Enhanced electrochemical performance of salen-type transition metal polymer with electron-donating substituents
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Salen-type Schiff base transition metal monomers with substituents Ni (CH3-salen), Ni (CH3O-salen), and Ni (Cl-salen) have been synthesized and electro-polymerized onto the indium tin oxide substrate electrodes. The effect of electron-donating groups on the electrochemical performance of the polymer is studied. Electron-donating groups enhance the electrochemical activity of the salen-type Schiff base during the electropolymerization process. SEM images show that the morphology of poly [Ni (CH3O-salen)] is nanobelt with a width of 200–500 nm. The cyclic voltammetry plots indicate that the strong electron-donating methoxy group facilitates the polymerization of the salen-type Schiff base. Thus, Ni (CH3O-salen) shows a higher doping level than other three polymers. XPS measurement is conducted to investigate the polymerization process and the mechanism of energy storage. It is proved that the azomethine nitrogen group (−N=CH−) matters a lot in the polymerization and energy storage process. In brief, the azomethine nitrogen group was affected by the introduction of the electron-donating group so that extra redox peaks appear in the cyclic voltammetry plots. There is no chemical valence change of nickel, and the nickel atom worked as a bridge in the system. The electro-donating substituent group activates the benzene ring of the polymer and facilitates the charge transfer and leads to poly [Ni(CH3O-salen)] that exhibits the highest doping level, charge-transfer ability, and electrochemical capacity characteristics than the polymer with weaker electro-donating or electro-withdrawing substituents (polyNi(Cl-salen)). At the current density of 0.1 mA cm−2, the specific capacitance of poly [Ni(CH3O-salen)] is 270.1 F g−1, higher than that of poly [Ni(salen)](136.7 F g−1), poly [Ni(CH3-salen)](148.1 F g−1), and poly [Ni(Cl-salen)](106.0 F g−1).
KeywordsSchiff base Electron-donating substituent group Charge storage mechanism Supercapacitors
This work is financially supported by the National Natural Science Foundation of China (No. 51372021), and National Natural Science Foundation of China (No. 51772025 and No. 51572024).
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