# NiO-Based Electronic Flexible Devices
## 3 NiO-Based Flexible Devices
### Supercapacitors and Electrodes  
Good flexibility as well as good electrochemical performances are achieved with NiO based cable-like all-solid-state flexible supercapacitor (CAF) [98]. In a recent report, a highly flexible electrode was constructed by achieving a sandwich of an interlayer of metallic glass (MG) and two outer layers of nanoporous nickel coated with nickel oxide/hydroxide (np-NiOxHy@Ni). The assembling of the sandwich was achieved in several steps, schematically reported in Figure 2. Metallic glass ribbons were obtained by arc-melting of pure metals (99.99 wt.%) under argon to yield the master alloy of Ni\({}_{40}\)Zr\({}_{20}\)Ti\({}_{40}\), followed by rapid quenching of the re-melted alloys onto a spinning copper roller [99; 100] (Figure 2a). The outer layers were generated by immersing the as-spun ribbons in 0.05 M HF solutions for 4 h at 298 K to synthesize sandwich-like flexible np-NiOxHy@Ni/MG/np-NiOxHy@Ni composite electrodes (Figure 2b). Cable-like all-solid-state flexible supercapacitors (CAFSs) were prepared by soaking the nanoribbons in a gel obtained by heating an aqueous solution of PVA/KOH (Figure 2c). Then, two ribbons were twined on a rubber rod in the opposite direction and coated with gel electrolyte (Figure 2d). Finally, the CAFSs were prepared by wrapping with a thin cling film. Electrochemical measurements indicate that the electrode is essentially a battery-type material, also exhibiting pseudocapacitive behavior with a capacitance of 778 F cm\({}^{-3}\) at 1 A cm\({}^{-3}\) in KOH solutions and a retention of 80.3% when the current density increases by 128-fold, as well as a cycle stability of 100% capacitance retention after 8000 cycles. \begin{table}
\begin{tabular}{c c c} \hline
**NiO Morphology** & **Capacitance** & **Reference** \\ \hline _Nanoparticles_ & 423 F/g at 0.5 mA/cm\({}^{2}\) & [84] \\ \hline _Nanoparticles_ & 1012 mF/cm\({}^{2}\) at 1 mA/cm\({}^{2}\) & [85] \\ \hline _Nanowires_ & 670 F/g at 1 A/g & [86] \\ \hline _Nanoplates array_ & 1124 F/g at 2 A/g & [87] \\ \hline _Microspheres_ & 1482 F/g at 0.5 A/g & [88] \\ \hline _NiO_**Composites** & & \\ \hline _NiO_@_ccO_2_ (5 wt/\%) Flower-like microspheres_ & 2155.6 F/g at 1 A/g & [89] \\ \hline _NiO_@_CNT_ & 2480 F/g at 0.5 A/g & [90] \\ \hline _NiO_@_GNS_/_PANI_ & 1409 F/g & [91] \\ \hline _NiO_/_GNS_ & 1050 F/g & [92] \\ \hline _M_-_NiO_-_G_//_AC_ & 154.0 F/g & [93] \\ \hline \end{tabular}
\end{table} 
Table 1: Capacitance of NiO with various morphologies and composites. The flexibility of the CAFSs was tested by performing cyclic voltammetry (CV) measurements with electrodes subjected to bending or twisting. The capacitance slightly increases by bending a 4 cm long electrode, until the distance between the extremes reaches 2 cm (nearly a U shape). With a further decrease of the distance, the capacitance slightly reduced. A capacitance retention of 96.4% could be obtained when the electrode was twisted into a shape of an \"8\". In addition, a capacitance retention of 92.4% was obtained after 1500 bending cycles. Dealloying was also used for preparing a flexible Co(OH)\({}_{2}\)/NiO\({}_{x}\)H\({}_{y}\)@Ni hybrid [101] with interwoven Co(OH)\({}_{2}\) nanopetals electrochemically deposited on the skeleton surface of np-NiO\({}_{x}\)H\({}_{y}\) electrodes. This was achieved by electrodeposition, using a three electrode cell immersed in a Co(NO\({}_{3}\))\({}_{2}\cdot\)6H\({}_{2}\)O aqueous solution, where np-NiO\({}_{x}\)H\({}_{y}\)@Ni substrate acts as working electrode, a platinum plate is used as counter electrode and Ag/AgCl as reference electrode. Owing to the hierarchical porous structure and synerge effect of Co(OH)\({}_{2}\) and NiO\({}_{x}\)H\({}_{y}\) electroactive materials, the Co(OH)\({}_{2}\)/np-NiOxHy@Ni hybrid electrode delivered a voltage window of \(-\)0.1-0.6 V and a specific capacitance of 1421.1 F cm\({}^{-3}\) at 0.5 A cm\({}^{-3}\), higher than the single transition-metal electrodes.