Overview of hybrid membranes for direct-methanol fuel-cell applications

The direct-methanol fuel cell (DMFC), a type of polymer-electrolyte membrane fuel cell, has lately received much attention because of its potential applicability as a good alternative power source for the future. In order to achieve commercially viable performance goals for the DMFC, a membrane with several important selective behaviors will need to be developed. Over roughly the past four decades, researchers have used the commercial Nafion membrane by DuPont as a proton-conductive membrane in DMFCs due to its chemical stability and high proton conductivity, as well as high mechanical strength. However, Nafion membranes also have several weaknesses such as high methanol permeability and an operational temperature limited to ∼100 °C or lower, and Nafion is also a very expensive material. Besides Nafion, there have been several engineering thermoplastic polymers such as poly(etheretherketone) (PEEK), polysulfone (PSF) and polybenzimidazole (PBI) used as alternative membranes due to their lower cost and very high mechanical and thermal stability in high temperature operation. To date, there has been continuous extensive research on developing a membrane which can fulfill all of the essential characteristics to yield the desired performance in DMFCs. In the course of this research, hybrid membranes have been developed by modifying the original membranes to produce new membranes with variously enhanced properties. This review discusses recent advances in hybrid membranes of two main types: Nafion-based and non-Nafion-based membranes. Recent achievements and prospect of applications also been included in this paper.