Structured polymer electrolyte blends based on sulfonated polyetherketoneketone (SPEKK) and a poly(ether imide) (PEI)

The performance and economics of proton-exchange membrane (PEM) fuel cells are highly dependent on the membranes used to separate the fuel and oxidant. While maintaining reasonable cost, the membrane must feature a number of desirable properties including high proton conductivity. Blends of polymers are one approach to tailoring PEM properties; however, blending to achieve mechanically and chemically robust membranes has generally resulted in reduced conductivity. The objective of this work was to demonstrate the use of field alignment of the proton-conducting domains to increase the conductivity in a polymer blend PEM. A blend of sulfonated poly(etherketoneketone) (SPEKK) and a polyether imide (PEI) was used to illustrate this method. Blends of SPEKK/PEI with a 3:7 mass ratio were aligned using electric field strengths varying from 0 to 30 V/mm and frequencies varying from 0 to 10 kHz. In general, the degree of alignment agreed with theoretical predictions for the alignment of drops or particles suspended in a fluid with a different dielectric constant, e.g., when the frequency of the applied ac field was increased, the threshold field for phase alignment increased and the diameter of the oriented columnar structures decreased. Alignment resulted in up to three orders of magnitude increase in conductivity at low humidity. By careful selection of temperature and residual solvent content, alignment was shown to be possible in the melt state, which is essential for an economic process for producing alignment-enhanced membranes.