Morphology and electrochemical properties of a gel blend polymer electrolyte based on PVDF-HFP/PEO blend

Hypothesis: In recent years, gel polymer electrolytes (quasi-solid state electrolytes) have attracted great attention as a suitable substitute for liquid electrolytes. On the other hand, ionic liquids could dramatically enhance the ionic conductivity of electrolytes. In this work, gel polymer electrolytes based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/poly(ethylene oxide) (PEO) blends (for application in dye-sensitized solar cells (DSSCs)) and imidazoliumbased ionic liquids were prepared. It is supposed that blending these two polymers could reduce the degree of crystallization and increase the porosity of the electrolyte blend to yield a higher electrolyte uptake and ionic conductivity. Methods: Polymer blend electrolytes were prepared in different blend ratios and in the presence of either one of the ionic liquids including BMII or BMIMBF4 through phase inversion method and their properties were investigated by differential scanning calorimetry (DSC), mercury porosimetry, electrolyte uptake, and morphology to optimize the blend ratio. Findings: It was found that the blend ratio of 60/40 (w/w) PVDF-HFP/PEO has the highest porosity and electrolyte uptake. Crystallization investigations by DSC showed that there is a direct relationship between the decrease of crystallinity of two polymers and the increment of electrolyte ionic conductivity. Electrolyte uptake gradually increased with increasing PEO component concentration up to 40 wt%, and reached a maximum of 98.49% and 89.48% for BMIMBF4 and BMII, respectively. Beyond this concentration, a decrease in electrolyte uptake was seen, which is an undesirable feature for the produced samples. In this blend ratio ionic conductivity was measured as 2.07 mS/cm and 1.78 mS/cm for PVDF-HFP/PEO/BMIMBF4 and PVDF-HFP/ PEO/BMII electrolytes, respectively.