Reactant flow management

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... 

Calculation of temperature distribution in the system was done. The consideration was given to several options of the cooling flows management, including the case when air-reactant is used as a heat-transport medium within both operating cells and additional cooling cells. 

In PEM fuel cells, the main operating limitation is the water management. Water is the main product of the cathode reaction, and is evacuated out through the cathode flow channels more details on PEM fuel cell operation and schematics can be found in the next chapter of this book. Failure to evacuate enough water causes the H2O condensation within the fuel cell which blocks the reactant flow channels. Removing water excessively, on the other hand, reduces the polymer electrolyte s ability to conduct protons. 

Apart from the flow management of reactants care has to be taken to avoid impurities in the reactant feeds. It has been established that impurities such as SO2, NO2, H2S, and O3 in the air stream affect the cell... 

A useful feature of the cold atmospheric PECVD environment is the potential for control of the polymerization process outcome. Energy management and control of reactant flow rates can be used to provide a broad spectrum of polymerization reaction outcomes ranging from essentially atomic to somewhat molecular. Atomic polymerization comprises cleavage of all bonds in the CVD material followed by addition of individual atoms to the building polymer layer on the surface of the deposition substrate. Molecular polymerization comprises cleavage of only the most labile bonds in the CVD precursor molecule followed by addition of the nearly intact precursor molecule to the building polymer on the surface of the deposition substrate. 

Distributions of water and reactants are of high interest for PEFCs as the membrane conductivity is strongly dependent on water content. The information of water distribution is instrumental for designing innovative water management schemes in a PEFC. A few authors have studied overall water balance by collection of the fuel cell effluent and condensation of the gas-phase water vapor. However, determination of the in situ distribution of water vapor is desirable at various locations within the anode and cathode gas channel flow paths. Mench et al. pioneered the use of a gas chromatograph for water distribution measurements. The technique can be used to directly map water distribution in the anode and cathode of an operating fuel cell with a time resolution of approximately 2 min and a spatial resolution limited only by the proximity of sample extraction ports located in gas channels. 

A catalytic process is a complex interplay of mnltiple phenomena, including the flow of reactive mixtures through a reactor, the respective diffusion of reactants and prodncts to and away from active sites, and the chemical transformation of reactants into prodncts at the active sites. Aside from issnes related to the transport and chemical transformations of reactants and prodncts, it is also necessary to manage the heat load in a catalytic system. In principle, any of the above-mentioned phenomena can be manipulated to affect the outcome of a catalytic reaction. 

Bipolar plates are integrating the functions of reactant distribution, current collection, and thermal management for each cell. For this purpose, the bipolar plates are containing distribution zones (flow fields) for fuel (hydrogen/methanol), oxidant (air) and coolant. The design of the flow fields must ensure spatially homogeneous reactant distribution across the active area as well as reliable and homogeneous removal of product water. 

Bipolar plates are an integral part of the PEFC water management. To maintain optimum reactant supply and product removal, the flow rate in the gas distribution zone must be high enough to remove water droplets forming inside the gas channels or other flow field structures. In case water droplets get stuck in the gas distribution field, starvation zones are forming which are severely affecting performance and endurance of the PEFC. 

The solid catalyst is flowing along with the reactants. It is therefore necessary to reduce the amount of displaced substance, for avoiding the transport of useless material and for limiting the problems associated with sedimentation and abrasion of reactor and equipments. Additionally, even when a solid is considered as disposable, the management of solid wastes prescribes a drastic reduction of their volume. 

Note that the flow field design of the bipolar plate is a critical factor for reactant supply and water management of stack and individual cells. As will be discussed later, a suboptimal flow field and manifold design can lead to an uneven distribution of reactants, and by this to fuel and air starvation. 

A combination of synergistic improvements in the catalyst, support, gas diffusion layers, membrane, and essentially the entire porous electrode structure in conjunction with bipolar plates/flow fields is expected to improve the mass-transport of reactant gases, protons, and water management. Thus, an increase in the peak current density (A/cm ) and peak power density (W/cm ) will result this in turn will lower the stack volume, the amount of catalyst, and membrane material used and raise the kW/L, kW/kg, and lower the /kW stack metrics. It should be noted that the rated or peak power for automotive stacks is based in part on maintaining an electrical efficiency of >50% this dictates that the cell voltage has to be maintained above 0.60 V. At this time, volumetric power densities of practical stacks in fuel ceU vehicles have been reported to be as high as 2 kW/L... 

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