Fabrication Of Spinel-type Mn-Co Bimetallic Oxide For Electrocatalytic Oxygen Reduction

A new generation of sustainable,clean and highly efficient energy conversion and storage technologies such as fuel cells and metal-air batteries has attracted much attention in recent years.However,kinetically sluggish ORR（oxygen reduction reaction）at the cathode is a bottleneck limiting the efficiency of such electrochemical devices.Thus,development of efficient ORR catalysts is vital to reduce the polarization loss.With superior activity and tunable nanostructure,the low-cost earth-abundant transition metal oxides are considered as promising alternatives to noble-metal Pt-based catalysts,among which spinel-type Mn-Co oxides(Mn_xCo_3-xO₄)have shown great potential due to their abundant composition,valence state,crystal phase and coordination structure.Aiming at the poor electrical conductivity and weak oxygen adsorption capability caused by intrinsically large band gap of metal oxides,supported Mn_xCo_3-xO₄ quantum dot and metal-defected Mn_xCo_3-xO₄ were constructed in this thesis.And the bimetallic synergy and defect effect were further investigated.Supported Mn_xCo_3-xO₄ quantum dot（<5 nm）was synthesized by a simple one-step process through precipitation-dehydration at low temperature.The incorporation of conductive carbon support and ultrasmall size of oxide nanoparticles enable the enhancement of electron transport and surface adsorption.Thus,Mn_1.5Co_1.5O₄/C exhibits comparable ORR activity to the benchmark Pt/C.Compared with the monometallic counterparts（i.e.Mn₃O₄/C and Co₃O₄/C）,bimetallic Mn_1.5Co_1.5O₄/C combines the advantages of both Mn and Co species,showing significant improvement in both activity and stability due to the optimization of active sites and the improvement of electrical conductivity.In order to further enhance the intrinsic activity of individual oxides for better practical application,metal defects were in-situ introduced into Mn_xCo_3-xO₄ via solvothermal-calcination method.The Mn-Co glycerolate precursors not only achieve controllable synthesis of bimetallic oxides with various compositions,but also play a key role in constructing metal-defected crystals for their O-rich lamellated crystal structures.The coexistence of V_Mn and V_Co is confirmed by XRD,XAFS and elemental analysis.Determined by the formation rate difference of Mn Gly and Co Gly,a unique Mn-enriched surface is formed after thermal calcination,leading to abundant surface active sites.Importantly,the introduction of metal defects provides more active sites with enhanced electrical conductivity and O₂ adsorption ability,leading to remarkably improved ORR activity and Zn-air battery performance which outperform the benchmark Pt/C.