Gas diffusion in thin porous catalyst layers of PEM fuel cells

Author: 
Date created: 
2018-10-30
Identifier: 
etd19946
Keywords: 
Fuel cell, modified Loschmidt cell, catalyst layer designs, gas diffusivity, operating conditions
Abstract: 

Oxygen molecules reach the reaction sites in the cathode catalyst layer (CL) of PEM fuel cells through diffusion, and the water vapor produced at the cathode leaves the reaction sites through diffusion. Therefore, the gas diffusivity of CL influences fuel cell performance. Uniform oxygen delivery to the Pt particles is one of the primary parameters ensuring high activity level of Pt particles and prolonging the CL lifetime. A sufficient supply of oxygen to the CL is required to achieve high current densities. Therefore, to reach high power outputs with low Pt loading, it is vital to understand the mechanism and improve the oxygen diffusion rate within CL and investigate the effects of different operating conditions on its performance. To investigate the effect of different CL designs and operating conditions on gas diffusivity, a modified Loschmidt cell was used to measure the gas diffusivity of CL. Also, the pore size distribution of CL was measured with N2 adsorption porosimetry. Moreover, the structure of CL was modeled through considering a packed-sphere model for carbon particles within agglomerates, and a network of overlapped spherical agglomerates forming the CL. The gas diffusion problem was solved analytically for the CL structure considering both Knudsen and molecular mechanisms. The results show that decreasing the ionomer content of CL from an ionomer to carbon weight ratio of 1.5 to 0.5 increases the relative diffusivity by 400%. Dry milling the catalyst powders for 48 hours led to 50% drop in the relative diffusivities of CL. Drying the catalyst ink on the support substrate at elevated temperatures improved gas diffusivity in some cases. The CL effective diffusivity is higher in higher operating temperature; however, its relative gas diffusivity is lower. High compressive loads (30 MPa or 50 MPa) reduces the CL diffusivity; however, in range of fuel cell operating condition (<5 MPa) the effect is negligible. The effect of gas relative humidity on the relative diffusivity of CL is negligible. On the other hand, liquid water reduces CL relative diffusivity. For example, a 25 wt. % water content in CL results in a 25% drop in relative diffusivity.

Document type: 
Thesis
Rights: 
This thesis may be printed or downloaded for non-commercial research and scholarly purposes. Copyright remains with the author.
File(s): 
Senior supervisor: 
Majid Bahrami
Department: 
Applied Sciences: School of Mechatronic Systems Engineering
Thesis type: 
(Thesis) Ph.D.
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