Para-azaquinodimethane-based quinoidal copolymers: Significant enhancement of electrochemical performances and stability with conformational planarity

Abstract

The electrochemical properties of quinoidal-donor (Q-D) alternating copolymers based on the new and attractive quinoidal unit para-azaquinodimethane (p-AQM) are investigated for the first time. A family of four polymers, namely PAQM3T, PAQM4T, PAQM2T-TT and PAQM2T-TVT, designed with different comonomers, is studied using cyclic voltammetry, galvanostatic charge/discharge cycling, electrochemical impedance spectroscopy and spectroelectrochemistry. While p-AQM molecules are known to be relatively unstable, it is demonstrated here that the Q-D polymers can be reversibly oxidized, indicating that p-doping does not generate highly reactive radical species on the methylene carbons of the alkoxy-substituted p-AQM units. This distinctive behavior of polymers vs small molecules can be attributed to a more efficient delocalization of radicals over the conjugated backbone. However, it is found that electrochemical performances and stability depend strongly on the nature of the donor unit. PAQM2T-TVT, having enhanced quinoidal character due to the presence of planar thiophene-vinylene-thiophene (TVT) units in its backbone, exhibits the highest specific and areal capacitance values (331 F/g and 83 mF/cm², respectively) as well as improved capacitance retention upon galvanostatic cycling, up to 92 % after 200 cycles. Moreover, PAQM2T-TVT also shows remarkably improved rate capability, withstanding current densities as high as 10 mA/cm², owing to higher electronic and ionic conductivity. This work suggests that the inclusion of monomer units inducing enhanced conformational planarity of the polymer backbone is crucial in optimizing the electrochemical properties and cyclability of p-AQM based polymers, making them promising materials for advanced electrochemical applications including (micro)supercapacitors and batteries.