A Salt Concentration-Gradient Cationic Hydrogel Based Moisture-Enabled Electric Generator for Sustainable Green Energy Production

Sustainable green electric power generation has become currently regarded as one of the most challenging and critical technologies to meet pollution issues and dramatically growing energy demands according to the development of state-of-the-art technologies. Hence, moisture-enabled electric generators (MEGs), first revealed in 2015, have emerged as one of the promising candidates for satisfying the requisites. MEGs can produce continuous direct power triggered by moisture, one of the forms of water. Since the energy resource of the MEGs exists anytime everywhere on Earth, it can break through chronic limitations of the current energy harvesting technologies such as piezoelectric, triboelectric, and thermoelectric generators requiring artificial external stimuli and confined environments. However, recently reported MEGs not only solely focused on high relative humidity conditions, about RH 80% ~ RH 95%, to form a huge water content gradient, but also innated large internal resistance enough to need additional external load for acceptable levels of power output.
In this work, we developed a moisture-enabled electric generator with self-assembled salt-gradient ionic hydrogel based on Fumion FAA-3 hydrogel (FAA gel) and LiBr salt to resolve these challenges. The LiBr salt could accumulate on the topside of the gel during the gelation process due to the Marangoni effect, and forming an ion concentration gradient by dissociation during the moisture absorption process. Therefore, the abundance of free ions inside the hydrogel released from the LiBr salt and functional group of FAA-3 could diminish the internal resistance up to 2 ~ 4k ohms as the level of typical commercial electronics (~1k ohms), producing remarkable electric power performances in a wide range of relative humidity conditions (42.1 mWm^-2 at RH 80%, and 13.8 mWm^-2 even at RH 30%). Thus, the MEG can cover 97% area of the globe based on the worldwide relative humidity map in 2023. These outstanding performances could be realized by synchronizing directions of several driving forces transferring both water molecules and free charge carriers inside the gel, and the efficiency of the open-circuit voltage surprisingly could remain constant in the atmosphere after 50 days without any degradation. Moreover, the applicability of this device was substantiated through diverse demonstrations in the atmosphere (~RH 50%) by connecting with commercial electronics, including smart film requiring high-working voltage (~40V). Consequently, we believe this work can suggest an important clue to overcoming existing environmental constraints and advancing sustainable and ubiquitous green energy harvesting technologies.