PNNL

Abstract

The oxygen reduction reaction (ORR) is the cathode reaction in fuel cells and its selectivity for water over hydrogen peroxide production is important for these technologies. Iron porphyrin catalysts have long been studied for the ORR, but their origins of selectivity are not well understood because the selectivity-determining step(s) usually occur after the rate-determining step. We report here the effects of acid pKa, acid concentration, and other solution conditions on the H2O2/H2O selectivity in electrocatalytic ORR by iron tetramesitylporphyrin Fe(TMP). The results show that selectivity reflects a kinetic competition in which the dependence on [HX] is one order greater for the production of H2O than H2O2. Based on such experimental results and computational studies, we propose that the selectivity is governed by competition between protonation of the hydroperoxo intermediate FeIII(TMP)(OOH) to produce water versus dissociation of the HOO– ligand to yield to H2O2. The data rule out a bifurcation based on the regioselectivity of protonation of the hydroperoxide, as suggested in the enzymatic systems. Furthermore, the analysis developed in this study should be generally valuable to the study of selectivity in other multiproton/multielectron electrocatalytic reactions.