The Ir-OOOO-Ir transition state and the mechanism of the oxygen evolution reaction on IrO2(110)

Abstract

Carefully assessing the energetics along the pathway of the oxygen evolution reaction (OER), our computational study reveals that the "classical" OER mechanism on the (110) surface of iridium dioxide (IrO₂) must be reconsidered. We find that the OER follows a bi-nuclear mechanism with adjacent top surface oxygen atoms as fixed adsorption sites, whereas the iridium atoms underneath play an indirect role and maintain their saturated 6-fold oxygen coordination at all stages of the reaction. The oxygen molecule is formed, via an Ir-OOOO-Ir transition state, by association of the outer oxygen atoms of two adjacent Ir-OO surface entities, leaving two intact Ir-O entities at the surface behind. This is drastically different from the commonly considered mono-nuclear mechanism where the O₂ molecule evolves by splitting of the Ir-O bond in an Ir-OO entity. We regard the rather weak reducibility of crystalline IrO₂ as the reason for favoring the novel pathway, which allows the Ir-O bonds to remain stable and explains the outstanding stability of IrO₂ under OER conditions. The establishment of surface oxygen atoms as fixed electrocatalytically active sites on a transition-metal oxide represents a paradigm shift for the understanding of water oxidation electrocatalysis, and it reconciles the theoretical understanding of the OER mechanism on iridium oxide with recently reported experimental results from operando X-ray spectroscopy. The novel mechanism provides an efficient OER pathway on a weakly reducible oxide, defining a new strategy towards the design of advanced OER catalysts with combined activity and stability.

Fig. 1. Computed grand-canonical stability diagram of adsorbate configurations on the IrO₂(110) surface in aqueous environment at pH = 0 with the fully *O-covered surface as reference system. Vertical dotted lines indicate the equilibrium potentials for the transition between lowest-energy adsorbate states (shown above with oxygen: red; hydrogen: white; iridium: pale golden). The total grand potential of the system is the “concave hull”, shown as a thin black dashed curve. Naming scheme: species at the top (t) and bridge (b) oxygen sites (two of each per simulated surface cell); V: vacant top oxygen site.

Fig. 2. Schematic of the catalytic cycle of the OER on IrO₂(110) according to the novel mechanism. For the dissociative adsorption of the second water molecule, a third surface oxygen entity is involved, indicated in brackets for the respective steps only.

Fig. 3. Comparison between the conventionally considered OER mechanism (golden color) and the novel mechanism (blue color) at pH = 0 and a potential of 1.53 V_NHE (a) and 1.23 V_NHE (b). Note that no correction of the O₂ DFT energy was applied, see Computational Methods, for which reason the initial and final states at the correct equilibrium potential in (b) are slightly misaligned. Results including such corrections are presented in Fig. S7 and S8 (ESI†). The intermediate adsorbate configurations are shown and denoted by the occupation of the two top oxygen sites per surface cell. If not otherwise indicated, the bridge oxygen sites are occupied by O + O. Reaction barriers of dissociative water adsorption (H₂O↓) and oxygen evolution (O₂↑) steps were estimated from climbing-image NEB calculations for the neutral systems, see Fig. 4. The zero energy reference was chosen at the respective lowest-energy surface configuration involving only *O and *OH, see Fig. 1. The absolute lowest-energy configuration OO + OO at 1.53 V_NHE is indicated by a dash-dotted line in (a). To facilitate conversion between different reference states, Table S6 (ESI†) presents their free energies versus the “clean” stoichiometric surface state as a common reference.

Fig. 4. Activation energy barriers from climbing-image NEB calculations for dissociative water adsorption on the *O covered surface (a), see reaction (RS5), oxygen evolution by *OO–OO* association (b), see reaction (RS9), and oxygen evolution by desorption (c), see reaction (RS8). Initial and final state energies are indicated by horizontal solid and dash-dotted lines, respectively. NEB calculations were performed for uncharged systems.

Fig. 5. Creation of the final O–O bond of the evolving O₂ molecule at the Ir–OOOO–Ir transition state: Isosurface plots of the local density of states (LDOS) integrated over different energy ranges (PARCHG file from VASP): (a) from −1.0 to 0.0 eV vs. E_Fermi, (b) from −2.0 to −1.0 eV vs. E_Fermi, and (c) from −3.0 to −2.0 eV vs. E_Fermi.

Fig. 6. Computed cyclic voltammogram (a) of an IrO₂(110) electrode at pH = 0, and integrated charge (b) per 2 × 1 surface cell with the fully *OH-covered surface (t: OH + OH b: OH + OH) as zero-charge reference state. The results including the “worst-case” OO energy penalty are shown as dash-dotted curves. Vertical dotted lines and numbered potential ranges have the same meaning as in Fig. 1. The most stable adsorbate configurations in the respective potential ranges are indicated with top and bridge occupations.