Analyzing Multiphase Flow in Membrane Electrode Assembly Using a Mixed Wettability Mathematical Model

• Author / Creator

  Zhou, Jie

• Improving fuel cell performance at high current density is critical for reducing stack size, cost and weight in automobile applications.
  A major source of performance losses at high current density is the accumulation of liquid water in the electrode which blocks reactant transport.
  Studying water management in proton exchange membrane fuel cell (PEMFC) is, therefore, critical in achieving the full potential of PEMFC.

  In this work, a novel multi-dimensional, non-isothermal, two-phase fuel cell model is present to study water management in a membrane electrode assembly (MEA).
  The porous electrode microstructural details are accounted for using a mixed wettability pore size distribution (PSD) model.
  The PSD model is used to predict two-phase related transport properties, such as saturation and relative permeability, based on the porous layer's microstructure and wettability.

  The proposed model has been used to study: (a) the effect of catalyst layer microstructure and wettability on fuel cell performance; (b) the role of the micro-porous layer (MPL); and (c) a new electrode architecture, i.e., the electrode coated membrane (ECM).

  Simulation results indicate that, when liquid water is present, reactant transport in the catalyst layer is the key factor limiting fuel cell performance.
  As shown by the model, improved water management in the catalyst layer can be achieved with: (a) a large hydrophobic contact angle; (b) a moderate hydrophilic volume fraction; and (c) a large PSD and small pores in the hydrophilic phase.

  The study on the MPL shows that, under fully humidified conditions, the reasons for the improved water management, after introducing an MPL in a fuel cell, are: (a) increased temperature, and (b) low saturation in the MPL.
  The increased temperature in the electrode results in enhanced evaporation and higher back-diffusion from the cathode to the anode, thereby, alleviates water accumulation.
  Since the MPL is mainly hydrophobic, water does not accumulate in its interior, thereby, creating in-plane transport pathways for the reactant.

  Finally, an inkjet printed electrode coated membrane (ECM) with ionomer based carbon MPL is studied. The ECM demonstrates better performance than the conventional MPL under dry conditions. However, under wet conditions, water accumulation is a problem for ECMs leading to reduced performance.
  The proposed model shows that, under wet conditions, better performance can be achieved for ECMs with: (a) thin MPL; (b) a high electrical conductivity; (c) a low MPL thermal conductivity; (d) a high porosity; (e) a moderate amount of hydrophilic percentage; and (f) a large PSD.

  In summary, this work presents a novel multi-dimensional, non-isothermal, two-phase fuel cell model that accounts for microstructural details by means of a mixed wettability PSD model.
  This model provides an ideal framework to study the microstructure effect of porous layers on water management.
  Knowing the microstructure effect, this work paves the way for designing porous layer microstructure with desired fuel cell performance.

• Subjects / Keywords

  • pore size distribution
  • membrane electrode assembly
  • water management
  • Two-phase model
  • wettability
  • microstructure

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3542JR01

• License

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