單步驟合成石墨烯材料與其在電子與電化學催化之應用

One-Step Synthesis of Graphene-based Materials for Electronic and Electrocatalytic Applications

曾祈昂(Chi-Ang Tseng)

指導教授 ： 陳逸聰

國立臺灣大學/理學院/化學研究所/博士(2019年)

https://doi.org/10.6342/NTU201902190

摘要

本研究利用二維材料石墨烯(graphene)建構出石墨烯三維球型結構，開發石墨烯包覆銅奈米粒子(MLG-CuNPs)、石墨烯中空球(GHBs)和氮參雜石墨烯中空球(N-GHBs)之材料，並將其應用於抗氧化電極(antioxidation)、電催化水分解產氫(hydrogen evolution reaction, HER)、染料敏化太陽能電池(dye-sensitized solar cell, DSSC)以及超級電容(supercapacitor)。本研究開發出單步驟且低溫沉積之化學氣相沉積方法(chemical vapor deposition, CVD)，可將上述材料沉積於各種基板上，例如：矽晶片(Silicon wafer, Si)、聚醯亞胺(Polyimide, PI)、碳布(Carbon cloth, CC)等。
(i) 將石墨烯包覆銅奈米粒子沉積於聚醯亞胺(MLG-CuNPs/PI)和矽晶片(MLG-CuNPs/Si)上，可做為抗氧化電極之應用，其電阻率分別為1.7 × 10-6 and 1.4 × 10-6 Ωm，電阻率低於文獻中報導的100倍。由於石墨烯包覆於銅奈米粒子表面，形成保護層，因此此電極具有良好的抗氧化特性。而石墨烯包覆銅奈米粒子沉積於聚醯亞胺之電極展現出優異的機械耐久度，並且成功利用此可饒曲電極組裝石墨烯電晶體(graphene-based field-effect transistor, G-FET)元件。另外，將石墨烯包覆銅奈米粒子沉積於碳布(MLG-CuNPs/CC)上，在電催化水分解產氫上有良好的催化性能及催化穩定性。利用石墨烯包覆銅奈米粒子當作催化電極，過電位(overpotential)只需施加-375 mV，即可產生∼10 mA cm-2的催化電流，在經過連續18小時的催化電解，石墨烯包覆銅奈米粒子之電極仍保有穩定之電流值，展現出其實際應用之價值。
(ii) 氮參雜之石墨烯中空球可於上述CVD方法中加入三聚氰胺當作氮前驅物來進行合成，作為無金屬之催化電極應用於染料敏化太陽能之陰極，進行碘還原催化(triiodide reduction)。氮參雜之石墨烯中空球解決了平面石墨烯相互堆疊之問題。藉由控制三聚氰胺揮發溫度，可控制石墨烯中空球中氮參雜之比例從8.7到14.0 %和不同的氮參雜型態(dopping state)。藉由不同氮參雜型態之討論，我們發現吡啶型氮(pyridinic nitrogen)和四級型氮(quaternary nitrogen)對碘還原有明顯的催化效應。由於氮參雜之石墨烯中空球提供高的表面積和氮參雜改善催化活性，降低電荷轉移之電阻(charge-transfer resistance)，因此將氮參雜之石墨烯中空球當作陰極之太陽能電池元件效率可達7.53%，接近利用標準白金電極製作之太陽能電池元件(7.70%)。
(iii)將高理論電容材料-氧化鈷(Cobalt oxide, CoO)電沉積在石墨烯中空球(GHBs)表面於碳布基板上，可作為質量輕、可撓性且高效能的超級電容電極。此超級電容電極(CoO-GHBs/CC)擁有高的比電容(specific capacitance)，以1 A g-1進行充放電，比電容可達2238 F g-1，且具有良好的充放電能力(rate capability)，即使以15 A g-1進行充放電，比電容仍能保持在1170 F g-1。另外，利用臨場電化學液相穿透式電子顯微鏡(in situ electrochemical liquid TEM)觀察氫氧化鈷(Co(OH)2)在石墨烯中空球表面上的成長，可發現氫氧化鈷均勻沿著石墨烯中空球表面生長，相反地，氫氧化鈷成長在碳電極上則會聚集成為團簇(cluster)。再者，利用超級電容電極(CoO-GHBs/CC)組裝成對稱性電容元件(symmetric supercapacitor)可展現出高的能量密度(energy density)(16 Wh kg-1 at 800 W kg-1)及高的功率密度(power density)(6000 W kg-1 at 8.2 Wh kg-1)，且電容元件也表現出優異的可撓性及充放電穩定性，在經過5000次充放電循環後，電容仍能維持在~100%。

關鍵字

石墨烯 ； 化學氣相沉積法 ； 抗氧化電極 ； 電化學產氫 ； 染料敏化太陽能電池 ； 超級電容 ； 臨場液相穿透式電子顯微鏡

並列摘要

In this thesis, we report a novel, one-step chemical vapor deposition (CVD) synthesis method to fabricate multi-layer graphene (MLG), MLG-wrapped copper nanoparticles (MLG-CuNPs), and nitrogen-doped graphene hollow nanoballs (N-GHBs) directly on various substrates (e.g., polyimide film (PI), carbon cloth (CC)), or silicon wafer (Si)). These synthesized materials have been widely applied to electronic and electocatalytic fields, such as antioxidation electrode, hydrogen evolution reaction, dye-sensitized solar cell, and supercapacitor.
(i) The MLG-CuNPs/PI and MLG-CuPNs/Si were used as antioxidation electrodes, which possess the electrical resistivities of 1.7 × 10-6 and 1.4 × 10-6 Ωm, respectively, of which both values are ~100-fold lower than earlier reports. Both MLG-CuNPs/PI and MLG-CuNPs/Si retained almost their conductivities after ambient annealing at 150 °C due to the remarkable protection of the Cu nanocores by MLG shell. The flexible MLG-CuNPs/PI exhibits excellent mechanical durability after 1000 bending cycles and can be used as promising source-drain electrodes in fabricating flexible graphene-based field-effect transistor (G-FET) devices. Moreover, the MLG-CuNPs/CC exhibits high performance and durability toward hydrogen evolution reaction (HER). The overpotential required to drive Jcathodic = 10 mA cm-2 for the MLG-CuNPs/CC electrodes is only -375 mV. The MLG-CuNPs/CC electrode remained stable after a continuous electrolysis test over 18 hours at Jcathodic of ∼10 mA cm-2.
(ii) N-GHBs/CC was used as an efficient metal-free electrocatalyst for dye-sensitized solar cell (DSSC) applications. The highly curved N-GHBs could avoid the self-assembly restacking of planar graphene sheets. By controlling the evaporation temperature of nitrogen precursor, the nitrogen-doping content of 8.7-14.0% and different nitrogen-doped configurations in N-GHBs could be adjusted. The results show that the pyridinic and quaternary nitrogens, rather than the total nitrogen doping level, in N-GHBs are mainly responsible for triiodide (I3-) reduction in DSSCs. For solar cell applications, the high surface area and heteroatomic nitrogens of GHBs can remarkably improve the catalytic activity toward the triiodide reduction, lower the charge-transfer resistance, and enhance the corresponding photovoltaic performance (7.53%), which is comparable to that of a standard sputtered Pt counter electrode-based cell (7.70%).
(iii) A binder free, flexible, and lightweight electrode is prepared for a high-performance supercapacitor, where a carbon cloth (CC) substrate was integrated with the active material of cobalt oxide (CoO) nanosheets and graphene hollow nanoballs (GHBs) synthesized via electro-chemical deposition and chemical vapor deposition (CVD), respectively. The high surface area and high conductivity of GHBs and excellent electrochemical activity of CoO nanosheets lead to a high specific capacitance of 2238 F g-1 and good rate capability (1170 F g-1 at 15 A g-1). In addition, the dynamic growth of Co(OH)2, a precursor of CoO, on GHBs to form CoO layers was investigated by in situ electrochemical liquid transmission electron microscopy (TEM). A solid-state symmetric supercapacitor (SSC) device using CoO-GHBs/CC samples as positive and negative electrodes exhibits outstanding flexibility, high power density (6000 W kg-1 at 8.2 Wh kg-1), high energy density (16 Wh kg-1 at 800 W kg-1), and cycling stability (~100% capacitance retention after 5000 cycles). The high power CoO-GHBs/CC electrode with the device merits of high specific capacitance, good rate capability, outstanding flexibility, high cycling stability, and high energy densities shows a promising future in energy storage technology.

並列關鍵字

Graphene ； CVD ； HER ； Supercapacitor ； DSSC ； in situ liquid TEM

參考文獻

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