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Abstract
Rutile RuO2 is a prime catalyst for the oxygen evolution reaction (OER) in water splitting. Whereas RuO2 is typically considered to be non-magnetic (NM), it has recently been established as being anti-ferromagnetic (AFM) at room temperature. The presence of magnetic moments on the Ru atoms signals an electronic configuration that is markedly different from what is commonly assumed, the effect of which on the OER is unknown. We use density functional theory (DFT) calculations within the DFT+U approach to model the OER process on NM and AFM RuO2(110) surfaces. In addition, we model the thermodynamic stability of possible O versus OH terminations of the RuO2(110) surface and their effect on the free energies of the OER steps. We find that the AFM RuO2(110) surface gives a consistently low overpotential in the range 0.4–0.5 V, irrespective of the O versus OH coverage, with the exception of a 100% OH-covered surface, which is, however, unlikely to be present under typical OER conditions. In contrast, the NM RuO2(110) surface gives a significantly higher overpotential of ∼0.7 V for mixed O/OH terminations. We conclude that the magnetic moment of RuO2 supplies an important contribution to obtaining a low overpotential and to its insensitivity to the exact O versus OH coverage of the (110) surface.
Highlights
Efficient catalysis of the oxygen evolution reaction (OER) is important in the electrochemical production of fuels for storage of renewable energy
Starting with bulk RuO2, we find that the AFM state is 74 meV per formula unit lower in energy than the NM state, with magnetic moments on the Ru atoms of ±1.18 μB, which is in agreement with previous work.[14]
By means of density functional theory (DFT) calculations, applying the DFT+U formalism, we model the oxygen evolution reaction (OER) on the AFM RuO2(110) surface and contrast our results with those obtained for NM RuO2
Summary
Efficient catalysis of the oxygen evolution reaction (OER) is important in the electrochemical production of fuels for storage of renewable energy. The overpotential calculated for AFM RuO2 decreases slightly to 0.41 V, for increasingly OH-covered surfaces, the third reaction step remaining the PDS These low overpotentials of different surface terminations agree well with the experimental results that RuO2 is an OER active material. As discussed above, switching on spin polarization creates a high-spin state (and magnetic moments) on the OER-active Ru atoms without adsorbate, or with OH/OOH adsorbates; see Figure 2c and d This lowers the total energies E*, E*OH, E*O, and E*OOH with respect to their low-spin NM counterparts. This can be traced to the fact that the adsorption energies of the intermediate species, OH, O, and OOH, do not change very much upon varying the O/OH coverage mix The exception to this rule is a fully OH-covered surface, because in that case the proton transfer from an adsorbed OOH species to the surface is blocked, which leads to a higher overpotential. The Pourbaix diagram shows that a fully OH-covered surface is unlikely to be present under typical OER conditions
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