Figure 1: A simplified representation of a Polymer Electrolyte Fuel Cell stack

Makers of fuel cell stacks say that the search is still on for the perfect bipolar plate. Perfection is seldom achieved, but the ideal solution may not be far off, thanks to the development of a range of conductive polymers by a British start-up called Bac2.

The development of ElectroPhen, Bac2's trade-marked material, began when it was seen as a low cost electrode material for potential use in advanced electrochemical water treatments. Since then a program has begun to optimize the material for fuel cell applications and the company is also planning its use in a range of other applications from electrostatic protective coatings to organic semiconductors and EMI shielding. ElectroPhen's heritage goes back to the birth of plastics. It has a polymeric structure that is basically phenolic, like that of Bakelite. Bakelite was developed during the first decade of the 20th century and used for its insulating properties in electrical fittings and appliances. By contrast, though the selective use of curing agents, ElectroPhen has conductive properties, which dramatically expands its potential uses. Ideal bipolar plate
The "ideal" bipolar plate
As the imperative of developing commercially viable sources of alternative energy breaks into the consciousness of public and politicians alike, the potential offered by fuel cells as a way of reducing dependency on fossil fuels and helping to manage climate change has been highlighted. The fuel cell concept was discovered in the 19th century, but its significance was starting to be realized in the 1960s and 70s when fuel cells were used as primary electrical sources by NASA on its space missions. At this time, its potential for military applications became apparent too. These tend to be "money-no-object" applications where the high price of raw materials mattered little. But it did prove the effectiveness of fuel cell technology, and pave the way for today's limited adoption in some public transport applications, industrial power supply back-up, and domestic power systems.

Looking ahead, it is accepted that fuel cells — particularly hydrogen or direct methanol types — are the most likely power technology to replace the internal combustion engine in all our vehicles, as well as becoming a cost-effective and environmentally clean alternative to batteries in portable electronics equipment, such as PCs and cellular phones. The only barriers to widespread adoption are cost and efficiency — best expressed as dollars per kilowatt of power, and kilowatts per cubic meter of volume — and physical strength or toughness. Key elements contributing to cost are the bipolar plates, which direct the gases to the reaction surface, and the MEAs (membrane/electrode assemblies). The bipolar plates also make the most significant contribution to the physical size of a fuel cell.

Figure 2: ElectroPhen bipolar plates like these have the potential to significantly reduce the cost and complexity of fuel cells]

Automotive traction and mobile electronics products are harsh environments, where long-term reliability is essential. This means that the ideal bipolar plate needs to be constructed from a material that has the structural integrity to enable the intricate features of the gas channels to be molded into it. It must also be robust, have minimal electrical resistance to the flow of current generated within the fuel cell stack, and cost very little. ElectroPhen
The ElectroPhen
Metal bipolar plates are ideal from an electrical point of view but require an expensive passivation process to prevent degradation from reaction with the catalyst, and a costly and time-intensive manufacturing process whereby the channels are etched or milled into the metal surface. Compressed graphite granules held in a resin are sometimes adopted. These resins, such as epoxy, are by nature insulators, so an absolute minimum must be used to ensure graphite particles make contact. The downside is that the softness of the graphite particles makes the structure brittle. Furthermore, the manufacturing process usually involves curing by heat, and so takes time, presenting problems for scaling to high volume manufacture.

ElectroPhen's raw state conductivity is in the order of 10⁹ (a billion times) more conductive than most common plastics, which means that far less conductive filler needs to be added to bring it to an acceptable conductivity for bipolar plates. This means that the strength of the ElectroPhen resin makes for a much tougher plate, and further modification with plasticizers, reinforcers, and conductive fillers enables the composition of ElecroPhen to be 'fine-tuned' for specific applications and customer requirements.

Other important physical characteristics of ElectroPhen are its thermal stability, resilience to temperature and inertness towards the catalyst. This presents the opportunity for stack manufacturers to safely explore the use of different, cheaper, catalyst materials which may demand higher temperatures at the reaction surface.

By virtue of its phenolic resin roots, ElectroPhen is cheap to manufacture, with the basic raw materials being widely available from major chemical suppliers. As a result, bulk quantities of raw materials, or better still, pre-mixes containing conductive fillers to Bac2's specification, can be supplied directly to molding companies. This minimizes the logistics and supply-chain overhead and ensures there will be no disruption to supply through multiple-sourcing.

Volume manufacturing
The trend currently is for fuel cell stack manufacturers to manufacture their own bipolar plates, having largely been forced to undertake their own R&D on the most suitable available materials. It is safe to say that volumes produced are only small, and that manufacturing techniques appropriate to these volumes, such as CNC milling or high temperature molding, may be applied. This is reflected in the high cost of stacks available on the market today.

However, we have to look ahead to the point at which the cost/efficiency barriers are behind us, and PEM fuel cells realize widespread adoption in automotive and electronic equipment applications. With stacks for automobiles likely to comprise more than two hundred MEA/bipolar plate assemblies, at the point where it becomes viable for the world's leading car manufacturers to introduce a fuel cell-powered vehicle to the mass market, the manufacturing requirement will rise rapidly to the magnitude of one million plates per day. Manufacturability on this scale needs to be considered by stack manufacturers currently pioneering the lower volume commercial markets.

ElectroPhen's room-temperature cure makes for easy scalability, from rapid-prototyping by CNC milling from pre-prepared blanks through to high volume compression or injection molding. Trials with hardener formulae have yielded cure times from as little as a few seconds up to tens of minutes, so it is easy to adapt the mix to suit the type of molding, size of the item, and operating cycle of the molding machine. The molding techniques are readily available and unsophisticated, presenting the opportunity for low-cost manufacture in developing countries where under-developed fossil fuel infrastructure reduces the entry barriers to adoption of a fuel cell based hydrogen economy.

Bac2 -- the future
Often in technological progression, the introduction of a new material or technique opens the door to a radical rethink on the technology itself. Early-stage experimentation at Bac2 reveals that it may be possible to reduce the number of fuel cell stack components, or simplify assemblies through the use of ElectroPhen conductive polymers and composites. The opportunities for significant cost reduction that leads to earlier adoption of fuel cell technologies are therefore tantalizingly close.

About the author

James Lewis, Non-executive Chairman. James (44) is an electronics engineer and entrepreneur who has worked in Europe and the United States. He co-founded Oxford Semiconductor in 1992 and headed up its sales and marketing activities, turning it into a global business with 150 employees. After returning to the UK from the US James formed Parallaxis (http://www.parallaxis.com) to apply his experience and knowledge in supporting start-up and small hi-tech companies to realize their full potential. James has a degree in electronic engineering from the University of London. Formed in 2002, Bac2 has recently completed a significant funding round, enabling it to embark on an extensive research and development program that we believe will ultimately see ElectroPhen becoming an industry standard for fuel cell materials. For more information: Mike Stannard, mike@bac2.co.uk .
www.bac2.co.uk