We will discuss the potential of mono- and few-layer graphene electrodes as versatile platforms for the exploration of new electrocatalyst design principles. By transferring cm² layers of CVD-grown graphene over metal deposits patterned on insulating substrates, and following passivation of pinholes and grain boundaries, we have measured enhanced rates of outer-sphere and inner-sphere electron transfer on sites modified with the sub-surface metal. Reactive imaging of these electrodes using scanning electrochemical microscopy was used to confirm our modification strategy and to highlight the possibility of using this strategy to spatially encode reactivity on surfaces. These electrodes enable a unique opportunity to modify the reactivity and mechanism of electrochemical reactions by means of electronic interactions that operate at scales coincident with their limiting thickness.

In a first study,^[1] we deposited bi-layer graphene on SiO₂ chips modified with thin deposits of Au and Pt. SECM imaging using reversible outer-sphere mediators (ferri/ferrocyanide and ferrocenemethanol) showed a five-fold increase in the heterogeneous electron transfer constant on sub-surface modified sites. These enhancements are likely a consequence of increased electronic density of states due to a donating effect from the substrate. Following this principle, we explored the impact on an inner-sphere process, the oxygen reduction reaction. Our results indicate that both the kinetics and mechanism of the reaction are sensitive to sub-surface modification, with the kinetics largely driven by the identity of the metal and the reaction mechanism dominated by graphene or adsorbed molecular catalysts.^[2] SECM imaging in the redox competition mode and in the H₂O₂ collection mode confirmed observations done on macro-electrode interfaces. The design principles derived from these experiments open new opportunities in the coupling of materials with different electrocatalytic properties using graphene as a mediating surface. We will discuss our recent progress expanding this toolbox for mitigating problems associated with several electrocatalytic reactions and exploring other reactions of interest, including the electrochemical reduction of CO₂.

[1] Hui, J.; Zhou, X.; Bhargava, R.; Chinderle, A.; Zhang, J.; Rodríguez-López, J. Electrochim. Acta 2016, 211, 1016-1023.

[2] Hui, J., et al., Rodríguez-López, J. In Preparation.