Polymer cuts the cost of fuel cells

ElectroPhen is much better suited to production of conductive composite bipolar plates than normal non-electrically-conducting polymers, says James Lewis, Chairman of Bac2.

Fuel cells combine hydrogen with oxygen (air) to generate clean electricity and heat, the only waste product being clean water.

However, the widespread adoption of fuel cells is 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.

A UK startup company called Bac2 has developed a new material - ElectroPhen - that promises to significantly reduce the cost of producing these 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 is much better suited to production of conductive composite bipolar plates than normal 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 has a polymeric structure that is basically phenolic, like 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 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 electronic 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.

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 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 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.

Today, a number of fuel cell stack manufacturers produce their own bipolar plates, having largely been forced to undertake their own R and 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.

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.

Stacks for automobiles are likely to comprise more than two hundred MEA/bipolar plate assemblies.

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.

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.

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