Theoretical Study of the Electrocatalytic Reduction of Oxygen by Metallocorroles

Abstract

Metallocorroles are transition metal complexes showing great promise as oxygen reduction reaction catalysts. The performance of metallocorrole catalysts is highly sensitive to the nature of the transition metal employed, although currently this dependence remains elusive. In the current work, we present a first principles density functional theory (DFT) investigation of the oxygen reduction reaction mechanism in acidic media using several first-row transition metal corroles. We show that the identity of the metal center, M, determines the relative formation free energies of the reaction intermediates, and thus the potential-determining step in the four-electron reduction process directly to water. For MMn, Fe, and Co, the hydroperoxyl intermediate is a thermodynamic maximum along the reaction path, whereas for Ni and Cu, the formation of the hydroperoxyl intermediate is a thermodynamic trap, with the following oxo intermediate being highly unstable. The formation of the oxo intermediate was carefully investigated using several flavors of DFT. The calculations suggest that in the oxo intermediate, the spin density on the oxygen atom increases from MMn to MCu, indicating a purely metal oxygen double bond for MMn, a single metal oxygen bond with an unpaired electron on the oxygen atom for MNi and Cu, whereas for MFe and Co, the spin density on the oxygen atom has intermediate values. When plotting the experimentally observed onset potentials as a function of the computed O2 adsorption free energies, a volcano-like plot is observed, indicating that for the best catalyst, [Co­(tpfcBr8)], a negative binding free energy is observed. A good correlation between the computed limiting and the experimental onset potentials indicates that the computationally proposed reaction intermediates are viable states along the reaction coordinate. The current work is expected to be of importance for the future design of efficient metallocorrole-based catalysts.

Control of Molecular Catalysts for Oxygen Reduction by Variation of pH and Functional Groups

“…To examine the first effect of the protonation on the ORR pathway, following the well‐known computational hydrogen electrode (CHE) approach, [30] the OOH and O 2 adsorption energies ( E ads ) were calculated for the three corroles in the presence of 4, 3, 2 and 0 H + on the N atoms of the corrole and reported in Table 2 [the adsorption energies are calculated with respect to H 2 and H 2 O; e. g., O 2 adsorption energy of CF 3 ‐Corrole is E (O2‐ CF3‐Corrole ) +2 E (H2) − E ( CF3‐Corrole ) −2 E (H2O) ; the data are reported in the Supporting Information]. The calculated E ads for the unprotonated corroles are comparable to those previously obtained for corroles and other molecular catalysts [7,16,31–33] . In the presence of protons decreases the adsorption energies of O 2 from 4.763 4.698 and 4.680 eV (relative to H 2 O) for the deprotonated CF 3 ‐ , ortho ‐ and para ‐Corroles to 4.445, 4.444 and 4.447 eV (relative to H 2 O) for the fully protonated CF 3 ‐ , ortho ‐ and para ‐Corroles , respectively.…”

“…In these studies, different metal centers and substituents were introduced to the corroles ring. Of the five first row transition metal centers studied in the past (Mn, Fe, Co, Ni and Cu), Co was found to be the most active [14,16,21] . In the present work, we used precise control of the active site environment of these corroles to analyze the relationships between the catalyst structure and performance and revealed the key role of protonation of the nitrogen atoms around the metal active site in determining the activity and durability of the catalyst.…”

“…In the past few years, we have investigated the catalytic activity of metallo‐corroles towards ORR in acidic and alkaline media [14–21] . In these studies, different metal centers and substituents were introduced to the corroles ring.…”

Molecular Control of Carbon‐Based Oxygen Reduction Electrocatalysts through Metal Macrocyclic Complexes Functionalization

“…Recent studies on metal corroles have found that when the central metal ion was Co(III), the catalyst exhibited the highest ORR activity. [ 97,98 ] The ORR activity of CNT‐supported Co(III) corroles in acidic, alkaline, and neutral electrolytes has been reported in many studies. [ 95,99–101 ] For example, Meng et al.…”

Enzyme‐Inspired Iron Porphyrins for Improved Electrocatalytic Oxygen Reduction and Evolution Reactions

“…[1,3] Inspired by nature, many synthetic Fe porphyrins and other related metal macrocycles have been designed and investigated as catalysts for O 2 reduction. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] From these studies, fundamental knowledge to boost catalyst performance is learned. For example, efficient electron transfer to Fe porphyrin sites and proper hydrogen bonding interactions with Fe-O 2 adducts can facilitate the 4e reduction of O 2 to water.…”