Controllable Syntheses And Electrochemical Properties Of Layered Electrode Materials Based On Cobalt Or Manganese Elements

With the increasing consumption of fossil energy,electrochemical water splitting to produce hydrogen has become the most potential conversion technology for the green energy.At the same time,rechargeable zinc-manganese batteries,a green energy storage substitute for lithium-ion batteries,have received widespread attention due to their high specific capacity and pollution-free.The development of energy conversion and storage technologies require suitable electrode materials to enhance their activity and durability.Therefore,the reasonable design and synthesis of electrode material with high catalytic performance is the goal of researchers.In recent years,layered transition metal compounds are expected to replace precious metal electrode materials for electrochemical water splitting and rechargeable zinc-manganese batteries.Layered bimetallic hydroxide（LDH）containing transition cobalt,due to its unique layered structure and abundant electrochemical activity center,was usually used as oxygen evolution reaction catalysts for water splitting.LDH materials have a large specific surface area,and cobalt ions in LDH can be used as the reducing agent to realize the growth of noble metal nanoparticles,which not only improves the dispersion of nanoparticles,but also enhances the conductivity and catalytic activity of LDH.In addition,LDH was also used as a precursor to prepare phosphides,nitrides and sulfides with the same morphology and structure,realizing the transformation from hydroxide to other compounds and benefiting for the optimization of its catalytic performance.Additionally,MnO₂ is widely used as battery positive electrode in electrochemical energy storage due to its low price and rich storage,and its electrochemical performance is closely related to its morphology and structure.Designing MnO₂ with different morphologies and structure is a popular strategy to improve the electrochemical performance of electrode materials.The contents of this paper are briefly described as follows:（1）Using metal-organic framework ZIF-67 as template and zinc nitrate as etchant to synthesize cobalt-zinc hydroxide under at room temperature,cobalt-zinc hydroxide with varied morphologies were achieved for enhancing their OER catalytic performance.The morphology and structure of cobalt-zinc hydroxide were characterized by SEM,TEM and XRD,and their OER activities were analyzed by electrochemical test.Results show that cobalt-zinc hydroxide with rod-like and lamellar morphology can be generated at room temperature,and the morphology of cobalt-zinc hydroxide can be adjusted by the temperature and the content of etchant.Electrochemical tests show that CoZn LDH is an efficient electrode material with good OER catalytic activity:the overpotential at 10 m A·cm^-2was only 388 m V,and the Tafel slope was only 110 m V dec^-1.（2）CoCoand CoNi LDH were prepared by using ZIF-67 as sacrificial template and cobalt and nickel nitrate as etchants.LDH was used as the carrier and reductant to grow Pd nanoparticles so as to improve the electrical conductivity and OER catalytic activity of LDH materials.Results show that Pd nanoparticles could be uniformly loaded on the surface of LDH,particularly at edges with higher particle density.XPS analyses suggest that the Co²⁺in LDH structure acted as a reductant.Experimental tests show that the Pd_2.83/CoNi_1.5LDH with the best OER performance:the overpotential at 10 m A·cm^-2 was only need of 281 m V,and the Tafel slope was only 76m V dec^-1.（3）CoNiP was prepared by using CoNi LDH as a precursor for investigating the composition-dependence of CoNi P OER catalytic activity and clarifying the real electrocatalytic active center for OER.Experimental results are as follows:as the atomic ratio of Ni:Co（y/x）increases,the OER activity of CoNi P first increased（0<y/x<1.5）and then decreased（1.5<y/x<3.5）.At y/x=1.5,the CoNi P nanocages exhibited the best OER performance with an overpotential of 278 m V at a current density of 10 m A·cm^-2 and a low Tafel slope of 67 m V dec^-1.Results also indicate that neither Ni OOH nor CoOOH can exclusively contribute for the OER activity,which should be attributed to the synergistic effect of NiOOH and CoOOH.（4）Hydrothermal method was used to prepare MnO₂/C composites through a redox reaction between KMnO₄ and carbonized ZIF-67 crystals(C_ZIF-67).The experimental results show thatδ-MnO₂ nanosheets were successfully planted over the surface of C_ZIF-67,and the thickness of MnO₂ layer is about 100 nm.The Co₃O₄nanoparticles formed during ZIF-67 carbonization can catalyze the growth of MnO₂.Electrochemical results show that the layered MnO₂/C_ZIF-67with an overpotential of547 m V at a current density of 10 m A·cm^-2,which is inferior to the commercial catalyst probably due to its poor conductivity.（5）MnO₂/C nanocomposites were prepared by in-site growth of MnO₂nanostructure on carbonized melamine sponges and used as cathodic electrode materials of alkaline Zn-manganese battery.Results show that the morphology and crystal structure of MnO₂ are closely related to the carbon material.As using the carbon foam（CF）as the carbon source,the nanosheet-likeδ-MnO₂/CF can be obtained,and as the carbon nanospheres wrapped carbon foam（GCF）used as carbon source,hierarchical nanospike-likeδ-MnO₂/GCF were formed.The optimized MnO₂/GCF composite delivered a capacity of 205 m Ah·g^-1at 1C,and shown excellent high-rate cycling stability in alkaline solution,maintaining 96%of the initial capacity at 1C and 90% at 5C after 400 cycles.