Design, Synthesis And Redox Performance Of Manganese-based Oxide Electrocatalysts

Zn–air batteries are consider to be one of the most promising electrochemical energy conversion devices to solve the energy and environmental crisis.In the past,the ORR reaction relied heavily on Pt-based catalysts,which are not only costly but also easily toxic,thus greatly limiting the large-scale application of zinc-air batteries.Therefore,the development of cheap,highly active and stable non-precious metal catalysts for ORR is of great importance to promote the large-scale application of Zn–air batteries.Manganese-based oxides are a class of oxides with abundant reserves,low prices,excellent safety and environmental friendliness,which have been widely applied in battery field.Manganese-based chalcogenide oxides exhibit unique properties due to their multivalent states,while single oxides are still limited in regulating the linkage between electronic state co-adsorption energies.Meanwhile,carbon-deficient based catalysts represented by transition metal and nitrogen co-doped carbon based materials(M-N-C)have been widely noticed for their tunable structure and controllable performance,while nitrogen-containing and metal precursors cannot effectively control the distribution of these active sites at high temperatures,such that the metal elements tend to agglomerate at high temperatures.In this thesis,carbon/non-carbon-based ORR catalysts with different compositional and structural characteristics were synthesized in a controlled manner using inexpensive and readily available experimental materials and mild and safe experimental methods through literature research and experimental design,and their morphology,properties,active sites and conformational relationships were systematically investigated.The construction of hybrid materials with interfacial interactions to enhance the catalytic efficiency was firstly achieved by atomic modulation and the optimization of the physical and chemical properties of the hybrid structures by exploiting the synergistic effects between different crystalline phases.Secondly,the effective exposure of active sites is achieved by modulating the coordination mode of catalysts and reducing the masking effect of complex species during the preparation process.The specific studies are as follows:(1)Due to the unique interfacial characteristics of heterogeneous catalysts,they have excellent performance in many electrochemical reactions.However,it is still a major challenge to unify the ideal interface characteristics and reasonable charge transfer characteristics of heterogeneous catalyssts.Based on the above issues,we developed the heterostructures of SrMn₃O_6-x-SrMnO₃ by epitaxial growth,which exhibit excellent electrocatalyst performance in oxygen reduction reactions(ORR).It was demonstrated by density functional theory(DFT)theoretical calculations and analysis that the formation of high-valent Mn^3+/4+facilitates the positive shift of the d-band position,which optimizes the adsorption and desorption capacity of ORR intermediates on the heterojunction surface,resulting in the improvement of catalytic activity.When SrMn₃O_6-x-SrMnO₃ catalyst as an air electrode catalyst for application in a rechargeable Zn–air battery,a high output voltage of 1.48 V and a power density of 116 mW cm^-2 were achieved,and the performance was comparable to that of the cell prepared using Pt/C+IrO₂ air electrode catalyst with better cycling stability.(2)M-N-C materials grown in situ on nitrogen-doped carbon by atomic doping are considered to be an excellent class of ORR catalysts,however,their activity and durability in both acidic and basic media are limited.Here,MnO-anchored Co-N-C materials were generated in situ by forming coordination chelates of Co and Mnwith nitrogen-doped carbon materials.Not only the charge transfer rate of the adsorbed intermediates is enhanced,but also the nitrogen active species in the carbon matrix can be effectively regulated.The results of theoretical calculations and experimental tests showed that compared with the reversible hydrogen electrode,MnO/Co-N-C exhibited excellent acidic and basic ORR performance with half-wave potentials of 0.87 V and 0.66 V in 0.1 M KOH and 0.1 M HClO₄,respectively,and after 1000cycles in 0.1 M HClO₄ and 0.1 M KOH solutions the half-wave potentials only appeared to be12.2 mV and 16.9 mV decreases,with obvious durability advantages.This work provides insight into the design of nonmetallic electrocatalysts at the atomic level from the perspective of active sites and catalyst carriers.