## 3 Results  
By controlling the initial ratio of Ni and Co in the reactants, the binary material experienced an evident morphological evolution from nanosheet to nanowire, investigated in detail by the SEM, as shown in Figure 1b-f. Figure 1a presents the bare nickel foam with a continuous porous three-dimension (3D) network. At a Ni:Co ratio of 9:1, the nanosheets were observed, as shown in Figure 1b. Partial nanosheets intersected among each other, displaying an ambiguous nanoflower-like feature. By further increasing content of Co, the morphology of the composite evolved into nanosheet completely, as shown in Figure 1c. Particularly, as more cobalt ions were devoted (intermediate Ni:Co ratio), nanowires appeared and were encapsulated around the nanosheets, as shown in Figure 1d. The constructed integrated geometry provided a large accessible surface area compared with sole nanosheet morphology. At a greater Co to Ni ratio, the nanosheet structures were replaced by uniform nanowires, as shown in Figure 1e. Finally, when Co became dominant (Ni:Co ratio of 1:9), it could be seen that sectional nanowires trended to aggregate, as depicted in Figure 1f. The distinct morphological evolution is ascribed to the competition of Ni and Co cations for hydroxide radicals [44]. The XRD patterns of as-prepared samples under different Ni:Co ratios are shown in Figure 2. The indexed planes of the diffraction peaks at corresponding 2\(\theta\) values can be assigned to Ni(OH)\({}_{2}\) (JCPDS No. 14-0117) and Co(OH)\({}_{2}\) (JCPDS No. 30-0443). The analogous peak positions are believed to be caused by the similar physical and chemical characteristics of Ni and Co [45]. When a larger Co source was applied, the peak signals became weaker and a lacking of partial peaks appeared, which is ascribed to the low crystallinity of the sample [46]. The XPS was carried out to verify the element valence state of the as-synthesized material. The typical XPS survey spectrum of the sample at a Ni:Co ratio of 5:5 is depicted in Figure 3a, where C, Ni, Co, and O were visible. The C element is due to the air exposure of the sample and can be referenced to calibrate the binding energy [47; 48]. In the high resolution XPS spectrum of Ni 2p, shown in Figure 3b, the peaks of Ni 2p\({}_{3/2}\) and Ni 2p\({}_{1/2}\) at binding energies of 855.6 and 873.5 eV (energy separation of 17.9 eV), as well as two obvious shakeup satellites (denoted as Sat. ), indicate the characteristics of Ni\({}^{2+}\)[49]. Figure 3c illustrates the core-level XPS spectrum of Co 2p, the peaks at binding energies of 781.4 and 796.9 eV standing for Co 2p\({}_{3/2}\) and Co 2p\({}_{1/2}\) (energy gap of 15.5 eV), identify Co as Co\({}^{2+}\)[50; 51]. The single peak located at 531.1 eV in the O 1s spectrum, shown in Figure 3d, is assigned to OH- [48; 52]. Hence, it can be concluded that Ni-Co double hydroxides were successfully formed under the present experimental conditions. Figure 1: Scanning electron microscope (SEM) images of the samples at different Ni:Co ratios: (**a**) bare nickel foam; (**b**) 9:1; (**c**) 7:3; (**d**) 5:5; (**e**) 3:7; (**f**) 1:9.  
in Figure 4a. All CV curves present a distinct pair of peaks, which involves three reversible faradaic redox processes, expressed as Equations (4)-(6) [41]:  
\[\text{Ni(OH)}_{2}+\text{OH}^{-}\leftrightarrow\text{NiOOH}+\text{H}_{2}\text{O}+ \text{e}^{-}, \tag{4}\]  
\[\text{Co(OH)}_{2}+\text{OH}^{-}\leftrightarrow\text{CoOOH}+\text{H}_{2}\text{O}+ \text{e}^{-}, \tag{5}\]  
\[\text{CoOOH}+\text{OH}^{-}\leftrightarrow\text{CoO}_{2}+\text{H}_{2}\text{O}+ \text{e}^{-}. \tag{6}\]  
The merged and indistinguishable redox peaks reveal the mixed uniformity of Ni and Co in the binary hydroxides [53]. Further, the peaks in anodic sweep move to lower potential when Co content increases, because the potential of Co transition is lower than that of Ni. In other words, the oxidation peak corresponding to Ni is closer to the voltage window limit [44; 54].