Developing Improved Earth-Abundant Electrocatalysts for the Reduction of Dioxygen through Mechanistic Insight

The oxygen reduction reaction (ORR) is an attractive route to alternative energy sources as well as environmentally friendly industrial processes. Development of earth abundant catalysts is necessary for the ORR to be a sustainable option. Motivated by the lack of synthetic non-heme Fe centered catalysts for the ORR, we have studied two non-heme Fe systems containing N3O ligand frameworks, Fe(PMG)Cl2 and Fe(tpytbupho)Cl2. Mechanistic analyses revealed that both systems operated via a 2+2 mechanism, where H2O2 was a discrete intermediate during catalysis before further reduction to H2O. Interestingly, the rate limiting step for Fe(PMG)Cl2 was cleavage of an off-cycle dimer species, while O2 binding to the reduced Fe center was rate limiting for Fe(tpytbupho)Cl2. Based on these results, a key factor in observed reactivity was the axial ligand trans to the O2 binding site.
Mn-based systems for the ORR are much less widely studied in comparison to Fe and Co. Because of the prevalence of Mn in nature, we undertook a mechanistic study examining two Mn complexes containing N2O2 ligand frameworks to understand how secondary coordination sphere interactions could tune Mn-based ORR. Analysis of these complexes using an ammonium proton source revealed that incorporation of a –OMe pendent relay group could shift selectivity to preferably producing H2O2. Interestingly, without added conjugate base strong hydrogen bonding with the pendent –OMe group suppressed catalysis. However, in the presence of added base, this suppression is mitigated and there is an accessible dimeric pathway that is controlled by the pKa of the Mn–H2O2 intermediate.
Redox-active organic molecules that are stable toward reactive oxygen species have drawn increasing attention for use as sustainable ORR catalysts. Here, two cationic organic molecules have been evaluated as ORR catalysts. First, an iminium-based compound was studied under both electrochemical and spectrochemical conditions with TFAH as a proton source and was shown to catalytically reduce O2 under electrochemical and spectrochemical conditions. We observed a divergence in mechanism, where under spectrochemical conditions, outer-sphere O2 reduction occurred to produce H2O2, whereas under electrochemical conditions in the presence of excess reduced catalyst, O2•– reacted via an inner-sphere mechanism to be further reduced to H2O. Then, a substituted phenanthroline diium compound was evaluated for the ORR using acetic acid derivatives to understand the dependence of ORR on acid strength. It was found that activity and rate-determining step was dependent on acid strength. Additionally, under certain conditions an off-cycle dimeric species was observed to be kinetically relevant. A mechanistic understanding of the controlling factors of the ORR is imperative to the development of efficient earth-abundant catalysts.