Fuel cells combine hydrogen with oxygen to generate "clean" electricity and heat, with 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-electrolytemembrane (PEM) fuel-cell stack (the most popular type) come from components called bipolar plates and end plates.
As a remedy, U.K. startup Bac2 developed a new material called ElectroPhen, which promises to significantly reduce production costs of those plates.
Bipolar plates and end plates interconnect individual cells and provide connections to the outside world. The bipolar plates conduct electricity, keep the reaction gases separated, and channel away waste water and heat from the reaction (Fig. 1).
Bac2's patent-pending ElectroPhen is better suited to production of conductive composite bipolar plates than normal non-electrically conducting polymers. 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. The company also plans to use it in a range of other applications, from electrostatic protective coatings to organic semiconductors and EMI shielding.
ElectroPhen has a polymeric structure that's basically phenolic, like Bakelite. Bakelite, developed during the first decade of the 20th century, was used for its insulating properties in electrical fittings and appliances. By contrast, through the selective use of curing agents, ElectroPhen 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). Bipolar plates also make the most significant contribution to the physical size of a fuel cell (Fig. 2).
Mobile electronic products and automotive applications are harsh environments that require long-term reliability. Therefore, the ideal bipolar plate must be constructed from a material with sufficient structural integrity.
Consequently, then, the intricate features of the gas channels can be moulded into the plate. The bipolar plate must also be robust, and have minimal electrical resistance to the flow of current generated within the fuel cell stack. In addition to that, its cost must be kept as minimal as possible.
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.
Sometimes compressed graphite granules held in a resin are adopted. By nature, these resins (such as epoxy) are 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. Thus it takes time, presenting problems for scaling to high-volume manufacture.
The moulding process also leaves a thin surface film of resin. As a result, it must be removed by an extra manufacturing step of abrasion. Due to Electro-Phen's conductivity, however, this step can be avoided all together.
ElecroPhen's raw state conductivity is on the order of 109 (a billion times) more conductive than most common plastics, which means that less-conductive filler must be added to bring it to an acceptable conductivity for bipolar plates. The ElectroPhen resin's strength makes for a tougher plate. And with further modification using plasticisers, reinforcers, and conductive fillers, the composition is capable of being "fine-tuned" for specific applications and customer requirements.
Other important physical characteristics of ElectroPhen are its thermal stability, resilience to temperature, and inertness toward the catalyst. As a result, stack manufacturers can safely explore the use of different, cheaper, catalyst materials that may require higher temperatures at the reaction surface.
Thanks to its phenolic resin roots, ElectroPhen is inexpensive to manufacture, with the basic raw materials being widely available from major chemical suppliers. Thus, 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 supplychain overhead, and ensures there will be no disruption to supply through multiple-sourcing.
Scaling for high volume
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. Because only small volumes are produced, 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 200 MEA/bipolar plate assemblies.
Manufacturability on this scale needs to be considered by stack manufacturers who are 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 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-cellbased hydrogen economy.