Unlocking the Potential of Redox-Active Copper Complexes in Thin Films via Proton-Coupled Electron Transfer for Enhanced Supercapacitor Performance

Abstract

Nanoscale redox-active molecular films are promising candidates for next-generation energy storage applications due to their ability to facilitate long-range charge transport. However, establishing stable and efficient electrode-molecule interfaces remains a critical challenge. In this study, the properties of redox-active copper-polypyridyl thin films covalently bonded to graphite rods are explored, investigating their potential as supercapacitors. Using an electrochemical grafting method, robust covalent interfaces are created, resulting in copper-polypyridyl films prepared on graphite rods and indium tin oxide (ITO) electrodes, exhibiting both Cu(II) and Cu(I) redox states. These redox-active mettalo-oligomeric films demonstrate a structural transition between octahedral and tetrahedral geometries around the Cu(II), and Cu(I), respectively contributing to their charge storage capabilities. The combination of an electrical double-layer capacitance and pseudocapacitance through Faradaic charge transfer is evaluated in different acidic electrolytes, showing significant capacitance enhancement. Notably, proton-coupled electron transfer (PCET) at free pyridine-N sites in Cu(I) polypyridyl complex is identified as a key factor in their distinct behavior in aqueous solutions, a finding supported by computational studies. This study shows the potential of binder-free thin films for efficient supercapacitor applications, with a maximum areal capacitance of 6.8 mF cm⁻2 in aqueous media, representing an 1840% improvement over bare graphite rods.