Fuel Cells

Fuel cells are now available, based on the use of zirconia as an oxygen ion conductor.

A fuel, typically hydrogen passes on one side of the zirconia electrolyte which is usually in the form of a tube or stack. The diffusion of oxygen ions through the electrolyte from the fuel interface to the air interface leads to the generation of an e.m.f.

It is anticipated that such devices will have a major influence on the supply of clean power in the next millennium.

Solid oxide electrolyte fuel cells have been developed by the use of stabilised zirconia as an oxygen ion conductor. The cell works in reverse to a hydrogen generator but at a somewhat higher temperature, approaching 1000°C. A diagram of this cell is shown in below.

Figure 1. Schematic diagram of a fuel cell using zirconia electrolyte. The oxidation of the fuel generates directly the electric current to the external circuit

The electrolyte is usually in the form of a tube with the fuel passing along one surface and the air along the other.

Diffusion of oxygen ions through the zirconia electrolyte allows the oxidation process to occur on the anode.

The ionisation of the oxygen molecule takes up to 4 electrons to the cathode thereby generating a current in conjunction with the oxidation of the fuel at the anode where the electrons are generated.

The open cell voltage at 1000°C given by the Nernst equation is approximately 1 volt and to provide higher current and voltages, the cells are combined both in parallel and in series.

The preferred fuel is hydrogen due to the cleanliness, but others are possible, and up to 90% fuel utilisation can be obtained.

In addition to the generation of a direct electrical current, the hot exhaust gas (~ 800°C) can be utilised in the normal fashion, thereby giving the fuel cycle a high overall efficiency.

Test performance of the cells for over 1000 hours has taken place with only a minor (<2%) voltage degradation. A crucial factor is the deposition of carbon on electrodes which can poison their efficiency; provided the correct fuel mixtures are maintained this problem can be avoided.

An interesting possibility lies in the use of such cells to carry out controlled oxidation of fuels such as natural gas. By predetermining the degree of oxidation of a gas such as methane or ethane, the oxidation product could be the alcohol or the ketone, and thus a resource such as natural gas could be used as a fuel to produce electricity and the by-product could be used as a chemical precursor.

Technox^® tubes and plates are now used by many organisations working on the development of zirconia fuel cells.