First-principles analysis of potential-dependent proton coupled electron transfer between polypyridyl-ruthenium complexes and oxygen-modified graphene electrodes

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Abstract

Proton coupled electron transfer reactions which are pervasive throughout electrochemistry control a number of energy conversion strategies. First-principles density functional theoretical calculations are used herein to examine proton coupled electron transfer between a homogeneous mononuclear polypyridyl-ruthenium catalyst used in the catalytic oxidation of water and surface ketone groups on oxygen-modified graphene electrode surfaces. The potential-dependent interface energies were calculated for two proton transfer states: RuIIIOH...O=C-graphene and RuIV= O···HO-C-graphene. The reactivity for interfacial proton coupled electron transfer was found to be controlled by functional groups that terminate surface defect sites as well as graphene edge sites. The energy gap between the two proton transfer states becomes smaller as the number of surface ketone groups increases. Ab initio molecular dynamics simulations clearly show that increases in the number of surface ketone groups increase the hydrophilicity of the graphene basal plane. This significantly decreases the energy for proton transfer, thus providing a low-energy path that extends over a wide range of potentials. The surface ketone groups at the armchair edges of the graphene plane appear to be the most reactive oxygens on the graphene surface as they lead to direct reversible proton coupled electron transfer between the polypyridal-Ru complexes and the C=O groups at the edges where the two proton transfer potential energy surfaces crossing at 0.2 V vs SHE. The graphene armchair edge helps to stabilize the RuIV= O···HO-C-graphene-edge proton transfer state which is an important step in water oxidation catalysis.

Original languageEnglish (US)
Pages (from-to)12097-12105
Number of pages9
JournalJournal of Physical Chemistry C
Volume118
Issue number23
DOIs
StatePublished - Jun 12 2014

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