Layer Design for PGM-Free Catalysts for Pemfcs: Impact of Electrical Conductivity & Diagnostic Tools
Authors: Li Yan-Sheng , Suthar Bharatkumar , Menga Davide , Gasteiger Hubert A. 
Published: December 2020

Recently, the notable efforts placed on investigating platinum group metal (PGM)-free catalysts (M-N-C catalysts, M = Fe, Co, & Mn) for the oxygen reduction reaction (ORR) have led to the possible development of low-cost proton exchange membrane fuel cells (PEMFCs).1 However, compared to PGM ORR catalysts, the lower turnover frequency and active site density of PGM-free catalysts generally require high-surface-area structures and high catalyst loadings to achieve reasonable low-current density performance in PEMFCs, thereby resulting in significant ionic, mass, and electronic transport overpotentials. 2 With regards to these transport resistances, an intriguing observation from the PEMFC performance with PGM-free cathode catalysts is the significantly higher electronic overpotential (iR-drop) when compared to PGM-based cathode catalysts (e.g., Pt/C). This increased iR-drop results in an increase of the high frequency resistance (HFR) due to the high electrical resistivity (ρe- in Ω/cm) of the thick PGM-free cathode catalyst layers. While ρe- of conventional Pt/C electrodes is rather low (~0.5-1 Ω/cm) and results in negligible overpotential losses, ρe- of PGM-free cathode layers can be higher by a factor of ~10-20 due to poor electrical conductivity of the active site (e.g., Fe-N-C) and the high-surface area structures of these catalysts. While several studies have revealed the impact of ionic and mass transport resistances of PGM-free cathode electrodes,3,4 the impact of electrical resistance is still unclear. A quantification of this effect is required for the rational design of PGM-free cathode electrodes, whereby the following three points would be of major interest: (1) The values of PGM-free catalysts vary widely, so that in a comparative study catalysts with high and low should be investigated. (2) When preparing electrodes with PGM-free catalysts that have a high value ρe-, conductive-carbon additives are commonly added, but there is no simple diagnostic tool that could be used for optimizing the amount of additive. (3) A quantitative model to predict the cell voltage loss due to the impact of ρe- would be desired. This can be accomplished by in-situ AC impedance analysis based on a transmission line model. 5 However, owing to higher ratio of ρe- to the ionic resistivity (ρH+) in PGM-free electrodes (>0.1/1), a modified approach needs to be developed to precisely quantify the voltage losses.6

In the present work, we present small-active area (5 cm2) single-cell H2/O2 PEMFC performance (differential-flow operation) combined with in-situ AC impedance and ex-situ 4-point-probe resistance measurement in order to elucidate the impact of the electrical resistivity of PGM-free cathode catalyst layer on performance. Two different catalysts with low and high ρe- are investigated. The results indicate that the lower one shows no significant change in cell voltage upon the addition of 7.5 wt% of conductive carbon (Figure 1a). However, with same wt% of carbon additive, the cell voltage for catalyst with high ρe- can be increased by 70 mV at 1 A/cm2, resulting in a ~30% improvement in peak power density (Figure 1b). Furthermore, we will present an in-situ AC impedance technique under blocking condition (N2 feed to cathode) that we developed to quantify ρe- and ρH+ of PGM-free cathode electrodes with and without carbon additive. Using these values, the cell-voltage gains due to the reduction of ρe- of a PGM-free cathode by carbon additive can be predicted (Figure 1c). At the end, a comparison between in-situ AC-impedance and ex-situ 4-point-probe measurements for PGM-free cathode electrodes will be presented and discussed. Overall, the study not only provides a new insight and methodology for electrode optimization of PGM-free cathode catalysts, but also reveals in some cases show significant impact of electrical conductivity on fuel cell performance with PGM-free catalysts.
ACKNOWLEDGEMENTS
This work is supported by the Fuel Cells and Hydrogen Joint Undertaking under grant agreement No. 779550 (PEGASUS). This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research.
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Figure 1