氧化石墨烯的電化學還原與超級電容應用之技術開發
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2021
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電化學還原是一種環保、節能且恢復氧化石墨烯(GO)特性的方法。然而，一般的電化學還原方法中，皆必須預先塗佈氧化石墨烯膜於電極表面，再以二極式或三極式系統給予負向偏壓進行還原處理，才能得到電化學還原氧化石墨烯(ERGO)膜，但後續僅能一次性的直接作為傳感器感測層或導電薄膜使用。因此，這種僅能獲得ERGO薄膜的電化學還原方法，無法被大量生產ERGO薄片與滿足產業應用需求。本研究提出一種結合機械式循環攪拌與電化學還原方法，將氧化石墨烯進行電化學還原，其方式是直接將GO薄片用作還原材料，不需預先將其製備成GO薄膜即可進行電化學還原。此外，更採用磷酸鹽緩衝溶液(K2HPO4/KH2PO4, PBS)作為電解質溶液，此即為生理食鹽水，具有環保、無毒、安全之特性。均質機攪拌棒同時作為電極使用，可實現循環攪拌與電化學還原之功能，以電源供應器施加負向偏壓，將GO薄片置於多孔性陶瓷濾筒之電解質溶液中，進行持續性機械攪拌與電化學還原而得ERGO薄片。本研究透過拉曼光譜分析儀(Raman spectroscopy)快速確認氧化石墨烯樣品的還原效果，再藉由X射線粉末繞射儀(XRD)分析判斷氧化石墨烯樣品結晶結構變化。另外，再以X射線光電子能譜儀(XPS)判別氧碳比值，以及掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)及原子力顯微鏡(AFM)等儀器設備，評估石墨烯薄片大小、表面形貌、層數及厚度狀況。最後，再以傅立葉轉換紅外線光譜儀(FTIR)量測官能基的變化。實驗結果顯示GO薄片經電化學還原成ERGO薄片，經由旋轉攪拌速度探討得知，攪拌速度愈快會使薄片尺寸變小，也導致 ID/IG 升高。從改變電解質濃度分析發現，濃度並非造成還原效果差異的主要影響因素。由改變偏壓大小之實驗得知，偏壓上升兩極反應速度加快，可縮短還原所需時間，在偏壓−17.5 V/2 h條件下，拉曼光譜中ID/IG從0.85增加到1.08。在XPS測量中C/O比值從氧化石墨烯中測得的2.02，增加到ERGO (17.5 V/2h)的3.10。再經XPS與FTIR比對其化學元素與鍵結形態，發現還原後含氧官能基團也明顯變少。上述結果歸納可知，本研究提出的電化學還原技術可獲得良好的GO薄片還原效果，且具有大規模生產ERGO薄片的潛力。本研究進一步利用ERGO薄片進行超級電容電極之電化學特性與性能評估。首先，由原始石墨(Graphite)、氧化石墨烯和ERGO進行電導率的比較，發現原始石墨電導率約為7.35×10-1 S·cm-1，而氧化石墨烯電導率7.92×10-4 S·cm-1明顯降低，經電化學還原後ERGO(10 V/8 h)則提升至3.83×10-1 S·cm-1，而ERGO (17.5 V/2 h)則再次提升至5.16×10-1 S·cm-1，而其電導率則比氧化石墨烯提升了約650倍。接著，進行循環伏安法(CV)與恆電流充放電法(GCD)評估，發現ERGO (17.5 V/2h)薄片之比電容值提升至191 F/g，高出氧化石墨烯17倍，也與熱還原氧化還原石墨烯(TRGO)的比電容值205 F/g相近。最後，再由電化學阻抗頻譜圖(EIS)分析各樣品材料與電解質界面之間反應行為，由奈奎斯特圖(Nyquist plot)圖形曲線可發現，氧化石墨烯與ERGO曲線形態有明顯不同。在高頻區之氧化石墨烯半圓直徑，經電化學還原後明顯變小，經串聯等效電路模擬其電荷轉移阻抗(Rct)，發現由氧化石墨烯的4.3 kΩ明顯變小至ERGO (17.5 V/2 h)的0.82 kΩ；ERGO (17.5 V/2 h)在低頻區之直線斜率也明顯變大，即代表擴散阻抗變小趨勢。綜合以上成果證實，本研究已成功開發一種利用安全無毒的PBS磷酸鹽緩衝溶液當作電解液，直接將氧化石墨烯薄片快速還原成ERGO薄片的方法。此種電化學還原技術，氧化石墨烯膜不必預先塗佈在陰極上再進行還原，容易放大生產條件，有機會實現大規模且高品質的ERGO薄片製備。此外，所製備之ERGO薄片進一步評估其各種電化學特性，也發現其擁有優異的電容性能表現，未來將有潛力擴大應用於高性能超級電容的開發。
Electrochemical reduction is an environmentally friendly and energy-efficient way to restore characteristics of graphene oxide (GO). However, in general electrochemical reduction methods, graphene oxide film must be pre-coated on the electrode surface, and then a bias is applied with a two-pole or three-pole system to perform the reduction to obtain the electrochemical reduction graphene oxide (ERGO) film, but this film can only be used as a sensor sensing layer or conductive film for a one-time usage. Therefore, this method of electrochemical reduction, which can only produce ERGO film, cannot be used to manufacture large quantities of ERGO flakes to meet the needs of industrial applications.This study proposes a method which combines mechanical cyclic stirring and electrochemical reduction for electrochemical reduction of graphene oxide. It uses GO flakes directly as a reduction material without the need to pre-prepare it into GO film to carry out electrochemical reduction. In addition, phosphate buffer solution (K2HPO4/KH2PO4, PBS) is used as electrolyte solution, which is essentially saline solution and is environmentally friendly, non-toxic and safe. Homogenizer stirrer rod acts as an electrode to realize the function of cyclic stirring and electrochemical reduction, applying bias from the power supply. The GO flakes is placed in electrolyte solution in a porous ceramic filter, ERGO flakes are produced by continuous mechanical stirring and electrochemical reduction. This study quickly confirms the reduction effect of graphene oxide samples by Raman spectroscopy and determines the crystallization structure changes of graphene oxide samples by X-ray powder diffraction (XRD). In addition, X-ray photoelectron energy spectrometer (XPS) is used to identify oxygen carbon ratio; equipment like scanning electron microscope (SEM), transmission electron microscope (TEM) and atomic force microscope (AFM) is used to evaluate the size, surface morphology, number of layers and thickness of graphene flakes. Finally, the change of the functional base is measured with a Fourier transform infrared spectroscopy (FTIR).Experimental results show that GO flakes is electrochemically reduced to ERGO flakes, and the study of rotary stirring speed shows that faster speed will make the flake size smaller, and also lead to ID/IG increase. Analysis of electrolyte concentration changes finds that concentrations are not the main factor contributing to the difference in the reduction effect. By changing the value of bias, experiment results show that higher bias raises the reaction speed at both electrodes, therefore shortens the reduction time. Under the bias of −17.5 V/2 h, the Raman spectroscopy ID/IG increased from 0.85 to 1.08. In XPS measurements, C/O ratio increased from 2.02 in graphene oxide to 3.10 in ERGO (17.5 V/2h). After that, XPS and FTIR were used to compare its chemical elements and the nature of bonding, it was found that oxygen functional groups decreased significantly after reduction. The above results show that the electrochemical reduction technology proposed in this study can achieve good GO flakes reduction results and has the potential for mass production of ERGO flakes.This study further used ERGO flake to evaluate the electrochemical properties and performance. First, it compared the conductivities of graphite, graphene oxide and ERGO and found out that graphite has a conductivity of about 7.35 × 10-1 S·cm-1, and graphene oxide has a much lower conductivity of 7.92 × 10-4 S·cm-1. After electrochemical reduction, conductivity of ERGO with 10 V/8 h bias increased to 3.83 × 10-1 S·cm-1, which increased again to 5.16 × 10-1 S·cm-1 with -17.5 V/2 h bias, an increase of about 650 times than graphene oxide. Next, the cyclic voltammetry (CV) and constant charge-discharge method (GCD) were used to perform evaluation, and the specific capacitance value of ERGO (17.5 V/2h) flakes increased to 191 F/g, 17 times higher than that of graphene oxide, close to the specific capacitance of 205 F/g of thermal reduction graphene oxide (TRGO). Finally, the reaction behavior between the sample material and the electrolyte interface is analyzed by electrochemical impedance spectroscopy (EIS). Nyquist plot graphics show that graphene oxide is significantly different from ERGO. After electrochemical reduction, the graphene oxide semicircle diameter in high frequency area is significantly smaller. The charge transfer impedance (Rct) is simulated by series equivalent circuit, the diameter of 4.3 kΩ of graphene oxide decreased significantly to 0.82 kΩ of ERGO (17.5 V/2 h); ERGO (17.5 V/2 h) also had a significantly larger linear slope in the low frequency zone, meaning the diffusion impedance shows a tendency of becoming smaller.Based on the above results, this study has successfully developed a method of rapidly reducing graphene oxide flakes into ERGO flakes using a safe and non-toxic PBS phosphate buffer solution as electrolyte solution. With this electrochemical reduction technology, graphene oxide film does not have to be pre-coated to the cathode for reduction, making it easy to be used under production conditions and have the opportunity to achieve large-scale and high-quality ERGO flakes preparation. In addition, the produced ERGO flakes are prepared to further evaluate their various electrochemical properties and it is found that they have excellent capacitance performance, with the potential to expand the development of high-performance supercapacitors in the future.
Electrochemical reduction is an environmentally friendly and energy-efficient way to restore characteristics of graphene oxide (GO). However, in general electrochemical reduction methods, graphene oxide film must be pre-coated on the electrode surface, and then a bias is applied with a two-pole or three-pole system to perform the reduction to obtain the electrochemical reduction graphene oxide (ERGO) film, but this film can only be used as a sensor sensing layer or conductive film for a one-time usage. Therefore, this method of electrochemical reduction, which can only produce ERGO film, cannot be used to manufacture large quantities of ERGO flakes to meet the needs of industrial applications.This study proposes a method which combines mechanical cyclic stirring and electrochemical reduction for electrochemical reduction of graphene oxide. It uses GO flakes directly as a reduction material without the need to pre-prepare it into GO film to carry out electrochemical reduction. In addition, phosphate buffer solution (K2HPO4/KH2PO4, PBS) is used as electrolyte solution, which is essentially saline solution and is environmentally friendly, non-toxic and safe. Homogenizer stirrer rod acts as an electrode to realize the function of cyclic stirring and electrochemical reduction, applying bias from the power supply. The GO flakes is placed in electrolyte solution in a porous ceramic filter, ERGO flakes are produced by continuous mechanical stirring and electrochemical reduction. This study quickly confirms the reduction effect of graphene oxide samples by Raman spectroscopy and determines the crystallization structure changes of graphene oxide samples by X-ray powder diffraction (XRD). In addition, X-ray photoelectron energy spectrometer (XPS) is used to identify oxygen carbon ratio; equipment like scanning electron microscope (SEM), transmission electron microscope (TEM) and atomic force microscope (AFM) is used to evaluate the size, surface morphology, number of layers and thickness of graphene flakes. Finally, the change of the functional base is measured with a Fourier transform infrared spectroscopy (FTIR).Experimental results show that GO flakes is electrochemically reduced to ERGO flakes, and the study of rotary stirring speed shows that faster speed will make the flake size smaller, and also lead to ID/IG increase. Analysis of electrolyte concentration changes finds that concentrations are not the main factor contributing to the difference in the reduction effect. By changing the value of bias, experiment results show that higher bias raises the reaction speed at both electrodes, therefore shortens the reduction time. Under the bias of −17.5 V/2 h, the Raman spectroscopy ID/IG increased from 0.85 to 1.08. In XPS measurements, C/O ratio increased from 2.02 in graphene oxide to 3.10 in ERGO (17.5 V/2h). After that, XPS and FTIR were used to compare its chemical elements and the nature of bonding, it was found that oxygen functional groups decreased significantly after reduction. The above results show that the electrochemical reduction technology proposed in this study can achieve good GO flakes reduction results and has the potential for mass production of ERGO flakes.This study further used ERGO flake to evaluate the electrochemical properties and performance. First, it compared the conductivities of graphite, graphene oxide and ERGO and found out that graphite has a conductivity of about 7.35 × 10-1 S·cm-1, and graphene oxide has a much lower conductivity of 7.92 × 10-4 S·cm-1. After electrochemical reduction, conductivity of ERGO with 10 V/8 h bias increased to 3.83 × 10-1 S·cm-1, which increased again to 5.16 × 10-1 S·cm-1 with -17.5 V/2 h bias, an increase of about 650 times than graphene oxide. Next, the cyclic voltammetry (CV) and constant charge-discharge method (GCD) were used to perform evaluation, and the specific capacitance value of ERGO (17.5 V/2h) flakes increased to 191 F/g, 17 times higher than that of graphene oxide, close to the specific capacitance of 205 F/g of thermal reduction graphene oxide (TRGO). Finally, the reaction behavior between the sample material and the electrolyte interface is analyzed by electrochemical impedance spectroscopy (EIS). Nyquist plot graphics show that graphene oxide is significantly different from ERGO. After electrochemical reduction, the graphene oxide semicircle diameter in high frequency area is significantly smaller. The charge transfer impedance (Rct) is simulated by series equivalent circuit, the diameter of 4.3 kΩ of graphene oxide decreased significantly to 0.82 kΩ of ERGO (17.5 V/2 h); ERGO (17.5 V/2 h) also had a significantly larger linear slope in the low frequency zone, meaning the diffusion impedance shows a tendency of becoming smaller.Based on the above results, this study has successfully developed a method of rapidly reducing graphene oxide flakes into ERGO flakes using a safe and non-toxic PBS phosphate buffer solution as electrolyte solution. With this electrochemical reduction technology, graphene oxide film does not have to be pre-coated to the cathode for reduction, making it easy to be used under production conditions and have the opportunity to achieve large-scale and high-quality ERGO flakes preparation. In addition, the produced ERGO flakes are prepared to further evaluate their various electrochemical properties and it is found that they have excellent capacitance performance, with the potential to expand the development of high-performance supercapacitors in the future.
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電化學還原氧化石墨烯(ERGO), 氧化石墨烯(GO), 磷酸鹽緩衝溶液(PBS), 電化學阻抗頻譜(EIS), Electrochemically reduced graphene oxide (ERGO), Graphene oxide (GO), Phosphate buffered saline (PBS), Electrochemical impedance spectroscopy (EIS)