## 5 Applications  
### Energy Storage  
The conventional energy sources are depleting rapidly and alongside creating several environmental issues such as global warming, air pollution, rising sea levels, thinning of ozone layer, etc., renewable energy sources, such as solar energy, hydropower, geothermal energy, ocean energy, etc., can help in meeting the energy demands of human society. However, high volatility, irregularity, security, and storage issues make it difficult for their global commercialization. To resolve these issues, researchers propose novel functional materials for efficient energy storage, which is reliable, cost effective, environmentally friendly, and is available easily and for long-term use. Until now, many materials and  
Figure 14: SEM images (**a**) HSR and (**b**) HEBM. Figure 13: Thermal conductivity of exfoliated h-BN@PDA/CNC composite and h-BN/CNC composites [103]. (RightsLink License obtained to use this Figure from CCC, Copyright Clearance Center, ACS Publishers, ACS Applied Nanomaterials). compositions, such as nanocomposites, ceramics, and carbon-based composites, have been tested in this regard. Lithium-ion batteries and supercapacitors have shown promise in energy storage devices in modern technologies due to their higher energy density, faster charging, and lower discharging rate, longer cycle life, and wide operating temperature ranges. In these energy storage devices, the active electrode materials play a crucial role as they directly participate in the electrochemical charging-discharging reactions. There are numerous synthesis techniques, such as hydrothermal route [107; 108], microwave synthesis method [109; 110], chemical vapor deposition technique [111; 112], etc., that have been used for synthesizing materials which have demonstrated enhanced energy storage capacity. Ball milling is one of the synthesis techniques that belongs to the class of mechanochemical processes for synthesizing micro and nano structured materials. Large scale production is one of the major advantages of this technique. Ball milled materials that include metal nanocomposites, ceramic composites, carbon-based materials, etc., are reviewed that have demonstrated satisfactory performance in energy storage mainly as metal-ion batteries like Li, Na, K, and supercapacitors and partly as hydrogen storage, thermal power storage materials, etc. Fuel cells as energy conversion devices and batteries or supercapacitors as energy storage devices are currently the thrust areas in the domain of energy research. The options of hydrogen production and hydrogen storage areas are also emerging as allied fields. Fuel cells and batteries have certain challenges in terms of reaction kinetics or energy and power density targets to be met, while in hydrogen storage, the uptake capacity as well as rate of adsorption/desorption matters. In case of the proton exchange membrane (PEM) fuel cells, the oxygen evolution and oxygen reduction reaction (ORR) should be effective reactions for a competitive cell performance. These are governed by the interactions between the nanomaterials of the electrodes and electrolytes (or gases) and, thus, the interfacial interactions become very important. In view of this, energy conversion and energy storage processes heavily depend upon the type of material, size of material, its phase, interconnections, conduction channels, bonding strengths, bonding environment, etc., as they either promote or disrupt the flow of charges in one way or the other. The techniques employed for synthesis of nanomaterials thus influence the performance of the device considerably. One of the distinctive properties of a ball-milling technique is the slow transformation of starting material into the final nanomaterial following a top-down approach. In this method, the phase changes occur gradually and at different physical sites of the sample. This inherently creates a host of defect sites in the nanomaterials.