Characterization of O2 Reduction and H2O2 Oxidation on Gold and Platinum Oxygen reduction occurred at less negative potentials on platinum, however this reduction proceeds via the one step pathway. The gold electrode reduced oxygen at more cathodic potentials but does so via the two-step pathway. A full explanation of the oxygen reduction reaction is given in the supplementary information (SI Eq. 1 -SI Eq. 13). Calibration plots of DO for both gold and platinum are also presented in supplementary information (SI Fig1 (a) -(d)) and exhibit sensitivities of 3.2 nA/ppm and 3.8 nA/ppm, respectively. The current measured on the reverse sweep at the platinum electrode, therefore after the platinum oxide has been reduced, is equivalent to the current measured on gold after the second reduction step, at approximately 30 nA. By not allowing the platinum oxide to form, this peak behavior is removed, highlighting that it is solely attributed to the oxide reduction (SI Fig2). Both IDEs have the same dimensions, so it was expected that the measured currents should be similar. Diffusional overlap was also observed to be occurring such that the IDE electrodes were acting as one larger electrode as opposed to an array of smaller ones.