Fuel cells are said by many people to offer almost the ideal source of power for applications ranging from mobile telephones to automobiles.

A simple reaction between the hydrogen-based fuel and atmospheric oxygen generates electricity, with heat, non-polluting water and carbon dioxide being the only by-products, depending on the exact process. Several types of fuel cell have been proposed but they are all based around the concept of two electrodes separated by a solid or liquid electrolyte. A typical cell produces around 0.6 to 0.7V, so large numbers of cells have to be stacked together to produce useful power levels.
The European Commission has recently established the Hydrogen and Fuel Cell Technology Platform "to facilitate and accelerate the development and deployment of cost-competitive, world class European hydrogen and fuel cell based energy systems and component technologies for applications in transport, stationary and portable power." Such a technology platform should encourage better working between the public and private sectors, between research establishments and industry, and it should ensure that policy-makers are closely involved as well.
One of the key areas of fuel cell development is the bipolar plates. These serve two principle roles: one is to act as a conductor for the electrical energy and the other is to channel the flow of gases to ensure that the electrode is adequately supplied with reactants. The gases flow in a number of finely detailed convoluted flow field channels, typically 0.5 to 2mm wide and up to 1mm deep, formed in the surface of the plate.

A conventional plate design might contain five parallel channels that follow a serpentine route, with the channels produced by computer-numerically-controlled (CNC) machining. However, two recent developments by the UK's Morgan Fuel Cell, part of the Morgan Crucible Company, have led to a plate design that boosts power output by 16percent.
Known as Biomimetic bipolar plate technology, the design mimics the structure seen in animal lungs and plant tissues to allow the gases to flow in a far more efficient way (Fig. 1). The basic principle is that larger channels feed a series of smaller channels and the highly branched flow field distributes gas through a fine system of capillaries. No less than 250 capillary elements are incorporated in the latest design, with 1500 capillary channels.

Improved performance

Compared with serpentine channels, the Biomimetic plate reduces the pressure drop and ensures a more even distribution of gas across the plate, thereby giving improved uniformity for the current density and allowing more power to be extracted from the cell. A reduced pressure drop also implies that lower-specification (and cheaper) fans can be used to pump the gases.
Another benefit of the Biomimetic design is that water management can be improved, water being a by-product of the fuel cell process.
Alongside the geometric innovations, Morgan Fuel Cells has also developed the Electroetch manufacturing technology for forming the channels in the surface of the carbon plates.
Effectively a high-precision grit-blasting technique, the Electroetch process cuts the channels in a single plate in minutes. Moreover, the system can be used as a rapid prototyping technique for taking CAD designs and converting them into a prototype plate in about two hours.
The company says that there is no reason why this technology could not be scaled-up to produce production volumes of plates, and one of the beauties of the technique is that cost is not related to flow field design complexity.
The process utilises a polymer mask that is deposited via screen-printing or as a photo-resist film that can be exposed to replicate the desired flow field. The plate and mask is then placed inside the etching chamber where the channels are cut.
A useful side-effect of the Electroetch process is that the flow channels are radiused at the bottom, benefiting the gas flow. This is in contrast to machined channels that have near-square corners.
Electroetching can produce features as small as 150microns and tolerance can be held to within ±25microns. The minimum track width and plate thickness are 0.2mm and 1.0mm respectively.
While tests have already shown an increase in power of 16 per cent, the researchers believe that further improvements are possible. Morgan Fuel Cell has so far concentrated on the graphite bipolar plates used in PEM (proton exchange membrane) type fuel cells favoured for automotive and general power replacement applications. However, Biomimetic flow field designs are potentially applicable to ceramic and metal bipolar plates, and the core design has already been adapted for use in direct methanol fuel cells and may yet find applications within solid oxide fuel cell systems.