From the coin-like batteries that power wristwatches to the large fuel cells under the hood of electric cars, batteries play a crucial role in sustaining our technology-driven world. But as anyone who has ever owned a smartphone knows, batteries degrade with frequent usage, gradually losing their capacity to hold energy with each charge-discharge cycle.
Supercapacitors, a related class of energy storage devices, provide high power density and long life cycles and are not prone to the same rapid degradation as traditional batteries. The catch is their performance relies on charge-carrying electrolytes that trade stability for efficiency. Liquid electrolytes that efficiently conduct electrical charges pose safety risks due to solvent leakage, while solid polymer electrolytes tend to have low conductivity at room temperature.
“The electrolytes used in supercapacitors require both mechanical rigidity and high ionic conductivity,” noted Jason Lim, an A*STAR National Science Scholarship recipient in 2007 and 2013 and currently an Emerging Group Leader at A*STAR’s Institute of Materials Research and Engineering (IMRE). Gel polymer electrolytes with multiple phases that share the best properties of liquid and solid polymers are a promising solution. Thus in this work, Lim and fellow IMRE scientist Derrick Fam explored polyurethanes as a new class of polymer candidates for gel electrolyte applications. However, polyurethanes are typically made using catalysts containing highly-toxic metals such as tin.
To address this quandary, Lim and Fam led a team to build safer polyurethane-based gel polymer electrolytes using non-toxic tetrabutylammonium (TBA) and potassium halide salts as catalysts.
“TBA salts have better solubility in organic solvents, which facilitates easy dissociation to free up the halide ions to perform the catalysis,” Lim explained. “After demonstrating that the TBA salts work, we explored the usage of potassium halide salts instead as these are a lot cheaper and much more readily available.”
The researchers then devised a mixture that could dissolve small amounts of the potassium halide salts into their separate ions. By enabling efficient crosslinking among the polymer components, these catalysts could produce polyurethane gels in mere minutes.
“Despite the crosslinked nature of the polyurethanes, it still retained sufficient porosity for a liquid electrolyte to infiltrate the polymer network. This enabled the resulting gel electrolyte to possess excellent ionic conductivity while still retaining the structural properties of the solid polymer matrix,” Lim added.
Given the affordable and common nature of the halide salts used, the team hopes the technique will allow for easier access to novel polymers with multifunctional properties. Besides serving as potential supercapacitors for electric vehicles, these nontoxic gel electrolytes may pave the way for biocompatible energy systems powering medical implants.
“Our research opens up avenues towards synthesizing materials sustainably and simply, and potentially lower the cost of these energy storage devices,” Lim concluded. “We are also looking at expanding the use of these gels to other energy storage devices as their structural rigidity will be useful for many different applications.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).
