Identification of Surface Reactivity Descriptor for Transition Metal Oxides in Oxygen Evolution Reaction

Abstract

A number of important reactions such as the oxygen evolution reaction (OER) are catalyzed by transition metal oxides (TMOs), the surface reactivity of which is rather elusive. Therefore, rationally tailoring adsorption energy of intermediates on TMOs to achieve desirable catalytic performance still remains a great challenge. Here we show the identification of a general and tunable surface structure, coordinatively unsaturated metal cation (MCUS), as a good surface reactivity descriptor for TMOs in OER. Surface reactivity of a given TMO increases monotonically with the density of MCUS, and thus the increase in MCUS improves the catalytic activity for weak-binding TMOs but impairs that for strong-binding ones. The electronic origin of the surface reactivity can be well explained by a new model proposed in this work, wherein the energy of the highest-occupied d-states relative to the Fermi level determines the intermediates' bonding strength by affecting the filling of the antibonding states. Our model for the first time well describes the reactivity trends among TMOs, and would initiate viable design principles for, but not limited to, OER catalysts.

Unraveling the Intrinsic Structures that Influence the Transport of Charges in TiO2 Electrodes

“…[106] Both DFT simulation results and temperature programmed desorption of O 2 demonstrated that the surface reactivity of TiO 2 increases monotonically with the density of coordinatively unsaturated titanium cation (donated as Ti CUS ). This trend is further directly verified by electrochemical tests as shown in Figure 31.…”

“…On the other hand, experimental results usually found that the oxygen deficient TiO 2 could work stably even in harsh oxidation conditions such as OER. [106] Recently, new finding gives a better explanation by showing that the excess charge associated with O V is not constraint to the O V sites but delocalized. [108] Moreover, the slightly reduced TiO 2 surface could only be re-oxidized in air at temperatures as high as 500 °C.…”

“…The good stability can be attributed to the delocalization of unpaired d-electrons (which form band gap states slightly below the Fermi level), [109] which improves the overall surface reactivity and stability of O V sites. [108] The electronic origin of the enhanced surface reactivity due to oxygen deficiency, excess charges or newly formed band gap states could be well-explained by a new model proposed by Tao et al [106] Assuming that the interaction between O and transition metal oxide (TMO) is mainly contributed from the coupling of O 2p to the highest occupied d-states (denoted as E d ), the determining factor is the filling of antibonding states. As the bonding states are generally far below Fermi level (E F ) and thus should be fully filled, whereas the filling of antibonding states depends on the relative energy of E d to E F (denoted as (E d − E F )).…”

Recent Progress on Multimetal Oxide Catalysts for the Oxygen Evolution Reaction

“…[41,42] Specifically, the features of the metal valence states (d states) and oxygen valence states (p states), which are close to the Fermi level and thus participate in the bond formation, determine the adsorption energies of metal oxides. [43] According to theoretical studies, foreign metals have two primary effects on the electronic structure: the ligand effect and the strain effect. [44][45][46][47][48] The ligand effect refers to the perturbations of the electronic structure induced by charge transfer between neighboring metals.…”

“…Several electronic structure factors have been used as descriptors representing the OER efficiency, including features (energy, filling, and width) of the electronic states, [43,49,50] the metal-oxygen covalency, [51,52] and the number of electrons with specific symmetry (Figure 2b). [18,53] Such descriptors enable simple prediction of efficiency without rigorously calculating the four-step energies in the OER.…”