PNNL

Abstract

The oxygen reduction reaction (ORR) is a multi-proton/multi-electron transformation in which dioxygen (O2) is reduced to water or hydrogen peroxide and serves as the cathode reaction in most fuel cells. The ORR involves up to nine substrates (O2 + 4e– + 4H+) and thus requires navigating a complicated reaction landscape, typically with several high-energy intermediates. Many catalysts can perform this reaction, though few operate with fast rates and at low overpotentials (close to the thermodynamic potential). Attempts to optimize these parameters, both in homogeneous and heterogeneous (electro)catalytic systems, have focused on modifying catalyst design and understanding thermodynamic/kinetic relationships between catalytic intermediates. One such method for analyzing and predicting catalyst reactivity and efficiency has been the development of “scaling relationships.” Here, we share our experience deriving and utilizing molecular scaling relationships for soluble, iron porphyrin-catalyzed O2 reduction in organic solvents. These relationships correlate turnover frequencies (TOFs) and effective overpotentials (?effs), properties uniquely defined for homogeneous catalysts. Following a general introduction of scaling relationships for both homogeneous and heterogeneous electrocatalysis, we detail the requirements for deriving such scaling relationships: i) defining the thermochemistry and ii) identifying the rate and rate law of the catalyzed reaction. We then show how to connect these thermodynamic and kinetic parameters to reveal multiple molecular scaling relationships for iron porphyrin-catalyzed O2 reduction. For example, the log (TOF) responds steeply to changes in ?eff that result from different catalyst reduction potentials (18.5 decades/V), but much less dramatically to changes in ?eff that arise from varying the pKa of the acid buffer (5.1 decades/V). A key takeaway from this Account is that a single scaling relationship is not always sufficient for describing molecular (electro)catalysis, whe Here, we share our experience deriving and utilizing molecular scaling relationships for soluble, iron porphyrin-catalyzed O2 reduction in organic solvents. These relationships correlate turnover frequencies (TOFs) and effective overpotentials (?effs), properties uniquely defined for homogeneous catalysts. Following a general introduction of scaling relationships for both homogeneous and heterogeneous electrocatalysis, we detail the requirements for deriving such scaling relationships: i) defining the thermochemistry and ii) identifying the rate and rate law of the catalyzed reaction. We then show how to connect these thermodynamic and kinetic parameters to reveal multiple molecular scaling relationships for iron porphyrin-catalyzed O2 reduction. For example, the log (TOF) responds steeply to changes in ?eff that result from different catalyst reduction potentials (18.5 decades/V), but much less dramatically to changes in ?eff that arise from varying the pKa of the acid buffer (5.1 decades/V). A key takeaway from this Account is that a single scaling relationship is not always sufficient for describing molecular (electro)catalysis, where catalyst identity and reaction conditions can correlate. Using these multiple scaling relationships, we demonstrate that the metrics of turnover frequency and effective overpotential can be predictably tuned to achieve faster rates at lowered overpotentials. This account uses related, anthological stories about our research on iron porphyrin-catalyzed ORR to show how molecular scaling relationships can be derived and used for any (electro)catalytic reaction. Such scaling relationships are powerful tools and connect the thermochemistry, mechanism, and rate law for a single catalytic system. We hope that the following examples show the utility and simplicity of this approach for understanding catalysis, enabling direct comparisons between catalyst systems, and optimizing catalytic processes.