Extracellular redox-active compounds, flavins and other quinones, have been hypothesized to play a major role in the delivery of electrons from cellular metabolic systems to extracellular insoluble substrates by a diffusion-based shuttling two-electron-transfer mechanism. Here we show that flavin molecules secreted by Shewanella oneidensis MR-1 enhance the ability of its outer-membrane c -type cytochromes (OM c- Cyts) to transport electrons as redox cofactors, but not free-form flavins. Whole-cell differential pulse voltammetry revealed that the redox potential of flavin was reversibly shifted more than 100 mV in a positive direction, in good agreement with increasing microbial current generation. Importantly, this flavin/OM c- Cyts interaction was found to facilitate a one-electron redox reaction via a semiquinone, resulting in a 10 ³ - to 10 ⁵ -fold faster reaction rate than that of free flavin. These results are not consistent with previously proposed redox-shuttling mechanisms but suggest that the flavin/OM c- Cyts interaction regulates the extent of extracellular electron transport coupled with intracellular metabolic activity.

Hexavalent iridium (Ir ^VI ) oxide is predicted to be more active and stable than any other iridium oxide for the oxygen evolution reaction in acid; however, its experimental realization remains challenging. In this work, we report the synthesis, characterization, and application of atomically dispersed Ir ^VI oxide (Ir ^VI - ado ) for proton exchange membrane (PEM) water electrolysis. The Ir ^VI - ado was synthesized by oxidatively substituting the ligands of potassium hexachloroiridate(IV) (K ₂ IrCl ₆ ) with manganese oxide (MnO ₂ ). The mass-specific activity (1.7 × 10 ⁵ amperes per gram of iridium) and turnover number (1.5 × 10 ⁸ ) exceeded those of benchmark iridium oxides, and in situ x-ray analysis during PEM operations manifested the durability of Ir ^VI at current densities up to 2.3 amperes per square centimeter. The high activity and stability of Ir ^VI - ado showcase its promise as an anode material for PEM electrolysis.