Enhanced electrochemical performance of salen-type transition metal polymer with electron-donating substituents

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).