Molecular Cobalt Catalysts for O2 Reduction to H2O2: Benchmarking Catalyst Performance via Rate-Overpotential Correlations

The oxygen reduction reaction catalyzed by homogeneous cobalt macrocycles typically leads to selective 2e(-)/2H(+) reduction of O-2 to H2O2. Variations in the reaction conditions make it difficult to compare the performance characteristics of different catalysts, however, and limits the ability to leverage insights to design improved catalysts. Here, we show that free energy relationships between the logarithm of the turnover frequency [log(TOF)] and the effective overpotential (eta(eff)) for the ORR enable systematic comparison of the catalytic performance of diverse Co-macrocycles under a variety of reaction conditions. The study is initiated by evaluating the ORR log(TOF)/eta(eff) correlation for a series of Co(porphyrin) catalysts. The data show that these catalysts exhibit a different linear free energy relationship relative to previously reported Co(N2O2) complexes and that this difference correlates with different rate laws associated with the two different classes of catalysts. These linear relationships are then compared to log(TOF)/eta(eff) data for a diverse collection of other homogeneous cobalt ORR catalysts reported previously in the literature, and the collective analysis shows how different catalyst systems and their performance may be compared, even when the reactions are conducted under different conditions. This benchmarking method is recommended as a general strategy for systematic comparison of other (electro)catalysts and catalytic reactions.