Glassy carbon electrodes were functionalized with redox-active moieties by condensation of o-phenylenediamine derivatives with o-quinone sites native to graphitic carbon surfaces. Electrochemical and spectroscopic investigations establish that these graphite-conjugated catalysts (GCCs) exhibit strong electronic coupling to the electrode, leading to electron transfer (ET) behavior that diverges fundamentally from that of solution-phase or surface-tethered analogues. We find that (1) ET is not observed between the electrode and a redox-active GCC moiety regardless of applied potential. (2) ET is observed at GCCs only if the interfacial reaction is ion-coupled. (3) Even when ET is observed, the oxidation state of a transition metal GCC site remains unchanged. From these observations, we construct a mechanistic model for GCC sites in which ET behavior is identical to that of catalytically active metal surfaces rather than to that of molecules in solution. These results suggest that GCCs provide a versatile platform for bridging molecular and heterogeneous electrocatalysis.

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.

Carbon electrodes chemically modified with molecular active sites are potent catalysts for key energy conversion reactions. Generally, it is assumed that these molecularly modified electrodes operate by the same redox mediation mechanisms observed for soluble molecules, in which electron transfer and substrate activation occur in separate elementary steps. Here, we uncover that, depending on the solvent, carbon-bound cobalt porphyrin can carry out electrolysis by the non-mediated mechanisms of metal surfaces in which electron transfer and substrate activation are concerted. We chemically modify glassy carbon electrodes with cobalt tetraphenylporphyrin units that are anchored by flexible aliphatic linkages to form CH-CoTPP. In acetonitrile, CH-CoTPP displays a clear outer-sphere Co(II/I) process which catalyzes the H2 evolution reaction by a step-wise, redox-mediated reaction sequence. In contrast, clear surface redox waves are not observed for CH-CoTPP in aqueous media and H2 evolution proceeds via a non-mediated, concerted proton-electron transfer reaction sequence over a wide pH range. The data suggest that, in aqueous electrolyte, the CoTPP fragments reside inside the electrochemical double layer and are electrostatically coupled to the surface. This coupling allows CH-CoTPP to carry out catalysis without being pinned to the redox potential of the molecular fragment. These studies highlight that the simple adsorption of molecules can lead to reaction mechanisms typically reserved for metal surfaces, exposing new principles for the design of molecularly-modified electrodes.

We synthesize porphyrins that are strongly electronically coupled to carbon electrodes by condensing diaminoporphyrin derivatives onto o-quinone moieties native to graphitic carbon surfaces. X-ray photoelectron and absorption spectroscopies along with electrochemical data establish the formation of a high-fidelity conjugated pyrazine linkage to the surface with preservation of the metalloporphyrin scaffold. Using the O2 reduction reaction (ORR) as a probe, we find that conjugation dramatically enhances the rate of catalysis. A graphite-conjugated Co porphyrin, GCC-CoTPP, displays an onset current density of 10 μA/cm2 at 0.72 V versus the reversible hydrogen electrode, whereas a nonconjugated amide-linked Co porphyrin onsets at 0.66 V. This corresponds to an order of magnitude enhancement in the activation-controlled turnover frequencies for ORR upon conjugation. This work establishes a versatile platform for examining the emergent reactivity of porphyrins strongly coupled to metallic electrodes.