The interactions of catalytically functionalized groups with graphene-based electrodes control complex interfacial processes encountered in electrochemical energy conversion systems. However, our knowledge of the atomic/nanoscale reactivity at such functional interfaces remain scarce due to the incomplete understanding of interfacial structures and dynamic processes encountered in operando conditions. In this talk, we will present a systematic study of the solution-based functionalization of epitaxial graphene by the adsorption of a quinone-based molecules, phenanthrenequinone (PQ), combining electrochemical characterization, high-resolution interfacial X-ray scattering andab-initio density functional theory calculations. Our results reveal that while PQ deposited on pristine graphene is unstable to electrochemical cycling, the prior introduction of defects and oxygen functionality (hydroxyl and epoxide groups) to the graphene basal plane by exposure to an oxygen plasma effectively stabilized its non-covalent functionalization by PQ adsorption. The PQ molecules adsorb by lying down parallel to the basal plane, resembling the graphene layer stacking (as measured in-situ with X-ray reflectivity) and are further stabilized by hydrogen bonding with terminal hydroxyl groups that form at defect sites within the graphene basal plane. PQ retains its expected redox activity associated with the proton coupled electron transfer (PCET) reaction. The resultant PQ adsorption structure is essentially independent of electrochemical potential. These results highlight a novel approach that uses non-covalent interactions to enhance functionalities of the otherwise chemically inert graphene. Such an approach is expected to be widely applicable to many functional adsorbate/2D-crystal systems. The interfacial structures and processes illuminated in this work lay a foundation for understanding relevant interactions between aromatic functional groups with graphene via our in-situ X-ray approach, providing unique insights into the atomistic interactions at fluid-solid interfaces for catalytic energy systems.