Photothermally Enhanced Hydrogel‐Based Thermoelectrochemical Cells for Stable, Passive Harvesting of Low‐Grade Heat

Low‐grade waste heat (<100°C) is an abundant yet underutilized energy resource due to its low exergy and the inefficiency of conventional conversion technologies under small temperature gradients and passive operation. Here, a photothermally enhanced thermoelectrochemical cell (PTEC) is reported, enabled by a high‐ionic‐conductivity double‐network hydrogel electrolyte incorporating a guanidinium‐boosted ferro/ferricyanide redox couple. Integration of photothermal carbon electrodes with a quasi‐solid architecture establishes stable temperature gradients under both thermal and solar excitation, without active cooling or liquid electrolytes. The PTEC delivers an instantaneous thermopower of 4.5 mV K − 1 and a normalized power density of 5 mW m − 2 K − 2 under passive cooling, surpassing prior hydrogel‐based thermoelectrochemical cells and approaching state‐of‐the‐art liquid systems. Long‐term operation reveals diffusion‐limited decay that stabilizes at steady current densities, with impedance analysis identifying mass transport as the dominant bottleneck. Pulsed‐load operation mitigates ion depletion, enhancing cumulative charge extraction by 2.5× and average power by >60% relative to continuous discharge. A nine‐cell array directly recharges an electrochemical system using only low‐grade heat ( T H = 50°C). This work elucidates key mechanisms governing sustained performance and demonstrates a robust, scalable platform for passive harvesting of ubiquitous low‐grade heat toward self‐powered distributed electronics.