Fuel Cells on Continuous Duty

It runs and runs and runs. With 25,000 hours, the Jülich hightemperature fuel cell has achieved a new endurance run world record in June 2010. Good preconditions in order to provide not only cars or laptops with power in the future, but also homes and industrial processes. For fuel cells are hot candidates for a future and also more decentralised power supply. They feature high degrees of electric efficiency of up to 60 percent and do not emit harmful fumes. “Fuel cells are already equipped for the average 5,000 hours life-span of a car motor, yet in the household or for industrial applications life-spans of above 40,000 hours often are desired and this they do not achieve as yet”, says Dr. Robert Steinberger-Wilckens, head of the fuel cell project at the Forschungszentrum Jülich.

Fuel cells obtain electric power from charge separation of hydrogen and oxygen on specially coated electrodes and the ensuing reaction of the positively charged hydrogen with the negatively charged oxygen to form water. Similar to a battery, these electro-chemical energy converters also lose performance over time, amongst other reasons, because the reactivity of surfaces decreases. At the Forschungszentrum Jülich, fuel cells operating at high temperatures are being researched. In doing so, the electrolyte is not liquid but consists of a ceramic membrane layer between the electrodes. It allows the negatively charged oxygen particles to pass from one electrode to another only at temperatures from 600 to 1,000 degrees Celsius. The high-temperature cells have higher degrees of efficiency and the high temperatures can be used for further processes by way of cogeneration systems.

Furthermore, they can be operated with various fuels – in addition to hydrogen also with natural gas or biogas. “However, the high temperature brings with it also some challenges, for the materials age faster”, says Steinberger-Wilckens. For example, the ceramic membrane loses its conductivity for oxygen because the crystalline structure slowly changes. And the so-called metallic interconnectors connecting several cells to form a more powerful stack oxidise more easily and can evaporate material, which deposits in the reactive layers and clogs these.

In order to prevent this, the Jülich researchers have decisively improved the materials and thus created the preconditions for their endurance run record. Thanks to newly developed ceramics, the operations temperature could be lowered. This in turn allows the use of cheaper steel for the interconnectors. In close cooperation with industry partners, two special steels were developed, which in combination with optimised protective layers render the cell much more robust. Also, they have the same expansion behaviour as ceramics, so that only minor mechanical tensions occur during heating. “The performance gain is distinct and a major step towards application”, Steinberger- Wilckens sums up.

If the fuel cell really is to play a major part in the provision of energy at some stage, it requires not just smooth operation. It is also important to secure the supply of fuel. For this, hydrogen, yet also methane from natural gas, gas from purification plants or landfill gas or diesel reformates can be considered. The main problem appears to be storing the hydrogen and the (energy) effort in producing the fuel.

Whereas cars could be powered entirely by hydrogen in the future, many heavy goods vehicles, ships or aeroplanes will continue to be powered by fossil fuels for the foreseeable future. Even so, fuel cells are an efficient alternative for alternators or generators to provide power on board for cooling units, air conditioners or avionics. For this purpose, they can be operated with the already available fuels. The key to this is a so-called reformer, which produces a hydrogenous gas as fuel for the fuel cells from diesel fuel or kerosene. Jülich researchers from the team around Dr. Ralf Peters developed an especially effective and long-lived reformer for this purpose.

The trick: Diesel fuel is sprayed in very fine droplets into a specially shaped chamber and mixed with hot water vapour and air, before it is conveyed – as in automotive catalytic converters – over a reactive surface, where it is separated into hydrogen, carbon monoxide and carbon dioxide. In order to optimise the hydrogen yield, the researchers calculated the complex flow characteristics at the Jülich super computer and thus developed the innovative reformer. For it is only in combination with such elements that fuel cells can unfold their versatile talents.