Metal–Oxygen Hybridization Determined Activity in Spinel-Based Oxygen Evolution Catalysts: A Case Study of ZnFe_2–xCr_xO₄

Abstract

A good understanding of the correlation between electronic properties and catalytic performance is vital to the rational design of active oxygen evolution reaction (OER) catalysts. Here, a volcano-shaped correlation between the OER activity and the Cr substitution amount was found for spinel oxides ZnFe 2−x Cr x O 4 (x = 0−2), in which Zn 2+ resides in tetrahedral (Td) sites while Fe 3+ and Cr 3+ occupy octahedral (Oh) sites. Such a relationship probably is because Cr substitution tunes the e g occupancy of Oh-site transition metals (TM Oh ) via the Oh−Oh superexchange effect. Density functional calculations further revealed the hybridization degree between the TM Oh 3d and the O 2p states, and a volcano-shaped trend was also found in the variation of TM Oh 3d-O 2p hybridization with the amount of Cr substitution. The good correlation between the OER activity and the hybridization highlights the important role of metal−oxygen hybridization in determining the OER activity of these spinel oxides.

Optimal Geometrical Configuration of Cobalt Cations in Spinel Oxides to Promote Oxygen Evolution Reaction

“…An anodic redox peak at about 1.43 V appeared in MgCo 2 O 4 , which was attributed to Co 3+ /Co 4+ couples (Figure a) . For CoCr 2 O 4 (Figure b), the first oxidation peak at about 1.10 V is corresponding to the oxidation of Co 2+ to Co 3+ and the second oxidation peak at about 1.30 V is attributed to Cr 3+ /Cr 4+ couples . The redox peak of Co 2+ /Co 3+ is at around 1.30 V (anodic) in Co 2 TiO 4 (Figure c).…”

Significance of Engineering the Octahedral Units to Promote the Oxygen Evolution Reaction of Spinel Oxides

“…Positive correlation between the degree of Fe–O hybridization and OER performance has been built . It is worth noting that increased TM–O hybridization (defined as the percentage of O 2p states that hybridize with TM 3d states) is expected to give increased TM–O covalency . In different crystalline structures, the geometric tilting of FeO 6 octahedra alters the Fe−O−Fe bond angles and bond lengths, and therefore give Fe–O bond different hybridization degree.…”

The Effect of Cation Mixing in LiNiO2 toward the Oxygen Evolution Reaction

“…Further studies showed a strong correlation between OER activity and oxygen p‐band center relative to the Fermi level through theoretical calculations [26] . Other parameters, including cations in octahedral sites and transition metal 3d – O 2p hybridization, have been widely studied by metal doping or partial element substitution [27–30] . Surface reconstruction mediated by transition metals has been reported to form active reaction sites, such as oxyhydroxide sites on the catalyst interface [31,32] .…”