Enlarged CoO Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction

Abstract

Cobalt-containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen-deficient perovskites. Here, a systematic study of spinel ZnFe Co O oxides (x = 0-2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO is developed. The distinctive OER activity is found to be dominated by the metal-oxygen covalency and an enlarged CoO covalency by 10-30 at% Fe substitution is responsible for the activity enhancement. While the pH-dependent OER activity of ZnFe Co O (the optimal one) indicates decoupled proton-electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p-band center relative to Fermi level governed by the spinel's cation deficient nature.

Shifting Oxygen Charge Towards Octahedral Metal: A Way to Promote Water Oxidation on Cobalt Spinel Oxides

“…Their catalytic activity depends on the electronic structure.I naspinel oxide lattice,e acho xygen anion is shared amongst its four nearest transition metal cations,ofwhich one is located within the tetrahedral interstices and the remaining three cations are in the octahedral interstices. [3,4] However,t he redox of late transition metal oxides (e.g., Coand Ni-based) could involve both the transition metal and oxygen ligand due to the increased orbital hybridization between TM 3d and O2 p. [2,5] Earlier reports had demonstrated that the energy in TM 3d orbital cannot be treated in isolation from O2pwhen there is significant overlap between TM 3d and O2 p. Recent studies on oxygen-deficient perovskite oxides reveal that the oxygen anion could also act as the redox partner in OER. The charge of oxygen anion tends to shift toward the octahedral-occupied Co instead of tetrahedraloccupied Co,w hich hence produces strong orbital interaction between octahedral Co and O. Thus,t he OER activity can be promoted by pushing more Co into the octahedral site or shifting the oxygen charge towards the redox-active metal center in CoO 6 octahedra.…”

“…[1b, 2] Better understanding of the OER reaction on TM oxides is necessary to this end. Spinel oxides, ah uge crystal family for oxygen electrocatalysis, [2,4,8] require more complex analysis because the tetrahedral and octahedral interstice can accommodate the same TM. [3] Thec onventional perception of oxygen evolution regards the redox-active metallic center as the active site and it is the redox ability of TM that mediates the transition of [M n+ ÀOH ad ]/[M n+1 ÀO ad ]d uring OER.…”

“…In order to meet the broader goal of sustainability,e xploring earthabundant transition metal (TM) oxide catalysts have been prioritized. [3,4] However,t he redox of late transition metal oxides (e.g., Coand Ni-based) could involve both the transition metal and oxygen ligand due to the increased orbital hybridization between TM 3d and O2 p. [2,5] Earlier reports had demonstrated that the energy in TM 3d orbital cannot be treated in isolation from O2pwhen there is significant overlap between TM 3d and O2 p. Recent studies on oxygen-deficient perovskite oxides reveal that the oxygen anion could also act as the redox partner in OER. It has been found that the surface redox-active centers in TM oxides play ak ey role in oxygen electrocatalysis.…”

“…[3] Thec onventional perception of oxygen evolution regards the redox-active metallic center as the active site and it is the redox ability of TM that mediates the transition of [M n+ ÀOH ad ]/[M n+1 ÀO ad ]d uring OER. Spinel oxides, ah uge crystal family for oxygen electrocatalysis, [2,4,8] require more complex analysis because the tetrahedral and octahedral interstice can accommodate the same TM. [6] Direct evidence for lattice oxygen participated OER using in situ 18 Oi sotope labelling mass spectrometry has been given by Alexis et al [6b] Thefact that the oxygen anion can also act as the redox-active center emphasizes the importance of considering TM À Obond as the redox-active building block.…”

“…Theg eometric distribution of Co was determined by analyzing the Fourier transform (FT) of extended X-ray absorption fine structure (EXAFS) spectra ( Figure 1b and Figure S2). [2] The absorbing atom, if distributed to only the tetrahedral site, gives one interatomic metal-metal distance at % 3.0 . [2] The absorbing atom, if distributed to only the tetrahedral site, gives one interatomic metal-metal distance at % 3.0 .…”