Water management PEMFC

PEMFC Polymer 80-110 60 High current and power densities, long operating life C02 intolerance, water management, noble catalyst Transportation, cogeneration... 

From these examples, it can be seen that water content has a strong effect upon proton conductivity. Thus, it is clear that water management is an important factor for efficient PEMFC operation. It will be discussed in Section 3.2.3. 

In PEMFCs, Ralph et al. [86] tested a Ballard Mark V single cell with two different DLs a carbon cloth (Zoltek PWB-3) and a carbon fiber paper (Toray TGP-090) all the other operating conditions stayed the same for bofh cases. It was observed that the carbon cloth demonstrated a distinct advantage over the CFP at high current densities (>600 mA/cm ), while at low current densities both DLs performed similarly. If was claimed fhaf this was because the CC material enhanced mass transport properties and improved the water management within the cell due to its porosity and hydrophobicity. 

An example of a transparent PEMFC was presented by Spemjak, Prasad, and Advani [87], who used a 10 cm transparent fuel cell to investigate different cathode DL materials (with and without MPLs) influence on water management. The FF channels had a single-path serpentine design with rectangular channel cross sections 1 mm deep and 0.8 mm wide. In these researchers study, the analyzed images corresponded to those in the lower section of the cathode s active area (closest to the outlet) because most of the water droplets were observed in this area away from the inlet. To observe how different DLs affected the water transport in the anode, this side was also visualized (see Section 4.3.3.2). 

T. Jian-hua, S. Zhao-yuan, S. Jin-song, and S. Zhong-qiang. Preparation of water management layer and effects of its composition on performance of PEMFCs. Energy Conversion and Management 49 (2008) 1500-1505. 

J. Shi, J. Tian, C. Zhang, and Z. Shan. A novel method for the preparation of a PEMFC water management layer. Journal of Power Sources 164 (2007) 284-286. 

Because of its lower temperature and special polymer electrolyte membrane, the proton exchange membrane fuel cell (PEMFC) is well-suited for transportation, portable, and micro fuel cell applications. But the performance of these fuel cells critically depends on the materials used for the various cell components. Durability, water management, and reducing catalyst poisoning are important factors when selecting PEMFC materials. 

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... 

X. Huang, X. Wang, J. Preston, L. Bonville, H.R. Kunz, M. Perry, D. Condit, effect of water management schemes on the membrane durability in PEMFCs. ECS Trans. 16, 1697 (2008)... 

In PEMFC systems, water is transported in both transversal and lateral direction in the cells. A polymer electrolyte membrane (PEM) separates the anode and the cathode compartments, however water is inherently transported between these two electrodes by absorption, desorption and diffusion of water in the membrane.5,6 In operational fuel cells, water is also transported by an electro-osmotic effect and thus transversal water content distribution in the membrane is determined as a result of coupled water transport processes including diffusion, electro-osmosis, pressure-driven convection and interfacial mass transfer. To establish water management method in PEMFCs, it is strongly needed to obtain fundamental understandings on water transport in the cells. 

As shown above, in-situ TDLAS measurements show its unique potential to investigate localized water transport in operating PEMFCs, leading to fundamental understanding on water management. It is also capable of monitoring localized phenomena with good temporal resolution. 

The GDL is located on the back of the CL in order to improve gas distribution and water management in the cell. This layer has to be porous to the reacting gases, must have good electronic conductivity, and has to be hydrophobic so that the liquid produced water does not saturate the electrode structure and reduce the permeability of gases. The GDL needs to be resilient and the material of choice for the PEMFC is usually carbon fiber, paper or cloth, with a typical thickness of 0.2-0.5mm [74,75], This macroporous support layer is coated with a thin layer of carbon black mixed with a dispersed hydrophobic polymer, such as P I LL, in order to make it hydrophobic. This latter compound can, however, reduce the electronic conductivity of the GDL, and limit the three-phase boundary access. 

It has been demonstrated that EIS can serve as a standard analytical diagnostic tool in the evaluation and characterization of fuel cells. Scientists and engineers have now realized that the entire frequency response spectrum can provide useful data on non-Faradaic mechanisms, water management, ohmic losses, and the ionic conductivity of proton exchange membranes. EIS can help to identify contributors to PEMFC performance. It also provides useful information for fuel cell optimization and for down-selection of the most appropriate operating conditions. In addition, EIS can assist in identifying problems or predicting the likelihood of failure within fuel cell components. 

The presence of water is critical for operation but in current PEMFCs proper water management is a delicate issue and poor control can greatly reduce the efficiency of the device. An excess of water can flood the catalyst and porous transport layers impeding the transport of reactants and eventually drowning the fuel cell. At low water content, the polymer electrolyte membrane can become a poor conductor and the reactivity at the electrodes is affected. Local hot spots arising due to the inefficient operation result in early degradation of the cell. ... 

S.H. Ge, X.G. Li, and l.M. Hsing. Water management in PEMFCs using absorbent wicks. Journal of the Electrochemical Society 151, B523-B528 2004. 

FCS water management is the key factor for an efficient and reliable operation of a PEMFC stack. Membrane hydration control and water balance for a durable operation of FCS are the main objectives of this sub-system, whose design and control issues, strictly connected to thermal management but also to reactant subsystem components, are discussed in Sect. 4.5. The possibility of interactions between the wet and warm cathode outlet stream and the components of thermal and water management sub-systems is also discussed. 

Another issue for PEMFCs is that of water management. Whereas, dehydration of the membrane leads to a reduction in proton conductivity, an excess of water may lead to the electrode being flooded. 

Hydrated Nafion 117 (175 pm thick) has conductivities in the range of 10" S cm" at 80°C. Nafion has a high electroosmotic drag coefficient that can lead to problems with water management in a PEMFC. Nafion has been... 

---