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Electrochemical reduction of dioxygen (O2) and carbon dioxide (CO2) are two examples of redox reactions that have captured interest as renewable energy solutions. For example, fuel cell technologies demand improvement for the reduction of O2 to H2O selectively at low energy costs. The basic requirements for transformations of O2 and CO2 are input of multiple protons (H+) and electrons (e–) (e.g., 4H+/4e– for O2 to H2O). The first half of my talk will focus on the O2 reduction reaction (ORR) using heterogeneous catalytic platforms. The work starts from Fe-tetra(aryl)porphyrins, which are known as ORR catalysts (producing H2O). However, they exhibit high overpotentials and poor stability, making them ill-suited for application. In comparison, heterogeneous Co-derivatives show ORR at lower overpotentials but make mostly H2O2 which is undesirable for fuel cell applications. My research presents asymmetric Fe- or Co-porphyrin derivatives bearing a single pendant proton relay. Work on heterogeneous ORR using these asymmetric metalloporphyrins shows improvement in catalytic stability on graphite electrodes without compromising selectivity. Noteworthy, Co-derivatives are better catalysts than Fe analogs and use of asymmetric porphyrins shifts selectivity from the production of H2O2 to H2O.
The electrochemical CO2 reduction reaction (CO2RR) provides a route to turn a greenhouse gas into value-added products. The 2H+/2e– reduction of CO2 to CO is the central attention because CO could be used as a precursor to produce many products, including fuels. Benchmark Fe-porphyrins bearing up to eight proton relays are known for homogeneous CO2RR. However, my research addresses that an iron porphyrin with one proton relay at a meso-position can achieve a similar benchmark depending on the choice of solvent. In addition, these asymmetric metalloporphyrin derivatives are excellent heterogeneous CO2RR catalysts once immobilized on graphite surfaces, with activation of CO2 near the thermodynamic potential in aqueous electrolyte. More kinetic insights of these catalytic models and a mechanistic outline for heterogeneous CO2RR will also be discussed. |