## 4 Challenges and Future Opportunities of IL-Based Gel  
IL-based gels offer a combination of simple preparation, a wide electrochemical potential window, high ionic conductivity, and enhanced stretchability. However, IL-based gels often suffer from poor mechanical strength such as stiffness, pinholes, and low toughness. In addition, some IL-based gels prepared from hydrophilic ILs or polymers are unstable in a humid environment. By contrast, IL-based gels composed of hydrophobic ILs possess excellent voltage stability and air and water resistance and are noncorrosive to metal electrodes. Although the conductivity of IL-based gels is lower than that of pristine ILs, researchers are exploring IL-based gel electrolytes as stable alternatives for Li batteries, SCs, actuators, sensors, wearable electronics, and internet of things. Meanwhile, MSCs that can be directly placed on chips are widely used as a power source for micro-/nano-electronic devices by instantly providing effective peak power. The urgent need for trendy, portable, smart, wearable devices and real-time communication devices has led scientists to design and commercialize IL-based microelectronics in recent years. Thus, the future possibilities offered by IL-based MSCs are worth exploring. The potential and future opportunities of IL-based gel application in electrochemical devices are shown in Figure 9.  
To date, Li batteries with IL-based gel are the most reported. This opens up great opportunities for IL-based gel modifications to other types of batteries. For SCs, power density and energy density are the critical parameters. Most of the existing SCs suffer from lower energy density compared to the Li battery. However, Li batteries often encounter a drawback of low power density, short lifecycles, and low safety. Advanced hybrid capacitors (HCs) are a new class of energy storage system consisting of a battery-type anode and a capacitive cathode that have attracted great attention by researchers and have begun to develop rapidly. In HCs, the anode undergoes redox reactions or reversible lithiation/delithiation, whereas the cathode stores energy through the electric double-layer capacitance. HCs can offer the long lifespan of SCs, the high energy density of batteries, and the longer cycling life of electronic devices [143]. The ever-increasing demand for power density and energy density systems in hybrid vehicles has necessitated the development of advanced HCs in recent years. Hence, the environmentally friendly IL-based advanced hybrid supercapacitors have great possibilities for next-generation energy storage devices [143; 144]. Currently, IL-based gel electrolytes are also being used in solar cell applications [145; 146]. Usually, IL-based gels are applied as a solid-state electrolyte or solid polymer electrolytes for energy storage device applications. The appropriate choice of electrode materials, IL, and the matrix of the IL-based gel can shed light on developing and exploiting novel smart and sustainable energy storage devices and more geometrically complex electronics. Besides electrochemical device application, another great potential of IL-based gels is to be used in electrochromic devices and as a heterogenous catalyst and biocatalyst [16; 147; 148]. IL-based gels as biocompatible materials were found to be effective in minimizing drug degradation and loss in drug delivery systems [16]. Intelligent structural design and optimization-based performance are required, for which it is important to carry out extensive and thorough experimentation. Furthermore, complex synthesis routes and device preparation processes need to be avoided. Digitalization of synthetic routes is required to create touchless automatic fabrication, such as via 3D printing or additive manufacturing. Some works on 3D printable IL-based gels and composites have been reported [149; 150; 151; 152], however, progress is very limited, mainly due to the limited compatibility of IL and monomeric systems and suitable polymerization processes for 3D printing.