超級電容器與空氣電極材料製備分析及其電化學性質研究

Abstract

本研究進行了超級電容器以及空氣電極材料的製備、鑑定，以及電化學性能的分析量測。我們設計了一種能鈦金屬孔罅電極 (Titanium Cavity Electrode; TCE) 來加速測試程序的進行，該電極只需要使非常少量的材料樣品，就能夠迅速地呈現出試樣本質的電化學特徵。因此，能夠清楚地用來評估該材料使用於超級電容器的可行性。所研究的材料除了奈米碳膠囊 (Carbon NanoCapsules; CNCs) 之外，尚包括了可取得的商業化碳黑產品，諸如BP2000以及Vulcan XC72R等。另外，我們也利用RuO2•xH2O-CNCs 和奈米顆粒之 Ag-CNCs材料製成電極獲得了偽電容 (pseudocapacitance) 和空氣電極的相關性質。 TCE是從原始設計用來量測粉體材料電化學性質的孔罅微電極 (Cavity Microelectrode; CME) 改良而成的一種電極結構，TCE不僅保有使用微量的粉體之優點又能精確地控制粉體重量。因此，TCE的實驗再現性令人滿意，能用來有效地過篩出可供選用的碳基材料。在所進行研究的幾種碳材之中，CNCs具有糾結相通的石墨烯層以及中空結構，意味著將具有較佳的導電性以及高質量密度特質，這是讓我們特別感與趣。CNCs是在缺氧的氣氛之下，使用乙炔和氧的混合氣體以火焰燃燒法所製備而成的奈米材料。初合成的CNCs直徑約為10~25nm之間 ，BET比表面積大約300 m2/g。經過適當的處理之後，表面積可提高到2019 m2/g且中巨孔比表面積佔92.6% 。實驗結果發現，在1N H2SO4電解液中其比電容值落於60 ~200 Fg-1範圍。 以共沈澱法所製備之RuO2．xH2O/CNCs，於1N H2SO4電解液中比電容值可達到490 F/g。然而，不管在大氣中或者是在水熱環境中進行後續處理，其循環壽命依然呈現出明顯衰減的情形。Ru-Ta/Ti電極中的Ru-Ta二元複合氧化物材料其比電容性能的表現象優於純Ru氧化物。其中金屬元素莫耳比為8:2(Ru：Ta)時所製備之氧化物塗層具有最高的比電容值，其比電容值約為350 F/g。RuTa二元複合氧化物電容材料與碳簇電容材料最大的差異在於其更為優異的導電度以及高功率充放電性質。相反地，PANI/Ti具有很高的比電容值，在低掃描速率時甚至可達到600 F/g以上，但卻受到掃描速率的影響非常顯著，循環壽命也短。這意味著PANI電容材料雖然具有高比電容值，但在仍侷限於法拉第反應的限制，並不適合在高功率的應用。 以BP2000商品、經表面改質處理之後的自製CNCs材料、RuO2.xH2O/CNCs複合材料等電容材料，添加2.5% 石墨為導電材料、10% PTFE及2.5% PVA為粘著劑為能夠兼顧電極結構強度、電解液親和性以及導電性能的配方組合。所製備4x5 cm尺寸的二極式超級電容器，其性質相較於可取得市售超級電容性商品在等效串聯電阻值(Equivalent Series Resistance; ESR)及自放電率兩個參數遠優於自製的電容器，不過在單位面積比電容及洩漏電流性能上自製的電容器則有較佳的表現。

This study is concerned with the synthesis, characterizations, and electrochemical analysis of materials for supercapacitors and gas diffusion electrodes applications. To expedite the testing process, we design a titanium cavity electrode (TCE) that employs a relatively small amount of samples for quick determination on essential electrochemical parameters so their prospects on supercapactors can be evaluated fairly. The samples under study include carbon nanocapsules (CNCs) and commercially available carbon blacks such as BP2000 and Vulcan XC72R. In addition, we prepare electrodes made of RuO2•xH2O-CNCs and nanoparticulate Ag-CNCs and obtain relevant properties for pseudocapacitance and gas diffusion electrodes. The TCE is a modified version of cavity microelectrode, which is originally designed for determining the electrochemical properties of powders. The TCE not only retains advantages of using a minute amount of powders but also provides a better control in sample weighting. Therefore, the TCE enables impressive experimental reproducibility for efficient screening of promising carbonaceous candidates. Among many materials under studies, the CNCs are of particular interests because it composes entangled grapheme layers encapsulating a hollow core which infers better electrical conductivity and higher energy density per unit mass. The CNCs are prepared by a flame combustion method using a mixture gas of C2H2 and O2. The diameter for the as-synthesized CNCs is in the range of 10–25 nm with a BET surface area of 300 m2/g. After appropriate treatments, the surface area can be further increased to 2019 m2/g with mesopores surface area of 92.6%. As a result, the specific capacitance in 1 N H2SO4 electrolyte is ranged between 60 to 200 F/g. The RuO2•xH2O-CNCs is synthesized from the coprecipitation method which reveals a capacitance of 490 F/g in 1 N H2SO4 electrolyte. However, the RuO2•xH2O-CNCs demonstrates serious decline in life cycle regardless of post-treatments in air or hydrothermal environments. In contrast, the specific capacitance of Ru-Ta binary oxide compound in the Ru-Ta/Ti electrode is much superior to pure Ru oxides. With a molar ratio of 8:2 (Ru:Ta), the sample displays the highest specific capacitance around 350 F/g. We realize that the notable advantage of Ru-Ta binary compound oxide over conventional carbonaceous capacitor materials is its improved electrical conductivity enabling facile charging and discharging in high power mode. In contrast, the PANI/Ti has a high specific capacitance (600 F/g) at relatively low scan rate but its value declines considerably with increasing scan rates along with deteriorating life cycles. This indicates that the contributing current for capacitance is limited by Faradaic reaction so its adoption in high power applications is rather slim. The recipes of using BP2000, modified-CNCs, or RuO2•xH2O-CNCs as a capacitor material, with the addition of 2.5 wt% graphite for conductive improvement, 10 wt% PTFE and 2.5 wt% PVA to fabricate porous electrodes is able to combine strength, electrolyte affinity and conductive properties. For comparison purpose, the 4x5 cm2 two-pole type supercapacitors are prepared and evaluated along with commercialized supercapacitor products in both specific capacitance and leakage current. Unfortunately, the ESR and self-discharge rate for our samples are inferior to those from commercial products.