Electrochemistry Fuel cells currently rely too heavily on expensive platinum catalysts to reduce oxygen. Cheaper metals still have not matched the efficiency of platinum. Pegis et al. studied a series of well-defined iron porphyrin complexes in solution, in the interest of discerning which factors would be most productive to optimize. They found that the turnover frequencies correlated with overpotential, despite rate-limiting oxygen binding. Further grounds for optimization emerged from distinctions between the iron redox properties and the proton transfer characteristics of substituent interactions in the second coordination sphere. ACS Cent. Sci. 2, 850 (2016).