Research On Preparation And Electrocatalytic Water Splitting Performance Of Supported Co Single-atom Catalysts

With the excessive use of fossil fuels（includeing coal,oil,and natural gas）,the content of CO₂and other toxic gases in the atmosphere continues to rise,leading to many types of problems such as sea level rise,greenhouse effect,environmental pollution and so on.In addition,traditional fossil is nonrenewable,the total amount of global fossil energy cannot meet the needs of people.Consequently,it is urgent to exploit a new type of clean and sustainable energy.Hydrogen energy has become the focus of attention due to its unique advantages.However,obtaining hydrogen energy by electrolysis of water,designing and synthesizing efficient electrocatalysts is still a huge challenge.In this thesis,we have selected single-atom catalysts（SACs）as catalysts for electrocatalytic water splitting,using biomass carbonl,graphene oxide,and MOFs with high specific surface area as SACs supports,single atoms are anchored on the suppors by using N,S and O atoms of the supports.It can effectively avoid the aggregation of single atoms,facilitate better dispersion of metals at the atomic scale.The research specific details of this paper are mainly divided into the following three parts:（1）N-doped porous biomass carbon was used as the support of SACs,Cobalt nitrate hexahydrate was used as the cobalt source and the coordination bond between cobalt nitrate and o-phenylenediamine,to form a coordination compound.Subsequently,a cobalt single-atom catalyst Co-N-C was synthesized using a high-temperature calcination coordination compound strategy.We adjusted the amount of cobalt nitrate to investigate the influence of the loading amount of Co atoms on the performance of the catalysts.The characterization results of the obtained catalysts show that Co elementin the Co-N-C is uniformly dispersed on the porous carbon surface in single atom,which indicates that we have successfully prepared a single-atom based Co-N-C catalyst.The Co-N-C catalysts were used for electrochemical hydrogen and oxygen evolution reactions,it can be seen that the HER and OER performance of the material under alkaline conditions have been greatly improved after loading with Co single-atoms.When adding 300 mg of cobalt nitrate,the catalytic performance is the best.In 1.0 mol L^-1KOH solution,it exhibits ultra-high electrochemical hydrogen evolution activity,at a current density of 10 m A cm^-2,the overpotential of electrochemical hydrogen evolution is only 151.31 m V,the slope of Tafel calculated from LSV curve is 31.19 m V dec^-1.After 10 h of i-t testing,the current density of the catalyst has almost no change,demonstrating that300Co-NPC-800 not only has high electrochemical catalytic activity,but also has good stability.（2）In order to improve the electrochemical water splitting activity of the catalyst,atomic-scale metal Co was doped into 2H-Mo S₂and then supported onto graphene to obtain the product r GO-Mo S₂-Co _X.The morphology and structural characteristics of the sample were characterized by using XRD and TEM characterizations.It was found that the as-obtained still maintained its original morphology after the introduction of Co,indicating that the structure of the monoatomic catalyst was not collapsed during the preparation process.The HAADF-STEM image shows white highlights representing isolated single atoms of Co.Electrochemical tests show that the sample r GO-Mo S₂-Co_0.75has excellent electrochemical HER and OER performances in 1.0 mol L^-1KOH electrolyte.The HER overpotential is only 89.42m V with a Tafel slope of 50.22 m V dec^-1,which is similar to that of Pt/C electrode.The OER overpotential is 31 m V with a Tafel slope of 78.05 m V dec^-1,showing a excellent OER performance.In addition,the sample also has a fast kinetic reaction rate and excellent electrocatalytic stability.（3）Using MOFs material MIL-101（Cr）-NH₂as a carrier,single atomic catalysts（Co@MIL-101（Cr）-NH₂and Pt@MIL-101（Cr）-NH₂）were synthesized by using a simple hydrothermal method to load metal Co and Pt on MIL material.In order to explore the performance of dual-single-atom catalysts and reveal the synergistic catalytic effects between dual single-atoms,we also prepared Co/Pt@MIL-101（Cr）-NH₂by using the same method.It was found that the dual-single-atom catalyst exhibited surprising electrochemical HER activity in both acidic and alkaline environments.Under acidic conditions,the hydrogen evolution overpotential of the dual-single-atom catalyst Co/Pt@MIL-101（Cr）-NH₂is 96.5 m V.Under alkaline conditions,the hydrogen evolution overpotential is 97.2 m V.However,the OER reaction is different,The prepared double monatomic catalyst has OER performance only in acidic environment,but has low oxygen evolution activity in alkaline environment.The results show that the double monatomic composite prepared by this synthesis route is more suitable for hydrogen evolution reaction.