Aug
25
Modeling the Fuel Cell From the Lung
August 25, 2010 | 2 Comments
Norwegian Academy of Science and Letters scientists propose that inspiration using the layout of the lung improves the energy efficiency and saves the catalyst material of a polymer electrolyte membrane (PEM) fuel cell. The group calculated results for a single cell in a stack that shows the amount of catalyst could be reduced by a factor of 4-8, while the energy efficiency can be increased by 10-20% at high current densities. These are quite significant numbers. The main catalyst in research is platinum – a stunningly expensive metal.
The math and commentary have been published in the August 10th issue of the ACS journal Energy & Fuels. The news, a bit obtuse this writer will admit, is still quite significant. Coverage is worldwide if seemingly subdued – there aren’t a lot of journalists that are going to realize the significance of reducing the catalyst needs by factors in the 4 to 8 range.
Fractal Branched Fuel Cell Layout. Click image for more info.
It’s still theory for now, using sophisticated math for getting to the point. Stay with me: the process method uses the theorem of equipartition of entropy production to maximize energy efficiency. The gas supply and water outlet systems, designed to produce entropy uniformly, have a fractal structure inspired by the human lung. The tree-like gas distributor engraved in the fuel cell bipolar plates may eliminate the need for porous transport layers.
Mathematical solutions are provided for the optimal height, macroporosity, and nanoporous column width of the electro-catalytic layer beneath the gas supply system. The paper shows that the optimal macroporosity of the catalytic layer is equal to 1/2 for the model chosen and that the optimal height of the catalytic layer depends upon the coefficient for first-order reaction kinetics at the cathode, the diffusion constant for oxygen in the gas phase, and the oxygen concentration of the inlet flow.
In lay terms that is trying to say the layout of the passages for the fuel gas, such as hydrogen, and the dimensions of the passages have been identified using the math work by the group. Using the materials discussed in the paper indicates that the amount of catalyst can be reduced by a factor of 4−8, while the energy efficiency can be increased by 10−20% at high current densities.
Typical MEA Layout in a Fuel Cell. Click image for more info.
Compared to passageway layouts such as in a radiator or other ideas, and then trying to get the fuel working through a comparatively massive membrane acting as the porous transport, the lung based distribution with the activity taking place in the distribution routes offers a great increase is using catalyst resources.
Today polymer electrolyte membrane fuel cells are usually built with five active layers, all with different characteristic pore sizes. The central layer is a gas-tight ion-exchange membrane (typically Nafion) filled with water, in which protons conduct charge and where water transport takes place by electro-osmosis and diffusion. Two catalytic layers on both sides of the membrane are nanoporous with dispersed platinum, carbon, and polymer; the carbon grains (20-40 nm) form agglomerates of 200- 300 nm size, leading to pores of 20-40 nm inside the grains and 40-200 nm pores in the void space between agglomerates. These three layers comprise the membrane electrode assembly (MEA).
The MEA is covered on both sides with a porous transport layer (PTL), having micrometer-sized pores. Supplying oxygen fast enough through the PTL and the catalytic layer to the active sites can be problematic. The lack of a fast supply leads to gradients perpendicular as well as parallel to the membrane, resulting in loss in potential; clogging of pores by water can give similar losses. The Norwegians work seeks to avoid those problems. Quote from the paper:
“To find an optimal structure starting from scratch, with all geometrical variables free, is not trivial. It is known, however, that the human lung as a gas distributor is characterized by the very same conditions (as given by eqs 9 and 10). The human lung has two flow regimes: one for convective flow and one for diffusion. The structure is such that the entropy production is constant in both parts, indicating that the entropy production is minimal for the total structure. This is exactly the situation that we want to achieve and why we take the bronchial tree as a source of inspiration…The gas supply system for the fuel cell should be compared to the first part of the bronchial tree.”
A bit of connecting the dots genius in involved here with more than a bit of objective analysis on the problems.
The paper’s authors offer, “The presented methodology is general and applies to any type of catalyst in a nanoporous catalytic layer. . . We have thus presented a method that predicts that significant catalyst savings are possible. . . The results remain to be validated experimentally by building a cell, proving that a better energy efficiency can indeed be realized in practice for the proposed structure. It is important to establish all cell characteristics, because the loss at low potentials may not only be due to mass-transfer limitations.”
Another look at the two graphics suggests the group should get some funding for proving up the concept. The issue is fundamentally controlling the fuel and the intersection with the catalyst. The math looks good, the engineering quite a challenge for the test cell, and the promise quite high for getting fuel cells closer to mainstream use.
Moreover, the authors aren’t playing favorites on the fuel front either; the door is open from hydrogen on up. The Norwegians have a great idea on the table and congratulations are in order for a very sharp innovation. It will be very interesting to see how this idea might drive fuel cell costs down.
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