This review covers the recent advances of ionic thermoelectric (i-TE) materials which offer a compelling alternative to traditional thermoelectrics due to their excellent ionic thermopower, low thermal conductivity, and abundant material options.

The efficient and economical conversion of low-grade waste heat into electricity has promising potential to combat the greenhouse effect and expedite the shift towards sustainable development. This study presents an innovative and appealing approach through the utilization of lignin, an abundant waste product derived from the paper and pulp industry, to develop hydrogels as compelling and sustainable materials for application in ionic thermoelectricity. Various compositions were evaluated to examine the impacts of varying lignin concentrations, types of electrolytes, concentrations of crosslinkers, and electrolyte concentrations on the ionic thermoelectric performance of the hydrogels. The optimized lignin-derived hydrogel, infiltrated with a 6 M KOH electrolyte, demonstrates high ionic conductivity (226.5 mS/cm) and a superior Seebeck coefficient of 13 mV/K. This results in a remarkable power factor (3831 µW/m·K²) that leads to an impressive Figure of merit (ZT_i) (3.75), surpassing most of the existing state-of-the-art materials and making it the most efficient sustainable ionic thermoelectric material reported until now. These findings underscore the exceptional performance of lignin-based hydrogels in the realm of low-grade waste energy harvesting applications. The present study contributes to address the challenges posed by waste heat through effectively harnessing low-grade waste heat through the utilization of sustainable lignin-based hydrogels while reducing the reliance on fossil fuels and minimizing greenhouse gas emissions. Graphical Abstract

Wood‐based ionic conductive membranes have emerged as a new paradigm for low‐grade thermal energy harvesting applications due to their unique andtailorable structures. Herein, a lignin‐derivedionic conducting membrane with hierarchical aligned channels is synthesized viaa double network crosslinking approach. Their excellent thermal stability andsuperior swelling ratio allow their optimization as low‐grade heat recovery technologies. Several vertically aligned nanoscaleconfinements are found in the synthesized membranes, contributing towardenhanced ionic diffusion. Among all the combinations, the membrane comprising69.2 wt.% of lignin and infiltrated with 0.5 m KOH exhibits anexceptional ionic figure of merit (ZTi) of 0.25, relatively higher ionic conductivity(51.5 mS cm^‒1), lower thermal conductivity(0.195 W m^‒1·K), and a remarkable ionic Seebeck coefficientof 5.71 mV K^‒1 under the application of an axialtemperature gradient. A numerical model is also utilized to evaluate theveracity of experimental observations and to gain a better understanding of thefundamental mechanisms involved in attaining such values. These results displaythe potential of lignin‐basedmembranes for future thermal energy harvesting applications and are a new facetin thermoelectric energy conversion which is certain to pave the way forfurther investigations on sustainable ionic conductive membranes.

The recent development in the field of wearable electronics has increased the demand for batteries as power sources which are subjected to periodic recharging and replacement. Therefore, the next challenge is to design new systems for sustainable energy to power portable electronic devices that can be easily integrated into textiles such as thermoelectric generators that can convert waste heat into electricity. Looking at this scenario, this work shows a methodology to prepare thermoelectric textiles by electrochemical polymerization of poly(3,4‐ethylenedioxythiophene) (PEDOT) on felt fabrics. The polymerization of PEDOT is carried out utilizing three different counterions, LiClO₄, 1‐butyl‐3‐methylimidazolium hexafluorophosphate (PF₆), and 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (BTFMSI) to provide a complete understanding of the role of the counterion in the thermoelectric properties. The electrical conductivity and Seebeck coefficient are dependent on the counterion, reaching the maximum ZT for PEDOT polymerized in presence of BTFMSI due to an improvement of carrier mobility. In addition, the manufactured textile thermoelectric device showed an outstanding power output (6.5 µW) compared to the previous devices reported to date.

In this study, we have improved the power factor of conductive polymer nanocomposites by combining layer-by-layer assembly with electrochemical deposition to produce flexible thermoelectric materials based on PEDOT/carbon nanotubes (CNTs)—films. To produce films based on CNTs and PEDOT, a dual approach has been employed: (i) the layer-by-layer method has been utilized for constructing the CNTs layer and (ii) electrochemical polymerization has been used in the synthesis of the conducting polymer. Moreover, the thermoelectric properties were optimized by controlling the experimental conditions including the number of deposition cycles and electropolymerizing time. The electrical characterization of the samples was carried out by measuring the Seebeck voltage produced under a small temperature difference and by measuring the electrical conductivity using the four-point probe method. The resulting values of the Seebeck coefficient S and σ were used to determine the power factor. The structural and morphological analyses of CNTs/PEDOT samples were carried out using scanning electron microscopy (SEM) and Raman spectroscopy. The best power factor achieved was 131.1 (μWm⁻¹K⁻²), a competitive value comparable to some inorganic thermoelectric materials. Since the synthesis of the CNT/PEDOT layers is rather simple and the ingredients used are relatively inexpensive and environmentally friendly, the proposed nanocomposites are a very interesting approach as an application for recycling heat waste.