## 4 Applications of Fe\({}_{3}\)O\({}_{4}\) Nanoparticles
### Energy Conversion and Storage Devices  
Due to their various advantageous characteristics, such as low cost, natural abundance, ecofriendliness, electrochemical activity, and high theoretical capacity, Fe\({}_{3}\)O\({}_{4}\) is a potentially useful anode material for supercapacitors and lithium-ion batteries. With good conductivity (10\({}^{2}\)-10\({}^{3}\)\(\Omega^{-1}\) cm\({}^{-1}\)) and a high theoretical capacitance (approximately 347 F/g at 1.2 V), Fe\({}_{3}\)O\({}_{4}\) has been used in high-energy-density storage supercapacitors via redox reactions [20; 155]. Nanostructured materials constructed from Fe\({}_{3}\)O\({}_{4}\) NPs and carbon-based materials are promising hybrid materials that achieve a high energy density and robust electrochemical performance. Several hybrid nanostructures, including a 3D network of Fe\({}_{3}\)O\({}_{4}\) NPs/reduced graphene oxide nanosheets [156], Fe\({}_{3}\)O\({}_{4}\)@carbon nanosheets [157], Fe\({}_{3}\)O\({}_{4}\) nanosphere-decorated graphene [158], and Fe\({}_{3}\)O\({}_{4}\) nanospheres coated with nitrogen-doped carbon [159], have produced encouraging results in the construction of high-performance supercapacitors. Using Fe\({}_{3}\)O\({}_{4}\) as a conductive core, nanocomposites of conductive ferroelectric core-shell Fe\({}_{3}\)O\({}_{4}\)@BaTiO\({}_{3}\) nanoparticles loaded into the ferroelectric poly(vinylidene fluoride-cohexafluoropropylene) (P-(VDF-HFP)) polymer matrix were prepared to increase the permittivity of the polymer and increase the degradation resistance of the polymer-based capacitor. The maximum energy storage density of this thin film reaches 7.018 J/cm\({}^{3}\) upon exposure to an electric field of 2350 kV/cm [160]. Similar core-shell Fe\({}_{3}\)O\({}_{4}\)@BaTiO\({}_{3}\) NPs were also incorporated into a PVDF polymer matrix and exhibited a remarkable energy density storage of 16 J/cc under an electric field of 430 kV/mm [161]. In addition to capacitor/supercapacitor applications, Fe\({}_{3}\)O\({}_{4}\) NPs have been integrated into various nanoarchitectures/nanocomposites for use as anode materials for rechargeable lithium-ion batteries. In these systems, Fe\({}_{3}\)O\({}_{4}\) participates in an electrochemical reaction with lithium, as indicated in Equation (3) below [162], which allows Li cation insertion/extraction. Theoretically, the capacity of Fe\({}_{3}\)O\({}_{4}\) can reach \(\sim\)900 mA h g\({}^{-1}\), which is even higher than the capacity of commercial graphite (\(\sim\)372 mA h g\({}^{-1}\)). Optimizing the sizes and shapes of nanoparticles [163; 164; 165; 166] and fabricating carbon hybrid materials of Fe\({}_{3}\)O\({}_{4}\)[167; 168; 169; 170] efficiently increase the conductivity, structural integrity, and performance of devices. Graphenenanosheet-wrapped Fe\({}_{3}\)O\({}_{4}\) particles prepared by [PERSON] and coworkers showed improved stability and an excellent rate [168]. [PERSON] et al. designed 3D graphene foams (GFs) cross-linked with graphene sheet (GS)-encapsulated Fe\({}_{3}\)O\({}_{4}\) nanosphere Fe\({}_{3}\)O\({}_{4}\)@GS/GF, which exhibited a high reversible capacity of 1059 mAh g\({}^{-1}\) over 150 cycles [171]. Thus, due to its superior properties, Fe\({}_{3}\)O\({}_{4}\) represents a potential oxide material for integration into energy conversion and storage devices.  
Electrochemical reaction (3):  
\[\text{Fe}_{3}\text{O}_{4}+8\text{ Li}^{+}+8\text{ e}^{-}\leftrightarrow 3\text{Fe}+4\text{ Li}_{2}\text{O} \tag{3}\]