Composites lead the way in fuel cell manufacture
TWI Bulletin, September - October 2009
Combining strength with electrical power - the structural battery
Paul Burling obtained a Higher National Certificate in Production Engineering with Merits and Advanced Mathematics from Cambridge College of Art and Technology. He has extensive experience in project management of large commercial projects world wide, which demand in depth knowledge of composite material and adhesives for military and commercial applications.
His expertise covers most engineering technologies but he specialises in composite materials and adhesives. He also has knowledge of working with plastics and ceramics, as well as design, fabrication, joining and costing of composite materials using novel production techniques for many manufacturing industries.
Composite structures are gaining popularity in a variety of industries for a number of reasons. They have good mechanical strength at lower densities and in comparison with classic materials, better electrical insulation properties, resistance to corrosion and ease of use. In most applications they offer passive structural volume to the product with functional elements, such as electronic systems mounted thereon. As Paul Burling describes, there are many types and forms of composites that have evolved over the years. An explanation of these is given below.
Composite sandwich materials or structures are a specific form of composite consisting of discrete materials bonded together to form a board or panel. Such sandwich panels are generally made up of two outer skins with a core material positioned between them. In some cases, multiple core materials may be used and skin materials may be incorporated between the outer skins.
Skin materials can be composite laminates or metal sheets for example carbon, Kevlar, glass, aluminium, stainless steel, mild steel. Indeed, aluminium sheets can be laminated with composite prepregs to form a structure similar to plywood. The choice of skin will depend on the end-use application.
Core materials such as foam, honeycomb 3D fabricated structures, are an important material for light weighting. Honeycomb is the generic name for a range of products incorporating a honeycomb or hexagonal form. Round, elliptical and square cell forms have been explored, but a hexagonal shape is preferred. Honeycomb core can be created from Aramid paper (Nomex, Tyvec), aluminium foil, craft paper, thin glass laminates and thermoplastic sheets. The material can either be corrugated or bonded at the cell nodes, expanded and heat set to form a hexagonal structure.
Experiments to combine these materials and tailor them for specific needs have been carried out over many years. As new matrix/resins fibres are developed, engineers combine them to produce the optimum structure for their requirements. Many engineers appreciate the benefits of composites, they can see their advantages over traditional materials, but may have been put off because of both the cost of materials and the cost of change in the manufacturing procedures.
Now there is an additional requirement to take these materials and add value, as most industry sectors are looking for ways to improve their products by incorporating additional functionality. The technology explained below shows how composite materials can be formed with integral fuel cells to provide power to structures containing them.
Power and multifunctional materials - the raw technology
Fuel cells are electrochemical devices that convert chemical energy to electrical energy without combustion. Unlike a battery, a fuel cell can continuously produce electricity for as long as fuel is supplied to it. Proton-exchange membrane fuel cells, known colloquially as PEMFCs, analogous to polymer electrolyte membrane fuel cells, are low temperature, compact fuel cells that have been developed for transport applications as well as for portable applications such as mobile phones.
A PEMFC comprises an ion exchange membrane (a thin polymer membrane, such as NafionTM) sandwiched between, but in contact with, an anode and a cathode. Fuel such as hydrogen gas is introduced at the anode, forming protons that pass through the membrane. An oxidant, oxygen, is introduced simultaneously at the cathode, where the spontaneous reaction takes place to produce electricity. When using pure hydrogen the cell's only by-product is water.
The anode and cathode are typically carbon supported platinum based catalysts. These may be printed on porous carbon paper which forms a gas diffusion layer (GDL). The three-layer (or five layer incorporating two GDLs) anode-membrane-cathode assembly, or membrane electrode assembly (MEA), is typically interposed between electrically conductive graphite separator plates. The graphite plates collect current, facilitate the access of the fuel and oxidant to the anode and cathode surfaces, respectively, and provide for the removal of water formed during the operation of the cell. Field flow plates are often expensive to manufacture, typically requiring machining of, sometimes complex, gas flow paths in solid graphite plates.
A plurality of MEAs are usually configured together to form a stack wherein supply and discharge manifolds of fuel and oxidant gases respectively are coupled across the stack at alternating surfaces. The fuel cell stack is normally enclosed in a housing. The stack, housing, and associated hardware make up the fuel cell unit.
For portable applications a variant of the PEMFC, the direct methanol fuel cell (DMFC), has been used. The DMFC operates on methanol fed directly to the anode of the device. A small compact version has been developed that is suitable for hand-held electronic devices such as laptops, mobile phones and digital audio players.
The vast majority of PEMFCs are prototypes and demonstration units. To move PEMFCs into large scale commercial use, even with the incorporation of various material improvements being progressed will require design changes for manufacturing. Also PEMFCs are relatively fragile and may not be self-supporting thus support of the PEMFCs when assembling the fuel cell unit is especially important.
When considering the introduction of fuel cells into an industrial application they are conventionally viewed as a stand-alone unit that is incorporated into the structure in much the same way as a battery. To provide the power output required a prescribed number of fuel cells is stacked, housed into a unit and then incorporated into the assembled structure. By way of example, Toshiba has developed the world's smallest DMFC that can be incorporated into an electronic hand-held device in the same manner as a battery and can run non-stop provided the integrated fuel tank is topped up.
Industry is driving towards ever more advanced, locally powered and multifunctional products - strident examples being military UAVs and non-combustion powered vehicles.
In view of the above, TWI has developed an innovation (patent granted) which aims to make a structural component that is multifunctional, combining the benefits of composite and fuel cell technologies. The functionality of a passive structural component, the composite sandwich structure, is enhanced through the use of the core material as a gas or liquid transfer mechanism for an embedded PEMFC. This technology provides a structural component that is multifunctional, enabling greater use of fuel cells in confined spaces while also providing housing for PEMFCs.
This approach removes many of the drawbacks associated with current methods of combining PEMFCs with laminate or sandwich structures, such as excluding the requirement for complex housing design. Integrating the PEMFC into a laminate or sandwich structure helps prevent the damage or loss of fuel cell functionality when assembling the PEMFC stack and enables the construction of complex field flow plates to achieve good gas transfer to the electrodes by specific machined channels in the composite laminate. The structure is also able to store gas or liquid, depending on the volume available, and enables multiple fuel cells, working either in isolation or as a collective, to be supplied with fuel in the form of gases or liquids.
The example shown (Fig.1) can be incorporated and formatted in such a way that power is provided to specific areas of a structure. This feature can help eliminate wiring, providing weight savings to the overall structure, and can also be important for devices that need to be isolated or self contained, such as plug and play devices. Being easily removed, they can be treated as a collective or individually, enabling electrical devices to be easily upgraded or removed completely.
Fig.1. Working model bolted together for assessment. The production unit would be fully bonded and be seen as a composite laminate
TWI has a long history of innovation across many technologies, with a broad interest in joining technology in both the fuel cell and composites fields. When a disruptive innovation appears, especially in an area of such current interest as this, TWI will seek to protect it for a number of reasons, most importantly to ensure that Members and the wider community see the most benefit from the new technology.
By seeking appropriate protection, developing appropriate commercialisation routes through running projects, seeking partners and licensing the technology, TWI tries to stimulate the rapid uptake of inventions and help incorporate them into the next generations of products. In the case of this innovation, which exhibits potential benefits in fuel cell and composite technology, TWI has been granted the patent applications (see PCT publication no. WO2007036705) and will seek partners and licensees throughout military, aerospace, automotive, consumer and other markets. Part of the TWI strategy in this respect is to ensure that technology does not get bogged down. Where, in another organisation, a patented technology would probably become part of a protective, excluding strategy, TWI seeks to spread a properly developed and researched technology as widely as possible. TWI does not make products. Its mission is to deliver world-class services in chosen fields to meet the needs of its global membership and associated community. Properly protecting and managing its innovations is an important part of that aim.
TWI is now in pursuit of partners to take this to market. The initial feasibility study has shown that a very simple single device embedded into a composite can provide power similar to an AA 1.5 volt battery. To learn more about this exciting technology why not contact bud@twi.co.uk
