Design Of Spinel Oxygen Electrocatalysts For Zinc-Air Battery

Oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?are the key processes for various electrochemical energy conversion devices?e.g.,fuel cells and metal-air batteries?.However,the sluggish multiple electron transfer process of ORR and OER greatly limit the practical applications in these energy conversion technologies.Therefore it is imperative to develop low-cost,high active and robust oxygen electrocatalysts for these energy conversion systems.Spinel-type?AB₂O₄?complex oxides with multiple cations can show a variety of chemical ordering between cations at the A or B sites,thus exhibiting various electrocatalytic properties.Notably,the efficient electrocatalytic process should consider the influence of the macroscopic and microscopic structures on the catalyst in the mass and charge transfer process.However,the general spinel oxides as electrocatalysts cannot meet the actual requirements of the device service because of their low activity density and low activity strength.Inspired by the above considerations,a variety of carbon nanotube-based spinel composite catalysts have been successfully developed.Carbon nanotubes with high conductivity,large specific surface area and good mechanical properties can be used as the growth substrate for spinel.In addition,through the combination of experimental exploration and theoretical guidance,we further proposed the potential value of redox-inert Zn_Td²⁺and Fe³⁺_Oct cation as auxiliary elements,confirming that the strength of Co³⁺_Oct active site was largely enhanced.The specific contents are as follows:?1?A carbon-based(Co_0.72Fe_0.28)_Td(Co_1.28Fe_0.72)_OctO₄ compound catalyst?Co₂FeO₄/NCNTs?with excellent bifunctional oxygen catalysis has been designed and prepared.Magnetic characterization and spectroscopy are used to demonstrate that Fe³⁺_Oct-O-Co³⁺_Oct is the main charge exchange mode in the catalyst.The interaction between O₂ and spinel AB₂O₄ is determined by the electronic structure of the transition metal B ions and the B-O bond character.Through the corresponding spectral data and magnetic property analysis,the introduction of Fe³⁺_Oct destroyed the long-term ordered structure of the original Co₃O₄ crystal,and the resulting J-Taylor effect effectively stretched the Co _Oct-O bond length.The rate of the traditional charge transfer step largely depends on the coordination relationship between local metal atoms.The oxygen catalytic correlation process is a type of reaction sensitive to geometric local structure.The interaction between the active site and the oxygen molecule should be at a moderate level of intensity.The DFT calculation resultsconfirm that the longer Co-O bond in Co₂FeO₄ system is unstable due to the accumulation of electrons in the high-energy anti-bond orbital,so the formed Co³⁺-O transition state shows less energy,thus enabling the ORR process rapidly.?2?A novel compound catalyst Zn_0.4Ni_0.6Co₂O₄/NCNTs?ZNCO/NCNTs?with excellent bifunctional catalytic activity has been designed and prepared,which is even surpass those of NiCo₂O₄/NCNTs and ZnCo₂O₄/NCNTs.The Zn²⁺substitution can effectively optimize the physical structure of oxides,resulting in that the Zn_0.4N_i0.6Co₂O₄/NCNTs has a large specific surface area and abundant structural edges,thus enhancing the ORR/OER performance.Furthermore,we propose the presence of potential dual-reinforcement mechanism and obtain activity trends at experimentally relevant electrochemical conditions.Cation segregation can be observed on the spinel after OER test.Specifically,plentiful Zn cation defects are randomly distributed on the catalyst surface.According to the DFT transition state calculation,the newly formed V_Zn-O-Co allows more suitable binding interaction between the active center Co and the oxygenated species,resulting in the superior ORR performance.Due to the Zn²⁺segregation properties,ZNCO/NCNTs has a high compatibility with the liquid-flow zinc-air battery system,and demonstrating the high rate capability,high energy density(109.1 mW cm^-2),and long-term charge-discharge stability?100 h?.