Portland, Ore. - Supercharging fuel cells by premixing the fuel with oxygen holds out the promise of superslim fuel cells that can be manufactured on a printing press. Inventor Generics Group Ltd. (Cambridge, England) believes the new design, which eliminates the need for bulky metal separation plates, will shrink the cell size while increasing power density tenfold.
"The compact mixed-reactant [CMR] fuel cell is a platform approach that is applicable to every type of fuel cell," said Michael Priestnall, technical director of CMR at Generics. "Our premise is that if you change the catalysts so that they are selective, then you can relax many of the restraints about not allowing the fuel and oxygen to mix."
In normal fuel cells, the fuel and oxygen are kept in separate, adjacent compartments. In the first compartment, the fuel enters and reacts with the anode catalyst, which breaks the fuel's molecular bonds so that its electrons can flow out the anode to power equipment. In the second compartment, oxygen flows over the cathode catalyst, which uses the electricity's return line from the load to bond oxygen to the leftover fuel's free hydrogen, resulting in water as exhaust.
Platinum is typically the choice for both the anode and cathode, because it "is very good at doing both fuel oxidation and oxygen reduction," said Priestnall. "You need to keep the fuel and the oxygen completely separate; otherwise they will both react on the same platinum electrode, producing combustion instead of electricity."
To keep the fuel and oxygen separate, traditional fuel cells put metal flow-field plates between adjacent cells. One side of the flow-field plate supplies fuel to the anode of the next fuel cell, while the other side supplies oxygen to the cathode of the previous fuel cell. The anode and the cathode are separated by a solid polymer membrane to segregate the fuel and oxygen.
Whereas those traditional designs measure about 2 mm thick, according to Generics, the CMR fuel cell eliminates the metal flow-field plate completely and uses a thin, perforated polymer membrane.
"The membrane material in a typical fuel cell is 100 microns thick-at a cost of about $300 per square meter. But our perforated membranes only need to be about 1 micron thick," Priestnall said. "The cost of membrane material in a normal fuel cell is over $500 per kilowatt. We have brought that down to $15/kW."
A selective catalyst allows the mixing of fuel and oxygen. Both the anode and the cathode of a fuel cell have to be coated with a catalyst that induces the correct reaction-either breaking down fuel into hydrogen molecules at the anode or bonding free hydrogen to oxygen at the cathode with the electrical return line. "If instead of using platinum for both reactions you use two different catalysts that are selective, then you can allow the fuel and oxygen to mix," said Priestnall.
Many catalysts have already been discovered that act selectively to perform just the anode or cathode function of a fuel cell, and more are under investigation. So far, biological systems have produced the most candidates. There are many to choose from, since enzymes are routinely used by living cells to selectively catalyze almost any specific chemical reaction.
Ceramics and metal alloys are also being tested as selective catalysts. Once selective anode and cathode catalysts are chosen, the rest of the CMR fuel cell is constructed in almost the same way as a traditional fuel cell, except that the flow-field plates are eliminated and the remaining components are perforated to allow the fuel and oxygen to flow through them.
"The optimal way of creating our fuel cells would be on a roll-to-roll printing press," which would make it possible to create extremely thin devices that nevertheless produce electricity when a fuel-and-air mixture flows through them, Priestnall said. If Generics can prove that this patented method works, then it claims that every other fuel cell maker will have to either license the CMR process or invent its own version.
"With our approach you can print with [anode] catalyst ink, then on top of that you write with electrolyte ink," said Priestnall. Atop that is printed the cathode catalyst ink, and "on top of that, carbon ink. Then you just repeat that whole sequence."
Generics is working on a 50-W portable demonstration system to show fuel cell makers that the idea can be engineered into an efficient, commercial design. It plans to begin showing the prototype within 18 to 24 months.