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  1. This review article presents the recent progress in the area of synthesis of quaternized aryl ether-free polyaromatics for alkaline membrane fuel cells.
  2. Fuel cells are attractive devices that convert chemical energy into electricity through the direct electrochemical reaction of hydrogen and oxygen. Intermediate temperature fuel cells operated at 200–300°C can simplify water and thermal managements, enable the use of non-precious or low-loading precious metal catalysts and provide insensitivity toward fuel and air impurities such as carbon monoxide. However, the performance of current intermediate temperature fuel cells is poor due to a lack of highly-conductive membrane electrolytes and optimal electrodes designed for these fuel cells. We demonstrate high-performing intermediate temperature fuel cells that use SnP 2O 7–polymer composite membranes and a quaternary ammonium-biphosphatemore » ion-pair coordinated polymer electrolyte in the electrodes. The peak power density of the fuel cell under H 2 and O 2 reached 870 mW cm -2 at 240°C with minimal performance loss under exposure to 25% carbon monoxide.« less
  3. The adsorption of the ionomer components significantly impacts the activity of hydrogen oxidation reaction (HOR) catalysts under high pH conditions. Two specific adsorptions, i.e., (i) cation-hydroxide-water co-adsorption and (ii) phenyl group adsorption, on the surface of platinum group metal HOR catalysts were identified as the performance-limiting factors for anion-exchange membrane fuel cells (AEMFCs). Here, we review the main characteristics of the surface adsorptions, their impact on the performance of HOR half-cell and AEMFC, and the mitigation strategies. This review emphasizes important aspects in the design of electrocatalysts and ionomers for improved AEMFC performance.
  4. Stabilizing transition metals (M) in MPt alloy under acidic conditions is challenging, yet crucial to boost Pt catalysis toward oxygen reduction reaction (ORR). We synthesized ~9 nm hard-magnet core/shell L1 0-CoPt/Pt nanoparticles with 2–3 atomic layers of strained Pt shell for ORR. At 60°C in acid, the hard-magnet L1 0-CoPt better stabilizes Co (5% loss after 24 hr) than soft-magnet A1-CoPt (34% loss in 7 hr). L1 0-CoPt/Pt achieves mass activities (MA) of 0.56 A/mg Pt initially and 0.45 A/mg Pt after 30,000 voltage cycles in the membrane electrode assembly at 80°C, exceeding the DOE 2020 targets on Pt activitymore » and durability (0.44 A/mg Pt in MA and <40% loss in MA after 30,000 cycles). Lastly, density functional theory calculations suggest that the ligand effect of Co and the biaxial strain (-4.50%/-4.25%) of the Pt shell weaken the binding of oxygenated species, leading to enhanced ORR performance in fuel cells.« less
  5. We report that addition of flow fields to carbon paper electrodes in a vanadium redox flow battery (VRFB) can improve the peak power density through uniform distribution of electrolyte in the electrodes. However, it is unclear whether flow fields have a similar effect with graphite felt electrodes, as VRFBs with felt electrodes reported in literature show a large anomaly in obtained power density. In this work, we evaluate three flow fields; viz. serpentine, interdigitated and conventional (without flow pattern) type with felt electrodes and compare their performance with a serpentine flow field using carbon paper electrodes under identical experimental conditions.more » The conventional flow field provides highest energy efficiency (75%) followed by serpentine (64%) and interdigitated (55%) at 0.2 A cm -2 attributable to the deteriorating electrolyte distribution in the electrodes. Computation fluid dynamic simulations confirm the experimental finding of worsening electrolyte distribution (conventional < serpentine < interdigitated). A power density of 0.51 W cm -2 at 60 mL min -1 flow rate is obtained for serpentine and conventional flow fields with felt electrodes; comparable to the highest power density reported in literature for high performing zero-gap flow field architecture. Finally, this paper gives comprehensive insights on flow fields for VRFBs that can be extended to other flow batteries.« less
    Cited by 1
  6. Alkaline membrane fuel cells (AMFCs) show great potential as alternative energy conversion devices to acidic proton exchange membrane fuel cells (PEMFCs). Over the last decade, there has been significant progress in the development of alkaline-stable polyaromatic materials for membrane separators and ionomeric binders for AMFCs. However, the AMFC performance using polyaromatic ionomers is generally poor, ca. a peak power density of <400 mW cm -2. We report a rational design for polyaromatic ionomers which can minimize undesirable phenyl group interaction with hydrogen oxidation catalysts. The AMFC using a newly designed aryl ether-free poly(fluorene) ionomer exhibits a peak power density ofmore » 1.46 W cm -2, which is approaching that of Nafion-based PEMFCs. This study further discusses the remaining challenges of high-performing AMFCs.« less
    Cited by 1
  7. Here, an effective method to enhance the durability of polymer electrolyte membrane fuel cells (PEMFCs) is reported. PEMFC performance loss is mitigated by exposing the electrodes of fuel cells to dry nitrogen gas periodically at high temperature. This method extends the lifetime of fuel cells significantly compared to their non-treated counterparts. The impact of treatment temperature and exposure time on PEMFC durability is reported, using potential cycling accelerated stress tests. The enhanced durability is attributed to the suppression of “ionomer relaxation” that occurs under the nearly water saturated operating conditions of a PEMFC cathode.
  8. Material interactions at the polymer electrolytes–catalyst interface play a significant role in the catalytic efficiency of alkaline anion-exchange membrane fuel cells (AEMFCs). The surface adsorption behaviors of the cation–hydroxide–water and phenyl groups of polymer electrolytes on Pd- and Pt-based catalysts are investigated using two Pd-based hydrogen oxidation catalysts—Pd/C and Pd/C-CeO 2—and two Pt-based catalysts—Pt/C and Pt-Ru/C. The rotating disk electrode study and complementary density functional theory calculations indicate that relatively low coadsorption of cation–hydroxide–water of the Pd-based catalysts enhances the hydrogen oxidation activity, yet substantial hydrogenation of the surface adsorbed phenyl groups reduces the hydrogen oxidation activity. The adsorption-driven interfacialmore » behaviors of the Pd- and Pt-based catalysts correlate well with the AEMFC performance and short-term stability. Finally, this study gives insight into the potential use of non-Pt hydrogen oxidation reaction catalysts that have different surface adsorption characteristics in advanced AEMFCs.« less
    Cited by 2
  9. We report in this article a detailed study on how to stabilize a first-row transition metal (M) in an intermetallic L1 0-MPt alloy nanoparticle (NP) structure and how to surround the L1 0-MPt with an atomic layer of Pt to enhance the electrocatalysis of Pt for oxygen reduction reaction (ORR) in fuel cell operation conditions. Using 8 nm FePt NPs as an example, we demonstrate that Fe can be stabilized more efficiently in a core/shell structured L1 0-FePt/Pt with a 5 Å Pt shell. The presence of Fe in the alloy core induces the desired compression of the thin Ptmore » shell, especially the 2 atomic layers of Pt shell, further improving the ORR catalysis. This leads to much enhanced Pt catalysis for ORR in 0.1 M HClO 4 solution (both at room temperature and 60°C) and in the membrane electrode assembly (MEA) at 80°C. The L1 0-FePt/Pt catalyst has a mass activity of 0.7 A/mg Pt from the half-cell ORR test and shows no obvious mass activity loss after 30,000 potential cycles between 0.6 V and 0.95 V at 80°C in the MEA, meeting the DOE 2020 target (<40% loss in mass activity). Here, we are extending the concept and preparing other L1 0-MPt/Pt NPs, such as L1 0-CoPt/Pt NPs, with reduced NP size as a highly efficient ORR catalyst for automotive fuel cell applications.« less
    Cited by 15Full Text Available
  10. Slow hydrogen oxidation reaction (HOR) kinetics on Pt under alkaline conditions is a significant technical barrier for the development of high-performance hydroxide exchange membrane fuel cells. Here we report that benzene adsorption on Pt is a major factor responsible for the sluggish HOR. Furthermore, we demonstrate that bimetallic catalysts, such as PtMo/C, PtNi/C, and PtRu/C, can reduce the adsorption of benzene and thereby improve HOR activity. In particular, the HOR voltammogram of PtRu/C in 0.1 M benzyl ammonium showed minimal benzene adsorption. Density functional theory calculations indicate that the adsorption of benzyl ammonium on the bimetallic PtRu is endergonic formore » all four possible orientations of the cation, which explains the significantly better HOR activity observed for the bimetallic catalysts. In conclusion, the new HOR inhibition mechanism described here provides insights for the design of future polymer electrolytes and electrocatalysts for better-performing polymer membrane-based fuel cells.« less

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