# Carbon Allotropes/Epoxy Nanocomposites as Capacitive Energy Storage/Harvesting Systems
## 1 Introduction  
The social challenges of resource depletion and climate change call for a cleaner, more efficient use of resources and energy. The need for energy efficiency is leading to the introduction of novel and improved materials and the design of new low-cost, low-weight environmentally friendly device structures that can harvest and/or store and/or convert energy with higher efficiency. By these means, increased autonomy and support for diverse applications within mobile devices that range from portable (or even wearable) electronics to hybrid electric vehicles can be achieved [1]. Innovation in dielectric polymer nanocomposites, which have the intrinsic ability to store energy at the nanoscale level, could lead to structures with high performance having a profound impact on the longer-term plans for more energy-efficient technology [2]. Multifunctional polymer composites could be employed as structural energy devices, where the matrix and filler work synergistically, leading to improved functional behavior [3, 4].  
Polymer nanocomposites are considered to be advanced technological materials in the fields of electronics, aviation, automobile and aerospace industries [5, 6] due to their low weight, low cost, easy processing and remarkable mechanical, thermal, electrical and magnetic properties that can be effectively tailored by controlling the filler type and content [7, 8, 9, 10]. The combination of polymers that exhibit high dielectric breakdownstrength with the high values of dielectric permittivity of the ceramic fillers leads to a composite system with enhanced dielectric properties [11]. In addition, dispersed ceramic nanoparticles inside the polymer matrix can act as a distributed network of nanocapacitors where energy can be stored and harvested via a fast charge/discharge process [8; 12; 13]. The large interfacial area between the polymer matrix and the nanoinclusions is a dominant factor that dictates the dielectric and energy performance of nanocomposite materials due to interfacial polarization [14; 15], which is the primary mechanism for most capacitor applications.  
Conductive elements have been employed to increase the dielectric permittivity of polymer matrices and enhance their energy storage performance [16; 17]. However, increased conductivity values may increase leakage currents and diminish energy efficiency. Semiconducting ceramic nanoparticles, especially perovskite ferroelectric oxides such as BaTiO\({}_{3}\) and SrTiO\({}_{3}\)[12; 13; 18], have been extensively investigated for their potential energy storage performance along with piezoelectric polymers such as PVDF [3; 19; 20; 21]. Even though there are many studies in the literature that investigate the potential energy storage capacity of polymer nanocomposites [14; 22; 23; 24], little work has been done to determine the amount of recovered energy [8; 12; 13; 25]. Furthermore, the disparity of the fabrication processes and/or different functionalization methods makes the direct comparison of the reported results difficult.  
The main goal of this study is to determine the energy efficiency in identically prepared nanocarbon-filled epoxy nanocomposite systems by investigating the effects of filler concentration temperature, DC voltage and different types of filler content upon the stored and harvested energy. For this reason, nanocomposites comprising an epoxy resin as a matrix and different carbon allotropes as the reinforcing phase (carbon black, multi-walled carbon nanotubes, graphene nanoplatelets and nanodiamonds) were prepared and studied at various filler contents. The nanocomposites were characterized by several techniques to determine their operational window and structural integrity.