The ever-present threat of global climate change has inspired study of proton reduction to hydrogen gas for use in fuel cells, and the reduction of carbon dioxide to CO or other hydrocarbons for use as fuel in a closed carbon cycle. One molecule that has shown electrochemical competence to catalyze both the hydrogen evolution reaction (HER) and the CO2 reduction reaction (CO2RR) is Fe(0) tetraphenyl porphyrin (FeTPP). We present the use of cyclic voltammetry and bulk electrolysis to investigate the role of co-substrates in the modulation of push-pull reaction dynamics for electrochemical HER and CO2RR by FeTPP. Addition of nitrogen containing bases to FeTPP with acetic acid (AcOH) under argon shows dramatic changes in potential-dependent current response and acceleration of kinetic rates for hydride formation and hydrogen release during HER. In these systems, ligand strength and Brønsted basicity demonstrate independent effects on the rates of each step of HER. Application of these co-substrates to electrochemical CO2 reduction by FeTPP using weak acids showed a drastically different effect on reaction dynamics. Bulk electrolysis experiments reveal divergent pathways for CO2 reduction where formate can be produced with up to 80% efficiency. These results introduce a new mechanism of formate production from CO2 that does not proceed via hydride insertion and shines light on the interplay of multiple system components in the design of improved catalytic systems.

Fine tuning HER and CO2RR reaction dynamics via co-substrate addition to Fe tetraphenyl porphyrin

2019 Fall National Meeting

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