Liquid water accumulation

After testing a number of DLs with and without MPLs, Lin and Nguyen [108] postulated that the MPL seemed to push more liquid water back to the anode through the membrane. Basically, the small hydrophobic pores in the MPL result in low liquid water permeability and reduce the water transport from the CL toward the DL. Therefore, more liquid water accumulated in the CL is forced toward the anode (back diffusion). This reduces the amount of water removed through the cathode DL, decreases the number of blocked pores within the cathode diffusion layer, and improves the overall gas transport from the DL toward the active zones. 

Schmitz et al. [184] tested various carbon fiber papers with different thicknesses as cathode DLs in PEM fuel cells. It was observed that the cell resistance dropped when the thickness of the DL increased thus, thicker materials are desired in order to improve the electrical conductivity. It was also mentioned that the optimal thickness for the DL is usually between the thinnest and the thickest materials because the two extremes give the lowest performance. In fact, in thin DLs, the water produced can fill pores within the material, resulting in flooding and the blockage of available flow paths for the oxygen. Similarly, Lin and Nguyen [108] concluded that thinner DLs (without MPLs) were more prone to liquid water accumulation than thicker ones. 

In recent years, the use of transparent fuel cells has increased substantially due to the need for a better understanding of liquid water accumulation on the surface of the DLs and flow through the FF channels. In most transparent cells, either the cathode or the anode (or both) has transparent polycarbonate end plates that act as windows and sit on top of the corresponding FF plates. These plates are normally thin and made out of metal, such as stainless steel or gold-plated brass, and their thickness is equal to the depth of the FF channels (i.e., the charmels are machined all the way through the plate). Thus, the transparent end plate also acts as part of the charmels. 

The challenge for modeling the water balance in CCL is to link the composite, porous morphology properly with liquid water accumulation, transport phenomena, electrochemical kinetics, and performance. At the materials level, this task requires relations between composihon, porous structure, liquid water accumulation, and effective properhes. Relevant properties include proton conductivity, gas diffusivihes, liquid permeability, electrochemical source term, and vaporizahon source term. Discussions of functional relationships between effective properties and structure can be found in fhe liferafure. Because fhe liquid wafer saturation, 5,(2)/ is a spatially varying function at/o > 0, these effective properties also vary spatially in an operating cell, warranting a self-consistent solution for effective properties and performance. 

A transparent PEM fuel cell with a single straight channel was designed by Ma et al.11 to study liquid water transport in the cathode channel (this study is also mentioned in Section 2.5). The pressure drop between the inlet and outlet of the channel on the cathode side was used as a diagnostic signal to monitor liquid water accumulation and removal. The proper gas velocities for different currents were determined according to the pressure drop curves. 

Trying to stay warm and dry while active outdoors in winter has always been a challenge. In the worst case, an individual exercises strenuously, sweats profusely, then rests. During exercise, liquid water accumulates on the skin and starts to wet the clothing layers above skin. Some of the sweat evaporates from both the skin and the clothing. Depending on the... 

Results HT without MPL increases liquid water accumulation at electrode, limiting oxygen transport to catalyst and lowering cell voltage, also decreases water at GDL HT with MPL addition suppresses water accumulation at electrode, increasing current increasing air permeability of GDL increases current, also improving start-up performance Liquid water, temperature, GDL thickness and porosity... 

The steps involved in modeling performance and water balance in CCLs are indicated in Figure 8.2 [50, 51]. At the materials level, it requires constitutive relations between random composition, dual porous morphology, liquid water accumulation, and effective physico-chemical properties, including proton conductivity, gas diffusivities, liquid permeabilities, electrochemical source term, and vaporization source term. The set of relationships between structure and physico-chemical properties has been discussed in [3, 47, 50-51]. Since the liquid water saturation S (z) is a spatially var5dng function at jf,>0, these physicochemical properties become spatially varying functions in an operating cell. This demands a self-consistent solution for non-linearly coupled properties and performance. 

In the past, studies of the macrohomogeneous model have explored the effeets of thickness and composition on performance and catalyst utilization. At the outset, it should be noted that these works neglected the effects of liquid water accumulation in pores on performance. The specific effects due to the complex coupling between porous morphology, liquid water formation, oxygen transport, and reaction rate distributions will be discussed in Section 8.5.5. The results presented in this section are only valid at sufficiently small current densities, for which liquid water accumulation in secondary pores is not critical. 

The effects of the porous structure and liquid water accumulation on steady-state performance were explored in [50, 51], employing the struetural pieture and the corresponding framework model of eatalyst layer operation, as diseussed in Section 8.5.2. 

Figure 8.15. Plot of cell potential vs. fuel cell current density, (/o), indicating the effect of liquid water accumulation in the CCL on performance (soUd hne). The interplay of liquid water accumulation in pores and impeded oxygen transport causes the transition from the ideally wetted state to the fully saturated state (dotted tines), as indicated [51]. (Reprinted from Electrochimica Acta, 53.13, Liu J, Eikerting M. Model of cathode catalyst layers for polymer electrolyte fuel cells The role of porous structure and water accumulation, 4435— 46, 2008, with permission from Elsevier.)...

This system has two issues. First, hquid water cannot be fully injected for evaporation and so liquid water accumulation is a problem. Second, the dynamic response of the system is slow. 

With increasing current density, the liquid water accumulation in secondary pores is bound to increase. This process depends on their pore size and wetting angle. The wettability of secondary pores is, therefore, vital for controlling the water formation in CCLs. If all of the secondary pores were considered hydrophilic, the liquid water saturation would be given by an integral expression containing the differential PSD, dXp r)/dr,... 

The effects of porous structure and liquid water accumulation on steady-state performance of conventional CCLs were explored in Eikerling (2006) and Liu and Eikerling (2008). In these modeling works, uniform wetting angle was assumed in secondary pores, with a value 0 < 90°. The full set of equations presented in the section Macrohomogeneous Model with Constant Properties are solved with the following boundary conditions ... 

These phenomena may provide a clue to elucidate the funetion of the MPL, as the difference in the performance appears to be caused by liquid water accumulating at the interface between the CL and MPL (or GDL) surfaces. This will be diseussed in the next section. 

Regarding the ionomer (PFSl) content in a hydrophilic CL, the optimal amount and distribution of the ionomer in the CL is a tradeoff among three requirements (i) maximum contact between the ionomer and the Pt particles to guarantee proton transport, (ii) minimal electron resistance, and (iii) minimal gas transport resistance. Normally, gas transport can be affected by both decreased porosity due to the presence of a solid ionomer and liquid water accumulation due to the hydrophilicity of the CL. When carbon-supported platinum (Pt/C) is used as the catalyst, the carbon particles have a much larger surface area than the Pt particles, so only if the carbon surface is covered by the ionomer can contact between the ionomer and the Pt particles be ensured. This indicates that the ratio between the ionomer and the carbon in the CL is quite important for achieving high performance. The suggested ratio of ionomer to carbon I C) is about 0.8 1.0, which is calculated based on the assumption that the ionomer forms a thin layer ( 1 nm) on the carbon surface. 

Liquid-Phase Accumulation and Pore Blockage Limitation In low-temperature fuel cells such as the PEFC, liquid water accumulation and blockage in the pores of the electrolyte, diffusion media, or flow channels of the anode or cathode can reduce the transport rate of reactant to the catalyst. Voltage loss by this phenomenon is generically ismioAflooding. This topic is discussed in greater detail in Chapters 5... 

Figure 6.17 Side-by-side comparison of (a) flooded and (b) unflooded liquid water distribution. Images are taken using neutron imaging technology. The brighter areas represent liquid water accumulations. Upon close inspection, the flow pattern in the fuel cell is visible. This is a result of accumulation of liquid water under the lands, where there is typically a high liquid saturation. (Adapted from Ref. [10].)...

Flooding Condition Adjustment When the water balance is accumulating liquid water mass, a periodic ejection of droplets can maintain the water balance in the fuel cell since the liquid droplets are so dense compared to gas phase ejection. An illustration of the process of water buildup and ejection from the DM is shown in Figure 6.19. In the steady state, the water from generation must be exactly balanced by that removed. Although water droplet ejection is a periodic process, a H2 PEFC can be operated in a net flooding condition and still achieve relatively stable performance with periodic ejection. If the liquid water accumulation restricts gas-phase flow to the catalyst surface, performance instability will occur, however, until a new equilibrium is achieved. 

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