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

Abstract

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 Nafion ionomer directly onto Nafion membranes, sandwiched between two identical mesoporous layer coated gas diffusion layers (GDLMPL). Relative humidity was varied for the MEA's containing various Nafion contents (MEAM; M: Nafion wt. percentage per mass of Pt). The MEA20 had low charge transfer (RCT) and high ionic (Rion) resistances, while MEA60 showed high RCT and low Rion. Both generated inferior performances to MEA30, MEA40, and MEA50. These observations simply ruled out RCT or Rion to independently explain the fuel cell performance. However, the MEA30, MEA40, and MEA50 had a combination of low RCT and Rion, all producing power densities larger than 1.1 W cm−2. This indicates that both RCT and Rion would effectively contribute to the performance of the MEA's, where the maximum power density sharply dropped when sum of the normalized RCT and Rion (i.e. RT, total resistance) was ∼0.5 Ω or higher. It appears that the RT may be a reliable while simple parameter for assessing the ketjenblack-based TKK with Nafion as ionomer for PEMFC. Such diagnostic tool can be further evaluated and extended to other catalysts with different supports and ionomer compositions.