Influence of MPL Structure Modification on Fuel Cell Oxygen Transport Resistance

Proton exchange membrane fuel cell (PEMFC)) technology is the basis for a promising future automotive powertrain. However,   PEMFC cost must be reduced and durability must be improved. The greatest opportunity to reduce cost is to run the fuel cell at high current density while not considerably lowering the cell voltage [1], which requires excellent mass (both oxygen and water) transport in the fuel cell electrode, as well as low ohmic loss. In this work we report the measurement of the resistances to oxygen transport in the fuel cell cathode electrode and the impact of microporous layer (MPL) structure modification on the oxygen transport.

The oxygen transport resistance is measured by the limiting current density method [2-4] in a specially designed 5 cm² cell using a variety of O₂ concentrations. The total oxygen transport resistance in a fuel cell cathode sums the resistance from the diffusion media and that from the catalyst layer (CL). The CL resistance includes the Knudsen diffusion resistance and the ionomer permeation resistance: R_tot = R_DM + R_CL = R_DM + R_CL,Knud + R_CL,ion, where  R_DM is quantified by varying the cathode gas pressure [2], and R_CL,Knud and R_CL,ion can be differentiated by varying the temperature [4].

The effect of MPL modification on the oxygen transport resistance has been studied. Straight holes of 20 mm diameter through the MPL, but not through the carbon fiber substrate, were made by a laser perforation technique. The oxygen transport resistances in the cell with the perforated MPL on the cathode were measured and compared to those in the cell with the baseline MPL.   

The total oxygen transport resistance depended on the water production rate, revealing a dry region and a dry-to-wet transition for both MPLs in response to the oxygen concentration increase. The MPL modification had little influence on the total transport resistance in the dry region, while the perforated MPL significantly reduced the transport resistance in the dry-to-wet transition region, indicating the perforated MPL improved the cathode water management. It is not clear at this moment whether the increased transport resistance under the wet condition occurs in the diffusion medium or in the catalyst layer. The transport resistance in the CL can only be obtained for the dry conditions and the MPL modification shows little influence on the CL resistance. A trend of decreasing CL resistance with increasing temperature was observed. This temperature effect is mainly attributable to the oxygen permeation in CL ionomer, while the Knudsen diffusion in CL secondary pores is only weakly dependent on temperature.

References:

1. DOE Roadmap, 2014.
2. D.R. Baker, D.A. Caulk, K.C. Neyerlin, M.W. Murphy, J. Electrochem. Soc., 156, B991 (2009).
3. D.A. Caulk and D.R. Baker, J. Electrochem. Soc., 157, B1237 (2010).
4. N. Nonoyama, S. Okazaki, A.Z. Weber, Y. Ikogi, T. Yoshida, J. Electrochem. Soc., 158, B416 (2011).