Impact of Graphene or Reduced Graphene Oxide on Performance of Thermoelectric Composites

: In recent years, worldwide research has been focused on clean and sustainable energy sources that can respond to the exponentially rising energy demands of humankind. The harvesting of unused heat in relation to automotive exhaustion, industrial processes, and home heating is one possible way of enabling the transformation from a fossil fuel-based society to a low-carbon socioeconomic epoch. Thermoelectric (TE) generators can convert heat to electrical energy thanks to high-performance TE materials that work via Seebeck effects when electricity appears between the cold part and the hot part of these materials. High ﬁgure of merit ( ZT ) TE material is characterized by high electrical conductivity and Seebeck coefﬁcient, together with low thermal conductivity. This article aims to summarize ZT values reported for chalcogenides, skutterudites, and metal oxides with graphene (G) or reduced graphene oxide (rGO), and intends to understand the relationship between the addition of G-rGO to composites and ZT variation. In a majority of the publications, ZT value increases with the addition of G/rGO, although the relative growth of ZT varies for different material families, as well as inside the same group of materials, with it often being related not to a G/rGO amount but with the quality of the composite.


Introduction
In view of the current climate change problem, thermoelectric (TE) energy conversion is considered particularly attractive for harvesting electricity from solar heat or for recovering waste heat [1][2][3][4]. The use of waste heat as source of electrical energy in vehicles is also an attractive option for major automotive manufacturers, who actively pursue this technology to implement thermoelectric generators (TEGs) in car exhaust systems [5]. Moreover, it can significantly reduce the amount of CO 2 in industrial production processes. Furthermore, silent, reliable, and versatile TEGs without moving parts [6] can be used to power portable electronic devices as additional sources of energy.
Thus, TEGs hold a tantalizing promise of greater energy efficiency by providing a robust and clean option for waste heat recovery and its conversion into useful electrical energy through the Seebeck effect [7][8][9][10]. Moreover, their size can be easily varied, since TEGs consist of TE modules, which include multiple n-and p-type semiconductor couples connected electrically in series and thermally in parallel [11]. Both types of TE materials are widely studied and characterized by the dimensionless figure of merit at the absolute temperature, T: Accordingly, efficient TE materials should possess a great Seebeck coefficient, S, and a high electrical conductivity, σ, but a low thermal conductivity, k. Then, a high voltage

Chalcogenides
Among the chalcogenide family, Bi-and Te-based alloys are the most studied p-and n-type TE materials used in both bulk and thin film commercial applications [32]. That is because they possess a combination of relatively high Seebeck coefficient with great electrical conductivity and low thermal conductivity, which results in an elevated TE figure of merit (ZT).
In 2013, Liang et al. [33] and Dong et al. [34] first fabricated chalcogenide Bi 2 Te 3 with G, and PbTe with rGO. Both research groups used a spark plasma sintering (SPS) process for the densification of the obtained TE materials. Moreover, in both cases the studied composites presented slightly decreasing relative densities when the G or rGO contents increased. Moreover, the grain sizes of the pristine ceramics were found to be larger than those of the composites, as can be seen in the scanning electron microscopy (SEM) images in Figure 1 for Bi 2 Te 3 , without and with graphene [33]. At the same time, the decrease in the grain size enhances the grain boundary phonon scattering, which is beneficial for the reduction in thermal conductivity [33]. The maximum ZT values were reported to increase from 0.16 to 0.21 at 475 K, with 0.2 vol.% G addition to Bi 2 Te 3 [33], and from 0.12 to 0.7 at 670 K, with 5 wt.% GO addition to PbTe [34].
C 2021, 7, x FOR PEER REVIEW 3 of 20 mance on chalcogenides, skutterudites, and metal oxides with G-rGO additives are presented.

Chalcogenides
Among the chalcogenide family, Bi-and Te-based alloys are the most studied p-and n-type TE materials used in both bulk and thin film commercial applications [32]. That is because they possess a combination of relatively high Seebeck coefficient with great electrical conductivity and low thermal conductivity, which results in an elevated TE figure of merit (ZT).
In 2013, Liang et al. [33] and Dong et al. [34] first fabricated chalcogenide Bi2Te3 with G, and PbTe with rGO. Both research groups used a spark plasma sintering (SPS) process for the densification of the obtained TE materials. Moreover, in both cases the studied composites presented slightly decreasing relative densities when the G or rGO contents increased. Moreover, the grain sizes of the pristine ceramics were found to be larger than those of the composites, as can be seen in the scanning electron microscopy (SEM) images in Figure 1 for Bi2Te3, without and with graphene [33]. At the same time, the decrease in the grain size enhances the grain boundary phonon scattering, which is beneficial for the reduction in thermal conductivity [33]. The maximum ZT values were reported to increase from 0.16 to 0.21 at 475 K, with 0.2 vol.% G addition to Bi2Te3 [33], and from 0.12 to 0.7 at 670 K, with 5 wt.% GO addition to PbTe [34].
At the same time, in 2013, Chen et al. first studied Cu-based chalcogenide TE material CuInTe2 in composite, with G obtained from expanded graphite with mass ratios of 80:1 and 40:1 [35]. The authors reported that, in incorporating G sheets into the CuInTe2 matrix, thermal conductivity was successfully decreased, thus leading to an enhanced ZT of 0.4 at 700 K for CuInTe2 composite, with G in 80:1 ratio, compared to 0.39 at 700 K for pristine CuInTe2 [35]. Further, two Cu-based chalcogenide TE materials such as Cu2SnSe3 [36] and Cu2ZnSnS4 [37] were modified by G with different concentrations. The lattice thermal conductivity of these chalcogenides decreased in composites with a low G amount, effectively resulting from the phonon scattering by the graphene interface, similar to Bi2Te3 with G [33] or PbTe with GO [34]. However, when the fraction of graphene exceeds a certain value, the thermal conductivity of the composites starts to increase; this is because of an increase in the interface thickness instead of interface area due to the tendency to aggregation of graphene nanosheets reported in both these works [36,37]. The maximum figure of merit for Cu2SnSe3 with 0.25 vol.% G was 0.44 at 700 K [36], and ZT of 0.5 at 623 K was obtained for Cu2ZnSnS4 with 0.75 wt.% G [37]. At the same time, in 2013, Chen et al. first studied Cu-based chalcogenide TE material CuInTe 2 in composite, with G obtained from expanded graphite with mass ratios of 80:1 and 40:1 [35]. The authors reported that, in incorporating G sheets into the CuInTe 2 matrix, thermal conductivity was successfully decreased, thus leading to an enhanced ZT of 0.4 at 700 K for CuInTe 2 composite, with G in 80:1 ratio, compared to 0.39 at 700 K for pristine CuInTe 2 [35]. Further, two Cu-based chalcogenide TE materials such as Cu 2 SnSe 3 [36] and Cu 2 ZnSnS 4 [37] were modified by G with different concentrations. The lattice thermal conductivity of these chalcogenides decreased in composites with a low G amount, effectively resulting from the phonon scattering by the graphene interface, similar to Bi 2 Te 3 with G [33] or PbTe with GO [34]. However, when the fraction of graphene exceeds a certain value, the thermal conductivity of the composites starts to increase; this is because of an increase in the interface thickness instead of interface area due to the tendency to aggregation of graphene nanosheets reported in both these works [36,37]. The maximum figure of merit for Cu 2 SnSe 3 with 0.25 vol.% G was 0.44 at 700 K [36], and ZT of 0.5 at 623 K was obtained for Cu 2 ZnSnS 4 with 0.75 wt.% G [37].
Besides the SPS Bi 2 Te 3 reported in 2013 by Liang et al. [33], in 2016, Agarwal et al. used simple mixing and pressing of commercial Bi 2 Te 3 with 0.05 wt.% commercial G without sintering or other processing [38]. Investigating obtained composites by nanoscale atomic force microscopy, conductive atomic force microscopy, Kelvin probe force microscopy and scanning thermal microscopy, ZT was reported to be up to 0.92 at 402 K according to these local technique studies [38].
In addition to pristine Bi 2 Te 3 , its alloys Bi x Sb y Te 3 (x = 0.36-0.5, y = 1.5-1.64) [39][40][41][42][43][44] and Bi 2 Te 2.7 Se 0.3 [45] were also modified by G or rGO and studied. All of them have shown enhanced ZT after the addition of G or GO, up to certain concentrations. ZT values for chalcogenides with G or rGO are presented in Table 1 and in Figure 2 for comparison. Table 1 shows also their thermal and electrical conductivities, as well as their Seebeck coefficient values. Table 1. The composite preparation methods and conditions. The highest value of ZT (ZT max ) at the corresponding temperature (T ZTmax ), as well as the thermal conductivity (k), electrical conductivity (σ) and Seebeck coefficient (S) at that temperature reported for composites of chalcogenide TE materials with graphene or reduced graphene oxide. 400 K [39,40,[42][43][44], in contrast to p-type composites of Cu-based chalcogenides (Cu2SnSe3, CuInTe2 and Cu2ZnSnS4) and G-rGO [35][36][37], with ZT continuously increasing up to the limit of the measured T range of 650-700 K. At the same time, reported n-type chalcogenide TE materials with G-rGO have also shown dissimilarities in the temperature variation of ZT. Evident low-T peak is detected for Bi2Te2.7Se0.3 [45] and for Bi2Te3 [38], while no peak is seen for SPS densified Bi2Te3 with G [33], or for PbTe with GO [34].    Table 1.
As it turned out, the method of TE material preparation can also affect the ZT value. Particularly, Bi 0.5 Sb 1.5 Te 3 made using microwave-induced solvothermally synthesized Bi 2 Te 3 and Sb 2 Te 3 nanoplates with 0.1 vol.% G have shown a maximum ZT of 1.13 at 360 K, in comparison to ZT of 1.24 at 360 K for the same Bi 0.5 Sb 1.5 Te 3 obtained by a ball-milling process with the same amount of G [39].
At the same time, the composite of Bi 2 Te 3 nanowires (NW) fabricated by a wetchemical synthetic route (WCSR) with rGO can have slightly higher room-temperature ZT of 0.4 than that of 0.3 for a composite made of Bi 2 Te 3 powder with the same 1 wt.% rGO [46], as can be seen in Table 1. Performance enhancement was connected to higher electrical conductivity and lower thermal conductivity of G-rGO composites with nanowires. Smaller dimensions of the NWs, and, thereby, a higher interface/boundary number in Bi 2 Te 3 -NW composite with G than this number in the Bi 2 Te 3 -powder composite with G, was suggested as an explanation for the low thermal conductivity [46].
As can be seen from Figure 2, the highest ZT of 1.54 at 440 K was reported by Li et al. for Bi 0.4 Sb 1.6 Te 3 , with 0.4 vol.% exfoliated G [41]. Other p-type Bi x Sb y Te 3 chalcogenides with G-rGO have shown lower ZT values with a visible peak at a relatively low T, up to 400 K [39,40,[42][43][44], in contrast to p-type composites of Cu-based chalcogenides (Cu 2 SnSe 3 , CuInTe 2 and Cu 2 ZnSnS 4 ) and G-rGO [35][36][37], with ZT continuously increasing up to the limit of the measured T range of 650-700 K. At the same time, reported n-type chalcogenide TE materials with G-rGO have also shown dissimilarities in the temperature variation of ZT. Evident low-T peak is detected for Bi 2 Te 2.7 Se 0.3 [45] and for Bi 2 Te 3 [38], while no peak is seen for SPS densified Bi 2 Te 3 with G [33], or for PbTe with GO [34].

Skutterudites
Skutterudites with a general formula of MX 3 (M = Co, Rh, or Ir; X = P, As, or Sb) have an open frame ("cagey") structure with a body-centered cubic unit cell and a network of corner-sharing octahedra, each of which consists of one M atom in the center and six X atoms vertices [47,48]. Such a structure has motivated works on filling the cages with diverse atoms, in conjunction with the routine substitutional doping efforts, to tailor specific application functions described in many articles, including reviews [22,49]. Another strategy to enhance the TE performance of materials such as skutterudites involves the incorporation of a secondary phase, particularly 2D graphene or reduced graphene oxide, into the core-shell nanocomposites with 3D network wrapping structures [30]. Among the composites of skutterudites with G or rGO, materials such as pristine CoSb 3 [50] and related Yb 0.27 Co 4 Sb 12 [30], Ce 0.85 Fe 3 CoSb 12 [51], and La 0.8 Ti 0.1 Ga 0.1 Fe 3 CoSb 12 [52] were reported.
In 2013, Feng et al. first reported p-type nanocomposite with GO added to CoSb 3 during the solvothermal process, performed at 290 • C for 12 h [50]. After hot pressing under 80 MPa at 600 • C for 2 h, the CoSb 3 bulk hybrid with rGO has shown homogeneously embedded graphene in the nanostructured CoSb 3 matrix (see Figure 3a,b). The obtained ZT value of 0.81 at 800 K for CoSb 3 with 1.5 wt.% GO was found to be more than twice as high as that of bare CoSb 3 [50]. Reported enhancement of ZT for CoSb 3 with the addition of a small G amount was attributed to significantly increased carrier's concentration and their mobility, and, thereby, electrical conductivity. Moreover, the well dispersed graphene in the nanostructured CoSb 3 matrix prepared by solvothermal route also contributed to the diminished lattice thermal conductivity [50]. In addition, ZT ≈ 0.45 at 650 K was reported for CoSb 3 also prepared by a solvothermal process with 1 wt.% commercial G, but at lower temperature of 240 • C during 24 h, with further sintering at 500 • C by Yadav et al. [53].
an open frame ("cagey") structure with a body-centered cubic unit cell and a network of corner-sharing octahedra, each of which consists of one M atom in the center and six X atoms vertices [47,48]. Such a structure has motivated works on filling the cages with diverse atoms, in conjunction with the routine substitutional doping efforts, to tailor specific application functions described in many articles, including reviews [22,49]. Another strategy to enhance the TE performance of materials such as skutterudites involves the incorporation of a secondary phase, particularly 2D graphene or reduced graphene oxide, into the core-shell nanocomposites with 3D network wrapping structures [30]. Among the composites of skutterudites with G or rGO, materials such as pristine CoSb3 [50] and related Yb0.27Co4Sb12 [30], Ce0.85Fe3CoSb12 [51], and La0.8Ti0.1Ga0.1Fe3CoSb12 [52] were reported.
In 2013, Feng et al. first reported p-type nanocomposite with GO added to CoSb3 during the solvothermal process, performed at 290 °C for 12 h [50]. After hot pressing under 80 MPa at 600 °C for 2 h, the CoSb3 bulk hybrid with rGO has shown homogeneously embedded graphene in the nanostructured CoSb3 matrix (see Figure 3a,b). The obtained ZT value of 0.81 at 800 K for CoSb3 with 1.5 wt.% GO was found to be more than twice as high as that of bare CoSb3 [50]. Reported enhancement of ZT for CoSb3 with the addition of a small G amount was attributed to significantly increased carrier's concentration and their mobility, and, thereby, electrical conductivity. Moreover, the well dispersed graphene in the nanostructured CoSb3 matrix prepared by solvothermal route also contributed to the diminished lattice thermal conductivity [50]. In addition, ZT ≈ 0.45 at 650 K was reported for CoSb3 also prepared by a solvothermal process with 1 wt.% commercial G, but at lower temperature of 240 °C during 24 h, with further sintering at 500 °C by Yadav et al. [53].    [30]. SEM (see Figure 3c) and transmission electron microscopy (TEM) analysis (see Figure 3d) revealed that rGO embedded in boundaries of Yb 0.27 Co 4 Sb 12 was wrapped and 3-5 nm thick. Thus, 3D-rGO network wrapping architecture dramatically reduced the lattice thermal conductivity due to enhanced interparticle and intraparticle phonon scattering effects, and simultaneously enhanced the Seebeck coefficient due to the energy filtering effect of the grain boundary semiconductive rGO layer with nanometer thickness, resulting in high ZT [30]. Moreover, both when GO was added to Ce 0.85 Fe 3 CoSb 12 [51] and when G was grown in-situ by plasma-enhanced chemical vapor deposition (PECVD) in La 0.8 Ti 0.1 Ga 0.1 Fe 3 CoSb 12 [52], the values of ZT increased in comparison to pure CoSb 3 or CoSb 3 with G or rGO, as can be seen in Table 2 and in Figure 4. Table 2. The composite preparation method and conditions, the highest value of ZT (ZT max ) at the corresponding temperature (T ZTmax ), as well as the thermal conductivity (k), electrical conductivity (σ) and Seebeck coefficient (S) at that temperature reported for composites of skutterudite TE materials with graphene or reduced graphene oxide. Here STS-solvothermal synthesis; CS-chemical synthesis; HM-Hummer's method; PECVD-plasma-enhanced chemical vapor deposition; HP-hot-pressing; CSM-conventional sintering method; SPS-spark plasma sintering.   Table 2. Figure 4 presents that all the reported skutterudite materials combined with G-rGO show increase of ZT with the temperature. Moreover, p-type materials can show a peak of ZT at 700-800 K in contrast to the only reported n-type skutterudite Yb0.27Co4Sb12 with rGO that presented no peak or saturation up to 850 K. In addition, the highest ZT reported for p-type (Ce,Fe)CoSb12 with rGO (ZT of 1.06 at 700 K for Ce0.85Fe3CoSb12 [51]) is very close to ZT obtained for more complex multicomponent p-type skutterudites without addition of G or rGO such as (La,Ce,Fe)CoSb12 (ZT of 1. 15 Table 2. Figure 4 presents that all the reported skutterudite materials combined with G-rGO show increase of ZT with the temperature. Moreover, p-type materials can show a peak of ZT at 700-800 K in contrast to the only reported n-type skutterudite Yb 0.27 Co 4 Sb 12 with rGO that presented no peak or saturation up to 850 K. In addition, the highest ZT reported for p-type (Ce,Fe)CoSb 12 with rGO (ZT of 1.06 at 700 K for Ce 0.85 Fe 3 CoSb 12 [51]) is very close to ZT obtained for more complex multicomponent p-type skutterudites without addition  [55]). Moreover, the highest ZT value of 1.52 at 850 K reported for unique n-type skutterudite Yb 0.27 Co 4 Sb 12 with rGO [45] is not far from ZT of 1.8 at 823 K reported for the more complicated n-type (Sr,Ba,Yb) y Co 4 Sb 12 with 0.91 wt.% In 0.4 Co 4 Sb 12 [56]. Thus, TE performance similar to that of complex skutterudites can be obtained by adding G or rGO to simpler structures.

Metal Oxides
In contrast to chalcogenides, skutterudites or other TE materials, metal oxides are not so deeply studied yet and there are only few well known p-type materials such as Na x CoO 2 and Ca 3 Co 4 O 9+δ , which have been found very promising especially in high T range, benefiting from their layered structure. BiCuSeO as well as n-type CaMnO 3 -and SrTiO 3 -based materials are also among the most studied TE oxides [57][58][59]. There are several reports since 2015 on the influence of G-rGO on n-type SrTiO 3 -or ZnO-based materials, TiO 2 or BaTiO 3 [31,[60][61][62][63][64][65][66][67][68][69], summarized in Table 3 and presented in Figure 5, but there is no available report on p-type metal oxide TEs modified by G-rGO. Table 3. The composite preparation method and conditions. The highest value of ZT (ZT max ) at the corresponding temperature (T ZTmax ), as well as the thermal conductivity (k), electrical conductivity (σ) and Seebeck coefficient (S) at that temperature, reported for composites of metal oxide TE materials with graphene or reduced graphene oxide.   Table 3.
A composite of commercial powder of pure SrTiO3 with rGO densified by SPS was studied by Feng et al., which reported a ZT value of 0.09 at 760 K for 0.64 vol.% GO [31]. That is higher than the ZT of 0.05 at 673 K reported for the similar composite densified by SPS, but with SrTiO3 powder obtained by conventional solid-state reaction method and mixed with 0.7 wt.% GO by Rahman et al. [60]. However, in both cases, the authors reported significantly enhanced ZT after the addition of GO. Feng et al. also reported that room-T electrical conductivity of SrTiO3 densified in vacuum by SPS increases from 18 S/m to 1633 S/m, after the addition of rGO and without any additional reducing process [31]. Based on electron energy-loss spectroscopy analysis and low-angle annular dark field imaging, Feng et al. concluded that rGO in composite with SrTiO3 serves as a carbon source that promotes the formation of oxygen vacancies via a mild reaction with oxygen atoms on the surface of SrTiO3 grains, and that is the reason behind the highly increased carrier density for SrTiO3 with rGO [31]. Table 3. The composite preparation method and conditions. The highest value of ZT (ZTmax) at the corresponding temperature (TZTmax), as well as the thermal conductivity (k), electrical conductivity (σ) and Seebeck coefficient (S) at that temperature, reported for composites of metal oxide TE materials with graphene or reduced graphene oxide.   Table 3.

TE
A composite of commercial powder of pure SrTiO 3 with rGO densified by SPS was studied by Feng et al., which reported a ZT value of 0.09 at 760 K for 0.64 vol.% GO [31]. That is higher than the ZT of 0.05 at 673 K reported for the similar composite densified by SPS, but with SrTiO 3 powder obtained by conventional solid-state reaction method and mixed with 0.7 wt.% GO by Rahman et al. [60]. However, in both cases, the authors reported significantly enhanced ZT after the addition of GO. Feng et al. also reported that room-T electrical conductivity of SrTiO 3 densified in vacuum by SPS increases from 18 S/m to 1633 S/m, after the addition of rGO and without any additional reducing process [31]. Based on electron energy-loss spectroscopy analysis and low-angle annular dark field imaging, Feng et al. concluded that rGO in composite with SrTiO 3 serves as a carbon source that promotes the formation of oxygen vacancies via a mild reaction with oxygen atoms on the surface of SrTiO 3 grains, and that is the reason behind the highly increased carrier density for SrTiO 3 with rGO [31].
Moreover, although the pristine SrTiO 3 was doped by many different elements, such as La, Nd, Sm, Gd, Dy, Y, Er Pr, Yb, Ta, Nb, etc., to enhance ZT [57,59], the G-rGO effect on ZT was studied only on SrTiO 3 doped by La, incorporated at Sr-site [61], or by Nb in Ti-position [62,63], or simultaneously doped by La and Nb [65]. In general, the ZT of Nband simultaneously La,Nb-doped SrTiO 3 composites with G-rGO were found to increase with the temperature, and the highest ZT values were reported at the highest measured T of these composites without an obvious peak (see Figure 5).
The addition of 0.6 wt.% GO to SrTi 0.90 Nb 0.10 O 3 , prepared by the conventional method, leads to ZT of 0.24 at 1160 K, as was reported by Okhay et al. [62]. At the same time, almost zero ZT was detected by Dey et al. for SPS SrTi 0.85 Nb 0.15 O 3 with 0.5 wt.% GO; that was significantly enhanced, however, to 0.5 at 1200 K by further increase of GO amounts up to 1.5 wt.% [64]. Considering Sr vacancy as a defect that can serve for phonon scattering, and thus decrease thermal conductivity and increase ZT, Okhay et al. studied composites of rGO with nonstoichiometric Nb-doped SrTiO 3 with 2 mol.% Sr vacancies, achieving the highest ZT of 0.29 at 1160 K for Sr 0.98 Ti 0.90 Nb 0.10 O 3 with 0.6 wt.% GO [62]. Following that direction, Wu et al. increased Sr deficiency to 7 mol.%, thus obtaining a ZT of 0.22 at 800 K for Sr 0.93 Ti 0.90 Nb 0.10 O 3 + 0.6 wt.% GO nominal composition [63]. However, such high Sr deficiency should result not only in the formation of Sr vacancies but in the segregation of the titanium oxide secondary phase [70].
On the other hand, a ZT of 0.36 at 1023 K was reported by Lin et al. for Sr-deficient La-doped strontium titanate La 0.067 Sr 0.9 TiO 3 with 0.6 wt.% G, obtained by exfoliation of graphite [61]. The high ZT value is accompanied by the highest electrical conductivity and the lowest thermal conductivity in Table 3. The observed enhancement of electrical conductivity in La-doped SrTiO 3 with graphene loading was explained by the formation of a percolation network and the graphene-facilitated reduction of La-doped SrTiO 3 at the grain boundaries, resulting in faster electronic transport. Similar to other TE materials, Lin et al. also posited that the introduction of G lead to significant reduction in grain size of the nanocomposites from 2.2 µm for pristine La-doped SrTiO 3 to 412 nm when 0.6 wt.% G was added. Such nanotexturing introduces significant lattice scattering and, hence, reduces the thermal conductivity of the composite [61]. However, the reason for the decrease in thermal conductivity on cooling below 500 K reported by Lin et al., for La 0.067 Sr 0.9 TiO 3 with 0.6 wt.% G, and thereby for room temperature ZT with a value of 0.42, is still unclear, since no other report on graphene-modified SrTiO 3 -based TE materials, including that from the same group [65], has presented such variation.
Moreover, although both La-doped SrTiO 3 and Nb-doped SrTiO 3 with G-rGO present in Figure 5 and Table 3 rather high ZT, from 0.22 to 0.5, either the addition of solventderived flakes of graphene nanoplatelets to or the mixing GO prepared by the modified Hammer method with Sr 0.8 La 0.067 Ti 0.8 Nb 0.2 O 3-δ composition, was reported to lead to ZT that does not exceed 0.07 at 1000 K [65].
In addition to the described above metal oxide TE materials, Chen et al. reported a ZT of 0.28 at 1173 K for Al-doped ZnO with 1.5 wt.% GO [67], which is much higher than 0.04 at 1073 K reported for pure ZnO with 1 wt.% G [66], or even 0.064 at 1050 K for TiO 2 with 4 wt.% GO [68] and 0.08 at 600 K for BaTiO 3 with 1.7 wt.% GO [69].
In summary, according to the aforementioned reports, the ZT of TE materials tends to be enhanced by G-rGO addition. However, the systematic studies of the G-rGO concentration influence on the TE performance of composite materials show that ZT decreases after the application of a certain concentration of G or rGO, for all material types. Such an optimal concentration of G-rGO, at which the highest ZT can be achieved, is individual for each composite and is reported to be between 0.1 and 1.4 vol.% (for chalcogenide Bi 0.5 Sb 1.5 Te 3 [37] and for skutterudite Ce 0.85 Fe 3 CoSb 12 [51], respectively) or from 0.05 to 4 wt.% (for chalcogenide Bi 2 Te 2.7 Se 0.3 [45] or Bi 0.48 Sb 1.52 Te 3 [40] and for metal oxide TiO 2 [68], respectively). However, the trend is general, and it deserves a comparative consideration involving all ZT components in the next section.

Relationship between the Thermoelectric Performance of Materials and the Presence of Graphene or Reduced Graphene Oxide
According to Equation (1) for the dimensionless figure of merit, ZT depends on electrical and thermal conductivity, as well as on the value of the Seebeck coefficient. As it was described above, the Seebeck coefficient decreases with increase in charge carrier concentration. Thus, a high S can be observed in dielectric materials, such as undoped metal oxide, particularly SrTiO 3 . After the addition of G-rGO to insulating pure SrTiO 3 , its electrical conductivity increases and absolute value of S decreases as can be seen in Figure 6a, as reported by Rahman et al. [60] and supported by Wu et al. [63]. However, σ of SrTiO 3 can be significantly increased not only by the addition of G-rGO but also by Nb or La doping process [61,62,64]. Then, the addition of G-rGO to an already semiconductive material, such as heavily doped SrTiO 3 , chalcogenides and skutterudites, does not lead to serious variations of S, as shown in Figure 6b-d, respectively, although they could be slightly enhanced in the case of Yb 0.27 Co 4 Sb 12 skutterudite (see Figure 6d) due to the energy filtering effect [30]. Figure 6a, as reported by Rahman et al. [60] and supported by Wu et al. [63]. However, σ of SrTiO3 can be significantly increased not only by the addition of G-rGO but also by Nb or La doping process [61,62,64]. Then, the addition of G-rGO to an already semiconductive material, such as heavily doped SrTiO3, chalcogenides and skutterudites, does not lead to serious variations of S, as shown in Figure 6b-d, respectively, although they could be slightly enhanced in the case of Yb0.27Co4Sb12 skutterudite (see Figure 6d) due to the energy filtering effect [30]. (nSTN15) and their composites with 0.6 wt.% GO, indicated, respectively, as STN10 + rGO, nSTN10 + rGO and nSTN15 + rGO (reprinted from [62] with permission from Elsevier); (c) Bi0.36Sb1.64Te3 with 0, 0.1, 0.4, 0.8 vol.% GO (black, red, blue, and magenta dots and lines, respectively) (reprinted from [42] with permission from Elsevier); and (d) Yb0.27Co4Sb12 with addition 0, 0.72, 1.8, and 3.6 vol.% rGO (reprinted from [30] with permission from Royal Society of Chemistry).
In accordance with the abovementioned statements, the electrical conductivity σ of rather resistive metal oxides is significantly enhanced after G-rGO addition, as can be seen in Figure 7a, in comparison to less notable changes of σ for initially conductive chalcogenides or skutterudites, as shown in Figure 8a. Moreover, there are several studies on chalcogenides (Bi0.44Sb1.56Te3 [43], Bi0.5Sb1.5Te3 [44]) and skutterudites (Yb0.27Co4Sb12 [30], Ce0.85Fe3CoSb12 [51]), with a reported decrease in electrical conductivity after the addition of G-rGO. In these articles, the authors stressed that the G-rGO nm-layer located between the grains can grow and form agglomerates, with increasing rGO content that leads to decreased charge carrier mobility and, thereby, electrical conductivity because of the boundary scattering effect. At the same time, both the carrier concentration and its mobility were typically increased with the addition of G-rGO in the metal oxides (see Figure 7b,c, respectively). However, in the case of the chalcogenides or skutterudites, the carrier concentration was reported only to change insignificantly, while the mobility of the carriers was not strongly raised in general with the increase of rGO content (see Figure 8b). and their composites with 0.6 wt.% GO, indicated, respectively, as STN10 + rGO, nSTN10 + rGO and nSTN15 + rGO (reprinted from [62] with permission from Elsevier); (c) Bi 0.36 Sb 1.64 Te 3 with 0, 0.1, 0.4, 0.8 vol.% GO (black, red, blue, and magenta dots and lines, respectively) (reprinted from [42] with permission from Elsevier); and (d) Yb 0.27 Co 4 Sb 12 with addition 0, 0.72, 1.8, and 3.6 vol.% rGO (reprinted from [30] with permission from Royal Society of Chemistry).
In accordance with the abovementioned statements, the electrical conductivity σ of rather resistive metal oxides is significantly enhanced after G-rGO addition, as can be seen in Figure 7a, in comparison to less notable changes of σ for initially conductive chalcogenides or skutterudites, as shown in Figure 8a. Moreover, there are several studies on chalcogenides (Bi 0.44 Sb 1.56 Te 3 [43], Bi 0.5 Sb 1.5 Te 3 [44]) and skutterudites (Yb 0.27 Co 4 Sb 12 [30], Ce 0.85 Fe 3 CoSb 12 [51]), with a reported decrease in electrical conductivity after the addition of G-rGO. In these articles, the authors stressed that the G-rGO nm-layer located between the grains can grow and form agglomerates, with increasing rGO content that leads to decreased charge carrier mobility and, thereby, electrical conductivity because of the boundary scattering effect. At the same time, both the carrier concentration and its mobility were typically increased with the addition of G-rGO in the metal oxides (see Figure 7b,c, respectively). However, in the case of the chalcogenides or skutterudites, the carrier concentration was reported only to change insignificantly, while the mobility of the carriers was not strongly raised in general with the increase of rGO content (see Figure 8b). leads to decreased charge carrier mobility and, thereby, electrical conductivity because of the boundary scattering effect. At the same time, both the carrier concentration and its mobility were typically increased with the addition of G-rGO in the metal oxides (see Figure 7b,c, respectively). However, in the case of the chalcogenides or skutterudites, the carrier concentration was reported only to change insignificantly, while the mobility of the carriers was not strongly raised in general with the increase of rGO content (see Figure 8b). According to Figure 9, reported values of the thermal conductivity tend to decrease at least until a certain amount of G-rGO as for undoped SrTiO3 (see Figure 9a) [60] as for donor-doped SrTiO3 (see Figure 9b) [62], chalcogenide (see Figure 9c) [42] or skutterudite (see Figure 9d) [30] composites. Such a decrease can be related to the enhanced interface phonon scattering, which is due both to the secondary phase of G-rGO at the grain boundaries of TE materials and to the enlarged number of grain boundaries caused by grain growth inhibition by G-rGO addition. Thus, thermal conductivity shows rather common behavior with G-rGO addition for all three TE material systems overviewed According to Figure 9, reported values of the thermal conductivity tend to decrease at least until a certain amount of G-rGO as for undoped SrTiO3 (see Figure 9a) [60] as for donor-doped SrTiO3 (see Figure 9b) [62], chalcogenide (see Figure 9c) [42] or skutterudite (see Figure 9d) [30] composites. Such a decrease can be related to the enhanced interface phonon scattering, which is due both to the secondary phase of G-rGO at the grain boundaries of TE materials and to the enlarged number of grain boundaries caused by grain growth inhibition by G-rGO addition. Thus, thermal conductivity shows rather According to Figure 9, reported values of the thermal conductivity tend to decrease at least until a certain amount of G-rGO as for undoped SrTiO 3 (see Figure 9a) [60] as for donordoped SrTiO 3 (see Figure 9b) [62], chalcogenide (see Figure 9c) [42] or skutterudite (see Figure 9d) [30] composites. Such a decrease can be related to the enhanced interface phonon scattering, which is due both to the secondary phase of G-rGO at the grain boundaries of TE materials and to the enlarged number of grain boundaries caused by grain growth inhibition by G-rGO addition. Thus, thermal conductivity shows rather common behavior with G-rGO addition for all three TE material systems overviewed here, in contrast to electrical conductivity and the Seebeck coefficient.  ZT was also found to be usually enhanced by G-rGO addition until a certain amount, as shown in Section 2. For further analysis, we have used the published data to calculate the relative increase of ZT as a ratio between the figure of merit difference for TE material without G-rGO (ZTmax without G-rGO), and its composite with G-rGO (ZTmax with G-rGO) in relation to ZTmax without G-rGO: 100% (2) Calculated values of the increment of ZT for chalcogenide, metal oxide and skutterudite TE materials are presented in Tables 4-6 and summarized in Figure 10. ZT was also found to be usually enhanced by G-rGO addition until a certain amount, as shown in Section 2. For further analysis, we have used the published data to calculate the relative increase of ZT as a ratio between the figure of merit difference for TE material without G-rGO (ZT max without G-rGO ), and its composite with G-rGO (ZT max with G-rGO ) in relation to ZT max without G-rGO : Calculated values of the increment of ZT for chalcogenide, metal oxide and skutterudite TE materials are presented in Tables 4-6 and summarized in Figure 10.    It is obviously seen in Figure 10 that the lowest relative increase of ZT is calculated for the composites with the highest initial ZT before the addition of G-rGO and vice versa. In the case that the materials have shown very low ZT before the addition of G-rGO, their composite with graphene or reduced graphene oxide can show a high increment of ZT. Moreover, such a tendency can be found in all chalcogenide, skutterudite or metal oxide TE materials (see Figure 10). Thus, the fabrication of TE composites using the addition of up to 5 wt.% or 1.4 vol.% G or rGO can be an alternative, but not incremental method, for the preparation of high-performance TE materials.

Conclusions
Analyzing the available data on chalcogenide-, skutterudite-and metal oxide-based composites with graphene or reduced graphene oxide, it is possible to conclude that the addition of rather low amount of G-rGO is able to increase their final ZT, mainly enhancing the phonon scattering at grain boundaries of all these materials and increasing the charge carrier concentration and mobility in some of them, particularly in oxide thermoelectrics. However, that is mainly valid in the case of TE materials with low initial ZT. When TE material is already nanostructured and possesses high electrical conductivity, such an addition does not enhance ZT significantly. Thus, G-rGO works mainly as an optimizer of the intrinsic performance of TE materials.   It is obviously seen in Figure 10 that the lowest relative increase of ZT is calculated for the composites with the highest initial ZT before the addition of G-rGO and vice versa. In the case that the materials have shown very low ZT before the addition of G-rGO, their composite with graphene or reduced graphene oxide can show a high increment of ZT. Moreover, such a tendency can be found in all chalcogenide, skutterudite or metal oxide TE materials (see Figure 10). Thus, the fabrication of TE composites using the addition of up to 5 wt.% or 1.4 vol.% G or rGO can be an alternative, but not incremental method, for the preparation of high-performance TE materials.

Conclusions
Analyzing the available data on chalcogenide-, skutterudite-and metal oxide-based composites with graphene or reduced graphene oxide, it is possible to conclude that the addition of rather low amount of G-rGO is able to increase their final ZT, mainly enhancing the phonon scattering at grain boundaries of all these materials and increasing the charge carrier concentration and mobility in some of them, particularly in oxide thermoelectrics. However, that is mainly valid in the case of TE materials with low initial ZT. When TE material is already nanostructured and possesses high electrical conductivity, such an addition does not enhance ZT significantly. Thus, G-rGO works mainly as an optimizer of the intrinsic performance of TE materials.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.