# NiO-Based Electronic Flexible Devices
###### Abstract  
Personal, portable, and wearable electronics have become items of extensive use in daily life. Their fabrication requires flexible electronic components with high storage capability or with continuous power supplies (such as solar cells). In addition, formerly rigid tools such as electrochromic windows find new utilizations if they are fabricated with flexible characteristics. Flexibility and performances are determined by the material composition and fabrication procedures. In this regard, low-cost, easy-to-handle materials and processes are an asset in the overall production processes and items fruition. In the present mini-review, the most recent approaches are described in the production of flexible electronic devices based on NiO as low-cost material enhancing the overall performances. In particular, flexible NiO-based all-solid-state supercapacitors, electrodes electrochromic devices, temperature devices, and ReRAM are discussed, thus showing the potential of NiO as material for future developments in opto-electronic devices.  
NiO; flexible devices; synthesis; supercapacitors; electrodes; solar cells; electrochromic devices; temperature devices; ReRAM +
Footnote †: journal: Applied  
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Footnote 1: _Applied_  
and high optical transmittance [46; 47; 48; 49]. However, high sheet resistance, low work function of pristine material [49], poor mechanical properties in presence of defects [50], and high strain limits curtail the actual graphene applications [51].  
Combining flexibility requirements and enhanced performances may be achieved by integrating nanomaterials with the desired properties with suited flexible substrates resistant to deformations, mechanical stress, and of long duration.  
Transition metal oxides and particularly NiO have the advantage of high storage capacitance [52], optical and electrical properties [53], transport properties on surfaces [54] as well as in perovskite solar cells [55]. Their nanoparticles synthesis may be achieved in several, environmentally friendly ways [56; 57; 58; 59; 60; 61; 62; 63], with several shapes, arrangements, and super-structures [64]. Depending on the specific size and morphological characteristics, they can be employed in the construction of several types of devices for different purposes, which include chemosensing [65; 66; 67; 68], supercapacitors devices [69; 70; 71], solar cells [72; 73], and resistive memories [74; 75; 76].  
Integrated NiO-based flexible materials can be achieved in several ways. NiO nanoparticles or thin films can be deposited on flexible materials such as PET (polyethylene terephthalate) or PEN (polyethylene naphthalate) substrates; NiO nanoparticles can be embedded in the flexible materials in the preparation phase, and NiO leagues can be de-alloyed [77] with a specific procedure that ensures ultimate pliability.  
In the present mini-review, an overview of NiO synthetic methods and properties are reported as well as recent progresses in the fabrication and performances of NiO-based flexible devices. In particular, the latest constructions will be presented of NiO-flexible supercapacitors, electrodes, electrochromic devices, solar cells, temperature sensor devices, and resistive random-access memory.