Study On The Catalytic Performance Of Cobalt-based Nanosheets For Hydrogen Production By Electrolysis Of Water

Two-dimensional nanomaterials have high application value in the field of energy storage and conversion due to high specific surface area,excellent electrical conductivity and special unsaturated atoms on the surface.However,two-dimensional nanosheet often exhibit slow mass transfer and easy stacking.Meanwhile,most porous nanosheets are prepared by etching or calcining at high temperature.Herein,a simple cyanogel-NaBH4 method is developed to effectively prepare porous cobalt-based nanomaterials.In this thesis,three cobalt-based nanosheets,including ultrathin porous Co3O4 nanosheets,ultrathin holey Fe doped Co3O4 nanosheets on nickel foam and ultrathin Fe doped CoP nanosheets on the nickel foam,are synthesized and their catalytic properties are studied and analyzed in detail.The main research contents are as follows:1.Ultrathin porous Co3O4 nanosheets for the oxygen evolution reaction.We present an etch-free one-step approach to directly synthesize the ultrathin porous Co3O4 nanosheets(Co-UNMs)by employing a CoCl2/K3Co(CN)6 cyanogel as the reaction precursor in the sodium borohydride solution.The results of the atomic force microscopy(AFM)show that the thickness of the Co-UNMs is only 1.5 nm.In addition,high resolution transmission electron microscopy(HRTEM)shows that a large number of pores with ca.1.5 nm diameter are uniformly distributed at the surface of the nanosheets.At the same time,electrocatalytic test results show that the Co-UNMs have excellent activity for the oxygen evolution reaction(OER)in alkaline media,such as a low overpotential of 307 mV at 10 mA cm-2,and attractive durability,which is much better than commercial RuO2 nanoparticles.2.Direct chemical synthesis of ultrathin holey iron doped cobalt oxide nanosheets on nickel foam for oxygen evolution reaction.Due to the geometric and electronic effects,bimetallic materials can regulate the catalytic activity of electrocatalysts.Therefore,we introduce FeCl3 into the formation process of COCl3/K3Co(CN)6 cyanogel on the basis of the Co-UNMs,and successfully in-situ growth of atomically thick Fe doped Co3O4 holey nanosheets on nickel foam(Fe-Co3O4 H-NSs/NF).Benefiting from ultrathin thickness(1.5 nm),numerous holes,and synergistic effect between Co and Fe atoms,Fe-Co3O4 H-NSs/NF provide a large specific surface area(199.12 m2 g-1)and highly active catalytic sites for the OER.Meanwhile,nickel foam substrate with three-dimensionally porous structure and high conductivity accelerates molecules/ions/gases transportation and electron transfer.Consequently,Fe-Co3O4 H-NSs/NF with optimal Co/Fe composition show super electrocatalytic performance for the OER,such as an overpotential as small as 204 mV at 10 mA cm-2,which is much better than the Co-UNMs.3.Ultrathin iron doped cobalt phosphide nanosheets for overall water splitting.We have successfully synthesized ultrathin Fe doped CoP nanosheets on the nickel foam(Fe-CoP NSs/NF)by cyanogel-NaBH4 method and succedent high temperature phosphidation treatment.The introduction of P not only maintains the structural characteristics of Fe-Co3O4 H-NSs/NF,but also significantly improves OER performance of the Fe-CoP NSs/NF,such as a low overpotential of 89 mV at 10 mA cm-2.In addition,Fe-CoP NSs/NF display the excellent electrocatalytic properties for hydrogen evolution reaction(HER)in alkaline media,such as a low overpotential of 67 mV at 10 mA cm-2.More importantly,Fe-CoP-NSs/NF can be used as bifunctional electrocatalysts for overall water splitting in alkaline electrolyte.