# NiO-Based Electronic Flexible Devices
## 4 Electronic Flexible Devices Based on Other Oxides  
Flexible supercapacitors are typically achieved by embedding an active material in a flexible support, usually a carbon-based material, including active carbon, graphene, carbon nanotubes, aerogel and hydrogels [112; 113], unless techniques such as the dealloying can be applied. More recently, the use of paper supports was implemented for one-time only devices [114]. Several transition metal oxides (TMO), alone and in association with conducting polymers, were probed on flexible supports to achieve supercapacitors. ZnO/carbon nanofibers attained by electrospinning showed a specific capacitance > 100 F\({}^{-1}\) and retention capability of 93.90% after 2000 continuous charge/discharge cycles [115]. A porous carbonized cotton/ZnO/CuS composite electrode showed a specific capacitance of 1830 mF cm\({}^{-2}\) at 2 mA cm\({}^{-2}\) and 74% retention with the current density increasing from 2 to 10 mA cm\({}^{-2}\) and a loss of 14.8% of the initial capacitance after 5000 cycles [116]. An Fe\({}_{2}\)O\({}_{3}\)/Graphene aerogel hybrid electrode was used to build a highly flexible all-solid-state symmetric supercapacitor device, which could be bent by several angles providing a high specific capacitance, i.e., 440 F\({}^{-1}\) and 90% capacitance retainment over 2200 cycles [117]. Confined anatase TiO\({}_{2}\) nanoparticles were achieved in conductive activated carbon interconnected nanopores. The assembled cells with porous carbon cathodes displayed a specific capacity of \(\approx\)140 mAh g\({}^{-1}\) at slow charge and \(\approx\)60 mAh g\({}^{-1}\) at a 3.5 s fast charge [118]. Compared to the most recent NiO-based supercapacitors or electrodes, these systems show either a slightly lower performance or lower retention of properties upon deformation. Flexible temperature devices have been fabricated using many different types of active materials on flexible supports deposited with different techniques. The most common active materials in temperature sensors are Pt and Au. They were deposited on a polyimide film after a peeling process, for monitoring the temperature in the range 20-120 \({}^{\circ}\)C and displayed a TCR value of 0.0032 \({}^{\circ}\)C\({}^{-1}\)[119]. Cr and Au thin layers were sputtered on pre-stretched PDMS to minimize the tensile strain [120] and create a reversible bendable and stretchable flexible temperature device. Ag lines on polyimide were evenly distributed by inkjet printing and constituted a bendable and flexible sensor for measuring body surface temperatures in the 20-60 \({}^{\circ}\)C range, with an average sensitivity of 2.23 \(\times\) 103 \({}^{\circ}\)C\({}^{-1}\) and less than 5% hysteresis [121]. ReRAMs are typically constituted of a top and a bottom electrode interlayered either by a semiconductor or by an insulating layer. The ReRAM performances are usually related to the interlayer, though the choice of the electrodes also plays a role. As for the interlayers, they are usually categorized into two groups, namely organic and inorganic. The organic interlayers include, among the others, silk protein/fibroin, nanocellulose and album [122; 123; 124]. These devices, however, tend to be rather humidity sensitive and not very stable in long terms operations. Metal oxides are typical inorganic interlayers, and a vast amount of electrode interlayer combinations was probed. Bipolar ReRAM devices were achieved, for instance, based on Pd/HfO\({}_{\mathrm{x}}\)-Ag NPs/TiN [125], or Ag/MnO-Ta\({}_{2}\)O\({}_{\mathrm{S}}\)/Pt [126]. The former operates at a set voltage of 2.2 V and reset voltage at \(-\)2.2 V, the latter at a set voltage of 0.8 V and reset voltage at \(-\)1.1 V. Unipolar ReRAM are more difficult to achieve. Some examples are provided by Al/HfO\({}_{\mathrm{x}}\)/Al (set voltage 1.8 V, reset voltage 0.8 V) [127] and Pt/ZnO/Pt (set voltage 1.1-2.3 V, reset voltage 0.4-1 V) [128]. The issue of flexibility of ReRAM, however, is difficult to tackle, also due to the necessity of incorporating the electrodes in the device.