| Layered double hydroxides(LDHs)are kind of resource-rich,low-cost nanomaterial with superior oxygen electrocatalytic activity.However,poor conductivity,easy agglomeration in nucleation,and limited specific surface area of LDHs severely affected its wide application.Therefore,there is a need for a nanomaterial having a special function to increase the electrical conductivity and effective specific surface area of the composite.Based on the hydrothermal method,the preparation conditions of LDHs are optimized.Afterwards,three different composite catalysts were prepared by combining LDHs with three different carbon materials by a simple hydrothermal method.The research contents are as follows:By simple hydrothermal method,when the total molar ratio of urea to metal ions is 5,the hydrothermal temperature is 140 ℃,and the total molar ratio of ammonium fluoride to metal ions is 5,resulting in a uniform spherical microscopic morphology.Moreover,LDHs have superior oxygen reduction activity with an onset potential of 0.71 V and a Tafel slope of 95 mV/dec.NiFe-LDH/GO composite catalyst has the unique structure of flowers growing on the flake.When the amount of GO is 180 mg,NiFe-LDH/GO-180 composite catalyst has good crystallinity and superior oxygen reduction activity and stability with an onset potential of 0.88 V and a Tafel slope of 82 mV/dec.After 32,000 s of chronoamperometry,the composite catalyst current is maintained at 83%.The NiFe-LDH/HG composite catalyst has a similar structure as described above.When the amount of HG is 10 mg,NiFe-LDH/HG-10 composite catalyst owes good crystallinity and superior oxygen reduction activity: onset potential is 0.78 V and the Tafel slope is 96 mV/dec.Differently,the NiFe-LDH/CNT composite catalyst has a unique structure with a tube into the flower.When the amount of CNT is 120 mg,the NiFe-LDH/CNT-120 composite catalyst shows good crystallinity and superior oxygen reduction activity: onset potential is 0.81 V and the Tafel slope is 88 mV/dec.Relative to LDHs,the composite catalyst exhibits excellent activity and good stability mainly due to the following reasons: The unique structure of the composite catalyst can avoid the layer-by-layer stacking of the nanosheets,which cause partial hole-hole blocking problem.so that composite catalyst manifests better oxygen reduction activity.LDHs combined with carbon materials,reducing the agglomeration and increasing the exposure to active sites of LDHs.The carbon material has good electrical conductivity and promotes contact between the active site and the electrolyte to accelerate ion/electron transfer.The carbon material provides an effective way for the active sites of LDHs micron flowers to enter the reactants and electrolytes,which is beneficial to the catalyst layer-to-layer mass transfer process.In addition,the strong interaction between the carbon material and the LDHs material makes the composite catalyst have better chemical stability. |
