Electrochemistry of graphene: not such a beneficial electrode material?

Abstract

We critically evaluate the reported electro-catalysis of graphene using inner-sphere and outer-sphere electrochemical redox probes, namely potassium ferrocyanide (II) and hexaammine-ruthenium(III) chloride, in addition to L-ascorbic acid and β-nicotinamide adenine dinucleotide. Well characterised commercially available graphene is utilised which has not been chemically treated, is free from surfactants, and as a result of its fabrication has an extremely low oxygen content allowing the electronic properties to be properly de-convoluted. Surprisingly we observe that graphene exhibits slow electron transfer towards the electrochemical probes studied, effectively blocking underlying electron transfer of the supporting electrode substrate likely due to its large basal and low edge plane content. Such observations, never reported before, suggest that graphene may not be such a beneficial electrode material as widely reported in the literature. Density Functional Theory is conducted on symmetric graphene flakes of varying sizes indicating that the HOMO and LUMO energies are concentrated around the edge of the graphene sheet, at the edge plane sites, rather than the central basal plane region, consistent with experimental observations. We define differentiating coverage-based working regions for the electrochemical utilisation of graphene: ‘Zone I’, where graphene additions do not result in complete coverage of the underlying electrode and thus increasing basal contribution from graphene modification leads to increasingly reduced electron transfer and electrochemical activity; ‘Zone II’, once complete single-layer coverage is achieved, layered graphenevizgraphite materialises with increased edge plane content and thus an increase in heterogeneous electron transfer is observed with increased layering. We offer insight into the electrochemical properties of these carbon materials, invaluable where electrode design for electrochemical sensing applications is sought.