Cu-based coordination polymer@GO for supercapacitor and solid-state proton conduction applications

Abstract

Redox-active linkers containing conjugated coordination polymers (CPs) are promising materials for energy storage/conversion devices owing to their superior multi-electron redox property compared to conventional CPs. Further incorporation of conducting materials with CP can be an effective approach towards upgrading their electrochemical performance. In this work, a redox-active N,N-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNDI) linker self-assembled with Cu(II) ion has been synthesized, introducing a 3D supramolecular structure of [Cu(DPNDI)(NO3)2(CH3CN)]n, (Cu-CP). Benefitting from the advantage of graphite oxide (GO) and redox-active Cu-CP, herein, we report the fabrication of Cu-CP@GO composite through a solvothermal approach. Cu-CP@GO exhibited an excellent specific capacitance of 178 F g–1 at 0.6 A g–1 in a three-electrode configuration, significantly higher than that of pristine Cu-CP. Furthermore, a two-electrode symmetric device made of Cu-CP@GO revealed a high specific capacitance of 93 F g–1 at 0.5 A g–1 and a cyclic stability of 96% after 5000 cycles (50 h), in spite of in-situ generation of radical anions during redox transitions. This was attributed to excellent charge delocalization in the π-network of the NDI ligand and GO. Additionally, Cu-CP@GO demonstrated high solid-state proton conductivity of 1.4×10-3 S cm-1 at 95 °C and 95% RH, along with a low activation barrier of 0.33 eV, revealing proton migration by the Grotthuss mechanism. This exceptional supercapacitor performance and solid-state proton conductivity of Cu-CP@GO are attributed to the enhanced electrical conductivity led by the synergistic effect between CP and GO.