A graphene oxide polymer brush based cross-linked nanocomposite proton exchange membrane for direct methanol fuel cells

Abstract

Functional polymer brush modified graphene oxide (FPGO) with functional linear polysiloxane brushes was synthesized via surface precipitation polymerization (sol-gel) and chemical modification. Then, FPGO was covalently cross-linked to the sulfonated polysulfone (SPSU) matrix to obtain novel SPSU/FPGO cross-linked nanocomposite membranes. Meanwhile, SPSU/GO composite membranes and a pristine SPSU membrane were fabricated as control groups. Reduced agglomeration of the inorganic filler and better interfacial interaction, which are benefit to increase diffusion resistance of methanol and to generate continuous channels for fast proton transportation at elevated temperature, were observed in SPSU/FPGO cross-linked membranes. Moreover, the enhanced membrane stability (thermal, oxidative and dimensional stability) and good mechanical performance also guaranteed their proton conducting durability. It is noteworthy that the SPSU/FPGO-1 cross-linked membrane possesses the best comprehensive properties among all the prepared membranes and Nafion®117, it acquires the highest proton conductivity of 0.462 S cm^-1 at 90 °C under hydrated conditions together with a low methanol permeability of 1.71 × 10^-6 cm² s^-1 at 30 °C. The resulting high membrane selectivity displays the great potential of the SPSU/FPGO cross-linked membrane for DMFCs application.

Scheme 1. Synthesis procedure of PGO, D^H₄/4-chlorostyrene cyclic cross-linker and FPGO.

Scheme 2. Preparation procedure of SPSU/FPGO cross-linked membrane.

Fig. 1. (a) FTIR spectra of GO, PGO and FPGO, (b) TGA curves of GO and FPGO, (c) XRD patterns of GO and FPGO, (d) AFM images and height profiles of PGO.

Fig. 2. Cross-sectional SEM images of (a) pristine SPSU membrane, (b) SPSU/GO-3 composite membrane, (c) and (d) SPSU/FPGO-3 cross-linked membrane.

Fig. 3. (a) XRD patterns of pristine SPSU, SPSU/GO composite and SPSU/GO cross-linked membranes, (b) TGA curves, (c) DTG curves, (d) water uptake curves for various membranes at different temperature, (e) area swelling curves of SPSU and hybrid membranes at 70 °C.

Fig. 4. Temperature-dependent proton conductivity of (a) Nafion®117 and SPSU/GO composite membranes under hydrated condition, (b) Nafion®117 and SPSU/FPGO cross-linked membranes under hydrated condition, (c) and (d) Arrhenius plots of proton conductivity of pristine SPSU membrane, SPSU/GO membranes and SPSU/FPGO cross-linked membranes.