Topological Transformation Route Of Cobaltbased Nanomaterials And Its Applications In Energy Conversion

Excessive consumption of fossil fuels not only causes serious environmental problems,but also drastically reduced energy reserves.Therefore,it is imminent to explore the clean and renewable energy.The use of resource—water,which is converted into hydrogen with high added value by means of electrochemical conversion,has great value.Advanced energy conversion systems,such as water electrolysis and rechargeable Zn—Air batteries(ZABs),include hydrogen evolution reaction(HER),oxygen evolution reaction(OER)and oxygen reduction reaction(ORR).However,the slow kinetics of these multi-electron-proton coupling reactions hinders the widespread use of energy conversion devices.The exploration of suitable catalysts is of great significance in promoting the reaction kinetics and alleviating energy requirements.Cobalt(Co)is one of the most abundant elements on the earth.It is located in the fourth period of the periodic table,and belongs to Group VIII,whose electronic arrangement is 3d⁷4s².The oxidation state of Co includes Co²⁺and Co³⁺.The redox potential of Co metal is the closest to the reduction potential of oxygen to water(E=1.23 V vs.RHE).Therefore,cobalt-based nanomaterials have attracted wide attention as non-noble metal electrocatalysts with good electrocatalytic activity.Topological transformation route is an effective strategy for the synthesis of nano-catalysts.It uses pre-prepared nano-materials as precursors,and through appropriate modification and regulation,guides the generation of specific nano-catalysts.Topological transformation route can effectively ensure the accuracy of the catalyst morphology,and prevent the agglomeration behavior of the catalyst.It plays an important role in preparing a catalyst with a specific morphology.So this paper is committed to using the topological transformation route to prepare a series of cobalt-based nano-catalysts,thereby generating a wider range of applications in energy conversion devices.The specific research works are as follow:?:A Fe-doped CoP porous nanowire array electrocatalyst material was prepared by topological transformation route.Fe³⁺,Co²⁺and Co³⁺have similar ionic radii.By doping Fe,the introduced d electrons of Fe can transfer with Co ions,and the charge density of CoP is adjusted at the atomic scale.The OER and HER overpotentials of Fe-CoP are279 mV and 126 mV at current density of 10 mA cm^-2,respectively.The regulation by Fe cations also reduces the overpotential of water splitting.?:N-doped CoS_1.97 hexagonal nanosheets catalyst with secondary structure was prepared by topological transformation route.During the introduction of N anion into cobalt carbonate,the morphology of the precursor was maintained without causing structural changes.While generating a new N-Co-S coordination structure,the charge density of CoS_1.97 is regulated.In addition,the generated secondary structure increases the active surface area and the number of active sites of the catalyst.With the regulation of N anion,N-CoS_1.97 has good OER performance.Under different current densities and test temperatures,the testing of alkaline ZABs can achieve charge and discharge cycles for 71 h and the stability of constant current discharge for 55 h.?:A MoS₂ hydrogen evolution catalyst with activated basal plane was prepared by topological transformation route through the regulation of Co₉S₈ coordination mode.The calcination in hydrogen and the transformation of Co₉S₈ coordination mode from octahedron to tetrahedron accelerate the dissociation of water and the hydrogen evolution reaction rate.At the same time,defects were appeared and the number of active sites were increased in MoS₂.This MoS₂ also has good hydrophilicity,which can be directly used as a self-supporting catalyst without a polymer binder.The basal plane of MoS₂ is activated by the transformation of the Co₉S₈ coordination mode.So in solution of pH 14.0 at a current density of 10 mA cm^-2,an overpotential of 84 mV and good stability can be reached.