Carbon foam for mitigating water management issues in low temperature fuel cells
Published on May 2010 by Catherine Lepiller, PhD, for Pragma Industries
In low temperature fuel cells, two-phase (liquid water and air) flow mal-distribution in the gas channels is further complicated by the generation of water at discrete spots and at different rates on the cathode catalyst layer. Uneven local current density may therefore deteriorate the flow distribution in channels, which in turn alters the power output.
Two papers recently published in the Journal of Power Sources stand out by their innovative approach for addressing this issue. Both propose the use of carbon foam material either as or in the bipolar plate of a PEM fuel cell. Their preliminary results are quite interesting.
The first approach by J. Chen at the University of California (J. Chen, J. Power Sources, 195 (2010), 1122-1129) has been to fill the parallel channels of a conventional graphite end plate with porous carbon foam and use visualization images to compare the two-phase flow under different regimes representative of a real fuel cell. After verifying that the porous media inserts in the channel did not modify the homogeneity for two-phase flow, it was clearly observed that the mal-distribution was much less severe in the device with porous channels vs. standard hollow channels. This was attributed to the random arrangement of water droplets in numerous tiny pores (~ 100 µm) of the carbon foam network. Despite overall low porosity (3%) the carbon foam inserts featured self-adjustment capacity to the amount of water in flow channels: under steady-state operation, over the whole volume of porous media, there would be a number of pores retaining liquid water whatever the air flow velocity (down to ~ 10% at high values), but the rest would remain free anytime for air to pass. Although the pressure drop with this configuration is 4-fold higher than in hollow parallel channels, the benefits are more significant, and particularly the mitigation of flooding/drying phenomena thanks to the water self-adjustment allowed by porous carbon foam inserts.
The Canadian team at the Queen’s Royal Military College Fuel Cell Research Centre (J. Kim and N. Cunningham, J. Power Sources, 195 (2010), 2291-2300) has also selected carbon foam for its feasibility study. Unlike in the previous work, such porous material was considered as whole cathodic bipolar plate, and not merely as insert in conventional channels. They used Reticulated Vitreous Carbon foam with different pore sizes, and compared the single cell performance of their new design (3.5 mm thick carbon foam at the cathode and serpentine flow-field at the anode) with that of a conventional fuel cell with serpentine channels on both sides. Consistently with J. Chen, they found that the RVC foam fuel cell offers advantages over the conventional design especially at low operating temperatures and fully humidified gases. Moreover, the parametric study of geometrical parameters of the carbon foam (thickness, average pore size) under various operating conditions has confirmed that those with small pore sizes in the range of 100 µm are best suited for water management because they provide higher electrical conductivity to the material. Lower gas permeability also enhances forced convection in addition to diffusion as well, thereby increasing air penetration and residence time in the gas diffusion layer for a more efficient electrochemical reaction. Fuel cell tests show comparable performances in terms of power density and long-term (250 h) stability.These two complementary studies agree to point to a new possible design of fuel cell bipolar plates including carbon foam, at least at the cathode side: both low porosity and isotropic characteristics of this material favor 3-D mass transfer and extend the effective GDL area accessible by the reactant flow through the porous medium to the catalyst layer. Under flooding conditions, the two-phase flow is made more uniform in a sustainable way. Maybe a first step towards good water management at low temperatures?