These Ni@graphene catalysts are prepared using the improved simple synthesis method mentioned in section 2.2. The number of all prepared catalysts is M1/M2@C-a-b-c-d, where M1=Ni; M2=NiO; a is the pyrolysis temperature; b is the oxidation temperature; c is the oxidation time and d is the solvent. Figure shows representative scanning electron microscope (SEM) images of Ni@C-700-EtOH catalyst and Ni@C-700-EtOH(recycled) catalyst. As shown in Figures and, the surface of Ni@C-700-EtOH catalyst has an obvious pore structure, while the pore structure of Ni@C-700-EtOH (recycling) catalyst deteriorates slightly after recovery but is still evenly distributed. Scanning electron microscope (SEM) image shows that the Ni@C-700-EtOH sample (Figure ) has formed nanospheres with large pores with a uniform diameter of 600 nm. Further high-resolution (HR) TEM analysis revealed that the nanospheres are composed of metal nanoparticles (NPs), which are completely coated with a graphene shell (Figure and). The Ni@C-700-EtOH metal NPs had a uniform size of generally 100-200 nm with a d-spacing of 0.20 nm, which corresponds to the (111) plane of the Ni alloy (Figure ). For the Ni@C-700-EtOH (recycled) catalyst, the intensity of the weak peak NiO did not increase (Figure ), indicating that the reduction reaction process did not lead to the oxidation of the catalyst. Through statistical analysis of high-resolution transmission electron microscopy (HR-TEM), the graphene shells on nickel nanoparticles are very thin, and >90% consists of a few layers.