Mass transport in polymer electrolyte membrane fuel cells using natural convection for air supply

Abstract

A fuel cell converts chemical energy into electricity and heat through electrochemical reactions. Polymer electrolyte membrane fuel cells (PEMFCs) are approaching commercialization in many applications, including transportation, stationary power, and portable devices. In this thesis, the focus was on small-scale PEMFCs, in which natural convection is used as the air supply method.

A cell design with straight vertical cathode channels was studied using experimental and modeling methods, in order to obtain a quantitative insight into mass transport phenomena and to identify the performance limiting processes. The variation of mass transport conditions over the active area of the cell was studied using a current distribution measurement system, which was based on the use of a segmented current collector. The accuracy of the method was analyzed by experimental work and numerical simulation. In order to quantify the local mole fractions of water and oxygen, and the velocity of buoyancy-driven air flow in the cathode channel, a numerical model was developed to describe mass transport in the cathode channel and the gas diffusion layer. Water transport across the polymer membrane was studied by measuring the fraction of product water exiting through the anode. The results give indication of the variation of net water transport coefficient across the active area. The redistribution of water along with the hydrogen flow was also observed. The effect of ambient temperature and relative humidity on cell performance was investigated in a climate chamber. For stack research, a measurement approach was developed for determining the ohmic voltage losses of individual cells in a stack by the current interruption method.

As an overall conclusion, it was found that the cell design should be improved especially from the point of view of water management. In order to reduce flooding problems, the cross-section and length of the cathode channels were identified as key parameters to be optimized. It was also found that mechanically rigid gas diffusion layer materials are advantageous for designing an optimized geometry. In addition, it was found that the choice of the anode flow geometry can be used to control the distribution of water across the active area.