Organic-inorganic (polysiloxane) crosslinked sulphonated poly(ether ether ketone ketone) hybrid membranes for direct methanol fuel cells

Abstract

A series of Ph-m-SPEEKK/polysiloxanes organic-inorganic hybrid proton exchange membranes (HMs) were prepared. The Ph-m-SPEEKK was synthesized from 2-phenylhydroquinone and 1,3-bis(4-fluorobenzoyl)benzene, and sulphonated in a controlled way until a 1.60 meq g−1 ion exchange capacity was reached. The crosslinked polysiloxane phase (PSP) was prepared by the sol-gel route from PDMS as the linear fraction and phenyltrimethoxysilane (PTMS) as crosslinker, the non-crosslinked PSP contained only PDMS. The crosslinked PSP was characterized by FTIR and solid state 29Si NMR, which revealed that the PTMS is fully crosslinked in the PSP structure. SEM micrographs of the HMs showed no apparent macro phase separation of the organic-inorganic components. The macroscopic properties of the HMs investigated were water uptake, swelling, methanol permeability and proton conductivity. The methanol permeability of the crosslinked HMs decreased to a limiting value between 16.3 and 17.9 × 10−7 cm2 s−1 at 60 °C. A continuous increase of the proton ion conductivity (64.1–90.0 mS cm−1 at 80 °C and 95% RH) was achieved with the addition of increasing amounts of the crosslinked PSP content as compared with the non-crosslinked HM. The HM with a 20 wt% of PSP presented a proton conductivity of 90.0 mS cm−1 and an activation energy for proton transport of 17.2 kJ mol−1, both comparable to Nafion 117 and in a single DMFC test the electrode assembly fabricated with this HM reached a maximum power density of 38.0 and 30.5 mW cm−2 for 2 and 5 M methanol concentrations, respectively, higher than Nafion 117 and the pristine Ph-m-SPEEKK membranes.