The Ni@N/C catalysts were prepared using our previous report with some modification. Briefly, Ni nitrates, citric acid, and different amine sources were mixed in EtOH under vigorous stirring. Then the formed greenish gel was dried and calcined, yielding N-doped graphene encapsulated Ni@N/C catalysts (Scheme S1). As shown in Figure of the representative scanning electron microscopy (SEM) images and transmission electron microscope (TEM) images, different amine sources affected the catalyst's surface significantly. The Ni@N/C-5 catalyst prepared from urea had a denser structure, while the other three catalysts Ni@N/C-1, Ni@N/C-4, Ni@N/C-6 prepared from melamine, ammonium citrate, 1,2 propylene diamine respectively had looser structure, also have the obvious pore structures, this could be further confirmed by the BET measurements of the catalysts. As shown in Figure , the nitrogen source can affect the metal nanoparticles' size dramatically. The Ni@N/C-1 has the uniform size of the metal species with 12.5 nm, while the Ni@N/C-4 prepared with ammonium citrate has the smallest particle size 5.9 nm. The Ni@N/C-5 has an uneven metal nanoparticle size, and some of the metal species could be seen aggregation. The Ni@N/C-6 prepared with the aliphatic amine 1,2 propylene diamine has a very irregular metal particle size distribution. The further zoom of the selected area of Ni@N/C catalyst could see the formation of a thin gra-phene layer structure around the Ni. The catalyst prepared by using melamine has an obvious thin-layer graphene structure, and the graphene wraps the active Ni metal with about 5 numbers of carbon layers. While the catalysts Ni@N/C-4 and Ni@N/C-5 prepared by using ammonium citrate or urea as the nitrogen source have fewer carbon layers, and the structure of the carbon layers is incomplete, and some carbon layers are destroyed. For the catalyst prepared with 1,2 propanediamine, no obvious carbon-coated metal structure was observed, but the presence of a graphene carbon layer was observed. In addition, the TEM-EDS analysis and high-angle annular dark-field EDS (HAADF-EDS) elemental mapping of Ni@N/C-1 exhibited a very similar distribution, which further indicated the co-existence of both N and Ni species in the Ni@N/C-1 catalyst. The elements of C, N, and O were well dispersed over the Ni nanoparticles according to the elemental mapping of the Ni@N/C-1 catalyst, indicating the presence of graphene layers around the nanoparticles. The C, O, N, and Ni atoms were also distributed homogeneously over the other prepared Ni@C catalysts (Figure ). As shown in the PXRD profile of different catalysts (Figure ), the Ni@N/C-1 has the highest intensity which showed clearly the Ni (111), Ni (200), Ni (220) metal species. Also, the graphene shell C (002) can be observed, which agrees with the TEM analysis. The intensity of the Ni (111), Ni (200), Ni (220) metal species peaks is lower in the catalysts Ni@N/C-4 and Ni@N/C-6, which indicated that the metal species were dispersed well in the support. While here, it also needs to mention, the nitrogen sources have a great effect on the metal dispersion in the graphene support. The catalyst Ni@N/C-4 which was prepared from ammonium citrate has a higher loading of Ni metal, according to the ICP and SEM-EDS analysis (Table , S2), but it has better dispersion than the Ni@N/C-1 catalyst prepared from melamine. For the catalyst Ni@N/C-6, which was prepared from the aliphatic amine 1,2 propylenediamine, it had the lowest Ni loading only 8.36% (Table ), and also the Ni metal species with smaller nanoparticle size and better dispersion in the support, which following the TEM analysis. The X-ray photoelectron spectroscopy (XPS) was carried out to elucidate the chemical states of the Ni metal elements (Figure after deconvolution. This was lower binding energy than the reported Ni/AC and the even N doped Ni/AC-N-0.5 catalysts, which indicated that the stronger electron density of the Ni atom in Ni@N/C-1 increased dramatically for the reported two catalysts. While the other two Ni@N/C-4 and Ni@N/C-5 had a similar figure trend with the presence of metallic Ni 0 peaks at BE 852.3 eV (Ni2p3/2) after deconvolution. Moreover, the presence of a large quantity of high oxidation state of Ni 2+ (II) species was also evident by the peaks at BE 854.3-854.5 eV (Ni2p3/2), BE 856.6-857.7 eV (Ni2p1/2) and BE 871.3-872.1 eV (Ni2p3/2). All of these observations indicated that the active metal species Ni 0 were easily oxidized to Ni 2+ (II) during the preparation. Furthermore, the N1s spectrum of Ni@N/C-x was recorded between 395.0 eV and 410.0 Ev. Deconvolution of the N1s spectra of Ni@N/C-1 can be fitted into three peaks of nitrogen species which were assigned to pyridinic-N, pyrrolic-N, graphitic-N respectively. It is proved that when the N content in the support increased, a transfer from N to Ni became more intense. As shown in the TEM-EDS element, when the nitrogen source changed from urea, ammonium citrate, and melamine, the content of N in the catalyst Ni@N/C-5, Ni@N/C-4, Ni@N/C-1 increased from 1.09%, 1.56% to 2.93% gradually. The stronger interaction between the N-doped support and nickel, which comes from the enhanced electron transfer, could improve the stability and activity of the catalyst. As we all know, when the electron acceptor of pyridinic-N species interacts with metal atoms, we can observe the positive binding energy shift. But for when the graphitic-N species donates electrons to the neighboring metal atoms, instead, we can obtain the negative binding energy shift. And this incensement of electron density could increase the Ni catalyst's selectivity. Furthermore, The C1s electrons were deconvoluted into the main four peaks centered at BE 284.2, 285.7, 286.8, 288.3 eV, which can be assigned to graphitic carbon C-C sp 2 , C-C sp 3 /-C-N, C-O, and C=O type of carbons respectively. To know the surface physical properties of the catalysts, we carried out the nitrogen adsorption isotherms of series Ni@N/C catalysts' samples. As illustrated in Figure -3i, all the nitrogen-doped Ni@N/C catalysts showed the typical type IV reversible adsorption isotherm curve, which indicates the existence of mesoporous. The Brunauer-Emmett-Teller surface area of Ni@N/C-1, Ni@N/C-4, Ni@N/C-5, and Ni@N/C-6 was calculated to be 297.6, 435.4, 200.7, and 591.9 m 2 *g -1 respectively (Table ). The nitrogen sources affected the catalysts' BET surface area significantly. The catalyst Ni@N/C-5 prepared from urea has the smallest surface area of 200.1 m 2 *g -1 , with the largest pore diameter of 6.03 nm. While for the catalyst Ni@N/C-6 prepared from aliphatic amine 1,2 propylenediamine, it had the largest BET surface area of 591.9 m 2 *g -1 and the smallest av-erage pore diameter of 3.24 nm. Although the Ni@N/C-4 catalyst had a higher loading of Ni (32.1%) than the Ni@N/C-1 catalyst (Ni loading 25.0%), due to its smaller Ni particles and well dispersion in the support, it had a higher BET surface area of 435.4 m 2 *g -1 than Ni@N/C-1 catalyst (297.6 m 2 *g -1 ). According to the quenched solid density functional theory (QSDFT) model, the pore-size distribution of these Ni@N/C-1 samples reveals main peaks in the range of approximately 3 nm to 5 nm.