Low-grade heat (below 373 K) is abundant and ubiquitous, but mostly wasted due to the lack of cost-effective recovery technologies. The liquid-state thermocell (LTC), an inexpensive and scalable thermoelectric device, may be commercially viable for harvesting low-grade heat energy if its Carnot-relative efficiency (ηr) reaches ~5%, which is a challenging metric to achieve experimentally. We used a thermosensitive crystallization and dissolution process to induce persistent concentration gradient of redox ions, a highly enhanced Seebeck coefficient (~3.73 mV K–1), and suppressed thermal conductivity in LTCs. As a result, we achieved a high ηr of 11% for LTCs near room temperature, and a substantially decreased cost-performance of our LTC. Our device demonstration offers promise for cost-effective low-grade heat harvesting.

Solar-enabled steam generation has attracted increasing interest in recent years because of its potential applications in power generation, desalination, and wastewater treatment, among others. Recent studies have reported many strategies for promoting the efficiency of steam generation by employing absorbers based on carbon materials or plasmonic metal nanoparticles with well-defined pores. In this work, we report that natural wood can be utilized as an ideal solar absorber after a simple flame treatment. With ultrahigh solar absorbance (∼99%), low thermal conductivity (0.33 W m K), and good hydrophilicity, the flame-treated wood can localize the solar heating at the evaporation surface and enable a solar-thermal efficiency of ∼72% under a solar intensity of 1 kW m, and it thus represents a renewable, scalable, low-cost, and robust material for solar steam applications.

Thermogalvanic cells offer a cheap, flexible and scalable route for directly converting heat into electricity. However, achieving a high output voltage and power performance simultaneously from low-grade thermal energy remains challenging. Here, we introduce strong chaotropic cations (guanidinium) and highly soluble amide derivatives (urea) into aqueous ferri/ferrocyanide ([Fe(CN)6]4−/[Fe(CN)6]3−) electrolytes to significantly boost their thermopowers. The corresponding Seebeck coefficient and temperature-insensitive power density simultaneously increase from 1.4 to 4.2 mV K−1 and from 0.4 to 1.1 mW K−2 m−2, respectively. The results reveal that guanidinium and urea synergistically enlarge the entropy difference of the redox couple and significantly increase the Seebeck effect. As a demonstration, we design a prototype module that generates a high open-circuit voltage of 3.4 V at a small temperature difference of 18 K. This thermogalvanic cell system, which features high Seebeck coefficient and low cost, holds promise for the efficient harvest of low-grade thermal energy.