Homogenous Electrocatalytic Oxygen Reduction Rates Correlate with Reaction Overpotential in Acidic Organic Solutions

Abstract

Improved electrocatalysts for the oxygen reduction reaction (ORR) are critical for the advancement of fuel cell technologies. Herein, we report a series of 11 soluble iron porphyrin ORR electrocatalysts that possess turnover frequencies (TOFs) from 3 s–1 to an unprecedented value of 2.2 × 106 s–1. These TOFs correlate with the ORR overpotential, which can be modulated by changing the E1/2 of the catalyst using different ancillary ligands, by changing the solvent and solution acidity, and by changing the catalyst’s protonation state. The overpotential is well-defined for these homogeneous electrocatalysts by the E1/2 of the catalyst and the proton activity of the solution. This is the first such correlation for homogeneous ORR electrocatalysis, and it demonstrates that the remarkably fast TOFs are a consequence of high overpotential. The correlation with overpotential is surprising since the turnover limiting steps involve oxygen binding and protonation, as opposed to turnover limiting electron transfer commonly found in Tafel analysis of heterogeneous ORR materials. Computational studies show that the free energies for oxygen binding to the catalyst and for protonation of the superoxide complex are in general linearly related to the catalyst E1/2, and that this is the origin of the overpotential correlations. This analysis thus provides detailed understanding of the ORR barriers. The best catalysts involve partial decoupling of the influence of the second coordination sphere from the properties of the metal center, which is suggested as new molecular design strategy to avoid the limitations of the traditional scaling relationships for these catalysts.

Mechanisms of Two‐Electron versus Four‐Electron Reduction of Dioxygen Catalyzed by Earth‐Abundant Metal Complexes

“…The two linear correlations of log(TOF) vs. η Fe/ORR shown in Figure C have inverse slopes of 57 and 54 mV per decade in TOF for the left and right correlations, respectively. This is quite close to the Nernstian slope of 59 mV per decade in concentration, indicating that the energy change in the driving force is directly reflected in the energy change in the transition state of ORR …”

“…8 DMF is represented in purple, and the conditions are identical to those specified in panel B. Reprinted with permission from Ref. . Copyright 2016, American Chemical Society.…”

“…Plots of log(TOF) vs. E Fe(III/II) for the various iron catalysts revealed linear free‐energy relationships (LFERs) within each solvent system (Figure B) . The separate LFERs are unified as two LFERs when the log(TOF) values are plotted against the overpotentials of the oxygen reduction reaction (ORR), η Fe/ORR = − E Fe(III/II) (Figure C) . The two linear correlations of log(TOF) vs. η Fe/ORR shown in Figure C have inverse slopes of 57 and 54 mV per decade in TOF for the left and right correlations, respectively.…”

A Carbon Electrode Functionalized by a Tricopper Cluster Complex: Overcoming Overpotential and Production of Hydrogen Peroxide in the Oxygen Reduction Reaction

“…This value is more positive than the onset reduction potential reported for any Cu complex modified electrode at this pH. [17] Although it is more appropriate for ah omogenous system, where the current density is constant at h beyond E 1/2 ,i tw ould still be meaningful for comparison of different molecular catalysts.F or the catalyst 2 modified SPCE at pH 7, h Cu/ORR = E O 2 =H 2 O ÀE Cu(ll/l) ffi0.68 Vv s. RHE at pH 7(E O 2 =H 2 O = 1.23 Vvs. [17] Although it is more appropriate for ah omogenous system, where the current density is constant at h beyond E 1/2 ,i tw ould still be meaningful for comparison of different molecular catalysts.F or the catalyst 2 modified SPCE at pH 7, h Cu/ORR = E O 2 =H 2 O ÀE Cu(ll/l) ffi0.68 Vv s. RHE at pH 7(E O 2 =H 2 O = 1.23 Vvs.…”

Efficient Solar‐Assisted O2 Reduction Using a Cofacial Iron Porphyrin Dimer Catalyst Integrated into a p‐CuBi2O4 Photocathode

“…Due to their relevance in biological ORR systems, mononuclear Fe porphyrins have received extensive attention to understand the ORR mechanism of biological enzymes and to develop efficient non‐precious metal‐based catalysts for fuel cells . Dinuclear Fe porphyrin dimers, however, have not yet been investigated for the ORR, although their dinuclear Co analogues had been extensively investigated, in which two Co ions capture O 2 through coordination and facilitate O−O bond cleavage, enabling catalytic electrochemical 4 e − reduction of O 2 to H 2 O .…”