石墨烯及其复合材料对溶解氧还原反应的影响
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摘要
溶解氧还原反应（Dissolved oxygen reduction reaction, ORR）在金属海洋腐蚀与新能源技术中扮有重要角色，不同材料上的ORR机理研究是电化学领域的研究热点之一。石墨烯作为一种新型碳纳米材料，能够降低ORR的过电位和增大电流，但其背后的作用机理尚不清晰。本论文采用循环伏安法、旋转圆盘电极伏安法、旋转圆环圆盘电极伏安法等电化学方法对石墨烯上的ORR行为进行深入研究，在动力学计算的基础上，提出作用机理。并根据作用机理，采用杂原子掺杂和复合材料构建的方法，进一步提高其ORR催化性能，实现较低过电位下O_2的四电子还原。主要成果与结论如下：
（1）明确了制备方法对石墨烯ORR性能的影响。与电沉积法制备的石墨烯相比，化学还原石墨烯上的ORR具有更高的起始电位和更大的电流，因而具有更好的性能。
（2）提出了不同电解液介质中化学还原石墨烯对ORR的作用机制。化学还原石墨烯能够降低ORR的过电位、增大电流、提高动力学电流密度，但在不同电解液介质中的作用机理不同。在0.1M KOH溶液中，化学还原石墨烯改变了ORR的路径，使得O_2到HO_2–的还原在较低的过电位下发生。在3.5%NaCl溶液中，化学还原石墨烯能够催化中间产物H2O_2的分解，使得O_2的四电子还原在较低的过电位下发生。
（3）发现了电解液组成和电沉积参数对电沉积石墨烯的制备及ORR性能有重要影响。以LiClO_4为支持电解质时，不能成功制得石墨烯修饰电极。当支持电解质为NaCl时，恒电位和循环伏安扫描方法均可实现石墨烯在电极表面的沉积。恒电位法制备的修饰电极ORR催化活性高、稳定性差；循环伏安法制得的电极具有较好的催化活性与稳定性，且其性能与扫描圈数密切相关。
（4）建立了氮掺杂石墨烯的氮含量、微观结构与ORR性能之间的关系。以不同比例的氧化石墨和尿素为原料，通过水热反应制备了不同氮含量和微观结构的氮掺杂石墨烯。发现氮含量为7%左右、缺陷密度适中（拉曼光谱中D带与G带强度比在1左右）时，氮掺杂石墨烯具有最高的活性。氮含量过低，活性位点数量不足；氮含量过高，缺陷密度高，阻碍载流子的传输。
（5）构建了化学还原石墨烯与氧化锰的复合材料，且实现两组分的优势互补。氧化锰的ORR活性受晶型和形貌的影响，不同材料的活性顺序为：β-MnO_2微棱体<无定型MnOx纳米颗粒<-MnO_2纳米线。在化学还原石墨烯与-MnO_2纳米线复合材料中，前者使得ORR反应过电位减小，后者催化HO_2–的分解，从而实现较低过电位下O_2的四电子还原。
（6）揭示了化学还原石墨烯与Co(OH)_2复合材料上的ORR机理，提出化学还原石墨烯的电化学活化。以Co(NO_3)_2为电解液在化学还原石墨烯表面沉积能够促进HO_2
分解的Co(OH)_2时，化学还原石墨烯的微观结构发生变化进而得到活化。活化后的化学还原石墨烯能够进一步降低反应过电位并催化HO_2分解，其与Co(OH)_2的协同作用使得O_2的四电子还原在低的过电位下发生。
Dissolved oxygen reduction reaction (ORR) plays an important role in metal marinecorrosion and new energy conversion technologies, and the mechanism study of ORR ondifferent materials is one of the hottest topics in the field of electrochemistry. Graphene, anew carbon nanomaterial, can decrease the overpotential of ORR and increase its current.However, the mechanism behind these phenomena has not been clear. In this thesis, intensivestudies have been done to investigate the ORR on graphene with the help of electrochemicalmethods, including cyclic, rotating disk electrode, and rotating ring-disk electrodevoltammetry, and the mechanism is proposed based on the kinetics calculation. Givenproposed mechanism, hetero-atoms doping and composites constructing are adopted toimprove the ORR properties to achieve4-electron reduction of O_2at lower overpotentials.The main results and conclusions are as follows:
(1) The effect of preparation methods on the activities of graphene towards ORR hasbeen defined. In comparison with electrodeposited graphene, graphene prepared via chemicalreduction (G-CR for short) leads to higher beginning potential and larger current of ORR, andtherefore possesses better properties.
(2) The mechanism of G-CR towards ORR in different electrolytes has been proposed.G-CR can decrease the overpotential, increase the current, and improve the kinetic currentdensity of ORR, but functions differently in different electrolytes. The ORR pathway hasbeen changed by G-CR in0.1M KOH solution, which leads to the reduction of O_2to HO_2–atlower overpotentials. While in3.5%solution,4-electron reduction of O_2at loweroverpotentials is achieved via catalytic decomposition of H2O_2by G-CR.
(3) Electrolyte compositions and electrodeposition parameters have great influence onthe preparation of electrodeposited graphene and its ORR properties. When LiClO_4is utilizedas supporting electrolyte, graphene modified electrodes can not be obtained. Theelectrodeposition of graphene can be achieved by potentiostatic and cyclic voltammetricmethods with NaCl as supporting electrolyte, in which modified electrodes via the formermethod possess high ORR activity and low stability, and the latter gives electrodes with goodactivity and stability, and their properties are dependent on the number of scanning cycles.
(4) The relationship between the nitrogen content and microstructure of nitrogen-dopedgraphene and ORR activity has been established. Nitrogen-doped graphene materials withdifferent nitrogen contents and microstructure have been synthesized via hydrothermalreaction of graphite oxide and urea with different mass ratios. Nitrogen-doped graphene witha nitrogen content of ca.7%and a moderate defect density (the intensity ratio of D band to Gband in Raman spectrum has a value of ca.1) gives the best ORR activity. If nitrogen contentis lower, active sites available are not enough, and higher nitrogen content brings moredefects to hamper the transport of current carriers.
(5) The composite of G-CR with manganese oxide has been constructed, and the roles ofthese two components are fully played. The ORR properties of manganese oxide materials areclosely related to their crystallographic structures and morphologies, which follow an order ofβ-MnO_2miciroprism (6) ORR mechanism on the composite of G-CR with Co(OH)_2has been defined, andelectrochemical activation of G-CR is proposed. The microstructure of G-CR has beenchanged during the electrodeposition of Co(OH)_2with catalytic activity towards HO_2decomposition on G-CR surface from Co(NO_3)_2electrolyte, resulting in the activation ofG-CR. The activated G-CR brings a further decrease in overpotential and catalyticdecomposition of HO_2, and its combination with Co(OH)_2gives rise to4-electron reductionof O_2at lower overpotentials.
（1）明确了制备方法对石墨烯ORR性能的影响。与电沉积法制备的石墨烯相比，化学还原石墨烯上的ORR具有更高的起始电位和更大的电流，因而具有更好的性能。
（2）提出了不同电解液介质中化学还原石墨烯对ORR的作用机制。化学还原石墨烯能够降低ORR的过电位、增大电流、提高动力学电流密度，但在不同电解液介质中的作用机理不同。在0.1M KOH溶液中，化学还原石墨烯改变了ORR的路径，使得O_2到HO_2–的还原在较低的过电位下发生。在3.5%NaCl溶液中，化学还原石墨烯能够催化中间产物H2O_2的分解，使得O_2的四电子还原在较低的过电位下发生。
（3）发现了电解液组成和电沉积参数对电沉积石墨烯的制备及ORR性能有重要影响。以LiClO_4为支持电解质时，不能成功制得石墨烯修饰电极。当支持电解质为NaCl时，恒电位和循环伏安扫描方法均可实现石墨烯在电极表面的沉积。恒电位法制备的修饰电极ORR催化活性高、稳定性差；循环伏安法制得的电极具有较好的催化活性与稳定性，且其性能与扫描圈数密切相关。
（4）建立了氮掺杂石墨烯的氮含量、微观结构与ORR性能之间的关系。以不同比例的氧化石墨和尿素为原料，通过水热反应制备了不同氮含量和微观结构的氮掺杂石墨烯。发现氮含量为7%左右、缺陷密度适中（拉曼光谱中D带与G带强度比在1左右）时，氮掺杂石墨烯具有最高的活性。氮含量过低，活性位点数量不足；氮含量过高，缺陷密度高，阻碍载流子的传输。
（5）构建了化学还原石墨烯与氧化锰的复合材料，且实现两组分的优势互补。氧化锰的ORR活性受晶型和形貌的影响，不同材料的活性顺序为：β-MnO_2微棱体<无定型MnOx纳米颗粒<-MnO_2纳米线。在化学还原石墨烯与-MnO_2纳米线复合材料中，前者使得ORR反应过电位减小，后者催化HO_2–的分解，从而实现较低过电位下O_2的四电子还原。
（6）揭示了化学还原石墨烯与Co(OH)_2复合材料上的ORR机理，提出化学还原石墨烯的电化学活化。以Co(NO_3)_2为电解液在化学还原石墨烯表面沉积能够促进HO_2
分解的Co(OH)_2时，化学还原石墨烯的微观结构发生变化进而得到活化。活化后的化学还原石墨烯能够进一步降低反应过电位并催化HO_2分解，其与Co(OH)_2的协同作用使得O_2的四电子还原在低的过电位下发生。
Dissolved oxygen reduction reaction (ORR) plays an important role in metal marinecorrosion and new energy conversion technologies, and the mechanism study of ORR ondifferent materials is one of the hottest topics in the field of electrochemistry. Graphene, anew carbon nanomaterial, can decrease the overpotential of ORR and increase its current.However, the mechanism behind these phenomena has not been clear. In this thesis, intensivestudies have been done to investigate the ORR on graphene with the help of electrochemicalmethods, including cyclic, rotating disk electrode, and rotating ring-disk electrodevoltammetry, and the mechanism is proposed based on the kinetics calculation. Givenproposed mechanism, hetero-atoms doping and composites constructing are adopted toimprove the ORR properties to achieve4-electron reduction of O_2at lower overpotentials.The main results and conclusions are as follows:
(1) The effect of preparation methods on the activities of graphene towards ORR hasbeen defined. In comparison with electrodeposited graphene, graphene prepared via chemicalreduction (G-CR for short) leads to higher beginning potential and larger current of ORR, andtherefore possesses better properties.
(2) The mechanism of G-CR towards ORR in different electrolytes has been proposed.G-CR can decrease the overpotential, increase the current, and improve the kinetic currentdensity of ORR, but functions differently in different electrolytes. The ORR pathway hasbeen changed by G-CR in0.1M KOH solution, which leads to the reduction of O_2to HO_2–atlower overpotentials. While in3.5%solution,4-electron reduction of O_2at loweroverpotentials is achieved via catalytic decomposition of H2O_2by G-CR.
(3) Electrolyte compositions and electrodeposition parameters have great influence onthe preparation of electrodeposited graphene and its ORR properties. When LiClO_4is utilizedas supporting electrolyte, graphene modified electrodes can not be obtained. Theelectrodeposition of graphene can be achieved by potentiostatic and cyclic voltammetricmethods with NaCl as supporting electrolyte, in which modified electrodes via the formermethod possess high ORR activity and low stability, and the latter gives electrodes with goodactivity and stability, and their properties are dependent on the number of scanning cycles.
(4) The relationship between the nitrogen content and microstructure of nitrogen-dopedgraphene and ORR activity has been established. Nitrogen-doped graphene materials withdifferent nitrogen contents and microstructure have been synthesized via hydrothermalreaction of graphite oxide and urea with different mass ratios. Nitrogen-doped graphene witha nitrogen content of ca.7%and a moderate defect density (the intensity ratio of D band to Gband in Raman spectrum has a value of ca.1) gives the best ORR activity. If nitrogen contentis lower, active sites available are not enough, and higher nitrogen content brings moredefects to hamper the transport of current carriers.
(5) The composite of G-CR with manganese oxide has been constructed, and the roles ofthese two components are fully played. The ORR properties of manganese oxide materials areclosely related to their crystallographic structures and morphologies, which follow an order ofβ-MnO_2miciroprism
引文
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