釩液流電池（VRFB）為目前十分有潛力的大型儲能系統之一，但其較高的生產成本與較低的能量效率仍限制了其實用性。然而，開發具有低成本與高活性的電催化觸媒或電極材料在釩液流電池的效能提升上至關重要。
因此，本篇研究中選用低價的氧化鈮金屬和氧化石墨烯為前驅物，利用回流及水熱法，將氧化鈮均勻分散於氧化石墨烯表面，接著於管型高溫爐以450、850及1050 °C的高溫燒結。於1050 °C的燒結溫度，因氧化鈮與氧化石墨烯之間的碳熱還原反應，五氧化二鈮被還原為二氧化鈮，同時在高溫下反應生成碳化鈮。氧化石墨烯之表面氧官能基應在高溫時分解，進而形成還原氧化石墨烯，但此篇研究利用碳熱還原法，使得氧化石墨烯在經過高溫燒結後仍可保留大量表面氧官能基。氧官能基為釩離子氧化還原反應的電化學活性點，在氧化石墨烯表面製造大量氧官能基可以有效提升電化學活性，另外，在五氧化二鈮還原過程中產生大量氧缺陷，可以有效吸附氧官能基，形成活性點，碳化鈮則具有高導電性。
為了證明NbO2/NbC/GO之協同效應，以多種電化學量測方法分別測量商售二氧化鈮、商售碳化鈮，及氧化石墨烯，NbO2/NbC/GO在所有測試樣品中對VO2+/VO2+及V2+/V3+表現出最佳電化學活性。最後，以釩液流單電池充放電進一步說明，使用NbO2/NbC/GO作為電催化觸媒之電池分別在40和80 mA cm−2 的充放電電流密度下，展現出81.9 % 及76.5 % 的能量效率，相較於未處理之碳氈及經過前處理之碳氈，具有卓越的電催化效果。另外，以80 mA cm−2的電流密度進行100次充放電的穩定性測試後，並未有明顯的效率下降，可得知NbO2/NbC/GO作為電催化觸媒不但具有良好的效果，同時也擁有良好的穩定性。

關鍵字：釩氧化還原液流電池；電催化活性；碳熱還原反應；電極表面改質

Vanadium redox flow battery (VRFB) is one of the most promising large-scale energy storage system. However, high production cost and low energy efficiency still limit its practicability. Therefore, the development of low-cost and high-activity electrocatalyst is essential to enhance the performance of VRFB. In this study, low-cost niobium oxide and graphene oxide are used as reactants to synthesis NbO2/NbC/GO. The niobium oxide is uniformly dispersed on the surface of graphene oxide by refluxing and hydrothermal method, followed by annealing the sample with varies temperature ranges of 450, 850 1050 °C in a high temperature tubular furnace. At the annealing temperature of 1050 °C, niobium pentoxide is reduced to niobium dioxide due to the carbothermal reduction reaction between niobium oxide and graphene oxide, and simultaneously reacts at to form niobium carbide. Moreover, the surface oxygen functional groups of graphene oxide will be decomposed at high temperature to form reduced graphene oxide. However, in this study we use carbothermal reduction method to retain the large amount of surface oxygen functional groups on graphene oxide after high temperature annealing, which act as the electrochemical active sites for the vanadium ion redox reactions. In addition, a high content of oxygen vacancies is generated during the reduction of niobium pentoxide, which can effectively adsorb the oxygen functional group to form active sites, and the presence of niobium carbide increases conductivity of the electrocatalyst.
In order to show the synergistic effects of NbO2/NbC/GO, various electrochemical measurements are performed to compare with commercially available niobium oxide, niobium carbide, and graphene oxide. Among all samples, NbO2/NbC/GO exhibits the best electrochemical activity toward VO2+/VO2+ and V2+/V3+ redox reactions. Finally, the charge-discharge test further demonstrates the battery using NbO2/NbC/GO modified graphite felt (GF) as the electrode exhibits the energy efficiency of 81.9 % and 76.5 % at the current densities of 40 and 80 mA cm−2, respectively. The NbO2/NbC/GO modified GF demonstrates an excellent electrochemical activity compared to untreated and pretreated GF. In addition, after 100 cycles of charge-discharge stability test at a current density of 80 mA cm−2, there is no significant decay in energy efficiency. The NbO2/NbC/GO as an electrocatalyst not only possesses outstanding electrochemical activity but great stability in strong acid electrolyte.
Keywords: Vanadium redox flow battery; electrocatalytic activity; carbothermal reaction; electrode surface modification

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