氮摻雜石墨烯之合成與鑑定於氧氣還原反應之應用

Synthesis and Characterization of N-doped Reduced Graphene Oxide for the Oxygen Reduction Reaction Applications

吳笙卉(Wu. Sheng Hui)

指導教授 ： 胡啟章

國立清華大學/工學院/化學工程學系/碩士(2015年)

摘要

本研究主要為利用石墨烯與氮原子相互結合形成氮摻雜石墨烯(N-rGO)，將之應用於氧氣還原反應(ORR)的研究。研究中利用旋轉環-盤電極(RRDE)計算電子轉移數，藉此了解ORR的機制。
　　本研究的主題分為兩個部分。第一個部分為探究使用RRDE時如何選定適當的環電位(Ering)以偵測ORR之中間體。首先對不同材料進行RRDE實驗，測試材料包括含有20 %白金之碳粉 (20 %, Pt/C)、α相之二氧化錳(α-MnO2)、α相之二氧化錳添加碳粉XC-72 (α-MnO2/XC-72)、XC-72及N-rGO，比較Ering設定為1.03 V、1.23 V及1.48 V (vs. RHE)所得電子轉移數並進行比較。結果得知，對於電子轉移數趨近於4的Pt/C、α-MnO2及α-MnO2/XC-72，當Ering = 1.48 V (vs. RHE)，所計算之電子轉移數較1.03 V及1.23 V (vs. RHE)低，然而Ering設定為1.03 V或1.23 V (vs. RHE)所偵測到的電子轉移數則最接近Koutecky-Levich plot (K-L plot)結果，初步推測1.03 V至1.23　V (vs. RHE)之Ering最為可信。從環電流(IR)隨著Ering從0.9 V至1.6 V (vs. RHE)的線性掃描伏安圖(LSV)得知，Ering於1 V至1.3 V (vs. RHE)之間IR幾乎不受Ering影響，而大於1.3 V (vs. RHE)之電位IR隨著Ering上升而增加，當pH值提高時此現象又更加明顯，由此判斷高電位下IR增加可能是OH-氧化所致，因此大於1.3 V (vs. RHE)之電位範圍並不適合用於偵測ORR中間體。由本研究之分析方法可找到最佳Ering設定值，並增加RRDE實驗的準確性。
　　第二部分利用微波輔助水熱法(MAHS)製備N-rGO，並探討ORR之應用。由於金屬空氣電池和過氧化氫(H2O2)電化學產生兩大重要應用，透過25-1部分因素實驗設計法(FFD)分析(A)微波反應溫度、(B)功率、(C)持溫時間、(D)吡咯單體(pyrrole monomer)與三聚氰胺(melamine)中的氮莫耳數比及(E)分子總莫耳數此五個實驗條件於ORR表現之影響。透過平方和百分比及變異數分析得知(A)微波反應溫度和(E)分子總莫耳數對於電子轉移數影響最深。陡升途徑實驗(PSA)中，透過改變(A)微波反應溫度和(E)分子總莫耳數的條件，可有效控制電子轉移數從2.94至3.93，3.93接近4個電子轉移的結果對應用於金屬空氣電池有相當大之助力；陡降途徑實驗(PSD)則是藉由改變(C)持溫時間及(D)pyrrole monomer與melamine中的氮莫耳數比，將電子轉移數從2.94降至2.34，此結果利於進行2個電子轉移反應產生H2O2以及電化學降解有機物之應用。加上XPS圖譜分析不同氮摻雜結構對ORR的影響，以及透過SEM及TEM觀察巨觀及微觀的形貌特徵，並利用STEM的EDS元素分布分析碳、氮、氧元素的分布情形，最後藉由拉曼光譜分析N-rGO和r-GO的排列性及缺陷程度。
最後一章為PSA/PSD的實際應用。PSA之N-rGO作為鋅空氣電池的觸媒並塗布在氣體擴散電極(gas diffusion electrode, GDE)作為電池陰極，以2 mA cm-2定電流做放電測試，電子轉移數最接近4的N-rGO (n=3.93)測得放電電壓為1.235 V；另一方面，PSD之N-rGO則作為產生H2O2的觸媒並塗佈於GDE，施予一定電位0 V (vs. RHE) 1個小時，再利用DPD (N,N-diethyl-p-phenylenediamine)法測得H2O2的濃度，電子轉移數最低的N-rGO (n=2.34)測量H2O2的濃度為25.27 mg L-1，電流效率高達43 %。本研究亦藉由UV-vis吸收光譜的變化觀察橙黃G (Orange G, OG)的電化學降解情形，藉此了解有機物的分解效果，定電位為-0.2 V (vs. RHE)的OG脫色效果最佳，波長為478 nm的吸收度為原始OG溶液吸收度的0.38倍。

關鍵字

石墨烯 ； 氧化石墨烯 ； 氮摻雜氧化石墨烯 ； 氧氣還原反應 ； 旋轉盤環電極

並列摘要

This thesis mainly focuses on the synthesis and fabrication of nitrogen-doped reduced graphene oxide (N-rGO) and its application towards oxygen reduction reaction (ORR). The rotating ring-disk electrode (RRDE) voltammetry was applied to study ORR.
In the first part, the electrocatalytic activities of various ORR catalysts including 20 % platinum on Vulcan XC-72 carbon black (Pt/C), manganese dioxide in the α phase (α-MnO2), XC-72 carbon black, α-MnO2/XC-72, and N-rGO were investigated using RRDE technique. During RRDE measurements, the potentials of the Pt ring electrode (Ering) were fixed at 1.03 V, 1.23 V and 1.48 V (vs. RHE) respectively. The results exhibits that the electron transfer number of Pt/C, α-MnO2 and α-MnO2/XC-72 were independent of the Ering between 1.03 V and 1.23 V (vs. RHE). When Ering was varied to 1.48 V (vs. RHE), the electron transfer number decreased. Electron transfer number was also confirmed by Koutecky-Levich plot (K-L plot), which is close to the RRDE results measured at Ering of 1.03 V and 1.23 V (vs. RHE) inferring that Ering between 1.03 V and 1.23 V is more reliable. Moreover, some by-product formed during the reduction reaction of the disk electrode can be only oxidized at the Ering higher than 1.48 V (vs. RHE). To investigate how the ring current (IR) can be changed by the Ering, Ering was scanned from 0.9 V to 1.6 V (vs. RHE). From the linear sweep voltammograms (LSV) of the ring electrode, IR exhibits an small difference between 1 V and 1.3 V (vs. RHE) and large difference between 1.3 V and 1.48 V (vs. RHE). To confirm the species oxidized at the higher Ering, the pH value of the electrolyte was increased and the difference between 1.3 V and 1.48 V became significant indicating the oxidation of OH-; therefore, the choice of the Ering for RRDE should be in the potential range of 1 V to 1.3 V.
In the second part, we propose an approach to fabricate N-rGO by microwave-assisted hydrothermal synthesis (MAHS). During the fabrication, melamine and pyrrole were used as nitrogen sources. While applying 25-1 factorial design of experiments and path of steepest ascent/descent (PSA/PSD) experiments, five factors including (A) N-doping temperature, (B) microwave power, (C) holding time of the microwave at the doping temperature, (D)　different nitrogen molar ratio of melamine and pyrrole, and (E) total concentration were considered in order to find the optimal values of electron transfer number for ORR. The highest electron transfer number was 3.93 whereas the lowest one was near 2.34. The structures and distributions of nitrogen doped onto r-GO were examined by the x-ray photoelectron spectroscopic (XPS) analysis. The layer-by-layer morphology and the high degree of defects of N-rGO were characterized by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), and Raman spectroscopy.
The N-rGO of PSA and PSD was further examined for ORR applications such as zinc-air battery discharge test and hydrogen peroxide (H2O2) electro generation. The N-rGO of PSA was coated on the gas diffusion electrode (GDE) as the cathode for the zinc-air full cell. The results shows that the highest zinc-air battery cell voltage of 1.235V, when the current density maintained at 2 mA cm-2.
The N-rGO of PSD was also coated on the GDE and used as a working electrode to generate H2O2. The concentration of 25.27 mg L-1 was reached in 1 hr at constant potential of 0 V (vs. RHE). The highest current efficiency was calculated to be 43 %. A higher concentration was reached at -0.2 V (vs. RHE); however, the current efficiency was only 16.4 %. The H2O2 generation was utilized for electrochemical catalytic decomposition of organics. The degree of degradation of orange G (OG) was analyzed by UV-vis absorption spectra. These results show that the relative absorbance of the azo π-conjugation structure of OG was only 0.38 at a constant potential of -0.2 V (vs. RHE) inferring with a high degree of decoloration.

並列關鍵字

Graphene ； Graphene oxide ； N-doped graphene oxide ； Oxygen reduction reaction ； Rotating ring-disk electrode

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