Functionalized graphene sheets (FGSs) with oxygen-containing sites are shown to perform comparably to platinum as catalytic counter electrodes in dye-sensitized solar cells (DSSCs), achieving conversion efficiencies of 5.0% and 5.5% at 100 mW cm⁻² AM1.5G simulated light. The catalytic activity of FGSs towards the reduction of triiodide is interpreted using a new electrochemical impedance spectroscopy (EIS) equivalent circuit that matches the observed spectra features to the appropriate phenomena. Cyclic voltammetry (CV) measurements show that increasing the amount of oxygen-containing functional groups can improve the apparent catalytic activity of FGSs. Additionally, a commercially available FGS-based ink is demonstrated to serve as a catalytic, flexible, and electrically conductive counter electrode material for DSSCs. The study provides insights into the role of oxygen-containing functional groups and the impact of material tuning on the catalytic performance of FGSs.Functionalized graphene sheets (FGSs) with oxygen-containing sites are shown to perform comparably to platinum as catalytic counter electrodes in dye-sensitized solar cells (DSSCs), achieving conversion efficiencies of 5.0% and 5.5% at 100 mW cm⁻² AM1.5G simulated light. The catalytic activity of FGSs towards the reduction of triiodide is interpreted using a new electrochemical impedance spectroscopy (EIS) equivalent circuit that matches the observed spectra features to the appropriate phenomena. Cyclic voltammetry (CV) measurements show that increasing the amount of oxygen-containing functional groups can improve the apparent catalytic activity of FGSs. Additionally, a commercially available FGS-based ink is demonstrated to serve as a catalytic, flexible, and electrically conductive counter electrode material for DSSCs. The study provides insights into the role of oxygen-containing functional groups and the impact of material tuning on the catalytic performance of FGSs.