Comprehensive Characterization of PGM-Free PEM Fuel Cells Using AC and DC Methods
Authors: Reshetenko Tatyana V. , Serov Alexey , Kulikovsky Andrei , Atanassov Plamen 
Published: February 2020

In the past decade, a lot of efforts have been done to develop a platinum group metal-free (PGM-free), cathodic materials for proton exchange membrane fuel cells (PEMFCs) as alternative to classic Pt/C cathodes [1, 2]. At present, Fe-N-C systems are considered as one of the most promising catalysts for oxygen reduction reaction (ORR) with peak power density of about 0.5 W cm-2 [3]. The key problems in Fe-N-C electrodes relate to ORR mechanism which can proceeds through a combination of 2- and 4-electron pathways depending on operating current density, and to mass transport processes occurring in relatively thick electrodes structure. Herein, we report comprehensive study of Fe-N-C electrocatalyst integrated into cathodic catalyst layers of PEMFC with various composition configurations using AC and DC methods. We fit the physics-based impedance model, which takes into account oxygen transport in the cathode catalyst layer (CCL), gas diffusion layer (GDL) and flow-field channels to the experimental impedance spectra, and obtain main kinetic (Tafel slope) and transport parameters (proton conductivity and oxygen mass transport coefficients) of PGM-free PEMFCs in application-relevant conditions.

Electrochemical evaluation has been performed using a segmented cell system and a test station developed at Hawaii Natural Energy Institute. Membrane electrode assemblies (MEAs) consisted of Fe-N-C (with varied catalyst and ionomer loadings) cathode gas diffusion electrodes (GDE), Pt anode GDE (0.1 mgPt cm-2, Alfa Aesar) and Nafion XL. The MEA was operated under galvanostatic control of the whole cell current, 80°C, 304 kPa back pressure, 100% relative humidity for the both electrodes and varied cathode stoichiometry. Details of the impedance model used in the present work have been reported in [4].

Fig. 1 a) shows a representative SEM image of the Fe-N-C GDE cross-section. The cathode thickness was found to be 120-140 μm for a sample with 3.0 mg cm-
2 catalyst loading (Fig. 1 a), while a catalyst content to 1.5 mg cm-2 caused appropriate decrease in the thickness to 70 μm. The independence of electrode thickness from ionomer content (35 vs 45 wt. %) may indicate the preferential distribution of Nafion® inside of large pores of Fe-N-C catalyst synthesized by sacrificial support method.

The results obtained from AC-impedance and IV data clearly showed that the largest problem of all the electrodes is high absolute value of the kinetic Tafel slope, which varies from 100 to 250 mV/dec for current densities in the range of 0.025 to 0.4 A cm-
2. This parameter is responsible for faster decay of the cell polarization curve as compared to Pt/C-based PEMFCs. The mean proton conductivity σ

p
and CCL oxygen diffusivity Dox
increase with the cell current density by a factor of ten (Fig. 1 b, c). Both effects take place also in Pt/C systems [5]; however, the absolute value of Dox
in the Fe-N-C electrodes is an order of magnitude higher, than in the Pt/C cathodes (0.5-2.5×10-
4 cm2 s-1) [6]. The growth of σ

p
with current partly is due to accumulation of liquid water in the CCL; however, it may also be due to ionization of acidic molecules on carbon surface at lower cell potentials. A comparison of different CCL configurations shows that each catalyst loading value requires its own optimum ionomer content in the catalyst layer. Detailed analyses of the PGM-free PEMFC performance and its correlation with AC-impedance results will be presented and discussed.

ACKNOWLEDGEMENTS

We gratefully acknowledge ONR (N00014-18-1-2127), DOE EERE (DE-EE0008419) and Hawaiian Electric Company. The authors are thankful to Rohan Gokhale for preparing some GDE samples, Tina Carvalho for assisting with SEM, Günter Randolf and Jack Huizingh for valuable help in the system operation.

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Figure 1