Improving ionic conductivity in fuel cells

Materials World magazine
1 Oct 2008
A 3D view of the zirconia/strontium titanate interface

Spanish researchers hope to enhance the efficiency of solid oxide fuel cells (SOFCs) by producing a super lattice that improves ionic conductivity near room temperature by a factor of almost 100 million. Solid oxide fuel cells rely on the passage of oxygen ions.

Using thin film technologies, scientists at the Universidad Complutense and the Universidad Politécnica in Madrid, both in Spain, have combined yttria stabilised zirconia (a typical fuel cell electrolyte material) with strontium titanate (an insulator) as an alternative to conventional electrolyte materials that include zirconia, samarium and gadolinium.

‘The crystalline structure of the two materials is very different. If you grow one on top of the other, the disorder promotes ionic diffusion,’ says Dr Jacobo Santamaria, lead researcher on the project. The disordered activity at the interface of the two materials creates numerous spaces that allow the ions to travel more quickly and easily from the cathode to the anode than in other electrolyte materials, which have narrower pathways.

This conductivity is maintained even when close to room temperature, claims Santamaria. Existing materials have to achieve high temperatures of 800-1,000ºC, restricting application into the full range of heat and power products. These temperatures increase resistance and constrain the performance of the device, putting stress on the seals between the compartments and placing high demands on the materials used. They also increase the cost of the end product.

Further analysis

This improved conductivity, however, is not yet fully understood. Santamaria says, ‘The empty spaces at the interface assist conduction but this is probably not the only reason’. The structural characteristics of the material have been probed using a 300kV Z-contrast scanning transmission electron microscope at the Oak Ridge National Laboratory in the USA.

Dr Steve Skinner, a specialist in SOFC electrolytes and electrodes at Imperial College London, UK, applauds the project. He describes it as ‘an exciting piece of work. If it enhances conductivity, it could open up all sorts of avenues for exploitation’.

But many unknowns are still to be clarified. In particular, Skinner argues that the scientists need to prove the conductivity is ionic. The Spanish team says DC measurements have proved negative, showing that the conductivity was likely to be ionic. ‘But we cannot definitely prove it’s due to oxygen [yet],’ says Santamaria.

Another concern is lowering of the temperature. ‘If you reduce the temperature, you have to develop oxygen reduction catalysts, because the gaseous oxygen reduction, necessary for the fuel cell to operate, is tricky at room temperature,’ explains Skinner.

Santamaria agrees, ‘Our discovery relates only to the conductivity of the electrolyte. Engineering problems remain to be resolved, for instance, how to speed up electrode reactions’.

The team will also continue combining different materials along the same principles. Santamaria claims that it should be possible to produce all of these materials on an industrial scale, as they are already used in the microelectronics industry.

Skinner adds, ‘Hetero-structural films have implications for other technologies too – including batteries. Materials that only operate at higher temperatures could now be used at lower [ones]. This development could be significant’.