There are many types of fuel cells that convert a source fuel into electrical energy through chemical reactions in an electrolyte. One common type is the proton-exchange membrane fuel cell, which delivers chemical energy from the reaction between hydrogen and oxygen to form water. This type of cell relies on a membrane that can pass protons while keeping the gases separated. The polymer known as Nafion is usually used for this purpose: it combines perfluorinated chains with sulfonate groups and has high proton conductivity and good stability. However, such polymers have poor thermal stability and high gas permeability, which are disadvantages for application in fuel cells. Now, Takuya Tamura and Hiroyoshi Kawakami from the Tokyo Metropolitan University in Japan1 have made membranes composed of sulfonated copolyimide that have high proton conductivity, low oxygen permeability and good chemical and thermal stability.

Fig. 1: Scanning electron microscopy image of electrospun nanofibers.From Ref. 1. Reproduced with permission. © 2010 ACS

The composite membrane developed by Tamura and Kawakami is made up of sulfonated polyimide nanofibers aligned within a matrix of sulfonated polyimide. The fibers (Fig. 1) are formed by ‘electrospinning’, a method that exploits the electrostatic interactions between the different domains of the copolymers. The arrangement of sulfonate groups creates a proton channel structure within the electrospun nanofiber, and the highly uniform alignment of the polymers in the fiber provides high mechanical strength.

Embedding the aligned nanofibers within the composite membrane resulted in a tenfold increase in proton conductivity in the direction parallel to the nanofibers. This enhanced conductivity may be a result of the large amount of water bound within the material. Preliminary results for an experimental fuel cell based on this composite membrane showed improved power density and polarization curves compared with similar non-fiber membranes.

One drawback at present is that it is only possible to measure proton conductivity in the direction parallel to the membrane surface, rather than across the thickness, which is the direction of proton current in the membranes used in electrolyte-based fuel cells. In the future, the researchers hope to establish a way to measure the proton conductivity of the membranes in the thickness direction, or fabricate membranes with a transverse fiber alignment.