The effect of SiO₂ additives on solid hydroxide ion-conducting polymer electrolytes: a Raman microscopy study

The effect of SiO₂ additives on the conductivity and longevity of an alkaline tetraethylammonium hydroxide (TEAOH)-poly(acrylamide) (PAM) polymer electrolyte was investigated. Electrochemical impedance spectroscopy (EIS) and Raman microscopy studies were performed for TEAOH-PAM with micro-sized (mSiO₂) or nano-sized (nSiO₂) additives under highly hydrated and under ambient conditions. At a high relative humidity (RH) of 75%, nSiO₂ significantly increased the ionic conductivity of OH^-, achieving 25 mS cm^-1, while mSiO₂ had little influence (10 mS cm^-1). Further investigation at lower RH (45%) revealed that dehydration of TEAOH led to crystallization and lower conductivity of the polymer electrolytes. The degree and rate of crystallization in the different systems varied greatly: mSiO₂ accelerated the process while nSiO₂ delayed it. Using characteristic signatures obtained from Raman microscopy, a correlation between the ionic conductivity and the structural differences among these systems has been established and an explanation for the impact of the SiO₂ additives has been proposed.