Enhancement of metallomacrocycle-based oxygen reduction catalysis through immobilization in a tunable silk-protein scaffold

Fuel cells convert chemical energy into electrical current with the use of an oxidant such as oxygen and have the potential to reduce our reliance on fossil fuels. To overcome the slow kinetics of the oxygen reduction reaction (ORR), platinum is often used as the catalyst. However, the scarcity and expense of platinum limits the wide-spread use of fuel cells. In the search for non-platinum oxygen reduction catalysts, metallomacrocycles have attracted significant attention. While progress has been made in understanding how metallomacrocycle-based molecules can catalyze the ORR, their low stability, remains an on-going challenge. Here we report an immobilization strategy whereby hemin (iron protoporphyrin IX, heme b) is converted into an oxygen reduction catalyst which could be operated for over 96 h, with turnover numbers >10⁷. This represents a 3 orders of magnitude improvement over the best reported iron porphyrin ORR catalyst to date. The basis for this improvement in turnover is specific binding of the heme within a recombinant silk protein, which allows for separation of the porphyrin active sites. Use of the silk protein provides a scaffold that can be engineered to improve selectivity and efficiency. Through rational design of the heme binding site, a > 95% selectivity for a four-electron reduction of oxygen to water was obtained, equal to the selectivity obtained using platinum-based catalysts. This work represents an important advance in the field, demonstrating that metallomacrocycle-based ORR catalysts are viable for use in fuel cells.