雙效型氣體擴散電極與高表面積碳凝膠製備及電化學分析

Abstract

本研究利用檸檬酸鹽法(amorphous citrate precursor)合成化學劑量比為La0.6Ca0.4Co0.8Ru0.2O3粉末。此外，利用固態燒結法(solid-state reaction)合成La0.6Ca0.4CoxRu1-xO3的鈣鈦礦氧化物（x = 0、0.2、0.4、0.6、0.8及1）粉末。由X光粉末繞射(XRD)可觀察到主相為鈣鈦礦的La0.6Ca0.4CoO3，表示成功的將Ru4+插層入La0.6Ca0.4CoO3相中，元素組成(EDX)分析結果亦與合成時比例一致。在鹼性KOH溶液中，利用H2O2分解反應測試其催化效能，La0.6Ca0.4Co0.8Ru0.2O3展現出優於La0.6Ca0.4CoO3之催化活性，推測在氧氣還原反應中，Ru摻雜後催化活性被提升。類似的情形可利用氧氣還原極化曲線佐證，實驗中以Black Pearl 2000 (BP2000)當作氣體擴散層的觸媒載體。在恆定電流長時間放電評估，La0.6Ca0.4Co0.8Ru0.2O3/BP2000皆有穩定的電流輸出。另一方面，在氫氣氧化(hydrogen evolution reaction)與氧氣還原(oxygen reduction reaction)極化曲線反應中，La0.6Ca0.4CoxRu1-xO3/BP2000相較於La0.6Ca0.4CoO3/BP2000皆表現出較佳的雙效催化活性，在SSR法La0.6Ca0.4CoxRu1-xO3/BP2000系統中，La0.6Ca0.4Co0.4Ru0.6O3/BP2000具有最佳的雙功能催化活性。 研究中，採用機械研磨法研磨La0.6Ca0.4CoO3與IrO2粉末，成功的製備出La0.6Ca0.4CoIr0.25O3.5-δ觸媒。另一方面，利用檸檬酸鹽法合成在鹼性電解液下具有氧還原與氧生成的La0.6Ca0.4Co0.8Ir0.2O3雙效觸媒。粉末XRD圖中展現出以La0.6Ca0.4CoO3為主相，表示Ir4+成功進入鈣鈦礦Co的位置。在電化學分析中， La0.6Ca0.4CoIr0.25O3.5-δ/CNCs與La0.6Ca0.4Co0.8Ir0.2O3/CNCs觸媒相對於La0.6Ca0.4CoO3/CNCs，不論是放電或充電電流對電壓極化曲線皆有較優越的性能表現，比較Ru與Ir摻雜入La0.6Ca0.4CoO3中，兩者的還原反應電催化性能相近。此外，以表面電位(Zeta Potential)測量表面電位，La0.6Ca0.4Co0.8Ir0.2O3相對於La0.6Ca0.4Co0.8Ir0.2O3有顯著變化。最後，以連續3小時為一循環重複氧還原反應/靜置/氧氧化反應，La0.6Ca0.4Co0.8Ir0.2O3/CNCs觸媒呈現穩定與持續耐久度。 對於提升電化學活性，除了合成適當的觸媒，開發適當的觸媒載體是另一關鍵議題，因此，本研究利用間苯二酚-甲醛(resorcinol-formaldehyde)的縮合反應合成碳常壓凝膠，做為電化學電雙層電容探討研究，其中反應中選擇醋酸為催化劑，樣品濃度R:F為1:2，R:C比例控制為5:1與10:1，接下來經過溶劑交換、熱裂解與二氧化碳活化。在溶劑的交換過程可避免乾燥過程中結構收縮，再經過二氧化碳活化後，我們能到得到多孔性的碳結構，其表面積達到3419 m2g-1。由SEM觀察到相互交連的泡沫狀結構，由BET證實有大量的微孔與中孔。電容特性與動力學探討部分，使用鈦孔穴電極做電化學分析量測，所使用的技術有定電流充放電法(Current Reversal Chronopotentiometry, CRC)，循環伏安法(Cyclic Voltammetry, CV)， 以及交流阻抗法(Electrochemical Impedance Spectroscopy, EIS)法。由CV與CRC結果得知，我們所合成出的碳常壓空氣凝膠相對於市售的BP2000具有較佳的比電容與持續耐久度。舉例來說，在電位區間為0-1 V，1 Ag-1得充放電條件下，比電容值為324.8 Fg-1，相對來說，沒有經過活化處理的樣品，雖然有類似的相型態，但表面積與比電容大幅降低至449 m2g-1以及34.7 Fg-1。

We employed an amorphous citrate precursor (ACP) method to synthesize stoichiometric La0.6Ca0.4Co0.8Ru0.2O3 powders. Besides, a variety of La0.6Ca0.4CoxRu1-xO3 perovskite oxide (x=0, 0.2, 0.4, 0.6, 0.8, and 1) were fabricated by solid-state reaction (SSR) method to form oxide powder with various ruthenium (Ru) ratios. X-ray diffraction profiles (XRD) of the as-synthesized powders exhibited the major phase identical is La0.6Ca0.4CoO3, indicating successful incorporation of Ru4+ at the Co cation sites. ACP-derived La0.6Ca0.4Co0.8Ru0.2O3 exhibited a higher H2O2 decomposition rate in KOH solution as opposed to that of ACP-derived La0.6Ca0.4CoO3, which suggested an improved catalytic ability for the oxygen reduction reaction (ORR). In ORR and hydrogen evolution reaction (HER) I-V polarization curves, the SSR-derived La0.6Ca0.4CoxRu1-xO3/BP2000 revealed an enhanced bi-functional catalytic ability in comparison with those of La0.6Ca0.4CoO3/BP2000. La0.6Ca0.4CoIr0.25O3.5-δ was prepared by a mechanical alloying process from mixtures of La0.6Ca0.4CoO3 and IrO2. The ACP method was employed to prepare perovskite La0.6Ca0.4Co0.8Ir0.2O3 as a bi-functional electrocatalyst for ORR and HER in an alkaline electrolyte. The XRD pattern of the as-synthesized powders exhibited the major phase is La0.6Ca0.4CoO3, indicating successful incorporation of Ir4+ at the Co cation sites. Supported on carbon Nanocapsules (CNCs), the La0.6Ca0.4CoIr0.25O3.5-δ and La0.6Ca0.4Co0.8Ir0.2O3 particles demonstrated superior performances than those of La0.6Ca0.4CoO3 in both charging and discharging I-V polarizations. For Ru and Ir doped into La0.6Ca0.4CoO3, the electrochemical capabilities displayed similar performance for the ORR. In life time determinations, La0.6Ca0.4Co0.8Ir0.2O3/CNCs delivered a stable and sustainable behavior with moderate degradation. In addition to synthesis of suitable electrocatalyst, the other critical issue is to identify appropriate material as electrocatalyst support. Therefore, a resorcinol–formaldehyde (R-F) condensation reaction catalyzed by acetic acid (C) is employed to prepare the carbon ambient gels for electrochemical double layer capacitors. The samples was fabricated with a R:F ratio of 1:2 and R:C ratios of 5:1 and 10:1, followed by solvent exchange, pyrolysis, and CO2 activation. The solvent exchange allowed negligible structure contraction upon drying, and after CO2 treatment, we were able to produce porous carbons with a surface area of 3419 m2g-1. Electrochemical analysis including cyclic voltammetry (CV), current reversal chronopotentiometry (CRC), and impedance spectroscopy are conducted using a titanium cavity electrode so relevant capacitive characteristics and kinetic parameters could be determined. Both CV and CRC results indicate specific capacitances and life time behaviors are comparable or even better than those of BP 2000.