Low cost and high-performing platinum group metal-free (PGM-free) cathodes have the potential to transform the economics of polymer electrolyte fuel cell (PEFC) commercialization. Significant advancements have been made recently in terms of PGM-free catalyst activity and stability. However, before PGM-free catalysts become viable in PEFCs, several technical challenges must be addressed including cathode's fabrication, ionomer integration, and transport losses. Here, we present an integrated optimization of cathode performance that was achieved by simultaneously optimizing the catalyst morphology and electrode structure for high power density. The chemically doped metal−organic framework derived Fe−N−C catalyst we used allows precise tuning of the particle size over a wide range, enabling this unique study. Our results demonstrate the careful interplay between the catalyst primary particle size and the polymer electrolyte ionomer integration. The primary particles must be sufficiently large to permit uniform ionomer thin films throughout the surrounding pores, but not so large as to impact intraparticle transport to the active sites. The content of ionomer must be carefully balanced between sufficient loading for the complete catalyst coverage and adequate proton conductivity, while not being excessive and inducing large oxygen transport losses and liquid water flooding. With the optimal 100 nm size catalyst and ionomer loading, we achieved a high power density of 410 mW/cm 2 at a rated voltage and a peak power density of 610 mW/ cm 2 in an automotive-relevant operating condition.

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Platinum group metal (PGM)‐free oxygen reduction reaction (ORR) catalysts for proton exchange membrane fuel cells (PEMFCs) continue to demonstrate significant advances in catalytic activity. Unfortunately, the most promising catalysts exhibit initial rapid activity loss as well as significant sustained activity decay. Herein, it is shown that PGM‐free cathode fuel cell performance can be partially recovered by an in situ electrochemical method by restoring a percentage of the original active sites or activating originally inactive sites. The same approach is also shown to increase the initial activity if applied to a pristine PGM‐free cathode. This is achieved by applying low constant potential holds to the cathode in the absence of oxygen in the cathode, by either flowing nitrogen or blocking the delivery of air and removing the residual oxygen by the ORR. Two types of active sites are further differentiated with degradation mechanisms characterized by different populations and timescales. It is hypothesized that the recovery of sites is due to the reduction of Fe3+ to Fe2+, restoring a percentage of the original active sites or activating originally inactive sites. This reduction has additionally shown to generate the removal of oxygen adsorbates at the catalyst surface.

Platinum group metal (PGM)-free oxygen reduction reaction (ORR) catalysts for proton exchange membrane fuel cells (PEMFCs) continue to demonstrate significant advances in electrocatalytic activity. However, the initial cell high performances cannot be retained over an extended period. We show that fuel cell performance can be electrochemically recovered in-situ, without having to disassemble or disconnect the fuel cell. This was achieved through a set of experiments that included cyclic voltammetry with variations in upper voltage limits and constant potential holds with variation of voltage and time durations. We found that by depriving the cathode of oxygen and applying a low voltage (less than 0.25 V) to the cell, performance can be recovered. Using a reaction kinetics model analysis, the activity decay as well as the recovery and activity enhancement were separated into two sets of sites, one set with a rapid performance decay and a second with a more durable activity. Through a detailed analysis of potential dependence, we show that the voltage requirements to recover the two sites are also different, with the stable sites requiring a lower, more reductive potential that generates greater amounts of reduction current. We hypothesize this is due to electrochemically cleaning the surface of the catalyst from a poisoning species and restoring a percentage of the original active sites. Furthermore, our CV analysis shows the recovery procedure increases the amount of redox active Fe species.

Polymer electrolyte fuel cells offer zero-emissions energy conversion but are commercially limited by cost and durability. Stack cost can be reduced by using platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR). PGM-free catalysts have lower volumetric activity relative to PGM catalysts and consequently require thicker catalyst layers. Thicker catalyst layers result in increased oxygen transport resistance and flooding in the cathode, which must be considered to accurately model PGM-free fuel cells and quantify the distinct transport resistance. For this purpose, we have developed a well validated model to elucidate relative contributions to oxygen transport resistance. This two-phase, transient, non-isothermal, channel-to-channel model contains cathode catalyst layer morphology informed by plasma-focused ion beam (P-FIB) SEM. Furthermore, the model includes agglomerate treatment of volumetrically active catalyst primary particles coated by ionomer. The structure of the agglomerate model is well suited for describing the catalyst layers containing metal organic framework (MOF) derived metal-nitrogen-carbon catalysts that are roughly spherical and volumetrically active for ORR. The MOF-derived Fe-N-C catalyst used in this study’s experiments contains atomically dispersed active sites and uniform, tunable primary particle size. In this presentation, we will report on our investigation of the performance sensitivity to a wide variety of operating conditions and MEA design specificiation, including cell temperature, gas pressures, cathode thickness, hydrophobic pore fractions and wettability, catalyst primary particle size, ionomer content, and ionomer equivalent weight. Non-dimensional characteristic parameter analysis on the model results is used to distinguish the transition between proton conductivity limited and oxygen transport limited performance, and indicate critical values that affect the the through plane ultilization of the catalyst. This information and studies on the impact of active site density help us set catalyst-specific targets for meeting fuel cell performance goals. With highly active PGM-free catalysts, we can model the impact of parameter changes on transport with experimental validation. DOE ACKNOWLEDGEMENT This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under award number DE-EE0008076. The authors gratefully acknowledge research support from the Electrocatalysis Consortium (ElectroCat), established as part of the Energy Materials Network under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under contract number DEEE0008076.