felt, GF)，有效改善石墨氈電極的親水性，Nafion®離子聚合物與GF重量比為0.9之NGF N90電極，置於VRFB電解液可達完全潤濕，並具良好電化學反應性能，能量效率高達73.08 ％，放電容量為2.015 Ah。經30次充放電循環後，衰退率僅約3 ％。 針對監測VRFB充電狀態(state of charge, SOC)，提出新穎的超音波速度傳感測定模式，使用2 M硫酸配置釩離子電解液，於不同電解質濃度，測量感測超音波速度值與操作溫度和濃度之關係。於10~90 ％ SOC範圍，超音波速度之實驗量測值與經驗方程式預估值十分吻合，誤差僅±2 ％。 開發數學模型描述VRFB三維多孔電極之電池反應與釩離子交叉混和效應，對比電池充電曲線之模擬數值與實驗數據，充電階段於10~80 ％ SOC，實驗電壓值相對於模擬值僅0.324 ％誤差，驗證模擬結果之優異擬真性。模擬結果推定於80 ％ SOC，V(III)與V(IV)離子濃度量值上升為無擴散混和效應之1.08與1.29倍。

Vanadium redox flow battery (VRFB) is a promising electrical energy storage device, particularly for use with intermittent-type renewable energies. The content of this thesis is divided into three parts. A new modification approach by impregnation with a Nafion® ionomer solution significantly improves the wettability and substantially enhances the activity of the graphite felt (GF) electrode. The NGF N90 electrode with a weight ratio of Nafion® ionomer/GF = 0.9 is fully wetted in the electrolyte solution and exhibits good performance. The energy efficiency is as high as 73.08% and the discharge capacity is about 2.015 Ah. It decays only about 3% after 30 charge-discharge cycles in operation. A novel ultrasonic velocity sensing approach is proposed and investigated to monitor the state of charge (SOC) of VRFB. The vanadium ion solutions are prepared in 2 M H2SO4 aqueous solution. Their ultrasound velocities measured in the electrolyte solutions are found to be both temperature- and concentration-dependent. A master model equation fits the experimental results of both charging and discharging stages quite well within the SOC range of 10~90 % with ±2 % errors with respect to the predicted values. A 3D isothermal model for VRFB is developed. The redox reactions in a porous electrode are formulated. The simulated battery charging curve is compared with the experimental results within the SOC range of 10~80% with 0.324 % errors with respect to the predicted values. The numerical result is validated against the experiment data. The simulation results indicate that the vanadium ion cross-mixing effect is significant at higher SOC. With vanadium crossover effects, the concentration of V(III) and V(IV) ions are increased to 1.08 and 1.29 times at 80 % SOC.