## 1 Introduction  
The enormously increasing energy requirements and the limited availability of energy sources motivate the development of efficient and safe energy storage devices [1, 2]. Supercapacitors, as next-generation high-performance energy storage devices, significantly make up the shortfall of conventional physical capacitors and batteries, and are extremely suitable for superior power density and high charge/discharge rate required applications [3, 4, 5], such as electrical vehicles and stop-go driving models [6, 7]. In addition, supercapacitors also possess an ultralong service life of up to 10,000 cycles and excellent operating temperature adaptability from \(-40\)\({}^{\circ}\)C to \(+70\)\({}^{\circ}\)C [8, 9]. Accordingly, supercapacitors have sparked extensive interest in recent years. Two types of supercapacitors, divided by energy storage mechanisms, are termed as electric double-layer capacitors and pseudocapacitors, respectively [10, 11]. Currently, the electric double-layer capacitors, by operating reversible ions' adsorption/desorption to store energy, suffer from lower specific capacitance [12, 13], which fails to meet the ever-growing demand. In contrast, pseudocapacitors contribute to higher specific capacitance by reversible faradaic redox reactions [14, 15] and have become the research hotspot in supercapacitors fields in past decade. Among various transition metal oxides/hydroxides for the electrode materials of pseudocapacitors, Ni(OH)\({}_{2}\) and Co(OH)\({}_{2}\), in virtues of definite redox transitions, superior theoretical specific capacitances (ca. 3750 F/g for Ni(OH)\({}_{2}\) and ca. 3460 F/g for Co(OH)\({}_{2}\)), earth-abundant resources, and environmental friendliness [16, 17], have attracted much attentions. For instance, the Ni(OH)\({}_{2}\) microspheres,synthesized by [PERSON] et al., presented the specific capacitance of 1280.9 F/g at current density of 0.5 A/g [18]. [PERSON] et al. reported the Co(OH)\({}_{2}\) nanoflakes delivering 1636 F/g at 0.5 A/g [19]. However, the achieved specific capacitances by single Ni(OH)\({}_{2}\) or Co(OH)\({}_{2}\) so far are far lower than the theoretical values, consequently hampering their practical use. To address the issue, novel binary Ni-Co hydroxide was studied and has been shown to outperform the corresponding single hydroxide, owing to unparalleled advantages, such as stronger layered orientation, reduced resistance, increased active sites generated by valence interchange or charge hopping between cations, synergistic redox reaction, moderate redox potential, and decreased redox peak potential difference [20; 21; 22; 23; 24; 25; 26; 27; 28]. Unfortunately, even though numerous efforts were devoted, the specific capacitances up to now are still unsatisfied, which are mainly caused by undesirable morphology [29; 30; 31; 32; 33] and involved binders [34; 35; 36; 37; 38]. Moreover, with respect to high rate capability, cycling stability, good mass loading, and facile method, previous progress to date rarely shows all of these characteristics. For example, the Ni-Co double hydroxide nanosheets prepared by [PERSON] et al. reached ultrahigh 2682 F/g at 3 A/g, but the capacitance loss was more than 35% at 20 A/g [39]. Similarly, the Ni-Co double hydroxides microspheres by [PERSON] and co-workers displayed 2275.5 F/g at 1 A/g, nevertheless the capacitance sharply faded to 1007.8 F/g at 25 A/g [40]. Besides, [PERSON] et al. developed flower-like Ni-Co binary hydroxides, in which only 73.8% capacitance could be maintained after 3000 cycles [41]. [PERSON] and co-workers fabricated Ni-Co hydroxide nanostructures, which demonstrated 1760 and 1468 F/g at 1 and 20 A/g, as well as 87.3% capacitance retention after 2000 cycles, whereas remarkable criteria were obtained at an exaggerated low mass loading of 0.23 mg/cm\({}^{2}\)[42]. In addition, the multi-step and complex routes employed in certain cases further hindered their commercial application.