Mechanism and Tafel Lines of Electro-Oxidation of Water to Oxygen on RuO2(110)

How to efficiently oxidize H2O to O-2 (H2O -> 1/2O(2) + 2H(+) + 2e(-)) is a great challenge for electrochemical/photo water splitting owing to the high overpotential and catalyst corrosion. Here extensive periodic first-principles calculations integrated with modified-Poisson-Boltzmann electrostatics are utilized to reveal the physical origin of the high overpotential of the electrocatalytic oxygen evolution reaction (OER) on RuO2(110). By determining the surface phase diagram, exploring the possible reaction channels, and computing the Tafel lines, we are able to elucidate some long-standing puzzles on the OER kinetics from the atomic level. We show that OER occurs directly on an O-terminated surface phase above 1.58 V vs NHE, but indirectly on a OH/O mixed phase below 1.58 V by converting first the OH/O mixed phase to the O-terminated phase locally. The rate-determining step of OER involves an unusual water oxidation reaction following a Eley-Rideal-like mechanism, where a water molecule from solution breaks its OH bond over surface Os with concurrent new O-OH bond formation. The free energy barrier is 0.74 eV at 1.58 V, and it decreases linearly with the increase of potential above 1.58 V (a slope of 0.56). In contrast, the traditionally regarded surface oxygen coupling reaction with a Langmuir-Hinshelwood mechanism is energetically less favored and its barrier is weakly affected by the potential. Fundamentally, we show that the empirical linear barrier similar to potential relation is caused by the linear structural response of the solvated transition state to the change of potential. Finally, the general strategy for finding better OER anode is also presented.