Modulating Electrocatalysis on Graphene Heterostructures: Physically Impermeable Yet Electronically Transparent Electrodes

The electronic properties and extreme thinness of graphene make it an attractive platform for exploring electrochemical interactions across dissimilar environments. Here, we report on the systematic tuning of the electrocatalytic activity toward the oxygen reduction reaction (ORR) via heterostructures formed by graphene modified with a metal underlayer and an adlayer consisting of a molecular catalyst. Systematic voltammetric testing and electrochemical imaging of patterned electrodes allowed us to confidently probe modifications on the ORR mechanisms and overpotential. We found that the surface configuration largely determined the ORR mechanism, with adlayers of porphyrin molecular catalysts displaying a higher activity for the 2e^- pathway than the bare basal plane of graphene. Surprisingly, however, the underlayer material contributed substantially to lower the activation potential for the ORR in the order Pt > Au > SiO_x, strongly suggesting the involvement of the solution-excluded metal on the reaction. Computational investigations suggest that ORR enhancements originate from permeation of metal d-subshell electrons through the graphene layer. In addition, these physically impermeable but electronically transparent electrodes displayed tolerance to cyanide poisoning and stability toward long-term cycling, highlighting graphene as an effective protection layer of noble metal while enabling electrochemical interactions. This work has implications in the mechanistic understanding of 2D materials and core-shell-type heterostructures for electrocatalytic reactions.