Synthesis And Modification Of Cobalt-based Chalcogenides And Its Applications In Electrocatalytic Water Splitting

To solve the energy crisis and environmental problems,electrocatalytic water splitting is becoming a facile and safe method to generate hydrogen energy as sustainable green energy.However,limited by the sluggish kinetics of hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）occurring at the cathode and anode,the actual water splitting voltage is usually much higher than the theoretical value of 1.23 V.Nowadays,the efficient HER catalysts are mainly Pt-based noble-metal materials,while the OER catalysts are Ir/Ru and their oxides.The scarcity of noble-metal reserves has led to high commercial price,which limits its large-scale application.Therefore,it is urgent to develop cheap catalysts with high activity and stability,which is a promising research topic in the era of"hydrogen economy".Cobalt-based chalcogenides（TMCs）with higher redox reactivity and intrinsic conductivity,facile transition of Co²⁺/Co³⁺redox as well as various crystal structures,are attractive substitutes to noble-metal electrocatalysts.Ceria（CeO₂）,exhibits excellent oxygen mobility and oxygen storage capacity benefit from the facile transition between Ce³⁺and Ce⁴⁺.Thus,ceria plays an important role in both thermal catalysis and electrocatalysis as support materials and co-catalysts.There are some effective strategies to increase the number of active sites,modulate the electronic structure and enhance conductivity via heterostructure constructing,morphology and defect engineering etc.,thus improving the catalytic performance of materials.In this work,we have designed series efficient free-standing electrocatalysts for water splitting in alkaline media via component optimization and heterostructure construction.Meanwhile,combined with in-situ spectra and theoretical calculations,the fine structures and structure-performance relationships are analyzed,providing a new strategy for the rational design of efficient advanced heterostructure electrocatalysts.The specific research works are as follow:Ⅰ:Cobalt-based chalcogenides(Co₃O₄,S-Co O,CoS_1.97)nanosheets array electrocatalysts with different concentrations of S were synthesized by topological transformation route.The introduction of S species significantly increases the electrochemically active area and conductivity of the materials,and accelerates the charge transfer process.Among them,CoS_1.97 exhibits the highest activity with the overpotential of 331 m V and 202 m V for OER and HER at 10 m A cm^-2.Furthermore,the CoS_1.97 will transform into Co OOH andβ-Co（OH）₂ species via the processing of OER and HER with distinct morphology evolution.Ⅱ:CeO₂/CoS_1.97 heterostructure electrocatalysts with different spatial architectures were designed and prepared for OER.The CeO₂-Ce S_1.97（CeO₂ supporting）has the highest number of Co²⁺active sites and lattice disorder,effectively reduces the energy barrier of reconstruction to Co OOH active phase,thus displays the excellent performance with the overpotential of 264 m V at 10 m A cm^-2 as well as long-term durability for 150 h at 1000 m A cm^-2 ensured by the rapid desorption of O₂ and stable heterogeneous interfaces.On the other hand,the strong surface electron interaction endows CoS_1.97-CeO₂（surface decorated）with the highest Co³⁺ratio,charge transfer and strong adsorption of OH^-,promoting a deeper restructure of Co OOH and the formation of Co O₂ species,which further improves the OER kinetic with the Tafel slope of only 49 m V dec^-1.This work offers a new perspective for the functionalization of the spatial architectures and rational design and synthesis of advanced electrocatalysts.Ⅲ:A"Hydrangea"-like bi-functional electrocatalyst of CoS_1.97-NiS₂-CeO₂nanosheets clusters was designed and prepared.The interaction of Ce³⁺and F^-ions remarkably reduces the size of the nanosheets,thus effectively promoting the exposure of edge active sites.The construction of the atomic-level interface increases the concentrations of defect,optimizes the charge density distribution of Co and Ni sites,thus enhancing the electron transfer to adsorption intermediates.It only needs the overpotential of 320 m V to reach 50 m A cm^-2 for OER,and 122 m V at the current density of 10 m A cm^-2 for HER.Furthermore,when directly used as electrodes for water spitting,it only needs the voltage of 1.66 V to drive the current density of 10 m A cm^-2 and exhibits a long-term durability of 100 h at 100 m A cm^-2.