Mechanism of water-evaporation-induced electricity beyond streaming potential

Since its first discovery in 2017, evaporation-induced electricity has attracted extensive attention and shown significant advantages in green energy conversion. While the streaming potential-related electrokinetic effect has been intensively explored and widely recognized as the underlying mechanism, the role of coupling between water molecules and charge carriers in the material remains elusive. Here we show through carefully designed experiments that the streaming potential effect indeed plays a role but can only contribute about half to the total water-evaporation-induced voltage occurring within the partially-wetted region of the carbon black film where the solid-liquid-gas three-phase interface exists. It is also shown that water evaporation from carboxyl and amino-functionalized carbon black films produces opposite voltage signals. Detailed first-principles calculations unveil that the adsorption of water molecules can lead to reversed charge transfer in the carboxyl and amino-functionalized carbon substrates. Finally, an evaporation-driven charge transport mechanism is proposed for the induced electricity mediated by the coupling between water molecules and charge carriers in the material. The results reveal the important role of direct interaction between water molecules and materials, deepening our understanding of the mechanism for evaporation-induced hydrovoltaic effect beyond streaming potential.