Carbon Allotropes/Epoxy Nanocomposites As Capacitive Energy Storage/Harvesting Systems

applied sciences

Article Carbon Allotropes/Epoxy Nanocomposites as Capacitive Energy Storage/Harvesting Systems

Sotirios G. Stavropoulos , Aikaterini Sanida and Georgios C. Psarras *

Smart Materials & Nanodielectrics Laboratory, Department of Materials Science, School of Natural Sciences, University of Patras, 26504 Patras, Greece; [email protected] (S.G.S.); [email protected] (A.S.) * Correspondence: [email protected]; Tel.: +30-2610-996-316

Featured Application: Lightweight, low cost, ﬂexible systems for fast storing and retrieving electrical energy.

Abstract: The present work aims at the development and characterization of carbon/polymer matrix nanocomposites, which will be able to operate as compact materials systems for energy storage and harvesting. Series of polymer nanocomposites employing different types of carbon allotropes (carbon black nanoparticles, multi-walled carbon nanotubes, graphene nanoplatelets and nanodiamonds) were developed varying the filler type and content. The energy storage ability of the systems was examined under AC and DC conditions to evaluate the influence of temperature, DC voltage and different types of filler content upon the stored and harvested energy. Experimental data confirmed the ability of the examined systems to store energy and release it on demand via a fast charge/discharge process. The addition of carbon nanoparticles significantly enhances the energy density of the systems. The coefficient of energy efficiency (neff) was determined for all systems, Citation: Stavropoulos, S.G.; Sanida, reaching up to 80% for the nanocomposite with 5 phr (parts per hundred resin per mass) carbon A.; Psarras, G.C. Carbon black content. In order to examine the optimal operational conditions of the systems, their structural Allotropes/Epoxy Nanocomposites integrity and thermomechanical properties were also investigated by means of static tensile tests, as Capacitive Energy Dynamic Mechanical Analysis (DMA) and Differential Scanning Calorimetry (DSC). Storage/Harvesting Systems. Appl. Sci. 2021, 11, 7059. https://doi.org/ Keywords: polymer nanocomposites; energy materials; carbon black; carbon nanotubes; graphene 10.3390/app11157059 nanoplatelets; nanodiamonds; thermomechanical properties; energy efficiency; energy storage/harvesting

Academic Editors: Sergey Nesov and Petr Korusenko

Received: 24 June 2021 1. Introduction Accepted: 29 July 2021 The social challenges of resource depletion and climate change call for a cleaner, more Published: 30 July 2021 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 Publisher’s Note: MDPI stays neutral and environmentally friendly device structures that can harvest and/or store and/or with regard to jurisdictional claims in convert energy with higher efficiency. By these means, increased autonomy and support published maps and institutional affil- for diverse applications within mobile devices that range from portable (or even wearable) iations. 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 Copyright: © 2021 by the authors. be employed as structural energy devices, where the matrix and filler work synergistically, Licensee MDPI, Basel, Switzerland. leading to improved functional behavior [3,4]. This article is an open access article Polymer nanocomposites are considered to be advanced technological materials in distributed under the terms and the fields of electronics, aviation, automobile and aerospace industries [5,6] due to their conditions of the Creative Commons low weight, low cost, easy processing and remarkable mechanical, thermal, electrical Attribution (CC BY) license (https:// and magnetic properties that can be effectively tailored by controlling the filler type creativecommons.org/licenses/by/ and content [7–10]. The combination of polymers that exhibit high dielectric breakdown 4.0/).

Appl. Sci. 2021, 11, 7059. https://doi.org/10.3390/app11157059 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 7059 2 of 14

strength 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 dom- inant 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 BaTiO3 and SrTiO3 [12,13,18], have been extensively investigated for their potential energy storage performance along with piezoelectric polymers such as PVDF [3,19–21]. Even though there are many studies in the literature that investigate the potential energy storage capacity of polymer nanocomposites [14,22–24], 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.

2. Materials and Methods 2.1. Materials Four nanocomposite systems were prepared by employing carbon allotropes as ﬁllers in an epoxy matrix. In particular, Epoxol 2004 was supplied by Neotex S.A., Athens, Greece, while ﬁllers were obtained from Plasmachem GmbH. The type of the employed epoxy was a two-component low-viscosity resin consisting of epoxy prepolymer (diglycidyl ether of bisphenol A (DGEBA)) along with the curing agent (aromatic amine). The physical properties of the carbon allotropes, according to the manufacturer’s datasheet, are presented in Table1.

Table 1. The employed carbon allotropes as ﬁllers.

Fillers Size Speciﬁc Surface Carbon black (CB) 13 nm 500 m2/g nanoparticles Graphene nanoplatelets (GnP) Thickness: 1–4 nmLateral size up to 2 µm 700–800 m2/g Multi-walled carbon Length: 1–10 µm (3–15 walls)Outer 240 m2/g nanotubes (MWCNT) diameter: 5–20 nmInner diameter: 2–6 nm Nanodiamonds grade G (ND) 4 nm 290–360 m2/g

2.2. Fabrication Method For the preparation of the nanocomposites, the carbon allotropes were initially dispersed into the prepolymer and stirred inside a sonicator for 5 min. Then, the curing agent was added at a 2:1 (prepolymer/hardener) mixing ratio, and the whole mixture was stirred by hand in a sonicator for 10 min before it was poured into suitable silicon moulds to be cured for seven days at room temperature. After polymerization, the samples underwent a post-curing treatment at 100 ◦C for 4 h in order to complete the crosslinking process. Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 15

2.2. Fabrication Method For the preparation of the nanocomposites, the carbon allotropes were initially dispersed into the prepolymer and stirred inside a sonicator for 5 min. Then, the curing agent was added at a 2:1 (prepolymer/hardener) mixing ratio, and the whole mixture was Appl. Sci. 2021, 11, 7059 3 of 14 stirred by hand in a sonicator for 10 min before it was poured into suitable silicon moulds to be cured for seven days at room temperature. After polymerization, the samples underwent a post-curing treatment at 100 °C for 4 h in order to complete the crosslinking Filler contentprocess. for all systems Filler content was 0, 0.1,for all 1, 3,systems 5, 7 and was 10 phr0, 0.1, (parts 1, 3, per5, 7 hundredand 10 phr resin (parts per per hundred weight). A schematicresin per representation weight). A ofschematic the fabrication representa processtion of of the the nanocomposites fabrication process is of the depicted in Figurenanocomposites1. is depicted in Figure 1.

Figure 1. Schematic representation of the fabrication process of the nanocomposites. Figure 1. Schematic representation of the fabrication process of the nanocomposites.

2.3. Characterization2.3. Characterization Techniques Techniques 2.3.1. Morphological2.3.1. Morphological Characterization Characterization The morphologyThe andmorphology the quality and of the the quality developed of the nanocomposites developed nanocomposites were examined were examined via Scanning Electronvia Scanning Microscopy Electron (SEM) Microscopy using an(SEM) EVO us MAing an 10 apparatusEVO MA 10 provided apparatus by provided by Carl Zeiss. Carl Zeiss.

2.3.2. Thermomechanical2.3.2. Thermomechanical Characterization Characterization The thermomechanicalThe thermomechanical properties of the nanocompositesproperties of the were nanocomposites assessed by Differential were assessed by Scanning CalorimetryDifferential (DSC) Scanning using TACalorimetry Q200 and Dynamic(DSC) using Mechanical TA Q200 Analysis and Dynamic (DMA) Mechanical using TA Q800,Analysis both provided (DMA) byusing TA Instruments.TA Q800, both For provided the DSC tests,by TA samples Instruments. were placed For the DSC tests, in an aluminumsamples crucible, were while placed an emptyin an aluminum aluminum crucible crucible, while served an asempty a reference. aluminum Addi- crucible served tionally, the mechanicalas a reference. properties Additionally, of the nanocomposites the mechanical were properties examined withof the static nanocomposites tensile were tests at room temperatureexamined with with static a 5 mm/min tensile tests rate, usingat room an Instrontemperature 5582 apparatus,with a 5 mm/min provided rate, using an by Instron. TheInstron parameters 5582 apparatus, of the DSC provided and DMA by are Instron. shown inTheTables parameters2 and3, respectively.of the DSC and DMA are shown in Tables 2 and 3, respectively. Table 2. DSC parameters.

Table 2. DSC parameters. DSC Temperature rangeDSC 0–100 ◦C ◦ Heating rateTemperature range 5 C/min 0–100 °C Module time Data Heating rate 5 °C/min Nitrogen ﬂow 50 mL/min Module time Data Table 3. DMA parameters. Nitrogen flow 50 mL/min

DMA Temperature range Ambient–100 ◦C Heating rate 5 ◦C/min Frequency 1 Hz Operating mode Three-point bending Appl. Sci. 2021, 11, 7059 4 of 14

2.3.3. Energy Storage AC Measurements The electric response of the nanocomposites under AC conditions was studied via Broadband Dielectric Spectroscopy (BDS) using an Alpha-N Frequency Response Analyzer and a Novotherm system for the temperature control. All experimental parameters for the BDS measurements are given in Table4. The sample was placed inside the dielectric cell BDS 1200, forming a sandwich capacitor. Frequency scans were conducted under successive isothermal conditions. The experimental data were recorded using Windeta software. Devices, cell and software were obtained from Novocontrol Technologies.

Table 4. BDS parameters.

BDS Frequency range 10−1–107 Hz Temperature range 30–160 ◦C Temperature step 5 ◦C/min Stabilization time 60 s Vrms 1 V

DC Measurements The energy storage/harvesting tests under DC current measurements were performed using a high-resistance DC meter (Agilent 4339B). The experimental set up included an automatic measurement process controlling the charging/discharging sequence. Table5 presents the parameters of DC measurements. The samples were put in a parallel-plate capacitor conﬁguration. Experimental data were obtained via suitable software.

Table 5. DC measurements’ parameters.

DC Measurements Applied Voltages 50, 100, 250 V Charging time 60 s Discharging time 600 s Temperature Ambient

3. Results and Discussion 3.1. Morphological Characterization The quality of the surface of nanocomposites along with the dispersion and distribution of nanoparticles in the polymer matrix is vital for the enhancement of their performance. Any cracks, voids or air trapped inside the matrix during the fabrication process would be detrimental to both the mechanical as well as the electrical properties of the nanocomposites. Scanning Electron Microscopy was employed to examine the quality of the fabricated systems. Figure2 shows representative SEM images for the nanocomposites with 3 phr ﬁller content. Obtained SEM images conﬁrm the high quality of the systems since no apparent air, cracks or voids are present. Uniform distribution and dispersion of the nanoparticles inside the polymer matrix was achieved, and the formation of large agglomerates was avoided.

3.2. Thermal and Mechanical Characterization Differential Scanning Calorimetry was employed for the investigation of the thermal properties of the nanocomposites. An endothermic step-like process was recorded in the thermograms of all examined systems, which was identified as the transition from the glassy to the rubbery state of the polymer matrix. The characteristic glass transition temperature for all systems was determined at the point of inflection using suitable software provided by TA instruments. The results are listed in Table6. The variation of the glass transition temperature with filler content can give an insight into the different interactions occurring inside the materials. Increasing Tg values indicate strong interactions between the Appl. Sci. 2021, 11, 7059 5 of 14

nanoparticles and the macromolecules that obstruct the cooperative segmental movement of the polymer chains. All nanocomposites exhibit higher glass transition temperature Appl. Sci. 2021, 11, x FOR PEER REVIEWvalues than the neat epoxy, which is a sign of the good adhesion of the nanofillers to5 of the 15 polymeric matrix. Carbon black nanocomposites exhibit a progressive increase in Tg values with filler content while, on the other hand, nanocomposites filled with nanodiamonds show a remarkable stability across the board. At high concentrations for the GnP and ofMWCNT-filled the systems since nanocomposites, no apparent air, the cracks interactions or voids between are present. adjacent Uniform nanoparticles, distribution due and to dispersiontheir aspect of ratio,the nanoparticles become stronger, inside the thus polymer cancelling matrix part was of achieved, the hindrance and the of formation the chain ofmobility large agglomerates and slightly loweringwas avoided. the values of the glass transition temperature.

(a) (b)

Figure 2. SEMSEM images images of of the the nanocomposites with 3 phr: ( (aa)) CB; CB; ( (b)) MWCNT; MWCNT; ( c) GnP; ( d) ND content.

3.2.Table Thermal 6. Glass and transition Mechanical temperature Characterization as determined via DSC test, for all examined systems. Differential Scanning CalorimetryCB was empl GnPoyed for the MWCNT investigation of the ND thermal propertiesSpecimens of the nanocomposites.◦ An endothermic◦ step-like process◦ was recorded◦ in the Tg ( C) Tg ( C) Tg ( C) Tg ( C) thermograms of all examined systems, which was identified as the transition from the Epoxy 44.6 44.6 44.6 44.6 glassy to the rubbery state of the polymer matrix. The characteristic glass transition 0.1 phr 43.7 52.0 51.7 53.2 temperature1 phr for all systems 46.4 was determined 53.3 at the point of 51.8 inflection using 54.0 suitable software3 phrprovided by TA instruments. 55.6 The resu 53.9lts are listed in 53.7 Table 6. The variation 53.5 of the glass5 phrtransition temperature 56.1 with filler content 58.8 can give an 52.1 insight into the 53.3 different interactions7 phr occurring inside 56.3 the materials. 55.3 Increasing Tg 51.7 values indicate 53.7 strong interactions10 phr between the 57.4nanoparticles an 54.9d the macromolecules 50.2 that obstruct 52.4 the cooperative segmental movement of the polymer chains. All nanocomposites exhibit The mechanical properties of polymer nanocomposites are dictated by several param- higher glass transition temperature values than the neat epoxy, which is a sign of the good eters including the individual properties of the constituents and the distribution of ﬁllers in adhesion of the nanofillers to the polymeric matrix. Carbon black nanocomposites exhibit the polymer matrix, as well as interfacial bonding and the fabrication method. The addition a progressive increase in Tg values with filler content while, on the other hand, of carbonaceous nanoparticles enhances the mechanical properties of the nanocomposites nanocomposites filled with nanodiamonds show a remarkable stability across the board. as demonstrated by the static and dynamic tests (Figures3 and4, respectively). The ten- At high concentrations for the GnP and MWCNT-filled nanocomposites, the interactions sile modulus was determined as the slope of the stress–strain curve in the elastic region. between adjacent nanoparticles, due to their aspect ratio, become stronger, thus cancelling part of the hindrance of the chain mobility and slightly lowering the values of the glass transition temperature.

Appl. Sci. 2021, 11, 7059 6 of 14

Figure3 presents the Young’s modulus and tensile strength versus filler content for all examined nanocomposites. Error bars denote the standard deviation of the representative results. The Young’s modulus increases predictably with filler concentration for all systems except the nanocomposites filled with nanodiamonds. The systematic increase in the tensile modulus values is attributed to the intrinsic stiffness and rigidness of the nanoparticles and to the interfacial bonding between the matrix and the nanoinclusions, which affects the effectiveness of the load transfer from the polymer matrix to nanofillers [26,27]. It can be seen that the addition of very small amounts of carbon nanotubes, with very large aspect ratio and stiffness, in the polymer matrix leads to the largest modulus enhancement of all examined systems. In the case of nanodiamond-filled nanocomposites, the Young’s modulus increases until the 5 phr specimen, where it obtains the maximum value and then diminishes significantly, indicating that a higher content of nanodiamonds favors their aggregation. The macroscopic properties of heterogeneous polymer nanocomposites are governed by the local stress distribution around the inclusions [28] that form a network that is able to bear the mechanical load. The tensile strength of the nanocomposites decreases with filler content for all examined systems, exhibiting lower values of strength than the neat epoxy, as depicted in Figure3. Nanoparticles seem to act as stress raisers, which is a common problem in particulate-filled composites [29]. The nanocomposites with MWCNT and GnP nanoparticles maintained higher values of tensile strength compared to the systems with 0D nanoreinforcements.

Figure 3. (a) Young’s modulus; (b) tensile strength versus filler content for all examined nanocomposites. Figure4 depicts the variation of the storage modulus and loss modulus (insets) as a function of temperature for all examined systems. The maximum values of the storage modulus follow the trend of the results for the elastic modulus. The increase of E0 values with filler content is ascribed to the effective load transfer from the polymer matrix to the filler, as a consequence of the fine dispersion and strong adhesion of filler in the matrix. Similarly to static tests, there is a significant increase in the storage modulus values with the addition even of a low amount of MWCNTs. This behavior reflects the intrinsic stiffness of carbon nanotubes and the formation of a load-carrying network inside the polymer matrix. There are several studies in the literature [30–32] reporting a sudden increase in stiffness for similar systems and suggesting the concept of a percolation threshold for the mechanical properties of polymer nanocomposites in an attempt to explain the high ranges of mechanical properties and the level of nanoparticle networks in polymer nanocomposites. However, this does not seem to be the case in the examined systems, as theoretical and computational results [30–32] indicate that such a threshold lies further beyond the electrical threshold, which in the examined MWCNTs system is 4.71% w/w [16]. A sharp decline in the storage modulus values suggests the transition of the polymer matrix from the glassy to the rubbery state, where the nanocomposites lose their load retention Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 15

Figure 3. (a) Young’s modulus; (b) tensile strength versus filler content for all examined nanocomposites.

Figure 4 depicts the variation of the storage modulus and loss modulus (insets) as a function of temperature for all examined systems. The maximum values of the storage modulus follow the trend of the results for the elastic modulus. The increase of E′ values with filler content is ascribed to the effective load transfer from the polymer matrix to the filler, as a consequence of the fine dispersion and strong adhesion of filler in the matrix. Similarly to static tests, there is a significant increase in the storage modulus values with the addition even of a low amount of MWCNTs. This behavior reflects the intrinsic stiffness of carbon nanotubes and the formation of a load-carrying network inside the polymer matrix. There are several studies in the literature [30–32] reporting a sudden increase in stiffness for similar systems and suggesting the concept of a percolation threshold for the mechanical properties of polymer nanocomposites in an attempt to explain the high ranges of mechanical properties and the level of nanoparticle networks in polymer nanocomposites. However, this does not seem to be the case in the examined systems, as theoretical and computational results [30–32] indicate that such a threshold Appl. Sci. 2021, 11, 7059 lies further beyond the electrical threshold, which in the examined MWCNTs system7 of 14 is 4.71% w/w [16]. A sharp decline in the storage modulus values suggests the transition of the polymer matrix from the glassy to the rubbery state, where the nanocomposites lose their load retention capacity. These step-like transitions are distinguished by the capacity.formation These of peaks step-like in the transitions loss modulus are distinguished spectra. by the formation of peaks in the loss modulus spectra.

(a) (b)

(c) (d) Figure 4. Comparative plots of the storage modulus as a function of temperature for the nanocomposites with (a) carbon Figure 4. Comparative plots of the storage modulus as a function of temperature for the nanocomposites with (a) carbon black; (b) MWCNT; (c) GnP; (d) ND. Insets depict the respective loss modulus as a function of temperature. black; (b) MWCNT; (c) GnP; (d) ND. Insets depict the respective loss modulus as a function of temperature.

3.3.Energy Storage and Harvesting 3.3.1. AC Measurements The ability of a nanocomposite to store electrical energy is expressed by its energy density [22]. The energy density, for all examined systems, was calculated via Equation (1):

1 U = ε ε0E2, (1) 2 0

−12 where ε0 is the dielectric permittivity of the free space, which is equal to 8.854 × 10 F/m, ε0 is the real part of permittivity and E is the field’s intensity. The calculations were made at kV constant electric field E = 1 m . In linear dielectric materials, polarization is proportional to the applied electric field, and the energy stored per volume increases with increasing permittivity, at constant applied field. The variation of energy density as a function of frequency and temperature for all nanocomposites with 3 phr filler content is depicted in Figure5. The illustrated behavior is representative of all examined systems. The energy density increases with temperature as the induced thermal energy facilitates the molecular mobility and the alignment of the dipoles with the externally applied electric field, thus increasing the achieved polarization. At high temperatures and low frequencies, energy density attains high values due to interfacial polarization. Polarization—and therefore also the energy density values—diminish Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 15

3.3. Energy Storage and Harvesting 3.3.1. AC Measurements The ability of a nanocomposite to store electrical energy is expressed by its energy density [22]. The energy density, for all examined systems, was calculated via Equation (1): U= ε εʹΕ, (1)

where ε0 is the dielectric permittivity of the free space, which is equal to 8.854 × 10−12 F/m, ε’ is the real part of permittivity and Ε is the field’s intensity. The calculations were made at constant electric field E = 1 . In linear dielectric materials, polarization is proportional to the applied electric field, and the energy stored per volume increases with increasing permittivity, at constant applied field. The variation of energy density as a function of frequency and temperature for all nanocomposites with 3 phr filler content is depicted in Figure 5. The illustrated behavior is representative of all examined systems. The energy density increases with temperature as the induced thermal energy facilitates the molecular mobility and the alignment of the

Appl. Sci. 2021, 11, 7059 dipoles with the externally applied electric field, thus increasing the achieved8 of 14 polarization. At high temperatures and low frequencies, energy density attains high values due to interfacial polarization. Polarization—and therefore also the energy density values—diminish rapidly with increasing frequency as permanent and induced dipoles rapidlydo not with have increasing sufficient frequency time to as align permanent themse andlves induced with the dipoles rotating do not external have sufﬁcient field. At timeintermediate to align themselves frequencies with and the temperatures, rotating external a relaxation ﬁeld. At process intermediate arises which frequencies is related and to temperatures,the glass to rubber a relaxation transition process of the arises polymer which matrix, is related also to known the glass as toα-relaxation. rubber transition of the polymer matrix, also known as α-relaxation.

(a) (b)

Figure 5. The variation of energy density as a function of frequency and temperature for the nanocomposites with 3 phr: (a) CB; (b) MWCNT; (c) GnP; (d) ND content.

The relative energy density function, deﬁned by Equation (2), was employed to highlight the contribution of the nanoreinforcements and to estimate the percentage of the stored energy in the nanocomposites compared to the energy stored in the matrix. Its normalized character eliminates the inﬂuence of geometric factors.

Ucomposite Urelative = (2) Uresin ◦ Figure6 showcases the variation of U relative as a function of temperature at 160 C for all examined systems. In general, the ability of the nanocomposites to store energy increases with filler content due to the higher values of conductivity for the sp2 hybridized carbon nanoparticles, such as MWCNTs, and dielectric permittivity for the sp3 hybridized nanodiamonds [16] and the induced heterogeneity of the systems. The nanocomposites with carbon black and MWCNT filler content exhibit enhanced energy storage ability that reaches up to hundreds and thousands of times, respectively, the ability of the neat epoxy matrix at the maximum point of the function at the highest filler concentrations. The enhanced performance of MWCNT and carbon black systems, especially at intermediate and high concentrations, can be attributed to the increased values of conductivity of Appl. Sci. 2021, 11, 7059 9 of 14

these systems. A thorough conductivity analysis of these systems reported in [16] shows that the signiﬁcant increase of conductivity corresponds to a very narrow variation of the conductive phase content. This increase can be described by means of percolation theory, which predicts that at a critical concentration (also called the percolation threshold), a conductive path is formed, allowing the migration of charge carriers throughout the material [16,33]. The abrupt increase of conductivity appears as the result of the transition from the insulating state, where limited contacts between conductive sites exist, to the state where a conductive network is formed, via physical contacts of the conductive inclusions (metallic-type conduction). The determined values of critical concentration were found to be 4.71% w/w for the MWCNT systems and 5.26% w/w for the carbon black systems. The dependence of conductivity on temperature and the Variable Range Hopping model were employed in this study [16], revealing that at concentrations higher than the percolation threshold, hopping and metallic-type conduction coexist. On the other hand, the effect of Appl. Sci. 2021, 11, x FOR PEER REVIEWthe addition of nanodiamonds on the energy storage capabilities of the systems is almost10 of 16

negligible. The peak formed at low frequencies is attributed to interfacial polarization due to the accumulation of free charges at the interface between the ﬁller and the polymer matrix, while the secondary peaks at higher frequencies correspond to the different dynamics of the α-relaxation process between the neat matrix and the nanocomposites.

(a) (b)

relative ◦ FigureFigure 6. 6.The The variationvariation ofof UUrelative asas a afunction function of of temperature temperature at at 160 160 °C Cfor for the the nanocomposites nanocomposites with with (a) (carbona) carbon black; black; (b) (bMWCNT;) MWCNT; (c) ( GnP;c) GnP; (d) ( dND.) ND.

3.3.2. DC Measurements The calculation of the of the stored and retrieved energy was performed by integrating the time-dependent current functions, under charging and discharging conditions, via Equation (3):

E= = , (3) where E is the stored energy at the capacitor, Q the charge and C is the capacitance of each nanocomposite as evaluated via the BDS measurements at the lowest frequency [12]. A more detailed insight into the rationale and the methodology can be found in [8,12]. Figure 7 depicts representative graphs of the stored and harvested energies for the nanocomposite systems with 3 phr filler content. The success of the energy storage/harvesting process is confirmed in the above diagrams for all examined systems. All charging curves are above the corresponding discharge curves. Systems quickly acquire high charging and discharge values (fast charge/discharge)—a characteristic feature of instantaneous power density capacitive systems. This ability to store and deliver a large amount of energy nearly instantaneously, sometimes called “pulse power”, cannot be delivered by storage technologies such as batteries and has potential applications in numerous commercial and military devices that require increased power density as well as higher charge/discharge current capabilities. The stored and recovered energies increase significantly with increasing voltage since the applied field lowers the Appl. Sci. 2021, 11, 7059 10 of 14

3.3.2. DC Measurements The calculation of the of the stored and retrieved energy was performed by integrating the time-dependent current functions, under charging and discharging conditions, via Equation (3): R 2 1 Q2 1 ( I(t)dt) E = = , (3) 2 C 2 C where E is the stored energy at the capacitor, Q the charge and C is the capacitance of each nanocomposite as evaluated via the BDS measurements at the lowest frequency [12]. A more detailed insight into the rationale and the methodology can be found in [8,12]. Figure7 depicts representative graphs of the stored and harvested energies for the nanocomposite systems with 3 phr filler content. The success of the energy storage/harvesting process is confirmed in the above diagrams for all examined systems. All charging curves are above the corresponding discharge curves. Systems quickly acquire high charging and discharge values (fast charge/discharge)—a characteristic feature of instantaneous power density capacitive systems. This ability to store and deliver a large amount of energy nearly instantaneously, sometimes called “pulse power”, cannot be delivered by storage technologies such as batteries and has potential applications in numerous commercial and military devices that require increased power density as well as higher Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 15 charge/discharge current capabilities. The stored and recovered energies increase significantly with increasing voltage since the applied field lowers the local potential barriers, thus facilitating the migration of the charges inside the nanocomposites.

(a) (b)

(c) (d) Figure 7. The stored and harvested (discharging procedure) energies as a function of time and voltages for the Figure 7. The stored and harvested (discharging procedure) energies as a function of time and voltages for the nanocompos- nanocomposites with 3 phr: (a) CB; (b) MWCNT; (c) GnP; (d) ND content. ites with 3 phr: (a) CB; (b) MWCNT; (c) GnP; (d) ND content. Figure 8 presents the comparative plots for the stored and retrieved energies for all examined systems at 100 V. Both energies increase with filler content due to the enhanced electrical properties of the carbonaceous nanoparticles compared to the polymer matrix as well as the interfacial effects due to the heterogeneity of the systems. It should be noted that, for the carbon black and MWCNT systems, above a critical concentration, reported as the percolation threshold in a previous study [16], the nanocomposites transition from the insulating to the conducting state. Therefore, no charge/discharge process could be established since the charge carriers do not accumulate inside the material but rather travel through it, either solely via a conducting path or in conjunction with hopping mechanisms [33]. However, the nanocomposite with 3 phr MWCNT concentration still exhibits the optimum performance. Appl. Sci. 2021, 11, 7059 11 of 14

Figure8 presents the comparative plots for the stored and retrieved energies for all examined systems at 100 V. Both energies increase with ﬁller content due to the enhanced electrical properties of the carbonaceous nanoparticles compared to the polymer matrix as well as the interfacial effects due to the heterogeneity of the systems. It should be noted that, for the carbon black and MWCNT systems, above a critical concentration, reported as the percolation threshold in a previous study [16], the nanocomposites transition from the insulating to the conducting state. Therefore, no charge/discharge process could be

Appl. Sci. 2021, 11, x FOR PEER REVIEWestablished since the charge carriers do not accumulate inside the material but rather12 of 15 travel through it, either solely via a conducting path or in conjunction with hopping mechanisms [33]. However, the nanocomposite with 3 phr MWCNT concentration still exhibits the optimum performance.

(a) (b)

FigureFigure 8. 8. Comparative plots plots for for the the harvested harvested energy, energy, at at100 100 V, V,versus versus time time for forthe thenanocomposites nanocomposites with with(a) carbon (a) carbon Black; Black;(b) MWCNT; (b) MWCNT; (c) GnP; (c) ( GnP;d) ND. (d Insets) ND. depict Insets depictthe stored the (charging stored (charging procedure) procedure) energies energies as a function as a functionof time for of the time respective for the systems. respective systems. AnotherAnother critical critical factor factor for the valuationfor the ofvaluation the nanocomposites’ of the energynanocomposites’ storage/harvesting energy performancestorage/harvesting is the calculation performance of the is coefficientthe calculation of energy of the efficiency coefficient (neff of), whichenergy is efficiency defined as(n theeff), ratiowhich of is the defined recovered as the to ratio stored of the energy reco withvered parameters to stored energy the charging with parameters voltage and the timecharging [8,12 ].voltage Figure and9 presents time [8,12]. the Figure results 9 for pres theents coefficient the results of for energy the coefficient efficiency of at energy the sameefficiency instant at timesthe same of t =instant 5, 10 andtimes 30 of s int = both 5, 10 charging and 30 s andin both discharging charging procedures. and discharging The coefficientprocedures. of energyThe coefficient efficiency of diminishes energy efficien over timecy diminishes for all systems, over confirming time for all once systems, more theconfirming fast charge/discharge once more the nature fast of charge/discharge the examined systems. nature n effof increasesthe examined with filler systems. content neff forincreases the nanocomposites with filler content with carbon for the black nanocomposites and MWCNT with filler carbon content. black The and optimum MWCNT energy filler efficiencycontent. wasThe notedoptimum for the energy nanocomposite efficiency withwas 5noted phr carbon for the black nanocomposite content with coefficientwith 5 phr (n ) values of 80 % at 100 V and 5 s. Unfortunately, once more, the nanocomposites carboneff black content with coefficient (neff) values of 80 % at 100 V and 5 s. Unfortunately, once more, the nanocomposites filled with nanodiamond particles fail to record a decent performance, as their respective coefficient drops even below that of the polymer matrix. Appl. Sci. 2021, 11, 7059 12 of 14

Appl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 15

ﬁlled with nanodiamond particles fail to record a decent performance, as their respective coefﬁcient drops even below that of the polymer matrix.

(a) (b)

(c) (d) Figure 9. The coefficient of energy efficiency as a function of the filler content at the same instant times of t = 5, 10 and 30 Figure 9. The coefficient of energy efficiency as a function of the filler content at the same instant times of t = 5, 10 and 30 s s for the nanocomposites with (a) carbon black; (b) MWCNT; (c) GnP; (d) ND, at 100 V. for the nanocomposites with (a) carbon black; (b) MWCNT; (c) GnP; (d) ND, at 100 V.

4.4. ConclusionsConclusions InIn thethe presentpresent study,study, seriesseries ofof polymerpolymer nanocompositesnanocomposites employingemploying differentdifferent typestypes ofof carboncarbon allotropesallotropes werewere successfullysuccessfully developeddeveloped varyingvarying thethe fillerfiller typetype andand content.content. SEMSEM imagesimages confirmedconfirmed the the quality quality of of the the fabricated fabricated systems systems and and revealed revealed fine fine nanodispersions nanodispersions of theof fillersthe fillers in the in polymer the polymer matrix. Thematrix. thermal The andthermal mechanical and mechanical properties wereproperties investigated were byinvestigated means of staticby means tensile tests,of static Dynamic tensile Mechanical tests, Dynamic Analysis Mechanical and Differential Analysis Scanning and Calorimetry.Differential Scanning All nanocomposites Calorimetry. exhibit All nanocomposites higher glass transition exhibit temperature higher glass values transition than thetemperature neat epoxy, values implying than strong the neat adhesion epoxy, of impl theying nanofillers strong toadhesion the polymer of the matrix. nanofillers Storage to andthe polymer elastic moduli matrix. attain Storage higher and values elastic with moduli the additionattain higher of the values reinforcing with the nanoparticles. addition of Thethe energyreinforcing storage nanoparticles. ability of the The systems energy under storage AC conditions ability of was the determined systems under by means AC ofconditions Broadband was Dielectric determined Spectroscopy by means in of a wideBroadband frequency Dielectric and temperature Spectroscopy range. in Energya wide densityfrequency increases and temperature with temperature range. and Energy attaines density the highest increases values with at low temperature frequencies dueand toattaines interfacial the highest polarization. values The at storedlow frequencie and harvesteds due energyto interfacial of the nanocomposites polarization. The was stored also evaluatedand harvested under energy DC conditions of the nanocomposites by employing was a charge/discharge also evaluated under procedure. DC conditions The energy by storageemploying and a harvesting charge/discharge ability ofprocedure. the nanocomposites The energy storage increases and with harvesting filler content ability due of the to thenanocomposites higher values ofincreases dielectric with permittivity filler content and/or due the to conductivity the higher of values the nanoinclusions. of dielectric Thepermittivity nanocomposites and/or withthe conductivity carbon black andof the MWCNT nanoinclusions. filler content The exhibit nanocomposites enhanced energy with storagecarbon abilityblack and that exceedsMWCNT several filler timescontent the exhi storedbit enhanced energy of theenergy polymer storage matrix. ability On that the otherexceeds hand, several the addition times the of nanodiamondsstored energy failsof the to enhancepolymer thematrix. energy On storage the other capabilities hand, the of addition of nanodiamonds fails to enhance the energy storage capabilities of the systems. Finally, the coefficient of energy efficiency (neff) was determined for all systems. The filler Appl. Sci. 2021, 11, 7059 13 of 14

the systems. Finally, the coefficient of energy efficiency (neff) was determined for all systems. The filler content enhances the energy efficiency of the examined systems, reaching the highest value of over 80% for the 5 phr carbon black nanocomposite.

Author Contributions: Conceptualization and visualization were conducted by S.G.S. and G.C.P.; measurements, methodology, validation and data analysis were performed by S.G.S. and A.S.; writing and editing were performed by S.G.S., A.S., G.C.P.; project administration and supervision was performed by G.C.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are included in the article. Additional data are available upon request. Acknowledgments: The present research work was supported by the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT) under the HFRI PhD Fellowship grant (GA. No. 2327). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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