New Insights from Electrochemical Diagnostics Pertaining to the High Current Density Performance of Pt-Based Catalysts
Authors: Neyerlin K.C. , Christ Jason M , Zack Jason W. , Gu Wenbin , Kumaraguru Swami , Kongkanand Anusorn , Kocha Shyam S. 
Published: February 2020

Significant advances in the development of electrocatalysts that exceed the DOE target of 440 mA/mgPt (0.90V, 80oC, 100% RH, ptotal=150 kPa) for ORR activity have placed fuel cells on a promising path towards achieving the DOE target of a 0.1 mgPt/cm2
elec cathode loading by 2020. However, unanticipated voltage losses that manifest at high current density and low Pt loading have prevented the attainment of the 0.125 gPGM/kWrated 2020 target.1 While some of the observed voltage loss for low Pt electrodes (< 0.1 mgPt/cm2
elec) has been attributed to oxide dependent kinetics that manifest at the lower iR-free potentials (<0.75V),2 it has been observed that a significant portion of this unanticipated loss stems from a transport resistance local to or associated with electrochemically available Pt surface area.1 While the exact cause of this phenomenon remains unknown, studies have demonstrated that this loss both; 1) scales with total Pt surface area1 and 2) can be associated with the incorporation of ionomer into the cathode electrode.3Several diagnostics have been developed and applied to PEMFC materials in an attempt to isolate the exact cause and/or quantify the magnitude of this so termed “local Pt resistance”.4
-6 However insightful the results of these studies may be, the idealized conditions under which the diagnostics have been performed leave questions as to the relevance of the extracted resistance values. In this work we will expand the parameter space used previously for oxygen limiting current measurements4as a means to elucidate trends and gain further insight into the cause and magnitude of the local Pt resistance at relevant PEMFC operating conditions (e.g. cell temperature, oxygen partial pressure and relative humidity).
Experiments were performed using a 5cm2 differential cell with MEAs consisting of Pt/V and Pt/HSC at loadings of 0.2 – 0.05 mg Pt/cm2. An example measurement is shown in Figure 1 where clear increases in local Pt resistance (RO2,local) can be observed commensurate with; 1) an increase in oxygen partial pressure (pO2) and 2) a decrease in relative humidity (RH).
Figure 1. Local Pt resistance (RO2,local) as a function of oxygen partial pressure (pO2) and relative humidity (RH). Data was obtained on 50 wt% Pt/HSC electrodes of 0.05, 0.10 and 0.25 mgPt/cm2nominal loading, which were diluted with carbon to maintain a uniform thickness.

Acknowledgements: This work was funded in part under the DOE FC-PAD consortium and also by the U.S. Department of Energy under CRADA #CRD-14-539.

References

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Figure 1