Feature

Fuelling the Future

Fuel cells offer an important, efficient energy source.

By Dr N. M. Sammes

Fuel cells have attracted considerable interest over the past decade as highly effective and environmentally acceptable sources of electrical energy. They provide a means of electro-chemically converting the chemical energy of fossil fuels into energy without having to burn the fuel, producing water, electrical energy and, on occasion, carbon dioxide.

Among all the different types of fuel cells, it is believed that solid oxide fuel cells (SOFCs) offer the widest potential for applications and high system efficiency.

SOFCs are highly efficient electrochemical devices that can operate at atmospheric pressures and temperatures in excess of 1000^oC to produce electricity from fossil fuels such as coal gas, natural gas, or distillate gas. The temperature from the exhaust is high enough for it to be used for further electrical production in co-generation applications. This pushes the efficiencies up past 80%.

In an SOFC, oxygen ions are formed at an air electrode (cathode), where they move through a solid electrolyte to the fuel electrode (anode). Here the oxygen reacts with the fuel gas with the onset of electricity. Internal or external steam reforming of methane (natural gas) to carbon monoxide and water are the main processes used for the feed at the anode compartment of the system.

There are currently three main designs for the SOFC system:

• the tubular design, as used by Westinghouse in the US. Two 25-kilowatt units are being tested by Osaka Gas and Tokyo Gas in Japan.
• the monolithic design. This was used by Argonne National Laboratories in the United States, but has now been discarded as being too difficult. A redesigned 1-kilowatt monolithic system is currently being built in Japan.
• the planar design, currently under development in New Zealand.

Each design has its advantages and disadvantages, but the cost factor, in terms of dollars per kilowatt produced, is one of the most important. Westinghouse is now selling 25-kW units for $300,000 ($12,000/kW) for the basic unit, although Osaka Gas paid over US$3,000,000 for the unit and the licence. This high cost is perceived as being due to the very high material, development and fabrication costs of the cell.

A programme has now been set up at the University of Waikato to investigate the feasibility of SOFC technology for New Zealand. The planar SOFC design has been selected because of its high power density and low specific material consumption as well as relative ease of fabrication.

The idea is to build a modular unit that can be added to other units to increase the total power output. Initial work on single cell systems, using a 5-centimetre-square electrolyte sheet, has given very promising results at 1000^oC using a hydrogen/steam fuel.

The technology at this stage is focused on ceramic fabrication. There are many steps involved in producing a ceramic fuel cell. Firstly, the electrolyte has to be fabricated. This can be done a number of ways. We have chosen to tape cast a thin film of zirconia-based electrolyte and fire the "green" ceramic at temperatures in excess of 1800^oC. The electrodes are then air-brushed onto both sides of the electrolyte, although other coating techniques are being investigated, including screen printing.

The anode is based on a metal or metal oxide ceramic metal composite (cermet) due to its cheap cost, its durability, its high catalytic activity and its relatively high conductivity. The cathode is usually based on a metal oxide material for the same reasons.

The next challenge is the high temperature sealing of the electrolyte. The sealant has to withstand continuous running temperatures in excess of 950^oC -- quite a tall order. This is still a problem not yet fully resolved. However, we have fabricated single cells and successfully tested them under hydrogen/steam gas feed. The target of this work is to build a 100-watt stack system by the end of next year.

Other fuel supplies, such as renewable energy resources, are also being investigated. These differing fuels do, of course, have their own effects on the efficiency of the cell. We need to isolate compatible fuels so that this country can confidently enter the next century without the restrictions on power generation and usage inherent over the past several years.

Nigel Sammes is a lecturer in Materials Science at the University of Waikato.