Human lungs and leaves show way to more power

Tom Shelley reports on how lessons learned from nature could be of great benefit in what is expected to be the next generation of automotive power sources

Fuel cells are able to produce significantly higher outputs when the gas channels in their plates are made to imitate passages in animal lungs and plant leaves.

Applying the same approach to the design of osmotic water purifiers and other chemical processing operations should bring equal benefits.

The idea comes from Morgan Fuel Cell, which makes electrode plates out of size graded graphite granules, bound by a few per cent of epoxy or polyphenylene sulphide for cells fuelled by methanol as opposed to hydrogen. The manufacturing process involves extrusion as tube, followed by slitting, rolling flat and curing. Channels are then produced by the company's patented ElectroEtch technique, which relies on precision grit blasting. In the past, gas channels have been made to follow a serpentine path. A fuel cell requires hydrogen to be supplied to the anode side of the electrolyte, and oxygen containing air to the cathode, where water is formed as a result of the cell reaction.

Dr Mark Turpin, global director of Technology explains, "We realised by looking at how animal lungs and plant leaves breathe, that a structure of large distribution channels feeding progressively smaller capillaries is the most efficient way to distribute reactants. So we mimicked this approach in the Biomimetic plate, with a highly branched flow field that distributes gas through a fine system of capillaries. This structure reduces the pressure drop found in the industry-standard serpentine design of flow field and ensures a more even delivery of gas across the bipolar plate, so that more power can be extracted from the fuel cell. Initial results are very promising, with tests already confirming a 16% increase in peak power, and we are certain that even more significant improvements can be made."

The plates are primarily intended for use in the proton exchange membrane type fuel cells favoured for future automotive and general power replacement applications. Turpin told Eureka that current densities are now up to 1.5 A/cm2 and lives are getting up to 2,000h at those current densities. The next generation of Ballard Power products are expected to achieve 20,000h. (It is already possible to achieve much higher life expectancies but only by working at very low current densities and overall system weights too high for automotive use).

Biomimetic flow fields incur the same costs to make as serpentine patterns and are potentially equally applicable to ceramic and metal bipolar plates. The core design has been adapted for use in direct methanol fuel cells and may find application within solid oxide fuel cell systems. The base idea is of potential usefulness in any system when gas or liquid has to be supplied or applied to one side of a membrane with which it interacts. Mother Nature knows best.

Morgan Fuel Cell c/o Thermal Ceramics
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* A structure of large distribution channels feeding progressively smaller capillaries is the most efficient way to distribute reactants

* Applied to fuel cell electrodes, it permits a 16% increase in peak power output so far, with greater increases anticipated in future

Tom Shelley

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