The difference between the ORR activity on Au(100) and that on Au(111) would be another interesting problem for further study. Previously it has been attributed to difference in the OH* adsorption energy, while the relevant reaction barriers are still unknown. There is also the possibility that the barrier for the outer sphere electron transfer is larger on Au(111) since the potential of zero charge is larger on Au(111) (~0.5 eV) than that on Au(100) (~0.3 eV).