Boosted Thermogalvanic Thermopower upon Solid-to-Liquid Phase Transition

Thermogalvanic cells offer scalable low-grade waste heat recovery using tunable electrode-dependent thermopower and electrolyte-dependent thermal conductivities. However, the use of single-phase electrodes thermodynamically curb the entropy difference, limiting the thermopower enhancement. Here, we show that phase transforming electrodes achieve significantly enhanced thermopower using the melting phase transition of bulk Na_xK alloys. Under both temporal and spatial temperature gradients, the electrodes exhibit significantly increased thermopower up to 26.1 mV/K across the melting point and the generated voltages of 261 mV under 10 K temperature gradient. We also show that stabilizing the liquid metal electrode-electrolyte interface plays a critical role in evaluating the thermopower associated with the phase transition. The strategies demonstrated in this work suggest potential design guidelines towards optimizing thermogalvanic cells to specific temperature ranges.