Mediated Electron Transfer for ORR Catalysis with Transition-Metal Macrocycles

The search for a non-precious-metal based, oxygen reduction reaction (ORR) catalysts for fuel cells has grown due to rising prices of the catalysts used today which is derived from their scarcity. Although very active, platinum, which is considered to be the state-of-the-art catalyst for ORR, is not used in biological systems almost at all, especially not in processes which involve the catalysis of ORR. One of the reasons may be availability, another may be reaction selectivity. Instead, biological systems usually use transition metal complexes to catalyze such reactions. These are normally a part of an elaborate system of electron donors, electron acceptors and electron mediators, described in electron transfer chains and cycles. Since the beginning of the search for non-precious-metal catalyst, one family of catalysts has always been interesting as a possible candidate to replace precious metal catalysts – transition metal macrocycles (TMMs). This is due to the significant roles these macrocycles play in biological systems, which in many cases involve ORR catalysis.

In the past years we have worked on the study of the interaction between various TMMs and redox active molecules known as electron donors in catalytic cycles. The interaction between quinones and other possible electron donors such as imidazoles and thiophenes have shown to enhance the ORR activity of TMMs. Figure 1 shows an RRDE measurement obtained with cobalt porphyrin (CoTPPS) interacting with hydroquinone on the surface of carbon electrode. In comparison to its ORR activity without the hydroquinone layer on the carbon (not shown), the CoTPPS’s half wave potential is shifted by more than 400mV to a more positive potential. This shift is attributed to the interaction with the electron donor.

In this talk, we will present our most recent work on the study of the interaction between TMMs and electron donors and its effect on ORR electrocatalysis.