Superexchange Effects on Oxygen Reduction Activity of Edge‐Sharing [Co_xMn_1−xO₆] Octahedra in Spinel Oxide

Abstract

Mn-Co containing spinel oxides are promising, low-cost electrocatalysts for the oxygen reduction reaction (ORR). Most studies are devoted to the design of porous Mn-Co spinels or to strongly coupled hybrids (e.g., MnCo O /N-doped-rmGO) to maximize the mass efficiency. The lack of analyses by metal oxide intrinsic activity (activity normalized to catalysts' surface area) hinders the development of fundamental understanding of the physicochemical principles behind the catalytic activities. A systematic study on the composition dependence of ORR in ZnCo Mn O (x = 0.0-2.0) spinel is presented here with special attention to the role of edge sharing [Co Mn O ] octahedra in the spinel structure. The ORR specific activity of ZnCo Mn O spans across a potential window of 200 mV, indicating an activity difference of ≈3 orders of magnitude. The curve of composition-dependent ORR specific activity as a function of Co substitution exhibits a volcano shape with an optimum Mn/Co ratio of 0.43. It is revealed that the modulated e occupancy of active Mn cations (0.3-0.9), as a consequence of the superexchange effect between edge sharing [CoO ] and [MnO ], reflects the ORR activity of edge sharing [Co Mn O ] octahedra in the ZnCo Mn O spinel oxide. These findings offer crucial insights in designing spinel oxide catalysts with fine-tuned e occupancy for efficient catalysis.

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

“…[2] The absorbing atom, if distributed to only the tetrahedral site, gives one interatomic metal-metal distance at % 3.0 . [2,10] Thef act that the Co K-edge of CoAl 2 O 4 and Zn K-edge of ZnCo 2 O 4 show one FT peak at % 3.0 indicates normal spinel, which (i.e., (Co) Td [Al 2 ] Oct O 4 ,( Zn) Td [Co 2 ] Oct O 4 )a re consistent with literature data. [4,11] Figure 1c shows the OER CV curves of CoAl 2 O 4 ,Z nCo 2 O 4 ,a nd Co 3 O 4 in 1m KOHa t as can rate of 10 mV s À1 .T he OER current is normalized by oxide BET surface area ( Figure S3-5) and the extracted Tafel plots are displayed in Figure 1d.…”

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

“…TheF Tp eaks at 2.5 ~3 reflects the interatomic metal-metal distances of TM Oct -TM Oct at % 2.5 and TM Td -TM Td /TM Td -TM Oct at % 3.0 . [2,10] Thef act that the Co K-edge of CoAl 2 O 4 and Zn K-edge of ZnCo 2 O 4 show one FT peak at % 3.0 indicates normal spinel, which (i.e., (Co) Td [Al 2 ] Oct O 4 ,( Zn) Td [Co 2 ] Oct O 4 )a re consistent with literature data. [2,10] Thef act that the Co K-edge of CoAl 2 O 4 and Zn K-edge of ZnCo 2 O 4 show one FT peak at % 3.0 indicates normal spinel, which (i.e., (Co) Td [Al 2 ] Oct O 4 ,( Zn) Td [Co 2 ] Oct O 4 )a re consistent with literature data.…”

Electrochemical Oxidation of Nitrogen towards Direct Nitrate Production on Spinel Oxides

“…Two interatomic metal–metal distances at around 2.5 Å (TM oct –TM oct ) and 3 Å (TM td –TM td or TM td –TM oct ) for Co and Fe K‐edges were observed, while only one metal–metal distance of around 3 Å was found for Zn K‐edge. This indicates that Zn cations only occupy the tetrahedral sites . The near normal spinel structure was further confirmed by EXAFS curve fitting results (Table S3).…”

“…The near normal spinel structure was further confirmed by EXAFS curve fitting results (Table S3). The exclusive occupation of octahedral sites by Fe and Co cations suggests that the main catalytic contribution of the as‐prepared spinel oxides should be attributed to the edge‐sharing [Fe x /2 Co 1− x /2 O 6 ] octahedral unit …”