Hybrid materials based on conductive polymers for energy harvesting

Abstract

One of the objectives of the United Nations (UN) for Sustainable Development 2030 is to obtain affordable and clean energy. For this reason, improvements in energy efficiency and green energy sources are strongly recommended as optimal solutions to reduce carbon dioxide emissions and thus reduce our society's carbon footprint. Carbon emissions have quadrupled since 1950 and currently contribute around 80% of greenhouse gas emissions. As a result of this change, the global climate is experiencing changes in global rainfall trends and a reduction in the polar ice caps. With the impact of global climate changes becoming more severe, governments are trying to reduce carbon emission levels and achieve sustainable development in their countries through the use of renewable energy sources. They harness natural resources such as sunlight, rain, geothermal heat, and waves to produce clean energy without greenhouse gas emissions. The electricity generation results indicate that the trend is increasingly sustainable as the commitment to renewable energy increases annually. However, the most consumed primary energy sources worldwide continue to be oil and its derivatives (83.15%), and only 5.7% of the global energy consumed comes from renewable energy sources. In addition, energy consumption fell in 2020 by around 4.5% compared to the previous year due to the crisis caused by Covid-19. However, energy consumption is expected to increase in the coming years, according to the International Energy Agency (IEA). It is vitally important that this increase in energy demand by society goes hand in hand with improved energy efficiency in order to meet the UN's sustainable goals. Only in this way will it be possible to achieve Net Zero Emissions for the 2050 Scenario in the 2020-2030 period. On the other hand, it is vitally important to improve energy efficiency since it is estimated that 62% of the fuel used to generate energy is lost as heat. Also, energy is lost as heat in power plants during energy conversion processes, and only 5% of energy is used in homes. Therefore, finding ways to recover all this wasted energy is imperative. Some of this lost energy can be recovered by harvesting energy and converting it into electrical energy. The three main phenomena that can recover energy such a: piezoelectricity, which can convert mechanical stretching into electrical current; triboelectricity, which can produce electrical energy through frictional contact between different materials; and thermoelectricity, which can recover electrical energy from losses of heat In recent years, this last phenomenon has become the most promising way to improve energy efficiency, since it is a property of semiconductors that can convert a temperature gradient into electrical current and vice versa. The Figure of Merit, ZT gives the thermoelectric efficiency of a material. The most widely used thermoelectric materials for commercial applications have been developed thanks to significant advances in the synthesis of new materials and structures with improved thermoelectric performance. It has sought to improve the Figure of Merit by reducing the thermal conductivity of the network. One way to achieve this goal is through the so-called phonon glass-electron crystal strategy, in which the material must conduct heat like glass, but electricity like a crystal. The result of this research are materials such as skutterudites and clathrates. Another strategy to improve thermoelectric efficiency is by reducing the thermal conductivity of the network, based on the use of materials with low-dimensional structures. Hicks and Dresselhaus, in 1993, demonstrated the potential use of quantum wells to improve the Figure of Merit. As a result of this work, materials such as Bi2Te3/Sb2Te3, PbSeTe or SiGe reached ZT values around 2. However, these inorganic materials have several drawbacks, such as the high cost of production, the toxicity of some of the elements used, and the scarcity of raw materials. All these drawbacks make inorganic thermoelectric materials unsuitable from the point of view of sustainable energy development. For this reason, many studies have focused on the search for efficient and environmentally friendly thermoelectric materials. One of the potential candidates for room temperature applications is organic semiconductors, particularly conducting polymers, due to their abundance, low cost, flexibility, and easy modification. In addition, conductive polymers provide other benefits. From an environmental point of view, conductive polymers are mainly composed of carbon, which is an abundant, low-cost and non-toxic element. These properties imply that obtaining thermoelectric materials from conductive polymers is much more sustainable than traditional inorganic materials. Furthermore, from a chemical point of view, conductive polymers can be easily modified to provide additional functionalities, and their flexible properties help large print areas. Another advantage is that conducting polymers often have a thermal conductivity (0.1 – 1 W m-1 K-1) below the thermal conductivity of metals and inorganic semiconductors. All of this makes conductive polymers ideal candidates for the next generation of thermoelectric materials because it is possible to obtain low-cost, large-area, flexible devices for low-grade thermal energy harvesting. However, despite the significant increase in the thermoelectric efficiency of conductive polymers in the last decade, the Figure of Merit is still much lower than the ZT of inorganic materials. Therefore, it is necessary to find new strategies to improve the efficiency of conductive polymers such as: 1) optimization of the doping level, 2) improvement of the ordering of the polymeric chains, 3) obtaining hybrid organic-inorganic materials.