Enhanced Thermoelectric Performances of CNTs-Reinforced Cement Composites with Bi0.5Sb1.5Te3 for Pavement Energy Harvesting

Driven by the huge thermal energy in cement concrete pavements, thermoelectric (TE) cement has attracted considerable attention. However, the current TE cement shows poor performance, which greatly limits its application. Herein, a series of Bi0.5Sb1.5Te3/carbon nanotubes (CNTs) co-reinforced cement composites have been prepared, and their TE properties were systematically investigated. It was shown that the addition of Bi0.5Sb1.5Te3 particles can effectively improve the TE properties of CNTs-reinforced cement composites by building a better conductive network, increasing energy filtering and interfaces scattering. The Bi0.5Sb1.5Te3/CNTs cement composites with 0.6 vol.% of Bi0.5Sb1.5Te3 exhibits the highest ZT value of 1.2 × 10−2, increased by 842 times compared to that of the CNTs-reinforced cement composites without Bi0.5Sb1.5Te3. The power output of this sample with the size of 2.5 × 3.5 × 12 mm3 reaches 0.002 μW at a temperature difference of 19.1 K. These findings shed new light on the development of high-performance TE cement, which can guide continued advances in their potential application of harvesting thermal energy from pavements.


Introduction
Cement is one of the most abundantly produced and basic structural materials, which is widely used in infrastructures, including roads, bridges and buildings [1][2][3]. Due to the low heat dissipation rate of cement, the cement-related infrastructures store large-scale thermal energy after exposure to a large quantity of sun irradiation [4,5]. This results in structural damage in the cement-related infrastructures [6], the growing intensity of urban heat island effect and increasing energy consumption of the air conditioning system in summer [7,8].
In fact, one of the best choices is using thermoelectric (TE) materials to convert largescale thermal energy into green electricity via Seebeck effect [9][10][11][12]. It is well known that the performance of a TE material relies on the dimensionless figure of merit ZT = σα 2 T/κ, where σ, α, κ, and T are the electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature, respectively [13]. Good TE cement should possess high ZT value to improve the conversion capability from heat to electricity. Therefore, it is of significance to maximize the ZT value by enhancing the electrical transport properties (σ and α) and reducing thermal transport properties (κ). From the discovery of the Seebeck effect in carbon-fiber-reinforced concrete [14], numerous research studies have been carried out to explore various high ZT cement-based functional TE materials [15][16][17][18][19][20][21]. Unfortunately, the performances of most kinds of TE cement are less than 1.0 × 10 −5 due to the poor electrical transport properties of the cement matrix.
Using carbon nanotubes (CNTs) as functional additives can significantly increase the electrical conductivity of cement because of their super σ and better conductive channel [22,23]. The σ of cement with 1 wt.% CNTs possessed a theoretical value of 3 S/m, increased by Nanomaterials 2022, 12, 3883 2 of 12 about 3000 times that of the cement without CNTs [24]. However, its α is primitive [25]. Wei et al. investigated the effect of functional additives (e.g., carbon materials [26,27] and metallic oxide [28,29]) on the TE performance of cement and found that the nano-metallic oxide increased the α. Liu et al. [30] discovered that forming a better electric conductive path was found to be very effective in tuning the σ of Bi 2 Te 3 and carbon-fiber-reinforced cement composites. Ghosh et al. [31] reported that a graphene and ZnO nanoparticle double doping method can simultaneously increases the σ and α of cement composites. The relatively high α is due to the quantum confinement effect of the nanostructured ZnO. Besides, the κ of TE materials with multi-scale architecture is lower due to the phonon scattering at grain boundaries [32][33][34]. Therefore, the combination of multi-additives and multi-scale architecture could further improve TE performance.
Herein, we choose CNTs and multi-scale Bi 0.5 Sb 1.5 Te 3 particles as co-reinforced phase in cement composites, taking into account that Bi 0.5 Sb 1.5 Te 3 is the commercial p-type TE materials near room temperature. Remarkably, it has many attractive features, such as excellent σ, considerably large α and low κ [35]. It demonstrates that optimizing the Bi 0.5 Sb 1.5 Te 3 contents can not only enhance the σ values of cement composites by forming a shorter conductive network, but can also adjust α and κ via the enhanced scattering with multi-scale Bi 0.5 Sb 1.5 Te 3 particles. As a result, the largest ZT value of Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites with 0.6% Bi 0.5 Sb 1.5 Te 3 reaches 1.2 × 10 −2 at 400 K, increased by 842 times compared to that of the CNTs-reinforced cement composites without Bi 0.5 Sb 1.5 Te 3 . We also present the power generation performance of the optimized Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites. The results introduce a new promotion of the TE cement application for harvesting green electricity from pavement.

Preparation of Bi 0.5 Sb 1.5 Te 3 /CNTs Cement Composites
The process for producing Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites was as follows. First, multi-scale Bi 0.5 Sb 1.5 Te 3 particles were prepared via the melt-quenching and grounding method; the detailed process was reported elsewhere [36].
Second, multi-scale Bi 0.5 Sb 1.5 Te 3 particles and CNTs were treated by ultrasonic dispersion for 20 minimums in water to obtain Bi 0.5 Sb 1.5 Te 3 /CNTs dispersion. After that, cement, active agent, water reducer, dispersant, and defoamer were added into the dispersion, and then mixed by the mechanical stirrer of 10 minimums to form Bi 0.5 Sb 1.5 Te 3 /CNTs cement pastes. The weights of CNTs, water, active agent, water reducer, dispersant, and defoamer to cement were fixed at 1:1000, 1:2, 1:20, 1:200, 6:1000, and 1:2000. The doping content of multi-scale Bi 0.5 Sb 1.5 Te 3 particles was controlled at the volume fractions of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 vol.% to cement, respectively. Proportions for the cement pastes are listed in Table 1. Final, the prepared cement pastes were poured into a polytetrafluoroethylene die and compacted with a vibrator to obtain dense Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites. Then, the demolded samples were cured in air at the relative humidity of about 95% for 28 days. To eliminate the effects of pore solution on the TE properties, all Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites were dried at 105 • C for 12 h after curing.

Characterization and Measurement
The constituent phases of the samples were determined by powder X-ray diffraction (XRD, PANalytical X' Pert PR) using the Cu K α radiation (λ = 0.15418 nm). The microstructures and chemical compositions were analyzed by using scanning electron microscopy (SEM, TESCAN Mira4) and electron probe micro-analyzer (EPMA, JEOL JXA-8230).
The σ, α, work voltage (V), currents (I), and power output (P) of the as-prepared Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites were measured by a self-made equipment, as shown in Figure 1.
ducer, dispersant, and defoamer to cement were fixed at 1:1000, 1:2, 1:20, 1:200, 6:1000, and 1:2000. The doping content of multi-scale Bi0.5Sb1.5Te3 particles was controlled at the volume fractions of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 vol.% to cement, respectively. Proportions for the cement pastes are listed in Table 1. Final, the prepared cement pastes were poured into a polytetrafluoroethylene die and compacted with a vibrator to obtain dense Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites. Then, the demolded samples were cured in air at the relative humidity of about 95% for 28 days. To eliminate the effects of pore solution on the TE properties, all Bi0.5Sb1.5Te3/CNTs cement composites were dried at 105 °C for 12 h after curing.

Characterization and Measurement
The constituent phases of the samples were determined by powder X-ray diffraction (XRD, PANalytical X' Pert PR) using the Cu Kα radiation (λ = 0.15418 nm). The microstructures and chemical compositions were analyzed by using scanning electron microscopy (SEM, TESCAN Mira4) and electron probe micro-analyzer (EPMA, JEOL JXA-8230).
The σ, α, work voltage (V), currents (I), and power output (P) of the as-prepared Bi0.5Sb1.5Te3/CNTs cement composites were measured by a self-made equipment, as shown in Figure 1.  and TC are the hot-side and cold-side temperature. The P was calculated by using the equation P = VI. The V, ΔV and I were measured by the Source Meter instrument (SMI, Keithley 2400). The hot-side temperature (TH) and cold-side temperature (TC) were recorded by a data-acquisition module, as shown in Figure 1. During σ measurements, the TH and TC were fixed, and the SMI instrument was set to different workloads to measure  . The hot-side temperature (T H ) and cold-side temperature (T C ) were recorded by a data-acquisition module, as shown in Figure 1. During σ measurements, the T H and T C were fixed, and the SMI instrument was set to different workloads to measure the I and V. It was evaluated by using the two outer thermocouples for temperature (T) measurement and the inner electrodes for open circuit voltages (∆V) measurement. During α measurements, the T H and T C were controlled by the heating stage and flowing cold water. When the heating stage and the flowing cold water started working, the ∆T = T H − T C increased. Meanwhile, the SMI instrument was connected to the samples to measure the ∆V. Thus, the σ, α, V, I, and P were obtained.
The κ was calculated using the equation κ = λρC p , where λ is the thermal diffusivity coefficient, C p is the specific heat capacity, and ρ is the bulk density of the material. The λ was measured using a laser-flash technique (Netzsch LFA-427) with the circular block in a flowing Ar atmosphere. The C p was measured using a Q20 differential scanning calorimeter. The ρ was obtained via the standard Archimedes method. Figure 2a shows the SEM image of CNTs. It is obvious that the CNTs display a tubular structure. Diameter distribution of CNTs reveals normal distribution. The average diameter of CNTs is approximately 27 nm (Figure 2b). From the XRD pattern of CNTs (Figure 2c), it can be seen that all the diffraction peaks of CNTs can be indexed to the JCPDS 75-2078 file for C, which reveals that the powder is comprised of single-phase CNTs. Figure 2d and the inset of Figure 2e show the SEM image of the Bi 0.5 Sb 1.5 Te 3 powders. It shows that the multi-scale Bi 0.5 Sb 1.5 Te 3 particles have a wide size distribution from nm to µm. The corresponding average grain size obtained from Figure 2d is approximately 7.8 µm (Figure 2e). The XRD pattern of the Bi 0.5 Sb 1.5 Te 3 powder shows all the diffraction peaks of the Bi 0.5 Sb 1.5 Te 3 powders, which can be indexed to the JCPDS 49-1713 file for Bi 0.5 Sb 1.5 Te 3 ( Figure 2f). This indicates that the single phase Bi 0.5 Sb 1.5 Te 3 has been successfully prepared.

Composition and Structure Characterization
in a flowing Ar atmosphere. The Cp was measured using a Q20 differential scanning calorimeter. The ρ was obtained via the standard Archimedes method. Figure 2a shows the SEM image of CNTs. It is obvious that the CNTs display a tubular structure. Diameter distribution of CNTs reveals normal distribution. The average diameter of CNTs is approximately 27 nm (Figure 2b). From the XRD pattern of CNTs (Figure 2c), it can be seen that all the diffraction peaks of CNTs can be indexed to the JCPDS 75-2078 file for C, which reveals that the powder is comprised of single-phase CNTs. Figure 2d and the inset of Figure 2e show the SEM image of the Bi0.5Sb1.5Te3 powders. It shows that the multi-scale Bi0.5Sb1.5Te3 particles have a wide size distribution from nm to μm. The corresponding average grain size obtained from Figure 2d is approximately 7.8 μm (Figure 2e). The XRD pattern of the Bi0.5Sb1.5Te3 powder shows all the diffraction peaks of the Bi0.5Sb1.5Te3 powders, which can be indexed to the JCPDS 49-1713 file for Bi0.5Sb1.5Te3 (Figure 2f). This indicates that the single phase Bi0.5Sb1.5Te3 has been successfully prepared. SEM images of (a) CNTs and (d) as-prepared Bi0.5Sb1.5Te3 powders. The corresponding size distributions, which were statistically analyzed from SEM images of (a,d), are shown along with (b,e), respectively. XRD pattern of them are also shown in (c) CNTs, and (f) as-prepared Bi0.5Sb1.5Te3 powders. The inset in (e) is the enlarged SEM image of (d). Figure 3 displays the XRD patterns of Bi0.5Sb1.5Te3/CNTs cement composites with different Bi0.5Sb1.5Te3 contents. It can be seen that all samples consist of portlandite (Ca(OH)2), ettringite (AFt), calcium silicate hydrate (C-S-H gel), alite (C3S), belite (C2S) and quartz (SiO2). However, the diffraction peaks of Bi0.5Sb1.5Te3 and CNTs are not detected, because their content is lower than the detection limit of the XRD method.   It is noted that the intensities of the peaks relating to portlandite, ettringite, calcium silicate hydrate, alite, belite, and silica change. The main peak intensities of portlandite at 18.1°, 34.1° increase with the increasing contents of Bi0.5Sb1.5Te3. The same trends of other hydrated products (ettringite, calcium silicate hydrate) are also shown in Bi0.5Sb1.5Te3/CNTs cement composites, whereas the peak intensities of non-hydrated products (alite, belite and quartz) gradually decrease with the increasing contents of Bi0.5Sb1.5Te3, indicating that multi-scale Bi0.5Sb1.5Te3 particles may accelerate the hydration of Bi0.5Sb1.5Te3/CNTs cement composites. Similar results can be found in Ref. [37]. Figure 4 displays the BEI photographs of the Bi0.5Sb1.5Te3/CNTs cement composites. It can be seen that the brightest objects are multi-scale Bi0.5Sb1.5Te3 particles and the darkest objects are CNTs. More multi-scale Bi0.5Sb1.5Te3 particles appear when the contents of Bi0.5Sb1.5Te3 increase from 0.0 vol.% to 1.0 vol.%. Significantly, the CNTs are dispersed uniformly for the samples at lower Bi0.5Sb1.5Te3 contents, but accumulate together for the samples with the 0.8 vol.% and 1.0 vol.% Bi0.5Sb1.5Te3, implying that higher Bi0.5Sb1.5Te3 particles content (≥0.8 vol.%) may cause a serious aggregation of CNTs during the cement hydration process.   It is noted that the intensities of the peaks relating to portlandite, ettringite, calcium silicate hydrate, alite, belite, and silica change. The main peak intensities of portlandite at 18.1 • , 34.1 • increase with the increasing contents of Bi 0.5 Sb 1.5 Te 3 . The same trends of other hydrated products (ettringite, calcium silicate hydrate) are also shown in Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites, whereas the peak intensities of non-hydrated products (alite, belite and quartz) gradually decrease with the increasing contents of Bi 0.5 Sb 1.5 Te 3 , indicating that multi-scale Bi 0.5 Sb 1.5 Te 3 particles may accelerate the hydration of Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites. Similar results can be found in Ref. [37].  It is noted that the intensities of the peaks relating to portlandite, ettringite, calcium silicate hydrate, alite, belite, and silica change. The main peak intensities of portlandite at 18.1°, 34.1° increase with the increasing contents of Bi0.5Sb1.5Te3. The same trends of other hydrated products (ettringite, calcium silicate hydrate) are also shown in Bi0.5Sb1.5Te3/CNTs cement composites, whereas the peak intensities of non-hydrated products (alite, belite and quartz) gradually decrease with the increasing contents of Bi0.5Sb1.5Te3, indicating that multi-scale Bi0.5Sb1.5Te3 particles may accelerate the hydration of Bi0.5Sb1.5Te3/CNTs cement composites. Similar results can be found in Ref. [37]. Figure 4 displays the BEI photographs of the Bi0.5Sb1.5Te3/CNTs cement composites. It can be seen that the brightest objects are multi-scale Bi0.5Sb1.5Te3 particles and the darkest objects are CNTs. More multi-scale Bi0.5Sb1.5Te3 particles appear when the contents of Bi0.5Sb1.5Te3 increase from 0.0 vol.% to 1.0 vol.%. Significantly, the CNTs are dispersed uniformly for the samples at lower Bi0.5Sb1.5Te3 contents, but accumulate together for the samples with the 0.8 vol.% and 1.0 vol.% Bi0.5Sb1.5Te3, implying that higher Bi0.5Sb1.5Te3 particles content (≥0.8 vol.%) may cause a serious aggregation of CNTs during the cement hydration process.    The C-rich areas can be found in Figure 5b,e, suggesting that these areas contain the CNTs phase. Te-rich areas can be found in Figure 5f, indicating that these areas contain Bi 0.5 Sb 1.5 Te 3 . For the sample without Bi 0.5 Sb 1.5 Te 3 particles, C is uniformly distributed, as shown in Figure 5b. However, when the content of Bi 0.5 Sb 1.5 Te 3 is 1.0 vol.%, the distribution of the CNTs in Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites become inhomogeneous, as shown in Figure 5e. This further reveals that the excessive Bi 0.5 Sb 1.5 Te 3 particles can lead to the aggregation of CNTs during hydration reaction. vol.%. The C-rich areas can be found in Figure 5b,e, suggesting that these areas contain the CNTs phase. Te-rich areas can be found in Figure 5f, indicating that these areas contain Bi0.5Sb1.5Te3. For the sample without Bi0.5Sb1.5Te3 particles, C is uniformly distributed, as shown in Figure 5b. However, when the content of Bi0.5Sb1.5Te3 is 1.0 vol.%, the distribution of the CNTs in Bi0.5Sb1.5Te3/CNTs cement composites become inhomogeneous, as shown in Figure 5e. This further reveals that the excessive Bi0.5Sb1.5Te3 particles can lead to the aggregation of CNTs during hydration reaction.  Figure 6a displays the electrical conductivity of Bi0.5Sb1.5Te3/CNTs cement composites at different Bi0.5Sb1.5Te3 contents. It can be seen that the σ increases with the increase in the Bi0.5Sb1.5Te3 content. The sample with 1.0 vol.% Bi0.5Sb1.5Te3 exhibits the largest σ value, which increases by 284 times compared to the sample without Bi0.5Sb1.5Te3. The α of Bi0.5Sb1.5Te3/CNTs cement composites are shown in Figure 6b. The positive α values indicate that the majority of the carriers of the Bi0.5Sb1.5Te3/CNTs cement composites are holes, exhibiting p-type conduction. α first increased and then decreased in the temperature range of 300 K to 400 K as the content of Bi0.5Sb1.5Te3 increased. The first increase in α of the samples means that the Bi0.5Sb1.5Te3 multi-scale particles have a positive effect on enhancing the electrical transport properties of CNTs-reinforced cement composites. However, the α of the samples decreased with further increases in the Bi0.5Sb1.5Te3 contents. Combining the evolution features of microstructure from BEI results, the CNTs are uniformly dispersed at lower Bi0.5Sb1.5Te3 contents, but accumulate together for the samples with the Bi0.5Sb1.5Te3 contents of 0.8 vol.% and 1.0 vol.%. We be- vol.%. The C-rich areas can be found in Figure 5b,e, suggesting that these areas contain the CNTs phase. Te-rich areas can be found in Figure 5f, indicating that these areas contain Bi0.5Sb1.5Te3. For the sample without Bi0.5Sb1.5Te3 particles, C is uniformly distributed, as shown in Figure 5b. However, when the content of Bi0.5Sb1.5Te3 is 1.0 vol.%, the distribution of the CNTs in Bi0.5Sb1.5Te3/CNTs cement composites become inhomogeneous, as shown in Figure 5e. This further reveals that the excessive Bi0.5Sb1.5Te3 particles can lead to the aggregation of CNTs during hydration reaction.  Figure 6a displays the electrical conductivity of Bi0.5Sb1.5Te3/CNTs cement composites at different Bi0.5Sb1.5Te3 contents. It can be seen that the σ increases with the increase in the Bi0.5Sb1.5Te3 content. The sample with 1.0 vol.% Bi0.5Sb1.5Te3 exhibits the largest σ value, which increases by 284 times compared to the sample without Bi0.5Sb1.5Te3. The α of Bi0.5Sb1.5Te3/CNTs cement composites are shown in Figure 6b. The positive α values indicate that the majority of the carriers of the Bi0.5Sb1.5Te3/CNTs cement composites are holes, exhibiting p-type conduction. α first increased and then decreased in the temperature range of 300 K to 400 K as the content of Bi0.5Sb1.5Te3 increased. The first increase in α of the samples means that the Bi0.5Sb1.5Te3 multi-scale particles have a positive effect on enhancing the electrical transport properties of CNTs-reinforced cement composites. However, the α of the samples decreased with further increases in the Bi0.5Sb1.5Te3 contents. Combining the evolution features of microstructure from BEI results, the CNTs are uniformly dispersed at lower Bi0.5Sb1.5Te3 contents, but accumulate together for the samples with the Bi0.5Sb1.5Te3 contents of 0.8 vol.% and 1.0 vol.%. We be- The α of Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites are shown in Figure 6b. The positive α values indicate that the majority of the carriers of the Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites are holes, exhibiting p-type conduction. α first increased and then decreased in the temperature range of 300 K to 400 K as the content of Bi 0.5 Sb 1.5 Te 3 increased. The first increase in α of the samples means that the Bi 0.5 Sb 1.5 Te 3 multi-scale particles have a positive effect on enhancing the electrical transport properties of CNTs-reinforced cement composites. However, the α of the samples decreased with further increases in the Bi 0.5 Sb 1.5 Te 3 contents. Combining the evolution features of microstructure from BEI results, the CNTs are uniformly dispersed at lower Bi 0.5 Sb 1.5 Te 3 contents, but accumulate together for the samples with the Bi 0.5 Sb 1.5 Te 3 contents of 0.8 vol.% and 1.0 vol.%. We believe that the decrease in α at the Bi 0.5 Sb 1.5 Te 3 contents of 0.8 vol.% and 1.0 vol.% comes from a positive effect of CNTs aggregation. The sample with 0.6 vol.% Bi 0.5 Sb 1.5 Te 3 shows the largest α of 40.66 µV/K at 350 K.

TE Properties of Bi0.5Sb1.5Te3/CNTs Cement Composites
The power factor (α 2 σ) is plotted as a function of the Bi 0.5 Sb 1.5 Te 3 content (Figure 6c), which is calculated via the measured electrical conductivity and Seebeck coefficient. A dramatic enhancement in α 2 σ is obtained in the sample with the 0.6 vol.% Bi 0.5 Sb 1.5 Te 3 at 400 K due to the remarkable increase in α and σ. This indicates that optimizing the doping content of Bi 0.5 Sb 1.5 Te 3 particles may effectively improve the electrical transport properties of Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites.
The κ of Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites are shown in Figure 7a. The κ of Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites first increases with the increase in the content of Bi 0.5 Sb 1.5 Te 3 . However, the κ decreases with further increases in the content of Bi 0.5 Sb 1.5 Te 3 in the range of 0.6 vol.% to 1.0 vol.%, revealing that excessive Bi 0.5 Sb 1.5 Te 3 particles could lead to a lower κ. The power factor (α 2 σ) is plotted as a function of the Bi0.5Sb1.5Te3 content (Figure 6c), which is calculated via the measured electrical conductivity and Seebeck coefficient. A dramatic enhancement in α 2 σ is obtained in the sample with the 0.6 vol.% Bi0.5Sb1.5Te3 at 400 K due to the remarkable increase in α and σ. This indicates that optimizing the doping content of Bi0.5Sb1.5Te3 particles may effectively improve the electrical transport properties of Bi0.5Sb1.5Te3/CNTs cement composites.
The κ of Bi0.5Sb1.5Te3/CNTs cement composites are shown in Figure 7a. The κ of Bi0.5Sb1.5Te3/CNTs cement composites first increases with the increase in the content of Bi0.5Sb1.5Te3. However, the κ decreases with further increases in the content of Bi0.5Sb1.5Te3 in the range of 0.6 vol.% to 1.0 vol.%, revealing that excessive Bi0.5Sb1.5Te3 particles could lead to a lower κ. As a result, the ZT values (Figure 7b) first increases and then decreases with increases in the content of Bi0.5Sb1.5Te3. The largest ZT reaches 1.2 × 10 −2 at 400 K for the Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites at 0.6 vol.% Bi0.5Sb1.5Te3, increasing by 842 times compared to the sample without Bi0.5Sb1.5Te3 particles. The dramatic enhancement in the ZT values is obtained in the sample with 0.6 vol.% Bi0.5Sb1.5Te3 due to the remarkable increase in σ and α. This work demonstrates that the addition of multi-scale Bi0.5Sb1.5Te3 particles is an effective approach to enhance the TE performance of CNTs-reinforced cement composites.

Evaluation of Bi0.5Sb1.5Te3/CNTs Cement Composites in Energy Harvesting from the Pavement
To demonstrate the potential application of the Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites, we measure the power generation performance of Bi0.5Sb1.5Te3/CNTs cement composites at 0.6 vol% Bi0.5Sb1.5Te3 with the size of 2.5 × 3.5 × 12 mm 3 . Figure 8a shows temperature difference (ΔT) dependences of the generated open-circuit voltage (Vo) for Bi0.5Sb1.5Te3/CNTs cement composites at 0.6 vol.% Bi0.5Sb1.5Te3, where the cold-side temperature (TC) remains at 295.7 K. When the ΔT increases to 288 K and 303 K, the Vo is about 2.0 mV and 4.4 mV, respectively.  As a result, the ZT values (Figure 7b) first increases and then decreases with increases in the content of Bi 0.5 Sb 1.5 Te 3 . The largest ZT reaches 1.2 × 10 −2 at 400 K for the Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites at 0.6 vol.% Bi 0.5 Sb 1.5 Te 3 , increasing by 842 times compared to the sample without Bi 0.5 Sb 1.5 Te 3 particles. The dramatic enhancement in the ZT values is obtained in the sample with 0.6 vol.% Bi 0.5 Sb 1.5 Te 3 due to the remarkable increase in σ and α. This work demonstrates that the addition of multiscale Bi 0.5 Sb 1.5 Te 3 particles is an effective approach to enhance the TE performance of CNTs-reinforced cement composites.

Evaluation of Bi 0.5 Sb 1.5 Te 3 /CNTs Cement Composites in Energy Harvesting from the Pavement
To demonstrate the potential application of the Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites, we measure the power generation performance of Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites at 0.6 vol% Bi 0.5 Sb 1.5 Te 3 with the size of 2.5 × 3.5 × 12 mm 3 . Figure 8a shows temperature difference (∆T) dependences of the generated opencircuit voltage (Vo) for Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites at 0.6 vol.% Bi 0.5 Sb 1.5 Te 3 , where the cold-side temperature (T C ) remains at 295.7 K. When the ∆T increases to 288 K and 303 K, the Vo is about 2.0 mV and 4.4 mV, respectively. Figure 8b  The Pmax value of Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites is not higher, but the amount of cement-related infrastructure on the earth is huge. For instance, when the temperature reduction in Bi0.5Sb1.5Te3/CNTs co-reinforced cement concrete pavements of 1 km in length, 10 m in width and 20 cm in depth is 19.1 K, the generated electricity remains at about 38 W. With further development of TE cement, the power output of TE cement concrete pavements is likely to be enhanced, which will further promote the application of TE technology in harvesting green electricity.

Conducting Behavior of Bi0.5Sb1.5Te3/CNTs Co-Reinforced Cement Composites
To clarify the conductive behavior of Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites, we investigate the C and Te elemental maps of the samples with 1.0 vol.% Bi0.5Sb1.5Te3. The analyses are carried out by extracting C and Te distribution from their elemental maps, and then overlaying Te onto C to explore the conductive network, as shown in Figure 9. When CNTs are dispersed in cement materials, some CNTs come into contact with each other and form a local conductive network. After addition of multi-scale Bi0.5Sb1.5Te3 particles, a tortuous but better conductive path can be formed between Bi0.5Sb1.5Te3 and CNTs. Therefore, the significant enhancement in the electrical conductivity should be attributed to the occurrence of a better conductive network in Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites. Figure 9. The elemental mapping of (a) C and (c) Te of Bi0.5Sb1.5Te3/CNTs cement composites at 1.0 vol.% Bi0.5Sb1.5Te3. The corresponding elemental distributions, which were extracted from (a,c), are shown in (b,d). The elemental distribution of (d) Te is overlayed on the elemental distribution of (b) C to explore the conductive network with (e) C+Te. The P max value of Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites is not higher, but the amount of cement-related infrastructure on the earth is huge. For instance, when the temperature reduction in Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement concrete pavements of 1 km in length, 10 m in width and 20 cm in depth is 19.1 K, the generated electricity remains at about 38 W. With further development of TE cement, the power output of TE cement concrete pavements is likely to be enhanced, which will further promote the application of TE technology in harvesting green electricity.

Conducting Behavior of Bi 0.5 Sb 1.5 Te 3 /CNTs Co-Reinforced Cement Composites
To clarify the conductive behavior of Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites, we investigate the C and Te elemental maps of the samples with 1.0 vol.% Bi 0.5 Sb 1.5 Te 3 . The analyses are carried out by extracting C and Te distribution from their elemental maps, and then overlaying Te onto C to explore the conductive network, as shown in Figure 9. When CNTs are dispersed in cement materials, some CNTs come into contact with each other and form a local conductive network. After addition of multi-scale Bi 0.5 Sb 1.5 Te 3 particles, a tortuous but better conductive path can be formed between Bi 0.5 Sb 1.5 Te 3 and CNTs. Therefore, the significant enhancement in the electrical conductivity should be attributed to the occurrence of a better conductive network in Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites. The Pmax value of Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites is not higher, but the amount of cement-related infrastructure on the earth is huge. For instance, when the temperature reduction in Bi0.5Sb1.5Te3/CNTs co-reinforced cement concrete pavements of 1 km in length, 10 m in width and 20 cm in depth is 19.1 K, the generated electricity remains at about 38 W. With further development of TE cement, the power output of TE cement concrete pavements is likely to be enhanced, which will further promote the application of TE technology in harvesting green electricity.

Conducting Behavior of Bi0.5Sb1.5Te3/CNTs Co-Reinforced Cement Composites
To clarify the conductive behavior of Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites, we investigate the C and Te elemental maps of the samples with 1.0 vol.% Bi0.5Sb1.5Te3. The analyses are carried out by extracting C and Te distribution from their elemental maps, and then overlaying Te onto C to explore the conductive network, as shown in Figure 9. When CNTs are dispersed in cement materials, some CNTs come into contact with each other and form a local conductive network. After addition of multi-scale Bi0.5Sb1.5Te3 particles, a tortuous but better conductive path can be formed between Bi0.5Sb1.5Te3 and CNTs. Therefore, the significant enhancement in the electrical conductivity should be attributed to the occurrence of a better conductive network in Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites.   Figure 10 shows the SEM images and EDS of Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites at 0.6 vol.% Bi 0.5 Sb 1.5 Te 3 . It can be seen that the fracture section of the samples is compact. The enlarged SEM image clearly indicates that a great variety of tube-like CNTs are distributed in the rough grain boundary of Bi 0.5 Sb 1.5 Te 3 in cement composites, as shown in Figure 10b. The randomly bridged Bi 0.5 Sb 1.5 Te 3 and CNTs in the co-reinforced cement composites can lead to a significant enhancement in the electrical conductivity via a better conductive network. The rough surfaces are very beneficial, decreasing the thermal conductivity by increasing the boundary scattering of phonons. In addition, the multiscale microstructures will serve as energy-dependent selective scattering centers to filter the low-energy carriers and improve the α. Hu et al. [38] observed the same phenomenon in the Bi 2 Te 3 -based TE materials. Thus, it can be concluded that the tunneling effect and phonons scattering in the co-reinforced cement composites are enhanced because of the presence of randomly arranged multi-scale Bi 0.5 Sb 1.5 Te 3 particles, which arises from enhanced conductive network, energy-dependent selective scattering and phonon scattering in the Bi 0.5 Sb 1.5 Te 3 /CNTs-co-reinforced cement composites.  Figure 10 shows the SEM images and EDS of Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites at 0.6 vol.% Bi0.5Sb1.5Te3. It can be seen that the fracture section of the samples is compact. The enlarged SEM image clearly indicates that a great variety of tube-like CNTs are distributed in the rough grain boundary of Bi0.5Sb1.5Te3 in cement composites, as shown in Figure 10b. The randomly bridged Bi0.5Sb1.5Te3 and CNTs in the co-reinforced cement composites can lead to a significant enhancement in the electrical conductivity via a better conductive network. The rough surfaces are very beneficial, decreasing the thermal conductivity by increasing the boundary scattering of phonons. In addition, the multiscale microstructures will serve as energy-dependent selective scattering centers to filter the low-energy carriers and improve the α. Hu et al. [38] observed the same phenomenon in the Bi2Te3-based TE materials. Thus, it can be concluded that the tunneling effect and phonons scattering in the co-reinforced cement composites are enhanced because of the presence of randomly arranged multi-scale Bi0.5Sb1.5Te3 particles, which arises from enhanced conductive network, energy-dependent selective scattering and phonon scattering in the Bi0.5Sb1.5Te3/CNTs-co-reinforced cement composites.

Mechanism of the Enhancement in TE Performance of Bi0.5Sb1.5Te3/CNTs Co-Reinforced Cement Composites
To better understand the mechanism of enhanced TE performance of CNTs reinforced cement composites with multi-scale Bi0.5Sb1.5Te3 particles, we draw up the schematic shown in Figure 11. For cement composites, the electrical conduction involves ions, electrons, and/or holes. The main conduction of pure cement is ionic with free water solution, including Ca 2+ , Na + , K + , OH − , SO4 2− . In this work, the cured cement composites were dried for at 105 °C for 12 h before the test. Under such circumstances, the cement paste is almost an insulating material due to no free water. When CNTs are dispersed in cement materials, some CNTs come into contact with each other to form a conductive network. The carriers (electrons and holes) will move along the conductive network or pass through the adjacent disconnected CNTs based on tunneling theory [39], which leads to an increment in σ of CNTs-reinforced cement composites. After addition of multi-scale Bi0.5Sb1.5Te3 particles, a tortuous but better conductive network can To better understand the mechanism of enhanced TE performance of CNTs reinforced cement composites with multi-scale Bi 0.5 Sb 1.5 Te 3 particles, we draw up the schematic shown in Figure 11. For cement composites, the electrical conduction involves ions, electrons, and/or holes. The main conduction of pure cement is ionic with free water solution, including Ca 2+ , Na + , K + , OH − , SO 4 2− . In this work, the cured cement composites were dried for at 105 • C for 12 h before the test. Under such circumstances, the cement paste is almost an insulating material due to no free water. When CNTs are dispersed in cement materials, some CNTs come into contact with each other to form a conductive network. The carriers (electrons and holes) will move along the conductive network or pass through the adjacent disconnected CNTs based on tunneling theory [39], which leads to an increment in σ of CNTs-reinforced cement composites. After addition of multi-scale Bi 0.5 Sb 1.5 Te 3 particles, a tortuous but better conductive network can be formed in Bi 0.5 Sb 1.5 Te 3 and CNTs. Hence, the electrical conductivity of Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites is further increased. be formed in Bi0.5Sb1.5Te3 and CNTs. Hence, the electrical conductivity of Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites is further increased. Figure 11. A schematic mechanism for multi-scale Bi0.5Sb1.5Te3 particles in enhancing the TE performance of CNTs reinforced cement composites.
Moreover, the multi-scale Bi0.5Sb1.5Te3 particles can enhance the low-energy electron filtering effect, thus improving the α of co-reinforced cement composites. However, the excessive Bi0.5Sb1.5Te3 particles lead to the aggregation of CNTs, thus lowering the energy-filtering effect of CNTs and α of co-reinforced cement composites. Additionally, for the multi-scale Bi0.5Sb1.5Te3 particles, there are a number of interfaces that can scatter the phonon, and thus lower the κ of Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites. Consequently, the multi-scale Bi0.5Sb1.5Te3 particles embedded in the CNTs-reinforced cement composites increase σ and α and lower κ. The increased ZT of Bi0.5Sb1.5Te3/CNTs co-reinforced cement composites should be responsible for the shorter conductive network, energy filtering and interfaces scattering by the Bi0.5Sb1.5Te3 multi-scale particles.

Conclusions
In this paper, a series of Bi0.5Sb1.5Te3/CNTs cement composites were prepared via a co-reinforced method, and their TE properties were systematically investigated. It was discovered that the Bi0.5Sb1.5Te3/CNTs-co-reinforced cement composites with 0.6 vol.% Bi0.5Sb1.5Te3 exhibit the highest ZT value of 1.2 × 10 −2 . The enhanced ZT value mainly originates from the formation of shorter conductive network, increased energy filtering, and defects scattering by multi-scale Bi0.5Sb1.5Te3 particles. The power output of this sample with the size of 2.5 × 3.5 × 12 mm 3 reaches 0.002 μW at a temperature difference of 19.1 K. This work introduces a successful route to develop high-performance TE cement, which can be extended to harvest thermal energy in pavements.
Author Contributions: Software, data curation, writing-original draft preparation, writing-review and editing, funding acquisition, H.Z.; methodology, investigation, H.L.; writing-review and editing, formal analysis, G.Q.; writing-review and editing, validation, P.X., H.Y. and J.C.; conceptualization and visualization, J.Z. All authors have read and agreed to the published version of the manuscript.  Moreover, the multi-scale Bi 0.5 Sb 1.5 Te 3 particles can enhance the low-energy electron filtering effect, thus improving the α of co-reinforced cement composites. However, the excessive Bi 0.5 Sb 1.5 Te 3 particles lead to the aggregation of CNTs, thus lowering the energyfiltering effect of CNTs and α of co-reinforced cement composites. Additionally, for the multi-scale Bi 0.5 Sb 1.5 Te 3 particles, there are a number of interfaces that can scatter the phonon, and thus lower the κ of Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites. Consequently, the multi-scale Bi 0.5 Sb 1.5 Te 3 particles embedded in the CNTs-reinforced cement composites increase σ and α and lower κ. The increased ZT of Bi 0.5 Sb 1.5 Te 3 /CNTs co-reinforced cement composites should be responsible for the shorter conductive network, energy filtering and interfaces scattering by the Bi 0.5 Sb 1.5 Te 3 multi-scale particles.

Conclusions
In this paper, a series of Bi 0.5 Sb 1.5 Te 3 /CNTs cement composites were prepared via a co-reinforced method, and their TE properties were systematically investigated. It was discovered that the Bi 0.5 Sb 1.5 Te 3 /CNTs-co-reinforced cement composites with 0.6 vol.% Bi 0.5 Sb 1.5 Te 3 exhibit the highest ZT value of 1.2 × 10 −2 . The enhanced ZT value mainly originates from the formation of shorter conductive network, increased energy filtering, and defects scattering by multi-scale Bi 0.5 Sb 1.5 Te 3 particles. The power output of this sample with the size of 2.5 × 3.5 × 12 mm 3 reaches 0.002 µW at a temperature difference of 19.1 K. This work introduces a successful route to develop high-performance TE cement, which can be extended to harvest thermal energy in pavements.