Ceramic art

Next-generation iron-oxide membrane could help fuels operate without loss of efficiency in higher temperatures and lower humidities. Siobhan Wagner reports.

An iron-oxide ceramic membrane could allow fuel cells to operate without losing efficiency as the temperature rises and the humidity drops.

This could benefit many fuel cell applications such as cars driven in extreme heat, submarines and remote weather stations.

Mark Wiesner, the project’s lead researcher at North Carolina’s Duke University’s School of Engineering, said current polymer cell membranes only function efficiently in humid environments.

‘If the polymer membrane dries out, its efficiency drops,’ he said. ‘We have developed a ceramic membrane made of iron nanoparticles that works at much lower humidities and higher temperatures.

‘The efficiency of current membranes drops significantly at temperatures over 190ºF,’ said Wiesner. ‘However, the chemical reactions that create electricity are more efficient at high temperatures, so this advance would be a big improvement for fuel cell technology.’

Wiesner said a higher working temperature is better for fuel cells because it reduces the fouling of catalysts when there are impurities in the fuel.

Cells that run on higher temperatures are also important for managing heat. ‘If you drove a fuel-cell car across the Sahara you’d want the internal temperature to be higher than the external,’ he said. ‘Otherwise you’d need a large additional heat management system.’

There are many types of fuel cells, but most generate electricity when an external fuel — most commonly hydrogen — and an agent chemically react. The membrane that separates the two parts of the cell is key in determining the efficiency.

‘A fuel cell is like a sandwich,’ said Wiesner. ‘The bread would be catalyst layers — an anode and cathode. Whereas the filling is the membrane.’

he said the membrane must transport protons efficiently from one catalyst layer to the other, and it has to insulate the two electrically.

Wiesner tested his membrane with a platinum catalyst at room temperature and measured its electrical properties after fuelling it with both hydrogen and methanol. He recorded astonishing results with methanol, but only meagre outcomes with hydrogen.

‘Our next step is to run this up to higher temperatures between 90ºF and 130ºF,’ he said. ‘Typically, performance improves with higher temperature.’

The membrane most commonly used today, known as Nafion, was discovered in the 1960s. As the temperature rises, the polymer becomes unstable and the membranes dehydrate, leading to a loss of performance.

In addition to its temperature and heat limitations, Nafion is also much more expensive to produce than the ceramic membrane, said Wiesner, adding that membranes make up as much as 40 per cent of the overall cost of fuel cells.

More costly than the membrane, however, is the catalyst. The most popular material being used for this is platinum, but there are projects looking for viable alternatives.

Wiesner said that in the future he could envisage his membrane working with a catalyst made of something other than platinum.

Fuel cells are currently used in submarines and remote weather stations because they have no moving parts, do not require combustion and can run unattended for long periods of time.

Wiesner said one unique application could be in back-up power generators that also produce small amounts of highly purified water for industrial processes.

‘Any combustion process will produce water,’ he said, ‘and this is basically a combustion process that is catalysed.’

Before Wiesner and his team begin they plan to study new ways of fabricating the membrane to improve its durability and flexibility.