Saw this one on the Popular Science Magazine.
Wonder if sugar subsitute works as well.
Fuel cell technology: A sweeter fuel
KEVIN KENDALL
Kevin Kendall is at the Department of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. e-mail: k.kendall@bham.ac.uk
doi:10.1038/nmat782
Eating sugar gives us a boost when we feel tired because our cells use it as fuel to produce energy. Likewise, sugar can now be used to produce power in artificial biological fuel cells that function in a physiological environment.
Engineering of fuel cells has greatly improved over the past 20 years, leading to the development of electric automobiles with zero emissions and improved performance, as required, for example, by the California Partnership in 2003. A fuel cell extracts electrical power directly from a fuel by converting the chemical gradient between fuel and oxygen into a voltage, usually across an electrolytic separator or membrane. Most fuel cells operate using a polymer electrolyte membrane to separate the fuel (pure hydrogen) from the oxidant (usually air) at temperatures around 100 °C. In addition, precious metal catalysts, such as platinum supported on carbon, are used to speed up both the anode and cathode reactions2, 3, and to draw off the electric current.
Fuel cells that operate on biological fuels are still in their infancy. But as an alternative to hydrogen, methanol can be used as a fuel, and has already been used in portable applications such as laptops and mobile phones. However, special anode catalysts are required to prevent poisoning of the electrodes with reaction side-products, and special electrolyte materials are needed to inhibit the leakage of fuel though the membrane. In a paper published by the Journal of the American Chemical Society, Mano and colleagues have now demonstrated that glucose, a much more complex molecule than either hydrogen or methanol, can be used to drive a miniature fuel cell4. Remarkably, this fuel cell was operated without a membrane to separate the oxygen from the fuel. Normally, fuel cells cannot function under these conditions.
Miniature fuel cells are not new and have been used in systems for detecting alcohol in breathalysers and in oxygen sensors for automobiles. For example, a spot of platinum ink about 1 mm square on a zirconia membrane has been used as an oxygen sensor5. Conceivably, even finer sensors could be made by making the spot of platinum smaller. The lower limit to the size of such a fuel cell is not known, but it is quite straightforward to make a cell with an area of 0.01 mm2, some 40 times less than the size reported by Mano and colleagues. So the small dimension of their cell is not particularly striking. Nonetheless, their fuel cell uses two carbon fibres — each 20 mm long — as electrode supports and current collectors, which could easily be made a hundred times shorter if the fuel cell needed to be miniaturized further.
The most surprising elements of Mano and colleagues' fuel cell are the use of glucose as the fuel and the single-compartment cell geometry (Fig. 1). Both these features required substantial advances in the catalyst materials used to coat the anode and cathode fibres. Glucose was first used in a fuel cell using enzyme electrodes in 1964 by Yahiro, Lee and Kimble6, and recent improvements were made by Persson and Gorton who electro-oxidized glucose on a carbon anode using the enzyme glucose hydrogenase7.
Fig. 1
Figure 1 | Configuration of the elements in the glucose-powered fuel cell constructed by Mano and colleagues4.
The electrodes are carbon fibres coated with crosslinked hydrogel films, each containing a different enzyme and redox polymer. The design of this single-compartment cell relies on the fact that enzymes are highly specific catalysts. Although there is no membrane to separate the fuel (glucose) from the oxidant (oxygen), the different enzymes on the anode and cathode only react with glucose and oxygen respectively.
Mano and colleagues used a different enzyme — glucose oxidase, obtained from the bacterium Aspergillus niger — immobilized on the surface of a carbon fibre (functioning as the anode). To transport the electrons from the glucose reaction site to the carbon fibre, a redox polymer was mixed with the enzyme and crosslinked on to the carbon fibre in the form of a swollen hydrogel. The enzyme used to catalyse the reduction of oxygen, bilirubin oxidase from the bacterium Trachyderma tsunodae, was mixed with a second redox polymer to form a hydrogel film around the cathode — a second carbon fibre — to transfer the electrons from the carbon to the oxygen.
These enzyme–polymer mixtures were stable for about one week and allowed power to be extracted from the glucose–oxygen electrode reactions. Less than 1 mW was obtained per square centimetre of cell, which is 1,000 times less than the power density required to drive a vehicle. But the glucose was dilute, only 15 mM, and the conditions were mild, 37 °C in physiological buffer; conditions not normally conducive to fuel cell operation. Although such a small power output is insufficient to drive a motor, it could be useful to operate sensors that could be implanted in a living organism.
The extraordinary feature of these experiments is that they were carried out in a single compartment cell. In other words, there was no membrane to separate the fuel electrode from the oxygen electrode. This means that fuel and oxygen were mixed throughout the cell and there was no chemical gradient to produce the fuel cell voltage. In equilibrium, there should therefore be no voltage or current from the cell. Thus, the cell produces power only because the anode and cathode catalysts can differentiate the fuel and oxidant reactant species. Such cells have not previously been successful in practice, although patents exist for fuel cells operating on mixed fuel and oxidant8.
The combination of enzymes and polymers on carbon fibres clearly works in the single-compartment situation designed by Mano and colleagues, but we don't really know why. The successful extraction of power from complex molecules under physiological conditions is a significant achievement in itself, and offers a new challenge — understanding precisely how the reactants, enzymes and polymers interact. A better understanding of the reaction mechanisms involved would help researchers to optimize this fuel cell to make functional devices that could be integrated into biological systems. |
