為了進一步探討HGNS-300 優異電化學表現之原因，本研究以XPS 探討HGNS-300 電極於不同鈉化/去鈉化電位之表面性質變化，證實於2~0.4 V 之電位區間，鈉離子會儲存於表面活性位置，此活性位置主要來自於表面含氧官能基，其反應式為：>C=O + Na++ e- ↔ >C-O-Na。再經由同步輻射XRD 探討HGNS-300 電極不同鈉化/去鈉化電位之結構變化，可證實其鈉化過程中會於電位低於0.4 V 開始進行插層反應，將鈉離子儲存於碳層間。由於HGNS-300 兼具插層機制與表面氧化還原機制，因此於電容量及高速電容維持率皆有優異的表現。;Holey reduced graphene oxide (RGO) is prepared by a Staudenmaier method, followed by a thermal reduction process. Amounts of functional groups on RGO can be controlled by the reduction temperature. In this study, electrochemical properties of the RGO electrodes are tested in an ethylene carbonate/diethyl carbonate mixed electrolyte containing 1 M NaClO4. The RGO reduced at 1100 oC (HGNS-1100) with a low content of functional groups shows a reversible capacity of 147 mAh/g (at 0.03 A/g). However, the RGO reduced at 300 oC (HGNS-300; with a higher surface functional group amount) shows a clearly higher capacity of 213 mAh/g at the same condition. With increasing the charge-discharge to 5, 10 and 20 A/g, a capacity of as high as 104, 83 and 58 mAh/g can be obtained, indicating an excellent rate capability. The functional groups may increase d-spacing and provide reaction sites for sodium ion storage, enhancing charge/discharge capacity. In addition, holey morphology can shorten the path of Na-ion diffusion, optimizing the rate capability. HGNS-300 shows the higher rate capability 44.6 %. GNS-300 (without holey morphology) only has 38.6 % at the same condition. It is also found that the RGO-300 electrode exhibits a capacity retention ratio of approximately 70 % after 100 cycles.
In order to study the reason of excellent electrochemical performance of HGNS-300, the methods of ex-situ XRD and ex-situ XPS are used to analyze the structure and surface properties change during charge/discharge process. It confirms that Na-ion will insert to carbon layers in the lower sodiation voltage (0.4~0.3 V). In the higher sodiation voltage 2~0.4 V, Na-ion will storage at surface active site from surface functional group. The reaction mechanism is “>C=O + Na+ + e- ↔ >C-O-Na”. Due to the two kinds of mechanism that mention in above paragraph, HGNS-300 can own both high capacity and excellent rate capability.
