Analysis of Gas Diffusion Layer and Flow Field Design using Neutron Radiography

Abstract

A carbon cloth gas diffusion layer (GDL) displays better performance than a carbon paper GDL under humidified conditions. A straight flow field displays better performance than a serpentine flow field. To investigate these phenomena, neutron radiography was used to compare the amount of liquid water that accumulated in test fuel cells. It was found that a larger amount of water accumulated in a carbon cloth GDL than in a carbon paper GDL. From the standpoint of cell performance, the carbon cloth GDL was less influenced by the accumulated water than the carbon paper GDL. It is assumed that the broader pore distribution of a carbon cloth GDL creates oxygen diffusion paths even if water accumulates in the GDL. With a serpentine flow field, water accumulated in the corner and the gas bypassed the flow field. These phenomena are the main causes of performance deterioration with a serpentine flow field.

Use of neutron imaging for proton exchange membrane fuel cell (PEMFC) performance analysis and design

“…The pore size distribution also seems to play a role in the GDL water retention, at least when comparing paper GDL to cloth GDL, each with an MPL. [16,26] In both studies, the overall cell water retention was about equal, but the paper GDL showed a lower limiting current. Using mercury porosimetry, the paper pore distribution showed a narrow peak corresponding to a bulk pore size of about 50 µm, while the cloth GDL showed a broad pore size distribution from a few micrometers up to about 100 µm.…”

“…Effect on cell performance of water condensation on anode and cathode sides. Yoshizawa et al [26] 2006 PSI 127 Effects of diffusion media type and flow field design on quantity of total accumulated water. Zhang et al [27] 2006 PSI 25 and 100 Effects of diffusion media type and flow field design on the relationship between material properties, liquid accumulation, and electrochemical performance.…”

“…Other studies examined different PTFE loadings in the MPL and bulk of the GDL, and from polarization curves determined that there is an optimum PTFE loading in the MPL for a given GDL substrate. [26,32] Neutron radiography was applied to demonstrate that the loss in performance was attributable to an increase in the GDL water retention.…”

“…In addition to the waste heat, the placement of cartridge heaters can alter the water content in the flow fields, suggesting that the usual assumption of uniform temperature distribution across the flow field is not always valid. [26] The impact on water retention is that this "local heating" can be a source of additional drying of the fuel cell. [11] Thus, it is possible that as the current density is increased, the waste heat aids in the evaporation at the MEA, resulting in a drier cell.…”

“…[11,16,19] An aspect of this slow response is that one can observe a hysteresis in the water content during sweep up and down of the current density. [25,26] For a given reactant stoichiometry, the critical saturation may only occur near the limiting current. As the current is reduced from near limiting current, there may not be sufficient gas flow to remove the liquid from within the GDL.…”