Tuning the Activity of Heterogeneous Cofacial Cobalt Porphyrins for Oxygen Reduction Electrocatalysis through Self‐Assembly

Abstract

We report a suite of coordination-driven self-assembled prisms for heterogeneous electrocatalytic oxygen reduction (ORR) differing in the molecular clips linking two porphyrin faces in a cofacial arrangement. ORR activities and selectivities of monomeric CoTPyP along with cofacial prisms Ox-Co, Oxa-Co, and Benzo-Co were probed using cyclic voltammetry and rotating ring-disk techniques. All species were immobilized as heterogeneous catalysts on glassy carbon electrodes using a Nafion ink method. The selectivities of Ox-Co, Oxa-Co, and Benzo-Co prisms towards H O as determined by RRDE were 87, 97, and 75 %, respectively. The current density of the Oxa-Co plateaus at five times that of Pt/C when normalized per Co/Pt. The high synthetic yield (79 %), competitive overpotential (η ≈800 mV) and high selectivity (%H O ≈97 %) of the Oxa-Co highlights how self-assembly can be used to address multi-electron multi-proton transformations using polynuclear catalysts.

Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(ii) porphyrin–porphyrin dyad

“…4, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] In addition to biologically relevant Fe porphyrins, Co porphyrins have also received great attention because of their high activity and stability. [19][20][21][22][23][24][25][26][32][33][34] However, mononuclear Co porphyrins are much less selective than Fe analogues to catalyze the 4e reduction of O 2 to water. Unlike Fe complexes, Co complexes in general catalyze the 2e reduction of O 2 because the peroxo intermediates of Co are challenging to undergo heterolytic O-O bond cleavage to generate terminal Co-oxo species that have large d electron counts and thus are high in energy.…”

“…34,[41][42][43][44] Additionally, for complexes of late-transition metals, such as Co, it is found that dinuclear complexes are typically more efficient than mononuclear analogues to catalyze the 4e reduction of O 2 . [20][21][22][23][24][25][26][45][46][47][48][49][50][51][52][53] This improvement is likely caused by the formation of peroxo-bridged dinuclear intermediates, 45,47 whose subsequent reduction can result in the 4e ORR. Despite the improvement in the selectivity, however, from mononuclear to dinuclear complexes, the enhancement of catalytic activity, in terms of both catalytic currents and overpotentials, is unsatisfactory.…”

Oxygen Reduction Reaction at Penta-Coordinated Co Phthalocyanines

“…For example ORR catalyzed by unsubstituted CoPc or by CoPc with the addition of one or more electron-donating or electronwithdrawing substituents adsorbed directly on graphite proceeds via a 2-electron transfer process (Zagal et al, 1992;Sun et al, 2014;Zagal and Koper, 2016;Ye et al, 2017;Abarca et al, 2019) whereas complexes with pendant residues that increase the electron back-bonding would promote a direct 4-e process as in Equation 2 (Steiger and Anson, 1995;Tse et al, 1997;Riquelme et al, 2018a,b). Co-cofacial bimetallic complexes and dimeric Co porphyrins that bind simultaneously both dioxygen atoms would also transfer directly 4 electrons to O 2 (Collman et al, 1980;Durand et al, 1983;Le Mest et al, 1997;Peljo et al, 2012;Swavey and Eder, 2013;Wada et al, 2013;Dey et al, 2017;Oldacre et al, 2018). Penta-coordinated complexes involve a Co center coordinated to 4 planar N atoms and an axial N belonging to a pyridine molecule as for example in VB12 seem to promote a 4-e process.…”

Probing the Roughness of Porphyrin Thin Films with X‐ray Photoelectron Spectroscopy

“…Depending on the central metal atom and potential side groups, porphyrins exist with a wide range of chemical, biological, and electronic functionalities. [8] Owing to their structural flexibility, they find widespread applications in electrocatalysis, [9] sensorics, [10] molecular electronics, [11] molecular mechanics [12] and dye-sensitized solar cells. [13] To be used in devices, the molecules have to be supported on solid substrates.…”