Hydrogel-based thermoelectrochemical cells for waste heat recovery under passive cooling conditions.

With global energy demands rising and the need to reduce greenhouse gas emissions, capturing low-temperature waste heat, which represents ≈ 60% of overall energy waste, offers a compelling pathway to sustainability. Thermoelectrochemical cells (TECs) are promising for converting low-grade heat into electricity but face limitations with liquid electrolytes, including inefficiency and instability under passive cooling. In this work, we introduce hydrogel-based TECs (HyTECs) as a solution to these challenges, leveraging their low thermal conductivity and permeability to sustain larger thermal gradients and stable operation across diverse conditions. We demonstrate that HyTECs achieve a power output of up to 3.5 μW cm^-2 under passive cooling with a hot temperature of 55-65 °C, comparable to those of state-of-the-art TECs under an externally applied thermal gradient of 10 K cm^-1. Through thorough experiments and multiphysics modeling, we attribute this performance to the hydrogel's ability to support stable convective cells that enhance redox species transport at the electrode interface. Systematic optimization of key parameters, including redox-pair concentration, electrode separation, and supporting electrolyte levels, revealed that a design with 20 mm electrode spacing, 0.4 M ferro-/ferricyanide, and 0.5 M KCl achieves a power output of 35 mW m^-2, a Seebeck coefficient of 3.5 mV K^-1, and a normalized power of 0.6 mW m^-2 K^-2. Furthermore, HyTECs exhibit robust performance across orientations (0°-150°) around hot pipes, with a 135° inclination delivering peak power due to enhanced convection and thermal gradients. This work establishes HyTECs as a viable platform for efficient waste heat recovery, providing a foundation for their deployment in real-world energy applications.