Chinese scientists have developed membranes that could improve direct methanol fuel cells (DMFCs). 

DMFCs oxidise methanol to produce small amounts of electricity over long periods, making them ideal as portable power supplies. A vital component of DMFCs is the membrane, which separates the two reactions of the cell while allowing protons to move between them. But sometimes unreacted methanol can also pass through the membrane, which reduces the fuel efficiency and performance. 'This methanol cross-over is an Achilles' heel for the implementation of the DMFC,' says Yohannes Kiros, an expert in fuel cells and energy at the Royal Institute of Technology, Stockholm, Sweden. 

Lei Li of Shanghai Jiaotang University has tackled this problem by modifying commonly-used perfluorosulfonic acid membranes. In these membranes, the polar sulfonic acid groups gather into clusters forming channels through which transport occurs. 'It is well known that the transport properties of the membranes are directly affected by the [density of sulfonic acid groups],' says Li. 

By varying this density, Li was able to find a membrane whose selectivity properties exceed those of current, commercially available ones. 'Compared with Nafion^® 117 membranes, our membrane has a similar proton conductivity but the methanol permeability is about 80% less,' he says. 

'This work dwells on one of the most important aspects of dealing with the methanol cross-over in DMFC,' says Kiros. However, he also notes that other factors such as operating conditions, methanol concentration and acid equilibration need to be investigated. 

Li acknowledges this need for further data: 'We are continuing to work on these membranes to improve their performance. Testing of our membranes in real DMFCs and optimization of the membrane formation process are now underway in our lab,' he says. 

Philip Robinson 

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