Modeling gas purge and three-phase transients in a polymer electrolyte fuel cell

Start-up of a polymer electrolyte fuel cell (PEFC) from subzero environment, commonly referred to as cold start, remains a major challenge for automotive applications. Water produced from PEFC operation can freeze at subzero temperatures, hindering reactant transport and covering reaction sites, thereby making the start-up extremely difficult or even impossible. Gas purge is most commonly used to enhance the cold start capability by removing residual water from a PEFC prior to engine shutdown. A closely related PEFC performance issue lies in the complex dynamic response of a PEFC near freezing point, immediately after a successful start-up (post-cold start). The presence of all three phases of water during ice melting calls for a three-phase transient model to understand the governing physics of PEFC post-cold start for effective prediction, control and optimization of cell performance, which is not yet explored. This work addresses these PEFC performance issues closely interconnected to the problem of cold start, namely gas purge and three-phase transients (including both cold start and post-cold start).;Gas purge process is investigated in detail using a previously developed two-phase, transient purge model based on the multiphase-mixture formulation. The effect of vapor diffusivity on purge effectiveness is assessed through the use of Helium purge as opposed to Nitrogen purge. The effects of purge temperature, porosity, tortuosity, wettability of the porous media and initial liquid saturation on purge effectiveness are delineated. Purge characteristics along the flow direction is investigated and a significantly slower water removal is observed towards the outlet section. Both 2D and 3D model predictions are compared against experiments and better agreement is observed at higher purge temperatures.;A one dimensional, three-phase, transient PEFC model is developed which is capable of capturing complex dynamic cell behavior starting from a subzero temperature towards normal operation. Ice formation at subzero temperature (cold start), ice melting at freezing point, and liquid water transport process are captured through a coupled heat and water transport model, and the cell performance near freezing point is analyzed. Using cold start part of this model, the effect of catalyst layer (CL) pore volume (or CL thickness) on both isothermal and nonisothermal cold start performance of a PEFC is studied. The cold start operational time or product water is shown to approach a minimum nonzero asymptotic value as the CL is gradually made infinitesimally thin. It is shown that the self start-up can be achieved even with an ultra thin (∼1 microm) CL, given certain adjustments to cell design and material properties such as cell thermal mass and membrane water diffusivity, and a design map for self-start-up of such a PEFC from various subfreezing temperatures is presented.;The effect of dynamic loading and the resulting electrode flooding and consequent performance loss in terms of voltage drop during post-cold start is studied. The model is calibrated and validated against experimental data and a reasonable agreement in terms of voltage dropdown and subsequent recovery time is observed. The effect of different operating parameters on post-cold start cell dynamics is elucidated. Finally, a simplistic algebraic model for water transport during post-cold start is hypothesized and presented, which shows a better predictive capability.