Linking interfacial energies with proton conductivity in sulfonated poly (ether ether ketone) nanocomposite

Abstract

So far, numerous studies have been conducted to enhance the proton conductivity of sulfonated poly (ether ether ketone) (SPEEK) as a proton exchange membrane (PEM) by incorporating nanofillers. However, the role of the filler-matrix interface in the proton conduction process has been inadequately understood. Herein, the SPEEK nanocomposite of polydopamine coated multi-walled carbon nanotube (P@MWCNT) was successfully fabricated and characterized by field emission electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR). The physicochemical properties revealed that the dimensional stability of nanocomposite was improved by 32% compared to the SPEEK due to lower water uptake. Moreover, the proton conductivity of SPEEK was significantly enhanced to 0.064 S cm−1 by adding 3.5% of P@MWCNTs. The contribution of the interface of P@MWCNT-SPEEK to proton conduction was explored through an innovative approach based on the interfacial energies. By polydopamine modification, the surface free energy (SFE) of nanotubes increased from 23.6 mJ/m2 to 29.4 mJ/m2, and the interfacial energy of SPEEK-P@MWCNT augmented by 192%. These findings suggest that due to the increase of interfacial energy between the P@MWCNTs and polymer chains, the transfer of protons along with the interface region of nanofiller and matrix is an acceptable mechanism.