**1. Introduction**

High heat flux generated by many electronic products has recently been comprehensively causing hotspot problems that need to be improved immediately with novel technologies with excellent performances. At present, many 3-C (Computers, Consumers, and Communications) electronic products employ heat pipe [1–5] and vapor chamber [6–11] thermal modules to become the standard equipment, so as to increase the heat dissipation efficiency of the goods and reduce the temperature of the heat sources [12–14]. These two-phase flow heat transfer facilities have high thermal conductivities compared to those of the large-footprint metal material heat sinks [15]. Wang et al. [16,17] showed that the maximum three-dimensional and effective thermal conductivity of the vapor chamber is up to 910 W/mK, many times that of the pure copper base plate at heat flux of over 100 W/cm2. If it can lower the temperature of the heat source simultaneously and then recycle the heat dissipated, it will be a great contribution to the green energy industry on the basis of conserving energy. Nowadays, as shown in the present paper, it is possible to utilize the temperature effects on electrochemical and thermal activities [18] to accomplish this intention, which can supply a cooling function for the electronic devices and simultaneously exploit the wasted heat energy to generate direct current electric power. Tan et al. [19]

**Citation:** Gang, Q.; Wang, R.-T.; Wang, J.-C. Estimations on Properties of Redox Reactions to Electrical Energy and Storage Device of Thermoelectric Pipe (TEP) Using Polymeric Nanofluids. *Polymers* **2021**, *13*, 1812. https://doi.org/10.3390/ polym13111812

Academic Editor: Vito Di Noto

Received: 22 April 2021 Accepted: 28 May 2021 Published: 31 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

found reasonable charging rates for lithium-ion battery management systems' design at different initial temperatures employing the thermal-electrochemistry coupled model, which investigates the temperature effects on electrochemical and thermal characteristics.

The first voltaic cell/pile of the world was developed by Alessandro Volta, an Italian physicist, between the end of the 18th century and the beginning of the 19th century. It can gather and provide a stable current via the electrochemistry reduction-oxidation (redox) reaction. Hereafter, in 1832, Michael Faraday, an English scientist, depicted in detail the electrochemistry phenomena about the electrolysis process between electric energy and chemical decomposition through solution containing ions. Wang et al. [20–22] utilized the metal oxide nanofluids as an electrolyte (thermoelectric nanofluids) compared with various aqueous solutions according to pH value, Zeta potential, viscosity, and IEP to display the best particle fraction, stability, and settled current output. These results indicated that the thermoelectric nanofluids (Al2O3 nanofluid) with temperature variation make use of the temperature effects on electrochemical and thermal activities, which depict the thermoelectric conversion function of temperature gradient into electric power generation in order to improve the application rate of the wasted thermal energy. Nanofluids are widely considered as the innovative nanotechnology-based heat transfer fluids, which have been certified for reforming the energy conversion procedure efficiency [23,24]. The creative notion of thermoelectric nanofluids is energetic and extremely dependable energy transformation that generates electricity in applications in which the heat will be dissipated. For the recent developments in thermal and electrical conductivities of this novel thermal fluid, various factors affect it, including the cluster of nanoparticle type, temperature, preparation methods, surfactant, and volume concentration. It was found that the rise in temperature and volume concentration of nanofluids generally led to linearly incrementing their thermal and electrical conductivities [25–27]. Heyhat and Irannezhad [28] created the thermoelectrical conductivity (TEC) ratio according to the acquired experimental data and thoroughly checked it. Results displayed that both the temperature and concentration have affirmative effects on the thermal and electrical conductivities of nanofluids. Geng et al. [29] discussed the effects of base fluid, temperature, solid volume concentration, and nanoparticle type on electrical conductivity of nanofluid and found that the electrical conductivity generally increases as temperature and solid volume fraction increase. Kim and Park [30] investigated the influence of the multi-walled carbon nanotubes (MWCNTs) nanofluid as an electrolyte on the energy storage capacity in vanadium redox flow battery, which was inspected and contrasted with the primitive electrolyte.

This study develops a thermoelectric pipe (TEP) device for the first time depending on the temperature effects on electrochemical and thermal activities having heat conduction performance and suitability, which adopted the polymeric nanofluid as an electrolyte. According to [21], the 2.0 wt.% titanium dioxide (TiO2) nanofluid had the best suspension stability and overall thermoelectric properties among the three nanofluids, including the Al2O3, ZnO, and TiO2, in the thermoelectric generation experiments. Pinchuk and Kuzmin [31] studied the effect of the addition of TiO2 nanoparticles to coal-water fuel on its thermophysical parameters. They suggested that the addition of the TiO2 nanoparticles in 0.5 to 4 wt.% increases the coal-water fuel thermal conductivity by 9% to 17%. Das et al. [32] noticed that use of TiO2 nanofluid reduces the wall temperature distribution as well as thermal resistance of thermosyphon and enhances the thermal conductivity compared to deionized water. Therefore, the nanofluids revealed higher thermal conductivity contrasted with base fluid and demonstrated an increase in the effective thermal conductivity with a reduction in particle size and with a growth in particle volume fraction. The present TEP composed of the polymeric nanofluid is capable of generating electromotive force and let-bearing heat dissipation at a temperature difference, and vice versa. Consequently, it is especially favorable for applications in the industrial waste heat and automotive waste heat used for recycling and reusing in order to reduce carbon dioxide emissions. The primary object of the present TEP is to provide a novel device that have high heat conduction performance and relatively good power generation suitability.
