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

Abstract

In the search for replacement of the platinum‐based catalysts for fuel cells, MN4 molecular catalysts based on abundant transition metals play a crucial role in modeling and investigation of the influence of the environment near the active site in platinum‐group metal‐free (PGM‐free) oxygen reduction reaction (ORR) catalysts. To understand how the ORR activity of molecular catalysts can be controlled by the active site structure through modification by the pH and substituent functional groups, the change of the ORR onset potential and the electron number in a broad pH range was examined for three different metallocorroles. Experiments revealed a switch between two different ORR mechanisms and a change from 2e− to 4e− pathway in the pH range of 3.5‐6. This phenomenon was shown by density functional theory (DFT) calculations to be related to the protonation of the nitrogen atoms and carboxylic acid groups on the corroles indicated by the pKa values of the protonation sites in the vicinity of the ORR active sites. Control of the electron‐withdrawing nature of these groups characterized by the pKa values could switch the ORR from the H+ to e− rate‐determining step mechanisms and from 2e− to 4e− ORR pathways and also controlled the durability of the corrole catalysts. The results suggest that protonation of the nitrogen atoms plays a vital role in both the ORR activity and durability for these materials and that pKa of the N atoms at the active sites can be used as a descriptor for the design of high‐performance, durable PGM‐free catalysts.

Application of Molecular Catalysts for the Oxygen Reduction Reaction in Alkaline Fuel Cells

“…During the past years, we have shown the great importance of the interaction between molecular catalysts and their support material and found that BP2000 is most suitable for macrocyclic compounds due to its high surface area and average pore size, which promotes cooperativity between catalytic sites, resulting in increased selectivity and lower ORR overpotential. − Herein, we present our study of ORR catalytic performance and durability under alkaline conditions using four different substituted iron phthalocyanines supported on BP2000. The results show that iron­(II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadeca (fluoro)­phthalocyanine (FeFPc) (Figure ) adsorbed on BP2000 (FeFPc@BP2000) has the highest catalytic activity of the four FePCs, higher than that described recently by others who used carbon nanotubes as the support, making it one of the best molecular catalyst–support system reported to date.…”

Orthogonal Design of Fe−N4 Active Sites and Hierarchical Porosity in Hydrazine Oxidation Electrocatalysts

“…Stability tests of corrole 1 @C800 show gradual loss of activity: 29 % of the starting HzOR current density are lost after 50 cycles (Figure 9e). On other N 4 macrocycles, this could arise from demetallation, especially since loss of iron is promoted by nitrogen protonation due to the high local proton concentration generated by the HzOR [66] . However, corroles are typically very stable towards hydrolysis [45,44] .…”

“…On other N 4 macrocycles, this could arise from demetallation, especially since loss of iron is promoted by nitrogen protonation due to the high local proton concentration generated by the HzOR. [66] However, corroles are typically very stable towards hydrolysis. [45,44] Alternatively, the iron center could be deactivated by the formation of inactive Fe IV -O-Fe IV dimers, due to the alkaline and oxidizing environment of the HzOR.…”

Corroles at work: a small macrocycle for great applications

“…This result evidenced the importance of the corrole dimensions on the ORR catalytic activity and led the researchers to [45][46][47] look to novel corrole-bearing substituents that can influence both the electronic and size-related properties of catalysts. In particular, the -CF 3 group as the meso-substituent attracted some attention because it is a small and strong electronwithdrawing substituent, two features that are particularly promising for the target application.…”

“…To further detail the different parameters that govern the ORR process, the influences of pH and peripheral substituents were studied via the comparison of the catalytic behavior of three Co corrole complexes (Fig. 12a-c), 46 with the support of DFT calculations.…”