Cement-Based Thermoelectric Device for Protection of Carbon Steel in Alkaline Chloride Solution

The thermoelectric cement-based materials can convert heat into electricity; this makes them promising candidates for impressed current cathodic protection of carbon steel. However, attempts to use the thermoelectric cement-based materials for energy conversion usually results in low conversion efficiency, because of the low electrical conductivity and Seebeck coefficient. Herein, we deposited polyaniline on the surface of MnO2 and fabricated a cement-based thermoelectric device with added PANI/MnO2 composite for the protection of carbon steel in alkaline chloride solution. The nanorod structure (70~80 nm in diameter) and evenly dispersed conductive PANI provide the PANI/MnO2 composite with good electrical conductivity (1.9 ± 0.03 S/cm) and Seebeck coefficient (−7.71 × 103 ± 50 μV/K) and, thereby, increase the Seebeck coefficient of cement-based materials to −2.02 × 103 ± 40 μV/K and the electrical conductivity of cement-based materials to 0.015 ± 0.0003 S/cm. Based on this, the corrosion of the carbon steel was delayed after cathodic protection, which was demonstrated by the electrochemical experiment results, such as the increased resistance of the carbon steel surface from 5.16 × 102 Ω·cm2 to 5.14 × 104 Ω·cm2, increased charge transfer resistance from 11.4 kΩ·cm2 to 1.98 × 106 kΩ·cm2, and the decreased corrosion current density from 1.67 μA/cm2 to 0.32 μA/cm2, underlining the role of anti-corrosion of the PANI/MnO2 composite in the cathodic protection system.


Introduction
Reinforced concrete is the most widely used material in marine engineering, but chloride ions migrate into porous concrete from the marine environment and cause steel corrosion. This problem would lead to the deterioration and failure of reinforced concrete structures and ultimately reduce the safe service of marine engineering [1,2]. Cathodic protection proves to be an effective method for protecting the reinforced concrete vulnerable to chloride pollution [3]. However, a series of problems, such as the consumption of metal materials, the waste of electric energy, and environmental pollution, would take place in traditional cathodic protection [4,5].
To solve the above problems, solar cells, closed-cycle steam generators, thermoelectric generators, and other sustainable energy sources have been applied in cathodic protection [6][7][8]. Thermoelectric generators made of BiSb, Bi 2 Te 3 , PbTe, SiGe, and other alloys can convert heat into electricity when there is a temperature difference. However, these The electrical conductivity of the PANI/MnO 2 composite and cement-matrix composites containing the PANI/MnO 2 composite was measured by a four-probe conductivity tester (FT-301, Rooko, Ningbo, China) at ambient temperature, where two copper plates adhered to the sample sides worked as current contacts and two copper meshes inserted into the sample worked as voltage connects. Thus, the electrical conductivity can be calculated by the obtained current, voltage, and sizes of the PANI/MnO 2 composite or the cement-matrix composites containing the PANI/MnO 2 composite sample. The apparatus and method used to measure the Seebeck coefficient of the PANI/MnO 2 composite or the cement-matrix composites containing the PANI/MnO 2 composite sample were the same as those used for MnO 2 powder in our previous study, as shown in Figure 1 [23].
The PANI/MnO 2 composite was added into a special plastic pipe and pressed to 5.0 MPa. The cement-matrix composites were grinded with emery papers (grade 100 and 600) to make the surfaces flat. The copper sheets and the sample were connected naturally by the gravity of the water tank on the top of the sample. The applied pressure of the water tank was approximately 2.5 MPa. In the apparatus, one copper sheet at the end of the sample was connected to a disciform resistance heater (0.05 K/s) and another copper sheet at the end was connected to flowing cold water. In addition, the two copper sheets were connected to a Fluke 289 C multimeter and two Type K thermocouples by copper wires to test the Seebeck voltage and temperature difference, respectively. Each determination was conducted five times to avoid accidental error. cement-matrix composites containing the PANI/MnO2 composite sample. The a and method used to measure the Seebeck coefficient of the PANI/MnO2 compos cement-matrix composites containing the PANI/MnO2 composite sample were as those used for MnO2 powder in our previous study, as shown in Figure 1 PANI/MnO2 composite was added into a special plastic pipe and pressed to 5.0 cement-matrix composites were grinded with emery papers (grade 100 and 600 the surfaces flat. The copper sheets and the sample were connected naturally by ity of the water tank on the top of the sample. The applied pressure of the water approximately 2.5 MPa. In the apparatus, one copper sheet at the end of the sa connected to a disciform resistance heater (0.05 K/s) and another copper sheet a was connected to flowing cold water. In addition, the two copper sheets were c to a Fluke 289 C multimeter and two Type K thermocouples by copper wires t Seebeck voltage and temperature difference, respectively. Each determination ducted five times to avoid accidental error.

Preparation of Cement-Based Thermoelectric Device for Cathodic Protection
The cement-based thermoelectric device mainly included cement-paste blo copper blocks, copper wires, and ceramic laminates, as shown in Figure 2. In th 24 hardened cement-paste blocks (40 × 40 × 160 mm) were connected in series b wire. A layer of silicone grease (3875, SINWE, Shenzhen, China) was applied be cement-paste block and the copper block to reduce the thermal resistance. In add side of the cement-paste block was covered with a layer of foam insulation to re loss during heat transfer.

Preparation of Cement-Based Thermoelectric Device for Cathodic Protection
The cement-based thermoelectric device mainly included cement-paste blocks (M6), copper blocks, copper wires, and ceramic laminates, as shown in Figure 2. In the device, 24 hardened cement-paste blocks (40 × 40 × 160 mm) were connected in series by copper wire. A layer of silicone grease (3875, SINWE, Shenzhen, China) was applied between the cement-paste block and the copper block to reduce the thermal resistance. In addition, the side of the cement-paste block was covered with a layer of foam insulation to reduce heat loss during heat transfer. PANI/MnO2 composite was added into a special plastic pipe and pressed to 5.0 cement-matrix composites were grinded with emery papers (grade 100 and 600 the surfaces flat. The copper sheets and the sample were connected naturally by ity of the water tank on the top of the sample. The applied pressure of the water approximately 2.5 MPa. In the apparatus, one copper sheet at the end of the sa connected to a disciform resistance heater (0.05 K/s) and another copper sheet a was connected to flowing cold water. In addition, the two copper sheets were c to a Fluke 289 C multimeter and two Type K thermocouples by copper wires t Seebeck voltage and temperature difference, respectively. Each determination ducted five times to avoid accidental error.

Preparation of Cement-Based Thermoelectric Device for Cathodic Protection
The cement-based thermoelectric device mainly included cement-paste blo copper blocks, copper wires, and ceramic laminates, as shown in Figure 2. In th 24 hardened cement-paste blocks (40 × 40 × 160 mm) were connected in series b wire. A layer of silicone grease (3875, SINWE, Shenzhen, China) was applied bet cement-paste block and the copper block to reduce the thermal resistance. In add side of the cement-paste block was covered with a layer of foam insulation to re loss during heat transfer.   The cathodic protection system mainly included a cement-based thermoelectric device, low thermostat, water bath, thermocouple, titanium mesh, and carbon steel electrode, as shown in Figure 3. A water bath, low thermostat, and thermocouple were used to maintain a temperature difference of 20 K between the two opposite surfaces of the cement-based thermoelectric device. The positive pole and negative pole were connected to the titanium mesh and carbon steel electrode, respectively. The carbon steel electrode was prepared by welding a carbon steel block (1.00 × 1.00 × 1.00 cm) to a copper wire. Five surfaces of the carbon steel block were coated by epoxy resin, and the rest of the surface was ground with emery paper (grade 1000). The titanium mesh and carbon steel electrode were immersed in the solution. Thus, the carbon steel electrode can be protected by the impressed current from the thermoelectric module. The simulated concrete pore solution was obtained by diluting an alkaline solution (0.6 mol/L KOH + 0.2 mol/L NaOH + 0.01 mol/L Ca(OH) 2 ) until the pH decreased to 12.5. To simulate the corrosion condition of seawater, 3.5 wt.% NaCl was added into the alkaline solution. The 3.5 wt.% NaCl-polluted simulated concrete pore solution was named NSCS. The cathodic protection system mainly included a cement-based thermoelectric device, low thermostat, water bath, thermocouple, titanium mesh, and carbon steel electrode, as shown in Figure 3. A water bath, low thermostat, and thermocouple were used to maintain a temperature difference of 20 K between the two opposite surfaces of the cement-based thermoelectric device. The positive pole and negative pole were connected to the titanium mesh and carbon steel electrode, respectively. The carbon steel electrode was prepared by welding a carbon steel block (1.00 × 1.00 × 1.00 cm) to a copper wire. Five surfaces of the carbon steel block were coated by epoxy resin, and the rest of the surface was ground with emery paper (grade 1000). The titanium mesh and carbon steel electrode were immersed in the solution. Thus, the carbon steel electrode can be protected by the impressed current from the thermoelectric module. The simulated concrete pore solution was obtained by diluting an alkaline solution (0.6 mol/L KOH + 0.2 mol/L NaOH + 0.01 mol/L Ca(OH)2) until the pH decreased to 12.5. To simulate the corrosion condition of seawater, 3.5 wt.% NaCl was added into the alkaline solution. The 3.5 wt.% NaCl-polluted simulated concrete pore solution was named NSCS.

Electrochemical Experiments
The open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and polarization potentiodynamics (PP) of the carbon steel electrode were tested with a PAR-STAT 2273 potentiostat/galvanostat in NSCS. A saturated calomel electrode and a platinum slice were immersed in the solution and used as the reference electrode and counter electrode, respectively.
The OCP of the carbon steel electrode was carried out once the cathodic protection was suspended and stopped when the potential did not change by more than 2 mV in 300 s. The EIS of the carbon steel electrode was carried out with sine wave circuit excitation (amplitude: 10 mV; frequency: 10 5 -10 −2 Hz). After that, the PP measurement was carried out with a scan rate of 1 mV s −1 from −250 mV to +250 mV versus the obtained potential from OCP.

Characterizations of PANI/MnO2 Composite
The PANI/MnO2 powders prepared with five different dosages of MnO2 were characterized by FT-IR. The obtained results showed that the dose of MnO2 powder had no effect on the location of the characteristic absorption peaks. Therefore, any one of the five synthesized products can be used to analyze the structure. The FT-IR transmission spectra of the PANI and PANI/MnO2 composite (C2) are shown in Figure 4. In the spectra of PANI, the absorption peaks near 1555 cm −1 and 1454 cm −1 correspond to the stretching vibrations of C=C in the quinone ring and benzene ring, respectively. The absorption peak

Electrochemical Experiments
The open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and polarization potentiodynamics (PP) of the carbon steel electrode were tested with a PARSTAT 2273 potentiostat/galvanostat in NSCS. A saturated calomel electrode and a platinum slice were immersed in the solution and used as the reference electrode and counter electrode, respectively.
The OCP of the carbon steel electrode was carried out once the cathodic protection was suspended and stopped when the potential did not change by more than 2 mV in 300 s. The EIS of the carbon steel electrode was carried out with sine wave circuit excitation (amplitude: 10 mV; frequency: 10 5 -10 −2 Hz). After that, the PP measurement was carried out with a scan rate of 1 mV s −1 from −250 mV to +250 mV versus the obtained potential from OCP.

Characterizations of PANI/MnO 2 Composite
The PANI/MnO 2 powders prepared with five different dosages of MnO 2 were characterized by FT-IR. The obtained results showed that the dose of MnO 2 powder had no effect on the location of the characteristic absorption peaks. Therefore, any one of the five synthesized products can be used to analyze the structure. The FT-IR transmission spectra of the PANI and PANI/MnO 2 composite (C2) are shown in Figure 4. In the spectra of PANI, the absorption peaks near 1555 cm −1 and 1454 cm −1 correspond to the stretching vibrations of C=C in the quinone ring and benzene ring, respectively. The absorption peak near 761 cm −1 corresponds to the bending vibration of the C-H out-of-plane. The absorption peaks near 1282 cm −1 and 1230 cm −1 correspond to the stretching vibrations of C-N [38]. Compared with the spectra of PANI, the positions of the absorption peaks corresponding to C=C, C-N, and C-H in the spectra of the PANI/MnO 2 composite shift slightly, which is caused by MnO 2 [39]. The absorption peaks near 1104 and 1043 cm −1 correspond to O-H stretching vibration caused by the H 2 O in the voids formed by Mn and O atoms. In addition, an absorption peak near 587 cm −1, corresponding to the bending vibration of Mn-O, is found in the spectra of the PANI/MnO 2 composite [39]. The reaction product is identified as a PANI/MnO 2 composite from the FT-IR results. near 761 cm −1 corresponds to the bending vibration of the C-H out-of-plane. The absorption peaks near 1282 cm −1 and 1230 cm −1 correspond to the stretching vibrations of C-N [38]. Compared with the spectra of PANI, the positions of the absorption peaks corresponding to C=C, C-N, and C-H in the spectra of the PANI/MnO2 composite shift slightly, which is caused by MnO2 [39]. The absorption peaks near 1104 and 1043 cm −1 correspond to O-H stretching vibration caused by the H2O in the voids formed by Mn and O atoms. In addition, an absorption peak near 587 cm −1, corresponding to the bending vibration of Mn-O, is found in the spectra of the PANI/MnO2 composite [39]. The reaction product is identified as a PANI/MnO2 composite from the FT-IR results.  Figure 5 shows the SEM images of the synthesized PANI/MnO2 composite, PANI, and MnO2. Among them, a, b, c, d, and e are the SEM images of C1, C2, C3, C4, and C5, respectively; f is the SEM image of pure PANI powder prepared without MnO2; g is the SEM image of pure MnO2 powder. It can be found that the microstructure of the five synthesized PANI/MnO2 composites were a mixture of rods and clusters and some clusters attached around the rods and cross-linked with each other. The purchased MnO2 powders are rod shaped with a diameter of approximately 70 nm and a length of approximately 1.2 μm. The rod shape is the typical morphology of MnO2, and the cluster shape is the typical morphology of PANI according to previous study [23,38]. The reaction product was identified as a PANI/MnO2 composite from the SEM results. In addition, some separated clusters appeared in C1, and some separated rods appeared in C5. This phenomenon indicates that a uniform PANI/MnO2 morphology cannot be obtained when the dose of MnO2 is out of a reasonable range in the reaction. In the polymerization reaction of aniline, the MnO2 nanorod worked as the substrate. Therefore, the PANI coated the MnO2 nanorods first and distributed among the PANI/MnO2 nanorods when it was abundant, according to the SEM images. The rods in the images exhibit a diameter of approximately 70~80 nm, indicating that the synthesized PANI/MnO2 composite is nanostructured.  , respectively; f is the SEM image of pure PANI powder prepared without MnO 2 ; g is the SEM image of pure MnO 2 powder. It can be found that the microstructure of the five synthesized PANI/MnO 2 composites were a mixture of rods and clusters and some clusters attached around the rods and cross-linked with each other. The purchased MnO 2 powders are rod shaped with a diameter of approximately 70 nm and a length of approximately 1.2 µm. The rod shape is the typical morphology of MnO 2, and the cluster shape is the typical morphology of PANI according to previous study [23,38]. The reaction product was identified as a PANI/MnO 2 composite from the SEM results. In addition, some separated clusters appeared in C1, and some separated rods appeared in C5. This phenomenon indicates that a uniform PANI/MnO 2 morphology cannot be obtained when the dose of MnO 2 is out of a reasonable range in the reaction. In the polymerization reaction of aniline, the MnO 2 nanorod worked as the substrate. Therefore, the PANI coated the MnO 2 nanorods first and distributed among the PANI/MnO 2 nanorods when it was abundant, according to the SEM images. The rods in the images exhibit a diameter of approximately 70~80 nm, indicating that the synthesized PANI/MnO 2 composite is nanostructured. Figure 6 shows the TGA curves of pure MnO 2 , the PANI/MnO 2 composites, and pure PANI. Pure MnO 2 exhibits a weight loss of 2.6% from 50 to 200 • C due to water evaporation and a weight loss of 6.7% from 200 to 800 • C due to oxygen release, as MnO 2 changes to Mn 2 O 3 under heating [39,40]. Pure PANI exhibits a weight loss of 5.0% from 50 to 200 • C due to water evaporation and a weight loss of 91% from 200 to 718 • C due to organic decomposition [39,41]. The residues of pure MnO 2 and pure PANI were 90.7% and 4.0%, respectively. C1, C2, C3, C4, and C5 exhibit a weight loss of water from 50 to 200 • C, and a weight loss from 200 to 800 • C due to oxygen release and organic decomposition, and leave 67.7%, 71.4%, 80.6%, 82.1%, and 84.4% residue, respectively. Based on the residue of pure MnO 2 , the PANI/MnO 2 composites, and pure PANI, the contents of PANI and MnO 2 in C1, C2, C3, C4, and C5 can be calculated, as shown in Table 2. It shows that the content of MnO 2 in the prepared PANI/MnO 2 composite increases with increasing doses of MnO 2 in the reaction. Some MnO 2 added in the reaction system would participate in the redox reaction as follows [39,42]: where MnO 2 acts as an oxidant and ANI acts as a reducing agent. Some MnO 2 turns into soluble Mn 2+ , which reduces the theoretical MnO 2 content in the PANI/MnO 2 composite.  Figure 6 shows the TGA curves of pure MnO2, the PANI/MnO2 composites, and pure PANI. Pure MnO2 exhibits a weight loss of 2.6% from 50 to 200 °C due to water evaporation and a weight loss of 6.7% from 200 to 800 °C due to oxygen release, as MnO2 changes to Mn2O3 under heating [39,40]. Pure PANI exhibits a weight loss of 5.0% from 50 to 200 where MnO2 acts as an oxidant and ANI acts as a reducing agent. Some MnO2 turns into soluble Mn 2+ , which reduces the theoretical MnO2 content in the PANI/MnO2 composite.   Figure 7 shows the Seebeck coefficient of the PANI/MnO2 composite with different weight fractions of MnO2 and pure MnO2. The Seebeck coefficient of the prepared PANI/MnO2 composites is lower than that of pure MnO2, indicating that MnO2 contributes more to the Seebeck coefficient than PANI in the composite. The Seebeck coefficient of the PANI/MnO2 composite reaches the maximum value of −7.72 × 10 3 ± 50 μV/K when the weight fraction of MnO2 in the composite is 77.7%. The obtained maximum Seebeck coefficient was less than that of compacted flake-shaped β-MnO2 powders (20,000-40,000 μV/K) from Song et al. [42]. It may be caused by the difference in structure, size distribution, and morphology of the MnO2 particles. The Seebeck coefficient of the PANI/MnO2 composite reaches the minimum value of −7.06 × 10 3 ± 60 μV/K when the weight fraction of MnO2 in the composite is 92.7%. When the weight fraction of MnO2 in the composite exceeds 77.7%, the Seebeck coefficient of the PANI/MnO2 composite gradually decreases

Thermoelectric Properties of PANI/MnO 2 Composite and Cement-Matrix Composites
Containing the PANI/MnO 2 Composite Figure 7 shows the Seebeck coefficient of the PANI/MnO 2 composite with different weight fractions of MnO 2 and pure MnO 2 . The Seebeck coefficient of the prepared PANI/MnO 2 composites is lower than that of pure MnO 2 , indicating that MnO 2 contributes more to the Seebeck coefficient than PANI in the composite. The Seebeck coefficient of the PANI/MnO 2 composite reaches the maximum value of −7.72 × 10 3 ± 50 µV/K when the weight fraction of MnO 2 in the composite is 77.7%. The obtained maximum Seebeck coefficient was less than that of compacted flake-shaped β-MnO 2 powders (20,000-40,000 µV/K) from Song et al. [42]. It may be caused by the difference in structure, size distribution, and morphology of the MnO 2 particles. The Seebeck coefficient of the PANI/MnO 2 composite reaches the minimum value of −7.06 × 10 3 ± 60 µV/K when the weight fraction of MnO 2 in the composite is 92.7%. When the weight fraction of MnO 2 in the composite exceeds 77.7%, the Seebeck coefficient of the PANI/MnO 2 composite gradually decreases with the increasing weight fraction of MnO 2 . This changing trend may be caused by the increased pores in the compacted PANI/MnO 2 composite powder, as the particle uniformity decreased with increasing the MnO 2 fraction [42]. Figure 8 shows the electrical conductivity of the PANI/MnO 2 composite with different weight fractions of MnO 2 and pure MnO 2 . Compared with the PANI/MnO 2 composite, the electrical conductivity of pure MnO 2 is negligible, indicating that PANI is the main conductivity source of PANI/MnO 2 . The electrical conductivity of the PANI/MnO 2 composite fluctuates between 1.2 ± 0.03 and 1.9 ± 0.03 S/cm. The electrical conductivity of the PANI/MnO 2 composite decreases with increasing the weight fraction of MnO 2 when it is high. In the PANI/MnO 2 composite, PANI serves as a fast conductive path for electron transport, so it is easy to see that the electrical conductivity decreases with increasing the MnO 2 content [43]. The electrical conductivity of the PANI/MnO 2 composite increases with increasing the weight fraction of MnO 2 when it is low. It may be caused by the aggregation of PANI particles according to the SEM image (Figure 5a). The PANI cluster in the PANI/MnO 2 composite increases the conduction path in the compacted powder for testing. In addition, the PANI cluster increased pores in the compacted PANI/MnO 2 composite powder, resulting in the decrease of conduction channels [42]. When the weight fraction of MnO 2 in the composite was 77.7%, the electrical conductivity of the PANI/MnO 2 composite reached a maximum value of 1.9 ± 0.03 S/cm. The obtained Seebeck coefficient and electrical conductivity of the materials in this work are similar to the composites from Wei, Sampad Ghosh, and some other researchers [44][45][46]. The Seebeck coefficient and electrical conductivity of those compacted composites with complex conductive "networks" inside are higher than those of the simple composite system, such as the unidirectional fibre-reinforced composites [47].   Figure 8 shows the electrical conductivity of the PANI/MnO2 composite with different weight fractions of MnO2 and pure MnO2. Compared with the PANI/MnO2 composite, the electrical conductivity of pure MnO2 is negligible, indicating that PANI is the main conductivity source of PANI/MnO2. The electrical conductivity of the PANI/MnO2 composite fluctuates between 1.2 ± 0.03 and 1.9 ± 0.03 S/cm. The electrical conductivity of the PANI/MnO2 composite decreases with increasing the weight fraction of MnO2 when it is high. In the PANI/MnO2 composite, PANI serves as a fast conductive path for electron transport, so it is easy to see that the electrical conductivity decreases with increasing the MnO2 content [43]. The electrical conductivity of the PANI/MnO2 composite increases with increasing the weight fraction of MnO2 when it is low. It may be caused by the aggregation of PANI particles according to the SEM image (Figure 5a). The PANI cluster in the PANI/MnO2 composite increases the conduction path in the compacted powder for testing. In addition, the PANI cluster increased pores in the compacted PANI/MnO2 composite powder, resulting in the decrease of conduction channels [42]. When the weight fraction of MnO2 in the composite was 77.7%, the electrical conductivity of the PANI/MnO2 composite reached a maximum value of 1.9 ± 0.03 S/cm. The obtained Seebeck coefficient and electrical conductivity of the materials in this work are similar to the composites from Wei, Sampad Ghosh, and some other researchers [44][45][46]. The Seebeck coefficient and electrical conductivity of those compacted composites with complex conductive "networks" inside are higher than those of the simple composite system, such as the unidirectional fibre-reinforced composites [47]. Due to the Seebeck coefficient and electrical conductivity results, the PANI/MnO2 composite (C2) was used as the thermoelectric component in the cement-matrix composites. The Seebeck coefficient and electrical conductivity of the cement-matrix composites Due to the Seebeck coefficient and electrical conductivity results, the PANI/MnO 2 composite (C2) was used as the thermoelectric component in the cement-matrix composites. The Seebeck coefficient and electrical conductivity of the cement-matrix composites containing different dosages of PANI/MnO 2 composite are shown in Figures 9 and 10, respectively. The Seebeck coefficient and electrical conductivity of the cement-matrix composites increases with increasing the dose of PANI/MnO 2 composite. When the PANI/MnO 2 composite content was 5.0 wt.% of the cement, the electrical conductivity of the cement-matrix composites reached a maximum value of 0.015 ± 0.0003 S/cm, and the maximum Seebeck coefficient was −2.02 × 10 3 ± 40 µV/K. The obtained maximum Seebeck coefficient was higher than the reported values, such as −880 µV/K of cement composites with 1 wt.% expanded n-type nitrogen-doped CNTs, 746 µV/K of cement composites with 3 wt.% expanded graphite, 168.12 µV/K of cement composites with 1 wt.% reduced graphene oxide, and 36.3 µV/K of cement composites with 0.45 wt.% Bi 2 Te 3 [14,44,48,49]. It is mainly caused by the rod-shaped PANI/MnO 2 composite, acting as a one-dimensional nanostructured material. The decreased dimension would dramatically increase the gradient of the density of states relative to the energy near the Fermi energy and increase the Seebeck coefficient finally [48]. However, it was less than that of cement composites with 5.0 wt.% MnO 2 powder (−3085 µV/K), which was caused by the low Seebeck coefficient of PANI [23]. The corresponding electrical conductivity maximum of the cement-matrix composites with 5.0 wt.% PANI/MnO 2 composite reached the maximum value (0.015 ± 0.0004 S/cm). At this content, PANI covered 1.91% of the volume of the cement matrix, resulting in a good conductive network according to the classical percolation theory [40]. The obtained maximum electrical conductivity was 10,000 times that of plain cement paste, 80 times that of cement composites with 5.0 wt.% MnO 2 powder, and it was higher than the reported values, such as 0.0064 S/cm cement composites with 3 wt.% expanded graphite, 0.0027 S/cm of cement composites with 1 wt.% reduced graphene oxide, 0.0011 S/cm of cement composites with 0.45 wt.% Bi 2 Te 3 , and a bit lower than the cement composites with 1 wt.% expanded n-type nitrogen-doped CNTs (0.0195 S/cm) [14,44,48,49]. The thermoelectric effect of the cement-matrix composites with added PANI/MnO 2 composite was more remarkable than that of those cement composites with added CNTs, expanded graphite, reduced graphene oxide, or Bi 2 Te 3 . In the cement-matrix composites with added PANI/MnO 2 composite, PANI contributes more to electrical conductivity and contributes less to the Seebeck coefficient than MnO 2 . Then, the cement-matrix composites with 5.0 wt.% or 2.98 vol.% PANI/MnO 2 composite were used to fabricate a cement-based thermoelectric device for cathodic protection with a current density of 0.41 mA/cm 2 .

Open Circuit Potential (OCP)
The open circuit potential of the carbon steel electrode immersed in NSCS for 21 d after depolarization is shown in Figure 11. The potential values of the carbon steel electrode were −684.1, −142.7, −157.3, −174.9, and −190.4 mV after depolarization for 0, 1, 2, 3, and 4 h, respectively. The potential value of the carbon steel in concrete with conventional

Open Circuit Potential (OCP)
The open circuit potential of the carbon steel electrode immersed in NSCS for 21 d after depolarization is shown in Figure 11. The potential values of the carbon steel electrode were −684.1, −142.7, −157.3, −174.9, and −190.4 mV after depolarization for 0, 1, 2, 3, and 4 h, respectively. The potential value of the carbon steel in concrete with conventional cathodic protection is usually between −763 mV and −1143 mV [49]. The received potential value of carbon steel was higher than that under conventional cathodic protection, as the power of the cement-based thermoelectric devices was less than that of conventional cathodic protection devices. The potential was more positive than −275 mV after depolarization for 4 h, indicating that the passive film on the surface of carbon steel was intact and no corrosion occurred on the surface of the carbon steel according to ASTM C876-2009. The potential attenuation was 493.7 mV after depolarization for 4 h, indicating that the cathodic protection system of the cement-based thermoelectric device can cause a negative potential shift of carbon steel and offer sufficient protection in the corrosion liquid according to NACE RP0169-96 and BS 7361-1-1991. cathodic protection is usually between −763 mV and −1143 mV [49]. The received potential value of carbon steel was higher than that under conventional cathodic protection, as the power of the cement-based thermoelectric devices was less than that of conventional cathodic protection devices. The potential was more positive than −275 mV after depolarization for 4 h, indicating that the passive film on the surface of carbon steel was intact and no corrosion occurred on the surface of the carbon steel according to ASTM C876-2009. The potential attenuation was 493.7 mV after depolarization for 4 h, indicating that the cathodic protection system of the cement-based thermoelectric device can cause a negative potential shift of carbon steel and offer sufficient protection in the corrosion liquid according to NACE RP0169-96 and BS 7361-1-1991. Figure 11. Open circuit potential of the carbon steel electrode immersed in NSCS for 21 days after depolarization.

Electrochemical Impedance Spectroscopy (EIS)
The electrochemical impedance plots of the carbon steel electrodes without and with Figure 11. Open circuit potential of the carbon steel electrode immersed in NSCS for 21 days after depolarization.

Electrochemical Impedance Spectroscopy (EIS)
The electrochemical impedance plots of the carbon steel electrodes without and with cathodic protection immersed in NSCS for 0 and 21 d are shown in Figures 12 and 13. The Nyquist plots show that the capacitive reactance arc of carbon steel in the high-frequency region is squashed, indicating that the corrosion reaction process is controlled by charge transfer resistance [50]. The equivalent circuit used to fit the EIS data of carbon steel is shown in Figure 14 [51,52]. The fitting results are shown in Table 3, where Rs represents the solution resistance, R f represents the film resistance of the carbon steel surface, and Rct represents the charge transfer resistance in the corrosion reaction. CPE 1 is composed of the film capacitance C f , and the dispersion coefficient n 1 represents the constant phase angle element. CPE 2 is composed of a double electric layer capacitor C dl , and the dispersion coefficient n 2 represents the constant phase angle element.         The corrosion tendency of carbon steel can be analysed by R ct and R f . R ct of the carbon steel without cathodic protection decreased rapidly, indicating that the corrosion was quite serious at 21 days [53]. R ct of the carbon steel with cathodic protection increased rapidly, indicating that the carbon steel was effectively protected by the cement-based thermoelectric device [53]. In addition, R f of the carbon steel with cathodic protection was approximately 100 times that of the carbon steel without cathodic protection at 21 days. This reveals that the passive film on the carbon steel surface was strengthened after cathodic protection [54][55][56]. Therefore, the cathodic current generated by the cement-based thermoelectric device proved effective for the protection of carbon steel.

Polarization Potentiodynamics (PP)
Polarization curves of the carbon steel without and with cathodic protection immersed in NSCS for 21 d are shown in Figure 15. It can be observed that the location of the polarization curves moved toward the current reduction and potential increase direction when cathodic protection was applied on the carbon steel electrode. This reveals that both the cathode and anode reaction corrosion rates are reduced [51,55,56]. The calculated values of the electrochemical parameters, such as corrosion potential (E corr ), corrosion current density (i corr ), anodic Tafer slope (β a ), and cathodic Tafer slope (β c ) from the polarization measurements are listed in Table 4. The i corr values of the carbon steel without and with cathodic protection were 1.67 and 0.32 µA/cm 2 , respectively. This revealed that the corrosion of carbon steel was delayed when the cathodic protection was applied. The corrosion current density of the carbon steel in concrete with conventional cathodic protection is usually between 0.1 to 0.2 µA/cm 2 [52]. The received current density value of carbon steel was larger than that under conventional cathodic protection, as the power of the cement-based thermoelectric devices was less than that of conventional cathodic protection devices. The corrosion rate is high when the corrosion current density is greater than 1 µA/cm 2 , and it is low when the corrosion current density is less than 0.5 µA/cm 2 [57]. Therefore, the cathodic protection provided by the cement-based thermoelectric device proved effective from polarization measurements.
To sum up, the carbon steel in alkaline chloride solution was effectively protected by the cement-based thermoelectric device according to the OCP, EIS, and PP experiments, although the anti-corrosion effect was poorer than conventional cathodic protection with impressed current. It provides a new method for protecting reinforced concrete in the simulated environment; however, it is currently far from practical application.