Evolution in design

Siàƒ¢n Green, Managing Editor

UK-based Morgan Fuel Cells has announced a ‘biomimetic’ technology breakthrough that promises to boost the power output and cut the manufacturing costs of fuel cells. Will this innovation bring fuel cell technology closer to commercialization?

The history of the fuel cell can be traced back to the 19th Century, yet in spite of this fact and several decades of intense development, fuel cell technology still seems a long way from commercialization. Fuel cell manufacturers believed that 2003-2005 would be when commercial production of their units for various applications would start, but the latter half of the decade is a more realistic prospect.

Nevertheless, development continues because fuel cells have considerable potential in a wide range of applications. In the power industry, their efficiency and emissions-free characteristics make them suitable for a wide range of applications: combined heat and power, distributed generation, storage and back-up applications, and powering the home. Their widespread commercial use is held back by two key factors: cost and reliability.

An innovation by a UK-based company promises to help overcome these obstacles, however. Morgan Fuel Cells (MFC), a subsidiary of Morgan Crucible Company, has announced a technology breakthrough that will boost the power output of fuel cells and at the same time cut manufacturing costs. The development, says MFC, brings forward the day when fuel cells will be commercially viable for mass-market automotive and general power applications.

The breakthrough is in the design of the bipolar plates that are a key component of fuel cells. Drawing its inspiration from the natural world, MFC has developed a ‘biomimetic’ bipolar plate, the design of which mimics the structures seen in plant tissues and animal lungs. The design results in more efficient and even distribution of gases, resulting in greater efficiencies, higher power outputs and greater reliability than ever seen before in fuel cells.

MFC says that it has already achieved increased power outputs of 16 per cent, and believes that more can be achieved.

Central role

Fuel cells generate electricity through a simple electrochemical reaction between the fuel ” hydrogen ” and oxygen from the atmosphere. Heat and water are the only byproducts. There are several different types of fuel cell but they are all based around the same basic design consisting of two electrodes separated by a solid or liquid electrolyte. A proton exchange membrane (PEM) cell typically used in power generation and vehicle applications uses a polymeric membrane as the electrolyte with carbon supported platinum electrodes. A single cell produces between 0.6 and 0.7 V, so a fuel cell ‘engine’ for a stationary power plant has to be built up from several hundred PEM cells stacked together.

Bipolar plates have two key roles in fuel cells. Firstly they are the method of introducing the gases into the fuel cell and ensuring that the electrodes are adequately supplied with reactants. On the cathode (positive electrode) side they supply air (or oxygen) and on the anode (negative electrode) side they provide the delivery of hydrogen (or fuel). The gases flow across the plate in a number of finely detailed convoluted flow field channels, typically 0.5-2.0 mm wide and up to 1 mm deep, formed in the surface of the plate.

The second function they play is to act as a conductor for the electrical energy, thereby allowing the current to flow from cell to cell through the stack. “And that’s why they are called bipolar,” explains Brendan Bilton, business development manager with MFC. “Two plates are placed together so that one side is the cathode and the other is the anode. Where one cell ends, another one begins.”

“The thing about bipolar plates is that they have to be resilient to any of the chemical reactions that are going on,” continues Bilton. “So you have a very strong oxidising side of the plate and a very strong reducing side. You therefore need a material that is good in both of those areas.”

Serpentine designs

Bipolar plates are typically used in PEM and direct methanol fuel cells. The industry standard design are known as serpentine bipolar plates in which the flow field channels are long and straight, running parallel up and down the plate. This design is largely the result of the manufacturing process, says Bilton. “These designs are made by a computer numerically controlled (CNC) machining route, where a computer controlled machine mills out a pattern to a pre-designed program. It’s basically a high speed drill that moves along the plate and cuts a channel into it.

Compared with a serpentine design (below) biomimetic plates (above) distribute gases more evenly across the plate resulting in even distribution of current. This helps to eliminate cold and hot spots and enhances power output and reliability
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“Because of the way that operates, the machine likes to have long, straight tracks and you therefore end up with a serpentine design, i.e. a long straight track going up one side of the plate, then turning and going back down again. It’s like a snake twisted over the full width of the plate to deliver the gas uniformly across the plate.”

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Although widely used in the fuel cell industry, this design brings a number of problems. A great deal of pressure is needed to blow the gas through the flow field channels because it has to travel anywhere from eight to 20 times the length of the plate, depending on how many turns there are. This high backpressure is achieved through the use of fans. In addition, because the gas travels such a long way, there can be a significant reduction in gas concentration from the inlet to the outlet.

“If you are using air in the fuel cell, the oxygen content as it comes into the plate will be 18 per cent,” explains Bilton. “But by the time it comes to the end of the plate, oxygen levels could be down to 12-13 per cent. You therefore won’t have as much oxygen for the electrochemical reaction so you won’t generate as much power. This results in a hot spot at the inlet and a cold spot at the outlet.”

These characteristics have implications for both efficiency and reliability.

Mimicking nature

The New Shorter Oxford English Dictionary defines ‘biomimetic’ as “mimicking a biochemical process”. In addressing the problems associated with serpentine designs, MFC took its lead from nature, which provides a perfect example through animal lungs and plant leaves.

MFC’s patented biomimetic bipolar plate technology mimics structures seen in the natural world to allow gases to flow through the plate in a far more efficient way than has been achieved before
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“A biomimetic plate is a flow field design that uses biological principles to diffuse the gas,” says Bilton. “If you look at any natural system, the way that gases or reactants are distributed over a large area is via a large central channel which then branches off into smaller and smaller vessels. That is a very efficient way of designing a gas delivery system and those are the types of designs that we have been able to develop and fabricate.”

MFC’s biomimetic design is much more complex than the standard serpentine design. The key advantage of the biomimetic plates is that the gas has to travel a much shorter distance and the pressures required are therefore lower. “We can reduce the backpressure because the gas is travelling a shorter distance across the plate,” says Bilton “Nature doesn’t use any pumps or compressors to distribute gas, so it’s a very efficient way of delivering the gas.”

Bilton continues: “If you reduce the back pressure you reduce the fan power requirements so your parasitic load is lower and you get more power out of the stack. There is a knock-on effect of having a very efficient gas delivery system and that is that you don’t need as much balance of plant to get the fuel cell to run. This improvement in efficiency and power output was a key selling point for our initial customers.”

The fact that the gases have to travel shorter distances in biomimetic plates compared to serpentine plates also results in more even gas distribution and therefore an even distribution of current across the plate. This in turn helps to eliminate cold and hot spots, which results in improved reliability in the fuel cell.

Serpentine designs are currently industry-standard for bipolar plates, which act as a conductor for the electrical energy and channel the flow of gases in the cell
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“If you take away the mechanical failure mechanisms ” for example these cells tend to be clamped very tightly together and can break ” one of the biggest failure routes is the fact that you get this unevenness across the membrane which over time means that parts of the membrane cease to operate effectively and this gradually magnifies into complete failure,” comments Bilton. “If the membrane is too hot it is prone to drying out and if it gets too dry it won’t work. Similarly if the membrane gets too wet then it floods and it also won’t work. So a fine balance must be achieved to keep the central membrane conductive and the fuel cell working.”

A tailored approach

Unlike serpentine designs, which are fairly standard from application to application and from manufacturer to manufacturer, MFC is able to tailor its biomimetic plates to meet the individual needs of its customers. This allows the flow fields to match the characteristics required in the fuel cell stack, but means that MFC has to work very closely with its customers.

“What we have found is that irrespective of the application, operating conditions within the stack itself are very different from one user to the next. What we have to do is to create a very strong relationship with that customer because we need to get information that they don’t usually let anybody else see. But that’s the only way that we can design a fully optimized flow field for their particular application.”

Etching a future

Another major advantage of the biomimetic plates is that they are manufactured using MFC’s patented ElectroEtch process, which can produce the plates at a fraction of the time and cost of conventional bipolar plate manufacturing processes. The ElectroEtch process should enable production costs of fuel cell systems to be reduced.

MFC has already achieved increased power outputs of 16 per cent from its biomimetic plates compared with standard serpentine designs, and believes that more can be achieved
Click here to enlarge image

The ElectroEtch process is a variation on shot blasting, which is a technique used in industry for a variety of applications ” cleaning surfaces of hard materials, for example. “That is the basis of our ElectroEtch process but we have done a lot of work in order to be able to control the process so that we can get tolerances that you normally get from machining a plate,” says Bilton.

Cost savings are achieved because it is a simple process which uses just compressed air and small amounts of grit. The flow field design is printed on to a polymer mask which is placed onto the plate. The mask is water soluble and is washed away by the etching process, leaving the flow field design behind. This means that very complex designs are just as easy to make as simpler serpentine ones.

“It is a very fast and efficient process, as fast as any moulding technology that is around and we are only at the development stage ” we haven’t gone to the production model yet.”

MFC’s lab-based etching machine is currently able to produce 20-30 small plates per hour, or 4-5 large plates per hour. The technology is easily scalable, however, and every time it is scaled up, the cost of producing plates will fall, says Bilton. MFC has ordered its next etching machine, which will increase its production capacity ten-fold.

A new standard

The development of the etching process enabled MFC to develop the biomimetic design. With ElectroEtch, the company realised that it could start making extremely complex branch networks that would enable efficient gas flow in fuel cells, and started to examine different flow field designs. “Of course nature was the benchmark here,” states Bilton. “After four billion years of evolution we felt that it was a little ahead of our computational modelling!”

MFC believes that the biomimetic design will become the new industry standard, and is prepared to license the technology to other parties. “We are very comfortable in saying that as the industry evolves we will not be the sole manufacturer and supplier of this technology. We are more than happy to discuss and come to reasonable commercial terms with anyone that wants to use this,” says Bilton. “Our internal plan when we came up with this concept three or four years ago was that we wanted to be the new ‘Intel inside’ in the fuel cell industry.”

MFC has now completed all of its own internal design and development work and is now starting to work with a number of launch customers to develop biomimetic plates for their fuel cell stacks. The company is designing flow field patterns for each customer’s system, and the customers will go forward with building new modules for testing.

“We’re very lucky because at the moment we have a range of customers covering the various different applications for fuel cell technology. We’ve got some people look at some very small plates for a portable application, some looking at medium sized plates for a stationary application and others looking at very large plates for vehicle applications.”

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