Gas Flow Channel

As we have mentioned before, the pressure drop in the reactant gas flow channels plays a critical role in the operation and performance of a fuel cell. The higher the pressure drop, the higher is the decrease in the reactant gas pressures in the bulk fluid flow, and this affects the gas distribution in the electrode layer in cases where pressure-driven advection flow is important. Additionally, a higher pressure drop in the gas flow channels results in higher pumping or parasitic power requirement of a fuel cell. 

A more detailed discussion of gas flow channel analysis and design is considered in Chapfer 10. 

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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... 

In-situ measurement technique of water vapor concentration in gas flow channels in PEMFCs using tunable diode laser absorption spectroscopy (TDLAS)31-36 is also shown with fundamental descriptions on its measuring principle and validity of a practical system. Localized current density and through-plane water-back transport index are obtained with variation of vapor concentration along the gas channel taken into account. Demonstrative results showing that effect of the micro porous layer (MPL) on variation of through-plane water-back transport index is shown in an operating PEMFC. 

Figure 3.36. Different design options for the gas flow channels (A) straight, (B) inter-digitated, (C) serpentine and (D) spiral. For the interdigitated design, the incoming flow must proceed through the gas diffusion layer to reach the outlet channel.

Nguyen, P., Berning, T., Djilali, N. (2004). Computational model of a PEM fuel cell with serpentine gas flow channels. /. Power Sources 130,149-157. 

Fig. 42. Schematic of regions considered in PEFC air electrode modeling, including (from left to right) gas flow channel, gas-diffusion backing, and cathode catalyst layer. Oxygen is transported in the backing through the gas-phase component of a porous/tortuous medium and through the catalyst layer by diffusion through a condensed medium. The catalyst layer also transports protons and is assumed to have evenly distributed catalyst particles within its volume [100]. (Reprinted by permission of the Electrochemic Society).

The backing in this model is treated with the Stefan-Maxwell equation to yield the partial pressure of oxygen at the backing/catalyst layer interface from the total pressure Ptot in the gas flow channel, the backing characteristic current density, 7b, and the mole fractions of water vapor and of oxygen, Xws and Xon, respectively ... 

Other elements of PEFC characeteristics which have been modeled recently are the variations of temperature and water content and the associated possible variations in protonic conductivity down the gas flow channel. These types of lateral thermal effects have been dealt with by Fuller and Newman [106] and by Nguyen and White [107]. 

Further modeling work done more recently reveals expected variations of both temperature and water content in a PEFC, together with the associated variations in protonic conductivity along the gas flow channel, that is, along the active surface area of the ME A. Such lateral distributions are evaluated and discussed in Ref. 38. 

Fig. 44 Schematic of regions considered in comprehensive modeling of the PEFC air electrode. From left to right gas flow channel, GDL, cathode catalyst layer. Oxygen in transported through the porous GDL by gas-phase diffusion through an inert mixture of nitrogen and water vapor. The catalyst layer is described in terms of effective transport characteristics of gas, protons, and electrons [13].

The XY plane section of a pin shows a close-up of the radial layout of a single pin. The innermost volume of the fuel pin is UN. Then there is a small gas gap followed by a layer of Rhenium. Next is the NblZr cladding and then the gas flow channel. Finally there is the matrix that the fuel pins are suspended in. A rhenium wire wrap is used to keep the fuel pins centered in the flow channels. 

Bipolar plate materials have historically been metals coated with corrosion-resistant layers or graphite with a seal treatment (to lower the gas permeability). In recent years, major efforts have been made for developing RP bipolar plates. The new plates usually have molded-in gas flow channels so that they can be fabricated rapidly and cost-effectively. However, the cost of bipolar plates ( 10/plate with 400 cm ) today is still too high to be applied to automotive and other civil power applications, and the conductivity is marginal. 

Diffusion of water along the membrane results in the distinctly different ignition fronts with counter-current flow. The water made at the cathode of the fuel cell is partitioned between the membrane and the cathode gas flow channel. The water in the membrane diffuses to the anode where the water activity is lower, and then it can enter the anode gas flow channel. 

Water is produced at the cathode/membrane interface, and it must be transported to the anode and cathode flow channels to be removed. At present, we do not have a direct measurement of the water activity in the membrane (the partial pressure of water in the membrane, p> embrane do know the water content in the effluent streams. The experimental data may be integrated from the known initial water content (after the water injection) to the steady state current and partial pressures of water. Integration of Eq. (3.4) gives the steady state membrane water content for known water partial pressure at the anode and cathode. Effective mass transfer coefficients for water from the cathode/membrane interface to the cathode gas flow channel and from the cathode/membrane interface to the... 

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