Atomically dispersed and nitrogencoordinated single metal sites embedded in carbon (denoted as M-N-C) have emerged as promising platinum-groupmetal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) cathode in proton-exchange-membrane fuel cells (PEMFCs). [1-5] The MN 4 (M: Fe, Co, or Mn) moieties have been theoretically predicted and then experimentally verified as the active sites in M-N-C catalysts. [6-13] Among the many studied precursors, zinc-based zeolitic imidazolate frameworks (ZIF-8s) are effective in creating atomically dispersed MN 4 sites embedded in defect-rich carbon during the hightemperature carbonization. [8,10,14-17] Despite their encouraging ORR activity demonstrated in aqueous acidic electrolytes recently, [18] the trend is often difficult to reproduce in the membrane electrode assemblies (MEAs) of PEMFCs using solid-state electrolytes (i.e., Nafion) (Table S1, Supporting Information). [19] Low catalyst utilization, severe carbon corrosion, and inferior mass transport within the Increasing catalytic activity and durability of atomically dispersed metalnitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction (ORR) cathode in proton-exchange-membrane fuel cells remains a grand challenge. Here, a high-power and durable CoN -C nanofiber catalyst synthesized through electrospinning cobalt-doped zeolitic imidazolate frameworks into selected polyacrylonitrile and poly(vinylpyrrolidone) polymers is reported. The distinct porous fibrous morphology and hierarchical structures play a vital role in boosting electrode performance by exposing more accessible active sites, providing facile electron conductivity, and facilitating the mass transport of reactant. The enhanced intrinsic activity is attributed to the extra graphitic N dopants surrounding the CoN 4 moieties. The highly graphitized carbon matrix in the catalyst is beneficial for enhancing the carbon corrosion resistance, thereby promoting catalyst stability. The unique nanoscale X-ray computed tomography verifies the well-distributed ionomer coverage throughout the fibrous carbon network in the catalyst. The membrane electrode assembly achieves a power density of 0.40 W cm −2 in a practical H 2 /air cell (1.0 bar) and demonstrates significantly enhanced durability under accelerated stability tests. The combination of the intrinsic activity and stability of single Co sites, along with unique catalyst architecture, provide new insight into designing efficient PGM-free electrodes with improved performance and durability.
We characterized the effects of cobalt (Co 2+ ) and other cation contaminants on the oxygen (O 2 ) transport properties of the PFSA ionomer used in polymer electrolyte membrane fuel cells (PEMFCs) and gained insight into the mechanisms by which contaminant cations inhibit O 2 transport. Such cations can be released by alloy catalysts and environmental conditions and pose a significant challenge to maintaining high current density performance with low platinum (Pt) loadings. We used a test cell capable of isolating the ionomer from the membrane electrode assembly (MEA), allowing for O 2 transport resistance (R O2 ) measurements using a limiting current technique. We contaminated ionomer membranes with Li + , Na + , Ni 2+ , Co 2+ , and Ce 3+ and found a general increase in R O2 for increased contamination levels and decreased water activity. In addition, our Co 2+ results indicated distinct concentrationdependent regimes. The other cation-form ionomers allowed us to separate the impacts of ion pair strength, multivalency, and reduced water uptake. We believe that these factors cause a more compressed hydrophilic domain and tortuous O 2 diffusion path, and a commensurate increase in R O2 . Finally, we studied the impact of Co 2+ on an operating PEMFC and found an increase in R O2 consistent with the results of our isolated membrane tests.
This work was performed to experimentally characterize the oxygen transport resistance effects of cobalt contamination of the ionomer inside of polymer electrolyte membrane fuel cells (PEMFCs). We created a test cell capable of decoupling the ionomer from the membrane electrode assembly, allowing for independent bulk and thin-film ionomer testing using limiting current techniques. Three levels of cobalt-doped bulk ionomers were tested in this study, including un-doped, minimally-doped (5% exchange), and fullydoped cases. The results showed an increase in oxygen transport resistance, both with a decrease in sample relative humidity and an increase in cobalt doping. These results were consistent with hypotheses derived from previous studies of the water content and ionomer structure effects of cationic contamination of the ionomer in PEMFCs.
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