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  • 學位論文

錳氧化物超高電容之放大

Study on Scaling up of MnO2 Supercapacitor

指導教授 : 吳乃立

摘要


本研究中首先以水系膠態電解質應用於氧化錳超高電容器。而膠態電解質是由高分子、鹽類、及去離子水所組成。Potassium polyacrylic acid (PAAK)、共聚物potassium polyacrylicacid-co-polyacrylamide (PAAK-co-PAAM)及polyacrylamide (PAAM)分別為膠態電解質中的高分子。這三種膠態電解質的離子導電度均可維持在10-1 S cm-1。如果和一般水溶液電解質相比較,由於高分子和氧化錳之間的作用力,使得在膠態電解質裡均能使氧化錳的電容量得到提升。 本研究的第二部分運用噴霧乾燥的技術,合成氧化錳、碳黑,PAA的複合材料。從實際應用面來說,超高電容除了比電容是一個評估標準之外,另一項評估標準為幾何電容密度。為了提高幾何電容密度,在單位面積上必須增加活性物質的含量,但是增加了活性物質含量之後,在高速充放電下電極的內部往往接觸不到足夠的電解液,所以造成幾何電容密度會隨著活性物質增加而減少。此複合粒子所含的PAA為超吸水高分子,目的就是為了要解決在高掃描速率下電極內部電解質不足的問題。在增加單位面積活性物質含量到4.4毫克時,此複合材料電極的比電容仍能保持在250 F/g,另一方面,當增加活性物質含量時,沒有PAA的氧化錳電極比電容就下降的非常明顯。再者,此複合材料的電極幾何電容密度也能隨著活性物質含量增加而增加。 為了要使單位面積的活物含量再增加,本研究中使用了泡沫鎳網當作電流收集板,由於泡沫鎳網具有95%的孔隙度,可以填入大量的活性物質,單位面積的電容也從1.1 F提升到4 F,以鎳網為電流收集板的電極,單位面積活性物質的量最多可以達到35毫克;和鈦片薄膜電極相比,最多只有4毫克。活物量大幅的增加,電解質離子的質傳阻力變得相當重要,所以使用多孔性的碳黑為了改善電解液供應不足的問題。最後製作大面積的電極,總電容可以達到64 F。而在這部分的研究中可以發現,整個電極的孔隙扮演了非常重要的角色,如果要保有較高的比電容,活物的量還是會受到限制,所以活物量和電極孔隙度必定存在一個最適值。而在泡沫鎳網電極中,雖然和鈦片薄膜電極相較之下比電容是下降的,但是在加有吸水高分子的電極中,在高掃描速率下還是能保有較多的電容。 本研究最後一部份是在改善氧化錳在低電位下不可逆的反應,因為在低電位下會產生Mn(II)這個不具有電化學活性的梨子,所以在製作對稱式的電極時,其工作電位往往會被限制住,其工作電位範圍大約在0.8V,為了增加電位範圍,在電解質裡加入鈦離子為了抑制Mn(II)的生成以及增加電子轉移的效率。氧化錳複合材料全電極電容,在1.2V的電位範圍下經過3000次的循環之後,在有加入鈦離子的電解質溶液仍然具有非常好的電性。

並列摘要


First, aqueous gel electrolytes have been successfully applied to the MnO2•nH2O supercapacitors. Each gel polymer electrolyte consists of polymer, salt, and water. The polymers, Potassium polyacrylic acid (PAAK), potassium polyacrylic acid-co-polyacrylamide (PAAK-co-PAAM), and polyacrylamide (PAAM) were used in the gel electrolytes. All the gel electrolytes still maintain high ionic conductivities in the order of 10-1 Scm-1. Compared to the MnO2 in liquid electrolytes, the capacitance can be enhanced by the interactions between the polymers and the MnO2. Second, for the MnO2 supercapacitor, the problem of capacitance reduction with increasing oxide loading can be solved to a great extent by introducing superabsorbent polymer, namely polyacrylic acid (PAA), to form new composite powders composed of MnO2, carbon black, and PAA. Besides, the capacitance of oxide in the composite electrode is also much higher than that of the electrode without PAA. One way to increase the geometric capacitance density is to increase the oxide loading per unit area. As mentioned previously, the capacitance is reduced with increasing oxide loading because the inner electrode is not enough wetted. The specific capacitance of the composite electrode is about 250 F/g with oxide loading of 4.4 mg. On the other hand, the specific capacitance of the electrode without PAA suffers from severe reduction with increasing oxide loading. In order to further increase the geometric capacitance, Nickel foam is used as current collector which has high porosity of 95%. Therefore, large amount of active materials can be put into the porous current collector. The largest amount of active materials per unit area in Nickel foam is 35 mg/cm2 while that is 4 mg/cm2 on Titanium current collector. From the preliminary results, it indicates that although PAA can help to improve the diffusion of electrolyte ions within thick electrodes, the porosity of electrodes becomes critical in Nickel foam-based electrodes. In this part, the excellent capacitance retention can be realized by replacing the XC72 carbon black with the pearl2000 porous carbon black. Regardless of the fact that the higher porosity leads to higher charge transfer resistance, the factor that inner electrode can get enough wetted in Nickel-foam based electrodes is more important. The last part of this research is to improve the stability of MnO2 under low potential. The capacitance fading of MnO2 supercapacitor under negative polarization below 0 V (versus Ag/AgCl) is due to the formation of electrochemically inactive Mn(II) and limits the potential window no greater than 0.8 V. Adding the metal redox couple, Ti(IV)/Ti(II), as a charge meditator into electrolyte increases the charge transfer efficiency of some specific reactions and suppressed the formation of Mn(II). Therefore, the stability of the MnO2 symmetric supercapacitor can be improved after 3000 cycles over an operating voltage window of 1.2 V.

參考文獻


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