Impedance spectroscopy assessment of catalyst coated Nafion assemblies for proton exchange membrane fuel cells

We report on the employment of the electrochemical impedance spectroscopy (EIS) to characterize catalyst coated membrane (CCM) electrode assemblies (MEA) as a function of ionomer percentage versus relative humidity, in proton exchange membrane fuel cell (PEMFC). The catalyst layer was fabricated by spraying TKK catalyst containing Nation ionomer directly onto Nation membranes, sandwiched between two identical mesoporous layer coated gas diffusion layers (GDL(MPL)). Relative humidity was varied for the MEA's containing various Nation contents (MEA(M); M: Nation wt. percentage per mass of Pt). The MEA(20) had low charge transfer (R-CT) and high ionic (R-ion) resistances, while MEA(60) showed high R-CT and low R-ion. Both generated inferior performances to MEA(30), MEA(40), and MEA(50). These observations simply ruled out R-CT or R-ion to independently explain the fuel cell performance. However, the MEA(30), MEA(40), and MEA(50) had a combination of low R-CT and R-ion, all producing power densities larger than 1.1 W cm(-2). This indicates that both R-CT and R-ion would effectively contribute to the performance of the MEA's, where the maximum power density sharply dropped when sum of the normalized R-CT and R-ion (i.e. R-T, total resistance) was similar to 0.5 Omega or higher. It appears that the R-T may be a reliable while simple parameter for assessing the ketjenblack-based TKK with Nation as ionomer for PEMFC. Such diagnostic tool can be further evaluated and extended to other catalysts with different supports and ionomer compositions. (C) 2018 Elsevier Ltd. All rights reserved.