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

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AbstractNanoscale 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.