Use of metal foams in Direct Methanol Fuel Cells (DMFCs)

Metal foams are routinely used in structures to enhance stiffness and reduce weight over a range of platforms. In direct methanol fuel cells, the controlled porosity and high electrical conductivity of metal foams provide additional benefits. In this design, a traditional MEA is sandwiched between two metal foam flow fields and covered with a composite skin, resulting in a strong lightweight structural element that can also produce auxiliary power. Apart from structural properties, compared to traditional flow fields, metal foams offer advantages in enhancing two-phase flow and current-collecting capacity

Performance studies were conducted with direct methanol fuel cells incorporating metal foams as the flow field. The influence of the foam pore size and density on cell performance was investigated. The performance of similar density metal foams but with different pore sizes was non-monotonic due to the opposing trends of electrical contact and CO2 removal with pore size. In contrast, for metal foams with the same in-plane pore size, the performance improved with increasing density. Because the cell operates in a diffusion-dominated regime, its performance showed a strong dependence on methanol concentration and a moderate dependence on methanol flow rate.

The feasibility of using metal foams as a GDL was also explored. It was found that metal foam performs better as a GDL compared to cloth and metal mesh. . It was found that the metal foams perform better as GDLs compared to cloth and metal mesh. Ni foam has good electrical conductivity compared to that of carbon cloth in which enhances performance. Although the mass transfer from the metal foam to the underlying gas diffusion layer (GDL) is diffusion-dominated, it is found that at a fixed methanol concentration, the limiting current density increases with increasing methanol flow rates. This unexpected result is attributed to the more efficient removal of product CO2 from the GDL. A methodology is developed to estimate the effective diffusion coefficient of methanol in the anode diffusion layer from limiting current density measurements, and to extract the fraction of GDL volume occupied by CO2.

A possible drawback of using metal foam flow fields is their susceptibility to corrosion. This issue must be addressed before they can be effectively used as multifunctional composite materials for structural and power requirements.

Figure 1. Calculated CO2 void fraction within the pores of the GDL at different methanol flow rates
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Figure 2. Transparent operational DMFC incorporating metal foams
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Figure 3. Metal foams (a) 10 pores per linear inch (ppi), 20 ppi and 40 ppi (left to right) with 6-8% density.
(b) Cross sectional view of 6-8%, 12-16% and 18-24% density (bottom to top) with 20 ppi.
(c) SEM image of 20 ppi metal foam.
Arisetty S., Prasad A.K., and Advani S.G., "Metal foams as flow field and gas diffusion layer in direct methanol fuel cells," Journal of Power Sources, Vol. 165, pp. 49-57, February 25, 2007. doi:10.1016/j.jpowsour.2006.12.008