Synthesis of Nanoscale Catalysts for Fuel Cells Using Polymeric Metal Complexes with Schiff Bases

Grant Number: 14-29-04057

Project Description

Catalysts based on transition metal oxides are widely used in low-temperature fuel cells for the oxygen reduction reaction (ORR) and considered as promoters or even alternatives to platinum catalysts in alcohol fuel cells. Numerous catalyst production methods exist; most suffer from large particle sizes, structural evolution over time, need for elevated temperatures, and multi-stage synthesis protocols.

Typically, these methods yield powders, making uniform deposition onto high-surface-area porous substrates challenging. The method proposed here generates active catalyst directly from films of polymeric metal complexes by hydrolysis or thermal decomposition.

Key features:

• Precursor: Polymeric films of metal-Schiff base complexes on conductive substrates
• Polymers may contain a single metal or mixtures of metals, deposited via electropolymerization for controlled thickness and uniform coverage
• Removal of organic ligands from the polymer matrix (by thermolysis or alkaline hydrolysis) leaves metal oxides on the substrate
• Homogeneous metal atom distribution in the original film yields nanoscale hydroxide/oxide particles or thin films of catalytic material with tunable composition and dimensions

For alkaline fuel cells, the catalyst can be synthesized in situ at device start-up from the precursor film—electrodes remain inactive and stable until use.

Advantages:

• Catalyst composition is widely tunable by precursor choice
• The technique creates uniform coatings of nanostructured catalysts by a simple process, improving fuel cell efficiency and reducing cost

Project Report

This project developed catalytic coatings fixed to substrates without binders, using N₂O₂-type salen ligand polymeric complexes with strong adhesion to carbon and metal surfaces (via chemisorption).

Achievements:

• Synthesized monomeric complexes and performed their polymerization, obtaining thick films of cobalt complexes and unprecedented cobalt-nickel & copper-nickel mixed films
• Created catalytic layers via:
  1. Low-temperature chemical synthesis of hydroxide coatings from polymeric precursors
  2. Synthesis of oxide-coated noble metal nanoparticles using polymeric films as templates and precursors
  3. Formation of oxide coatings via pyrolysis of polymeric precursors

Outcomes:

• Developed thin, nanodispersed catalyst layers on carbon supports
• Produced non-noble metal oxide ORR catalysts with high specific activity (750 A/g)
• Created bimetallic platinum/metal oxide catalysts yielding ORR current of 550 A/g(Pt) at platinum load ≤10 μg/cm².
• Demonstrated scalability of the deposition methodology in aluminum-air and zinc-air battery prototypes
  • Power in model systems with project-derived platinum catalyst was 25% higher than standard Pt catalysts, at significantly lower Pt content
  • Zinc-air cell with the project’s nickel oxide catalyst (in carbon-nitrogen matrix) had E.M.F. 1.4 V and discharge voltage ~1.25 V at 2 mA/cm²—only 40–50 mV less than a Pt/C cathode (20% Pt by mass)

Conclusion:
Project goals were achieved; binder-free supported catalytic coatings were successfully tested in electrochemical cells and real device prototypes.