TWO-PHASE TRANSPORT AND PREDICTION OF FLOODING IN POLYMER ELECTROLYTE FUEL CELLS

Open Access
Author:
Pasaogullari, Ugur
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 23, 2005
Committee Members:
  • Chao Yang Wang, Committee Chair
  • Ali Borhan, Committee Member
  • Fan Bill B Cheung, Committee Member
  • Matthew M Mench, Committee Member
Keywords:
  • PEFC
  • flooding
  • two-phase
  • GDL
  • MPL
  • modeling
Abstract:
Performance of polymer electrolyte fuel cells (PEFC) is known to be limited by transport of reactants and products. At high current densities, excessive amount of water is generated and condenses, blocking the pores of porous electrodes with liquid water, limiting the reactant transport to active catalyst. This phenomenon, known as “flooding” becomes the limiting mechanism of PEFC performance. In order to predict flooding and its effects on PEFC performance, first a theory of liquid water transport in these porous electrodes, namely gas diffusion media (GDM) is developed, and effects of porous structure and wettability, such as bi-layer construction, is analyzed. Two-phase heat transfer in GDM is also investigated. Based on this understanding, a multi-dimensional, multi-component, two-phase mathematical model is developed. The model accounts for simultaneous two-phase flow, transport of species, and electrochemical kinetics. Multiphase mixture (M) formulation is then utilized to efficiently model the two-phase transport in the porous electrodes. Governing equations are developed using a single domain formulation, generating a single set of governing equations valid for all components of PEFC. The resulting governing equations are solved with a finite-volume based computational fluid dynamics (CFD) technique, using a general-purpose commercial CFD software. The proposed model investigates PEFC behavior under various operating conditions, primarily exploring the two-phase flow physics in the cathode GDM. Multidimensional simulations reveal that flooding of porous cathode decreases oxygen transport rate to the cathode catalyst layer and causes a substantial increase in cathode polarization. Furthermore, the humidification level of reactant streams and the inlet flow rates are found to be key parameters controlling PEFC performance, two-phase flow and transport characteristics. It is found that minimization of specific performance limitations such as dry-out and flooding depends upon not only material characteristics but also the optimization of these operating parameters.