BIPOLAR PLATES: CUTTING COSTS

A new polymer could help in the struggle to make fuel cells
economically viable.

A start-up company based in the UK – Bac2 – has developed a new material called ElectroPhen, which promises to significantly reduce the cost of producing bipolar plates, and thus reduce the overall cost of fuel cell stacks; this could help with the ultimate aim of helping fuel cell mass adoption, maintains the company’s chairman, James Lewis.

Hydrogen fuel cells provide efficient energy without damaging
emissions, and with heavy backing from major governments, they look set to provide an answer to the environmental problems of fossil fuels, and concerns over oil and gas supplies. Already in use in London buses and Japanese homes, these fuel cells combine hydrogen with oxygen (air) to generate electricity and heat, with the only waste product being clean water.

Originally invented as far back as 1839, fuel cells first came to prominence when they were used by NASA as a power source
in its space missions of the 1960s and 1970s, successfully flying on over 100 missions.

These proved the effectiveness of the technology, and led to a large array of prototype and development applications in
transport, portable and stationary power applications.

In recent years the first of these have seen commercial adoption as auxiliary generators for military or leisure use as well as industrial power back-up. As sales volumes increase
in these niche sectors and corresponding manufacturing costs come down, so other commercial markets will open up leading
ultimately to the mass volume markets of consumer electronics goods and automotive transport.

However, the widespread adoption of fuel cells is currently limited by the cost of key components. In particular, up to 30%
of the cost, and 75% of the weight, of a Polymer Electrolyte Membrane (PEM) fuel cell stack (the most popular type) is due to components called the bipolar plates and end plates.

This is where Bac2’s ElectroPhen comes into play, reportedly cutting the cost of producing bipolar plates, and thereby reducing the cost of fuel cell stacks.

Such cost reductions will help fuel cells– particularly hydrogen or direct methanol types – to be the most likely power technology to replace the internal combustion engine in all our vehicles, as well as becoming an affordable and ‘green’ alternative to batteries in portable electronics such as PCs
and mobile phones.

Bipolar plates

Bipolar plates and end plates interconnect individual cells and provide connections to the outside world. The bipolar plates
conduct electricity, keep the reaction gasses separated and channel away waste water and heat from the reaction.

Bac2’s patent pending ElectroPhen material is ideally suited to easier and lower cost production of conductive composite bipolar plates than existing composite plates made from non-electrically conducting polymers. The development of ElectroPhen began when it was seen as a low cost electrode
material for potential use in advanced electrochemical water treatments. Since then a programme has begun to optimise 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. ElecroPhen’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, through the selective use of curing agents, ElecroPhen has conductive properties, which dramatically expands its potential uses.

The barriers to widespread adoption of fuel cells are cost and power density – best expressed as dollars per kilowatt of power, and kilowatts per cubic metre 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.

Mobile electronics products and automotive applications 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 sufficient structural integrity so that the intricate features of the gas channels can be moulded into it. It must also be robust, have minimal electrical resistance to the flow of current generated within the fuel cell stack, and be very low cost.

A new material

From an electrical point of view, metal bipolar plates are ideal. However, they 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 as little as possible must be used to ensure graphite particles make contact. The disadvantage is that the softness of the graphite particles makes the structure weak; appearing brittle. Furthermore, the manufacturing process usually involves curing by heat, and so takes time, presenting
problems for scaling to high volume manufacture.

The moulding process also leaves a thin surface film of resin which has to be removed by an extra manufacturing step of abrasion. Because of ElectroPhen’s conductivity this step can be avoided all together.

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

Other important physical characteristics of ElecroPhen are its thermal stability, resilience to temperature and inertness
towards the catalyst. This means that stack manufacturers can safely explore the use of different, cheaper, catalyst materials that may require higher temperatures at the reaction surface.

Due to its phenolic resin roots, ElecroPhen 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 moulding companies. This minimises the logistics and supply-chain overhead and ensures there will be no disruption to supply through multiple-sourcing.

Scaling for high-volume production

Today, a number of fuel cell stack manufacturers produce their own bipolar plates, having largely been forced to undertake their own R&D on the most suitable available materials. The volumes produced are only small, so manufacturing techniques appropriate to these volumes, such as CNC milling or high temperature moulding, may be applied. This is reflected in the high cost of stacks available on the market.

However, when the cost/efficiency barriers of fuel cells are overcome, PEM fuel cells will realise widespread adoption in automotive and electronic equipment applications and the volumes required will grow dramatically. 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.

As stacks for automobiles likely to comprise more than two hundred MEA/bipolar plate assemblies, the number of cars built does not need to be particularly high to reach this kind of number. Manufacturability on this scale needs to be considered by stack manufacturers currently pioneering the lower volume commercial markets.

ElecroPhen’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 moulding. 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 moulding, size of the item, and operating cycle of the moulding machine.

The moulding techniques are readily available, 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.

Final thought

ElectroPhen has the potential to become an industry standard for the construction of fuel cells.

A new material or technique often creates the opportunity for a radical rethink on the technology itself. Early-stage experimentation at Bac2 has revealed that using ElectroPhen conductive polymers and composites may make it possible to reduce the number of fuel cell stack components, or simplify assemblies.

The opportunities for significant cost reduction that leads to earlier adoption of fuel cell technologies are therefore tantalisingly close.

www.bac2.co.uk