Microfluidic Salinity Gradient-Induced All-Day Electricity Production in Solar Steam Generation

Synergistic generation of freshwater and electricity using solar light would be an ideal solution for global freshwater challenges and energy demands. Recently, interface solar steam generation has been considered one of the promising cost-effective alternatives for freshwater generation. Here, we have systematically maintained the salinity gradient within two-legged paper-based microfluidic channels to transport wastewater from the reservoir to the evaporator surface and generate electricity all-day-long.

Flowing seawater (3.5 wt % NaCl) on one leg and tap water on the other of the water-conducting channels connected to a conical evaporator, we achieved an average open-circuit voltage (V_OC) of 150 mV and a short-circuit current of 6.5 μA across each channel along with a water evaporation efficiency of 88%. As the V_OC depends only on the ion concentration gradient within the channel in the direction perpendicular to the water flow, the electricity generation persists throughout the day and can be tuned by varying the salinity. Increasing the salt concentration of the seawater to 20 wt %, the V_OC increased to 250 mV in a single channel.

In an evaporator connected with four such channels, we achieved a maximum output power density of 9.9 mW m^–2 in a series combination without sacrificing the evaporation rate. Furthermore, removing agglomerated salt from the evaporator surface, we harvested salt at a rate of 0.33 kg m^–2 h^–1. Therefore, our approach provides an alternative way of freshwater generation, salt harvesting, and all-day-long electricity production simultaneously.