Browsing by Author "Cullen, David A."

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    Amino-tethering synthesis strategy toward highly accessible sub-3-nm L10-PtM catalysts for high-power fuel cells
    (Elsevier, 2023-03) Gong, Qing; Zhang, Hong; Yu, Haoran; Jeon, Sunghu; Ren, Yang; Yang, Zhenzhen; Sun, Cheng-Jun; Stach, Eric A.; Foucher, Alexandre C.; Yu, Yikang; Smart, Matthew; Filippelli, Gabriel M.; Cullen, David A.; Liu, Ping; Xie, Jian; Earth and Environmental Sciences, School of Science
    Because of the poor accessibility of embedded active sites, platinum (Pt)-based electrocatalysts suffer from insufficient Pt utilization and mass transport in membrane electrode assemblies (MEAs), limiting their performance in polymer electrolyte membrane fuel cells. Here, we report a simple and universal approach to depositing sub-3-nm L10-PtM nanoparticles over external surfaces of carbon supports through pore-tailored amino (NH2)-modification, which enables not only excellent activity for the oxygen reduction reaction, but also enhanced Pt utilization and mass transport in MEAs. Using a low loading of 0.10 mgPt·cm−2, the MEA of PtCo/KB-NH2 delivered an excellent mass activity of 0.691 A·mgPt−1, a record-high power density of 0.96 W·cm−2 at 0.67 V, and only a 30-mV drop at 0.80 A·cm−2 after 30,000 voltage cycles, which meets nearly all targets set by the Department of Energy. This work provides an efficient strategy for designing advanced Pt-based electrocatalysts and realizing high-power fuel cells.
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    Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells
    (Springer Nature, 2022) Liu, Shengwen; Li, Chenzhao; Zachman, Michael J.; Zeng, Yachao; Yu, Haoran; Li, Boyang; Wang, Maoyu; Braaten, Jonathan; Liu, Jiawei; Meyer, Harry M., III; Lucero, Marcos; Kropf, A. Jeremy; Alp, Esen E.; Gong, Qing; Shi, Qiurong; Feng, Zhenxing; Xu, Hui; Wang, Guofeng; Myers, Deborah J.; Xie, Jian; Cullen, David A.; Litster, Shawn; Wu, Gang; Mechanical and Energy Engineering, Purdue School of Engineering and Technology
    Nitrogen-coordinated single atom iron sites (FeN4) embedded in carbon (Fe–N–C) are the most active platinum group metal-free oxygen reduction catalysts for proton-exchange membrane fuel cells. However, current Fe–N–C catalysts lack sufficient long-term durability and are not yet viable for practical applications. Here we report a highly durable and active Fe–N–C catalyst synthesized using heat treatment with ammonia chloride followed by high-temperature deposition of a thin layer of nitrogen-doped carbon on the catalyst surface. We propose that catalyst stability is improved by converting defect-rich pyrrolic N-coordinated FeN4 sites into highly stable pyridinic N-coordinated FeN4 sites. The stability enhancement is demonstrated in membrane electrode assemblies using accelerated stress testing and a long-term steady-state test (>300 h at 0.67 V), approaching a typical Pt/C cathode (0.1 mgPt cm−2). The encouraging stability improvement represents a critical step in developing viable Fe–N–C catalysts to overcome the cost barriers of hydrogen fuel cells for numerous applications.
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    Effects of Ink Formulation on the Structure and Performance of PGM-Free Catalyst Layer in PEMFCs
    (IOP, 2021) Li, Chenzhao; Liu, Shengwen; Zeng, Yachao; Liu, Yadong; Wu, Gang; Cullen, David A.; Xie, Jian; Mechanical and Energy Engineering, Purdue School of Engineering and Technology
    Platinum group metal (PGM) catalysts are the major electrocatalysts for oxygen reduction reaction (ORR) in the polymer electrolyte membrane fuel cells (PEMFCs). The cost becomes unaffordable if the PEMFC is in massive application. The PGM-Free catalyst shows very promising activity in rotation disk electrode (RDE) testing. The half-wave potential could reach 0.91 V versus standard hydrogen electrode (SHE). However, in a membrane electrode assembly (MEA), the performance of PGM-Free catalysts is not good enough to replace the PGM catalysts. Since the PGM-free catalysts are so different from the PGM catalysts in terms of catalytic activity, stability, surface conditions, particle size, etc., the fabrication of PGM-Free catalyst MEA cannot simply copy the method of making PGM MEA. We proposed a novel method of fabricating PGM-Free catalyst MEA, so that the intrinsic catalyst activity from RDE can be translated into MEA performance. The method is based on the catalyst coated membrane (CCM) method using optimized ionomer to carbon (I/C) ratio and solvent mixture of catalyst ink. Using this method, the PGM-free catalyst MEA achieved the current density 44.9 mA cm-2 at 0.9 V iR-free in H2/O2 and 150 mA cm-2 at 0.8 V in H2/air, which surpassed the performance targets of US Department of Energy (DOE)for PGM-Free catalyst MEA. The property (solvent composition, dispersion of catalyst and ionomer in an ink), structure (pore structure) and the MEA performance have been characterized using, mercury intrusion porosimetry (MIP), MEA testing. A property-structure-performance relationship has been established.