Electrocatalytic Dioxygen Reduction by Carbon Electrodes Noncovalently Modified with Iron Porphyrin Complexes: Enhancements from a Single Proton Relay

Abstract

Oxygen reduction in acidic aqueous solution mediated by a series of asymmetric iron (III)-tetra(aryl)porphyrins adsorbed to basal- and edge- plane graphite electrodes is investigated. The asymmetric iron porphyrin systems bear phenyl groups at three meso positions and either a 2-pyridyl, a 2-benzoic acid, or a 2-hydroxyphenyl group at the remaining meso position. The presence of the three unmodified phenyl groups makes the compounds insoluble in water, enabling catalyst retention during electrochemical experiments. Resonance Raman data demonstrate that catalyst layers are maintained, but can undergo modification after prolonged catalysis in the presence of O2 . The introduction of a single proton relay group at the fourth meso position makes the asymmetric iron porphyrins markedly more robust catalysts; these molecules support higher sustained current densities than the parent iron tetraphenylporphyrin. Iron porphyrins bearing a 2-pyridyl group are the most active catalysts and operate at stable current densities ≥1 mA cm(-2) for over 5 h. Comparative analysis of the catalysts with different proton relays also is reported.

A Pyridinic Fe-N4 Macrocycle Effectively Models the Active Sites in Fe/N-Doped Carbon Electrocatalysts

“…Cyclic voltammograms of Fe-N-C, (phen2N2)FeCl, (OEP)FeCl, and (Pc)FeCl recorded in the absence of O2 provide insight into the redox potential of the metal center. For (OEP)Fe(Figure 4, orange), we observe a very broad redox feature with E1/2 = 0.27 V (all potentials are reported vs the reversible hydrogen electrode, RHE), similar to reported values for carbon-supported iron porphyrin complexes 113,135. For (Pc)Fe(Figure 4, aqua), we observe a broad redox feature with E1/2 = 0.61, similar to reported values for carbon-supported iron phthalocyanine complexes (Figure S19).…”

A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

“…We note that the Fe(III/II) redox couple of (OEP)FeCl is quite broad with a peak-to-peak separation of 315 mV. We attribute this to sluggish Fe(III/II) redox self-exchange through the film, but nonetheless, we note that the observed E 1/2 value is comparable to other carbon-adsorbed iron porphyrin compounds 17 , 56 , 77 , 78 . For (Pc)FeCl (Fig.…”

“…While this species is catalytically inactive in the acidic electrolyte (Supplementary Fig. 6 ) 56 , 57 , it provides a good spectroscopic model for Fe-N-C materials and the analogous [(OEP)Fe] 2 O and [(Pc)Fe] 2 O were synthesized for comparison 55 , 58 , 59 . The 57 Fe Mössbauer spectrum of the [(phen 2 N 2 )Fe] 2 O complex is a narrow doublet (Supplementary Fig.…”

Nature-inspired electrocatalysts and devices for energy conversion

“…imitating the proton transfer pathways and hydrogen bonding groups in cytochrome c oxidase 149,[175][176][177][178][179] demonstrated high selectivity (greater than 90%) and reactivity (rate constant greater than 10 7 M À1 s À1 ) for ORR at pH = 7 when adsorbed on an edge-plane graphitic electrode. 180 The measured rate constant of oxygen reduction was approximately two orders of magnitude higher than the heme/Cu complexes, [181][182][183][184][185][186] suggesting that the distal basic residues stabilize the Fe III -OOH intermediate species and enable heterolytic cleavage via protonation of their distal oxygen, promoting a four-electron ORR route. 180 Another promising example of a bio-imitating, porphyrinbased ORR electrocatalyst mimicking the active site of oxygenactivating heme-containing enzymes, such as cytochrome c oxidase, is an axial-imidazole coordinated iron porphyrin, covalently grafted on multi-wall carbon nanotubes (MWCNTs).…”