Graphite Conjugation Eliminates Redox Intermediates in Molecular Electrocatalysis

Abstract

The efficient interconversion of electrical and chemical energy requires the intimate coupling of electrons and small-molecule substrates at catalyst active sites. In molecular electrocatalysis, the molecule acts as a redox mediator which typically undergoes oxidation or reduction in a separate step from substrate activation. These mediated pathways introduce a high-energy intermediate, cap the driving force for substrate activation at the reduction potential of the molecule, and impede access to high rates at low overpotentials. Here we show that electronically coupling a molecular hydrogen evolution catalyst to a graphitic electrode eliminates stepwise pathways and forces concerted electron transfer and proton binding. Electrochemical and X-ray absorption spectroscopy data establish that hydrogen evolution catalysis at the graphite-conjugated Rh molecule proceeds without first reducing the metal center. These results have broad implications for the molecular-level design of energy conversion catalysts.

Strong Electronic Coupling of Graphene Nanoribbons onto Basal Plane of Glassy Carbon Electrode

“…This simple electrochemical treatment provides a promising method towards deposition of molecular species to the carbon electrode and can be useful to the field of molecular enhancement of heterogeneous electrocatalysis. 40 We now seek to answer the following question: does deposited GNR behave like the molecular species in solution or does it undergo the field-induced electrochemistry observed previously [23][24][25] for molecular species that are in strong electronic communication with the electrode? To address this, we compared Pourbaix diagrams for as-deposited GNRs (first CV scan) and monolayer GNRs (second CV scan).…”

“…As mentioned, earlier, the stronglycoupled system is expected to only undergo processes that are overall charge neutral  each protonation of the surface nitrogen sites triggers the immediate charge compensation by electrons from the carbon electrode. [23][24][25] Thus, the absence of the twoelectron, four-proton coupled chemistry in the acidic region is indicative of the strong coupling regime. The exact number of protons (m) and electrons (n) transferred during PCET is difficult to evaluate from Figure 2C because the ~59 mV/pH slope informs only of the m/n=1 ratio.…”

Molecular enhancement of heterogeneous CO2 reduction

“…As the Fermi energy level of electrode (E F ) is shifted, the redox potential of the metal active centre is shifted together and there is no change in the driving force for electron transfer between graphite and metallic active centre in GCCs. No outer-sphere electron transfer or reduction of metal active centre is observed in GCCs 38,39 . A potential drop between the GCC site and the solution was found, which induces ion transfers coupled with electron transfers between them 38 .…”

“…In contrast, the Surendranath group has recently reported molecular catalysts electronically linked to a graphitic electrode through conjugated aromatic pyrazine linkages. The authors found distinct electron transfer behaviours compared with those observed in weakly-immobilized systems 38,39 . Unlike homogeneous molecular catalysts with stepwise electron transfer and substrate activation, this graphite conjugated catalyst (GCC) induces concerted electron transfer and substrate activation 38 .…”

“…A potential drop between the GCC site and the solution was found, which induces ion transfers coupled with electron transfers between them 38 . The applied potential modulates the driving force for substrate activation 39,40 .…”