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 properties compared to conventional CPs. Further incorporation of conducting materials into CPs 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 the 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 a 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 exhibited a high specific capacitance of 93 F g⁻¹ at 0.5 A g⁻¹ and a cycling 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 a 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 resulting from the synergistic effect of CP and GO.