Development of ionic thermoelectric materials for low-grade heat harvesting

Abstract

The increasing global demand for sustainable energy solutions has spurred significant research interest in thermoelectric technologies. Ionic thermoelectrics (i-TEs), an emerging branch of thermoelectric materials, have garnered attention for their ability to harness waste heat and convert it into electricity. Unlike traditional electronic thermoelectrics, i-TEs operate based on thermos-electrochemical systerm, presenting unique opportunities for efficiency improvements and cost-effectiveness. However, their performance still needs improvement for commercialization and widespread adoption. To improve the Seebeck coefficient (S_i) of i-TE devices. Various mechanisms have been proposed and implemented, primarily centered around optimizing electrode materials and tailoring electrolyte compositions. However, the interactions between electrodes and electrolytes, critical for efficient energy conversion, have received comparatively less attention. A thermal-induced adsorption-desorption capacitor (TADC) triggered by the behaviors of selected species at the electrode-electrolyte interface is developed to utilize the interactions between electrode and electrolyte. The as-fabricated TADC exhibits an ultra-high Seebeck coefficient of 5 mV K-1 based on the single effect of adsorption-desorption, in a simple aqueous system with polyethyleneimine as the only solute. The adsorption coverage varies with the temperature change, leading to different surface ion densities and electrode potentials, generating a net output voltage between the two electrodes. This research highlights the significance of electrode-electrolyte interactions to enhance the thermoelectric performance of i-TEs, opening new avenues for sustainable waste heat recovery and energy conversion. Optimizations focusing on one aspect, electrode decoration, electrolyte modification or configuration design, provide only one-sided improvements and still fall short for practical applications. By introducing PEI to a K3Fe(CN)6/K4Fe(CN)6 thermocell, synergistic thermal-induced effects can be triggered, including adsorption-desorption on the electrode-electrolyte interface, selective crystallization at the cold side, and selective homogenous catalysis at the hot side. These effects work in tandem with the intrinsic thermogalvanic effect of Fe(CN)63-/4- redox couple. leading to a substantial increase in global thermopower from 1.3 mV K-1 to 6.83 mV K-1. The polyethyleneimine-mediated thermocell (PMTC) delivers reliable performance over 120-hour continuous operation demonstrating the potential of this approach. The proof-of-concept panel, consisting of 16 thermocells, generates over 5 V and 7.5 mW with a 50 K temperature difference successfully powering a smart window, Bluetooth earbuds, and a fitness tracker to meet the requirement of many practical senarios. Our findings highlight the importance of a well-designed additive in unlocking the full potential of thermocells for heat-to-electricity conversion with a detailed elucidation of the mechanisms of synergies and provide an effective design strategy for engineering thermocells.