The efficient interconversion of electrical to chemical energy requires the intimate coupling of electron transfer to substrate bond rearrangement. In molecular electrocatalysis, electron transfer and substrate activation are decoupled into separate elementary steps which imposes E1/2-rate scaling relationships that prevent access to high rates at low overpotentials. These redox mediated pathways are often presumed operative for molecules adsorbed to carbon electrode surfaces. Here, we uncover that carbon-bound cobalt porphyrins display both mediated and non-mediated reaction mechanisms depending on the electrolyte medium. We chemically modify glassy carbon electrodes with cobalt tetraphenylporphyrin anchored by an insulating, flexible linkage to form CH-CoTPP. In acetonitrile, CH-CoTPP displays a clear outer-sphere Co(II/I) redox process which catalyzes the H2 evolution reaction by a step-wise, redox-mediated reaction sequence. In contrast, non-catalytic redox features are not clearly observed for CH-CoTPP in aqueous media and the H2 evolution reaction proceeds via a non-mediated, concerted proton-electron transfer sequence across a wide pH range. The data indicate that, in aqueous electrolyte, the CoTPP fragments reside inside the electrochemical double layer and are electrostatically coupled to the surface. This coupling allows the adsorbed CoTPP to circumvent the common E1/2-rate scaling relationship that limits redox-mediated catalysis. These studies highlight that non-mediated concerted reaction pathways may contribute to the potent reactivity of chemically modified carbon electrodes.

Solvent dependent concerted proton-electron transfer by cobalt porphyrins tethered to carbon electrodes

ACS Spring Meeting 2021

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