Study of Cathode Catalyst Layer Parameters for HT-PEMFC Using Electrochemical Impedance Spectroscopy
Authors: Liu Shuai , Rahim Yasser , Wippermann Klaus , Janßen Holger , Lehnert Werner , Stolten Detlef 
Published: February 2020

One of the main hurdles in achieving optimum performance for the high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) is to come up with just the right combination of material parameters for membrane electrode assembly (MEA) construction. In this study, we investigate the effect of platinum (Pt) loading, cathode catalyst layer (CCL) thickness and the phosphoric acid doping level (PADL) on the performance of a polybenzimidazole (PBI) based HT-PEMFC MEA. The range for each parameter is carefully chosen to address the need of optimizing cell performance at minimum cost. These experiments build on some of the results obtained by Fang et al [1], but with different operating conditions and analysis procedure and a greater emphasis on CCL characterization. It is very important to reduce CCL thickness as it is a limiting factor for effective diffusion of oxygen to the so-called triple phase boundary (TPB). The CCL thickness is varied between 60 µm and 120 µm to analyze the reduction potential. The Pt loading should be as low as possible to reduce cost. The cathode loading for most commercially available HT-PEMFC MEAs is close to 1 mg/cm2. Therefore, this value is used as the upper limit, and 0.6 mg/cm2 is selected as the lower limit. PADL is vital in HT-PEMFC MEAs because phosphoric acid provides the protonic conductivity in the PBI based membrane. A higher doping level improves protonic conductivity, but also floods the pores within the CCL, thus impeding smooth diffusion of oxygen to available TPBs. A range of 15 mg/cm2 to 25 mg/cm2 is selected for the PADL based on literature values and previous experience. All the gas diffusion electrodes (GDE) are prepared by the doctor blade method. Two types of catalyst (20 wt% and 40 wt% Pt/C), spacers with different thicknesses and inks with different proportion of ingredients are used to control the thickness and Pt loading of GDEs. As for the PADL, the doping level of all the membranes is kept at 15 mg/cm2, and 25 mg/cm2 PA doped MEA is obtained by adding extra 5 mg/cm2PA on both cathode and anode GDEs.

A series of designed experiments similar to those presented in Rahim et al [2] is performed using the selected ranges of each parameter. A total of nine MEAs with different parameters are evaluated using polarization curves and electrochemical impedance spectroscopy (EIS) in single cells with an active area of 14.44 cm2. The impedance measurements are performed in the low current density regime (up to 100 mA/cm2) and with a very high cathode stoichiometry (operation as a differential cell – constant conditions inside the cell with hardly any depletion of oxygen between inlet and outlet). This makes it possible to characterize the CCL on the basis of characteristic fuel cell parameters by almost eliminating the mass transport losses within the CCL. Cell impedance is measured at four selected current densities of 10, 20, 50 and 100 mA/cm2. The impedance spectra are fitted with an equivalent circuit model (ECM) to extract the ohmic resistance (RΩ) of the cell, the activation resistance (Ract), the protonic resistance (Rp) and the double layer capacitance (Cdl) of the CCL. All of these cell parameters are found to be a strong function of current density in agreement with some of the observations made by Wippermann et al [3], especially concerning the RΩ.

Finally, a comparison is made between all MEAs and the optimum level for each material parameter is analyzed based on the obtained cell parameters and a discussion of the underlying physics.

References

[1] Liu F, Mohajeri S, Di Y, Wippermann K, Lehnert W. Influence of the Interaction between Phosphoric Acid and Catalyst Layers on the Properties of HT-PEFCs. Fuel Cells 2014;14:750–7. doi:10.1002/fuce.201300272.

[2] Rahim Y, Janßen H, Lehnert W. Characterizing membrane electrode assemblies for high temperature polymer electrolyte membrane fuel cells using design of experiments. Int J Hydrogen Energy 2017;42:1189–202. doi:10.1016/j.ijhydene.2016.10.040.

[3] Wippermann K, Wannek C, Oetjen HF, Mergel J, Lehnert W. Cell resistances of poly(2,5-benzimidazole)-based high temperature polymer membrane fuel cell membrane electrode assemblies: Time dependence and influence of operating parameters. J Power Sources 2010;195:2806–9. doi:10.1016/j.jpowsour.2009.10.100.