In our field of science, there are many unrevealed phenomena. The topic I am discussing is one of the examples from my PhD research. The basic concept of this study is that alginate gels or beads, crosslinked with high-valence cations such as aluminum and zirconium, exert a strong repulsive force on cations dissolved in seawater.
The developed materials were actually used for lithium recovery from seawater, which was the main topic of my PhD research. I will discuss this in more detail in future posts. Nonetheless, these materials prompted me to question if the repulsive force of the alginate composite is strong enough to create a salinity gradient inside the composite. If so, could I generate electricity in a simple manner?
Electricity is generated by an electrical potential difference. In the field of evaporation-driven desalination, electricity can be generated when electrolytes or seawater pass through a nanoporous path. For this, the size of this path should be close to the Debye length of the fluid, forming an electrical double layer by counterions against the surface charge. As hydrodynamic pressure is applied by the evaporation, counterions begin to transport downstream where an electrical potential difference forms, known as streaming potential. Details of this mechanism can be found in the study (link) conducted by my colleague, Dr. Jun Hong Park, during the same period I conducted this research.
In contrast, the intrinsic characteristics of alginate composites crosslinked with high-valence cations enable the rejection of seawater cations. During evaporation, water molecules move to the upper surface where the seawater evaporates, while the transport of cations is inhibited through the rejection by crosslinked cations. As a result, a salinity gradient forms, with the upper surface having a lower concentration than the bottom surface. This gradient facilitates the generation of electricity through an electrical potential difference. The nature of this salinity gradient depends on the type of crosslinked cations, as illustrated in the simple diagram below.
Based on this concept, I tested the electricity generation performance using tri-positive (3+) aluminum and tetra-positive (4+) zirconium crosslinked alginate composites. As expected, the zirconium-crosslinked alginate composite generates electricity more stably compared to the aluminum-crosslinked alginate. This result leads us to believe that we no longer need to create nanopores to generate electricity; instead, we can do so through a simpler method just by crosslinking alginate with these cations and incorporating polypyrrole, a light-absorbing and electrically conducting polymer.
While we generate electricity, we can also easily obtain fresh water during the solar evaporation process at the same time. I believe this novel phenomenon has the potential to advance our field of desalination. However, despite the simplicity of the method, the efficiency needs improvement. This study focuses to verify the feasibility of simultaneous generation of electricity and fresh water (and I did). So, those materials just had an evaporation rate of 1.15 kg/h/m2 and a conversion efficiency of 54.12% under the 1 sun solar irradiation condition. The efficiency varies depending on the types of incorporated polymers and the structures of the polymer networks, where a trade-off may exist between electricity generation and fresh water production.
Numerous studies have shown high performance in solar evaporation and electricity generation using various methods. I hope future studies will explore and expand upon these initial findings, optimizing the polymer compositions and network structures to enhance both electricity generation and water production efficiency. By tackling these challenges, we can potentially unlock more sustainable and cost-effective solutions in the field of desalination as well as electricity generation, benefiting communities worldwide.
This study was published in the journal of Desalination. For the details of this study, click Here (link).