Electrocatalytic dioxygen reduction with copper complexes

The oxygen reduction reaction (ORR) is the four electron and four proton reaction with oxygen to form water. This reaction, while very exothermic (-474 kJ/mol), remains a difficult reaction to harness due to the high O-O bond strength (485 kJ). Many proteins in nature couple the ORR to various substrate oxidations in order to provide energy, such as in respiration. Commercially, the ORR is important in a large number of systems, chiefly as the cathode reaction in a polymer electrolyte membrane fuel cell (PEMFC). While fuel cells have been in use since in their modern form since the 1960s, they are still not widely employed. The main factor preventing fuel cell proliferation is the high overpotentials for the ORR, the cathode reaction. Cathodes are typically constructed with Pt based catalysts, which exhibit ∼300 mV of overpotential, leading to high loading of Pt and therefore large costs for the fuel cell stack.;Using Nature as an inspiration, this dissertation explores the fundamental reaction mechanism of the oxygen reduction reaction as electrocatalyzed by copper complexes. The second chapter focuses on two similar complexes, [CuTPA] 2+ (TPA = tris(2-pyridylmethyl)amine) and [Cu2bistripic] 2+ (bistripic = 1,2-bis(6-(bis(6-methylpyridin-2-yl)methyl)pyridin-2-yl)ethane). CuTPA reacts with dioxygen to form an end-on peroxo complex, while Cu 2bistripic forms a side-on peroxo complex. It is found that both complexes likely react to form Cu2O2 complexes, with CuTPA exhibiting lower overpotentials for the ORR than Cu2bistripic. The low overpotential for the ORR by [CuTPA]2+ is attributed to the nucleophilicity of the end-on peroxo species, while the high overpotential for [Cu2bistripic] 2+ and the side-on peroxo complex, is due to its electrophilicity.;Attempts at understanding the ligand effects on the ORR are studied in Chapter 3. Many ligands based on the CuTPA motif were studied. A series of ligands that raise the CuI/II couple potential are found to have no effect on the overpotential for the ORR. Similarly, addition of hydrogen-bonding groups did not effect on the overpotential. Ligands with only alkyl functionality exhibited very high overpotentials, which is attributed to poor electron transfer into the alkyl-Cu complex. The rate determining step (RDS) is not the Cu II to CuI reduction, nor is it a protonation event. I speculate that the RDS is the reduction of a hydroperoxo intermediate.;The first step in the ORR catalytic reaction mechanism for [CuTPA] 2+ is examined in detail in Chapter 4. Using in-situ Raman spectroscopy, the initially electro-reduced CuTPA species is examined. Solution electrochemistry confirms that an insoluble species precipitates on the electrode surface upon a reduction of [CuTPA]2+. There is evidence that the reduced species is the cuprous complex, which then disproportionates into Cu metal and [CuTPA]2+. There also may be a hydrated perchlorate bridged species. The actual catalytically active species is cannot be readily identified, as the mixture of species is complex.