Temperature adaptive self-regenerating ionic thermoelectric cycles for time domain thermal energy harvesting

The rising demand for sustainable energy solutions has promoted significant interest in ionic thermoelectric materials, which convert low-grade waste heat into electrical energy through spatial temperature gradients. However, diurnal temperature variations, which offer potential for location-independent time-domain thermal energy, remain largely unexplored. To overcome the challenges of harvesting spatially limited thermal energy, this study presents an ionic thermoelectric cycle (t-ITC) designed for time-domain thermal energy harvesting, incorporating two gels with contrasting temperature coefficients. A temperature-adaptive self-regeneration (TASR) strategy is proposed to set the critical regeneration temperature (TCR) at the midpoint of temperature fluctuations, facilitating long-term device operation. The regeneration criteria are defined as neutralization of the electrochemical potential difference between separated cells, and a method based on shared counter-ion self-balancing is introduced. Employing a polyacrylamide-polyvinylpyrrolidone (PAM-PVP) matrix with KI3/KI and K3Fe(CN)6/K4Fe(CN)6 redox couples, both utilizing the same counter-ion K+ for regeneration, the t-ITC device attains a peak energy density of 3.28 kJ m–2 per cycle and a relative Carnot efficiency of 8.39% with 70% heat recuperation, under cycling conditions between 60 °C and 10 °C. This work highlights the potential of t-ITC devices for global-scale time-domain thermal energy on a global scale, across diverse environments, such as hot deserts and cold plateaus. The authors present a PAM-PVP hydrogel ionic thermoelectric device using KI3/KI and K3Fe(CN)6/K4Fe(CN)6 redox couples to harvest time-domain thermal energy without external charging, reaching 4% relative Carnot efficiency from 60 °C to 10 °C.