Brett, Atkinson, Brandon and Skinner of the Imperial College Fuel Cell Network, London, UK, look at how advances in materials and engineering are presenting new opportunities for solid oxide fuel cells.

Intermediate temperature solid oxide fuel cells hold the middle ground in the temperature scale of fuel cell operation

Fuel cells are electrochemical energy conversion devices that convert the chemical energy in fuel directly into electricity and heat without combustion. Simplistically, a fuel cell can be viewed as a cross between a battery, which converts chemical energy directly into electrical energy, and a heat engine, a continuously fuelled, air breathing device. There is a range of different fuel cell technologies, each with its own materials set and operation temperature, ranging from room temperature to over 1000 degrees Celsius. However, they all share the characteristics of high efficiency, no moving parts, quiet operation and low or zero emissions.

There is no consensus as to the optimal operating temperature of fuel cells; the preferred temperature of operation depends to a large extent on the application. However, there is significant effort to raise the operating temperature of polymer electrolyte fuel cells (PEFCs) and reduce the operating temperature of solid oxide fuel cells (SOFCs). PEFCs currently operate at around 80 degrees Celsius and are used in automotives, mobile phones and laptops. SOFCs operate at more than 800 degrees Celsius and use a ceramic oxide ion-conducting electrolyte to generate energy on a large scale.

Advances in the chemistry and processing of materials are allowing the operating temperature of SOFCs to be lowered into the so-called 'intermediate temperature' (IT) region of 500 to 750 degrees Celsius. The IT-SOFC opens up a new range of applications and opportunities for SOFCs in areas formally dominated by PEFCs, while maintaining the ability to operate on hydrocarbon fuels and produce high quality heat.

Operation in the IT range expands the choice of materials and stack designs that can be used compared to conventional high temperature (HT) SOFCs. Lower temperature operation affords more rapid start-up, improved durability, reduced system cost and more robust construction through the use of compressive seals and metallic construction materials (as opposed to the all-ceramic HT-SOFCs).

There are two main routes by which SOFCs can be used at lower temperatures while still attaining comparable performance to the higher temperature technology. The first involves reducing the thickness of the electrolyte to the order of a few 10s of micrometres, so ions can travel more easily through the fuel cell. Alternatively, the same result can be achieved by improving the electrolyte's ionic conductivity at lower temperatures and the electrodes' electrochemical performance.

The range of new applications for the IT-SOFC includes soldiers' personal power supplies, traction power for vehicles, remote telecommunications, power for isolated communities and back-up power units for trucks. However, it is the small-scale combined heat and power market where the IT-SOFC is particularly well suited. Operating on natural gas and with a heat-to-power ratio close to one, IT-SOFCs with an electrical power rating of about one kilowatt are expected to be popular as combined heat and power sources for use in the home. Indeed, IT-SOFCs have the potential to be the simplest fuel cell system and are a strong contender to be the first fuel cell technology to reach mass market.

As with all fuel cells, the cost of IT-SOFCs must be reduced for them to compete in the market with current technologies. Using less, and cheaper, material is necessary; moving to lower temperature operation represents a significant step in this direction. Scientists still need to develop IT-SOFCs with commercially meaningful levels of durability. Fundamental studies are improving our understanding of processes such as electrode sintering, anode-fuel interaction, electrocatalyst poisoning and the mechanical properties of electrolytes and support structures.

Read more in 'Intermediate temperature solid oxide fuel cells' in issue 8 of Chemical Society Reviews.