Title

Improved oxygen transport in hydrogen/air proton exchange membrane fuel cells

Date of Completion

January 2004

Keywords

Engineering, Chemical|Energy

Degree

Ph.D.

Abstract

Research was done to reduce oxygen transport limitations in proton exchange membrane fuel cells (PEMFCs) through understanding the role of the gas diffusion layer (GDL) on fuel cell performance in a wide range of operating conditions and designing the GDL for specific applications. The operating conditions of interest ranged from 60°C to 120°C in cell temperature and 0% to 100% in relative humidity. ^ Factors limiting the oxygen transport in the cathode gas diffusion layer were found through characterizing critical properties of the GDL. Linear empirical relationships for permeability coefficient versus limiting current were found at multiple operating conditions. Porosimetry measurement provided the pore size distribution for the gas diffusion layer, which helped in understanding the correlation between the permeability coefficient and the limiting current. ^ Convection was found significant in oxygen transport through the GDL even when using a conventional flow field pattern (i.e. not interdigitated). The influence of cell temperature, oxygen mole fraction, and relative humidity on the limiting current due to reactant gas transport under conditions where there is no significant flooding was evaluated. Cell relative humidity significantly affected the limiting current by reducing oxygen transport through the ionomer thin film of the cathode catalyst layer as the relative humidity decreased. A GDL with higher gas permeability in the micro-porous layer had a higher limiting current due to increased convection. ^ A novel GDL was shown superior to several commercial gas diffusion layers in elevated temperature and/or low relative humidity PEMFC operation. The in-house GDL enabled high performance at elevated temperature conditions. The in-house GDL also enabled PEMFCs to be operated at no external humidification with a minimal additional loss compared to near-saturated conditions at a cell temperature up to 80°C. ^ An analysis technique to evaluate six sources of polarization, including two concentration polarization losses, in hydrogen/air proton exchange membrane fuel cell was developed. The technique was straightforward and useful in diagnosing the sources of loss in membrane electrode assembly development work such as that presented here. ^