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The increasing energy demand and the ever more pressing need for clean technologies of energy conversion pose one of the most urgent and complicated issues of our age. Thermoelectricity, namely the direct conversion of waste heat into electricity, is a promising technique based on a long-standing physical phenomenon, which still has not fully developed its potential, mainly due to the low efficiency of the process. In order to improve the thermoelectric performance, a huge effort is being made by physicists, materials scientists and engineers, with the primary aims of better understanding the fundamental issues ruling the improvement of the thermoelectric figure of merit, and finally building the most efficient thermoelectric devices. In this Roadmap an overview is given about the most recent experimental and computational results obtained within the Italian research community on the optimization of composition and morphology of some thermoelectric materials, as well as on the design of thermoelectric and hybrid thermoelectric/photovoltaic devices.

Carbon nanotube/polymer composites have recently received considerable attention for thermoelectric (TE) applications. The TE power factor can be significantly improved by forming composites with carbon nanotubes. However, the formation of a uniform and well-ordered nanocomposite film is still challenging because of the creation of agglomerates and the uneven distribution of nanotubes. Here, we developed a facile, efficient, and easy-processable route to produce uniform and aligned nanocomposite films of P3HT and carbon nanotube forest (CNTF). The electrical conductivity of a pristine P3HT film was improved from ∼10–7 to 160 S/cm thanks to the presence of CNTF. Also, a further boost in TE performance was achieved using two additives, lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and tert-butylpyridine. By adding the additives to P3HT, the degree of interchain order increased, which facilitated the charge transport through the composite. Under the optimal conditions, the incorporation of CNTF and additives led to values of the Seebeck coefficient, electrical conductivity, and power factor up to rising 92 μV/K, 130 S/cm, and 110 μW/m K2, respectively, at a temperature of 344.15 K. The excellent TE performance of the hybrid films originates from the dramatically increased electrical conductivity and the improved Seebeck coefficient by CNTF and additives, respectively.

Organic materials have attracted considerable attention for thermoelectric (TE) applications. Given their potential as wearable power generators, there is an urgent need to develop organic TE materials that possess superior electronic properties as well as excellent mechanical and environmental stability. Here, we develop paper-based TE materials using the poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS), graphene nanoplatelets (GNPs), and a starch-based biopolymer as a binder for GNPs. The device fabrication consists of spraying the biopolymer/GNP ink onto the cellulose paper followed by spraying the PEDOT:PSS solution. Further enhancement of TE properties was obtained by adding an ionic liquid (IL), bis(trifluoromethane)sulfonimide lithium salt to the PEDOT:PSS solution. Upon addition of the IL, the electrical conductivity of as-fabricated PEDOT:PSS films increased nearly two orders of magnitude. The electrical conductivity increases with GNPs content due to formation of an effective electrical percolation network. Interestingly, incorporating GNPs simultaneously improves the Seebeck coefficient. Raman measurements suggest that the concurrent enhancement of the Seebeck coefficient and electrical conductivity might be related to the chemical bonding between the conducting polymer chains and the filler. In addition, these composites display remarkable flexibility at various bending angles and environmental stability without losing their original conductivity after three months of exposure to ambient conditions. Funding Agencies|Regione Lazio through ISIS@MACH [G10795, 69]; University of Rome "Tor Vergata" project THERMA (Grant Beyond the Borders) [2561]

Ionic thermoelectric supercapacitors (ITESCs) have recently been developed for converting low-grade waste heat into electricity. Until now, most reports of ITESCs have been focused on the development of electrolytes, which then have been combined with a specific electrode material. Here, it is demonstrated that the electrode is not only critical for electrical energy storage but also greatly affects the effective thermopower (S-eff) of an ITESC. It is shown that the same ion gel can generate a positive thermopower in an ITESC when using gold nanowire (AuNW) electrodes, while generating a negative thermopower when using poly(3,4-ethylendioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes. The achieved negative sign of the S-eff could be attributed to the Donnan exclusive effect from the polyanions in the PEDOT:PSS electrodes. After examining the thermovoltage, capacitance and charge retention performance of the two ITESCs, it is concluded that PEDOT:PSS is superior to AuNWs as electrodes. Moreover, a new strategy of constructing an ionic thermopile of multiple p- and n-type legs is achieved by series-connecting these legs with same electrolyte but different electrodes. Using interfacial effect at ionic gels/PEDOT:PSS electrode interface, an enhanced thermoelectric effect in ITESCs is obtained, which constitutes one more step towards efficient, low-cost, flexible, and printable ionic thermoelectric modules for energy harvesting. Funding Agencies|University of Rome Tor Vergata - Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linkoping University [2009-00971]; Swedish Research Council [2016-05990, 2016-06146]; Swedish Foundation for Strategic Research

Electrochemical exfoliation of graphite has recently attracted a big attention as a simple, fast and scalable method for the preparation of high quality graphene, but there are some drawbacks that hinder its application. Direct deposition is one of the most critical challenges that makes it difficult to deposit uniform, compact and large scale graphene thin films. This work develops a facile electrophoretic deposition route to fabricate exfoliated graphene (EG) film on Ag nanowires (NWs) networks with a controllable film thickness in nanometers scale. EG thin films are deposited with different applied potentials and times from an EG dispersion in N, N -dimethylformamide solvent. Since confirmed by light transmittance results, the increase in deposition time or applied potential lead to an increase in the number of deposited EG layers. The scanning electron microscopy results confirm the uniformity and compactness of EG films deposited on Ag NWs network. Furthermore, transparency and conductivity of Ag NWs films before and after the deposition of EG are investigated. It shows significant improvement in its performance, as a result, it leads to Ag NWs/EG hybrid films exhibiting a sheet resistance of R s = 30 Ω sq -1 , and 85% light transmittance, comparable to conventional transparent conductive oxide films.

Organic-based energy harvesting devices can contribute to a sustainable solution for the transition to renewable energy sources. The concept of ionic thermoelectrics (iTE) has been recently proposed and motivated by the high values of thermo-voltage in electrolytes. So far, most research has focused on developing new electrolytes with high Seebeck coefficient. Despite the major role of the electrode materials in supercapacitors and batteries, the effect of various electrodes on energy harvesting in iTE devices has not been widely studied. In this work, the conducting polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is investigated as the functional electrodes in iTE supercapacitors. Through investigating the thermo-voltage of iTEs of the same electrolyte with varying composition of PEDOT electrodes, it is identified that the different PSS content greatly affects the overall thermo-induced voltage coefficient, S-eff (i.e., effective thermopower). The permselective polyanion in the electrode causes cation concentration differences at the electrode/electrolyte interface and contributes to an interfacial potential drop that is temperature dependent. As a result, the overall thermo-voltage of the device possesses both an interfacial and a bulk contribution. The findings extend the fundamental understanding of iTE effect with functional electrodes, which could lead a new direction to enhance the heat-to-electricity conversion. Funding Agencies|Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; Knut and Alice Wallenberg Foundation (Tail of the sun); Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-05990, 2016-06146, 202005218, 2018-04037]

Porous network-like MnO2 thick films are successfully synthesized on a flexible stainless steel (SS) mesh using a simple and low-cost electrodeposition method followed by an electrochemical activation process. Morphology, chemical composition and crystal structure of the prepared electrodes before and after the activation process are determined and compared by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) analyses. The results show that the implementation of the electrochemical activation process does not change the chemical composition and crystal structure of the films, but it influences the surface morphology of the MnO2 thick layer to a flaky nanostructure. Based on the electrochemical data analysis, the maximum specific capacitance of 1400 mF (381 F g-1) and 3700 mF (352 F g-1) are measured for small (2.6 cm2) and large (10 cm2) surface area electrodes, respectively. In addition, a flexible symmetric MnO2//MnO2 solid state supercapacitor shows a capacitance of 0.3 F with about 98% retention at different bending angles from 0° to 360°.