Experimental Study of Transient Dynamics of PEM Fuel Cells

Open Access
Author:
Hussaini, Irfan Saif
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 14, 2010
Committee Members:
  • Prof C Y Wang, Dissertation Advisor
  • Chao Yang Wang, Committee Chair
  • Fan Bill B Cheung, Committee Member
  • Stefan Thynell, Committee Member
  • Jun Huang, Committee Member
Keywords:
  • relative permeability of diffusion media
  • water management
  • transient dynamics
  • visualization
Abstract:
This thesis describes an experimental study of the dynamics of water distribution and transport in polymer electrolyte membrane (PEM) fuel cells. The study is divided into four parts: cathode channel flooding and its impact on cell performance, role of membrane water transients, measurement of two phase air-water relative permeabilities of fuel cell diffusion media and development of a dynamic water management technique based on intermittent humidification. In the chapter on cathode channel flooding, an in-situ visualization study of flooding in cathode channels of an operating fuel cell is presented with the objective of identifying two-phase flow patterns and documenting the impact of flooding on fuel cell performance. Humidification levels, current densities and flow stoichiometries representing typical automotive fuel cell operation are used in this study. Observed flow patterns are presented in the form of a flow map. The impact of flooding is also presented in terms of measurable parameters like two-phase pressure drop coefficient and voltage loss. A new parameter called wetted area ratio is introduced to characterize channel flooding and liquid water coverage on a gas diffusion layer, and its repeatability with multiple tests is demonstrated. Transient response of fuel cell presents a challenging area of research in PEM fuel cells and little experimental data is available in fuel cell literature. The problem of transient response is complicated by the occurrence of several complex dynamic phenomena with time scales varying over several orders of magnitude. In this part of the study, response of PEM fuel cells to a step-change in load is investigated experimentally. Voltage undershoot, a characteristic feature of transient response following a step-increase in load, is shown to be due to transients of water distribution in the membrane and ionomers occurring at sub-second time scales. Use of humidified reactants as a means to control magnitude of voltage undershoot has been demonstrated. Further, response under step-decrease in current density has been explored to determine existence of hysteresis. Under sufficiently humidified conditions, response under forward and reverse step changes are found to be symmetric, but under low RH conditions, voltage undershoot is found to be twice as large as the overshoot. Air-water relative permeabilities of gas diffusion material play a pivotal role in water management. Modeling of liquid water transport through the GDL relies on knowledge of relative permeability functions in the in-plane and through-plane directions. However, experimental measurements of carbon based paper and cloth materials are not available in literature. In this part of the study, air and water relative permeabilities are experimentally determined as functions of saturation for typical GDL materials such as Toray-060, -090, -120 carbon paper and E-Tek carbon cloth materials in their plain, untreated forms. Saturation is measured using an ex-situ gravimetric method. Absolute and relative permeability functions in the two directions of interest are presented. Based on experimental observations of fuel cell response following change in gas RH, a novel method of water management of PEM fuel cells using intermittent humidification is developed. The goal of this method is to maintain the membrane close to full humidification, while eliminating any channel flooding. The method is tested on experimental test cells using both plain and hydrophobic GDL materials and an improvement in performance as compared to steady humidification is demonstrated.