Effect of Hydrophilic Treatment of Cathode Channel on Liquid Water Transport through Gas Diffusion Layer and Performance of PEFC

Water flooding and plugging on cathode side of polymer electrolyte fuel cells (PEFCs) lead to severe concentration overpotential due to oxygen transport limitation.  To alleviate these issues, it is necessary to elucidate liquid water transport through cathode gas diffusion layer (GDL) and design the optimum structure of GDL and gas channel.  In the previous works, many experimental efforts to visualize two-phase flow in operating PEFCs have been carried out by neutron [1], X-ray [2] and optical [3] techniques.  However, most of the conventional visualization studies investigated only the water behavior along the flow direction of a PEFC.  The through-plane behaviors of liquid droplets inside the cathode channel, including the emergence, growth and coalescence of droplets on the GDL surface and the attachment to the channel sidewall, cannot be exactly grasped.  Turhan et al. [4] analyzed the through-plane liquid storage, transport and flooding mechanism as a function of channel wall hydrophobicity with the use of high-resolution neutron imaging.  Results revealed that the channel wettability significantly affects the water transport from the diffusion media to the gas flow channels.  Hydrophilic channel walls enhance the liquid water suction from under-the-land locations into the channels.

In this study, the dynamic behaviors of liquid droplets inside the cathode channel are directly observed using a cross-sectional visualization cell, and the effect of hydrophilic treatment of cathode channel on the water transport through the GDL and the cell performance is investigated under operating conditions.  The cross-sectional visualization technique is very useful to evaluate the growth and attachment behaviors of liquid droplets and the channel blockage owing to liquid water in the gas channel.  In the experiment, the cell voltage and differential pressure between the cathode inlet and outlet are simultaneously measured, and the correlation between the liquid water behavior and the performance characteristics is discussed.

Fig. 1 presents the time-series variation of the cell voltage and differential pressure of an operating PEFC for two different wettabilities of cathode channel wall.  The contact angles of water of uncoated and hydrophilized channels are 76^o and 17^o, respectively.  The fuel cell operation is performed at 0.5 A/cm² for 1000 s.  In the case of uncoated channel, the cell voltage remarkably decreases for the first 350 s at low air stoichiometry, because water flooding proceeds inside the cathode catalyst layer (CL) and GDL.  Subsequently, both the voltage and pressure frequently fluctuate due to water plugging in the channels.  On the other hand, the hydrophilic treatment of the cathode channels reduces the voltage drop due to flooding.  Water droplets emerged from the GDL surface immediately attach to the hydrophilized channel sidewall and spread out on the wall surface.  Therefore, the liquid water removal through the GDL is accelerated, and the water accumulation at the CL/GDL interface is alleviated.  In addition, since the hydrophilic treatment of channels does not cause water plugging frequently, it drastically reduces the amplitude of voltage fluctuation.  Hydrophilization of cathode channel effectively controls through-plane water transport and decreases both water flooding and plugging.

References:   [1] D. Kramer, et al., Electrochim. Acta, 50, 2603 (2005).   [2] P.K. Sinha, et al. Electrochem. Solid-State Lett., 9(7), A344 (2006).   [3] K. Tüber, et al., J. Power Sources, 124, 403 (2003).   [4] A. Turhan, et al., Electrochim. Acta, 55, 2734 (2010).