The morphologies and microstructures of the synthesized MOFs are further investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM and TEM images show that the MOFs feature spindle-shaped micro/nanostructures (0.8–2.0 µm in length, 100–400 nm in diameter) with smooth surface (
Figs. 3a1–
e2) and solid nature (
Figs. 3a3–
e3). It is noteworthy that the introduction of Co ions has little effect on the structures and morphology of the MOFs that may be because the Co ion is similar in size to the original Ni ion. With the increase of the introduced Co ions, the sizes of the MOFs slightly decrease (
Figs. 3b1 and
b2) and then gradually increase (
Figs. 3c1,
c2,
d1,
d2,
e1 and
e2). The possible mechanism may be as follows. When a small amount of Co ions was introduced, the coordination ions is mainly Ni ions and the Co ions facilitate the coordination of Ni species, leading to fast nucleation and growth and affording smaller crystals. With the increasing amount of Co ions, the coordination competition of Co and Ni make the rate of nucleation and growth slow down, giving larger crystals [
18,
33]. Energy-dispersive X-ray (EDX) analysis shows the composition of Ni, Co, O, N and C in Co/Ni-MOF-1:15 (Fig. S2 in Supporting information). High-angle annular dark-field scanning TEM (HAADF-STEM) image and the corresponding elemental mappings demonstrate the homogeneous distribution of Ni, Co, O, N, and C elements (
Figs. 3f–
k), further verifying the successful preparation of Co/Ni-MOF-1:15. The mass ratios of Ni
2+/(Co
2++Ni
2+) are 0.941, 0.904 and 0.572 in the Co/Ni-MOF-1:15, Co/Ni-MOF-1:5 and Co/Ni-MOF-1:1 from Inductively coupled plasma-atomic emission spectrometry (ICP-AES) elemental analysis (Table S1 in Supporting information), which is in consistent with the introduced initial feeding materials, suggesting the successful formation of the bimetallic MOFs. N
2 sorption measurement is performed to confirm the Brunauer-Emmett-Teller (BET) surface areas and the pore structures of Ni-MOF, Co/Ni-MOF-1:15 and Co-MOF. The Ni-MOF, Co/Ni-MOF-1:15 and Co-MOF exhibit the calculated BET surface areas of 212.1, 229.0 and 17.75 m
2/g with pore volumes of 0.1975, 0.2004 and 0.058 cm
3/g, respectively (Fig. S3 in Supporting information). The larger surface area and higher porosity of Co/Ni-MOF-1:15 allow faster electrolyte diffusion and ion transport. To be noted, the much smaller BET surface area and pore volume for Co-MOF is because its structure was completely collapsed under activation condition [
30]. The thermal stability of Co/Ni-MOF-1:15 is investigated using thermogravimetric analysis (Fig. S4 in Supporting information). From room temperature to ~325 ℃, the gradual weight loss is due to the loss of guest DMF molecules and the DMF molecules trapped inside the pores. As the temperature increases, the decomposition of the organic components occurs. In addition, no obvious phase change is observed when the Co/Ni-MOF-1:15 is immersed separately in water and ethanol solution for 24 h (Fig. S5 in Supporting information), demonstrating its favorable stability in these solutions.