Catalyst layer freeze

Ge and Wang also visualized the fuel cell cathode during cold start.6 Using a silver mesh as cathode GDL, they observed that when product water was less than 0.56 mg/cm2 water was not seen on the catalyst layer surface and that when it reached 1.12 mg/cm2 the liquid water emerged from the catalyst layer surface. This is consistent with the roughly estimated value of cathode catalyst layer water storage capacity of M).5 mg/cm2. They also estimated from their experiment that the freezing-point depression of water in the catalyst layer was at most 2°C. Then, they made the fuel cell optical... 

Proper water management in proton exchange membrane fuel cells (PEMFCs) is critical to PEMFC performance and durability. PEMFC performance is impaired if the membrane has insufficient water for proton conduction or if the open pore space of the gas diffusion layer (GDL) and catalyst layer (CL) or the gas flow channels becomes saturated with liquid water, there is a reduction in reactant flow to the active catalyst sites. PEMFC durability is reduced if water is left in the CL during freeze/thaw cycling which can result in CL or GDL separation from the membrane,1 and excess water in contact with the membrane can result in accelerated membrane thinning.2... 

Delamination of the MPL from the GDL substrate has not been widely reported but may occur during freeze-thaw cycles, as occurs with catalyst-layer delamination from the membrane [131, 132]. A different situation occurs in the GDL/MPL, where the pore diameters are on the order of a micron or larger and the water is not hydrating the sulfraiic acid of the ionomer. The volume expansion caused by ice formation can produce large isotropic stresses that can damage the structure of the catalyst layer, the MPL, or the GDL. 

Li J, Lee S, Roberts J. Ice formation and distribution in the catalyst layer during freeze-start process—CRYO- SEM investigation. ECS Trans 2007 ll(l) 595-605. 

Cold start Shut-down strategy importance on freezing of process water on catalyst layer of MEA Results the degree of dryness in the stack significantly influences cold start-up ability, increasing dryness improves performance the optimal shut-down strategy allows start-up from -6 C without any performance loss, lower temperatures will see temporary performance loss Schiebwohl et ai, 2009... 

PEMFCs must have the ability to survive and start up from sub-freezing temperatures, also called cold start, to be deployed successfully in automobiles. Under freezing environmental conditions, water produced has a tendency to freeze in open pores in the catalyst layer and GDL, rather than being removed from the fuel cell, thus creating mass transport limitations that eventually result in the shutdown of PEFC operation. Figure 31.11 shows the accumulation of solid water in a fuel cell in sub-freezing operation detected by neutron imaging [34]. 

Water in the cathode catalyst layer may exist in multiple states such as absorbed in the ionomer, as vapor, and as sohd phases at sub-freezing conditions. Assuming phase equilibrium, solid water can emerge when the vapor pressure reaches the saturated level. Before that, most water produced from the ORR can be absorbed in the ionomer, which hereby can be defined as the first stage. The second stage is characterized by solid production and ice volume growth within the catalyst layer. 

In general, thin catalyst layers will have the lower proton resistance. However, they may also fill up more easily with water as is, for example, the case with the very thin NSTF electrodes by 3M, which only seem to function well at non-saturated conditions. Also for start-up under freezing conditions, a thicker electrode, or at least a higher pore volume, seems to be an advantage as complete filling with ice is even more detrimental. 

Catalyst layer porosity, where very small pores can prevent freezing due to freezing point depression. 

Crack-free catalyst layer to prevent water from freezing in the cracks. 

FIGURE 9.5 Effects of freezing-thaw cycles on the catalyst layer surface, (a) Fresh catalyst surface, (b) catalyst surface after freeze/thaw cycles. (Reprinted from /. Power Sources, 160, Guo, Q. and Qi, Z., Effect of freeze-thaw cycles on the properties and performance of membrane-electrode assemblies, 1269-1274, Copyright (2006), with... 

The current density reaches a maximal value when the ohmic or HFR resistance is minimal. At this time, the membrane at the cathode side is almost saturated and additionally produced water cannot be totally absorbed and thus begins to flow and to freeze in the CCL. However, no ice is formed at the anode catalyst layer (ACL). This has been observed in the literature by different means scanning electron microscopy (SEM) (Thompson, 2008a), MEA layers hydrophobicity (Oszcipok, 2005) and visual observations (Ge and Wang, 2007b). 

After the shutdown in the cold start, the cell was disassembled and the MEA surface of the cathode side and cross-sections of the MEA were observed as detailed in the previous Section, 1.5.2. Figure 1.39 shows pictures of the cathode catalyst surface at the two temperatures. At — 10°C, there are numerous ice layers across the MEA surface, while few ice crystals were observed at —20°C. These results indicate that at — 10°C the produced water transfers through the catalyst layer (CL) and freezes at the CL/MPL interface. At —20°C, most of the produced water appears to freeze in the CL. From the transport balance of water it was verified that the accumulated water amount was the same for a current density of 0.08 A cm at — 10°C and one of 0.02 A cm at —20°C. Therefore, it may be concluded that the differences in the ice formation characteristics are caused, not by the water amount produced, but by the differences in the start-up temperatures. To determine more details of the ice formation in the catalyst layer, cross-sections of the cathode catalyst layer were observed by CRYO-SEM. 

The above results indicate that the produced water initially diffuses into the membrane. After the membrane is saturated, water starts to accumulate in the cell and freeze. At temperatures below —20°C the accumulating water starts freezing immediately in the catalyst layer. It is therefore not possible to control the amount of water freezing in the locations where the ice does not affect the oxygen diffusion. This indicates that one solution to improve the cold start is to dry the... 

In cold starts, the produced water freezes inside the catalyst layer at ambient temperatures below —20°C, whereas at — 10°C it freezes at the interface between the catalyst layer and the MPL. To ensure a sufficient period of time for starting below —20°C, there are not many measures that can be taken other than ensuring that the membrane is in a dry state before a cold start. 

When residual water produced during fuel cell operation remains in the electrodes after the stack is shut down, problems can arise, particularly when the environmental temperature is <0 °C. When the stack is exposed to subzero conditions, the residual water will freeze, so the volume of the electrodes (in particular, the catalysts layers) will expand due to ice formation, which will lead to structural damage and decreased electrochemical active surface area. This has been reported as an additional degradation mechanism in PEM fuel cells [21]. However, if the PEM fuel cell is operated at high temperatures, less liquid water will remain in the electrode and thus decrease the impact of fuel cell structure failure caused by frozen water. 

There is limited, and sometimes contradictory, data in the literature available on the durability of the catalyst layer and the GDL under freeze/thaw cycling. Studies have revealed that even a free-standing hydrated catalyst layer can be subjected to cracking and peeling while cycling (six cycles) from -30 °C (Guo and Qi 2006). This damage was associated with a loss in the electrochemical surface area of the catalyst that can be avoided by drying the catalyst. 

As mentioned previously, the water content in the catalyst layer will be determined by the extent of drying of the fuel cell before freezing, so the various literature studies may have widely different starting water contents in the catalyst layer. Furthermore, because the saturated vapor pressure curve for water is a strong function of temperature, even cells that operate at undersaturated conditions can exhibit condensation and freezing as the temperature drops after shutdown. It is expected that liquid-water and ice contents will be highest at the periphery of the cell, as the cell should cool first at the outer perimeter, and water will be transported to the coldest regions, condense, and subsequently freeze. 

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