Proper understanding of the major limitations of current catalysts for oxygen reduction reaction (ORR) is essential for further advancement. Herein by studying representative Pt and non-Pt ORR catalysts with a wide range of redox potential (E) via combined electrochemical, theoretical, and in situ spectroscopic methods, we demonstrate that the role of the site-blocking effect in limiting the ORR varies drastically depending on the E of active sites; and the intrinsic activity of active sites with low E have been markedly underestimated owing to the overlook of this effect. Accordingly, we establish a general asymmetric volcano trend in the ORR activity: the ORR of the catalysts on the overly high E side of the volcano is limited by the intrinsic activity; whereas the ORR of the catalysts on the low E side is limited by either the site-blocking effect and/or intrinsic activity depending on the E.

Metal macrocycles are among the most important catalytic systems in electrocatalysis and biocatalysis owing to their rich redox chemistry. Precise understanding of the redox behavior of metal macrocycles in operando is essential for fundamental studies and practical applications of this catalytic system. Here we present electrochemical data for the representative iron phthalocyanine (FePc) in both aqueous and nonaqueous media coupled with in situ Raman and X-ray absorption analyses to challenge the traditional notion of the redox transition of FePc at the low potential end in aqueous media by showing that it arises from the redox transition of the ring. Our data unequivocally demonstrate that the electron is shuttled to the Pc ring via the Fe(II)/Fe(I) redox center. The Fe(II)/Fe(I) redox transition of FePc in aqueous media is indiscernible by normal spectroscopic methods owing to the lack of a suitable axial ligand to stabilize the Fe(I) state.

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Chemical structures of lithium and tetrabutylammonium (TBA) salt solutions in N,N-dimethylacetamide (DMAc) and N,N-diethylacetamide (DEAc), two high Donor Number organic solvents, have been studied. In LiX salt solutions (where X = PF 6 − , CF 3 SO 3 − , ClO 4 − and NO 3 − ), solvation occurs when the Li + bonds with the solvent's carbonyl group forming Li + [O=C(CH 3 )N(CH 3 ) 2 ] n X − ion pairs. Infrared and 13 C-NMR spectra are consistent with the ion pair being solvent-separated when the anion is PF 6 − , ClO 4 − or NO 3 − , and a contact ion pair in the case of CF 3 SO 3 − . Chemical interactions between TBA + and the solvents to form conducting solutions appeared to be dipolar in nature. Ionic conductivities of TBA + and Li + electrolytes were measured and correlated with their viscosities. In 0.1M TBAPF 6 /DMAc, the O 2 solubility and diffusion coefficient (3.09 × 10 −6 mol/cm −3 and 5.09 × 10 −5 cm 2 s −1 , respectively) measured using microelectrode technique are typical of values measured in several TBA + solutions. Microelectrode voltammetry revealed steady-state limiting current behavior for oxygen reduction reactions (ORR) in TBAX/DMAc electrolytes indicating a reversible ORR process. Conversely, microelectrode current-voltage data for ORR in LiX/DMAc electrolytes revealed irreversible behavior mainly ascribed to the blockage of the electrode surface by insoluble ORR products. The ORR in DMAc correlated with its high Donor Number and the overall process conformed to the Hard-Soft Acid-Base theory.

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