Numerical Simulation of the Interfacial Oxygen Transport Resistance for a PEMFC Cathode Incorporating Water Droplet Coverage

Abstract

The transport resistance of oxygen in a PEMFC flow field and gas diffusion layer interface was numerically investigated in a 3D model. This model solved for the convection of air and the diffusion of oxygen in the air in a single flow field channel. The interfacial oxygen transport resistance was expressed with the Sherwood number. For a dry channel, the numerically obtained Sherwood number was validated by the relationship between Nusselt and Sherwood numbers through the Lewis number. Following the validation, the model simulated conditions of a single water droplet that remained adhered on the flow field and gas diffusion layer interface. The effects of droplet size, flow velocity, and variations in vapor content in air due to changes in temperature were analyzed and discussed. For the selected range of droplet sizes and flow velocities, numerically obtained drag forces were compared against experimentally obtained adhesion forces. IntroductionProton exchange membrane fuel cells (PEMFCs) are electrochemical engines which transform the chemical energy in hydrogen into electrical energy. They are strong candidates to be the successor of internal combustion engines. Their environmentally friendly by-products provide them an important advantage against internal combustion engines. PEMFCs dissipate heat and produce water as their by-products. However, the liquid water produced in the PEMFC can partially block the pores of gas diffusion layer (GDL) and thereby hinder the reactant diffusion. Moreover, the heat generated needs to be removed without overcooling the system to avoid excess condensation of water vapor, which leads to the blockage of GDL pores. The management of these by-products is an active area of research to enhance the fuel cell performance as much as possible.PEMFCs produce high power output at high current and low voltage operation points. At these conditions and at a given current, the major source of voltage drop is the transport losses of oxygen on the cathode side. As the oxygen concentration at the catalyst layer decreases, the voltage of the cell decreases drastically. Due to this source of voltage drop, it is important for researchers to accurately model the concentration drop from the cathode flow field (FF) to the catalyst layer (CL). Down-the-channel performance model by Gu et al. is a pseudo-2D numerical model which incorporates a wide range of operation parameters but still treats them with simplifications so the model can be applied to large scale and solved conveniently (1). This performance model 10.1149/05002.0183ecst ©The Electrochemical Society ECS Transactions, 50 (2) 183-196 (2012) 183 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.49.23.145 Downloaded on 2015-03-15 to IPnumerically simulates the through-plane transport in a fuel cell which is also discretized in the down the flow field direction. The pseudo-2D nature of this model comes from the fact that down the channel discretized fuel cell sec...

Measurement of Net Water Drag Coefficients in Polymer Electrolyte Fuel Cells under Cathode-Dry Conditions

“…The mass transfer between the GDL and the channel in PEFCs has been analytically studied by Koz and Kandlikar. 35,36 Based on their numerical simulation, they obtained Sh = 3.36 for a flow channel whose geometry was 0.7 mm width and 0.4 mm depth, which agrees well with the value of 3.279 calculated using the Nusselt-Sherwood (Nu-Sh) analogy. 35 In addition, when no water droplets were present in the channel, their results clearly showed that Sh was steady at 3.36 regardless of change in flow velocity in the range from 1.59 to 15.89 m s −1 .…”

Interfacial oxygen transport resistance in a proton exchange membrane fuel cell air channel with an array of water droplets

“…This Sh FD is significantly lower than the ones previously recommended: Sh FD =5.274 [42] (value readjusted for the characteristic length of hydraulic diameter), 6.0 [43], and 5.411 [44]. Moreover, it was shown that local Sh can be larger than Sh FD downstream of the droplet [41]. As a result, a single droplet was found to decrease the interfacial O 2 resistance, a finding that is in agreement with the results of Chen et al [16].…”

“…In a later numerical study by Koz and Kandlikar [45], single, two and multiple droplet cases were simulated over a more refined parametric range compared to their previous work [41]. Two and multiple droplet simulations revealed the effect of droplet spacing on Sh and how Sh varied at each consecutive droplet,…”

“…The effect of droplets on Sh was studied numerically and shown to be important in the available literature. Koz and Kandlikar simulated a single droplet in an isolated 3D air channel by using a droplet-shaped obstruction [41]. The use of an obstruction allowed the steady-state solution of single-phase air flow and mass transport equations.…”

Advanced control of liquid water region in diffusion media of polymer electrolyte fuel cells through a dimensionless number

“…In the fully developed region, Sh can be assumed constant, ~3.66 [50]. Koz and Kandlikar found Sh to be 3.36 using 3D simulation [52].…”