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Multi-ionic Hydrogel with outstanding heat-to-electrical performance for low-grade heat harvesting.

Ionic thermoelectric (i-TE) materials have attracted much attention due to their ability to generate ionic Seebeck coefficient of tens of millivolts per Kelvin. In this work, we demonstrate that the ionic thermopower can be enhanced by the introduction of multiple ions. The multi-ionic hydrogel possesses a record thermal-to-electrical energy conversion factor (TtoE factor) of 89.6 mV K-1 and an ionic conductivity of 6.8 mS cm-1, which are both better than single salt contact hydrogel. Subsequently we build a model to explain thermal diffusion of the ions in multi-ionic hydrogels. Finally, the possibility of large-scale integrated applications of multi-ionic hydrogels is demonstrated. By connecting 7 i-TEs hydrogels, we obtained an open-circuit voltage of 1.86 V at ΔT = 3 K. Our work provides a new pathway for the design of i-TEs and low-grade heat harvesting.