Efficient chemical-to-electrical energy conversions are challenging but are key to sustainable energy technologies such as fuel cells. Energy conversion reactions like the oxygen reduction reaction (ORR; O2 + 4H+ + 4e− = 2H2O) have many steps and require navigating a complicated energy landscape. Through studies of the rates and mechanisms of soluble iron porphyrin ORR electrocatalysis in organic solvents, we have developed “molecular scaling relationships” to describe the various relationships between turnover frequency (TOF) and overpotential as a function of catalyst E1/2, acid pKa, etc. This talk will describe how such scaling relationships can be additive, to achieve “inverse” scaling relationships: faster TOFs are obtained with lowered overpotentials. ORR catalysis by the polycationic iron-(ortho-trimethylanilinium)porphyrin illustrates the power of this approach, and it provides a system in which to test the role of cationic groups in the second coordination sphere. The electrostatic effects are substantial and typically affect multiple steps in the catalytic cycle, often in indirect ways. Extensions of these approaches and conclusions to new catalytic systems will be discussed.

Understanding and optimizing molecular electrocatalysis through scaling relationships and electrostatics

ACS Fall 2021

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