Preparation Of Cobalt Phosphides Based Nanocomposites And Their Electrocatalytic Performance For Water Splitting

Energy crisis and environmental pollution are two major problems that human beings have to face.The development of new technologies and energies is the key to solve these two problems,also a research hotspot in this field.Such as solar energy,wind power,biomass energy and other new energies arises at the historic moment,in which the hydrogen,as a kind of efficient and clean sustainable energy,is considered as one of the most promising energy carriers in the future.Among the numerous methods of hydrogen production,electrocatalysis of water splitting has aroused great interest,because of it is efficient,green,sustainable,and the raw materials is abundant,the electricity can be satisfied by wind power and solar energy,as a result,electrocatalysis water splitting is an important means of energy transformation and storage.Although precious metals（such as Pt,Pd,Ru,etc.）possess excellent catalytic activity,its low reserve and high cost limit the large-scale application.To develop highly efficient and low cost electrocatalysts is the core of electrocatalysis water splitting and the focus of future research.This study takes the cobalt phosphides as the research object,to develop low cost,high catalytic activity and stability electrocatalysts as the goal,by regulating its morphology,structure and compositing with other materials,several excellent cobalt phosphides/carbon composites electrocatalysts were synthesized.Main contents and results are listed as follows:（1）Co₃O₄ nanoparticles are uniformly and densely dispersed on nitrogen doped reduced graphene oxides（Co₃O₄/N-rGO）by solvothermal method and low temperature heat treatment.CoP_x/N-rGO can be obtained by phosphorization of the Co₃O₄/N-rGO precursor,exhibiting superior electrocatalytic hydrogen evolution reaction（HER）activity in the alkaline media,with low onsetpotential of 44 mV,low overpotential of 104 mV(at 10 mA cm^-2),a small Tafel slope of 65 mV dec^-1 and long-term stability.CoP_x/N-rGO possesses better HER performance than CoP_x/rGO,Co₃O₄/N-rGO and Co₃O₄/rGO.The remarkable HER performance can be ascribed to the good electrical conductivity of rGO and the doping of nitrogen into the lattice of graphene introduces numerous defects to form pyridinic and pyrrolic,beneficial for the nucleation of CoP_x nanoparticles with a uniform distribution.More importantly,the pyridinic and pyrrolic species act as the extra HER active sites,helpful to enhance the electrocatalytic activities.Benefited from the synergistic effect of uniform distribution of CoP_x NPs and N-doping rGO sheets,the as-prepared CoP_x/N-rGO composite exhibits outstanding HER catalytic performance in alkaline electrolyte.This work provides a versatile route on the design of multifunctional composite materials for renewable energy research with low cost and high activity.（2）The organic metal framework of cobalt（ZIF-67）was prepared by simple co-precipitation method,and the nanoporous cobalt phosphide nanoparticles/carbon composite（CoP@C）was obtained by low temperature phosphorization of the hydrolysed ZIF-67 precusor.The morphology,structure and electrocatalytic performance were systematically studied,originally smooth surface dodecahedron ZIF-67 became a fold type of spherical surface,and finally transformed into nanoporous nanofilms and nanoparticles.The CoP@C composites showed excellent HER performance in alkaline condition,with low onsetpotential of 50 mV,low overpotential of 126 mV(at 10 mA cm^-2),a small Tafel slope of 56 mV dec^-1 and long-term stability.The superior HER performance can be attribute to the good electrical conductivity of the carbon framework,the nanoporous structure not only provides enough channels for permeation and diffusion of the electrolyte,but also increases the electrochemical effective area,which exposed more active sites for HER.The CoP nanoparticles prepared by low temperature phosphrization,could promote the adsorption of protons and formation of Co-hydrides.The small particles size of the CoP increases the surface atom number greatly,which means more active sites can be exposed.This study provides a new way to prepare porous two dimension nanocomposites materials.