Interfacial charge transfer mechanism of oxygen reduction reaction in alkali media: Effects of molecular charge states and triphenylamine substituent on cobalt porphyrin electrocatalysts

Promotion of Catalytic Oxygen Reduction Reactions: The Utility of Proton Management Substituents on Cobalt Porphyrins

“…In addition, F-CoPor/C attains an even smaller reduction current than XC-72R when comparing the j L values in Table and the CV plot in Figure S4, suggesting its poor ORR activity in alkaline electrolyte. While the n and HO 2 – % values of F-CoPor/C are different from those of XC-72R ( n = 2.4 and HO 2 – % = 80%), revealing that the porphyrin F-CoPor can participate in ORR and alter the selectivity …”

“…The adsorption affinities between Co­(III)–O and Co­(II)–O are different, which can further affect the O–O cleavage energy and then the ORR activity of the catalyst . In addition, previous theoretical studies revealed that for ORR, it is the change in the adsorption affinity of oxygen atoms to the metal core that the substituents on macrocyclic molecules affect. , In three porphyrin models, cyanoacetic acid has stronger electron-withdrawing ability than fluorine and acetic acid, which results in the negative shift of both HOMO and LUMO energies of CN-CoPor as compared to those of F-CoPor and AC-CoPor (Figure ). This may contribute to, at least partially, the better ORR performance of the CN-CoPor -based composite than the other two analogues.…”

“…It is noteworthy that the substrate of the composite can reduce oxygen in alkaline conditions, and the electrochemical behaviors of F-CoPor/C and AC-CoPor/C overlap that of − % = 80%), revealing that the porphyrin F-CoPor can participate in ORR and alter the selectivity. 41 We selected the best performing porphyrin composite CN-CoPor/C to examine the electrochemical stability in O 2saturated solutions according to the accelerated potential cycling method reported by Osmieri et al 42 Linear sweep voltammetry (LSV) sweeps were recorded in-between every 500 cycles of CV in an 1 V potential range, as shown in Figure S9. It is found that both current density and onset potential decrease clearly as more CV cycles exerted in both acidic and alkaline electrolytes.…”

Meso‐substituted Metalloporphyrin‐based Composites for Electrocatalytic Oxygen Reduction Reactions

“…[90] Two cobalt porphyrins with strong electron-withdrawing benzothiadiazole derivatives were coated on carbon black and examined in ORRs under alkaline conditions, exhibiting electron transfer numbers of 2.8 ~3.0 and enhanced response current as compared to the methylsubstituted counterparts. [91] Besides, altering the electron cloud density on the metal center core can be achieved by attaching a secondary coordination sphere. For example, markedly improved ORR activity of a Fe porphyrin 20 were reported by implementing a tethered imidazole for Fe binding, exhibiting competitive performance to Pt/Ir-based materials on a rechargeable Zn-air battery (Figure 6).…”

“…[111] When alkaline electrolyte is used for ORR measurements, triphenylamine is prohibited to be protonated and its benefit on charge transfer kinetics disappears, for instance, the study using compounds 38 and 39 in KOH electrolyte. [91] As a common phenomenon, the introduction of triphenylamine onto a porphyrin macrocycle often triggers re-localization of the dipole-induced partial charges, which may alter the charge states as well as the adsorption status at the reactive interfaces. [91] The charge sense on the triphenylamine part is affected by the molecular structure, in particular, the electron affinity strength of the neighboring units.…”