Abstract
Nanocrystalline multivalent metal spinels are considered as attractive non-precious oxygen electrocatalysts. Identifying their active sites and understanding their reaction mechanisms are essential to explore novel transition metal (TM) oxides catalysts and further promote their catalytic efficiency. Here we report a systematic investigation, by means of soft X-ray absorption spectroscopy (sXAS), on cubic and tetragonal CoxMn3-xO4 (x = 1, 1.5, 2) spinel oxides as a family of highly active catalysts for the oxygen reduction reaction (ORR). We demonstrate that the ORR activity for oxide catalysts primarily correlates to the partial covalency of between O 2p orbital with Mn4+ 3d t2g-down/eg-up, Mn3+ 3d eg-up and Co3+ 3d eg-up orbitals in octahedron, which is directly revealed by the O K-edge sXAS. Our findings propose the critical influences of the partial covalency between oxygen 2p band and specific metal 3d band on the competition between intermediates displacement of the ORR, and thus highlight the importance of electronic structure in controlling oxide catalytic activity.
Highlights
The oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER) on an oxygen-based electrode are essential for a wide range of electrochemical energy conversion and storage technologies, such as direct solar cell [1], electrolytic water splitting [2], rechargeable metal–air batteries [3], and regenerative fuel cells [4]
Restovic et al pointed out that the electrocatalytic activity in MnxCo3-xO4 of the ORR was correlated to the Mn content, and more precisely to the amount of Mn4+/Mn3+ pairs [18]. These results suggested that two metals in dual-metal spinel system and their redox pairs play different roles in influencing the catalytic performances for ORR or OER
The results indicate that lower Co/Mn ratio are more favorable to intrinsic catalytic activity
Summary
The oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER) on an oxygen-based electrode are essential for a wide range of electrochemical energy conversion and storage technologies, such as direct solar cell [1], electrolytic water splitting [2], rechargeable metal–air batteries [3], and regenerative fuel cells [4]. The intrinsic slow kinetics of ORR/OER is the obstacle for their application. It is a great challenge to seek for highly active catalysts to improve the efficiency of ORR and OER. Pt-alloy catalysts for the ORR [5] and iridium-oxide- or ruthenium-oxide-based catalysts for the OER [6]. Transition metal (TM) oxides and carbon materials with excellent electrocatalysts and high stability [7,8,9], owing many advantages such as high abundance, low-cost, easy prepared, and environmental friendliness, are considered as an alternative to noble metals. Spinel oxides have been widely used as catalyst for ORR and/or OER [10,11,12,13,14,15,16]. In pursuit of further enhanced oxygen electrocatalytic activity, it is necessary to understand the catalytic mechanism of TM oxides and to identify the activity site, which has attracted extensive research efforts
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