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Cationic Chain-Growth Polymerization: Mechanism00:57

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The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the...
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Ziegler–Natta polymerization is another form of addition or chain‐growth polymerization used for synthesizing linear polymers over branched polymers. The catalyst used for polymerization is the Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, who developed it in 1953. This catalyst is an organometallic complex of titanium tetrachloride and triethyl aluminum, with the active form of the catalyst being an alkyl titanium compound. Using the Ziegler–Natta...
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A battery is a galvanic cell that is used as a source of electrical power for specific applications. Modern batteries exist in a multitude of forms to accommodate various applications, from tiny button batteries such as those that power wristwatches to the very large batteries used to supply backup energy to municipal power grids. Some batteries are designed for single-use applications and cannot be recharged (primary cells), while others are based on conveniently reversible cell reactions that...
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Step growth polymerization involves bi or multifunctional monomers. Bifunctional monomers react to form linear step growth polymers, whereas multifunctional monomers react to form non-linear or branched polymers.
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The mechanism for anionic chain-growth polymerization involves initiation, propagation, and termination steps. In the initiation step, a nucleophilic anion, such as butyl lithium, initiates the polymerization process by attacking the π bond of the vinylic monomer. As a result, a carbanion, stabilized by the electron‐withdrawing group, is generated. The resulting carbanion acts as a Michael donor in the propagation step and attacks the second vinylic monomer, which acts as a Michael...
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On the Preparation and Testing of Fuel Cell Catalysts Using the Thin Film Rotating Disk Electrode Method
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Correlating Catalyst Growth with Liquid Water Distribution in Polymer Electrolyte Fuel Cells.

Preetam Sharma1, Douglas Aaron1, Pierre Boillat2

  • 1Electrochemical Energy Storage and Conversion Laboratory, Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee, Knoxville, TN, 37919, USA.

Small (Weinheim an Der Bergstrasse, Germany)
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Summary

Liquid water accumulation under flow field lands in polymer electrolyte fuel cells (PEFCs) accelerates platinum catalyst degradation. Optimizing flow field design is crucial for improving fuel cell durability and performance.

Keywords:
accelerated stress test (AST)catalyst growthfuel cellsneutron imagingwater distribution

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Area of Science:

  • Materials Science
  • Electrochemistry
  • Chemical Engineering

Background:

  • Polymer electrolyte fuel cells (PEFCs) are crucial for clean energy, but catalyst degradation limits their durability.
  • Understanding catalyst degradation mechanisms is vital for advancing fuel cell technology.

Purpose of the Study:

  • To investigate the impact of liquid water distribution on platinum (Pt) catalyst degradation in PEFCs.
  • To correlate liquid water accumulation patterns with heterogeneous Pt particle size distribution after accelerated stress tests (ASTs).

Main Methods:

  • Utilized accelerated stress tests (ASTs) on membrane electrode assemblies (MEAs) in varied cathode environments (N2 and air).
  • Employed high-resolution neutron imaging to map liquid water distribution.
  • Used synchrotron micro-X-ray diffraction (micro-XRD) to analyze Pt particle size distribution.

Main Results:

  • Liquid water preferentially accumulates in diffusion media, particularly under flow field lands, due to thermal resistance differences.
  • Aged MEAs showed increased water retention, linked to enhanced diffusion media hydrophilicity.
  • Significant heterogeneity in Pt particle size was observed, correlating with areas of high liquid water accumulation.

Conclusions:

  • Flow field design critically influences liquid water distribution and subsequent catalyst degradation.
  • Preferential water accumulation under flow field lands exacerbates Pt catalyst degradation.
  • Innovative strategies targeting water management are needed to enhance PEFC durability and performance.