Medium Effects Are as Important as Catalyst Design for Selectivity in Electrocatalytic Oxygen Reduction by Iron–Porphyrin Complexes

Several substituted iron–porphyrin complexes were evaluated for oxygen reduction reaction (ORR) electrocatalysis in different homogeneous and heterogeneous media. The selectivity for four-electron reduction to H₂O versus two-electron reduction to H₂O₂ varies substantially from one medium to another for a given catalyst. In many cases, the influence of the medium in which the catalyst is evaluated has a larger effect on the observed selectivity than the factors attributable to chemical modification of the catalyst. For instance, introduction of potential proton relays has variable effects depending on the catalyst medium. Thus, comparisons of selectivity results from supported and soluble molecular ORR electrocatalysts must be interpreted with caution, as selectivity is a property not only of the catalyst, but also of the larger mesoscale environment beyond the catalyst. Still, in all the direct pairwise comparisons in the same medium, the catalysts with potential proton relays have similar or better selectivity for the preferred 4<i>e</i>^– path

Molecular Cobalt Catalysts for O₂ Reduction: Low-Overpotential Production of H₂O₂ and Comparison with Iron-Based Catalysts

A series of mononuclear pseudomacrocyclic cobalt complexes have been investigated as catalysts for O₂ reduction. Each of these complexes, with Co^III/II reduction potentials that span nearly 400 mV, mediate highly selective two-electron reduction of O₂ to H₂O₂ (93–99%) using decamethylferrocene (Fc*) as the reductant and acetic acid as the proton source. Kinetic studies reveal that the rate exhibits a first-order dependence on [Co] and [AcOH], but no dependence on [O₂] or [Fc*]. A linear correlation is observed between log­(TOF) vs <i>E</i>_1/2(Co^III/II) for the different cobalt complexes (TOF = turnover frequency). The thermodynamic potential for O₂ reduction to H₂O₂ was estimated by measuring the H⁺/H₂ open-circuit potential under the reaction conditions. This value provides the basis for direct assessment of the thermodynamic efficiency of the different catalysts and shows that H₂O₂ is formed with overpotentials as low as 90 mV. These results are compared with a recently reported series of Fe-porphyrin complexes, which catalyze four-electron reduction of O₂ to H₂O. The data show that the TOFs of the Co complexes exhibit a shallower dependence on <i>E</i>_1/2(M^III/II) than the Fe complexes. This behavior, which underlies the low overpotential, is rationalized on the basis of the catalytic rate law

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

[Image: see text] 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 × 10(6) s(–1). These TOFs correlate with the ORR overpotential, which can be modulated by changing the E(1/2) 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 E(1/2) 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 E(1/2), 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

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

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

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

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

Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and <i>N</i>,<i>N</i>‑Dimethylformamide

A variety of next-generation energy processes utilize the electrochemical interconversions of dioxygen and water as the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Reported here are the first estimates of the standard reduction potential of the O₂ + 4<i>e</i>^– + 4H⁺ ⇋ 2H₂O couple in organic solvents. The values are +1.21 V in acetonitrile (MeCN) and +0.60 V in <i>N</i>,<i>N</i>-dimethylformamide (DMF), each versus the ferrocenium/ferrocene couple (Fc^+/0) in the respective solvent (as are all of the potentials reported here). The potentials have been determined using a thermochemical cycle that combines the free energy for transferring water from aqueous solution to organic solvent, −0.43 kcal mol^–1 for MeCN and −1.47 kcal mol^–1 for DMF, and the potential of the H⁺/H₂ couple, – 0.028 V in MeCN and −0.662 V in DMF. The H⁺/H₂ couple in DMF has been directly measured electrochemically using the previously reported procedure for the MeCN value. The thermochemical approach used for the O₂/H₂O couple has been extended to the CO₂/CO and CO₂/CH₄ couples to give values of −0.12 and +0.15 V in MeCN and −0.73 and −0.48 V in DMF, respectively. Extensions to other reduction potentials are discussed. Additionally, the free energy for transfer of protons from water to organic solvent is estimated as +14 kcal mol^–1 for acetonitrile and +0.6 kcal mol^–1 for DMF

Synthesis and Reactivity of Tripodal Complexes Containing Pendant Bases

The synthesis of a new tripodal ligand family that contains tertiary amine groups in the second-coordination sphere is reported. The ligands are tris­(amido)­amine derivatives, with the pendant amines attached via a peptide coupling strategy. They were designed to function as new molecular catalysts for the oxygen reduction reaction (ORR), in which the pendant acid/base group could improve the catalyst performance. Two members of the ligand family were each metalated with cobalt­(II) and zinc­(II) to afford trigonal-monopyramidal complexes. The reaction of the cobalt complexes <b>[Co­(L)]</b>^<b>–</b> with dioxygen reversibly generates a small amount of a cobalt­(III) superoxo species, which was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of the zinc complex Zn­[N­{CH₂CH₂NC­(O)­CH₂N­(CH₂Ph)₂}₃)]⁻ (<b>[Zn­(TN</b>^<b>Bn</b><b>)]</b>^<b>–</b>) with 1 equiv of acid occurs at a primary-coordination-sphere amide moiety rather than at a pendant basic site. The addition of excess acid to any of the complexes <b>[M­(L)]</b>^<b>–</b> results in complete proteolysis and formation of the ligands <b>H</b>_<b>3</b><b>L</b>. These undesired reactions limit the use of these complexes as catalysts for the ORR. An alternative ligand with two pyridyl arms was also prepared but could not be metalated. These studies highlight the importance of the stability of the primary-coordination sphere of ORR electrocatalysts to both oxidative <i>and</i> acidic conditions

Current status on the development of homogenous molecular electrocatalysts for oxygen reduction reaction (ORR) relevant for proton exchange membrane fuel cell applications

Oxygen reduction reaction (ORR) is an essential component in aerobic biological energy transduction, where the oxidation prowess of O2 is employed to harvest the energy stored in reduced carbon sources. This experimental blueprint is mimicked in renewable energy technology, such as fuel cell. However, the harsh chemical conditions encountered in fuel cells have restricted the direct use of fragile biological ORR catalysts: the copper-based oxidase enzymes. Thus, a number of homogeneous synthetic ORR catalysts were developed in the past few years that can be used directly as an alternative cathodic substance in fuel cell. In this review, we have depicted the rationale behind the evolution of various ORR catalysts along with their developmental history.by Afsar Ali, Divyansh Prakash and Arnab Dutt