Electrolyte Engineering of Quasi‐Solid‐State Thermocells for Low‐Grade Heat Harvest at Sub‐Zero Temperatures

Abstract The pursuit of sustainable energy technologies has led to considerable interest in waste heat harvest from various energy sources. Thermocells (TECs), using the thermogalvanic effect, hold high potential in converting low‐grade heat directly into electricity. Optimizing thermopower and ensuring adaptability in low or sub‐zero temperature conditions are crucial for the advancement of next‐generation TECs. To address these challenges, in this work, a composite hydrogel electrolyte incorporating ethylene glycol (EG) and Ti 3 C 2 T x MXene nanosheets is rationally engineered. EG boosts thermopower by increasing solvation entropy change and concentration ratio difference of redox ions; it also prevents freezing by disrupting hydrogen bonds among water molecules. Meanwhile, hydrophilic MXene nanosheets facilitate gelation process, improve mechanical strength, and further bond to water molecules to enhance anti‐freezing and moisture‐retaining capabilities. The TECs fabricated on this composite hydrogel electrolyte exhibit a notably increased thermopower of 2.04 mV K −1 and can be continuously operated at sub‐zero temperatures down to −40 °C. Electricity‐generating TEC windows are further demonstrated to harvest all‐day low‐grade heat via utilizing the temperature difference between the indoor and the outdoor. This study proposes an electrolyte engineering strategy for long‐lasting and reliable TECs that are suitable for low‐grade heat harvesting in extreme low‐temperature conditions.