Preparation Of Transition Metal-based Materials And Their Oxygen Electrocatalytic Performanc

Over the past half century,with the irresistible process of economic globalization and modernization of industrial production,people’s demand for energy is increasing day by day.At present,traditional fossil fuels are still the energy basis for our survival and development.Over-dependence has also led to serious environmental pollution and energy depletion problems.Accelerating the conversion of new and old energy has become the key to sustainable development.As a new energy,zinc-air battery has gradually become the focus of attention because of its relatively high power density and stable discharge voltage.The conversion of energy between chemical energy and electric energy has achieved"zero emission"and"sustainable"development.However,the kinetics of oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）in the charge and discharge reaction of zinc-air battery is slow,which seriously hinders its commercialization process.At present,precious metal catalysts Ru O₂/Ir O₂and Pt/C are considered to be the mainstream OER and ORR electrocatalysts respectively.However,precious metal catalysts are difficult to be widely used in industrial production because of their high cost,low natural reserves and poor stability.Therefore,it is of great significance to construct non-precious metal catalysts with low cost and high activity.Based on the above requirements,three kinds of Co or Ni-based transition metal composites with high activity and high stability were designed and prepared for OER and ORR.The details are as follows:（1）Nitrogen-doped carbon/carbon nanotube composites modified by oxygen-rich Co/CoO nanoparticles were prepared using Co-MOF as the precursor and melamine as the auxiliary synthesizer during pyrolysis,which was mainly used in ORR reaction.The ORR catalytic performance of Co/CoO@NC-CNTs was investigated under alkaline conditions（0.1 M KOH）.The results show that the initial potential of Co/CoO@NC-CNTs is 0.96 V（half-wave potential:0.88 V）and the limit current is 5.6 m A cm^-2.After 40 hours of stability test,the current attenuation is only 18.9%.The catalyst shows excellent activity and stability.The reduction of CoO to Co and the production of oxygen vacancy and nitrogen-doped carbon provide abundant active sites.In addition,a large number of carbon nanotubes formed are conducive to proton transport and gas adsorption and desorption.The reason for its high catalytic performance due to the synergistic effect of oxygen vacancy and nitrogen-doped carbon.（2）Ni₂P-modified nitrogen-doped carbon composites（V_p-Ni₂P@NC）rich in phosphorus vacancies were prepared for bifunctional catalytic reaction of ORR/OER.The electrocatalyst was prepared by using Ni-MOF as the precursor and treated by Ar plasma after pyrolysis.The whole treatment process is efficient and convenient,environmental protection and no pollution.V_P-Ni₂P@NC exhibits excellent ORR and OER properties in 1 M KOH electrolyte.Its initial potential is 0.95 V and the slope of Tafel is 51.2 m V dec^-1,which is comparable to that of commercial Pt/C catalyst(the initial potential is 0.95 V and the slope of Tafel is 38.7 m V dec^-1).The overpotential is 308 m V,the current density is 10 m A cm^-2,and the Tafel slope is 90.4 m V dec^-1,which is similar to the most advanced Ru O₂catalyst(overpotential 312 m V,Tafel slope71.2 m V dec^-1).The electronic interaction between phosphorus vacancy and nitrogen-doped carbon matrix improves the conductivity of the catalyst and provides a strong ORR/OER active site.It is better than most of the bifunctional catalysts reported at present.（3）Co LDH-CNT catalysts modified by CeO₂nanoparticles were prepared and used in OER.The addition of carbon nanotubes in the synthesis process of Co LDH makes the nanoflower structure have a higher specific surface area,which is conducive to the exposure of active sites.The doping of CeO₂nanoparticles makes the catalyst form two redox pairs of Co³⁺/Co²⁺and Ce³⁺/Ce⁴⁺,which improves the electrical conductivity and provides the active site for the formation of oxygen vacancy.The catalytic results show that the CeO₂@Co LDH-CNT catalyst only needs the low overpotential of 252 m V to reach the current density of 10 m A cm^-2,which is better than the commercial Ru O₂electrocatalyst.In addition,the catalyst also showed excellent long-term stability,and the current remained almost unchanged after 30 hours of testing under the current density of 10 m A cm^-2.