How is liquid water generated at the cathode transported away during fuel cell operation?
Published on March 2011 by Catherine Lepiller, PhD, for Pragma Industries
After my September’s science note about the beneficial role of the microporous layer in low temperature fuel cell’s water management, further highlights in the following will expand the subject to all porous components adjacent to the membrane. The catalyst layer and the gas diffusion layer – with or without a microporous layer – interplay and contribute to mass transport losses in different ways, especially on the cathode side.
How is liquid water generated at the cathode transported away during fuel cell operation? This question has proved difficult to address experimentally; nevertheless, smart ideas and clever lab setups, in conjunction with the ongoing development of more realistic non isothermal models, are improving our comprehension of the actual fuel cell physics for better component’s material design.
In recent years, significant work has been devoted to experimental and modelling studies of the water transport from the cathode catalyst layer (CCL) to the bipolar plate channels in liquid form by the capillary-fingering mechanism. As reported in my previous note, pore network models have clearly evidenced that the microporous layer (MPL) acts like a physical capillary barrier: it reduces the number and size of liquid water droplets formed in the cathode nano/micro interfacial pores to be directed towards the macroporous gas diffusion layer (GDL) and finally blown away with the exit air [1,2]. This process decreases the overall saturation in the GDL appreciably, and hence helps mitigate flooding issues. In short, the cathode microporous layer controls both the quantity – lowered by more water back diffusion to the anode through the membrane – and the morphology of liquid water removed through the diffusion media .
In his macroscopic model, Weber has examined limiting-case scenarios of water and gas transport and established numerically that liquid water saturation is not capable to affect gas phase diffusion in the bulk of standard commercial diffusion media noticeably under normal fuel cell operating conditions. Meanwhile, the model confirms that the MPL dominates the overall response of the porous medium, and interfacial capillary interactions between the MPL and the GDL, depending on both material properties, are more important than bulk GDL properties alone to control water management .
Still, beside capillary-dominated liquid water flow, vapor phase diffusion must not be overlooked. The main heat source in the fuel cell core is also located at the CCL where the irreversible oxygen reduction reaction occurs: the temperature is highest at the CCL and decreases towards the gas channels. Temperature gradients therefore do exist between the different porous components leading to evaporation/condensation of water. The exact location of phase change will depend on the operating conditions (temperature, relative humidity of inlet gases, current density) but also on the fuel cell material’s thermal properties. There is a vapor flux from the cathode to the bipolar plate through the MPL and then the GDL along the water concentration gradient.
Modelling work by a Russo-American team has focused on assessing these thermal effects with aim at clarifying the importance of an intermediate porous layer to aid vapor diffusive transport. A water-saturated transport plate as developed by UTC Power was simulated as bipolar plate . It was found that water transport depends on the combination of water vapor diffusion coefficients and thermal conductivities of the GDL components, and for composite GDLs (with MPL) the fraction of water removed in vapor phase is 90% and higher, depending on the MPL thickness and porosity. Assuming saturated conditions at both GDL interfaces (but no evaporation/condensation reactions in the porous layers), the main conclusion was that a composite GDL structure with low thermal conductivity near the cathode-GDL interface helps to sustain under-saturated conditions in the GDL bulk.
|These model predictions have been confirmed lately. In a recent invited lecture at the CARISMA conference in La Grande Motte (France), September 20th 2010, General Motors has presented the neutron imaging technique used as in situ water management tool to identify the key GDL and flow field parameters that impact liquid water accumulation. The starting point is that air outlet conditions as close as possible to saturation are desirable for maximizing water transport in vapor state – rather than liquid. It was shown that in a fuel cell operating at steady state, the exit air relative humidity levels off to a value that is to critical saturation in the GDL. The important point is that the critical saturation is strongly related to the GDL’s thermal conductivity: higher thermal conductivity will lead to higher critical saturation due to higher temperature gradient between the MEA and the GDL. Therefore, low- thermal conductive GDLs should be preferred in most cases. More specific tuning using for example a high-thermal GDL at the air inlet could also help to decrease MEA temperature and increase water condensation in this –generally water-deficient – region. General Motors and Rochester Institute’s teams have further examined the dominant water discharge mechanism at a variety of normal (dry and wet) operating conditions in additional experimental work . Several sets of experiments with various types of porous fuel cell components and different locations were designed to study vapor- and liquid-driven mechanisms independently (e.g., in Fig. 1 = Fig. 4 in ). Results again make it clear that liquid water accumulation against the CCL results in large mass transport losses: an MPL placed against the CL minimizes this phenomenon whereas an MPL within the GDL away from the CL exacerbates it. In the meantime the primary role of gas-phase water transport is confirmed, and GDL’s thermal properties are found preponderant in controlling the thermal and RH gradients driving water from the CCL. The authors suggest that the flux of water vapor could alone be sufficient to remove product water at high current densities even with saturated inlet gases. They propose that the primary role of the cathode MPL is to prevent liquid water condensed in the large pores of the GDL from pooling at the CCL interface rather than improving capillary-driven flow from the nano/micropores of the CCL. According to this hypothesis, water crosses the MPL in a vapor state and the presence of liquid water in the GDL is the result of subsequent condensation.
This is a sort of “paradigm shift” when considering capillary transport-based previous discussions. As often, reality is probably in-between: thermal conductivities of the porous layers (GDL and MPL) have no doubt a great influence on global water management in establishing the thermal gradient that will promote – or hinder – vapor-phase transport out of the fuel cell. Diffusive vapor flux is dominant especially at high temperatures and with low-thermal conductive porous layers. But the MPL is also a capillary transport component that controls liquid water migrating towards gas channels and limits saturation in the GDL. This role is more evident at low temperatures, when the thermal conductivity of the GDL substrate has much smaller influence on vapor transport, since the introduction of an MPL significantly improves performance regardless of substrate’s thermal conductivity (Fig. 2 = Fig. 12 in).
|Figure 1: Cross section micrograph of the Ketjen Black EV300J (KB, mean pore diameter 20 nm) on Pureblack SCD 205-110 (SG, mean pore diameter 80 nm) MPL deposited on carbon fiber paper GDL substrate|
|Figure 2: Polarisation performance with and without MPL at 40 and 80°C. (a) MPL coated on Toray carbon paper TGP-H-060 (high thermal conductivity k = 1.4 W/m K) and (b) MPL coated on Mitsubishi Rayon Co. MRC 105 (low thermal conductivity k = 0.3 W/m K)|
|Finally, neither liquid- or vapor-only water transport might be appropriate, especially when considering actual defects and cracks in the MPL. The network model by Medici et al.  has precisely demonstrated that the presence of small defects in an MPL has the combined effect of reducing the saturation level in the GDL while retaining a low injection pressure for the water transport toward the gas channels. This could enable water to cross the MPL in the liquid state and induce capillary-fingering in the GDL with low saturation, as experimentally observed!
Obviously, the water transport mechanism from the cathode catalyst layer is deeply connected to porous layer’s morphological and thermal properties, and there is not a single truth…
 J.H. Nam, K.-J. Lee, G.-S. Hwang, C.-J. Kim and M. Kaviany, Int. J. Heat Mass Transfer, 52 (2009), 2779-2791.