Effect of HNO₃ functionalization on large scale graphene for enhanced tri-iodide reduction in dye-sensitized solar cells

Improving the electro-catalytic activity of graphene has recently been the subject of intense research for high efficiency flexible energy storage and conversion devices. We report the synthesis of a large scale graphene film by a CVD method and its electro-catalytic activity by functionalization with HNO ₃ for a high efficiency electrochemical electrode in DSSCs. We found that HNO₃ functionalization on graphene enhances the tri-iodide reduction rate by three times in a dye sensitized solar cell compared to that of pristine graphene. The X-ray photoelectron spectroscopy (XPS) and ultra-violet photoemission spectroscopy (UPS) studies confirm the covalently attached C-OH, C(O)OH and NO^3- moieties to carbon atoms through sp ²-sp³ hybridization, and this results in the Fermi level shift towards p-type doping. We believe that the covalently attached functional groups cause the enrichment of the electro-catalytically active sites along with facilitating the charge transfer kinetics from graphene counter electrodes to redox couples. The enhanced catalytic effect of functionalized graphene offers insights into new types of electrode development opportunities in graphene based energy storage and conversion devices.